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 warm 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, cold fronts, occluded fronts, stationary fronts, dry lines and today we’ll focus on warm fronts.
On a weather map the warm front is a red line with small semi circles called pips attached and they point the direction that the warm front is moving, which is generally from south to north. In our part of the world, the northern hemisphere, the low pressures spin counterclockwise, so to their right the air comes from the south, and that’s the warm direction, and where you’ll find the warm front, to the right of the low.
An easy way to remember what a warm front looks like is a lesson we teach in Bill’s Weather 101s. When you walk out on a nice warm day it’s like Mother Nature giving you a nice big welcoming hug. Well when you see the warm front with the semi circles in front, they look like hugs. So the red warm front line has little hugs out in front.
Warm air is less dense than cold air, so it rides over the top of the retreating cold air mass. The warm air comes in aloft before it arrives at the surface in a process called overrunning. Where the warm and cold air come in contact, condensation can happen which creates clouds. When it occurs at the highest levels, it begins as cirrus clouds. As the slope works back toward the surface the clouds get lower and can be stratus or cumulus type depending the atmospheric energy available. Rain can also begin if the clouds are thick enough.
There are also times when energy and instability near the front are such that thunderstorms can occur. In particularly energetic cases when there is also jet streams and wind shear involved severe weather may occur near the warm front.