These are the questions I am asked again and again
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?
This is the number one most-asked question I get. And the answer is...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.
What was the weather like on a particular date?
This is my second most-asked question. I am always being asked what the weather was like on a certain date at a certain place,
or what the weather is usually like in a certain place at a particular time of year.
You can find various kinds of information about past weather online, but the nature
of information and how detailed it is will vary widely from one place to another. The best place to start is the home page of the National
Weather Service at
http://www.nws.noaa.gov
and use the map they provide. Click on the
map on the part of the country you need information for. This will take to the web pages
of the local National Weather Service office that has the information you need. Follow the link that says “Climate,”
usually on the left hand side of the page. Then
you can see what sort of data is offered, and for what cities. Be advised that you won’t be able to find
past weather information for every city or county that you want, but you can usually get
close. For example, climate information is
not available for Virginia Beach from the National Weather Service page, but it is
available for Norfolk. Likewise, information
isn’t available for Burlington County, NJ, but it is for Trenton, Atlantic City, and
Philadelphia, PA. For more detailed
information, you’ll probably need to contact your local National Weather Service
office. Look in the U.S. Government pages of your phone book under Department of Commerce,
NOAA, National Weather Service.
The National
Climatic Data Center has an extensive archive of past weather data available for a fee. Go to the NCDC web site for more information. http://lwf.ncdc.noaa.gov/oa/ncdc.html
If you simply
want average temperatures or record temperatures for a particular city, you can find the
information quite easily from this web site. Simply
type in the zip code or city name for which you want weather information near
the top of my
Forecast Page.
Look to the right on the weather.com page where it says "Today's Averages and
Records." There's a link below it that says "Historical Data." Click on that
link and you will see month-by-month average and record temperatures (plus
sunrise and sunset information.)
If hot air rises, why is it hot at the
ground and colder up high?
or
Since you're closer to the sun, why is it colder on a mountaintop than it is on the ground?
It’s true that warm air rises, but as it rises, it cools. That is partly because air pressure is greater near the ground than it is higher in the sky, and the higher the pressure, generally the warmer the temperature. Here’s an illustration: Imagine a group of acrobats stacked one on top of the other. The weight or pressure on the bottom acrobat would be much greater than the pressure on the top one. It’s the same with air molecules; those on the bottom have greater pressure on them than those above. When the molecules compress together, the air gets warmer. Have you ever used a bicycle pump? When you push the plunger down, you compress the air inside. After pumping awhile, you can feel the pump get warmer as the air inside heats up.
There’s also another reason it’s warmer near the ground. The sun warms the ground and the ground, in turn, heats the layer of air next to the ground. So most of the heat we feel is actually coming from the ground, which has been heated by the sun. Air itself is not very good at absorbing the sun’s energy directly, so, unlike the ground, the air does not warm very much as sunlight passes through it. However, as the warm air rises from the heated ground, the air pressure decreases, and the air cools. So the farther away you get from the ground, the cooler the temperatures are.
Why does it seem the
temperature drops as the sun is coming up in the morning?
Often the temperature does fall for a short time after sunrise, though it is only a degree or two at most. And even if the temperature doesn’t fall as the sun is rising, the air may actually feel colder to you. That is because we are used to it being colder when it’s dark and warmer when it’s light. So as the sky begins to brighten in the morning and the temperature remains nearly the same, we might perceive that it is colder than it actually is. Also, wind generally increases during the day. The air sometimes begins to stir just after sunrise, and an increase in wind would make us feel colder.
If there are no fronts or precipitation nearby, the daily temperature cycle is primarily controlled by the radiation budget. This is a comparison between the incoming radiation from the sun (sunlight) and the terrestrial radiation given off by the earth’s surface (felt as heat.) Think of the sunlight shining down on earth as the same as putting pennies into a jar. As long as you keep putting pennies in, the money adds up. In the same way, as long as the sun is shining down on earth, the amount of radiation adds up. Let’s say that at some point you decide to stop depositing pennies into the jar and begin to take them out. Even though you’re withdrawing pennies, you still may have a lot of money in the jar. Likewise, when the sun goes down, the incoming radiation from the sun stops, but there is still a lot of radiation that has been absorbed by the earth, so we still feel heat near the earth even after the sun goes down. At night, the “withdrawals” of terrestrial radiation continue, and the ground and the air near it cool. Earth’s surface is typically in radiation “debt” from a couple of hours before sunset to near sunrise. When the sun comes up and the “deposits” of incoming radiation from the sun equalize with the “withdrawals” of radiation from earth, we get the coolest temperature of the day. Sometimes in winter when the sun is low in the sky, the earth’s surface can remain in radiation debt longer, and the coldest temperature of the day can actually occur as much as an hour after sunrise. As the sun gets higher in the sky, earth’s surface is in radiation surplus (the deposits exceed withdrawals), so the ground and the air near it warm.
