vf-tropicom

Weather Terms Defined

SOI Defined

The Southern Oscillation Index (SOI) compares air pressure in Darwin, Australia (which is typically low), and in Tahiti (which is typically high). When Tahiti has lower-than-normal air pressure and Darwin has higher-than-normal air pressure for a sustained period, it indicates that an El Niņo event is occurring. The opposite indicates a La Niņa event. Note: on these plots, this indicator is expressed as the "negative" value of the usual SOI indicator so that it is comparable with the other indicators.

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Hurricane/Typhoon/Cyclone

Severe tropical cyclones with sustained surface winds greater than 64 knots (74 mph) ("hurricanes") are called "typhoons" in the western North Pacific and "cyclones" in the western South Pacific and the Indian Ocean.

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Harmattan Defined

The parching Harmattan winds are a continental part of the globe-encircling trade winds. These winds dominate the Sahara Desert and during certain times of the year, bring extremely dry, warm air to much of North and West Africa. In northern hemisphere summer, the winds blow moderate to strong from the Mediterranean southward and eastward to about 17 degree north latitude. In northern hemisphere winter, the winds blow from 30 degrees north latitude to coastal West Africa, sometimes reaching as far south as the equator.
(From The Encyclopedia of Climatology. John E. Oliver, Rhodes W. Fairbridge, editors.)

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Monsoon Defined

A monsoon wind is part of the atmospheric circulation (as opposed to a local wind) that has a persistent direction which changes seasonally. Generally this is caused by patterns of differential heating between a continent and adjacent ocean that change with the seasons. Monsoonal rains are caused when the land is warmer than the ocean. The warm air over the land rises causing a slight vacuum. Cooler, moist air from over the ocean moves in to fill this vacuum. When this moist air becomes heated by the warm land, it, too, rises, the moisture condenses, and rain eventuates.
(From The Glossary of Weather and Climate with Related Oceanic and Hydrologic Terms. Ira W. Geer, editor.)

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ITCZ | El Niņo | Cocoa

Meteorological Drought Defined

Drought is a period of insufficient water initiated by reduced precipitation. The impacts of drought on crops and society are crititcal but not easily quantified. The result is that "drought" does not have a universal definition. "Meteorological drought" is defined as a sustained period of deficient precipitation with a low frequency of occurrence. While crops may be damaged by lack of precipitation and high temperatures in just a few days, such short periods are not considered to be meteorological droughts. A three-month period is defined by the American Meteorological Society to be the shortest period that can be defined as a drought.

(From The American Meteorological Society)

MJO (Madden-Julian Oscillation) Defined

Tropical rainfall exhibits strong variability on sub-seasonal time scales, often cycling over a period of 30-60 days. These cycles are called "intraseasonal oscillations", "Madden-Julian Oscillations", "MJO", "30-60 day oscillations" or "30-60 day waves".

(From the Climate Prediction Center)

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Ethiopia's Keremt or Meher Rains Defined

Since Ethiopia and Eritrea are in the tropics, physical conditions and variations in altitude have resulted in a great diversity of climate, soil, and vegetation. Rainfall is seasonal, varying in amount, space, and time. There is a long and heavy summer rain, normally called the big rain or keremt, which falls from June-September. It is followed by the baga hot, dry period from October through February. In some areas there are short and moderate spring rains in March and April known as the little rains or belg. These rainy periods correspond to Ethiopia's primary and secondary agricultural seasons, known as the meher and belg.

(From FEWS)

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Ethiopia's Belg Rains Defined

In spring, a strong cyclonic center develops over Ethiopia and Sudan. Winds from the Gulf of Aden and the Indian Ocean highs are drawn towards this center and blow across central and southern Ethiopia. These moist, easterly and southeasterly winds produce the main rain in southeastern Ethiopia and the little spring rains to the east central part of the northwestern highlands. The little rains of the highlands falling from February through May are known as belg rains, referring to the second most important sowing season of the region.

