Wunderground Meteorologist Shaun Tanner

Lesson 3 part 2: Nighttime Cooling

By: shauntanner, 11:03 PM GMT on March 27, 2012

Okay, so it has been awhile. It is funny where the weather takes you if you let it. The weather has been quite busy lately, so I have been dashing around between tornadoes, West Coast storms, heat waves, and such. But, I am getting back to my lessons of teaching you all how the atmosphere works. My goal is to make you all experts by the time I am finished so you can make educated guesses about the atmosphere and climate change, rather than listen to experts on either side of the fence. If you are coming across this blog and want to get caught up, go ahead and look back at some of my old blogs from the end of 2011. They aren't the best writing you will ever read, but they are something.

Last time, we tackled the topic of daytime heating. The ground receives radiation from the Sun and then, in turn, it radiates energy into the atmosphere. The big note is that the Sun DOES NOT warm the atmosphere. The ground receives radiation from the Sun after passing through the atmosphere. Next, we will discuss nighttime cooling today.

Nighttime Cooling

Here's a big surprise...the atmosphere cools at night. Okay, I guess we are done with this part of the lesson. Everybody can go to sleep now.

For those of you who want to learn more, read on.

I am going to tell you something very simple, yet very complicated. It is a very important sentence that you have to remember throughout this lesson:

The temperature of an object will decrease as long as its emission of radiation is greater than its absorption of radiation.

There are some words in there you might not understand, so I will bring up the analogy of a bank account once again. Let's say your bank account balance goes up. That is the equivalent of your temperature increases. If your bank account balance goes down, your temperature decreases. When you put money into your bank account, that is the equivalent of you receiving radiation. If you withdraw money, then that is the equivalent of emitting radiation. With these ideas in mind, it is easy to see that if you withdraw (emit) more money than you deposit (absorb), then your back account will go down (temperature decreases). The opposite is also true.

So, why does temperature decrease at night? Don't think too hard about this one...THE SUN IS DOWN! Now, which part of the above absorption/emission problem is taken away when the sun sets? Absorption! So if you take away absorption, you are left with the surface only emitting radiation. If this is the only part of the equation, then of course temperature will decrease as long as the Sun is away from the surface.

The process at which the ground cools at night is called radiational cooling. It basically means that the ground cools because it radiates its heat away and into the atmosphere. But because the ground is a much, much better radiator of heat than the atmosphere, it loses its heat much quicker than the air. Thus, by late evening/early morning, the coldest part of the atmosphere is most likely to be the few feet closest to the ground. This is exactly why you should always build a survival shelter off the ground if at all possible. Even if you build the shelter a few inches off the ground you will be warmer than if you were to sleep directly on the ground.

So, if you think about it, a weird temperature profile sets up during the nighttime. When you have cold temperatures near the surface and warmer temperatures higher up in the atmosphere, this is called an inversion. There can be strong inversions in which a very cold surfaces transitions very quickly to warmer temperatures higher in the atmosphere, or there can be weak inversions that have a much slower change.

This brings up a very important topic. Inversions are very important to the atmosphere because they limit vertical movement. If you remember from an earlier discussion, cold air is heavier than warm air. That means cold air always wants to be below arm air. So, if you have an inversion with cold air below warm air already, that is the natural state of this. If you try to pull the cold air above the warm air, it will want to sink back down the surface. This, this inversion is inhibiting vertical movement. Because of this characteristic, inversions are often easy to spot even if you don't have a thermometer and a hot air balloon. Because of this inhibition of vertical movement, our normal tellings of vertical movement are not there. If you see large, billowy, vertical clouds, then you have great vertical movement in the atmosphere. However, if you see something like this:

Then this is telling of an inversion. Do you see how the clouds have a definite flat top? The clouds are being held down by the inversion.

Here is another example, but with pollution:

Very bad pollution episodes are usually accompanied by a strong inversion that prohibits the pollution from mixing high into the atmosphere.

So what nights do we get the strongest inversions?

Well, if you think about it, that is pretty simple. Since the temperature of the surface will cool as long as the Sun is away from warming it, then it would be logical that the coolest nights of the year occur in the Winter, with the nights are the longest. This is also when we experience the strongest inversions.


