MONSOON

• Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. Usually, the term monsoon is used to refer to the rainy phase of a seasonally-changing pattern, although technically there is also a dry phase.

•  'Monsoons' is derived from the Arabic word "mausim" which means season and it is a weather phenomenon which is common to Asia. Monsoons are defined as winds that change their direction with the change of seasons. Monsoons are seasonal reversals of wind direction that affect the Indian sub-continent and are caused by the following factors:

1. Extreme heating and cooling of large land masses in relation to the surrounding sea, which impacts on pressure and winds.

2. The Himalayas that is high enough to interfere with general atmospheric circulation.

3. Movement northwards of the ITCZ in summer in the northern hemisphere.

• The major monsoon systems of the world consist of the West African and Asia-Australian monsoons. The inclusion of the North and South American monsoons with incomplete wind reversal has been debated
• The term was first used in English in British India (now India, Bangladesh and Pakistan) and neighboring countries to refer to the big seasonal winds blowing from the Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall to the area. The south-west monsoon winds are called 'Nairutya Maarut' in India.

• Also defined as a  wind blowing part of the year from one direction, alternating with a wind from the opposite direction; -- a term applied particularly to periodical winds of the Indian Ocean, which blow from the southwest from the latter part of May to the middle of September, and from the northeast from about the middle of October to the middle of December.

• A monsoon is a seasonal prevailing wind which lasts for several months. monsoonal rainfall is considered to be that which occurs in any region that receives the majority of its rain during a particular season, and so monsoons are referred to in relation to other regions such as in North America, Sub-Saharan Africa, Brazil and East Asia. Monsoons are caused by the larger amplitude of the seasonal cycle of land temperature compared to that of nearby oceans. This differential warming happens because heat in the ocean is mixed vertically through a "mixed layer" that may be fifty metres deep, through the action of wind and buoyancy-generated turbulence, whereas the land surface conducts heat slowly, with the seasonal signal penetrating perhaps a metre or so. As monsoons have become better understood, the term monsoon has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical land regions of the earth.

• The Asian monsoons may be classified into a few sub-systems, such as the South Asian Monsoon which affects the Indian subcontinent and surrounding regions, and the East Asian Monsoon which affects southern China, Korea and parts of Japan.

Causes of Monsoon

Monsoons are an annually recurring weather phenomenon, triggered by the earth’s tilt in relation to the sun. Although they return every year, it is still impossible to tell the timing, duration, and quantity of rain each season, a fact that leaves impacted areas without accurate storm information. Monsoons are set by land and sea temperature differences. Land reflects the sun’s rays, heating air over land more rapidly. Water is able to absorb a lot of heat without itself changing temperature much, so air over water stays relatively cooler.

This fact is prevalent in Asia because the northern hemisphere has so much more land than the southern hemisphere, which is mostly ocean. During he summer, the earth is tilted at such an angle that the sun’s rays shine more directly on the northern hemisphere. The heat is absorbed by the land masses, warming the air above it. The hot air rises, and cooler ocean air rushes inland from the southern hemisphere to replace it. As it moves, it carries moisture with it, releasing it over land as the summer monsoon (also known as southwest monsoon). The cycle continues as the cooling air creates precipitation and releases more energy. This energy then heats the air, which rises and flows back to the sea, cools, descends, and rushes back to land to replace more warm, rising air. This monsoon is centered over continental Asia.

There is also a winter monsoon (also known as northeast monsoon), created during the winter when most of the sun’s rays shine on the southern hemisphere. During this season, the continents are cooler than the water, which retains absorbed heat. The air reverses circulation, with warm air rising over the oceans and cooler land air, called “cold surges,” rushing in to replace it. The cold surges pick up warm moisture as it travels across tropical waters only to release them over Indonesia, northern Australia, Sri Lanka, and the east Indian coast.

Rain Gauge Journal

Date: June 28, 2012

Weather Condition: occasional rains

Humidity: 74 %

Sunrise: 5 : 30 am 

Sunset: 6 : 29 pm

Rain Gauge Collection: .4 mm

Source: pagasa.dost.com.ph

        ITCZ         

The Intertropical Convergence Zone (ITCZ) is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together.

Ø  Also known by sailors as the DOLDRUMS               

Ø  was originally identified from the 1920s to the 1940s as the "Intertropical Front" (ITF), but after the recognition in the 1940s and 1950s of the significance of wind field convergence in tropical weather production, the term "ITCZ" was then applied. When it lies near the equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough, a usage more common in Australia and parts of Asia. In the seamen's speech the zone is referred as the doldrums because of its erratic weather patterns with stagnant calms and violent thunderstorms.

Ø  appears as a band of clouds, usually thunderstorms, that circle the globe near the equator. In the Northern Hemisphere, the trade winds move in a southwestern direction from the northeast, while in the Southern Hemisphere, they move northwestward from the southeast. When the ITCZ is positioned north or south of the equator, these directions change according to the Coriolis effect imparted by the rotation of the earth. For instance, when the ITCZ is situated north of the equator, the southeast trade wind changes to a southwest wind as it crosses the equator. The ITCZ is formed by vertical motion largely appearing as convective activity of thunderstorms driven by solar heating, which effectively draw air in; these are the trade winds.^[2] The ITCZ is effectively a tracer of the ascending branch of the Hadley cell, and is wet. The dry descending branch is the horse latitudes.

Ø  The location of the intertropical convergence zone varies over time. Over land, it moves back and forth across the equator following the sun's zenith point. Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures. Sometimes, a double ITCZ forms, with one located north and another south of the equator. When this occurs, a narrow ridge of high pressure forms between the two convergence zones, one of which is usually stronger than the other.

Ø  band of weather that circles the Earth at or near the equator. Weather within the ITCZ can be quite violent as the weather systems from the northern and southern hemispheres converge. Thunderstorms within the ITCZ can reach heights of 80,000 plus feet.

EFFECTS ON WEATHER

Variation in the location of the intertropical convergence zone drastically affects rainfall in many equatorial nations, resulting in the wet and dry seasons of the tropics rather than the cold and warm seasons of higher latitudes. Longer term changes in the intertropical convergence zone can result in severe droughts or flooding in nearby areas.

In some cases, the ITCZ may become narrow, especially when it moves away from the equator; the ITCZ can then be interpreted as a front along the leading edge of the equatorial air. There appears to be a 15-25 day cycle in thunderstorm activity along the ITCZ, which is roughly half the wavelength of the Madden-Julian Oscillation (MJO).

Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed. Early sailors named this belt of calm the doldrums because of the inactivity and stagnation they found themselves in after days of no wind. To find oneself becalmed in this region in a hot and muggy climate could mean death in an era when wind was the only effective way to propel ships across the ocean. Even today leisure and competitive sailors attempt to cross the zone as quickly as possible as the erratic weather and wind patterns may cause unexpected delays.