Previously Asked Questions

Weather Jobs, Weather Tools and Forecasting

More about Weather Forecasting

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Nick, why do we have weather?
Mrs. Smith
Pamona Elementary School
Oceanside, PA

Dear Ms. Smith's class,
Sometimes we concentrate so much on the details we forget the big picture. That's why I'm especially glad to answer your question. But bear in mind that entire volumes have been written in answer to your question, and I have only one page to do it!

There is one basic reason we have weather, and that is the sun. Most of the large-scale weather systems that we recognize on weather maps develop because the sun's energy is not distributed equally over Earth. Most of the time the sun shines most directly on low latitudes, with less heating of the polar regions. Earth is tilted on its axis at exactly the right angle to have seasons, with different parts of Earth being heated more or less during different times of the year. Further, land heats up faster than water, setting up temperature differences between oceans and continents. These unequal heatings create temperature and air pressure variations, winds, and ocean currents.

The winds transport heat from the tropical regions toward the poles in a never-ending effort to reach a thermal balance. In the process, fronts set up between warm and cold air masses, and a jet stream forms aloft in association with the fronts. Disturbances develop along the fronts and in the jet stream that trigger centers of low and high pressure. The fronts and low-pressure areas have rising motion that yields clouds and precipitation.

Earth is also unique in that all three forms of water-liquid, solid and gas-exist naturally. Heating from the sun helps evaporate water from the oceans as a source for clouds and precipitation, while the unending sun-powered circulations distribute Earth's water from its oceans to atmosphere to land and back and between the three forms. Heating from the sun can also trigger thunderstorms. Thunderstorm clusters over warm ocean waters can turn into hurricanes. The sun is behind all the changes in our weather and if it were to suddenly be extinguished, our weather machine would stop.
Nick

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Nick,
What tools do you use to track storms?

Where I work at The Weather Channel, we use several tools to help us track storms.  Some of the most important tools are the weather satellites orbiting the Earth.   The main satellites we use are called geostationary satellites.   They orbit the Earth at the same rate that the Earth rotates, so the satellites always stay above the same place over the earth.   They are up about 22 thousand miles (36,000 km) taking pictures of earth about every half hour.  We use these pictures to see which direction storms are going and whether they are getting stronger or weaker. 

Another valuable tool is Doppler radar.  Radars send out radio signals, which bounce off rain drops, hail stones and snow flakes, and reflect some of the signals back to the radar antenna.  The radar shows these reflections in picture form, so we can see where rain or snow is falling and how hard it is coming down.  

Probably the simplest and most reliable tool we use is one that has been used to track storms for thousands of years.  That tool is nothing more than a good set of eyes.  Weather observers all over the world report changes in the weather every hour, and even more often when the weather is changing rapidly.  The weather observers record the temperature, the wind direction, wind speed, air pressure and cloud cover.  We plot these observations on weather maps every hour to see where storms are moving.  Computers also use these observations in mathematical formulas to help forecast where storms will be the next day and the day after that. 

So the next time you watch The Weather Channel, I hope you will think about all the tools we use, and all the people who use their knowledge of the atmosphere to follow the path of a storm and forecast where it is going.

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I’ve been so interested in weather since I can’t even remember and want to attend a good college that will teach me the skills I need to become a meteorologist. What should I concentrate on studying the next couple of years to prepare?
Brandea
Garland, TX

Brandea,
You can prepare for a weather career by studying hard in science and math, and while you are in high school, take the advanced math and science courses too.  Weather includes chemistry and physics and it’s also valuable to know other sciences such as oceanography and geography too.  Work on your English and reading so you’ll learn how to speak and write well. This is especially important if you go into weather broadcasting, but good communication skills are very in any line of work you end up choosing. 

A lot of universities offer degree programs in meteorology and atmospheric sciences.  You can find a list of them at http://www.weather.com/education/student/colleges.html  In addition, the branches of the U.S. military offer extensive training in meteorology.  Good luck to you; you’ll probably have my job someday!

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Hi Mr. Walker, I am a 12th grade student at Kingsway Reg. High School and I am working on my senior paper.  I chose meteorology and I was curious about the education/training requirements, and a brief classification of the jobs in the field. Any information that I can receive would be great and very helpful.
Tommy
Swedesboro, NJ

Tommy,
One reason meteorology is a great field of study is because there are so many different branches of it, and so many different kinds of jobs out there.  The basic requirement to become a professional meteorologist is a Bachelor of Science degree in Meteorology or Atmospheric Sciences.  Some research or teaching positions require a Masters or Doctorate degree.

There are several branches of meteorology.  Among them are climatology: the study of global climate patterns and how they change, hydrology: which deals with rivers, flooding and water supplies, agricultural meteorology: which analyzes weather’s effect on farming, air quality: which looks at air pollution, biometeorology: which considers how weather affects the human body, and astrometeorology: which involves researching weather on other planets.

