Which Way to the Front?

"If you don't like the weather, just wait five minutes and it will change." We've all heard this quotation about fast-changing weather conditions in the Midwest. In truth, all areas of the country have this weather phenomenon in common. When flying, keeping up with changing weather conditions is paramount to safety. Let's examine the characteristics of various fronts that change our weather patterns daily.

Air masses adopt the characteristics of areas they overlay. One can imagine that an air mass over Canada in winter would be cold and dry. Similarly, those over the Caribbean Ocean would tend to be warm and moist. If air masses were always stationary, the art of weather forecasting would be simple. Each day's forecast would be, "Tomorrow's weather will be just like today's."

Air masses are put into motion, however, because of global pressure systems. Fronts, areas between two air masses, are named for the type of air mass causing the displacement. When relatively cooler air replaces warmer air, a cold front exists. "Relatively" is the keyword -- even if 90F air replaces 100F air, a cold front would exist. If warmer air overtakes colder air, this is a warm front. A stationary front is one that is not moving, while an occluded front occurs when two fronts overrun each other.

Each frontal passage has its own distinct characteristics, but all can be identified by two noticeable occurrences. Because of the dissimilar pressure pattern behind the front, the barometric pressure decreases until the front passes and then begins to increase. The most easily recognized indication of the passage is the shift in wind direction. Oddly enough, the change in temperature one would expect is not usually noticed for hours after the front has passed.

The most violent weather is associated with cold fronts. These fronts are usually fast moving and have a steep pressure gradient face. To illustrate, imagine a snow plow moving quickly along the highway. As the plow moves, the snow is forced abruptly upward. In this same way, a cold front uplifts the warmer air in its path. As this warm air is forced upward it cools rapidly, forming clouds and rain. If the air is particularly moist and unstable, rain clouds can form thunderstorms. These frontal thunderstorms can exist hundreds of miles ahead of a fast-moving cold front and can include hail, tornadoes, and turbulence. After the passage of a cold front, the weather is characteristically good for flying. The front acts as a windshield wiper and leaves behind clear cool skies with good visibility.

Warm fronts are usually more subdued. The gentler sloping face creates overcast skies and milder continuous precipitation. If a cold or warm front loses the propulsion created by the pressure gradient, it becomes stationary. In a stationary front the characteristic precipitation slowly dissipates. In an occluded front, the generally faster-moving cold front pushes into a warm front, resulting in overcast skies with imbedded rain showers or thunderstorms.

Because pilots are continuously moving from one location to another, they must constantly keep up to date on changing weather patterns. Flight crews are advised of current weather and expected changes through comprehensive weather briefings before all flights and are kept up to date with new forecasts and reports after departure. Numerous devices on board the aircraft monitor the changing weather. One of these, weather radar, allows the crew to see where the rain showers are and to plan the flight path accordingly. The objective of all of these trained professionals is to make sure the flight arrives on time, safely, and as smoothly as possible.

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