Rainfall patterns

Definition

Global rainfall patterns are produced by global scale atmospheric motions. In the tropics, warm air at the surface holds considerable moisture. As it is heated and rises, it cools, and the moisture condenses and falls out as rain. High in the atmosphere this air moves away from the equator to descend at latitudes near 30° north or south. As this dry, descending air approaches the surface, it warms, increasing its capacity to evaporate water. These latitudes contain many of the Earth’s major deserts: the Sahara, Kalahari, Australian, and Saudi Arabian, for example. Air high in the atmosphere that reaches either pole also cools and descends. Most of Antarctica is a desert, and the dry valleys there are among the driest places on Earth.

Each of these convective systems has a return flow directed toward the equator, across the surface of the Earth. Between them is a third convective system, complicated by the jet stream and other factors. In this system the upper atmosphere moves toward the equator, and the return flow across the surface moves away from the equator.

The Coriolis effect, a result of the Earth’s rotation, causes things in motion in the Northern Hemisphere to be deflected to the right and those moving in the Southern Hemisphere to be deflected to the left. Therefore, the return flows in the three cells described above, responsible for Earth’s surface winds, come from the east between the equator and 30° latitude, from the west between 30° and 45° latitude, and again from the east at latitudes greater than 45°. As these winds move across the oceans, they evaporate water. If they are brought onto a continent and then are forced up over a mountain range, the air they are moving will cool and rain will fall out. Moving down the far side of the mountain range, the now dry air will warm and evaporate moisture again. In this way, mountain ranges typically have an excess of rain (orographic rainfall) on their windward sides and a deficiency of rain (rain shadows) on their leeward sides.

Other winds are produced by the variation in temperature between continents and oceans. In the summer, continents are warmer and air rises above them, bringing moisture-laden air in from the oceans, and monsoon rains occur. In the winter, this pattern reverses, and little rain falls.

Significance for Climate Change

Humankind depends on patterns of rainfall—but not the total amount (most of which falls on the oceans) and not the amount that falls at a given location during a year (if it all comes in one torrential downpour, rain does more harm than good). What is important is the pattern, in both space and time, of the right amount of rainfall.

As climate changes, the geographically determined patterns of rainfall will probably change very little. The atmospheric convective cells will continue to operate, resulting in the variations in rainfall with latitude that are seen today and producing winds that will continue to blow in generally the same directions. On human timescales, mountains will stay the same, and hence rainfall and rain shadows will persist.

Rainfall patterns, however, are also affected by the strength and locations of persistent high and low pressure regions, which control the particular paths taken by various air masses. In the temperate zone (between 30° and 45°), the generally west-to-east flow of air produced by the large-scale convective systems is actually rather wavy, wandering north and south as it interacts with topography and with the persistent pressure systems set up by the various modes, or pressure seesaws, that exist in the atmosphere. The degree of waviness and where the waves occur change with the seasons (the excursions of the jet stream are larger in the winter) and with these modes.

Some of these modes are the El Niño-Southern Oscillation cycle, the Pacific-North American (PNA) cycle, the Pacific Decadal Oscillation (PDO), and the Northern (NAM), which may include the North Atlantic Oscillation (NAO). Although what controls these oscillations is not well understood, their timing and intensity are probably more likely to change as the climate evolves than the larger, geographically determined patterns.

Studies of for rainfall and temperature have attributed variations in the monsoons to fluctuations in the strength of the ENSO. The Medieval Warm Period (MWP), from roughly 900 to 1300 CE when the North Atlantic and northern and western Europe experienced warmer conditions on average, is thought by some to have resulted from a very persistent phase of the NAO, causing the winds that blow from the west over much of Europe to be deflected far to the south prior to reaching that continent.

Climate change has increased the amount of rainfall and the intensity of storms. According to the Environmental Protection Agency (EPA) in 2024, because of global warming and climate change, rainfall has come in the form of intense single-day events more often than in the past and in more places. From 1910 to 2023, the part of the country that experienced intense rainfall in one day had expanded by about one-half a percentage point each decade. Rising temperatures increase evaporation, which results in more frequent and intense storms. In terms of human suffering, changing rainfall patterns may well turn out to be more significant than temperature changes or sea-level rise.

Bibliography

"Climate Change Indicators: Weather and Climate." US Environmental Protection Agency (EPA), 27 June 2024, www.epa.gov/climate-indicators/weather-climate. Accessed 20 Dec. 2024.

Collier, Michael, and Robert H. Webb. Floods, Droughts, and Climate Change. Tucson: University of Arizona Press, 2002.

Gautier, C. Oil, Water, and Climate: An Introduction. New York: Cambridge University Press, 2008.

Kandel, Robert. Water from Heaven. New York: Columbia University Press, 2003.

Rohli, Robert V., and Anthony J. Vega. Climatology. Sudbury, Mass.: Jones & Bartlett, 2008.