How weather fronts work

A weather front is a place of collision of air masses, a boundary that is constantly in motion. In this new lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn more about what weather front is and how it works.

Let’s recall that the Earth’s atmosphere consists of air masses — air volumes varying in temperature and humidity, which constantly compete for the territory. If cold air moves into warm air territory, it is a cold front; if warm air moves into cold air territory, it is a warm front.

In forecasts, warm fronts are marked with red lines, cold fronts — with blue lines, with triangles and other figures indicating the direction and type of the front.

Over time, the front can slow down and stop as the mutual movement of air mass slows down, or there are mountains or sea on the way. The slowing down front, in which the air movement has practically stopped, is called stationary. If a cold front catches up with a warm front and closes in on it, it is an occluded front.

Types of weather fronts. Valerya Milovanova / Windy.app

What do fronts bring with them

Any front brings weather changes, including those of the strength and direction of the wind.

A cold front is more dangerous, as it often brings squally winds, thunderstorms and rains. As a rule, such bad weather comes suddenly, but it does not last long: a cold front moves rather quickly — up to 80 kilometers per hour. In the Northern Hemisphere, such fronts usually move from northwest to southeast, in the Southern Hemisphere - from southwest to northeast.

A warm front can also bring bad weather, but the rains can last the whole day or more, because its precipitation zone is much wider, and also it moves much slower than the cold one — 20-30 kilometers per hour or less. That’s because it’s harder for less dense warm air to squeeze out denser cold air. Wind increase in a warm front is not as strong as in a cold front. In the Northern Hemisphere, warm fronts usually move from southwest to northeast, in the Southern Hemisphere — from northwest to southeast.

The surface of the front is always sloping against the ground. In the case of a cold front, cold air jumps under warm air, forcing it to rise up. 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. Warm fronts often bring cloud-covered skies, high humidity, haze and fog, heat and rain for days, which is not good for pilots.

The calmest are stationary fronts: there are usually rather weak winds. The most dangerous are occluded fronts, as various dynamic processes can lead to the formation of a new cyclone.

What are the dangers and how can you protect yourself

If a warm front with poor visibility is unpleasant for planes, a cold front, or rather its squalls and thunderstorms, are more dangerous for unprepared people on the streets and yachtsmen. But if on the street it is always possible to take shelter from heavy rains and strong winds in solid buildings, at sea it is important to know in advance whether you will face bad weather, and it is better to avoid it.

Even before going to sea, you should check the weather forecast both on the route and in its vicinity. If you have internet access you should do it twice a day, you can also check the CAPE (Convective Available Potential Energy) index. If it is above 500, you should prepare for possible bad weather conditions.

If there is no internet, you can see a cold front coming from afar — it often looks like a dark wall of bad weather. A sudden pressure drop can also be a precursor.

Text: Windy.app team

Illustration: Valerya Milovanova, an illustrator with a degree from the British Higher School of Art an Design (BHSAD) of Universal University

Cover photo: Unsplash