Severe Storm Continues To Drench Eastern Plains

By: shauntanner, 6:46 AM GMT on March 20, 2012

As Weather Underground meteorologists watched and reported on the progress of a cold front that moved through Texas and Oklahoma on Monday, we were asked several times via our twitter account by people in Dallas, "where is this heavy rain and thunderstorms we were promised?" True, the cold front moved agonizingly slow across the Lone State State and its neighbor to the north, but it eventually did reach the Dallas/Fort Worth area with extremely heavy rain and severe thunderstorms. Over 2 inches of rain fell in Dallas Love Air Field. But, this was just the tip of the iceberg. The airport at Oklahoma City reported nearly 3 inches of rain, while areas to the east, including Tulsa, reported several more inches of rain. Thus, while there were tornadoes in parts of Texas, including a strong tornado that ripped through Devine and damaged some 50 houses, the most damaging part of the storm from Monday was the flooding it caused. Flash Flood Watches and Flood Watches and Warnings were posted from southern Texas through Missouri because of this heavy rain. When rain this heavy falls on any sort of ground, the ground simply cannot absorb it fast enough. Thus, it runs right off as groundwater that floods streets very easily.

What's Next?

The bad news is this storm was not a one-day event. As we have stated for several days, this storm will be a marathon event as it moves very slowly eastward. On Tuesday, expect the heaviest rain and strongest thunderstorms to move into Louisiana, Arkansas, and Missouri. Amazingly, the HPC forecast for Tuesday below is showing a bull's eye of rain of greater than 6 inches along the border of Louisiana and Arkansas. But, rain up to 3 inches are expected as far north as southern Missouri.

In addition, the Storm Prediction Center has issued a slight chance for severe weather for the Arkansas, Louisiana, eastern Texas area on Tuesday. The main worry for this area is the potential for very strong, possibly severe thunderstorms in the area. Strong tornadoes cannot be ruled out, but there is a decent chance that any tornadoes that do form will be mild. The severe thunderstorms that are expected will be accompanied by large hail that by themselves can cause great damage to the area.

With regard to temperature, the Northern Plains and Upper Midwest will finally see a bit of a cooldown, but record setting temperatures are still possible, as noted in the maximum forecast temperatures below. Seasonally warm temperatures will also continue along the eastern seaboard.

Flood Tornado

Massive Storm and Severe Weather Possible In Plains Monday

By: shauntanner, 6:18 AM GMT on March 19, 2012

As always, Weather Underground will be monitoring the progress of the storm that will be discussed in detail below. To get the most updated information on the storm, please follow @wunderground on twitter or "like" our Facebook page.

All corners of the United States will see interesting weather on Monday as an intensely strong storm moves into the Plains. If you have read my blogs before, you know I always start off looking at Weather Underground's Severe Weather Map.

Weather Underground's Severe Weather Map

The more colorful the map, the more active the weather the country will experience. So, I wanted to take each region of the country and explain what can be expected for Monday and beyond.

Southern and Central Plains

The region of the country from eastern Texas through Missouri and Kansas will likely be the hardest hit area of the country, so I will start here.

A tremendous amount of moisture will stream northward from the Gulf of Mexico into the area. When you couple this moisture with the beginning of severe season, this may turn out to be bad news. The Storm Prediction Center has issued a modate risk of severe weather in eastern Texas, and a slight chance for severe weather in surrounding areas.

This area could see typical weather associated with severe weather, including severe thunderstorms, extremely heavy rain, and tornadoes. Residents in this area should monitor local weather conditions and be prepared to take any precautions necessary to protect life and property.

This is also the same area that is under Flash Flood Watches and other Flood Watches in anticipation of a lot of rain over the next few days. The image below shows the forecast rainfall amounts for much of Monday. Note the bull's eye of heavy rain in northeast Texas and east Oklahoma. This forecast is calling for over 2.5 inches in Oklahoma, while the flood watches note that locally some areas could receive up to 6 inches of rain through Tuesday. That amount of rain falling in the area will most likely experience flooding, so please be aware of any flooded roadways and streams.

