Weather College Essay Example

aloft is towards the poles.

A low-pressure area of calm, light variable winds near the equator is known as the doldrums, equatorial trough, intertropical front, or the Intertropical Convergence Zone. When located within a monsoon region, this zone of low pressure and wind convergence is also known as the monsoon trough. Around 30° in both hemispheres air begins to descend toward the surface in subtropical high-pressure belts known as subtropical ridges. The sinking air is relatively dry because as it descends, the temperature increases but the absolute humidity remains constant, which lowers the relative humidity of the air mass.

This air mass is dry and subsident, or sinking through the troposphere, and sometimes reaches the ground. When this warm, dry air reaches the surface it is known as a superior air mass. The superior air normally resides over the top of maritime tropical air masses over oceans, forming a warmer and drier layer over the more moderate maritime tropical air mass below. When the temperature increases with height, it is known as a temperature inversion. When it occurs within a trade wind regime, it is known as a trade wind inversion.

The surface air that flows from these subtropical high-pressure belts toward the Equator is deflected toward the west in both hemispheres by the Coriolis Effect. These winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. Because winds are named for the direction from which the wind is blowing, these winds are called the northeast trade winds in the Northern Hemisphere and the southeast trade winds in the Southern Hemisphere. The Trade Winds

meet at the doldrums. As they blow across tropical regions, air masses heat up over lower latitudes due to more direct sunlight.

Those that develop over land (continental) are drier and hotter than those that develop over oceans (maritime), and travel northward on the western periphery of the subtropical ridge. Maritime tropical air masses are sometimes referred to as trade air masses. The one region of the Earth which has an absence of trade winds is the north Indian Ocean. Clouds which form above regions within trade wind regimes are typically composed of cumulus which extend no more than 4 kilometres (13,000 ft) in height, and are capped from being taller by the trade wind inversion.

Trade winds originate more from the direction of the poles (northeast in the Northern Hemisphere, southeast in the Southern Hemisphere) during the cold season, and are stronger in the winter than the summer. As an example, the windy season in the Guianas, which lie at low latitudes in South America, occurs between January and April. When the phase of the Arctic oscillation is warm, trade winds are stronger within the tropics. The cold phase of the AO leads to weaker trade winds. When the trade winds are weaker, more extensive areas of rain fall upon landmasses within the tropics, such as Central America.

During mid-summer in the Northern Hemisphere (July), the westward-moving trade winds south of the northward-moving subtropical ridge expand northwestward from the Caribbean sea into southeastern North America. When dust from the Sahara moving around the southern periphery of the ridge travels over land, rainfall is suppressed and the sky changes from a blue

to a white appearance which leads to an increase in red sunsets. Its presence negatively impacts air quality by adding to the count of airborne particulates.

Over 50% of the African dust that reaches the United States ffects Florida. Since 1970, dust outbreaks have worsened due to periods of drought in Africa. There is a large variability in the dust transport to the Caribbean and Florida from year to year. Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s. The Westerlies, anti-trades, or Prevailing Westerlies, are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing from the high pressure area in the horse latitudes towards the poles.

These prevailing winds blow from the west to the east, and steer extratropical cyclones in this general manner. Tropical cyclones which cross thesubtropical ridge axis into the Westerlies recurve due to the increased westerly flow. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. The Westerlies are strongest in the winter hemisphere and times when the pressure is lower over the poles, while they are weakest in the summer hemisphere and when pressures are higher over the poles.

The Westerlies are particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, or become more north-south oriented, which slows the Westerlies down. The strongest westerly winds in the middle latitudes can come in the Roaring Forties, between 40 and 50 degrees latitude. The Westerlies

play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse.

If the Earth were a non-rotating planet, solar heating would cause winds across the mid-latitudes to blow in a poleward direction, away from the subtropical ridge. However, the Coriolis effect caused by the rotation of Earth causes winds to steer to the right of what would otherwise be expected across the Northern Hemisphere, and left of what would be expected in the Southern Hemisphere. This is why winds across the Northern Hemisphere tend to blow from the southwest, but they tend to be from the northwest in the Southern Hemisphere.

