Thursday, October 24, 2013

Frost/freezing conditions expected the next couple of nights

With the issuance of our first Freeze Warning of the fall (for Fayette and Tipton Counties) and Frost Advisory for the rest of the metro except Tunica County, it's a good time to provide some meteorological insight on why these freezes are able to occur and remind you to protect any tender vegetation.

It's not too hard to reach sub-freezing temperatures in the winter, but at this time of year the atmosphere needs a few specific conditions to get there:
  1. Clear night
  2. Relatively dry air
  3. Close to calm winds
So it's no surprise that these are the conditions we're experiencing over the Mid-South as rural areas will begin to see these freeze effects.

A clear night is required because heat radiates off the surface of the Earth and into space most efficiently when there are no clouds to re-emit radiation back to Earth. [EP: Clouds act like a "blanket" at night trapping heat near the Earth's surface.  Areas outside the city, where concrete, buildings, etc. tend to hold heat more efficiently than grass, dirt, and vegetation, tend to be the coldest on clear nights.  This is why outlying areas are colder on cool nights than areas within a city.]

Relatively dry air is required because the amount of moisture in the air places a limit on how far the temperature can fall. The less moisture presence, the lower the temperature can get before reaching saturation. Once this happens condensation can occur which will further damage vegetation if the temperature is at or below freezing. [EP: Dewpoints are the measure of humidity we use to determine how far the temperature can fall at night. Tonight dewpoints will be in the mid to upper 20s.]

Last but not least, light winds are required because winds act to mix warmer air to the surface. Usually the coldest temperatures right at dawn are very close to the surface. This is a typical time for temperatures to be increasing with height in the lowest parts of the atmosphere rather than decreasing with height which is more typical during the day. This is because of radiational cooling which can only cool the air at the surface. [EP: Tonight's saving grace, keeping temperatures from dropping to freezing in the city, will be a wind of about 5 mph. A calm wind would mean even colder temps!]
NAM meteogram showing two nights of potential freezing in the Mid-South (This graphic is for the airport. Outlying areas will be cooler than shown above at night.)
Modeled temperatures and wind show favorable conditions for frost and freeze tonight and Friday night. The dotted green line represents the dew point which is the bound on how low the temperature can fall and also when condensation starts. Each night the temperature should reach this bound that will be near freezing. Winds are shown below that which are light throughout the period, even becoming calm during the overnight hours.
Weather Prediction Center's forecast of fronts and pressure systems for Friday morning
Light to calm winds are mostly due to the Mid-South being positioned under a high pressure. This is typical for a high pressure system because the pressure gradient is lowest near the center of the high.

As far as protecting vegetation, since sub-freezing temperatures are expected to be brief (a couple of hours or less), a hard freeze is not expected, and they will affect mostly rural areas, you can protect any vegetation that cannot be brought inside by putting a light cloth sheet over any plants. This acts to reduce radiational cooling, with the sheet playing the role of clouds in the atmosphere by keeping heat in. Just remember to remove this sheet in the early morning after temperatures begin to rise so that it can warm up quickly as well.

--William Churchill (Social Media Intern)

[Editor's comments are marked by EP and italicized.]

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