Meteorologists may work for the federal government to take weather observations, make forecasts, and issue warnings.  They may work for the armed forces to forecast weather for military exercises.  Some do research to better understand and forecast weather.  Meteorologists also work for utility companies, airlines, railroads and shipping companies, gas, electric and water utilities, sports teams and stadiums, and yes, even for broadcasting companies like The Weather Channel. 

Good luck in your weather pursuits!

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Mr. Walker,
What are fronts on a weather map? What do they tell us about our weather?
Sincerely,
Sue
Ms. Shelton's Aerospace Lab
Marietta, GA

Dear Sue,
Weather fronts are usually indicated on a weather map by blue and red lines. Like battle fronts, these lines mark boundaries where a “war” of sorts is going on between different kinds of air masses. Some air masses are cold, some are warm; some air masses are dry, others are moist. Along this battle front is where many of our thunderstorms and much of our precipitation occurs.

The blue lines with the triangles, called “barbs,” show a cold front, in which the cold air is advancing and digging under warm air ahead of it. The warmer air rises quickly, creating clouds, showers and storms. The barbs point in the direction the front is traveling.

The red lines with the half circles, called “pips,” indicate a warm front, where warm moist air is advancing over colder air. The pips also point which way the front is moving. When you see high cirrus clouds increasing, a warm front is often moving in, with the moisture increasing higher in the atmosphere working its way down toward the ground. Warm fronts usually bring lighter, steadier rain.

You may also see two other kinds of fronts. A stationary front is indicated by alternating blue and red lines with the barbs and pips sticking out in opposite directions. This is where neither the cold nor the warm air is advancing, and rain along the stationary front may keep falling for a long time and cause flooding.

An occluded front is drawn as a purple line with the barbs and pips facing the same direction. This is where a faster-moving cold front catches up with and overtakes a warm front, forcing the air in between the fronts to be pushed higher into the atmosphere and perhaps bring more clouds and rain.

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What causes the mercury in a thermometer to rise up?
Mary
Grade 4th - Mr. Benac’s Class
Whitehall Elementary School
Bowie, Maryland 

Today’s thermometers use either liquid mercury or alcohol, both of which expand (get bigger) when heated, and contract (get smaller) when cooled. These liquids are put inside the thin glass tube of a thermometer so they have nowhere to go but farther up into the tube when they expand and farther down when they contract. That’s why the mercury goes up when the temperature gets warmer and down when it’s cooler. The good thing about mercury and alcohol is that they become solid at temperatures well below the freezing point of water, so they can measure a wide range of temperatures, both cold and hot.

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Why is it difficult to distinguish snow, light drizzle, melting snow, and moderate rain using radar?
Heidi
Cedartown, GA

Heidi,

Weather radar is a wonderful tool for meteorologists, but as you say, it does have its limitations.  As you may know, radars send out radio signals from an antenna.  Those radio waves bounce off objects in the air, such as raindrops, snow crystals, hail stones and other precipitation.  Then the reflected radio waves travel back to the antenna and are electronically converted into pictures and colors showing the location and the intensity of   precipitation.  On radar images that The Weather Channel uses, a light green color shows light precipitation, and red shows heavy precipitation. We call this radar reflectivity. 

When interpreting radar images, something we have to keep in mind is that different types of precipitation sometimes have about the same reflectivity.  For example, the diameter of drizzle drops is very small, but they are dense enough to show up on the radar screen.  Snowflakes are bigger, but they are not as dense as liquid water, so the reflectivity is about the same as that of drizzle.  Untrained observers might therefore interpret the snow on the radar as drizzle.   When snow is melting, big wet snowflakes have more reflectivity and may show up on the radar looking like giant raindrops, and that might lead some radar observers to think moderate rain is occurring instead of wet snow.  The same would be true of wet hail; it might sometimes be mistaken for heavy rain.  This is why meteorologists must use ground observations, temperature readings and other tools together with radar to accurately know what kind of precipitation is occurring. 

Just to let you know that in the ever-changing world of technology, there is a new type of radar that can distinguish between precipitation types.  It’s called “multiparameter” or dual-polarization” radar.  It should be in wide use in a few years. 

Nick

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How can I explain to my students how to use a barometer to forecast weather?
Michelle
Jesup, GA 
 

Dear Michelle, 

Although it is not the only tool that needs to be used to get an accurate forecast, a barometer is very important in forecasting weather.  A barometer, as you probably know, measures air pressure. 

Please note that low and high pressure can’t always be used to forecast temperature, because high pressure can bring very warm weather or very cold weather, depending on where the air mass is coming from.  Low pressure is often found near fronts, where there are transitions between warm and cold temperatures.    

