Previously Asked Questions

Climate, Seasons and Weather Extremes

More About Seasons

Nick
My company is producing an outside event in Virginia Beach, VA on July 31 & Aug 1. I just want to know, in the past 5 years what was the weather like on those days so we can be prepared.?
Jim  

Nick,
I was wondering if you could send me information on what the weather was for March 16th, 2004 in Burlington County N.J. ?
Shawn
New Jersey

More than any other question, I am 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.  Near the top of my Forecast Page, simply type in the zip code or city name for which you want weather information.  At the weather.com site, click on the bar in the upper left that says "Yesterday."  Scroll down to the bar that says “Averages and Records.”   Click there and see the daily and monthly records and average temperatures for that city. 

Nick,
What is the hottest temperature we have ever seen in this country?
Ed
Kansas City, KS

 Ed,

The hottest temperature ever recorded in the United States is 134° F (56.7 C) in Death Valley, CA.  It occurred on July 10, 1913.  For several years, this was a world record until Al Aziziyah, Libya in the Sahara Desert reached 136° F (58C) on September 13, 1922. But Death Valley, CA is one of the hottest places in the world.   The summer of 1974 saw 134 days over 100 degrees.  The summer of 1996 had 40 days over 120°F! 

Nick

Mr. Walker,

What are some of the hottest and coldest temperatures ever recorded in Mexico? We are currently studying Mexico in Social Studies.
Matt in Mrs. Quinn's Period 2 Social Studies Class
Martin J. Ryerson Middle School
Ringwood, New Jersey

It can get very hot in Mexico, and pretty cold too. Mexicali in the desert just south of the California border is a hot spot, recording temperatures as high as 126 degrees F.  On the other hand, Los Lamentos in the high country south of the border with Arizona has seen a low temperature of 18 degrees below zero F.  Here are a few other Mexican cities and their temperature highs and lows, all in Fahrenheit: 

Mexico City Low: 25 High:93
Acapulco Low: 52  High 106
Monterrey Low: 23  High: 111
La Paz Low: 32  High: 108
Cancun Low: 45  High: 102
Hermosillo Low: 32  High: 122
Chihuahua Low: 5  High: 106

Hi Nick. I am a Spanish teacher and want to know if you can help me explain to students why there are only two seasons in the tropical zones of the world (rainy & dry).  Thanks!
Bonnie
New Canaan, CT

Close to the equator, temperatures do not vary much during the year, since the angle of the sun is much more uniform throughout the year than it is in the mid and upper latitudes. That's why they define their "seasons" by precipitation rather than by temperature. But in fact, many places in the world have a similar "wet" season and a "dry" season. (For example, the Pacific Northwest is wet in winter and dry in summer, the Southeastern U.S. is wet in summer, dry in winter.) But in those places where the temperature varies greatly between summer and winter, the changing of the seasons is more pronounced than in the tropics, and we have four distinct seasons.

What is a Jet stream?
Josh
Dacula, Georgia

I am a homeschool mom and I  was wondering if you can give me the reason it does not rain as much in the desert as it does other places. Thanks for all your help.
Mr. Lovern  

Dear Ms. Lovern, 
Deserts around the world are areas of very little average precipitation; in fact, there is less precipitation in deserts than evaporation from the soil and transpiration from the plants.  Deserts may actually receive heavy downpours of rain at times, but overall the rain is scarce. Streams and lakes which form in the heavy rains often dry up later.   

Deserts often occur under semi-permanent areas of high air pressure.  In high pressure, the air sinks, so clouds have trouble forming. 

Some deserts are on the lee sides of mountain ranges.  When rain clouds run into the mountains, the moist air rises up the sides of mountains, cools and condenses into clouds.  The clouds bring precipitation over the windward slopes of the mountains.   When the air descends on the lee side of the mountains, it sinks, compresses, warms and dries out, leaving the area on the other side of the mountains dry. 

Because there is very little moisture in the deserts ground already, there isn't much moisture to evaporate, even when the air does rise.  And without moisture to condense into clouds, there is no rain.  

One thing to keep in mind is that deserts are not always hot.  Since the air is dry and does not hold as much heat as humid air, temperatures often fall sharply at night in the deserts.  And remember that the biggest desert in the world is also the coldest place in the world, in Antarctica. 