What is the difference between scattered
and isolated showers?
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.
Why aren't the shortest days in
the year also the coldest?
or
Why aren't the longest days of the year the warmest?
The coldest part
of the year does lag about a month behind the shortest day, just as the warmest days come
a month or so after the longest days. To keep the answer simple, we won't consider warm or
cool air coming in from other places, but will assume the air at a particular location
stays in place, warming and cooling.
We could think of the air in
such a place as being like a bank account. If you add money to a bank account, it grows.
If you add heat to air it warms up. The Earth is always losing heat, like a bank account
that you're always taking some money from. If the amount of heat arriving from the sun is
exactly equal to the amount leaving, the temperature stays the same.
As days grow longer in the
summer, the balance tips to more heat arriving than leaving. This is like adding money to
the account faster than you are withdrawing it. The air grows warmer and warmer. On the
longest day, the amount of heat arriving is greatest. But, even after the days begin
growing shorter, the amount of heat arriving is more than the amount leaving. It's like
continuing to add more money to the bank account than you're taking out, even though you
are adding less than you were before.
Sometime in the late summer or
during the fall, depending on how far north of the equator you are, the heat
"account" is in balance. From then on, more heat is leaving than arriving and
the days grow colder. Now, you're taking out more than you're adding to the account.
In December, when days are
shortest, the "withdrawals" from the heat account are greatest. But even after
the days start growing longer, more heat is leaving than arriving. The heat account is
growing smaller, even though less heat is leaving. Eventually, however, you arrive at a
day when the amounts of heat leaving and arriving are in balance. Then, the amount of heat
being put into the account becomes greater than the amount being withdrawn. The air begins
warming up. Here's a graphic and text that help show
why longest days are not the warmest.
Why
does the sky appears to turn green
during a tornado?
The sky does
sometimes turn green when severe thunderstorms are present, but there may or may not be a
tornado forming. The green color is not
completely understood, but one theory states that clouds will turn a greenish color if 1)
The clouds contain a very high amount of liquid water drops and perhaps hail, and 2) the
thunderstorm forms early or late in the day near times of sunrise or more typically
sunset.
Here’s
how it works. Liquid water (as well as ice)
is actually slightly blue in color (meaning that it absorbs red light weakly.) However, this color is so weak that it requires a
thickness of tens of feet before it becomes apparent (much larger than the dimensions of a
glass of water, for example). However, scuba
divers can vouch for this, as objects seen beneath the surface of the water by more than
ten or twenty feet do indeed appear bluish.
As you know,
the sky turns a shade of orange or red at sunset. But
why? It’s because the distance that
sunlight travels through the atmosphere is much longer when the sun is low in the sky. Because atmospheric particles (dust, salt, smoke,
pollution) scatter blue light more than red, the longer path of the sunlight through the
atmosphere leaves sunlight depleted of blue and therefore rich in orange and red.
The
combination of reddish light from a horizon-hugging sun and the slightly blue color of
clouds filled with water can result in greenish light.
Since many severe thunderstorms do form late in the day, and many tornado-producing thunderstorms also contain abundant liquid water, the green sky is often associated with tornadoes, but again, it is not the tornado that causes it.
If the military set off a nuclear explosion in the path of a hurricane, would it
be strong enough to deflect the storm?
Every time we have major hurricanes that threaten the United States, I get this
question, or one similar to it. The idea you suggest has been considered, but
unfortunately even an atomic or hydrogen bomb is not powerful enough to weaken
the energy of a hurricane. The National Hurricane Center says that a hurricane
releases heat energy at a rate of 50 trillion to 200 trillion watts. (Trillion
in this case is a number followed by 12 zeros) This is the equivalent of a
10-megaton nuclear bomb exploding about every 20 minutes.