The crazy part is that this precipitation is just the beginning. Take a look a the following image, showing the 5-day precipitation for the country.

You are reading that correctly. The forecast is calling for over 10 inches of rain in a 5-day period through this week in parts of east Texas through Arkansas and southern Missouri. In addition, 8 inches is possible in Louisiana.

Intermountain West

This area includes Montana and Idaho. The center of the main storm will move out of eastern Montana and into Canada. Winter Storm Warnings are posted for much of Montana in anticipation of up to 2 feet of new snow in the mountainous areas, and up to 8 inches in the lower elevations. In addition, gusty winds up to 25 mph will blow around this newly-fallen snow, creating blizzard conditions in the state and surrounding areas.

Upper Midwest

With all of this stormy weather, you would think the warm weather in the East would be a distant memory. Not so. A warm pocket of air will move through the Dakotas and Minnesota, keeping maximum temperatures well above normal. Minneapolis will see afternoon temperatures in the mid-70s, which is 30 degrees above normal. Let me state that again. Minneapolis will see temperatures 30 DEGREES ABOVE NORMAL! Likewise, Fargo, ND will also be warm. There is even the possibility that the city could experience maximum temperatures in the low 80s. That could be 40 degrees above normal. Needless to say, daily record temperatures will likely be set throughout the region.

Forecast Maximum temperatures for Monday.

Southeast and Northeast

Warm temperatures will also continue in the Southeast with widespread 70s and 80s likely. While these temperatures are not completely from left field (afternoon temperatures will be "only" 10-15 degrees above normal), the Northeast will once again experience late Spring-like weather. Normal afternoon temperatures this time of year are in the lower 50s in New York, yet the city will experience in the lower 70s. Boston will only be in the mid-60s, which is still over 10 degrees above normal.

All-in-all, it will be a very, very busy week for the country, so please prepare yourself before it is too late.

Flood Tornado Winter Weather Winter Weather

Spring Heat Wave Hits Middle of United States

By: shauntanner, 3:46 PM GMT on March 14, 2012

For much of the Winter, the United States has been stuck in a nearly zonal pattern. What that means is that the flow from West Coast to East Coast has been generally flat. No zig-zags north or south. We have spoken a lot of the Arctic Oscillation and how the jet stream has been very reluctant to make any sort of southward jog down into the Lower 48 since December. The result of this is that the country has experienced a very warm Winter. So warm, in fact, some outlets are calling this the 4th warmest Winter on record for the country.

Well, fast-forward to now and we are beginning to see something quite different. With a low pressure trough over the West Coast (with more to come) and a high pressure ridge over the eastern half of the country, we are in a more progressive pattern. Progressive simply means that things are moving now. Rather than earlier in the Winter when things just seemed to be stuck in time. Trouble is, the progressive pattern is occurring in the West where several storms are pounding the area. In the East, however, the high pressure ridge seems to be stuck, and will continue to be stuck for the next several days.

This high pressure ridge is what I want to focus on right now as it continues to bring well-above normal temperatures basically from the Plains eastward. Below is the expected high temperatures for Wednesday.

Note the reds and orange colors in the Plains and Southeast. Those colors translate to 70s and 80s for these areas. Take Chicago, for instance. The city's normal high for today is 46 degrees with a record of 76 degrees set in 1995. Well, Chicago may be that record today as the forecast is set for 77 degrees.

Much of the Southeast has averages this time of year in the 60s. Birmingham's forecast is in the lower 80's, making a record-setting day possible there as well (the record for today is 84 degrees set in 1967).

If you enjoy this type of warm Spring weather, then the East is the place to be over the next 5 days or so. The extended forecast is calling for days with maximum temperatures in the 70s and 80s into next week. In the meantime, keep an eye on Weather Undergrounds new Record Extremes section as it tracks daily records set throughout the country. I have a feeling it will be quite active over the next few days.

Who is enjoying this Spring weather?


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Wunderground Meteorologist Shaun Tanner

About shauntanner

Shaun Tanner has been a meteorologist at Weather Underground since 2004.

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