Nick

Where in our 48 states I can live that has little cold and snow and little heat and humidity?
Patricia

It’s tough to find perfection, isn’t it?  Where it’s nice and warm, it can also be very humid.  Where it’s nice and dry, it’s often too hot.  For example, southern Florida usually has mild temperatures, but it’s also very humid in summer.  The least humid places in the country are often the hottest, such as the deserts in the southwestern U.S. where temperatures often exceed 100 degrees in summer.   

To find the places with the least amount of fluctuation between high and low temperature, you’ll need to look on the West Coast.  But that area can also be pretty humid in the winter.  If you can take that kind of humidity, that is, humidity without the heat, you can try Seattle, Portland, or San Francisco.  About the only place where you’ll find drier weather along with mild temperatures is Southern California.  Many people consider San Diego’s weather to be the most ideal, with dry sunny skies and average highs in the summer in the upper 70s and average lows in winter only around 50.  Unfortunately, it also has some of the most expensive real estate in the country, with the median price for a home at about a half million dollars.  Such is the price of perfect weather.

Nick

What's the strangest weather you've ever heard of?
4J class
Jeju-si, Korea 

Wow, I’ve heard of so many strange weather events, it’s hard to pick the absolute strangest.  But some of the weirdest weather involves the numerous documented cases of animals raining from the sky.  In 1901, a storm in Minneapolis, Minnesota brought not only rain, but a shower of frogs from the clouds.  In 1877, a South Carolina farmer reported several foot-long alligators falling from the sky.  In 1966, a man in Sydney, Australia was trying to get out of the rain when a large fish fell on him.  In 2001, red-colored rain fell on Kerala, India; apparently from a red fungus in the raindrops.  And that same year, corn husks rained down in a storm over Wichita, Kansas. 

There have also been several reports of wild temperature swings.  In 1943, Spearfish, South Dakota saw the temperature rise from four degrees below zero Fahrenheit to 45 degrees above in just two minutes.  An hour and a half later the temperature plunged back down to four below zero.  In 1916, the temperature at Browning, Montana went from 44 degrees to 56 below zero in one day.  That’s a one hundred degree drop! 

There are lots of stories of tornadoes doing strange things.  One of my favorites is from Edmunds County, South Dakota in 1955.  As her mother watched, a nine-year old girl was riding her pony when a tornado suddenly lifted them both into the air and out of sight.  The girl later landed unhurt in a ditch. 
Strange, huh?

Nick

We live in Northwest Indiana, and we've had an unusually warm winter in 2001-2002.  What are some possible reasons for the unseasonably warm weather?  What effects could the warm weather have on spring storms, wildlife, insect populations, and plants?
Mr. Hebert's 5th Grade Class
Paul Saylor Elementary
Portage, Indiana 

Dear Mr. Hebert’s class,

You are certainly not alone in your warm winter.  Almost the entire country has experienced warmer than average winter temperatures.  And for some people, especially those in the northeast United States, the months of December through February have been the warmest on record, and their records go back about 100 years!  Snows have been lighter than usual, and the Great Lakes have stayed free of ice cover.

 I am sure that scientists will be studying the winter of 2001-2002 for some time to come, analyzing what made this season so warm.  Only then will we have a more detailed idea of all the factors involved.  But we already have some clues when we look at the jet stream, the narrow “river” of fast-moving air several miles above the ground.  In the winter, the jet stream is partially responsible for transporting cold arctic air from Canada into the United States.  You have probably seen the jet stream pictured on a weather map, looking like a wavy line extending across the country.  In order to bring that air southward, the jet stream has to move northward far enough to tap that air, then dip southward, bringing that cold air down into the United States.  On a weather map this would look like a squiggly line with a large peak called a “ridge” near the West Coast and a big dip, called a “trough” across the East.  But for much of the winter, the jet stream pattern has been relatively flat, with no big moves northward or southward.  The cold air has stayed in Canada most of the winter, with only a few trips southward.

Another factor in our warm winter is the lack of snow cover.  When the ground is covered with snow, much of the sun’s energy is reflected off the white snow surface back into space without being absorbed by the ground.   This causes temperatures to be lower.  Because much of the ground has remained free of snow this winter, temperatures have been warmer than they would have been otherwise.

There have been some outbreaks of cold temperatures in March, so it’s a bit early to write off the winter completely. That late winter cool down was tough on early blooming plants and crops.   It may be too early to tell what effect, if any, this development will have on animal and insect populations.  The warm winter won’t dictate spring storms though, since they are more dependent on small-scale weather patterns.  Whatever happens, we’ll be talking about the winter of 2000-2001 for a long time to come.