The ability to forecast the weather using a barometer relates to the fact that low-pressure systems are associated with clouds and precipitation, while high-pressure systems generally have minimal clouds and precipitation.  This is because air rises in the low-pressure system and sinks in the high-pressure system.  In sinking air, clouds can’t form well; so high pressure usually means dry weather.   

The upward or downward motion in highs and lows doesn’t have much to do with lighter or denser air as it does with how the winds are coming together (converging) or pulling apart (diverging).  Winds generally rotate counterclockwise about a low-pressure system and clockwise about a high-pressure system in the Northern Hemisphere.  Because of friction, the air specifically spirals counter clockwise around and inward toward the area of low pressure.  That causes clouds and precipitation.  So when air pressure starts to fall, look for increasing clouds and a better chance of precipitation because a low-pressure system is heading toward you. 

Air spirals clockwise around and outward from a high-pressure system.  That outward flow induces air to sink down toward the surface in the high, which dissipates clouds and tends to make high-pressure areas relatively cloud free.   

One additional note.  The temperature change through the day and into the night as well as the varying pull of the moon can cause air pressure to vary a bit throughout the day.  The pressure tends to fall during the afternoon when heating makes the air less dense.  The pressure tends to rise toward sunrise when the cold air is heavier.  The amount of variation from these types of influences can be up to about .06 inches of mercury, so don’t interpret every small change in air pressure as meaning a change is coming in the weather. 

Thanks for your interest in weather!
Nick Walker

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On a weather map, they show curved lines, sometimes blue with triangles sticking out the front side and sometimes red with half circles sticking out. What do these line show and how do they affect our weather?

Bettie
Russellville, AR

Bettie,

The blue and red lines on weather maps indicate weather fronts.  Like a battle front, these lines mark a boundary where a “battle” of sorts is going on between different kinds of air masses.  Some air masses are cold, some are warm; some air masses are dry, others are moist.  

The blue lines with the triangles, called “barbs,” show a cold front, in which the cold air is advancing and digging under warm air ahead of it.  The red lines with the half circles, called “pips,” indicate a warm front, where warm moist air is advancing over colder air.   

You may also see two other kinds of fronts.  A stationary front shows alternating blue and red lines with the barbs and pips sticking out in opposite directions. This indicates an area where neither the cold and warm air is advancing. An occluded front is drawn as a line with the barbs and pips facing the same direction.  This is where a faster-moving cold front catches up with and overtakes a warm front, and forces the air in between the fronts to be pushed higher into the atmosphere.

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Nick,

I am interested in studying meteorology in college.  Can you recommend some schools with meteorology programs?
Kaitlyn
Clearwater, FL

There are several universities around the country with good meteorology programs.  You can get a list with links to the web pages of the colleges' programs from the National Weather Association by following this link.

I wish you the best in your studies!

Nick

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Hello,

My name is Emily and I am 13 years old. I am in 8th grade. One of the classes that I am in this year has explained some very interesting things to us. Some of them like the butterfly effect. Could you explain the butterfly effect to me in greater detail please? Thank you for taking the time to read my email and I hope you will post this so I can find out my answer.  

Emily,

The “butterfly effect” is the term given to the concept that small differences in atmospheric conditions can lead to big differences in weather later.   The term was first used by from Edward Lorenz, a meteorologist at MIT, in a 1972 paper titled "Predictability: Does the Flap of a Butterfly's Wings in Brazil set off a Tornado in Texas?"   His experiments in the 1960s concluded that the slightest difference in initial weather conditions made huge differences over time, and that these initial differences were often impossible for humans to detect.  At the time Lorenz’s concept was revolutionary and defied the assumptions of most physicists of his day.  To be sure, it is debatable whether a butterfly in Brazil can actually trigger a tornado in Texas.  But Lorenz’s concept is one reason why the accuracy of day to day forecasts decreases as they become for farther in advance.  The other main reason of course is that is simply impossible to accurately observe and model every inch of Earth’s atmosphere.

Nick

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We are planning to make a class mini weather station. But first, what we do is all have our assignments. Mine is to bring in the barometric pressure readings for the week.

Since my barometer was factory adjusted for sea level and I do live almost right on the beach, my only question is that since I live in a high rise, on the 8th floor, would that make any difference in adjusting the barometer?  Checking The Weather Channel’s readings for La Guardia and JFK airports, I get very similar readings, but the airport farther from me, La Guardia, has readings actually closer to those I get, after not making any adjustments.  Should I adjust my barometer further or for our purposes is it close enough?

