Previously Asked Questions

Rain and Humidity

More about Precipitation

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At our office we are having a discussion about how many gallons of rainwater falls over an acre if the rain amount equals one inch in a rain gauge.   Can you tell us?
Bruce A. Price, Topeka, KS


It’s a lot.  Here’s the math: one acre equals 43,560 square feet, and one cubic foot equals 7.481 gallons.  So if one inch of rain is one twelfth of a foot, this means the volume of water that falls in an acre is 43,560 divided by twelve, or 3,630 cubic feet.  That is equal to 27,156 gallons of water.  Quite an abundance!  Breaking that down to a smaller area, one inch of rain would be about five and a half gallons per square yard, certainly enough to water a thirsty flower bed or lawn. 

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How many water molecules does it take to make a water drop?
Anson, TX

According to the Water Resources Research Center at the University of Arizona, the average drop of water has 1,700,000,000,000,000,000 molecules or, in other words, 1.7 quintillion. 

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Dear Nick,
I am doing a science fair project for our school science fair. I won last year but sixth graders weren't allowed at state. So, this year I am hoping to win grand honors and go to state. But for this to happen, I need some expert advice to write my report. For my science fair project, I am making a cloud in a soda bottle. I did about thirty trials and they all worked. I was wondering how the cloud is formed?  
Incarnation School
Palos Heights
Grade 7

Dear Karli,
You are right; the experiment really works.  I’ll outline the steps of the experiment below, but first, some background:

In our atmosphere, clouds form when water vapor turns back into liquid water droplets.  That is called condensation.  It happens in one of two ways: when the air cools enough, or when enough water vapor is added to the air.   You’ve seen the first process happen on a summer day as drops of water gather on the outside of a glass of ice tea.  That’s because the cold glass cools the air near it, causing the water vapor in the air to condense into liquid.  Unlike the drops on the side of your glass though, the droplets of water in a cloud are so small that it takes about one million of them to form a single raindrop.   Most clouds form this way, but the cooling comes not from ice in a glass, but as the air rises and cools high in the sky. 

Also, our air has to be just a little bit dirty for clouds to form.   That’s because water vapor needs a surface on which to condense.  Fortunately, even the cleanest air has some microscopic particles of dust, smoke or salt for water droplets to cling to, so the air is rarely too clean for clouds to form.

To make a cloud in a bottle, get an empty two-liter plastic soda bottle and a match.  Put about an inch of water into the bottle, have a grownup light the match, hold it inside the bottle for a few seconds, and then drop it inside.  Quickly put the cap on and shake the bottle to fill the air inside with moisture.  Give the bottle a squeeze.  The increased air pressure will warm the air inside.  Then release the bottle and allow the lower air pressure in the bottle to cool the air inside.  As the air cools, the water vapor inside the bottle should quickly condense on the smoke particles, forming a little cloud.

Good luck in your science fair!

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Hi Nick,
Could you please tell me where dew comes from?
Thank You.
Little Rock, AR

Dew comes from moisture in the air. When the air near the ground cools overnight, plants, grass, and hard surfaces such as glass or metal also cool quickly. Those cool surfaces cause moisture in the air near them to condense from water vapor back to liquid water.
You've seen the same thing happen when drops of moisture collect on the outside of your glass of ice water. When the temperature of the surface cools to the dew point temperature--the temperature at which water vapor condenses to liquid moisture--the liquid forms on those surfaces. And that's dew.  By the way, when the temperature at the ground dips below freezing, water vapor changes directly into ice crystals.  That, as you probably have guessed, is frost.

Thanks for your interest in weather!

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Can you please explain the difference between dew point & humidity?
Babbitt, MN

 Dear Terry, 

Dew point and humidity are closely related.  Dew point refers to the temperature at which water vapor will condense into liquid water.  You have seen this happen when water droplets condense onto the outside of a glass of ice water.  When the glass gets colder than the dew point,  water vapor in the air condenses into liquid on the glass.   The higher the dew point temperature, the more moisture there is in the air.  The closer the dew point temperature is to the actual air temperature, the higher the relative humidity will be.  For example, if you notice in the morning that both the air temperature and the dew point temperature are 60 degrees, then relative humidity is 100%, and it is likely that fog is forming.   However, if the dew point temperature is 70 and the air temperature is 85 degrees, there is actually more moisture in the air even though the relative humidity is only  61%.   That’s why we call it “relative” humidity, because the humidity is relative to the air temperature.  That’s why we talk a lot on the air about the dew point, because it gives you more of an idea of how much moisture is in the air, and how sticky or how dry the air will feel to you.

