Polar depressions are spontaneous formations. It is not so simple to foresee and detect such natural systems using ordinary meteorological messages. Therefore, they pose a threat to mariners, air carriers and other vigorous human activities in the northern regions. How unpredictable and dangerous polar depression is, what this phenomenon is, let's look at it in stages.
Discovery history
Polar depression is a phenomenon that relates to a relatively small-scale weather system that is short-lived and characterized by low pressure. Formed over the oceans in both hemispheres on the side of the main polar front. In early studies, it was assumed that the main reason for its occurrence is thermal instability. But this statement turned out to be very far from reality. Later, the formation conditions were studied. This type of natural system was first discovered on meteorological images, which became available in the 60s of the last century.
At high latitudes, specialists were definitely a whole host of swirling clouds. They were traced over ice-free areas of the above-mentioned seas, over Labrador, as well as in the bays of Alaska. It is noted that the polar depression dissipates quite rapidly when entering land. The northern counterparts of the Antarctic cyclones are usually weaker, because along the entire continent they experience changes in different temperatures. Although sometimes even within the Southern Ocean one can observe the dynamism of this phenomenon.
Satellite images allow us to conclude that the polar depression is characterized by a variety of cloud forms, which can form in a spiral form from cloud bands enveloping the center, or take a comma image closer to the polar front. As a matter of fact, the degree of danger of a given weather phenomenon, its intensity and speed of propagation also depend on the structure.
Formation mechanism
When a wave begins to develop on the polar front, facilitating the penetration of the tropical stream into the air mass medium , a polar depression forms. Considering the system is moving eastward, a warm cyclone, whose air is trying to displace the cold, is different from the opposite, which follows it and rolls under the masses warmed by the sun. The result of such a movement of opposite elements is a decrease in pressure on the surface, the center of which is surrounded by isobars that are blown across by the wind.
As a result, air moves to the core of the depression upward and in a spiral overnight. As this process develops, the cold front approaches the warm one, which leads to the occluding phase. Despite the presence of air with a low temperature at the top and cyclonic movements indicated by isobars and wind direction, there is one frontal contrast on the surface in the form of a dividing line between the incoming flows located in the rear area of the depression. This leads to a transformation in the front. Depending on the nature of the processes that determine such a metamorphosis, occlusion is either cold or warm. The external manifestation of the cyclone on land depends on this.
Lifetime
The period of existence of this kind of weather system depends on how long the potential energy has to be transformed into kinetic. The polar depression is destroyed when the contrast of low and high pressure disappears between the air layers located in the neighborhood. Its rapid weakening occurs when displaced above the ice surface or when land approaches. Given the direct relationship with the rise of air and powerful winds, it can significantly affect the weather.
Weather effects
Since the air of warm fronts rises gradually, until it reaches stability, the formation of layered cloud forms. If cirrus clouds appear in the sky, then a warm front is nearby. As it approaches, the clouds become lower and more massive. Often layering portends a light rain over time turning into heavy rain. And by dinner, you can already expect a sunny sky in a cumulus.
The arrival of the cold front dramatically changes the weather. Cumulonimbus clouds similar to towers appear in the sky, bringing, as a rule, heavy rains and thunderstorms. Suddenly, the direction of the wind changes to north or northwest. The storm situation develops unexpectedly and in a short period.
What's the Difference?
What is the difference between the frontal depression of the Southern Hemisphere and the similar depression in the Northern Hemisphere? Almost nothing, although there is one important dividing line. In the first case, the wind on the warm front turns from north to northwest, and on the cold - from west to southwest, in the second - the movement occurs in the same way as the hands on the clock. But the peculiarity is that each polar depression is an individual phenomenon, that is, there is no idealized model that can describe it.
Forecasting Opportunity
It is possible to make a weather forecast for frontal depressions provided that a significant territory is covered by synoptic observations. For example, for the European part of the continent, the research area should expand to the west, including the adjacent areas of the Atlantic. After all, usually such natural systems have a speed of 1000 km per day. If observations are made in the upper atmosphere, this will greatly facilitate the work on the forecast in the sector where the cyclone is located.
It is quite common when frontal depressions join in large families, involving secondary formations in movement around the main stream. The most common are those that appear on the edge of cold air. Each subsequent representative of such a conditional family is located along a trajectory closer to the equator than its predecessor.