Thanks so much for any feedback.
Smiley

Queens, NY

Dear Smiley,

As you know, air pressure decreases with altitude, so the measured air pressure over land is usually less than what the value would be at sea level.   On weather maps, the raw air pressure observations at various locations and elevations are adjusted to what the air pressure measurements would be at those locations if the air extended all the way to sea level.  You’ll see slight differences in your barometer readings from those at the airports due to differences in elevation, location of high and low pressure centers and temperature.  Your eighth floor elevation may even make a slight difference.  But if your barometer is adjusted for your general elevation your readings are probably pretty close to what they should be.  Good luck on the class weather station!

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Nick, how did you become interested in weather?  What do I need to do to have a career in weather?
Karen
Seattle, WA

Karen,

The more I learn about weather, the more I want to know.  Since scientists make new weather discoveries all the time, there is always something new to learn.  That’s why meteorology is such a fascinating career for me.  But strangely enough, I got into weather sort of by “accident.”   For eight years I worked as a TV news reporter at various local stations.  I was working at a television station in Seattle when one of the weathermen left suddenly for another job.   My boss asked me to fill in for a few weeks until he found a replacement.  I liked the job so much that I started taking it more seriously.  Before I knew it, I was studying meteorology through Mississippi State University’s Broadcast Meteorology Program and weather became my new career.  Shortly thereafter, a life-threatening wind storm brought hurricane force winds to Seattle and I realized what a huge responsibility weather forecasting really is

If you’re interested in a meteorology career, don’t wait until you are grown up.  Start observing the weather around you every day.  Take notice of the different kinds of clouds and what kind of weather they bring.  Notice when the weather warms up and cools down, read books and watch The Weather Channel to find out what makes the weather change.  In school, prepare for a weather career by studying hard in science and math, and work on your English and reading so you’ll learn how to speak and write well.  Who knows?  Maybe you can take my place someday!

Nick

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Nick,
Winds often die down at sunset. When I sail on Lake Michigan, I notice that the winds usually return to their normal range within an hour or two after sunset. Do you have an explanation for both of these phenomena?

Louis
Chicago

Dear Louis, 

Part of the reason winds die down at night is because the ground surface cools down fast.  There is just not enough heat in the soil to compensate for how fast the heat radiates away from the surface.  Because of the cooling, the air near the ground becomes more stable; that is, there is not as much vertical motion of the air.  This keeps the faster winds higher in the atmosphere from mixing down to the ground, unless there is a weather system that could force them down to lower levels.  The air higher in the atmosphere doesn’t cool as quickly, so on occasion, a temperature inversion forms, in which the temperature at the ground surface is cooler than that of the air above.  But it is the lack of mixing between the air near the ground and the air higher in the atmosphere that is largely responsible for the slower wind at night.  As you know, air in contact with the ground is always slower than that above, because friction with the ground slows its speed.   Mixing during the day forces some of this faster wind down to ground level, but when mixing slows after sunset, so do the winds. 

The temperature of Lake Michigan’s water changes very little at night, if at all.  So air over the water does not cool as fast as that over land, and vertical mixing doesn’t stop over the water.  That’s one reason why winds over the lake don’t slow as much, and may actually increase slightly from the daytime wind speeds.  

Another factor could be sea and land breezes. Near large bodies of water during the day, the sea breeze can kick up in the afternoon when the land warms up more than the water. The warm air rises and cooler air from the water moves in toward land. At night, the land cools down more than the water a few hours after sunset, and the wind is reversed, with the breeze blowing from now cooler land toward the warmer lake or ocean. This may be one more reason why you notice the wind return after sunset over the water, but not over land.  

Nick
Thanks to Dr. Greg Forbes and Dr. Steve Lyons for their suggestions with this answer.

 

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My son is interested in finding out about Doppler Radar.   Here are some questions he has:
1) When did the doppler radar begin?
2) How does a doppler radar track a storm?

3) How does a doppler radar project images on screen?
 

Submitted by Ryan
Bradenton, FL

Ryan,

Doppler Radar gets its name from Austrian scientist Christian Doppler, who in the mid 1800s discovered that sound changes pitch as its source moves toward or away from a stationary observer.  You have probably heard the Doppler effect as a car with its horn blaring moves toward and then away from you.  The pitch of the horn gets lower as the car approaches and  then passes by. The same principle applies not only to sound waves, but to electromagnetic radiation too, including energy used in radar.  Though radar that uses the Doppler principle dates back decades,  in the late 1980s and 1990s, a network of Doppler radars was installed across the U.S. to aid weather forecasters. 

All weather radars send out radio signals from an antenna.  Those radio waves bounce off objects in the air, such as raindrops, snow crystals, hail stones and other precipitation.  Then the reflected radio waves travel back to the antenna and are electronically converted into pictures and colors showing the location and the intensity of    precipitation.  On radar images that The Weather Channel uses, a light green color shows light precipitation, and red shows heavy precipitation. 