Hurricanes are "heat engines" that depend on the temperature contrast between warmth at the ocean surface and cold air aloft. Knowing that, one wonders whether the heat from any kind of bomb would actually add to the storm's natural heat supply, making the storm stronger. In any case, it has been said that trying to heat the upper atmosphere with bombs in an effort to lessen the heat contrast would be as fruitless as trying to heat the city of Minneapolis in January by opening the windows of a house.
The best thing we can do right now is to learn to anticipate and prepare for nature’s power, since we have not found any ways to significantly change it. The Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory has answers to other questions about hurricane modification here.
Why do hurricanes seem to move east to west
and most US weather systems move west to east?
Most tropical systems, at least initially, basically move in an east to
west direction. To get the best answer for you
about why this is true, I asked The Weather Channel's tropical weather expert Steve Lyons
for his input. Here’s his answer:
“Non-tropical weather systems
(low pressure areas, gales and storms) generally move from west to east while tropical
cyclones (tropical depressions, tropical storms and hurricanes) generally move from east
to west. The answer to this is simple, they each move with the "steering"
current they are embedded in. The steering current basically pushes them along. In
non-tropical areas steering winds blow from west to east and hence so to do weather
systems. In tropical areas steering winds blow from east to west and hence so to do
tropical weather systems. The boundary that separates steering from west to east from
steering from east to west is the subtropical ridge of high pressure, typically located
near 30 degrees north latitude (farther south in winter and farther north in summer).
South of this ridge of high pressure we find "trade winds" (blowing from east to
west), north of this high pressure ridge we find "westerlies" (blowing from west
to east).
Frequently tropical cyclones will move generally toward the west but also move north of
about 30 degrees north latitude. When this happens we see the cyclone "re-curve"
and begin moving toward the east in the same direction as non-tropical weather systems do
in those locations. After all, a steering current will move tropical and non-tropical
weather systems about the same, if they are about the same depth.
Unfortunately the world is not so simple and we have changing winds and steering currents
in the east-west direction as well as in the north-south direction. It is for this reason
we see rather wild and highly varied tracks to tropical cyclones. Steering currents are
constantly changing in speed and direction due to continuously varying atmospheric weather
patterns.”
Has
there ever been a case where two hurricanes come together and merged into one larger
hurricane?
No, it's actually impossible for two or more hurricanes to merge together. But if two hurricanes or tropical storms do come close together, they can interact with one another. Their close proximity may weaken one or both of the storms. Or they may begin circling around each other. This is called the Fujiwhara effect, named after a Japanese meteorologist who first explained the phenomenon. In this case, the tropical cyclones begin to move around a center point between them. It's sort of like two people on opposite sides of a tetherball pole each holding onto a ball and walking around the pole. The resulting "dance" is an amazing thing to see on an animated weather satellite photograph! Go here to see a photo.
Who names the hurricanes and how do
they come up with them?
The
Go here for a list of hurricane and tropical cyclone names.
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?
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.
When the wind
direction says, for example, Northwest 5-10 miles an hour, does this mean out of
the northwest or toward the northwest?
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.
Can it ever be too cold to snow?
As long as there is moisture in the air and a way for it to rise and form
clouds, there can be snow, even in temperatures below zero. But very cold air
doesn’t have much moisture in it, and it is also dense and heavy, so clouds
don’t form unless the cold air rises up a mountainside or unless the cold air
blows across a body of water and collects moisture. Most heavy snowfalls occur
in temperatures 15 degrees Fahrenheit or above.
What is the warmest temperature at which snow can fall?
The answer to your question is like the answer to many....it depends. Here's why: Almost all precipitation begins as snow, as ice crystals in clouds absorb super-cooled water droplets (small liquid droplets in clouds), and grow big and heavy enough to fall from the cloud. As long as the air temperature is below freezing on the way to the ground, the precipitation will stay in snow form. Often, the layer of sub-freezing air reaches near the ground, but not quite. So even if the temperature at the ground is above freezing, the flakes may not have time to melt in the distance from the freezing air to the ground. So it's possible for this shallow warm layer to be 2-4 degrees (sometimes even more) warmer than the freezing temperature a short distance above. Obviously the snow would melt on contact with the warmer ground, or at least melt quickly after reaching the ground. But usually even in the case of wet melting snowflakes, the temperature at the ground is no warmer than a few degrees above freezing. One of our lead meteorologists here at The Weather Channel, Buzz Bernard, tells me that he has seen snow at temperatures as high as 39 degrees F (4 degrees C). But, he says, it usually cools down pretty quickly as the snow continues to fall.
