A warm front marks a weather boundary where a mass of warm, humid air advances and gradually rises over a retreating mass of colder, denser air. The warm air, being less dense, gently glides upward over the colder air, leading to a characteristic sequence of weather events.
Weather Before the Warm Front
As a warm front approaches, the first indications often appear high in the atmosphere, sometimes hundreds of miles ahead of the front itself. High-level clouds like wispy cirrus and then more widespread, sheet-like cirrostratus begin to form, gradually thickening and lowering. These clouds, which can create a halo effect around the sun or moon, signal the initial ascent of the warm, moist air over the colder air mass.
Following the high clouds, mid-level altostratus clouds typically appear, making the sky appear grayish and obscuring the sun or moon. As the warm air continues to override the cold air, these clouds deepen into thick, gray nimbostratus clouds, often bringing the onset of precipitation. This initial precipitation is usually light and widespread, often in the form of drizzle, light rain, or snow, beginning hours before the main frontal passage.
During this pre-frontal period, a gradual but steady decrease in atmospheric pressure is common, indicating the approaching low-pressure system associated with the front. Temperatures remain cool or cold in the area, although there might be a slight rise as the leading edge of the warm air mass begins to influence the upper atmosphere. The air can also become more humid.
Weather During Warm Front Passage
As the warm front itself moves through a location, noticeable changes occur. The characteristic precipitation intensifies into a steady, widespread, and often prolonged light to moderate rain, snow, or mixed precipitation, falling from the extensive nimbostratus cloud deck. This precipitation is continuous rather than showery, resulting from the consistent lifting of warm, moist air over the cold air wedge.
The most significant change during frontal passage is a distinct and often rapid increase in temperature as the warm air mass completely replaces the colder air at the surface. This temperature rise brings warmth and increased humidity. Visibility frequently decreases significantly due to the ongoing precipitation and potential fog.
Wind direction typically shifts as the front passes, often from an easterly or southeasterly direction to a more southerly or southwesterly flow in the Northern Hemisphere. Atmospheric pressure, which had been falling, usually levels off or may begin to rise slightly as the center of the low-pressure system moves away.
Weather After the Warm Front
Once the warm front has passed, the weather transitions into conditions characteristic of the warm air mass. Temperatures remain warmer, and humidity levels typically increase.
The steady precipitation associated with the front generally ends, though isolated showers or drizzle might persist for a period. The extensive cloud cover often begins to break up, leading to partial clearing or scattered, lower-level clouds. Winds continue to blow from a warmer direction, typically from the south or southwest.
Atmospheric pressure, having leveled off during the frontal passage, usually begins to rise slowly in the hours following the front’s departure. This rise in pressure indicates a more stable atmospheric condition.