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I have heard it said that fog takes away snow more quickly than rain or sunshine. Is this true and if so - why?  I am a home schooling mother and would like to be able to teach this to my children.

Southern New Brunswick

I consulted (as I often do) with The Weather Channel's severe weather expert Doctor Greg Forbes on this one.  He tells me that there is some truth to that "fog melts snow faster than rain or sunshine." But it is true only in certain cases.  

If the overall air temperature a few feet above the snow is the same, foggy conditions will melt more snow than sunny conditions.  Here’s why: The energy for melting the snow comes from the air and its contents (fog droplets, raindrops).  On a sunny day, when the relative humidity is low, the air can cool to a temperature below freezing.  With the lowered air temperature, less snow melts.  When skies are clear there tends to be a large swing between daytime and nighttime temperatures.  If the snow is dirty, there can be significant melting during the daytime, but refreezing at night.  In foggy conditions, there is little fluctuation in temperature between day and night, so melting can proceed around the clock.   

But the time of year may throw a monkey wrench into argument.   In December when the sun angle is low and nights are long (especially in northern latitudes) the sun might not melt the snow very fast.  But in the South and elsewhere in March, the sun can be a real snow-eater, with shady areas hanging on to the snow but sunny areas getting bare quickly.   

How does fog melt snow?  In fog, the relative humidity is 100% and any cooling of the air results in more fog drops condensing out, which gives off some heat to the air.  This slows the rate that the air cools.  Also, some of the energy of the above-freezing fog droplets helps melt the snow. 

The melting from rain versus fog is a hard one to determine.   If it is rain forming higher in the atmosphere and falling into a dry layer near the ground, then rain may evaporate in that drier air and cool.  In that case, fog would be more effective in melting the snow than the rain.  But if the rain is warm and the near-ground relative humidity is high, then the rain is likely to cause more melting than just fog. 

One other factor can reduce the snow pack without melting.    In dry, windy conditions, the snow can just "evaporate" from ice crystals to water vapor.  Technically we call this “sublimation.”  In this case, the snow just seems to disappear, leaving no water behind.

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Dear Nick,

I have asked this question a thousand times and no one has ever been able to answer it. My question is this: When it's raining outside, why is it NOT 100% humidity?  Doesn't humidity measure moisture in the air?
Thank you,
St. Paul, Minnesota 

You are right; humidity measures moisture in the air.   But it can be raining without 100 percent humidity.  And, the humidity can measure 100 percent without it raining. 

For clouds to form and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity. Some water from the rain evaporates into the air it's falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%.  

When air is saturated, or 100% humidity, rain may or may not form, since it takes more than saturated air to make rain.  Rain actually forms from two processes.  In one case, the drops simply collide with one another until they are big and heavy enough to cease being suspended in air and they fall to the ground.  The air may be completely saturated, but the drops are so small that they may remain suspended in the air and not fall as rain.  This is one reason why not all clouds produce rain.  In thick fog, the humidity is usually 100 percent, but the fog droplets are so small they don't form raindrops. 

Most rain actually starts from ice crystals in clouds, which draw moisture from very cold water drops.  The ice crystals grow in size and weight until they are big and heavy enough to fall from the cloud.  If the air at the earth's surface is cold enough, they remain in crystal form and fall as snow or melt on the way to the ground and fall as rain.  Again, the air in the cloud may be 100 percent humidity and this process may not happen.  Thus you have saturated air, but no precipitation. 

Sometimes you just have to ask a question at least one thousand and ONE times to get an answer!

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How do clouds come down to make fog?
Waterford, MI



We usually think of a cloud as something you see up in the sky.  So it might be natural to think that fog is a cloud that has “come down” to the ground. But the moisture that forms a cloud of fog is already at the ground even before you can see it. 

