Tuesday, August 5, 2008

Rain, Heat, Storms -- We've Got It All

Wow, was it ever wet in the Chicago-land area on Monday. Much of the region saw a widespread 1-3 inches of rainfall.

It was double that across northwest Indiana -- with 3 to 7 inches of rain falling across a 5-county area.

As I write this morning's blog heavy rain is falling across central and portions of southern Indiana, thanks to a MCS -- mesoscale convective system.

Now that is a weather term you can drop at your next dinner party and impress the socks off those you know.

The MCS is a complex of thunderstorms that become organized on a larger scale than just an individual thunderstorm. It normally persists for several hours and they usually form in the evening hours.

It must meet a certain size criteria -- which is about 60 miles in at least one direction.

As they persist into the overnight they often turn into MCC's, a mesoscale convective complex.

These are typically nocturnal (meaning overnight) and are associated with lots of rain and lightning, but wind is common too. MCC's usually have a large, circular cloud shield as viewed from a satellite.

Tropical Storm Edouard has arrived in coastal Texas. It will bring heavy rain and wind to much of Texas over the next few days.

We should see a really nice footprint on tomorrow's CoCoRaHS maps of the landfall in Texas, as well as in adjacent areas of southwest Louisiana.


So yesterday I teased you saying that we know both air pressure and air density decrease with height, but does temperature always cool with height?

Air temperature normally decreases from the surface of the Earth up to about 7 miles, or 36,000 feet.

This is due to the fact that sunlight warms the ground, and the ground in turn warms the air immediately above it.

The rate at which the air temperature decreases with height is called the lapse rate, and the standard (or average) lapse rate in this region of the lower atmosphere (the bottom 7 miles or so) is about 3.6°F for every 1,000 feet you rise in elevation.

Now you see why us Denverites go to the high country to play on the hot summer days in the city.

The above figures are only averages. There are definitely days when the air becomes colder more quickly as you move upward. This would "steepen" the lapse rate.

Some days the air cools more slowly with height, and the lapse rate would be less than the standard of 3.6°F.

And still yet, there are some days when the air warms with height, producing a condition known as a temperature inversion.

The region of the atmosphere (from the surface up to about 7 miles) contains all the weather we are familiar with on Earth.

It is kept well mixed by rising and sinking air currents -- and is called the troposphere.

Each day, hundreds of weather balloons are released around the world that carry a radiosonde.

The radiosonde measures important weather variables as it rises 100,000 feet into the air.

One of those variables is temperature -- and the data it sends back gives us a temperature profile of the atmosphere.

Although the overall trend in the troposphere is that temperature cools with height, there can be layers where the air is warmer than that above or below it.

This is part of the dynamic that drives our weather, and is very important when forecasting. A meteorologist always looks at the temperature profile of the atmosphere because at some point that air is going to mix out (thanks to air currents) and it could have an impact on what kind of weather we see at the surface.

Probably the best example I can give you that most have heard at some point in their life from their favorite local t.v. meteorologist is the "cap" in the atmosphere.

This is a layer of warm air aloft that acts like a block for cloud and thuderstorm development. However, as conditions change in the environment of the upper atmosphere and cloud growth can punch through the layer of warm air (this is called breaking the cap) --- things change rapidly and thuderstorms can grow rapidly.

The troposphere is just one layer of the atmosphere -- where we live and where the weather occurs.

Tomorrow we will explore the other 3 layers.

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