Global Precipitation
Precipitation averages just about 1 meter per year over Earth but, like
wealth, varies widely from place to place and from time to time. Rainfall is
generally most abundant near the equator where the air rises and
temperatures are high, small in the subtropics where the air routinely sinks,
and tiny in the polar regions where the frigid air has a tiny vapor capacity.
Annual Cycle of Global Precipitation
Global Winds and Precipitation
Global precipitation patterns are closely tied to the winds. Since precipitation is produced
by rising air,
Precipitation is abundant where air rises and scanty where air sinks.
Air rises where the,
1. Atmosphere is unstable, as in thunderstorms.
2. Surface winds converge, as in the ITCZ or low pressure areas.
3. Wind strikes a mountain slope.
Despite all the irregularities imposed by geography, world maps of precipitation are
dominated by a few major patterns, such as,
1. Precipitation (like temperature) generally decreases with latitude.
2. The tropics (0 to 10° lat) tend to be wet, the subtropics (20 and 30°), dry.
3. The interiors of continents surrounded by mountain ranges are dry.
4. West coasts are drier than east coasts in the tropics and wetter above about 45º.
When winds and/or temperatures change much over the seasons so does precipitation.
1. In the tropics summer (high Sun season) tends to be wet, winter (low Sun) dry.
2. On the west coasts in the mid latitudes winters tend to be wet, summers dry.
3. In the high latitudes, summers tend to be wet, winters dry.
The Tropical Rainbelt
As the ITCZ moves north and south in pursuit of the sun, it causes the pronounced
wind shifts and celebrated rainy seasons of the monsoons. When the sun and ITCZ
are furthest north around July, tropical rains fall from the equator to about 20° N
latitude. At the same time, the latitudes from around 5 to 30°S are bone dry. Then, as
the sun turns south the rain belt follows. In January, the rainbelt is centered south of
the equator. Rains fall from 0 to 20°S, while the latitudes from 5 to 30°N tend to be
bone dry. The ITCZ is wide enough to give the equator rain all year, but many places
near the equator have two distinct rainy peaks when the heart of the rainbelt passes
overhead. Thus, tropical climates are,
1. Rainy all year from 0 to 5° latitude.
2. Rainy during high sun and dry during low sun from 5 to 20°.
3. Dry all year from 20 to 30° except on the east sides of the continents.
Tropical Rainbelt: Monsoon
Wind systems and rainbelts
move with the sun.
This gives rise to rainy and
dry seasons, i. e. monsoons.
(mausim = season)
In January, the sun and
region of hot rising air
(ITCZ or tropical rainbelt)
are south of the equator
In July, the sun and region
of hot rising air (ITCZ or
tropical rainbelt) are north
of the equator.
West Coast Wind and Weather Changes
An annual cycle of wind and rain occurs on the west coasts of all the continents in
the midlatitudes. In summer, the subtropical highs over the oceans strengthen and
drift poleward and eastward, until they cover the west coasts with sinking,
downslope easterly winds. Thus summer has little or no rain between about 10° and
45° latitude. Further poleward, westerly winds and occasional passing low pressure
areas bring some summer rains. During winter, the highs weaken, retreat
equatorward, and move away from land. Westerly winds and passing lows from the
mid latitude rain belt then strike the west coasts with increased frequency, bringing
rains down to about 30° latitude. As a result, west coasts are,
1. Dry all year from about 10° to 30°.
2. Rainy in winter and dry in summer between about 30° and 45°.
3. Rainy all year above 45°.
Mid-Latitude West Coast Rain Belt
As the large subtropical
high and sub polar low
follow the sun, so do the
belt of onshore winds
and rising air.
This brings rainy winters
and dry summers to the
west coast of all
continents between
about 30o and 50o Lat.
latitude. Poleward of 50o
rain can fall all year.
Hot Season (mainly High Latitude) Rain Rule
In most parts of the world (Mid-Latitude West Coast or Mediterranean Climates are
the major exception) the hottest season is the rainiest (or snowiest) and the
difference between the warmest and coldest season increases with the annual
range of temperature because water vapor capacity doubles for roughly every 10ºC.
Temperature becomes the dominant factor in precipitation at most places in high
latitudes, because the annual temperature range is particularly large. Thus, at all
inland places above about 50º latitude and even coastal regions above about 60º
latitude, July or August is the rainiest month in the North Hemisphere and January
or February is the rainiest month in the South Hemisphere.
Warning: Runoff does not necessarily follow rainfall in high latitude locations
because evaporation is also largest in the hot months. Rain that evaporates
does not run into the rivers.
Seasonal Distribution and Timing of River Runoff
There are 3 factors that determine the timing of runoff.
1. Lag. It always takes time for water to reach the rivers and then flow downstream. The
longer the river the longer the lag between the time of rainfall or snowmelt. Since water
flows about 1 ms-1, it takes 106 s  12 days for water to flow down a river 1000 km long It
also takes time after a dry season for the ground to be replenished with water. There is
also a lag for the lowest flow when there are dry seasons. Throughout the dry season
the ground gets progressively drier so discharge decreases and reaches a minimum at
the beginning of the rainy season.
2. Snow and Freezing. In the mid and high latitudes and in high mountains away from
the equator, snow falls in winter and melts in spring. In general, the higher the latitude,
the higher the fraction of precipitation that falls as snow. This means that spring is
usually the season of maximum discharge. Around 40 latitude the month of maximum
flow ranges from March to April. Above 60 latitude (in Siberia, Alaska, and Northern
Canada) May to June are the months of maximum discharge. During the long frozen
winter the discharge continues to decrease and is minimum just before the snow melts.
3. Evaporation. Outside of the tropics temperature and evaporation are much larger in
summer than in winter. The large summer evaporation can seriously reduce runoff.
Therefore, in the middle latitudes, the lowest discharge is usually at the end of summer
or the beginning of fall, as the ground has grown drier all during the hot summer.