What is a weather front?
Lets start with the definition. The weather front is the boundary surface between two masses of air of different temperature and density, inclined under a small angle (0.5 to 3 °) to the Earth’s surface. Due to differences density, both air masses can not be mixed up immediately. Instead the warmer, lighter mass of air begins to rise above cool and denser air. This leads to the creation of a front atmospheric, which is the transition zone between them.
Atmospheric fronts usually move from west to east, because in medium latitudes, where the fronts they arise, Western winds prevail. However, especially in the lower layers of the atmosphere, the fronts can be modified by various elements of the geographical environment, such as mountains or large reservoirs Water.
What are four kinds of weather front?
The fronts are always accompanied by cloudiness and very often also rainfall. However, when the front leaves an area, they also occur changes in wind speed and direction, atmospheric pressure and air humidity. There are four types of fronts: cool front, front warm, occluded front and stationary front. The type of front depends on both from the physical characteristics of the air masses and the direction in which these masses are moving.
Cold front characteristics
The cool front is the boundary between the cold air that advances and the warm mass giving way. Mostly it is stormy, and the main type of clouds occurring within this front is cumulonimbus.
The cold air, as thicker, slides into the thinner warm air. Due to the fact that the cold air does not have to waste energy on climbing the mass of warm air, almost all its energy is used to move the front horizontally, therefore the cool front is a fast front, always faster than the warm front. The cool front occupies a belt with a width of only 50-75 km, and hot air pushed up at high speed must form a whirling cloud as a result.
Before the arrival of the cold front, we are in a warm mass of air. The first sign of approaching the cold front may be the occurrence of clouds altocumulus lenticularis (altocumulus in the shape of a lens). Just as the hand moved in the water causes the water to waver in front of him, the fast-moving cool front also causes air to ripple, and the clouds are the symptom of it.
At the head of the cold front we have a huge storm cloud cumulonimbus, which sucks the warm air from the front of the front, cleaning them on the occasion of all types of suspensions (characteristic for air in a warm mass). Therefore, during the passage of the cold front, visibility improves significantly. Cumulonimbus itself can not be noticed by poor visibility inside the warm air mass. The wind, on the other hand, intensifies and changes its direction to the opposite. It’s getting darker, the wind is gusty. The first lightnings may appear. The front cumulonimbus cloud is much more powerful and its lifespan is much longer than that which arises inside a homogenous mass of air due to local overheating of the ground. A strong storm precipitates and a thunderstorm begins, which passes relatively quickly. The end of the storm means that the front line has already passed.
After passing the front, the wind changes its direction by more than 50 degrees in relation to the direction in the warm air mass. Visibility improves considerably, it becomes even „crystal”.
The observer is at this moment in a cool mass of air with unstable equilibrium. Usually, after passing the front, on the horizon one can see the next shaft of storm clouds, sometimes mistakenly called the secondary front. It is not a front, but the effect of wavering in a cool mass of air. It is the smaller the farther the front is behind us, but the first storms after passing the front may be no less strong than the front storms.
Warm front characteristics
The warm front arises when the warmer air comes to cool air. Warm air „slipping” over the cold, ascending, it cools down. The condensation of water vapor occurs and the formation of a vast cloud system at the interface of the air masses. In the front part, at the most, Cirrus clouds appear, followed by clouds of the middle and low floors, respectively: Altostratus and Nimbostratus. The passage of the warm front is accompanied by relatively long-lasting continuous rainfalls, often covering an area from several dozen to several hundred kilometers in front of the front. Precipitation while passing the warm front is not as intense as in the case of a cold front, but they have a larger range (300-400 km).
The air behind the warm front is warmer and more humid than the air in front of it. In front of the warm front there are rain or snow, lasting longer than before the cold front (from several hours to several days), but less intense. After passing the warm front, the air becomes visibly warmer and more humid.
On synoptic maps, the warm front is marked with a red line with semicircles, indicating the direction of warm air movement.
Occluded front characteristics
The occluded front arises from the combination of a cold front and a warm front, and three different air masses meet in it: cold, warm and cool. There are two types of occlusion: occlusion with the character of a cold front and occlusion with the character of a warm front. In both cases, the cold front, moving faster, overtakes the warm front.
In the case of a cold occluded front, a cold front that carries very cold air overtakes the warm front. The very cold air is forced both into the warm air and into the cooler air between the warm front and the ground. The warm front becomes the upper warm front. The weather is initially the same as during the passage of the warm front, but later the weather changes and is characteristic of the passage of the cold front, with strong precipitation. Cloudy is mixed and is not characteristic of either the warm front or the cold front.
Occupational warming occurs when moderate cool air overtakes the warm front. In this case, advancing the cold air is still warmer than it is cold the air lying between the warm front and the surface of the earth.
The incoming cold air slides into the air very much cold, pushing hot air upwards. Cool front it becomes the upper front. The weather is similar to occurring during the passage of the warm front.
Warm occlusion arises when the air behind the cold front is warmer than the air in front of the warm front.
Cold occlusion is created when the air behind the cold front is cooler than the air in front of the warm front.
Stationary front characteristics
Stationary front is a transition zone between hot and cold air mass, not moving or moving at a speed of less than 2 m / s.
On synoptic maps, a stationary front is marked with a line with red semi-circles directed towards the mass of cold air and blue triangles towards the warm air (they mean what character the front will have when it moves to the indicated side).