With all the heat lately in California, I am wondering what the all-time hottest day was in Palm Springs, California.
Diana
La Quinta, California

Diana,
Palm Springs was one of at least six dozen cities that saw record highs this past weekend, with temperatures well above 110 degrees F.  But twice on record, Palm Springs has hit a whopping 123 degrees! The first time was on August 1, 1993, and again two years later on July 28, 1995. But as hot as that is, that’s still a far cry from the temperature recorded just 300 miles away in Death Valley back in 1913.  At 134 degrees F, it still holds the all-time record for the hottest temperature in the United States.

Nick,
I was told that the sun is stronger through the clouds and that is why we can get sun burned so much easier. Why would that be true?
Theresa
Milwaukee, WI
 

Theresa,
The sun's rays are not stronger through the clouds than through a clear sky, but it is true that we can get sunburned almost as easily on a cloudy day as on a sunny day, especially if those clouds are thin and high. People have a tendency not to pay as much attention to putting on sunscreen on a cloudy day because they think they
won't sunburn. But the sun's ultraviolet rays can shine through the clouds, and in the summertime, those rays can especially be harmful if we're out outside too long.
Here's a link to the Ultraviolet Index forecast for various cities so you can get an idea how easily you can be burned on any given day. http://www.atmos.washington.edu/data/uv_report.html

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

Nathan,

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.

Nick,
I am a home-school mom, and I have a question for you. Would you explain what "heating or cooling degree days" are?
Vicki
Moline, Illinois

Dear Vicki,
Heating and cooling degree days help energy experts calculate the demand for heating fuel and electricity.  If you keep good records, you can also use them yourself to determine how much more or less you’ll pay to heat or cool your home in one part of the country versus another, or how efficient your furnace, air conditioner, or insulation is. 

To calculate heating degree days, first get a day’s average temperature.  To do that, add together the high and low temperature for the day and divide by two.  If the number you get is above 65, there are no heating degree days for that day.  If it is less than 65, subtract the number from 65 and that will give you the number of heating degree days.

To find cooling degree days, get the average temperature for the day and subtract 65 from it. For example, if during the late fall the daily high is 50 and the low is 30, the average temperature is 40.  That day would have 25 heating degree days.  If during the summer the high temperature is 90 and the low is 70, the average temperature is 80 and that day would have 15 cooling degree days. 

Thanks for your question!

Why are the winter temperatures in a rural area usually lower than those in the big city?  We think it has something to do with the buildings in the big city, but we're not sure.  For example, the temperatures in Seagoville are usually lower than the ones in Dallas.
Mrs. Gattis's 3rd grade
Seagoville Elementary School
Seagoville, TX   

Dear Mrs. Gattis’s class,

What you have noticed is called the “urban heat island” effect.   You are right; temperatures outside the city are usually cooler than those in the city.  This is because concrete and asphalt get hotter than trees and grass during the day.   After sundown, the concrete and steel surfaces of the city cool slowly.  This keeps the city warmer longer into the night.  Tall buildings also limit the cooling of surfaces below.  Cars and factories also give off heat, further increasing the warmth in the city.

Outside the city, the sun shines down onto trees, lakes and soil.  A significant part of the sun’s energy goes toward evaporation of water from these surfaces, so the surfaces don’t warm up as much, and they cool faster at night.  If all other things are equal, you can usually count on temperatures in the country being  lower than in town.  The urban heat island effect is observed in all seasons. The effect is largest at night and early in the morning when it’s clear, calm and dry. Under these conditions, temperatures in city centers can be more than 10 degrees warmer than in outlying areas, so you may have to wear your jackets a little more often than your friends in the city. 

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.

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

Hi Nick, 
The reason I'm writing to ask a question is because I work with a few guys who think that their Navy education is better than my civilian one. The question on the line is: We know that the "official" first day of spring is March 21st. But what is the first day of spring for meteorologists? I believe it is March 1st. They tell me I'm wrong. Can you help straighten this out? Thanks!

Maria,
Well Maria, you win, sort of.   We usually talk about spring beginning with the spring equinox, which is around March 20 or 21st, depending on when exactly the equinox is, and what time zone of the world you are in. As far as I can tell, there is no "official" government-proclaimed start to meteorological spring.   However, most meteorologists think in terms of spring weather commencing on or about March 1^st.  That’s because the coldest three months of the year usually align with the calendar months of December, January and February, although some places in the U.S. see their cold weather begin in mid November.  Thus, meteorological spring goes March through May, summer from June through August, fall from Sept. through November, and winter from December through February.   These dates coincide better with the actual changes in the weather around the Northern Hemisphere than the dates of the summer and winter solstices and the spring and fall equinoxes. 

