Cool weather phenomena

This winter has brought some serious challenges for the deep south.

 

Impassable roads, disrupted air travel, power outages, and even snow!

 

Most folks seem familiar with the causes and the sometimes-serious effects of snow, sleet, and freezing rain. 

 

But cold air creates some more subtle phenomena, too. They are fascinating, and in some cases, absolutely beautiful. 

 

Have you ever wondered why frost covers your car’s windshield, forms on airplane wings – requiring that dreaded de-icing – or coats trees and grass with beautiful icy fuzz, sometimes even with air temperatures above freezing? 

 

Perhaps you have pondered the super-dense fog that sometimes forms over our waterways. 

 

And what makes those crazy jet tracks, or contrails,  and why do they hang up there in the air? 

 

A friend’s young daughter once pointed up at a contrail and said, “Daddy, an airplane scratched the sky.” 

 

How completely logical! I always smile thinking about that description, but these environmental occurrences are functions of the air’s humidity and changes in its temperature.

 

Let’s start with scraping frost from your car’s windshield after a night that never went below freezing. Where does that ice come from? 

 

A few things need to happen. First, the humidity of the air (how much moisture it contains) needs to be adequate. Then the temperature of the air near the ground needs to cool towards the dew point. The dew point is the temperature at which the air can hold no more water. 

 

At the same time, if the air temperature is not below freezing, clear skies can allow objects like your car’s windshield to radiate heat into space and drop below the temperature of the surrounding air or even below freezing. The moisture condenses out of the air, and it may freeze, depending on how cold the glass is.

 

Advection fog involves clouds or fog formed when moist air passes over a cool surface like winter waters or cold ground. The air close to the cold surface cools below the dew point, and fog results. This can be a very dense, but very shallow fog layer. 

 

Lastly, just how do airplanes “scratch the sky?” Contrails, short for condensation trails, are formed primarily by jet engine exhaust, and to a lesser extent, by low pressure areas created by the wings and engine. Somewhere around 35,000 feet is an ever-moving altitude band where the temperature is ideal for contrail formation. 

 

On some days, when both the temperature and the humidity are right, the water vapor in the exhaust stream cools, is joined by some of the existing moisture in the air and forms a trail of floating ice crystals. On rare days the trails seem to hang there forever, and one can see dozens of contrails marking up the sky from horizon to horizon.

 

Frost, fog and contrails are a few of the lesser-known products humidity and cold temperatures can produce.