When the air is very full of water vapor (water in invisible gas form), fog will form if a little more moisture is added to the air, or if temperatures cool enough to cause the water vapor to condense into water drops.   At night, a sky with no clouds may allow heat to escape up into space and the air near the ground to cool enough to cause that condensation.  It’s the same process that turns the moisture in your breath into a little cloud on a cold day. 

Fog can form in other ways too.  Sometimes winds blow warm moist air over a cold surface such as water or ice, which causes the moisture to condense into fog.  When cool air moves over a warm lake or pond, moisture may evaporate from the water’s surface and condense in the cool air.  This results in what is called “steam fog,” because it looks like steam rising from the lake. 

By the way, just as fog doesn’t “come down,” it doesn’t really “lift” either, even though we use that word when fog starts to go away.  As the ground begins to warm, the warmer temperatures cause the moisture in the fog nearest the ground to evaporate back to invisible water vapor.  It may still be foggy above our heads, so it looks as if the fog is “lifting,” but it’s actually evaporating and disappearing from the ground up.

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What actually causes it to rain?
Stockton, California

Sometimes we’re so concerned with more complicated issues that we ignore the basics.  That’s why I appreciate your question.  And it’s a good one, because many people assume that once a cloud forms, the cloud droplets simply fall back to Earth.  But a cloud droplet must grow about a million times bigger in order for it to turn into a raindrop.   

That can happen two ways.  The most common is called the “Bergeron Process,” named after the Norwegian meteorologist who first proposed it.  He discovered that cloud droplets can cool below freezing and still remain in liquid form.  In a cloud that contains these “supercooled” drops as well as ice crystals, the liquid drops begin to freeze to the ice crystals, forming larger ice crystals that finally get big enough to fall as snowflakes.  When the snowflakes meet warmer air on the way to the ground, they melt into raindrops. 

The other way rain can form is through a process called “collision and coalescence.”  This simply means the cloud droplets bump into each other (collision) and stick together (coalescence) until they’re big and heavy enough to fall to the ground.  This is how rain forms in clouds that aren’t cold enough for ice to form. 

Thanks for the question!

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Hello Nick,
My husband was wondering how much rain does it take to equal a foot of snow?
We were driving in a torrential downpour that dropped five to six inches of rain that evening. He asked how much snow we would've received had it been snow instead of rain. I had no idea so I came home to seek the answer online.   Thank you for your help.

A little bit of moisture goes a long way when it comes to snow.  The average ratio is about ten inches of snow per inch of rain.  However, this ratio is quite variable. If the air is warm, it may take the equivalent of as much as five inches of rain to get ten inches of wet snow, since the snow may be packing or melting quickly.  In extremely cold air, the ratio may be only about half an inch of rain equivalent for ten inches of “dry” fluffy snow.  Our severe weather expert Dr. Greg Forbes says he has seen ratios as much as 40 to one in some lake effect snows, or 40 inches of snow for one inch of water equivalent!  Just be glad all the moisture you saw in your heavy rain wasn’t in a winter storm!

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I am doing a weather project.  Can you give me some tips on how to make a weather measuring instrument?

Rexford Knysna, South Africa


One of the most useful and easy-to-make weather instruments is a rain gauge.  That’s because the rain gauges that meteorologists use are basically fairly simple devices.   An “official” rain gauge is little more than a large cylinder-shaped cup that collects rain when it falls. Several times a day, weather observers insert a measuring stick (which is just 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.   It’s interesting to see just how much rain you get over a period of time.

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Dear Nick,
We have two rain gauges.  I dumped the rain from one (it had 3.3 inches in it) into the other, and that rain gauge 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?  

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.

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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My boyfriend and I were having a discussion about rain. Our question is this: Can it rain at 26 degrees?  Or at any temperature below 32 F for that matter? Your help is greatly appreciated to settle our debate.

New York City

The falling precipitation could be rain at 26 F, but if it is liquid before it hits the ground, chances are that it would freeze as soon as it hits something, like the ground, or streets or trees.  So meteorologists refer to this type of precipitation as “freezing rain.”   This happens when the air temperature is above freezing somewhere between the clouds and the ground, but at freezing temperatures or below freezing at ground level.

