Dry season

Dry season

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The dry season is a term commonly used when describing the weather in the tropics
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

. The weather in the tropics is dominated by the tropical rain belt
Tropical rain belt
The rainfall, and hence the tropical climate is dominated by the tropical rain belt, which oscillates from the northern to the southern tropics over the course of the year, roughly following the solar equator; it is largely a manifestation of the ITCZ...

, which oscillates
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...

 from the northern to the southern tropics over the course of the year. The tropical rain belt lies in the southern hemisphere roughly from October to March, and during this time the northern tropics experience a dry season in which precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 is more rare, and days are typically sunny throughout. From April to September, the rain belt lies in the northern hemisphere, and the southern tropics experience their dry season. Under the Köppen climate classification
Köppen climate classification
The Köppen climate classification is one of the most widely used climate classification systems. It was first published by Crimea German climatologist Wladimir Köppen in 1884, with several later modifications by Köppen himself, notably in 1918 and 1936...

, for tropical climates
Tropical climate
A tropical climate is a climate of the tropics. In the Köppen climate classification it is a non-arid climate in which all twelve months have mean temperatures above...

, a dry season month is defined as a month where average precipitation is below 60 millimetres (2.4 in).

The dry season is characterized by its low humidity, and some watering holes and rivers drying up. Because of the lack of these watering holes, many grazing animals are forced to migrate due to the lack of water and feed to more fertile spots. Examples of such animals are zebras, elephants, and wildebeest
Wildebeest
The wildebeest , also called the gnu is an antelope of the genus Connochaetes. It is a hooved mammal...

. Because of the lack of water in the plants, bushfires are common.

Data shows that in Africa, the advent of the Dry Season coincides with a rise in the cases of measles
Measles
Measles, also known as rubeola or morbilli, is an infection of the respiratory system caused by a virus, specifically a paramyxovirus of the genus Morbillivirus. Morbilliviruses, like other paramyxoviruses, are enveloped, single-stranded, negative-sense RNA viruses...

--which researchers believe might be attributed to the higher concentration of people in the dry season, as agricultural operations are all but impossible without irrigation. During this time, some farmers move into cities, creating hubs of higher population density, and allowing disease to spread more easily.

The rain belt reaches roughly as far north as the Tropic of Cancer
Tropic of Cancer
The Tropic of Cancer, also referred to as the Northern tropic, is the circle of latitude on the Earth that marks the most northerly position at which the Sun may appear directly overhead at its zenith...

 and as far south as the Tropic of Capricorn
Tropic of Capricorn
The Tropic of Capricorn, or Southern tropic, marks the most southerly latitude on the Earth at which the Sun can be directly overhead. This event occurs at the December solstice, when the southern hemisphere is tilted towards the Sun to its maximum extent.Tropic of Capricorn is one of the five...

. Near these latitudes, there is one wet season and one dry season annually. On the equator, there are two wet and two dry seasons as the rain belt passes over twice a year, once moving north and once moving south. Between the tropics and the equator, locations may experience a short wet and a long wet season. Local geography may substantially modify these climate patterns, however.

New data shows that in the South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

n Amazon Rainforest
Amazon Rainforest
The Amazon Rainforest , also known in English as Amazonia or the Amazon Jungle, is a moist broadleaf forest that covers most of the Amazon Basin of South America...

, foliage growth and coverage varies in between the dry and wet seasons-with approximately 25% more leaves and faster growth in the dry season. Researchers believe that the Amazon itself has an effect in bringing the onset of the wet season; by growing more foliage, it evaporates more water. However, this growth appears only in the undisturbed parts of the Amazon, where researchers believe roots can reach deeper and gather more rain water. It has also been shown that the ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

levels differ between the dry and wet seasons in the Amazon, with the level being substantially higher in the dry season than in wet season.