The general circulation of atmospheric air is disrupted by variable winds resulting from regional and local variations in air temperature and pressure. An important factor modifying the overall distribution of air temperature and also atmospheric pressure on the globe is the distribution of land and water surfaces and therefore oceans, seas, and lakes. Over some areas of the Earth there is a marked seasonal change in the prevailing wind directions to opposite or near opposite directions. In intertropical latitudes, the reason for this change is related to the altitude of the sun’s peak. In extra-tropical latitudes, periodic changes in wind direction are caused by differences in the heating of land and water surfaces.
These periodic winds that change their direction at the end of summer and the beginning of winter are called monsoons. (Arabic: mau-sim-season, season). In connection with these changes, the monsoon circulation is distinguished, which is part of the global atmospheric circulation on the globe, or rather disrupts it. In the summer, a very deep baric low is formed over the strongly heated Asian land, and a baric high is formed over the oceanic water masses besieging this continent, which are much cooler. This disturbs the general direction of atmospheric circulation in these areas and produces local seasonal winds called monsoons. Monsoons change their direction to the opposite depending on the season.
Two types of monsoons can be distinguished:
- Summer (marine) monsoon with rainy weather associated with low pressure over land and high pressure over the sea
- Winter (land) monsoon with dry weather caused by high pressure over land and low pressure over the sea.
During the summer monsoon, the wind blows from the sea toward the land; in the winter monsoon, the opposite is true. In summer, the land heats up faster than the water, so the air pressure over it decreases. Violent winds blowing from the sea inland appear. In winter, lows form over warmer waters, which contributes to monsoons blowing from land seaward (winds blow from high to low). The wind that blows from the ocean to land during the warm season is the summer monsoon. During the summer half of the year, monsoons blow from sea to land, in winter from land to sea. They are formed as a result of large differences in temperature and therefore pressure between land and sea. In summer, the land heats up more than the sea. Above the land, an area of reduced pressure is created as a result, causing air to flow downward from the ocean to the land.
Overhead, on the other hand, air moves in the opposite direction at this time – from the land toward the ocean. During the cold season, the ocean is warmer than the land, causing the wind to blow in the opposite direction – from land to ocean. This is the winter monsoon. This wind is usually weaker than the summer monsoon. Monsoons develop most strongly in the zone of the southern coasts of Asia. They also make their presence felt in the Indian Peninsula, southeast, Asia, and a small part of Australia. This is due to the extremely large temperature differences created by the huge continent of Asia being strongly heated in summer and cooled during winter. The time of occurrence of the monsoon is very important for farmers. It is most important for them that the summer monsoon, bringing rains from over the ocean, arrives at the expected time. Long and heavy downpours begin in late May and early June. The desiccated land that has received water is prepared for crops. Heavy and violent rainfall irrigates the fields for five months. A dry period follows in late October, when the winter monsoon brings dry, moisture-deprived air from deep within the continent.
Breeze
Similar disturbances that the monsoons cause on an annual basis, on a daily basis, are caused by winds arising in areas of contact between land and sea and are known as breezes. A breeze is a wind blowing on the sea coast. Changes in wind direction are caused by differences in the heating of the land and sea. During the day, the land heats up faster than the water, so the warmer air over the land rises and is replaced by cooler and wetter air from the sea. So the daytime breeze (sea breeze) blows from the sea toward the land. At night, on the other hand, the water gives off heat faster than the land. The phenomenon repeats, but the wind blows in the opposite direction. The daytime breeze covers a layer of air 1 km thick. If the air temperature changes decisively, a sea breeze front can often be felt. The air then rises and clouds of clouds are formed. The arrival of a sea breeze can also contribute to the development of clouds that give precipitation. The night breeze (land breeze) brings dry air from over land onto the water. Its range is about 20-30 km from the shoreline toward the water. The land breeze usually begins around midnight and lasts until dawn. Due to smaller temperature differences between land and sea at night, the range of the night breeze is smaller than that of the day breeze. Thus, towards land it is significantly smaller and depends on the nature of the surface. The breeze circulation develops particularly intensively on the coasts in subtropical latitudes. In temperate latitudes, pronounced breezes are marked in summer in sunny and quiet weather. The breeze phenomenon has long been known and exploited by fishermen, who go out to fish at night with a land breeze and return with a sea (daytime) breeze. Breeze-like processes can also arise over large lakes. The depth of the body of water then plays a big role. Shallow lakes heat up faster reducing the temperature difference between land and water. Canada’s Great Lakes produce the same breeze phenomenon as the seas and oceans. Shallow lakes on flat terrain produce a weak breeze, but if they are surrounded by hills or mountains that heat up strongly during the day, then lake breezes tend to be more intense.
