Dry season inLaos. Longshadow of a dead tree with its branches on the dry fields ofDon Det, a sunny day with blue sky and white clouds, late afternoon.
Thedry season is a yearly period of low rainfall, especially in thetropics. The weather in the tropics is dominated by thetropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. Thetemperate counterpart to the tropical dry season issummer orwinter.
The tropical rain belt lies in the southern hemisphere roughly from November to March; during that time the northern tropics have a dry season with sparserprecipitation, and days are typically sunny throughout. From May to September, the rain belt lies in the northern hemisphere, and the southern tropics have their dry season. Under theKöppen climate classification, fortropical climates, a dry season month is defined as a month when average precipitation is below 60 millimetres (2.4 in).[1]
The rain belt reaches roughly as far north as theTropic of Cancer and as far south as theTropic of Capricorn. Near these latitudes, there is one wet season and one dry season annually. At theequator 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 or a long wet season; and a short dry or a long dry season. Local geography may substantially modify these climate patterns, however they can.
Data shows that in Africa the start of the dry season coincides with a rise in the cases ofmeasles—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 the disease to spread more easily.[3]
New data shows that in the seasonal parts of theSouth AmericanAmazon rainforest, foliage growth and coverage varies between the dry and wet seasons—with about 25% more leaves and faster growth in the dry season. Researchers believe that the Amazon itself has an effect in bringing[clarification needed] the onset of the wet season: by growing more foliage, it evaporates more water.[4] However, this growth appears only in the undisturbed parts of the Amazon basin, where researchers believe roots can reach deeper and gather more rainwater.[5] It has also been shown thatozone levels are much higher in the dry than in the wet season in the Amazon basin.[6]
