A glance at a weather map reveals streaks of colors and letters dotted all across the landscape. The letters- usually accounting for high and low pressure systems- were examined in detail previously. The streaks, however, reveal a scattering of weather fronts, each of which has different weather and climate impacts. 

 

 

Perhaps the most common and recognizable of fronts are warm and cold. Adorned by different colors and symbols, these two visible yet invisible lines have something in common. They are both boundaries between air masses, which can delineate differences in weather variables like temperature, wind, and dew point, to name a few. A front will be stronger if the two air masses it separates are very different.

 

 

Fronts will always come off low-pressure systems. The red, semicircle-adorned warm front is typically the “leader”, being the first sign that an area is under the influence of a low. In the Northern Hemisphere, warm fronts usually travel poleward. Cold fronts, while second in line, tend to travel faster than warm fronts due to the way they can lift air quickly, which we’ll get to in a moment. Colored blue with triangles pointing where the air is moving, these fronts travel from West to East in our neck of the woods.

 

 

Let’s figure out why fronts act the way they do. The best way to visualize it is to create a cross-section, as illustrated in the graphic above. Warm air behind the front slowly overtakes cold air ahead of the front, which is moving more slowly in the same direction. The warmer air, because it is less dense, rises over the colder air as it moves. 

 

 

As a result of its increased altitude, the air there cools off and its moisture condenses, forming clouds and possibly precipitation. The more air that is lifted, the more moisture is present, which is why a greater amount of rain falls as one gets closer to the boundary. This is why clouds gradually increase as a warm front nears, and validates the weather folklore that high, wispy clouds lead to rain in a day’s time.

The “in-between” area of the two fronts is called the warm sector. This area is known for being warmer and more humid than the surroundings, and generally pleasant conditions.

The leading boundary for cold air is known as a cold front. Dry, cool air forms a steeply sloping boundary under the warmer, moister air at the surface and lifts that up. In cases with a strong amount of lift, thunderstorms can develop. Cold fronts are the origin for a wide variety of severe storms, as they provide ample lift for their formation. In most cases, though, as the cold front departs, conditions immediately improve, with a notable decline in temperature and dew point.

 

 

As a low-pressure system matures, cold fronts will eventually catch up to a warm front. On the ground, the “warm sector” completely disappears, as the “nose” of warm air is now restricted to being above the ground. In this case, an occluded front forms, which is the boundary between two different masses of cold air. 

 

 

Because low-pressure systems rely on warm air to operate, this stage of a low-pressure cyclone’s development is typically the last. Looking at a map from above, occluded fronts are colored purple and feature a combination of semicircles and triangles, meant to be a “fusion” of both fronts’ look.

 

 

Apart from these three fronts, other ones with different characteristics can exist.

 

 

Stationary fronts are common in the tropics and can even be found in our area during the summer months. As the name suggests, these boundaries contain separate air masses on each side, but provide no movement as no side is strong enough to displace the other. As a result, each side of a stationary front is typically subject to prolonged unsettled weather, though not very strong.

Dry lines, common in the Southern Plains, separate two different air masses whose main difference is their moisture content. They are colored either yellow or orange, with semicircles facing the moist side of the front. In the Plains, this can separate stagnant continental air masses from moist ones coming from the Gulf. This can be another way that thunderstorms can develop in this area.