WeatherBill's Weather 101


Bill's Weather 101 at Home Edition: Cold Fronts

Posted at 6:25 PM, Apr 09, 2020
and last updated 2020-04-09 19:19:02-04

When you look at a weather map on TV or online there are certain items that are standard and stand out. They’ve been known about as long as the science of meteorology has been studied. We’re talking about the highs, lows and fronts. Today we’ll talk about cold fronts.
First of all, a front is simply the boundary between two different air masses. We’ll cover air masses another time. These boundaries come in different types, warm fronts, occluded fronts, stationary fronts, dry lines and today we’ll focus on cold fronts.

On a weather map the cold front is a blue line with small semi triangles called barbs attached and they point the direction that the cold front is moving, which is generally from north to south or west to east. In our part of the world, the northern hemisphere, the low pressures spin counterclockwise, so to their left the air comes from the north, and that’s the colder direction, and where you’ll find the cold front, to the left of the low.
An easy way to remember what a cold front looks like is a lesson we teach in Bill’s Weather 101s. When you walk out on a cold day, sometimes it feels like nature is just smacking you. It can be a sharp not always pleasant feeling. So think of the blue line with the sharp pointy triangles and you have the cold front.

Cold fronts tend to be very active places because that’s where the air masses essentially battle it out. The end result for us is usually rain and storminess.
Cold air is heavier and denser than warm air, so it’s able to push it out of the way. If you think of it in 3 dimensions, the cold front is essentially a plow. It moves in and plows into the warm air. Since the warm is lighter and less dense the plow of the cold front sends the warm air up where the water vapor in it can condense and form clouds. With strong and steep cold fronts, that plowing and lifting can be violent and that’s when we talk about thunderstorms that can be strong or severe. Other times the lifting is more gentle and we can just end up with rain or nothing much at all.

The best way to tell when a front has gone by your place is a shift in the wind. If you have a home barometer, you’ll also notice that it will start rising once the cold front has passed. That’s actually one of my preferred techniques to figure out the exact frontal position.

There are different types of cold fronts and there can be multiple types attached to the same area of low pressure. Most of the time we have polar fronts that come through. The air is moderately cold or cooler behind the front, and it’s drier. Occasionally in winter we’ll have Arctic fronts that follow. This is the very cold air and it’s also very dry. Sometimes we’ll have Pacific fronts come by. These airmasses come off the Pacific, dry out and moderate in temperature after they cross the Rockies. It’s when those ‘cold’ fronts go by we can actually end up being warmer because the air its replacing…and remember all the front is is a boundary between air masses…was polar in origin (came from Canada) and was initially colder. Structurally it’s a cold front, but in reality we warm up.