The temperate zone is located between the Arctic Circle and the tropics, therefore, both low latitude and high air masses influence its climatic conditions. In this article, we will characterize the types of air masses of the temperate zone.
Air masses and their types
The concept of air masses was developed in the first years after the First World War by Norwegian scientists. Currently, this phrase refers to huge volumes of air, which are characterized by some average values of temperature and humidity. This aerial object can have a length (width) of up to 1000 km and a height of several kilometers to 10 km. Inside this gigantic atmospheric formation, temperature and humidity can undergo changes, however, they are much smaller than the average values of these values in the neighboring air mass.
On our planet, the following types of masses are distinguished, depending on their temperature:
- arctic;
- polar;
- tropical
- equatorial.
It is obvious that the lower the air masses formed at lower latitudes, the higher its temperature will be.
In terms of humidity, two types of masses are distinguished:
- Continental (dry).
- Sea (wet).
Moderate air masses include all the types mentioned above, with the exception of the equatorial ones, which simply do not reach mid-latitudes.
Temperate limits
Before proceeding to consider the air masses of the temperate climatic zone, it is necessary to briefly describe the location of the latter.
According to the general classification, the temperate zone is located between the latitudes of the tropics (23 o ) and the polar circles (66 o ) in both hemispheres of the planet. This means that both low latitude (tropical) and high (arctic and polar) air masses influence its climate.
When considering the climate of temperate latitudes, they usually do this only for the Northern Hemisphere, since the waters of three oceans are located in most of the southern temperate latitudes.
Areas of formation of air masses that affect the climate of mid-latitudes
Air has a fairly low thermal conductivity. This means that for the formation of a homogeneous (homogeneous) air mass, which would acquire the characteristics of a surface (land or water), it needs to be relatively quiet for a while above it. Such conditions can be created if the following requirements are met:
- The surface does not have strong elevations (sea, deserts, plains).
- The air in this region has a fairly high pressure, otherwise it will be replaced by incoming masses from neighboring territories. Anticyclones correspond to this condition.
When considering the Northern Hemisphere, the following zones are distinguished that form air masses:
- North Pole;
- northern part of Eurasia (territory of Russia);
- North Canada
- Northern zones of the Atlantic and Pacific oceans.
Further in the article, we will characterize the moderate air masses listed in this list.
Arctic masses
They are formed in close proximity to the North Pole. Since temperatures here are below zero all year round and the water surface is covered with glaciers, such conditions lead to the formation of cold and dry air volumes. Its temperature ranges from -55 o C to -35 o C, and the amount of water vapor in it does not exceed 0.02 g per 1 kg of air.
These air masses are extremely stable. They never lead to the formation of precipitation, even if for this the conditions for the formation of ascending convection flows are created. On maps, arctic masses are usually denoted by Latin letters cA, where the letter c indicates their continental character (lack of humidity).
Polar masses
These are perhaps the most important air masses of the temperate zone, since their formation regions are entirely within its limits. Despite their name, the latitudes where they form and dominate are located between the 50th and 70th parallels. There are two main types of these masses, the characteristics of which are given below:
- Continental. The region of formation of moderate air masses of this type lies on the continents (the north of Eurasia and the northern part of North America). Their typical winter temperatures lie in the range of 20-35 o C below zero. These are quite dry volumes of air, since their moisture level does not exceed 0.6 g / kg. In summer, their air warms up to 5-15 o C, and humidity increases 10 times and reaches values of 6 g / kg. As these figures show, the polar masses remain cool and dry even in the summer. They are usually denoted by the symbol cP.
- Marine. They form in the northern Atlantic and Pacific oceans. The temperature of moderate air masses of this type fluctuates significantly less during the year than the similar value for cP. So, in winter its values are 0-10 o C, and in summer 2-14 o C above zero. These masses are relatively unstable and moist (10 g / kg). They are designated by the letters mP (m - marine).
The issue of stability of air masses
The classification and characteristics of moderate air masses considered are ideal or model. In fact, these atmospheric formations are in constant motion. Passing over a territory whose properties differ from the regions of mass formation, the latter change. To understand in which direction these changes will go, it is necessary to answer 2 questions:
- Does the surface over which the mass moves have a higher or lower temperature than it?
- Is this surface wetter or not?
When the mass has a lower temperature than the surface, its lower layers are heated, which creates favorable conditions for mixing the air in it using convection. That is, it is destabilizing. This process leads to precipitation if the air humidity is sufficient. To describe the described effect, use the Latin letter k.
The opposite situation: the mass is warmer than the surface. In this case, its lower layers are cooled, which creates an inversion of temperature at its base, this prevents the formation of convection flows. In this case, they say that the mass is stabilizing. This process is indicated on cards by the letter w. The effect of mass stabilization is the occurrence of fog and reduced visibility.
Temperate air masses in January and July
The easiest way to consider this issue is using a map. It is shown in the figure below.
The upper image corresponds to the zones of formation and movement of air masses in January. It can be seen here that stable cP dominates in the north of Eurasia and North America. The western part of Europe is influenced by the Arctic (see front AF) and marine cold polar masses (mPk), which, passing over the Atlantic Ocean, are saturated with water vapor. The latter, condensing, lead to the formation of heavy rains, snowfalls and hail. In southern Europe, in the Mediterranean, a warm tropical marine front (mTw) dominates.
The bottom figure shows the distribution of air masses in July. It shows that the air masses of the temperate zone in northern Eurasia in the middle of summer are characterized by a front of arctic (AF) and marine cold polar air (mPk), leading to the formation of rains. Continental polar air (cPk) descends to lower latitudes, while tropical continental dry mass (cT) completely dominates in southern Europe. In turn, most of the North American continent (with the exception of the north) is influenced by unstable tropical wet masses (mT).