Answers: Inside Weather Fronts

By Jack Williams ©2015

Q: In the November issue of AOPA Flight Training, you discuss extratropical cyclones, including the movement of cold air under warm air and vice versa. If I understand this correctly, the fronts themselves do not discriminate between the cold or warm air ahead of them.  How  do you explain movement under (cold into warm) or movement over (warm into cold)?

Kevin, Hickory, N.C.

A: I should have added one sentence to my November Flight Training article that would have answered your question: “Cold air is denser than warm air, which is why advancing cold air pushes up warm air and advancing warm air rides over cold air.”

Let me add a little more detail about this. Air’s density, which scientists measure in kilograms per cubic meter, increases as the temperature falls if the air pressure stays the same.

For example, if the pressure is 1013 millibars (mb) and the air’s temperature is -20 degrees Celsius (-5 Fahrenheit) the density is 1.35 kilograms per cubic meter, which is approximately 0.083 pounds per cubic foot. If the pressure remains at 1013 mb, but the temperature rises to plus 10 C (50 F), the density decreases to 1.25 kilograms per cubic meter, which is the same as 0.078 pounds per cubic foot.

This shows why we say that cold air is heavier than warm air–strictly speaking you should say that cold air is denser than warm air.

Cold air is advancing from left to right--the blue arrow. The green arrow indicates warm air that's being pushed up. NWS image.

Cold air is advancing from left to right--the blue arrow. The green arrow indicates warm air that's being pushed up. NWS image.

Another important key to understanding fronts is that when a mass of cold air advances into a mass of warm air or warm air advances into cold air, they do not mix with each other.

Thus, when cold air is advancing, it’s like a wedge that pushes the warm air up, as shown in the image to the left.

When the warm air is advancing the cold air becomes like a ramp that carries the warm air up.

The cold front image to the left is from the National Weather Service’s Jetstream Online School for Weather page on air masses and fronts. This page also has information on air masses. It also has an illustration of a warm front, but I didn’t use it because the colors make it a little confusing. If you do go to the page, you will see that the green arrow represents warm air riding over cold air as in the image above, but instead of being shown in blue, as above, the cold air is shown in a “warm” orange color.

Pages 110 and 111 of my latest book,  The AMS Weather Book, have graphics showing fronts and how they are organized into storms. The text on the pages before and after these graphics explains more about what’s going on.

Some of my past “The Weather Never Sleeps” columns for Flight Training Magazine also have more on winter storms and their fronts:

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