Rain drops and snow crystals are usually moving, and this is where the Doppler effect comes in.  Doppler radars measure not only the amount of radio waves returned to the antenna after bouncing off precipitation but also the frequency change of those radio waves, caused by movement of the precipitation.   This allows meteorologists to “see” the wind speed and direction within areas of precipitation.   This is particularly important when dealing with severe thunderstorms.   Weather forecasters can determine how strong a storm is, and even if the storm is likely to produce a tornado.  During strong thunderstorms, our severe weather experts are constantly watching Doppler radar images so we can warn of approaching storms. 

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How is rainfall calculated?
Beth
Grand Rapids, MI

Even with all the high-tech instruments we use to observe and forecast weather, sometimes the old ways are still the best. That's the case with measuring rainfall. The rain gauge that is used by meteorologists today was invented more than 100 years ago. It is a large cylinder-shaped cup that collects rain when it falls. Several times a day, weather observers insert a measuring stick (which is basically a kind of big ruler) into the cup, or they look at a measuring scale printed on the side of the rain gauge. They record how much rain falls over a certain period of time to the nearest one-hundredth of an inch.

You can make your own rain gauge and measure how much rain falls at your house. Here's how: With the help of an adult, cut the top part off a clear plastic soda pop bottle so the bottle is a straight cylinder from top to bottom. Put masking tape over the rough edges around the top so they won't cut your fingers. Using a ruler and a dark permanent marking pen, mark a scale on the bottle in inches, half inches, quarter inches, or centimeters. Place your rain gauge outside and away from walls, fences or buildings that could block the rain. After the rain stops, look at your rain gauge and write down how much rain fell at your house that day.

By the way, an inch of rain in your gauge may not seem like a lot, but think about this: An inch of rain means that your area had enough rain to cover the ground one inch deep. That equals more than five-and-a-half gallons of water for every square yard, or more than seventeen MILLION gallons of water per square mile!


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Mr. Walker, do meteorologists look at the maps when they are doing their forecast and see the things that are on it, or do they stand in front of a green screen and look at a TV off to the side?

Miss Romines Class
Elkins, WV

You obviously know our secret!  Like most on-camera meteorologists, we at The Weather Channel use what is called a “key wall” when presenting weather maps on the air.  “Keying” is the same effect that is used in the movies to put a false background behind an actor so the audience will believe he is in some location where he is not.  Here’s how it works: The key wall is a background that has been painted blue or green, which are the colors that work best for this camera trick.  The on-camera meteorologist stands in front of this wall and faces a television camera.  The camera is adjusted electronically to “see” every color except the exact shade of blue or green that the wall has been painted.  The camera automatically replaces that shade of blue or green with the computer-generated image of the weather map.  At home, the viewer sees the meteorologist standing in front of the weather map.  But in the studio, the visitor sees him or her standing in front of a blank wall.  We do this because it’s the best way to get a sharp clear picture of our weather maps.

   So how do the meteorologists know where to point?  Like you said, we actually watch ourselves on a TV screen, just like you do at home.   Just outside the camera’s view, there is one TV set to our right, another to our left, and still another mounted on the camera.  So no matter which direction we look, we can see a TV image of ourselves standing in front of the weather maps.  By the way, it’s not the same as seeing ourselves in a mirror, since on TV, right and left are not reversed like they are in a mirror.  But we get used to that pretty quickly.

   One other thing: What do you suppose would happen if one of us wore a suit, blouse, or tie that was exactly the same color of the key wall?  That’s right, the clothing would disappear and become our weather map!  So we must be very careful with the colors that we wear on the air.   I’ll never forget a St. Patrick’s Day when I was working in Seattle.  We used a green key wall and someone gave me a big green shamrock to wear on the air.  Without thinking, I put it on my lapel and went out in front of the camera.   Immediately I knew I had made a mistake when I looked over at the TV screen and saw on my lapel not the shamrock, but a shamrock-shaped hole through my chest!  I am determined to not make that mistake again.

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What do the numbers 29.58in/1002mb on your tropical maps tell us? What does that information mean?
Ms. J. Lenox's 4th grade class
McDonough, GA


When we show you the air pressure numbers inside a tropical depression, tropical storm or hurricane, we give the pressure readings in both inches of mercury and in millibars.  Inches of mercury are the familiar standard of air pressure measurement in the United States.  The scale refers to how high the mercury rises in a barometer due to air pressure.  Millibars, also called hectopascals, are the standard method of air pressure measurement for meteorologists.  Unlike an inch of mercury, a millibar is the unit of pressure in the metric system, like pounds per square inch would be the unit used to express air pressure in the U.S.  Since they refer to a direct measurement of pressure, millibars can be used in metric-based mathematics to calculate weather conditions.   One millibar is equal to .02953 inches of mercury.  To give you some perspective, the average pressure at sea level is 29.92 inches of mercury or 1013.25 millibars.
On the other hand, the air pressure in a category three hurricane is 27.91 to 28.47 inches on average, or 945 to 964 millibars.  Air pressure decreases with altitude, so the measured air pressure over land is usually less than what the value would be at sea level.  One important thing to remember is that when we refer to the air pressure on surface maps over land, the raw observations at various locations and elevations are adjusted to what the air pressure measurements would be at those locations if the air extended all the way to sea level. 
 