Valley and mountain winds
Mountainous regions also experience local winds that change direction throughout the day. Valley and mountain winds are winds caused by differences in the heating and cooling of valley bottoms and mountain slopes. The cause of valley and mountain winds is the difference between the temperature of the air adjacent to the slope and the temperature of the air above the valley floor at the same height. During the day, slopes especially those with a southern exposure heat up more strongly. The air also heats up, expands and rises up the slopes. A warm valley wind blows then. In place of the uplifted air come new portions of air from the lower parts of the slopes and from the valley floor. Valley winds are sometimes the cause of the formation of massive clouds of clouds over mountain ridges during the day. Valley winds usually appear just after sunrise and disappear completely at sunset. They can reach speeds of up to 20 km/h.
At night, however, there is a flow of heavier cooler air down the slopes toward the valley floor under the influence of gravity. A cool nighttime mountain wind blows downward. At the ground surface, the mountain wind is weaker than the valley wind and its speed can be up to 12 km/h. During sunny days there are distinct valley winds. On cloudless nights, however, there are mountain winds, which give a runoff of cooled air from the mountain side, where the wind has caused excess pressure, toward the valley with relatively lower pressure.
Fen
Fen is a strong, gusty, warm, dry wind blowing from the mountains. It has long been known in the Alps, from the German fohn. It occurs in the Carpathian Mountains, the Sudeten Mountains, the western Caucasus, the mountains of Central Asia, the Altai Mountains, the eastern slopes of the Rocky Mountains, as well as Greenland and Iceland. In different countries it bears local names, e.g. halny in the Tatra Mountains, polak on the Czech side of the Sudetes, or chinook in the Rocky Mountains. The condition for the formation of this wind is the difference in atmospheric pressure on both sides of the mountain barrier. This change forces the air to move. As the air meets the mountains it rises, cooling by about 0.6° per 100 meters of altitude. As the air rises, water vapor condenses – clouds and rains form. The already dry air crosses the barrier of the mountain peaks and descends on the other side towards the valleys.
As it falls, it warms up, by about 1° per 100 meters. Hence the foehn winds are dry and much warmer than the air at the same altitude on the opposite side of the mountains. The higher the mountain barrier, the greater this difference can be. The wind of the foehn type on the side of the lowland system is warm, dry, sometimes very gusty and strong, and the increase in temperature and decrease in humidity, generally referred to as the foehn effect, will still be felt at considerable distances from the mountains, when the foehn is blowing. This type of wind is recorded quite often, about a few dozen times a year. In winter, it causes rapid melting and even direct evaporation of snow. In summer, the phen effect sometimes causes rapid drying and leaf fall. Fen sometimes brings warming of 10-20°C per day. In Poland, this type of wind is found in the Carpathians and Sudetes. In the Sudetes, due to lower relative altitudes, the strength of the wind and the phen effect are weaker than, for example, in the Tatras. Regional names for phen winds include chinook in the Rocky Mountains, halny in the Tatras, fohn in the Alps, or zonda in the Andes.
Bora
Bora is a cool, dry and gusty wind blowing mainly on the Dalmatian coast. It arises most often in winter, when a high-pressure center forms over the land and low pressure prevails over the sea. The cold air accumulates behind the Dinaric Mountains, then crosses the mountain barrier and descends towards the coast. This wind as it passes over the sea becomes saturated with moisture. Hence the name, or more precisely bora scura (dark bora), is used on the Italian coast of the southern Adriatic Sea to describe the cool and moist wind blowing from the sea. In Novorossiysk on the Black Sea, winds of this type reach speeds of more than 100 km/h and are accompanied by air temperature drops in the mountains as low as -35°C. Very strong winds of the bora type also blow from the interior of Antarctica. Bora is also felt on Spitsbergen and New Earth. Mistral is the local name for the bora wind that is felt down the Rhone valley in Provence, as a cold wind falling from the Massif Central, or the foothills of the Alps towards the Mediterranean. Other names for this type of wind include Koshava in the Carpathians, or Santa Ana in California.
