GEOG 010, Section 015
November 12, 2003
Climatology of Sudan- Precipitation Patterns
Presented by Armand Silva, Jordan Strauss, and Matthew Zambelli
Our two locations of rainfall analysis lie within the African nation of Sudan. The
two stations are Khartoum, at 15.6° N 32.55°E, and Juba, at 4.87°N 31.60°E.
Over the past 30 years, precipitation in Khartoum has fluctuated within a range
between 0 and 20 cm per year. The exception was 1988 where the city received over 40
cm of rain. In Juba, precipitation fluctuated within a range between 70 and 130 cm per
year between 1967 and 1988. There was a significant decrease in rainfall in 1991 and in
1992 no rainfall was recorded. Afterwards, the annual precipitation amounts jumped
back and forth within a range between 20 and 80 cm per year.
Each of the two stations has a unique annual precipitation cycle. In Khartoum,
very little rainfall, if any, is recorded between the months of November and April. May
through October is the rainy season in the area, with peak precipitation in August. In
Juba, only December, January and February have little precipitation. March through
November have substantial rainfall, especially between April and October. Each month
between April and October have somewhat similar amounts of rainfall.
Latitude and atmospheric circulation are most responsible for the annual
precipitation cycles at each station. The movement of the Intertropical Convergence
Zone (ITCZ) and the latitude at which Sudan lies makes for a great amount of contrast in
climates. As the ITCZ moves in its north-south cycle during the year, the climates, and
therefore the precipitation patterns, are altered. Generally speaking, however, two
important climates can be looked upon as having a significant impact on rainfall in Sudan.
The first climate region is the tropical rain forest region located along the equator.
Equatorial regions get great amounts of precipitation because of the fact that the ITCZ is
located along the Equator as its “net” latitude over the year. During both the fall and
spring seasons, the ITCZ is always generally located along the equator. The ITCZ is a
low pressure trough. As a result of this, winds converge near the surface at the ITCZ and
then air is forced upward. This upwelling of air causes clouds to form, and along with
the clouds, precipitation. This means that equatorial regions are rainy most of the year,
thus tropical rain forests thrive in these areas. The second important climate region
affecting Sudan is the deserts. Deserts are formed as a result of the subtropical highs that
are located about 30 degrees on either sides of the equator. Winds diverging out of the
ITCZ aloft are then pushed downward toward the ground at the subtropical highs. This
compression of air causes the air to warm up. As the air warms, it stores more water
vapor, so as a result, when the air reaches the surface, it is both warm and dry. For this
reason, deserts occur at the subtropical highs. With these two climate regions and the
ITCZ in mind, the climate in Sudan can be explained. During the summer, the ITCZ
moves north until it reaches its most northerly position over the northern part of Sudan in
July. In response to this northerly position, moist air during the summer blows from the
south out of the rain forest areas to converge at the ITCZ. The convergence of air causes
great amounts of precipitation in northern Sudan during the peak summer months.
During the winter, when the ITCZ is located in southern regions of Africa, winds from
the desert areas to the north blow southward over Sudan. This air from the deserts is very
dry and carries little or no precipitation, so most of the country lies in drought during the
winter.
The annual precipitation cycles are very different for Khartoum and Juba. This is
so because these two locations are at significantly different latitudes. Sudan is a huge
country, spanning almost the entire northern tropics area. The northern border with
Egypt lies at about 22°N and the southern tip lies at about 4°N. Khartoum, at 15.6°N is
closer to the desert regions of the north, so is therefore located in a semi-arid steppe
region. As a result, there is little precipitation throughout the year, with the exception of
the peak summer months. Juba, at 4.87°N, is much closer to the equator and lies within a
tropical savanna climate. Khartoum and Juba have their precipitation peaks and lows at
about the same time of year, but because of the difference in latitude, Juba has more
months in which precipitation occurs. It is for this reason that Juba gets much more
precipitation over the year than does Khartoum.
As a result, the average annual precipitation between the two stations differs
significantly. In Khartoum, the average annual precipitation over the past 30 years is
11.14 cm, while in Juba it is 84.65 cm. Once again, latitude and atmospheric circulation
are the climatic controls that have determined the amount of rainfall each location has
gotten over the past years.