Water drops can remain liquid in our atmosphere even at temperatures below freezing.  These drops are called “supercooled.”  Drizzle, which is composed of small liquid droplets, can form as liquid and remain unfrozen even when temperatures are continually below freezing.  In these cases, the clouds form as tiny liquid drops, even though the air temperature is below 32 F.  This happens in relatively shallow clouds in which no part of the cloud has temperatures too far below freezing, so the cloud doesn’t contain any snow.  Sometimes the drops grow large enough to become freezing drizzle at the ground, or liquid drizzle if there is warm air below.  Or it might even be crunchy round snow pellets if the air is very cold below the cloud.  

In thicker clouds, rain usually starts as snow, which melts on the way down to turn to rain. If it never melts, you get snow. If it melts, then re-freezes in a layer of sub-freezing air closer to the ground, you get sleet. If the layer of cold air is very shallow, you can get freezing rain as described above. If that layer of sub-freezing air is very shallow and hasn't been around long enough to make the ground freeze, you'll get rain until the surfaces fall below freezing.

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Dear Nick,
We are studying the weather in Kindergarten. We have measured wind with our windsocks, temperature with thermometers, forecasted with our cloud charts, and weather watched with our binoculars (made out of toilet paper rolls!). One of our questions about the weather that we hope you can answer is:
What is the difference between hail, freezing rain, and sleet?
Thanks a bunch!
Ms. Vann's Kindergarten class. (all 22 of us)

Dear Ms. Vann’s students, 

The type of winter precipitation we get depends on the temperature inside the clouds and the temperature between the clouds and the ground.  In clouds that are cold enough for ice crystals to form, we can get snow.  Those cold clouds aren’t hard to find.  Even in the summer, most of our rain actually starts out high in the clouds as snow.  But in winter, the temperature of the air is sometimes cold enough all the way from the clouds to the ground, so snowflakes don’t melt into raindrops.  They stay in crystal form and we see snow pile up and schools close. 

Sometimes there is a layer of above freezing air in the clouds, then closer to the ground the air temperature is once again below freezing.  Snowflakes partially melt in the layer of warmer air, but then freeze again in the cold air near the ground.  This kind of winter precipitation is called sleet.  It bounces when it hits the ground.  If snowflakes completely melt in the warmer air, but temperatures are below freezing near the ground, rain may freeze on contact with the ground or the streets.  This is called freezing rain, and significant freezing rain is called an ice storm.  Ice storms are extremely dangerous because the layer of ice on the streets can cause traffic accidents.   Ice can also build up on tree branches and power lines, causing them to snap and our lights to go out. Hail forms in strong thunderstorms.  These storms contain very strong updrafts, which are winds blowing up through the thunderstorm clouds.   They can be as strong as one hundred miles per hour.  Those strong updrafts suspend rain in mid-air with temperatures around the raindrop of below 32 degrees.   Those cold temperatures allow the rain to freeze into small hailstones.  As more freezing raindrops get caught in the updraft, they collide with the hailstones, adding layer after layer of ice.  When hail becomes too heavy for the updrafts to keep it aloft, it falls to the ground.   In strong updrafts, the hail has time to collect lots of ice, so the hail is bigger.  In weak updrafts, the hail doesn’t have to get as big before it is able to fall to the ground.  On rare occasions, the updrafts can be strong enough so the hailstones can grow larger than softballs!  Keep up the great work, and thanks for your interest in weather!

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Dear Nick,
What is the difference between rain and showers? That is a question my 4th and 5th graders are asking in class.
Marina Del Mar School in Marina, CA.

Dear Mrs. Flores,
The word “rain” can mean any liquid precipitation, but it usually refers to a prolonged steady rainfall over a wide geographical area.   This usually occurs when the sky is covered with layers of clouds, perhaps as far as you can see. The rain might be heavy at times, but more often is light or moderate.  On a weather map, the rain may occur along or near a warm front, which marks a gradual contrast of warm air sliding up over cooler air. 

When we talk about showers on the other hand, we’re talking about more spotty precipitation for shorter amounts of time.  The showers may come from puffy clouds right over you, while a short distance away there might be sunshine or clear skies. Sometimes the showers can be very heavy, producing a lot of water in a short amount of time.  This comes from "unstable" rising and sinking air, caused by warm moist air at low levels and cool air higher up. Some air rises in a thin column to form tall clouds, which give brief heavy rain showers as they pass by. In between, skies may clear where the air sinks downward.