When pressures are lower over the poles, the strength of the Westerlies increases, which has the effect of warming the mid-latitudes. This occurs when the Arctic oscillation is positive, and during winter low pressure near the poles is stronger than it would be during the summer. When it is negative and pressures are higher over the poles, the flow is more meridional, blowing from the direction of the pole towards the equator, which brings cold air into the mid-latitudes. Throughout the year, the Westerlies vary in strength with the polar cyclone.

As the cyclone reaches its maximum intensity in winter, the Westerlies increase in strength. As the cyclone reaches its weakest intensity in summer, the Westerlies weaken. An example of the impact of the Westerlies is when dust plumes, originating in the Gobi desert combine with pollutants and spread large distances downwind, or eastward, into North America. The Westerlies can be particularly strong,

especially in the Southern Hemisphere, where there is less land in the middle latitudes to cause the progression of west to east winds to slow down.

In the Southern hemisphere, because of the stormy and cloudy conditions, it is usual to refer to the Westerlies as the Roaring Forties, Furious Fifties and Shrieking Sixties according to the varying degrees of latitude Due to persistent winds from west to east on the poleward sides of the subtropical ridges located in the Atlantic and Pacific oceans, ocean currents are driven in a similar manner in both hemispheres. The currents in the Northern Hemisphere are weaker than those in the Southern Hemisphere due to the differences in strength between the Westerlies of each hemisphere.

The process of western intensification causes currents on the western boundary of an ocean basin to be stronger than those on the eastern boundary of an ocean. These western ocean currents transport warm, tropical water polewards toward the polar regions. Ships crossing both oceans have taken advantage of the ocean currents for centuries. The Polar Easterlies (also Polar Hadley cells) are the dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the north and south poles towards low-pressure areas within the Westerlies at high latitudes.

Cold air subsides at the pole creating the high pressure, forcing a southerly (northward in the southern hemisphere) outflow of air towards the equator; that outflow is then deflected westward by the Coriolis effect. Unlike the westerlies in the middle latitudes, the polar easterlies are often weak and irregular. These prevailing winds blow from the east to the west. Effect

on the Climate: Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation.

In mountainous parts of the world subjected to consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending and generally warming, leeward side where a rain shadow is observed. In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desert like climate just downwind across western Argentina.

The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts A wind in low-latitude climates that seasonally changes direction between winter and summer is called a monsoon, and is a typical example of Seasonal Winds. Monsoons usually blow from the land in winter (called the dry phase, because it carries cool, dry air), and to the land in summer (called the wet phase, because it carries warm, moist air), causing a drastic change in the precipitation and temperature patterns of the area.

The word "Monsoon" originates from the Arabic mauzim, meaning season. It was first used to depict the winds in the Arabian Sea, but later it was extended for seasonally changing wind systems all over the world. The main reason for monsoons is the difference in the heating of land and water surfaces, which results in land-ocean pressure differences.

On a small scale, heat is transferred by land-sea breezes, to maintain the energy balance between land and water.

On a larger scale, in winter when the air over the continents is colder than over the oceans, a large, high-pressure area builds up over Siberia, resulting in air motion over the Indian Ocean and South China, causing dry, clear skies for East and South Asia. This is the winter monsoon. The opposite of this happens in summer; the air over the continents is much warmer than over the ocean, leading to moisture-carrying wind moving from the ocean towards the continent.

When the humid air unites with relatively drier west airflow and crosses over mountains, it rises, reaches its saturation point, and thunderstorms and heavy showers develop. This is the summer monsoon in Southwest Asia—wind blowing from the ocean to the continent with wet, rainy weather patterns. Although the most pronounced monsoon system is in eastern and southern Asia, monsoons can also be observed in West Africa, Australia, or the Pacific Ocean. Even in the southwestern United States, a smaller scale monsoonal circulation system exists (called North American Monsoon, Mexican monsoon, or Arizona monsoon).

The North American Monsoon is a regional-scale circulation over southwest North America between July and September, bringing dramatic increases in rainfall in a normally arid region of Arizona, New Mexico, and northwestern Mexico. It is a monsoonal circulation because of its similarities to the original Southwest Asian monsoon—the west or northwest winds turn more south or southeast, bringing moisture from the Pacific Ocean, Gulf of California and Gulf of Mexico. As the moist air moves in, it is lifted

up due to the mountains, which, combined with daytime heating from the Sun, causes thunderstorms.