If it is 0 degrees today, and it is supposed to be twice as cold tomorrow, how cold will it be? I
Zero times two is, of course, zero. And if you tried to figure this out by simple multiplication, you would get different answers, depending on whether you used the Fahrenheit or the Celsius scale. And what if the temperature goes down from two degrees to one? You might be tempted to say "it's twice as cold now." But what about when the temperature falls from 20 to 10? Is that still "twice as cold?" You see that it just doesn't work. Another consideration is that you can measure temperature fairly easily, but measuring coldness is a bit tougher, because you must take into account how fast the wind is blowing, which will make you feel colder than if the wind is calm.
But let’s try to get as close as we can to answering your question. If instead of the Celsius or the Fahrenheit scale, you used the Kelvin or absolute temperature scale, you might be able to come up with some sort of answer. The Kelvin scale measures heat energy, and starts at absolute zero, the temperature at which all motion of molecules stops. This is equal to minus 273.16 degrees Celsius. So zero degrees Celsius (32 degrees Fahrenheit) would be 273.16 degrees Kelvin. Twice as cold would be half of the heat energy measured in Kelvin, so if you divide 273.16 in half, you would get 136.58 degrees Kelvin. Converting that back to Celsius, you would get a temperature of minus 136.58 degrees Celsius, which is much colder than the world record low temperature of minus 89.6 degrees C set in 1983 in Antarctica. So it isn’t likely that the temperature will be half as low as zero Celsius anytime soon.
Does water freeze faster when the wind is blowing? Does my car radiator get colder when the wind is blowing? Do my plants get colder when the wind is blowing?
Although wind chill is calculated on how cold the temperature is and how strong
the wind is blowing, it affects only people and animals. It doesn't actually
make the temperature colder; it only makes it feel colder. Only
humans and animals can feel temperature, so water would still have to be 32 or
below before it freezes, no matter how strong the wind is. In fact, wind can
keep temperatures higher than they might otherwise be in calm conditions.
That’s why fruit growers often use giant fans to keep the air stirred up on cold
nights. This prevents cold air from settling in over their orchards and
freezing the fruit on the trees. You can see a table showing how the temperature
and wind speed creates wind chill by going here:
http://www.weather.com/encyclopedia/charts/wind_chill.html
I
saw a rainbow ring around the sun. What does that mean?
You
saw a halo, a single ring of color around the sun or moon. Believe it or not, halos actually occur more
frequently in nature than rainbows do. It is
formed by sunlight shining through ice crystals between the sun and the viewer. A 22-degree halo is the most common; formed when
light refracts (bends) around the edges of long ice crystals at a 22 degree angle. Sometimes the halo is white. Sometimes you can see red and orange in the
middle, with yellow and blue at the outer edges.
If a thin
cloud's ice crystals are in the right position, you might see arcs just above or below the
halo. The arcs form when light refracts inside long pencil-shaped ice crystals. Flat ice
crystals can produce an effect high above the halo called a circumzenithal arc -- or an
upside-down rainbow. Also when light refracts through flat horizontal ice crystals, you
might see bright spots of light along the right and left sides of a halo. These bright
spots are commonly called sun dogs; their scientific name is parhelia.
To see these
optical effects, do not look directly into the sun. You can damage your eyes and you won’t
see the colors anyway. Instead, block the sun from your view with your hand, a car visor,
the edge of a building or other object so you can just see the clouds around it.
Sunglasses also may help you see a halo -- but even with sunglasses, you'll need to block
the sun from your eyes.
When it's raining outside, why is it NOT 100% humidity? And how can it be 100% humidity outside when it's not raining?
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.
Can it rain at 26 degrees? Or at any temperature below 32 F for that matter?
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.
Why are storm clouds black?
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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