(From FEWS)

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Ethiopia's Baga Defined

Since Ethiopia and Eritrea are in the tropics, physical conditions and variations in altitude have resulted in a great diversity of climate, soil, and vegetation. Rainfall is seasonal, varying in amount, space, and time. There is a long and heavy summer rain, normally called the big rain or keremt, which falls from June-September. It is followed by the baga hot, dry period from October through February. In some areas there are short and moderate spring rains in March and April known as the little rains or belg. These rainy periods correspond to Ethiopia's primary and secondary agricultural seasons, known as the meher and belg.

(From FEWS)

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Somalia's Gu Defined

Rainfall in southern Somalia is bimodal, that is, there are two rainy seasons. Rainfall from April through August affecting Somalia's main harvest in September/October is called the "gu" rains. Sometimes these are also referred to as the "long rains."

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Somalia's Deyr Defined

Rainfall in southern Somalia is bimodal, that is, there are two rainy seasons. Rainfall from October through January affecting Somalia's secondary harvest in January/February is called the "deyr" rains. Sometimes these are also called the "short rains". (A fair amount of the deyr crop is recessional or irrigated.) Sometimes spelled "der".

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Somalia's Hagay Defined

Light coastal showers in southern Somalia falling after the gu and before the deyr from July through October are called "hagay" rains.

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Somalia's Karan Defined

Rainfall in northern Somalia (Somaliland) is unimodal, that is, there is only one rainy season. The seasonal rains from April through September are called the "karan".

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Kenya's Long-Rains Defined

Rainfall in Kenya is bimodal, that is, there are two rainy seasons. The main season rains begin in March and last through August affecting the harvest in October.

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Kenya's Short Rains Defined

Rainfall in Kenya is bimodal, that is, there are two rainy seasons. The secondary season begins in October and lasts through January affecting the harvest in February.

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Uganda's First-Season Rains Defined

Rainfall in Uganda is bimodal, that is, there are two rainy seasons. The "first-season" rains start in February/March and continue though mid-June in the south and early August in the north affecting the main harvest in June-August.

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Uganda's Second-Season Rains Defined

Rainfall in Uganda is bimodal, that is, there are two rainy seasons. The "second-season" rains start in July and end in October affecting the secondary harvest in November-January.

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Rwanda's Season A Rains Defined

Rainfall in Rwanda is bimodal, that is, there are two rainy seasons. The "season A" rains start in October/November and continue through January affecting the harvest in February.

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Rwanda's Season B Rains Defined

Rainfall in Rwanda is bimodal, that is, there are two rainy seasons. The "season B" rains start in January-March and continue through May/June affecting the harvest in June/July.

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Tanzania's Masika and Vuli Rains Defined

Tanzania has two major agroclimatic areas - the unimodal and the bimodal rainfall areas. The bimodal area has a short-rains minor cropping season, known as vuli, with planting around October/November and harvesting in late January/February; and a main, long-rains season, called masika, following immediately the vuli season, with planting in late February/March and harvesting in July/August. The bimodal areas extend over the northern and north-eastern regions, including Kilimanjaro, Arusha, Moshi, Tanga, as well as Kigoma, Kagera, Mwanza, Mara and parts of Morogoro, Mbeya, Dar es Salaam and Coast. Elsewhere in the country, i.e. the central and southern regions, rainfall is unimodal, with planting starting around November and harvesting taking place from May to July. In some regions, the unimodal or "seasonal rains" are also referred to as masika.

(From the Special Report on the FAO/WFP Crop and Food Supply Assessment Mission to Tanzania - August 1998)

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East Africa's Rainfall Seasons Defined

East Africa has a complicated pattern of seasonal rainfall. Far northern (Somaliland) and far southern (central and southern Tanzania) areas have unimodal rainfall patterns - that is, there is only one rainy season each year. Areas in between these two extremes have a bimodal pattern - two rainy seasons with dry periods between. In each country, patterns are somewhat different (scroll up for details), but for more northern areas (Somalia, Kenya, Ethiopia, Eritrea), the main rainy season occurs sometime between March and September, and the minor rainy season occurs sometime between October and February. For southern areas (Uganda, Rwanda, Burundi, Tanzania), the pattern is reversed.

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