The major characteristic of the climate is the contrast between
a relatively rainy season from November to April and very dry
weather for the rest of the year. With hot, dry, uniform summers
and cool, variable winters during which practically all of the
precipitation occurs, the country has a Mediterranean-style
climate. In general, the farther inland from the Mediterranean Sea
a given part of the country lies, the greater are the seasonal
contrasts in temperature and the less rainfall. Atmospheric
pressures during the summer months are relatively uniform, whereas
the winter months bring a succession of marked low pressure areas
and accompanying cold fronts. These cyclonic disturbances generally
move eastward from over the Mediterranean Sea several times a month
and result in sporadic precipitation.
Most of the East Bank receives less than twelve centimeters of
rain a year and may be classified as a dry desert or steppe region.
Where the ground rises to form the highlands east of the Jordan
Valley, precipitation increases to around thirty centimeters in the
south and fifty or more centimeters in the north. The Jordan
Valley, lying in the lee of high ground on the West Bank, forms a
narrow climatic zone that annually receives up to thirty
centimeters of rain in the northern reaches; rain dwindles to less
than twelve centimeters at the head of the Dead Sea.
The country's long summer reaches a peak during August. January
is usually the coolest month. The fairly wide ranges of temperature
during a twenty-four-hour period are greatest during the summer
months and have a tendency to increase with higher elevation and
distance from the Mediterranean seacoast. Daytime temperatures
during the summer months frequently exceed 36°C and average
32°C. In contrast, the winter months--November to April--bring
moderately cool and sometimes cold weather, averaging about
Except in the rift depression, frost is fairly common during the
winter, and it occasionally snows in Amman.
For a month or so before and after the summer dry season, hot,
dry air from the desert, drawn by low pressure, produces strong
winds from the south or southeast that sometimes reach gale force.
Known in the Middle East by various names, including the khamsin,
this dry, sirocco-style wind is usually accompanied by great dust
clouds. Its onset is heralded by a hazy sky, a falling barometer,
and a drop in relative humidity to about 10 percent. Within a few
hours there may be a 10°C to 15°C rise in temperature.
windstorms ordinarily last a day or so, cause much discomfort, and
destroy crops by desiccating them.
The shammal, another wind of some significance, comes
from the north or northwest, generally at intervals between June
and September. Remarkably steady during daytime hours but becoming
a breeze at night, the shammal may blow for as long as nine
days out of ten and then repeat the process. It originates as a dry
continental mass of polar air that is warmed as it passes over the
Eurasian landmass. The dryness allows intense heating of the
earth's surface by the sun, resulting in high daytime temperatures
that moderate after sunset.
Data as of December 1989