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Mr. Walker, what makes fog?

Decorah, IA


As you know, fog is a cloud that is at the ground.  It can form on clear nights when there is a lot of moisture in the air.  The cloudless sky allows heat to escape up into space and the air near the ground cools.  When the air cools, the moisture in the air condenses into a cloud.  It’s the same process that turns the moisture in your breath into a little cloud on a cold day.

Fog can form in other ways too.  Sometimes winds blow warm moist air over a cold surface such as water or ice, which causes the moisture to condense into fog.  When cool air moves over a warm lake or pond, moisture may evaporate from the water’s surface and condense in the cool air.  This results in what is called steam fog, because it looks like steam rising from the lake.  It’s kind of fun to be in the middle of fog because you are actually inside a cloud.  But it also means car and school bus drivers can’t see you very well, so be careful on a foggy morning.

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Is the weather and cloud formation affected by lakes or other large bodies of water?

Plano, TX

Yes.  Oceans and lakes provide a huge moisture source for clouds and fog and precipitation.  Some examples: When cold air from Canada blows over the Great Lakes, moisture from the warmer lake water rises, cools and condenses into clouds.  The clouds produce “lake effect” snow downwind on the lake’s shoreline and inland. 

Lake effect snow occurs over salt water too.  Often the Salt Lake City area will see lake effect snow as cold air blows across the Great Salt Lake.  You can also find “bay effect” snow on the New England coast when cold Canadian air blows over the waters of the Massachusetts Bay and Cape Cod Bay to deposit snow on Cape Cod.    The same effect can occur in Norfolk and Virginia Beach, Virginia downwind of Chesapeake Bay.  When temperatures aren’t cold enough for snow, we can see “lake effect” rain form in the same way.

But bodies of water don’t always help produce rain.  Sometimes warm winds blow warm moist air over a colder body of water, which causes the moisture to condense into fog or low clouds.  This is common along the west coast, where fog or low clouds often form in this way.  Small lakes or ponds don’t have much effect on the weather, but big ones do, and in a big way.

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Why are storm clouds black?
Russell Springs, KY

The color of a cloud depends on three things: the sun, how thick the cloud is, and where you are standing. If the cloud is between you and the sun and the cloud contains a lot of moisture, then less sunlight will shine through it, so it will look darker. You may have seen big tall cumulonimbus clouds that are very dark at the bottom. They are filled with moisture and can often bring heavy rain. But even if a cloud is thick, it may actually look white if you watch the cloud with the sun to your back. The cloud reflects the sun’s light and appears bright white. In any case, watch out for clouds that are very dark and very tall. These usually can produce a lot of rain in a short time, so when you see them, it would be a good idea to head indoors! 

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What is the term that describes when you see the black streaks of rain under a cloud, but the rain evaporates before reaching the ground?
Santa Monica, CA

You’re thinking of virga, which are streaks or wisps of precipitation falling from clouds but evaporating before hitting the ground.  The precipitation can be rain or ice particles.  As you said, it can be very dark, but it can also be light colored too, looking like wisps of cloud coming from altocumulus or altostratus clouds.  The precipitation falls into dry air below the cloud and evaporates.  Now and again, virga is mistaken for a tornado or funnel cloud because of the way it might be shaped
, but there is no direct danger from virga.   The danger is when the precipitation comes from a thunderstorm.  Because the rain doesn’t hit the ground, people may be tempted to stay outside as dangerous lightning strikes around them.


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Question: Are raindrops really shaped like teardrops like in the drawings we see?
Loveland, OH 

Dear Ian,

Drawings of raindrops are much prettier than reality.  The teardrop shape (round on the bottom and pointed on top) makes for a nice picture, but actually most raindrops are probably shaped more like tiny hamburger buns: round on tops and flat on the bottoms.  Because of the surface tension in the drop, which is the attraction of water molecules to one another, raindrops usually start out round.  As they fall from the clouds, larger raindrops encounter resistance from the air, which causes them to change shape.  Air pressure on the falling drop is greater on the bottom than on the sides, so the undersides of the drops tend to flatten out by the time they reach the ground.   The smallest raindrops stay rounder, since air resistance on them isn’t as great.  I’ll admit, however, that I prefer looking at a picture of a teardrop-shaped rain drop, even though it’s meteorologically incorrect! 

