When watching the weather report, you often hear of warm and cold fronts, but what exactly are those? In a general sense, a cold front is the point at which colder air moves into an area that had been previously warmer.
A hot, humid summer day may become much more agreeable as a cold front moves in, often bringing stormy weather with it. Winds behind a cold front, usually from the west or northwest, propel colder air forward, which pushes warm air up and creates cumulus clouds.
A warm front is the point at which warmer air moves into a colder area. The warm air rides up over colder air, often ascending 3,000 feet about 100 miles ahead of where it first began rising. Lower, thicker clouds exist at this point, while high, thin clouds can be a few hundred miles ahead of the front.
During winter, warm fronts sound like good news, but they can bring forth hazardous weather. A “wintry mix” of snow, sleet, and/or freezing rain requires relatively warm air during cold months. High above the earth, air temperatures are literally freezing—that is to say, below 32 degrees Fahrenheit.
An approaching winter warm front can result in above-freezing air being pushed up above freezing air, creating an inversion that results in above-freezing air sandwiched between the colder temps above and below it.
Meteorologists refer to this as an “elevated warm layer” or a “melting layer.” Snow beginning high above melts into rain as it enters this warmer air pocket, only to refreeze into tiny ice pellets, or sleet, as it plunges down into the thicker layer of cold air nearer to the ground.
Between December 15 and 16, 2005, such a warm front resulted in an ice storm in the southeastern US. A weather balloon launched near Roanoke, Virginia, on the morning of the 15th indicated a cold-warm-cold air sandwich, as well as sufficient water vapor for significant precipitation, foretelling imminent trouble.
By noon, weather reports ranged from 68 degrees and no precipitation over Panama City, Florida, to 27 degrees and sleet over Roanoke. Athens, Georgia, reported a temp of 34 degrees and rain, while Hickory, North Carolina, was at 32 degrees under freezing rain.
In Hickory, snow high above melted as it entered the warm pocket, then became super-cooled to below 32 degrees, while remaining liquid as it entered the lower cold air pocket. This instantly became ice as it landed. Over in Roanoke, the warm layer was thinner and the lower cold layer was thicker, which resulted in snow only partially melting before refreezing into sleet.
The National Weather Service declares an ice storm when freezing rain persists for several hours and results in a buildup of ice. When such an ice storm is forecast, folks are strongly advised to stay off the roads.
A far more common type of winter precipitation is snow. While capable of making roads hazardous, snow is usually much easier to deal with than ice. Clouds form where rising air causes moisture to ascend and prevents it from falling back down. When the drops grow too heavy for the air to hold them up, they fall to earth as snow.
Snow forms in many different ways, but it is not simply frozen rain. First, slowly rising air holds up the water drops and ice crystals. Next, water vapor condenses around a nucleus, remaining a super-cooled liquid, though it is below freezing.
Freezing rain occurs when super-cooled drops freeze as they enter below-freezing air. If a tiny particle is the right shape, it will turn to ice. When there is sufficient moisture in the air high above, temperatures between three and ten degrees above zero can cause individual crystals to grow into a six-armed crystal, or a dendrite.
As falling snow collides with super-cooled water, they stick together and freeze, known as riming, which transforms ice crystals into what’s called graupel, or soft hail. But if unrimed crystals fall into warmer air, they melt, and the water holds them together in large flakes.
Identical snowflakes do exist, but they’re very rare. The air temperature determines the shape of the crystals. Temperatures of 25 to 32 make for thin hexagonal plates. Temps of 21 to 25 result in long, thin needle shapes. Temps of 14 to 21 degrees make for hollow columns that are hexagonal cylinder shapes. Finally, temps of 10 to 14 degrees, make for sector plates, which are the classic six-armed snowflake shapes.
Snow forms all year-round, but most of the time we can’t see it. Rain or snow that evaporates into dry air before reaching the ground is called virga. You can often spot virga when you see precipitation extending from a distant cloud without making it all the way to the ground.
El Niño has resulted in above average temps this winter and it’s expected to last into spring, with our dry weather making snow harder to come by. That will change in February, as average precipitation is spread out across the entire month, resulting in small amounts of snow nearly every day. Very cold temperatures are expected early on, warming up towards Valentine’s Day.