Fronts on leading edge of air masses

In last week’s column, I decided to cover some of the basics of meteorology – what you could call Meteorology 101. The previous column covered the meaning and significance of the H’s and L’s often seen on a weather map (high and low pressure).

Just as common, are symbols meteorologists refer to as cold fronts, warm fronts and stationary fronts. Fronts merely mark the leading edge of differing air masses. If colder air is advancing, we call it a cold front; and when warmer air is advancing, we call it a warm front. I use the terms colder and warmer rather than cold and warm because air behind the cold front it is not necessarily cold, but it is merely colder relative to the air it is advancing upon. You can have cold fronts move through in the summertime, and you might see 80-degree air replacing 90-degree air. No one would call that air cold.

Sometimes, there are differing air masses, but neither is able to advance. In that case we mark the dividing line with what’s called a stationary front.

Why is the air moving in the first place? It is because nature likes to be in balance. On a global scale, areas near the equator receive much more direct sunlight than at the poles. That is why the farther south you travel (in the Northern Hemisphere) the warmer it gets. To equalize this imbalance in heat energy across the globe, warm air is constantly moving northward and cold air is constantly moving southward. The general circulation of the air from north to south is broken up into many smaller circulations.

The one we are most generally concerned with is the movement of air around an area of low pressure. In last week’s column I pointed out that air likes to move from higher to lower pressure. It does not make a direct beeline, however, to the center of the low. Other forces such as friction, the rotation of the earth and the centrifugal force cause the air to not only move in toward the low, but in a counterclockwise circulation around it. Ahead of the low, the wind usually has a southward component to it such as south or southeast. This would help move warmer air northward, and as such, a warm front is often attached somewhere on the east side of a low.

Behind the low, the wind usually has a northward component such as north or northwest. Likewise, a cold front usually trails behind a moving low pressure area. Having said all that, if you ever check your local winds after a cold front has passed through, they will almost always be southwest. How can that make sense? It turns out that local terrain has a huge influence on surface winds, and we can attribute the wind direction to the lay of the land. Higher up in the atmosphere, high enough where mountains don’t interfere, the winds eventually back to a westerly or northwesterly direction to usher in cooler air.

The reason we care about the location of fronts, is that they are often the focal point for precipitation. East of the Rockies, fronts of one type or another are often the focal point for severe thunderstorms. Stationary fronts are notorious for bringing on flooding conditions. Because the stationary front moves very little or not at all, rainfall can be focused over the same area for many days in a row. During the dry Inland Northwest summer, cold frontal passage is often bad news for fire fighters. Just ahead of the front, there may be dry lightening storms, while gusty southwest winds usually follow after the front passes through.