My father used to work as a TV anchor at a small network in Vermont right after college. I am sure he was good at reporting on news and local events, but one day the weathercaster called in sick and he was forced to do the stations weather segment. Without thinking too far in advance, he got up and was standing in front of a green screen pointing at various atmospheric features of which he had no knowledge about. Pointing out a specific item, he very solemnly told everyone in Burlington, VT to watch out for this approaching blue “spiky” line from the west. And moved on. Presumably Vermont survived the great attack of the spiky blue line, but what was he actually talking about that day?
Bane of Vermont, circa 1983.
On weather charts, blue lines with protruding triangles are denoting something called a cold front. Cold fronts are a very familiar atmospheric phenomenon and can sometimes result in extremely dramatic looking skies. Put simply, a cold front is simply a large mass of relatively colder air invading space occupied by relatively warmer air. Generally, they act like a snow plow to the air already in place by pushing it all upwards higher into the sky. It is this lifting action that creates the clouds and rain that weathercasters forecast for a frontal passage event.
Fronts in your mirror may be colder than they appear.
So what exactly happens when a cold front passes? Well, the obvious answer is that the temperature drops, since by its name, coldness is implied. Some other, more subtle things do occur though. Due to the genesis of a cold front, it sits in an area of low pressure. So as a front approaches, the atmospheric pressure around one will fall. This is because a cold front actually cannot move on its own. Rather, it must be attached to a low pressure system (those giant, red Ls that one might find on a weather chart) which in the northern hemisphere rotates all the air counter clockwise. This rotation of the air (wind) moves the cold air. As all the warm air gets lifted above (this is called Isentropic Lift, remember that; it might be a trivia question at your local bar next week) it cools rapidly which produces all of the moisture such as clouds and rain which can help make a cold front visible to the naked eye. Another telltale sign that a cold front has passed is the change in wind direction. As everything flows counter clockwise around a low pressure system, the cold front must too. The direction of movement will lend a change in wind direction to wind coming from the west-northwest. After the front has passed, certain things start to undo themselves. Mainly, the pressure will begin to rise as the low pressure system skirts away. Markedly colder temperatures will now inhabit the area and everyone might be slightly damper.
Cold air meet warm air. Warm air, prepare to get high(er).
There is a ton of science going on here that certain people have devoted their lives to studying that simply can’t be fully discussed in a mere blog post, but at least now anyone who read this is slightly better equipped to discuss that feature that my family name nearly incited as a horseman of the apocalypse.