Nick

Dear Nick,
Why does the temperature drop a few degrees just before sunrise in the cooler months of the year?
Vince
Hurst, Texas

Vince,
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, 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.

Nick Walker
 

We are a home school group in Fort Worth, TX. Our teacher's name is Mrs. Garza and our school name is the Golden Cross Academy. Our question is dealing with Texas weather. We are compiling information for a "State Notebook" and we need the average monthly high and low temperatures for the months of the year 2002.
We would appreciate your help.
Sincerely,
Mrs. Garza's home school group

Dear Mrs. Garza’s home schoolers, 
You can find various kinds of climate information online, but the kind of information and how detailed the information will vary widely from one place to another.   You’re in luck though.  The Office of the Texas State Climatologist at Texas A&M University has the data for the Dallas-Fort worth area at http://www.met.tamu.edu/met/osc/tx/tx2002.html

Click on the monthly summary links one by one and you will get tables of the monthly data that you are seeking in the columns labeled “Avg Max Temp” and “Avg Min Temp.” 

For other parts of the country you can often find climate information from local offices of the National Weather Service.  Start with their home page at http://www.nws.noaa.gov and use the map they provide to click on the part of the country you want.  Follow any links that say “Climate” to see what sort of data is offered.  Be advised that you won’t be able to find past weather information for all parts of the country, and where such information is available, it may be limited to only recent weeks or months. 

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 want average temperatures or record temperatures for a particular city, you can find the information quite easily on weather.com.  Simply type in the zip code or city name for which you want weather information.  Along with the ten-day forecast, you’ll see an orange bar that says “Averages and Records.”   Click there and see the daily and monthly records and average temperatures for that city. 

Good luck on your project! 

Nick Walker 

Nick, I was wondering why the temperature on the coast is usually higher in the winter compare to temperatures inland? Then why it is a total opposite in the summertime (cooler on the coast than inland)?  
Kate
New London, CT

Dear Kate, 

It is all because of the influence of the oceans on coastal climates.   Water warms up and cools down more slowly than land does.  It takes a lot of heat to warm an ocean, so during the summer, the ocean water will stay cooler than the land, and that cooler water will keep temperatures on the coast from warming as much as areas away from the water.  Likewise in winter, the ocean will hold onto heat longer than land, so that coastal temperatures will remain warmer than landlocked areas. 

The warming ocean influence takes place when the broad wind pattern over the regions pushes the warmer sea air onshore.  The cooling ocean influence can take place from those broad wind patterns too.  But even if a weak overall wind pattern would normally send the winds offshore, the cold air offshore can force itself onshore near the coast in localized winds called “sea breezes.” 

Compare Seattle, Washington on the waters of Puget Sound with Spokane, which is well inland in eastern Washington.  Their climates differ from one another partially because they are separated by the Cascade Mountains, but also because of the ocean’s influence.  Seattle’s average high temperature in July is 75, while Spokane’s is 83.  The average low temperature in Seattle in January is 35, whereas in Spokane, it’s 20.  As a rule, coastal areas will find their seasonal shifts in temperature much less dramatic than areas farther inland. 

Nick Walker

Mrs. Sturm's 2nd grade class from Ridge Park Elementary in Conshohocken, PA writes:

We know that the winter solstice is on December 21, 2001 and that is the shortest day of the year.  We would like to chart the sunrise and sunset times for the next week and a half to prove that the days are actually getting shorter.  Where would we find information on the internet telling the sunrise/sunset times?

Thank you!

Dear Second Graders,

You can find the sunrise and sunset times by going to the local weather page for your city at www.weather.com and then click on “Averages and Records” just above the Temperature Converter.  You will see that the shortest days of the year (in terms of daylight) are the days leading up to Christmas.  However, the shortest day of the year as calculated by sunrise/sunset times may not necessarily be the day of the winter solstice.  This is because the day of the winter solstice (as well as the summer solstice and the equinoxes) is not based on the sunrise and sunset times, but on the location of the center of the sun with respect to the earth.  Sunrise and sunset times are based on the minute the top of the sun touches the horizon when it rises and sets.  Another thing to keep in mind is that the sun is still actually below the horizon when you see the top edge of the sun at the horizon.  This is because Earth's atmosphere bends light rays, so we see the sun “rise” before it actually does. These astronomical phenomena add a few minutes to the hours of daylight.

