Weatherwatch: how frontal systems work (2024)

FAQs

How do weather fronts work? ›

A weather front is a transition zone between two different air masses at the Earth's surface. Each air mass has unique temperature and humidity characteristics. Often there is turbulence at a front, which is the borderline where two different air masses come together. The turbulence can cause clouds and storms.

A front is a weather system that is the boundary separating two different types of air. One type of air is usually denser than the other, with different temperatures and different levels of humidity. This clashing of air types causes weather: rain, snow, cold days, hot days, and windy days.

When you see a cold front on a weather map, it means a colder air mass is trying to replace warmer air. The front marks the leading edge of the cold air. The blue triangles always point in the direction that the front (and the cold air) is going. A red line with half-circles on one side signifies a warm front.

A warm front is when the surface boundary between a warm air mass and a cold air mass it is overtaking. A cold front separates a cold, dry air mass from a warm air mass. The cold air mass pushes under the warm air mass because of its higher density, forcing this warm air to rise. Thunder storms and rain can occur.

The frontal zone represents the leading edge of a wedge of cold/cool air. If the wedge is moving into an area of warmer air, the front is called a cold front. If the wedge is retreating and warmer air is moving into an area previously occupied by cool air, the front is termed a warm front.

As a cold front approaches, the barometric pressure usually falls. In the wake of a cold front, the barometric pressure usually rises. In extreme cases, temperatures can plunge dozens of degrees Fahrenheit in a matter of minutes and 30 degrees or more in a matter of hours.

The effects from a cold front can last from hours to days. The air behind the front is cooler than the air it is replacing and the warm air is forced to rise, so it cools.

Occluded Front

Occluded fronts are usually a sign that the parent weather system has reached its mature stage and is decaying (decreasing in intensity). They are indicated by a purple line with alternating triangles and half-moons on the side of its motion. The cold air mass is moving faster than the cool air mass.

In a warm occlusion, the cool air mass overtaking the warm front is warmer than the cold air ahead of the warm front, and rides over the colder air mass while lifting the warm air.

A wide variety of weather can be found along an occluded front, with heavy thunderstorms and tornadoes possible, but usually, their passage is instead associated with a drying of the air mass. Additionally, cold core funnel clouds are possible if the wind shear is significant along the cold front.

Is 1000 hPa high or low pressure? ›

On a weather map, a cold front is usually drawn using a solid blue line with triangles pointing in the direction of the warm air that will be replaced.

After a cold front passes, the cold air mass behind it brings cooler temperatures. The air is likely to be less humid as well.

Cold fronts move faster than warm fronts and can produce sharper changes in weather. Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary.

In the spring, these cold fronts can be very strong, and can bring strong winds when the pressure gradient is higher than normal. Cold fronts sometimes come through an area with little or no precipitation.

In a warm front, warm air still rises up, but gradually squeezes out the cold air underneath. A cold front brings bad weather first, but then — a clear sky full of cumulus clouds, good visibility as well as low humidity. It is also accompanied by updrafts which will help if you are going to float.

More commonly, the meeting of a cold front and a warm front will produce rainfall, because the contact of cold and warm will cause water vapor within either air mass to condense, forming water droplets (if the air mass is sufficiently cold enough, this precipitation will be snow or even hail instead of rain).