The monsoon is an important feature of atmospheric circulation, because large areas in the tropics and subtropics are under the influence of monsoons, bringing humid air from over the oceans to produce rain over the land. In highly populated areas (e. g. , Asia or India), this precipitation is essential for agriculture and food crop production. Sometimes a strong monsoon circulation can also bring flooding. Or, if the monsoon is late in a certain year, it can cause droughts. The most well known Monsoon’s occur in Asia, the Southwest Monsoon and the Northeast Monsoon. The Southwestern Summer Monsoons occur from June through September.

The Thar Desert and adjoining areas of the northern and central Indian subcontinent heats up considerably during the hot summers, which causes a low pressure area over the northern and central Indian subcontinent. To fill this void, the moisture-laden winds from the Indian Ocean rush in to the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas, creating winds blowing storm clouds towards the subcontinent. The Himalayas act like a high wall, blocking the winds from passing into Central Asia, thus forcing them to rise. With the gain in altitude of the clouds, the temperature drops and precipitation occurs.

Some areas of the subcontinent receive up to 10,000 mm (390 in) of rain. The southwest monsoon is generally expected to begin around the start of June and fade down by the end of September. The moisture-laden winds on reaching the southernmost point of the Indian Peninsula, due to its topography, become divided

into two parts: the Arabian Sea Branch and the Bay of Bengal Branch. The Arabian Sea Branch of the Southwest Monsoon first hits the Western Ghats of the coastal state of Kerala, India, thus making the area the first state in India to receive rain from the Southwest Monsoon.

This branch of the monsoon moves northwards along the Western Ghats with precipitation on coastal areas, west of the Western Ghats. The eastern areas of the Western Ghats do not receive much rain from this monsoon as the wind does not cross the Western Ghats. The Bay of Bengal Branch of Southwest Monsoon flows over the Bay of Bengal heading towards North-East India and Bengal, picking up more moisture from the Bay of Bengal. The winds arrive at the Eastern Himalayas with large amounts of rain. Mawsynram, situated on the southern slopes of the Eastern Himalayas in Shillong, India, is one of the wettest places on Earth.

After the arrival at the Eastern Himalayas, the winds turns towards the west, travelling over the Indo-Gangetic Plain at a rate of roughly 1–2 weeks per state pouring rain all along its way. June 1 is regarded as the date of onset of the monsoon in India, as indicated by the arrival of the monsoon in the southernmost state of Kerala. The monsoon accounts for 80% of the rainfall in India. Indian agriculture (which accounts for 25% of the GDP and employs 70% of the population) is heavily dependent on the rains, for growing crops especially like cotton, rice, oilseeds and coarse grains.

A delay of a few days in the arrival of the monsoon can

badly affect the economy, as evidenced in the numerous droughts in India in the 1990s. Effect: Bangladesh and certain regions of India like Assam and West Bengal, also frequently experience heavy floods during this season. And in the recent past, areas in India that used to receive scanty rainfall throughout the year, like the Thar Desert, have surprisingly ended up receiving floods due to the prolonged monsoon season. The influence of the Southwest Monsoon is felt as far north as in China's Xinjiang.

It is estimated that about 70% of all precipitation in the central part of the Tian Shan Mountains falls during the three summer months, when the region is under the monsoon influence; about 70% of that is directly of "cyclonic" (i. e. , monsoon-driven) origin (as opposed to "local convection"). Around September, with the sun fast retreating south, the northern land mass of the Indian subcontinent begins to cool off rapidly. With this air pressure begins to build over northern India, the Indian Ocean and its surrounding atmosphere still holds its heat.

This causes the cold wind to sweep down from the Himalayas and Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsula. This is known as the Northeast Monsoon or Retreating Monsoon. While travelling towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka. Cities like Madras, which get less rain from the Southwest Monsoon, receives rain from this Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu is

from the Northeast Monsoon.

In Southern Asia, the northeastern monsoons take place from December to early March when the surface high-pressure system is strongest. The jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow across southern Asia, creating dry air streams which produce clear skies over India. Meanwhile, a low pressure system develops over South-East Asia and Australasia and winds are directed toward Australia known as a monsoon trough.