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Nick,

When determining the temperature, is the thermometer in the shade or in the sun?  Isn't the temperature the same whether it is in the sun or in the shade?
Alan

Venice, Florida

Alan,

There are a lot of factors that go into temperature.  The temperature over hard surfaces such as pavement, roofs, and walls will be warmer than the temperature over flat grassy areas.  And direct sunlight can have a lot to do with that temperature difference.   To give an accurate air temperature, you always want to make sure your thermometer is shaded and away from hard surfaces.  This is because direct sunlight heats up most solid objects (including thermometers) more than the air away from them.  That’s why thermometers that are used for official readings are kept in shelters with slits in the sides.   This allows the air to flow through to the thermometer while keeping direct sunlight off the instrument.  Official thermometers are also up about four feet off the ground, over grass (but away from the shade of trees), and away from buildings or pavement, so the heat from those surfaces will not result in an inaccurately warm reading. 

Nick

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Dear Nick,

We have two rain gauges.  After a storm, one had 3.3 inches in it the other only measured 2.5 inches.  Why would the reading be different?  How is rain measured by these gauges?  We know it's in inches, but neither of the gauges measures properly with a ruler.  Why is that?  
Krista 

Dear Krista,

It is possible that two rain gauges not far from one another could measure different amounts.  Rain showers may be heavier in one location than in another fewer than 100 yards away.  Or one rain gauge may be near a building or other structure that blocks the rain from it if the wind blows from the direction of the structure.  Another explanation could be that if a few hours pass before you take the measurements, one gauge may have received more evaporation than the other, especially if one gauge is in the sun and another in the shade. 

Tiny variations in the wind, perhaps even caused by the adjacent rain gauge can influence how much rain falls in one versus how much rain is blown over the top or around it.

The opening of an official National Weather Service rain gauge (and perhaps the one you have too) is ten times the area of the opening of the measuring cylinder.   That means a ten-inch long cylinder would hold one inch of rain.  This allows us to measure rain in smaller increments, usually in hundredths.  NWS rain gauges are also surrounded by wind baffles, which allow more rain to fall into the gauge rather than blow over or past it.  Inexpensive rain gauges often used by weather enthusiasts can have much smaller openings and different ratios that make their measurements more erratic and sensitive to local wind variations.

Of course anyone can make a simple rain gauge without the ten-to-one ratio.  Here's how: With the help of an adult, cut the top part off a clear plastic soda pop bottle so the bottle is a straight cylinder from top to bottom. Put masking tape over the rough edges around the top so they won't cut your fingers. Using a ruler and a dark permanent marking pen, mark a scale on the bottle in inches, half inches, quarter inches, or centimeters. Place your rain gauge outside and away from walls, fences or buildings that could block the rain. After the rain stops, look at your rain gauge and write down how much rain fell at your house that day.

By the way, an inch of rain in your gauge may not seem like a lot, but think about this: An inch of rain means that your area had enough rain to cover the ground one inch deep. That equals more than five-and-a-half gallons of water for every square yard, or more than seventeen MILLION gallons of water per square mile!

Nick

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Dear Nick,
How do you forecast the weather? What kinds of machines do you use? How do you forecast the weather ahead of time?

Thank You,
Katie

Charlton, MA
 

Katie,
In order to forecast the weather, we first must know what the weather is doing right now. To do this, meteorologists use several tools: thermometers to measure temperature, barometers to measure air pressure, and anemometers to measure wind speed.  Meteorologists also launch weather balloons to get these measurements higher in the atmosphere.  There are also weather satellites orbiting Earth taking pictures of clouds and making other weather measurements.  These pictures help us see where and how weather systems are moving.  The satellites also help us get weather data over the oceans. In addition, we use weather radar to get information about precipitation and storms.   The radar unit shoots a radio signal into a cloud, which bounces back and shows us, among other things, if there is rain or snow falling and how intense the precipitation is.
 

But the best tools we have are our eyes, and the eyes of other weather observers. There are people all over the world making weather observations of clouds, precipitation, wind, and temperature.  They relay that information to other meteorologists all over the world so we can know how the weather is changing from place to place. Then we can plot that information on a weather map every hour and see how fast the weather is changing.

When we can see the direction and how fast air masses are moving, as well as where it is raining or snowing, we can get an idea of when and if it might move into the area that we are forecasting for.
 