Tropical cyclones
The formation of tropical cyclones is associated with a low-pressure system in which there are no atmospheric fronts. A necessary condition for the formation of cyclones is a water temperature above 26°C. Cyclones are formed only in latitudes of 5-25° in both hemispheres. The distance from the equator should be at least 500 km. The variation of wind speed with height throughout the troposphere should be small. This allows convective clouds to “organize” into a cyclonic system. A tropical cyclone is called a hurricane (in the Atlantic and eastern Pacific), typhoon (in the Pacific Northwest), or tropical cyclone (in the southwestern Indian Ocean). Tropical cyclones can cause catastrophic damage. In the strongest hurricanes, record wind speeds, estimated by pressure at the center, exceed 85 m/s (305 km/h). The strongest winds tend to blow in the halves of cyclones farther from the equator, because there the speed of movement of the system itself is added to the speed associated with cyclonic circulation. The center, that is, in the so-called eye of the cyclone with a diameter of up to several tens of kilometers, the sky is cloudless, and there is little wind. Strong ascending currents and the lowest pressure are noted there. Very strong winds swirl around the eye of the cyclone, accompanied by powerful clouds that bring torrential rains.
Glacial winds
These are cold gusty winds that always fall from the cold surfaces of glaciers to the warmer areas below. A necessary condition for the occurrence of such air movements is air compression caused by the settling of cold air from higher layers above the glacier, or ice sheet. Also important is the strength of the baric gradient, and the force of gravity. Glacial winds are found in mountains with large glacial fields, as well as in Greenland and Antarctica.
The water vapor content of the atmosphere is constantly changing. It is estimated that up to a height of 1.5 km there is about 50% of the total water vapor content. The troposphere contains 99% of the water vapor of the entire atmosphere.
The degree of cloud cover is assessed without specialized instruments, so “by eye” on a scale of 0 to 10 where 0 means no cloud cover at all, and 10 means full cloud cover. The types, kinds, and species of clouds are also determined at weather stations. The average annual cloud cover of the entire globe using this scale is estimated at 5.4 degrees. Over land it is 4.9 over the seas, while it is 5.8. In Poland, the average annual cloudiness is 6.4. A clear day is considered to be one whose cloudiness is below 2, the average cloudiness of a cloudy day is from 2 to 8, while a cloudy day is one in which the average cloudiness oceans above 8 degrees.
Types of precipitation
Huge clusters of tiny water droplets or ice crystals, which form clouds so long they merge together as long as they can’t stay in the air. Precipitation is then formed. Thus, precipitation is the product of condensation of water vapor falling on the Earth’s surface. The instrument used to measure precipitation is a rain gauge, while a pluvograph is used to record the level of precipitation on a permanent basis. Based on the genesis, we distinguish three types of precipitation:
- Orographic precipitation – precipitation associated with the vertical movement of air forced by the flow over mountains. Incoming air masses rise, cool and lose some water vapor through condensation.
- Convective precipitation – atmospheric precipitation caused by the rise of moist air heated from the ground and the formation of a low pressure center near the Earth’s surface.
- Frontal precipitation – atmospheric precipitation formed in the zone of the atmospheric front, where air masses of different temperatures meet. Depending on the type of front (warm, cool, occluded), the precipitation that occurs varies in character.
The most common precipitation in Poland is rain. It is a precipitation of water droplets of various sizes. The maximum size of raindrops is 7-8 mm. Larger drops break under the influence of air resistance. Continuous and long-lasting precipitation is the result of cloud development associated with the passage of atmospheric fronts. This type of precipitation is formed from Nimbostratus and Altostratus clouds. Very intense but short-lived precipitation is given by clouds of vertical structure, namely Cumulonimbus.
When there is a homogeneous cloud, whose parts, however, are of different sizes then there is precipitation of droplets of very small sizes called drizzle. Drizzle is most often formed from Stratus and Stratocumulus clouds. Precipitation of ice crystals rolled together is snow. When the air temperature is close to 0°C, then the ice crystals soften and easily fuse together to give large patches of snow. When the air temperature is lower then the ice crystals fuse together to form very fine six-pointed stars. The precipitation of white opaque lumps a few millimeters in size is called snow croup. It forms when there are a large number of cooled droplets in a cloud.
The croup comes from Nimbostratus, Cumulonimbus, and Stratocumulus clouds. Hail is precipitation in the form of flattened balls, or irregular lumps of ice. It is formed during summer from Cumulonimbus storm clouds. This type of precipitation falls with heavy rain. It forms when water droplets that, along with the ascending current, enter the upper strongly cooled layers of the cloud and freeze there. As it sinks downward due to its weight it enters the higher temperature zone. Droplets at this altitude, after coming into contact with the snow crystal, freeze and overgrow it, forming an ice ball. Sometimes the hail, thus overgrown, is moved several times into the upper layer of the cloud, which causes further overgrowth. Hail is most common in the temperate zone, less often in the tropics. The phenomenon does not occur at all in the circumpolar zone.