The U.S. Naval Observatory has more information about this at http://aa.usno.navy.mil/faq/docs/dark_days.html

Do seasonal changes take place at the same time worldwide?
Al
Pennsylvania

Al,

The changes occur at the same time worldwide, but not all areas see the changes at the same rate, and not all areas see as dramatic a change as others.  As one half of the world enters spring, another enters fall.   As one half begins summer, the other half begins winter.  

The seasons’ changes start with the sun and Earth’s orbit around it. The summer solstice in the Northern Hemisphere, or top half of Earth, is around June 20.   That's when the sun reaches its most northern point over our planet and the days are the longest of the year.  The rays from the sun beat almost directly down on the United States, Europe and most of Asia, and temperatures warm up.  In the Southern Hemisphere, or bottom half of Earth, the sun’s angle is very low and the days are short. June means the start of winter for Australia, South Africa and the southern parts of South America. 

As we move into September, the sun’s rays shine more directly on the center of Earth than on the top half.  Since areas near the equator don’t see as extreme a difference in sun angle from one season to the next, the seasonal changes are not as dramatic as in other areas farther north and farther south.  As Earth revolves around the sun, the Northern Hemisphere becomes tilted more away from the sun as fall turns to winter there.  Meanwhile the bottom half, the Southern Hemisphere, becomes tilted more toward the sun.  The beginning of autumn for the United States and Europe is the beginning of spring for people in Australia and the southern parts of Africa and South America.  By late December the sun is at its southernmost point over Earth.  That’s when it’s summer in the Southern Hemisphere and winter in the Northern Hemisphere. 

Nick,

I was wondering what the record snowfall for Houston is.
Jerry
Wisconsin 

Jerry,
According to Bill Read of the National Weather Service office in Houston the record snowfall, believe it or not, is twenty inches that fell February 14-15, 1895. This must have been an incredibly unusual pattern because record snows from Brownsville (six inches) into Louisiana (24 inches at Rayne, nine at New Orleans) occurred with this storm. According to Read, Houston's next biggest snowfall was 4.4 inches Feb 12-13, 1960.

Nick,
Why is it colder up in the sky than it is on the ground?   After all, you are closer to the sun higher up in the sky.
Chris
Schenectady, N.Y.

Chris,

You are right; it is usually colder higher in the atmosphere than it is at ground level. 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 adjacent air layer. So most of the heat we feel is actually coming from the ground, which has been heated by the sun. Heated bubbles of air rise and transfer some of the warmth to a deeper layer.  However, as the warm air rises, the air pressure decreases and the air cools.  So the farther away you get from the ground, the cooler the temperatures are. The atmosphere is not generally 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, clouds reflect some of the visible light back to space, and even some of this energy is reflected back to space by the Earth’s surface.

On a sunny day we can feel warmth on our body as some of the sun’s radiation is directly absorbed by our skin and clothing, just as energy is being absorbed by the ground.  A similar effect can be felt when you sit a few feet away form an open fire or fireplace.

Most of the sun’s energy comes in the form of visible light and infrared radiation.   A small portion of the sun’s energy comes in the form of ultraviolet radiation, which is the portion that causes sunburn and a skin cancer risk.  At night, the Earth gives back some of the daytime energy gained in the form of infrared radiation.

Nick

I'll be traveling to Barrow, Alaska soon.  I see low and high temperatures listed for Barrow.  When it is daylight 24 hours a day, what causes the variation in temperature?

Thanks,
Nita
Snellville, GA 

Nita,

In July,  Barrow, Alaska has average high temperatures in the mid and upper forties and average lows in the mid thirties.  But these temperatures can vary greatly, especially the high.  The temperature differences can come from several factors, the main factor being the sun’s position in the sky.  Even though there is daylight through the entire 24-hour period, the sun is much lower on the horizon in the hours around midnight, and much higher around noon.  As the sun rises higher in the sky, its rays shine more directly onto the ground, warming it.  As the sun lowers in the sky, the ground has a chance to cool.  During the hours around midnight, the outgoing energy from the ground is greater than the weak incoming sunlight. 

There may also be other factors that affect temperature, including weather fronts and cloud cover.  Clear skies would allow the maximum warming as the sun shines down unblocked, but it would also allow the maximum cooling, as any warmth would readily radiate from the ground into space.  A cloudy sky might prevent the sun from warming the ground as much, but may also hold heat close to the ground.  Cold fronts could bring colder air masses into the area, just as a warm front may bring some warmer temperatures.