The weather in the Midwest has been wild this summer. There have been many reports of tornado damage and severe flooding across several states in the region. Why does wild weather happen during this time of year?

It’s all about Heat

It is common knowledge that temperatures rise as winter changes into spring and summer. Higher temperatures means more heat, and heat is the driving force behind wild weather. This is why you don’t see thunderstorms in the winter.

Wild weather occurs when the air is warm, full of moisture, and unstable. It is easy to figure out when the air is warm and full of moisture, just think of a hot and humid day. How can you tell if the air is unstable?

Use your eyes

The air above us can either be stable or unstable. The easiest way to tell if the air is unstable is to look at the clouds in the sky. Tall and puffy clouds are a sure sign of unstable air. If the air is stable, the clouds will form a flat layer.

Here is a picture I took of a tall and puffy Thunderstorm cloud (called a Cumulonimbus) while flying over Florida. This cloud was able to get very tall and puffy because the air is very unstable.

Meteorologist (weather scientists) know when wild weather is coming by study charts and data from satellites in space. 

Take a look at this weather map from the National Oceanic and Atomspheric Adminstration (NOAA). What do you notice?

The “H” and “L” you see on the map refers to areas of High pressure and Low pressure. The weather person on TV will refer to this as “barometric pressure” and will say if it is rising or falling.

In general:

– If the barometric pressure is rising or is High, expect good weather.

– If the barometric pressure is falling or is Low, expect bad weather.

Notice that the areas of low pressure have a lot of rain and thunderstorm warnings.

Another thing to notice are the lines of blue flags and red half circles. These lines indicate a weather front. What is a weather front? 

A weather front is the boundary between two different air masses. Cold fronts are the line of blue flags and warm fronts are the line of red half circles. The direction that the flags and circles are pointing show the way the weather fronts are moving.

As a general rule, when opposite weather fronts (warm and cold) collide, wild weather will happen. Notice that there is an area shaded in yellow that warns “Severe Thunderstorms Possible” near where a warm front is cutting a cold front in half.

The Big Picture

The weather map shown above is a typical example of what happens during the summer over the middle of America. Cold dry air from the Artic moves south over Canada, while warm moist air from the Gulf of Mexico moves north. These air masses collide and cause wild summer weather. Sometimes the storms can be so big that is stretches across the U.S. from north to south.

These storms can pack a punch, which means it has energy in the form of heat and wind. I wonder if scientists can create a technology that drains a storm of this energy and transform it into electricity. This way, the storm won’t cause damage and people will have a natural source of power. What do you think?

Dr. Dave

Fog can be thought of as a cloud that is on the ground.  Clouds are usually very thick and almost impossible to see through, while fog can vary in thickness.

Did you know that Meteorologists (scientists that study the weather) officially define a day as foggy if the visibility is about 6/10 mile or less?  If the visibility is greater than 6/10 mile but less than 1 1/4 miles, it is officially call mist.  I don’t know who came up with those numbers but I know that fog poses a challenge to safe driving or flying.

Here is a really cool picture of a plane landing through the fog from www.airliners.net.

Recipe for fog

Fog typically occurs when the air is calm, cool, and full of water vapor.

Depending on where you live, and what season it is, your local TV weather person will give the temperature, dew point, and humidity.  The temperature tells us how hot or cold the day will be.  The dew point is the temperature the air needs to be cooled to form dew, fog, or clouds.

Both the dew point and humidity both tell us how much water vapor is in the air. 

Key points

• If the temperature and dew point are within a few degrees of each other, then it means that the humidity is high and it is probably raining, snowing, or foggy.
• If the temperature and dew point are far apart, it means the humidity is low.
• Humidity is relative to the temperature.  Cool air holds less water than warm air.  This means that a hot, humid summer’s day holds more water vapor than a cold, humid winter’s day. 

Putting it all together

Chicago recently experienced fog so thick that hundreds of flights were canceled into the area airports and there were many car accidents.  The fog came on the heels of serious snowfall resulting in 10 inches of snow on the ground.  A few days later the air began to warm slight above freezing and the snow to melt and evaporate.  This filled the air with water vapor and, because it was not windy, the air was clam enough for fog to form.

In an earlier post on Hurricanes and Latent Heat, I discussed condensation, the process that causes water to form on the outside of a cold glass of soda on a hot and humid day.  Did you know that fog and clouds form in a similar fashion?  It’s true!  Tiny particles of dust and pollution give water a surface to condense on to form visible moisture.

If you would like to read more about fog, this link to Wikipedia has a section that discusses it in greater detail.

-Dr. Dave