In addition to precipitation, condensation products can be found in the atmosphere, floating in the air and settling on objects. Sedimentary precipitation called atmospheric deposits. Among them can be distinguished dew, which appears when the Earth’s surface due to nighttime radiation cools down reaching the dew point temperature. The layer of air adjacent to objects near the Earth’s surface also cools, as a result of which the water vapor there is condensed. It settles in the form of tiny water droplets on the surface of soil and rocks, on grass, leaves.
Dew is formed during bright windless summer evenings. In areas of the intertropical zone, dew deposits are so abundant that they run off trees and rooftops further irrigating the soil. Another sediment whose mechanism of formation is similar, but which forms at sub-zero temperatures, is frost. This is a collection of ice crystals that often take the form of needles. It settles on grass, ground, and various surfaces.
Rime, on the other hand, is a silvery-white, crystalline deposit in the form of threads found on tree branches, telecommunications wires, nets, etc. It appears at various times of the day and night during cold weather, when an influx of warmer air will cause fog to form. Fog droplets, after coming into contact with strongly cooled objects, freeze to form ice crystals. They develop most strongly on the side of objects from which the wind blows. Rime is particularly abundant in mountainous areas. This deposit usually forms a layer of several centimeters, however, sometimes its thickness exceeds 1 meter. That’s when tree branches break under the weight of the rime, and high voltage wires are snapped.
When raindrops, or drizzle, fall on the Earth’s surface cooled below 0°C they freeze. Glaze is then formed. It is a smooth deposit of a dull color, or transparent. Glaze occurs on the surface of the ground, telecommunication wires, tree branches, or on airplanes causing icing on their wings, radar equipment. The formation of glaze can also paralyze road transportation.
Factors causing variation in precipitation on Earth
A very important influence on the variation of precipitation is the general atmospheric circulation on Earth. Steady winds carry moist air inland and dry air toward the ocean. In the equatorial low-pressure zone, due to ascending currents, the air cools, reaching the dew point temperature. The water vapor contained in this air condenses. Clump clouds are then formed, bringing heavy precipitation. The slight cloudiness in the tropical zone is accompanied by very strong solar radiation, while in the temperate latitudes there are lows that move according to the direction of the westerly winds.
In the polar zone, there are canopies of high pressure, which greatly limit the development of precipitation. The primary source of moisture on Earth is water bodies. As a result, drier air is observed inland than near the seas and oceans. As we move away from bodies of water, the amount of precipitation decreases. Over the oceans and coasts, ocean currents are an important factor causing the variation in precipitation. Warm currents warm the air, causing it to rise, creating low-level systems and precipitation. Cold currents, on the other hand, cause the air temperature to drop, increasing atmospheric pressure. Under such conditions, rain clouds do not develop. On the other hand, due to the cooling of the air, fog can form, as is the case in the fog deserts: Namib and Atacama.
Distribution of precipitation on the globe
The annual course of precipitation varies markedly with latitude. In the circumpolar belt, two periods with abundant precipitation (zenithal rains) are distinguished due to the change in the Sun’s place of elevation in the annual course. The wet periods are separated by relatively dry periods, when the Sun tops one of the tropics. In the tropical latitudes, precipitation is observed to occur here only during the Sun’s mountaintop for one period of the year. The subtropical zone is eminently dry.
Average annual precipitation often does not exceed 100 mm. The main reasons for this are descending currents, Low temperatures, and low-lying systems. In areas of intertropical latitudes, especially on the eastern coasts of the continents, there is an increased intensity of precipitation in the summer season associated with the occurrence of monsoons. In temperate latitudes, precipitation associated with baric lows dominates. In the vicinity of water bodies, precipitation is distributed almost evenly throughout the year giving a maximum at the turn of autumn and winter and a minimum in spring.
Inland, the highest precipitation totals are recorded in summer, as both low-level precipitation and convective precipitation caused by ascending currents over a strongly heated surface occur at these latitudes. In winter, on the other hand, seasonal high systems form in these areas, accompanied by dry and cold weather. In higher latitudes near land, the highest precipitation falls in summer, as the air then has a higher water vapor content. In polar latitudes near bodies of water, the highest precipitation occurs in winter due to the stronger activity of low-pressure systems. Areas that significantly increase annual precipitation values are mountainous areas.
The highest annual average rainfall of more than 11,000 mm has been recorded in Cherrapunji, India. In Poland, average annual precipitation totals are 600 mm. The lowest are recorded in the Polish Lowlands (about 500 mm), while the highest are in the Tatra Mountains (up to 1,700 mm).