We also take all these weather observations and put the numbers into several computers, which use complicated formulas to come up with a forecast for the next several days. Using these computer forecasts, using our knowledge of how the weather works, and using our knowledge of what the weather does normally under a certain set of circumstances, we make can come up with a forecast of weather for the hours and days to come.
 
Nick

 

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How does rainfall affect temperature?
Lemmonia

Dear Lemmonia,

Rainfall can affect temperatures in a number of ways, or perhaps not at all.  When it's raining, you're going to have clouds, and clouds will block the sunlight from warming the earth.   If it's cloudy for only a short time or late in the day after the earth has already been warmed, this may have no affect on temperatures, but if it's cloudy early in the day and stays that way for a long time, the earth will warm more slowly, so temperatures will be cooler.  In the evening or all night, this may work the other way.  If the earth has already been warmed and then clouds form in the late afternoon or evening, the clouds tend to hold the heat closer to earth and it's warmer, whereas if the sky cleared, the heat would escape into space and temperatures would cool.

Also, humid air can hold onto the heat more than dry air can.  So if it has rained recently making the air more humid, the temperature of the air is potentially higher than if it were not as humid.

One more thing, rain is often accompanied by a front.  If a warm front brings the rain, temperatures may actually warm up after the front passes through.  It it is a cold front that brings the rain, the temperatures mayl cool after the front passes through.  This really isn't related to the rain though, but only to the fronts that bring the rain.

Nick

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Nick,
Why does the U.S.A. use Fahrenheit when most of the world uses Celsius?
Zachary
Wisconsin

Zachary,
Scientist Gabriel Fahrenheit invented his scale in the 1700s. Zero degrees Fahrenheit was the coldest temperature that he could create with a mixture of ice and salt.  On his scale, the freezing point of water is 32 degrees and the boiling point is 212.  Mr. Fahrenheit invented the mercury thermometer and the scale eventually became widely used in areas such as the U.S. and Britain and Canada.
 

A few years later, Swedish astronomer Anders Celsius invented his scale.  He based it on a metric system of measurement instead of a British system. On it, the freezing point of water is zero and the boiling point is 100 degrees, so each degree is one-hundredth of the distance between the freezing point and the boiling point.  The U.S. has not embraced the metric system like the rest of the world has, so we continue to use the Fahrenheit scale out of habit. The Fahrenheit scale doesn't work with metric systems of measurement, so areas of the world that use the metric system use the Celsius scale.  

It is good to be familiar with both scales, since they both are widely used.

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Dear Mr. Walker,
I would like to know about weather satellites.  How do they constantly monitor the same area of Earth without moving?
Hugh
Westminster, CO 

Hugh,
The United States has two kinds of weather satellites in Earth’s orbit.  Geostationary satellites orbit at the same speed as Earth’s rotation, so they stay over the same spot all the time.  They are positioned right above the equator about 22,000 miles from Earth. Polar orbiting satellites orbit 530 miles above Earth over the North and South poles.  They are able to get closer views of Earth, as well as photographs of areas near the poles which the geostationary satellites can’t see as well.   

The U.S. launched its first weather satellite in 1960.  Right now, we use two geostationary satellites, one with a view of North and South America and most of the Atlantic Ocean, and the other aimed at North America and the Pacific Ocean.  A third satellite is in orbit as a spare, in case one of the others stops working.  In addition to showing meteorologists how storm systems move, weather satellites also allow us to estimate rainfall and snowfall amounts, and look at snow cover on land and ice cover on the oceans.  On-board cameras take two types of photographs.  A visible photo is taken with a normal camera, so we can’t see anything when that part of Earth is in darkness.  An infrared photograph senses different temperatures to give us its picture of Earth both day and night.  

The National Oceanic and Atmospheric Administration has more information about weather satellites, including those operated by other countries, here:
http://noaasis.noaa.gov/NOAASIS/ml/genlsatl.html

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When the wind direction says, for example, Northwest 5-10 miles an hour, does this mean out of the northwest or toward the northwest?
Deb
West Mifflin, PA

Deb,
Wind direction is very important in meteorology because it can determine whether cool or warm air will flow over an area, or whether moist or dry air will influence an area’s weather. When we talk about a northwest wind, what we mean is that the wind is blowing from the northwest (which would mean toward the southeast). Sometimes you might hear us say, "Wind is out of the northwest,” or “a northwesterly wind.”  It all means the same direction.

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Nick,
I understand that leaves turn upside down when a storm is coming. Why is that?
Kelli
Stony Point, NC
Kelli,

A common weather proverb says, “When leaves show their undersides, be very sure that rain betides.” It is true that before rain, some leaves, such as those of the poplar tree, curl up or turn upside down.  This is because before the rain comes, the humidity, or amount of moisture in the air, rises.  The moist air makes the stalks of the leaves softer, so they can be more easily lifted by the wind.  Different types of leaves will react differently to changes in the weather.  Horticulturists also say that dandelions, clover, and tulips all fold their petals just before rain. The reason for each is the increasing humidity in the air. 

