Weather fronts are responsible for the majority of our clouds and precipitation. However, the characteristics of this weather depend on the type of front, either cold or warm. By understanding the nature of each type of frontal system, you can more accurately predict the type of weather you will encounter. In the case of a warm front, you should definitely break out your umbrella and rain gear.
Warm Front Basics
Fronts represent the boundaries between colliding air masses. Warm fronts represent the transition zone where warm air is replacing colder air. These fronts typically form east of a center of low pressure, where the southerly winds of the low’s counter-clockwise rotation push the warmer air northward. Warm fronts generally move from the southwest to the northeast.
Structural and Behavioral Characteristics
As the warm air advances, the cold air acts as a gently sloping ramp. This ramp gently uplifts large areas of the warmer, less dense air. The slope of a typical warm front is 1:200, compared with the much steeper 1:100 slope of a cold front. This is why warm fronts are characterized by a much larger area of cloud cover and precipitation. Because the advancing warm air is less dense, it has a difficult time pushing the heavier cold air back. This is why warm fronts move more slowly than cold fronts, and contribute to the longer periods of clouds and precipitation.
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Cloud and Precipitation Characteristics
Clouds and precipitation can extend for hundreds of miles both in advance and behind the warm front. As a warm front approaches, you will typically first observe high, wispy cirrus clouds. As the front nears, the cloud layer will thicken and the base will drop. This sequence will typically consist of cirrostratus clouds, followed by altostratus, stratus and nimbostratus clouds. Following the passage of the warm front, stratocumulus clouds can form, eventually followed by clearing. Precipitation associated with a warm front is typically steady and light to moderate in intensity. Due to the slow speed of these fronts, the rain can last several hours or even several days. Warm fronts can occasionally produce thunderstorms with more intense precipitation.
Warm fronts are typically characterized by a transition from southeasterly to southwesterly winds. Unlike cold fronts, winds along the front itself are generally light and variable. Warm fronts, as their name implies, are also characterized by a rise in temperature, but also humidity. Because warm fronts generally involve low pressure centers, barometric pressure will fall as a warm front approaches. Behind a warm front, pressures will typically stabilize before eventually beginning to slowly rise again. Warm fronts are generally characterized by poor visibility due to low layers of overcast and steady precipitation.