Why so darn cold?

May 10, 2010; 9:45 AM ET
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A blast of cold air rolled into the region in the wake of showers and, in some cases, locally severe thunderstorms over the weekend. Many areas in the Upper Midwest and interior Northeast were smacked with unusual cold and a frost or freeze as a result.

Why?

Uneven heating of the Earth by the sun causes the jet stream to bend to the north and to the south.

The jet stream is a ribbon of strong winds, high above the Earth's surface.

On a typical day, there are many bends in the jet stream around the globe. If the jet stream bends to the north of your area, warm weather usually occurs. If it bends to the south, cold conditions are the rule.

The sharper or larger the bend, the more severe the conditions can be.

In the case of this past weekend, the jet stream took a big dive to the south. In addition to surface cold air being offered a direct southern route from Canada, air was also funneled down to the surface from high in the atmosphere, where temperatures are much lower than near the ground.

On a windy day, the air warms at the rate of 5.5 degrees per thousand feet, when descending in elevation. In this case, valleys are significantly warmer than ridges. Meteorologists call this air "well mixed."

However, on a clear night, it is the valleys and the low spots in the valleys, that are often significantly colder than the ridges. The cold air collects near the ground, as it is denser than the surrounding air. When this happens, winds from high in the atmosphere cannot penetrate this colder, denser air, and calm conditions develop.

In a clear, calm situation, the cold, dense air tends to flow downhill and fill up the valleys, or even low spots in your yard.

Meteorologists call this setup an "inversion," and it only needs to happen for a couple of hours for problems to develop.

Your porch thermometer may have a low of 35 degrees, but the temperature at ground level on your lawn, garden or flower bed may be 5 degrees lower or more, resulting in a frost or freeze. Meanwhile, on a nearby ridge, the temperature could be around 40 degrees.

Official temperatures are recorded at around 6 feet off the ground to avoid inaccurate readings during the daytime, when the ground temperature can be 30 degrees higher than the air temperature.

When the wind kicks in, the air is mixed and the temperature tends to even out from the ground on up.

What was with that wind?

The tremendous mixing of the air this past weekend also allowed some of the normally strong winds high in the atmosphere to work their way down to the surface in the form of powerful gusts.

The gustiest situations occur when winds aloft and at the surface blow from the same direction. This setup, known as "downward transfer of momentum," minimizes the effects of friction.

Jesse Ferrell has more about the high winds this past weekend in his blog.

What's ahead?

As for the threat of a frost or freeze tonight, the risk has passed for the Upper Midwest. However, light winds and clear skies farther east over much of New England and the eastern part of the mid-Atlantic could allow a temperature inversion to develop, yielding some of the lowest temperatures yet for the outbreak.

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