Bear in mind that this leafy “forecast” is only for the immediate future, that is, over the next few minutes to the next few hours, and only for the immediate area.

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Does the weather always move from west to east?
Paul
New York, NY

Paul,
Like so many questions, the answer to this one is: it depends.  In the middle latitudes of the Northern Hemisphere, which include the United States, most of Europe and much of Asia, the prevailing wind flow is from the west.  However, in much of the tropics in the Northern Hemisphere, winds are primarily from the east.    

But even over the U.S. we often see storm systems moving in ways other than the familiar west-to-east direction.  One reason for this is that Earth’s rotation tends to turn winds in the Northern Hemisphere clockwise around high pressure areas and counter-clockwise around low pressure areas.  So while large-scale storm systems may move basically from west to east, it’s often more like northwest to southeast or southwest to northeast.  And in the case of smaller-scale storms, they can move in any direction, depending on local winds and air pressure differences.

Nick

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Why is air pressure measured in inches?
Nina
Eatontown, NJ

Nina,

Although scientists give air pressure readings in other units of measurement called “millibars” or “hectopascals,” the National Weather Service still gives air pressure in “inches of mercury” because we have become so familiar with that measurement.   

The term “inches of mercury” dates back to the first barometers, which consisted of a glass tube in a dish of mercury.  There is no air in the tube, so the pressure of the air on the mercury in the dish causes the mercury to rise in the tube.  At Earth’s surface, that pressure forces the mercury up about 27 to 31 thirty inches into the tube, depending on the actual air pressure. The barometric pressure reading would be the actual distance in inches that the mercury rises in the tube. 

We still use mercury barometers, though nowadays we also have other barometers that don’t use mercury.  Even so, the familiar unit of measurement is still with us.

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Nick,

What is the difference between scattered and isolated showers?
Lee
Alpharetta, GA

Dear Lee,
Have you ever been in a car with blue sky overhead only to suddenly ride into a heavy rain storm?  Then after a few minutes, you drive right out of the rain into clear skies again.  These showers or thunderstorms are what we call “scattered.”

In the spring and summer, much of the country can see afternoon thunderstorms that seem, to many people, to come out of nowhere.  Clear skies suddenly become filled with tall puffy clouds which produce thunderstorms that can bring short, but sometimes heavy amounts of rain.  These storms come from quickly rising pockets of warm air, which carry water vapor high into the sky where the moisture condenses into thunderstorm clouds.   Usually the clouds are narrow, and the rain doesn’t come down over a wide area, but over only a few miles.  The storms “pop up” here and there, and due to their scattered nature, it’s hard to tell exactly where they will form at any given time. 

When there are very very few of these storms, we call them “isolated.”  A forecast for “isolated thunderstorms” means that most of the area will not see them, but a few storms may come and go quickly during the day, and they may last for only a few minutes or less.  Don’t cancel your baseball game on a forecast of isolated thunderstorms, but it’s a good idea to have a plan, just in case one of those storms moves over you. 

Nick

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Why is it difficult to distinguish snow, light drizzle, melting snow, and moderate rain using radar?
Heidi
Cedartown, GA

Weather radar is a wonderful tool for meteorologists, but as you say, it does have its limitations.  As you may know, radars send out radio signals from an antenna.  Those radio waves bounce off objects in the air, such as raindrops, snow crystals, hail stones and other precipitation.  Then the reflected radio waves travel back to the antenna and are electronically converted into pictures and colors showing the location and the intensity of   precipitation.  On radar images that The Weather Channel uses, a light green color shows light precipitation, and red shows heavy precipitation. We call this radar reflectivity. 

When interpreting radar images, something we have to keep in mind is that different types of precipitation sometimes have about the same reflectivity.  For example, the diameter of drizzle drops is very small, but they are dense enough to show up on the radar screen.  Snowflakes are bigger, but they are not as dense as liquid water, so the reflectivity is about the same as that of drizzle.  Untrained observers might therefore interpret the snow on the radar as drizzle.   When snow is melting, big wet snowflakes have more reflectivity and may show up on the radar looking like giant raindrops, and that might lead some radar observers to think moderate rain is occurring instead of wet snow.  The same would be true of wet hail; it might sometimes be mistaken for heavy rain.  This is why meteorologists must use ground observations, temperature readings and other tools together with radar to accurately know what kind of precipitation is occurring. 

Just to let you know that in the ever-changing world of technology, there is a new type of radar that can distinguish between precipitation types.  It’s called “multiparameter” or dual-polarization” radar.  It should be in wide use in a few years. 

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