Hot & Dry Weekend

It was a hot and dry start to the weekend with widespread 90s and 100s from the Rocky Mountains to the deep south.

Denver set a new record high of 104 degrees on Friday. Wichita Falls, TX tied their record high of 108 degrees. North Little Rock, AR (where I was born) tied their record high of 103 degrees.

I am writing this blog at 3pm on Saturday and we have set another record here in Denver at 102 degrees today.

But get this....we had a heat index of 95 when it was 102 degrees. The dewpoint was 27 and the relative humidity was 7%.

Jacksonville, AR (just down the road from N. Little Rock) was 102 degrees with a heat index of 113 degrees. The dewpoint was 73 degrees with a relative humidity of 40%.

I am SO glad I am not back home today.

The hot air is spreading east and north over the next few days, so places like Chicago and Minneapolis can expect a warm up.

Today's Lesson

We're ready to talk about density of the air. This is determined by the masses of all atoms and molecules and the amount of space between them.

In other words, density tells us how much matter is in a given space. (that is volume)

Mathematically, it looks like this.

Density = (mass ÷ volume)

There are considerably more molecules within the same size volume of air near the Earth's surface than at higher levels.

Air density decreases with height, rapidly at first, then more slowly the higher you go in the atmosphere.

Air molecules are always in motion. They bounce around off people, objects, even your plants and trees.

Each time an air molecule hits something, it gives a tiny push.

This small force (push) divided by the area on which it pushes is called pressure.

Pressure = (force ÷ area)

I bet by now you are thinking so how in the heck do you measure air?

Well luckily there is a standard to go by.

If you took a column of air (1 square inch in size) that extended from the ocean surface to the top of the atmosphere, it would weigh about 14.7 pounds.

Therefore, normal atmospheric air pressure near sea level is close to 14.7 pounds per square inch.

If more molecules were packed into that column, it'd become more dense or heavier, and since the air would weigh more pressure would go up. (high pressure)

The opposite would be low pressure if there were fewer molecules of air in that 1 square inch column.

Now in weather we don't typically measure air pressure in pounds per square inch. The most common unit used to measure air pressure is the millibar.

The hectopascal (hPa) is also widely used in science as is mercury (Hg) on television and in weather reports that go out to the public.

Here are the conversions for standard sea level pressure. All 4 of the following are equal.

14.7 pounds per square inch is standard sea level pressure

which converts to 1013.25 mb

or 1013.25 hPa

or 29.92 in. Hg

Tomorrow we'll dive a little deeper into air pressure.