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When a forecast is given, and it calls for 70% chance of rain, does it mean that you have 7 chances out 10 to receive rain or will 70% of your area receive rain?

Yes.  A probability of precipitation expressed as a percentage is a combination of two figures— the likelihood of precipitation in the area and the percentage of the area that is likely to get it.  Strictly speaking, a 70% chance of rain means that there is a 70% chance of measurable precipitation (0.01 inch or more) at a specific area in a twelve-hour period.  This figure would be used if there’s a 70% chance of rain over 100% of the area, or if rain is certain (a 100% chance) over 70% of the area.  Both scenarios would result in a 70% probability of precipitation. 

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Dear Nick,
I play a lot of golf. When it is very humid out, my playing partners say the ball does not go as far because the air is heavy. I always heard that humid air was not as heavy as cooler air. Can you offer me an explanation?
, Massachusetts

You are right. Humid air is slightly lighter or less dense than dry air. And if temperature, pressure, and wind are equal, your ball will go farther in humid air than when the air is dry. This is confusing to some people because we think of water as being heavier and denser than air.  That is true of liquid water, but humid air is not liquid; it is a gas.   

Since the early 19th century, scientists have known that a fixed volume of air always contains the same number of molecules.  When we add water molecules to air, some of the nitrogen and oxygen molecules must leave.  It just so happens that water molecules weigh less than nitrogen and oxygen molecules do.  So the more water molecules that are added to air, the lighter and less dense the air becomes.  

But bear in mind that temperature and pressure can have an even greater influence on air density than humidity. On a hot day, air molecules will move faster and farther apart, so the density of the air decreases and your golf ball will travel farther.  And because pressure increases the density of air, if you play a golf course in Denver, your ball will go farther in the less dense air at a mile-high course than it will at Pebble Beach at sea level.  

High, hot and humid—those are ingredients for golf ball distance.  However, if you play golf like I do, even these factors won’t help much! 

Have a great game!

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Is it possible for the dew point to be higher than the temperature?
Nichols Middle School

Middleboro, MA

Possible? Yes.  But does it typically happen? No. As you know, dew point refers to the temperature at which water vapor will condense into liquid water. The higher the dew point temperature, the more moisture there is in the air. The closer the dew point temperature is to the actual air temperature, the higher the relative humidity will be. When the dew point equals the temperature, you have 100% relative humidity, and the air is completely saturated.  Under normal conditions at Earth’s surface, water vapor will condense into liquid water droplets onto microscopic particles of dust or salt or smoke in the air (we call it “condensation nuclei”), and fog will form.  When condensation takes place, you can’t get anymore moisture into the air, so the dew point temperature can’t go any higher.  However, laboratory experiments have shown that when the air contains no condensation nuclei, you can put even more moisture into the air and raise the relative humidity as high as about 300 percent!  Some high clouds have been known to contain relative humidities a fraction above 100 percent, but down here where we live, there is plenty of dust in the air for water vapor to condense onto, and the dew point won’t get higher than the air temperature. 

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In the desert, why is the temperature during the day so hot, but at night so cold?
Huntsville, AL


The desert is a perfect example of how moisture in the air (or the lack of it) can affect temperature.  Dry air tends to heat up and cool down quickly, whereas humid air takes longer to heat up than dry air, and it tends to hold onto the heat more than dry air does.  So when the air is dry, temperatures can warm up quite a bit during the day, only to cool down at night.   

There is also something else to consider.  When the air is very dry, skies will most likely be clear.  During the day, clear skies allow the sun to shine and heat up the ground. As the ground radiates that heat, we feel the warmer temperature.  Then when the sun goes down, the ground radiates the heat out into space, and since there are no clouds to hold the heat close to Earth, temperatures cool.  In some of the higher elevations of the Southwestern United States, temperatures in the dry summer air can range from the 40s and 50s at night to the 90s during the day.


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