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Environmental Studies and Forestry
An air mass is a large volume of air with similar temperatures and humidity that converges in a large area, usually thousands of square kilometers. Air masses determine the weather in a given environment as well as the probability of formation of natural climatic disasters such as storms and cyclones in a given area. Air masses are classified depending on the source region, their characteristics, and area of the formation. Based on the above definition, there are six types of air masses: tropical continental, tropical maritime, polar continental, polar maritime, arctic maritime, and returning polar maritime.
Types of Air Masses and How They Interact to Form the Four Frontal Types (Part A)
Tropical continental air mass originates from the Sahara region in North Africa, which is a region of warm climate. It occurs during summer in the months of June, July, and August; however, it may occur at other times of the year. When there is a tropical air mass, temperatures rise to 30oC during the day and ranges between 15oC and 20oC during the night. Usually, its visibility is moderate since the air picks up pollutants as it passes over Europe and sand particles as it passes through the storms of Saharan dust. Occasionally, the dust is washed out in rain showers resulting in colored rain. As a result, cars are covered with a thin layer of dust after the rain.
Tropical maritime originates from the warm waters of the Atlantic Ocean and in the area between Bermuda and Azores. In this climate, the wind blows south-westerly across the British Isles. This air mass is warm and moist in lower areas. Although this air mass is unstable as it passes from its source region, it stabilizes as it passes through cooler waters where it becomes saturated with water. Consequently, it brings a drizzle as it reaches the British Isles and fog across hills and along the windward coasts. To the lee side of the mountain, the cloud breaks down to a fine sun during summer months. During winter, it raises the air temperatures to above the average degrees.
Polar continental originates from Russia and the snow fields of Eastern Europe. This air mass is considered to be a winter phenomenon since it becomes considerably warm during winter where it changes to a tropical continental air mass. Generally, this weather is normally dry and inherently cold as the air as moves from Europe to Britain. When the air mass reaches the South of Britain, it is characterized by severe frosts and clear skies. As it nears the coast of Britain, the air mass collects moisture that leads to showers of rain and in some cases snow. Moreover, it is associated with the low temperatures in Britain, where temperatures at times reach a low of -10oC at night. Sometimes, they remain below freezing point during the day.
Polar maritime originates from Greenland and Northern Canada and follows the north-westerly direction towards the British Isles. In its earlier stages, the air mass is cold and dry. However, as it moves over the waters of North Atlantic, which are relatively warm, its temperatures rise leading to instability in its occurrences. Generally, this air mass is characterized by frequent showers and rains. During winter, it is associated with hail and thunder across the northern and western isles of Britain. In summer, land temperatures rise higher than sea temperatures and heavy showers are experienced in Eastern England.
Arctic Maritime air mass is similar in many ways to the polar maritime air mass. However, it is cold and less moist since it passes over a shorter streak of the sea. This air mass rarely occurs in summer, and when it does, it leads to heavy rains and thunderstorms. The temperatures are considerably low during such times. During spring, the air mass is cold enough to cause hails in Scotland and the coasts exposed to the northerly winds. The Arctic Maritime air mass originates from the Arctic Ocean and the North Pole.
Finally, Returning Polar Maritime is similar to Polar Maritime only that it moves over a long streak of the sea taking southerly direction over the North Atlantic Ocean. Later, it moves north-easterly across the British Isles. As it moves southwards, the air becomes moist and unstable. However, as it advances north-eastwards, it becomes stable since the waters are cooler. Generally, weather across this air mass in Britain is dry.
When air masses meet, a sharp boundary is formed due to the intense differences in temperatures. This boundary is commonly referred to as a front. Four types of fronts are formed when air masses meet: stationary fronts, cold fronts, warm fronts, and occluded fronts. Stationary fronts occur when the two air masses do not move against each other at the point of the meeting. This situation happens when a warm air mass moves above a cold air mass so that there is no change. A cold front occurs when an advancing cold and dry mass of air displaces a warm and moist air mass, which is in most cases unstable. This movement leads to the formation of clouds and precipitation at the zone. After the meeting, there is usually heavy rains that end after a short while. Also, temperatures become very cold. A warm front occurs when a warm and moist air mass replaces a cold and dry air mass that is retreating from a region. This front is also characterized by precipitation that occurs in the form of rain or snow. In addition, the temperature rise is experienced in this kind of front. Finally, occluded fronts occur when a slow moving warm front is overtaken by a fast moving cold front. Occluded fronts are further classified into two: cold type occluded front and warm type occluded front. A cold type occluded front happens when the air ahead of the front is warmer than that which is behind the front. When the air ahead of the front is colder than that which is behind the front, a warm type occluded front appears. This type of front is associated with storms and heavy rains in places where it occurs.
Mid-latitude Cyclogenesis in the Context of Part A
Mid-latitude cyclones are large traveling atmospheric storms, 2000 kilometers in diameter. Normally, their centers are characterized by low atmospheric pressure. To elaborate, an average sea level cyclone has a pressure of 1013 millibars, but a typically intense mid-latitude cyclone has a surface pressure of 970 millibars, which is very low. Generally, these cyclones move from the west to east direction and last for about 3 to 10 days. Normally, mid-latitude cyclones are dominant in the earth’s mid-latitudes that form along the polar front. Mid-latitude cyclones form the interaction of cold polar and warm tropical air masses at the polar front. This kind of meeting results in the cyclonical uplifting of the warm air into the atmosphere vertically until where it meets the cooler air on the upper part of the atmosphere. The excess energy is transported from lower to higher latitudes. This cyclone moves a distance of 1200 kilometers in a day, eastwardly. Also, mid-latitude cyclones produce a wide variety of precipitation including rain, hail, snow, freezing rain, and pellets. The frozen types of precipitation occur during storms in the months of winter. Noteworthy, hail related to severe thunderstorms form in front or along the cold fronts during the months of winter and spring. The damaging effects of mid-latitude cyclones are less when compared to those caused by hurricanes.
Cyclogenesis, the process of forming a cyclone, begins by a weak disturbance along the stationary front where warm air from the south meets cold air from the Polar Regions. Warm air is uplifted into the upper atmosphere due to the collision, creating a spin that is cyclonic with a center that has low pressure. This center is associated with cold and warm fronts. In the middle stage, the intensity of the storms increase, and the pressure at the center continues to decrease. The last stage occurs when the warm front is overtaken by the cold front leading to the lifting of air on the warmer side into the atmosphere. The resulting boundary between cool and cold air masses is the occluded front. Days after occlusion, winds subsidize and the stationary from is re-established.
To sum up, the interactions of different air masses have both positive and negative effects on the climatic conditions of any region. Therefore, they are associated with different climatic conditions in different places and at different times of the year. In light of this, it is important for an individual to understand the theory behind them in order to predict the expected weather changes due to the effects of the interactions of air masses.