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IB World Geography Precipitation
People rely on precipitation to be “normal Normal rains and snowmelt are needed for consistent agriculture
All plants and animals are adapted to “normal” amounts of moisture for their environment. However, “normal”
does not always happen. Oftentimes, the various regions of the U.S. suffer tremendous differences in
precipitation. There have been many periods of drought in one region, and abundance of precipitation
sometimes even floods, in other regions in the same year. This fluctuation of drought and precipitation is
actually the “normal” pattern for our planet, and has been a common occurrence for millions of years. Because
we are all affected sooner or later, unusually dry or wet periods make headlines.
Precipitation is both a part of energy flows in the atmosphere, and a consequence of them. Precipitation is
essentially heat that has been released into the atmosphere and is returning to the Earth where it can absorb
more heat. This process is called condensation The amount of water that the atmosphere can actually hold is
called saturation vapor pressure At this point, precipitation begins.
Relative humidity tells us how wet the air actually is. It is expressed as a percentage of how much water the air
could hold at a given temperature This measurement fluctuates daily with changing air temperatures, because
the amount holds fairly steady while temperature rises and falls. This is why moisture falls as dew in the
nighttime air. The amount of water vapor has not changed, but the temperature has dropped therefore, the
relative humidity percentage is higher because the air is saturated.
Condensation is when the atmosphere produces clouds Clouds are able to form when water-vapor molecules
condense around tiny particles of dust, sea salt, pollen, and other particles. Clouds contain small droplets of liquid
water or particles of ice—depending on temperature, that are too light to fall. As the droplets become too large to
be supported by air currents, they fall to the surface as rain/snow
Causes of Precipitation:
1. Convection air warmer than its surroundings rises, expands, and cools by expansion.
2. Or ographic uplift wind forces air up over mountains.
3. Frontal uplift: air is forced up a boundary (front) between cold and warm air masses.
Convection precipitation often occurs on warm, bright, summer days. As the sun warms the ground moisture
rises into the atmosphere as a vapor transferring into the air, to rise through the cooler air above. As the warm
air raises, it begins to cool by a process called adiabatic cooling This processing cools air about 1 °C for every
100 meters of elevation. This process is self- reinforcing and causes clouds to grow rapidly, concluding with a
thunderstorm This is why the Southeastern U.S. often experiences rainfall in the afternoons of warmer
months (in “normal” years). In fact, convection storms are responsible for a large portion of the world’s
Orographic precipitation occurs when air rises over mountain regions As the air hits the mountains, it rises,
and cools adiabatically (by expansion The cooling causes condensation, and precipitation results. This is why
snow will begin to form on mountains when the months turn to cooler weather. After air has moved up the
windward side of the mountain and over the top, it descends on the leeward side. As it does, the relative
humidity drops significantly. The dry region on the leeward side of a mountain is called a rain shadow Many of
the world’s major deserts are arid because they are situated on the leeward side of a mountain range. Some of
the highest average US rainfall totals occur in the sierras with very dry regions in their rain shadows. The East
side of the Appalachi ans does not experience a rain shadow because the Gulf and Atlantic produces abundant
moisture for the region. http://guidesbyjulie.blogspot.com/
IB World Geography Precipitation
Frontal precipitation forms along a front, which is a boundary between two air masses. An air mass is a large
region of air with relatively uniform characteristics of temperature and humidity If formed over land, the mass
is called a continental mass; if formed over water, the mass is called a maritime air mass. When a cool mass
advances against a warm air mass, a boundary forms called a cold front and the warm air mass rises over the
cooler mass. When the warm air rises over the cold front, clouds usually are generated and intense
thunderstorms are likely to occur. When warm air masses advance, it is called a warm front The warm air
rises over the cooler air as if it were a ramp. Warm fronts often pass without precipitation however, in the
winter, this weather pattern is the source of freezing rain or snow
When dry continental air masses advance into an advancing tropical maritime air mass, the pressure changes
can create a vacuum that draws air into it, which circulates in a cyclone formation. Tornadoes create the
highest winds on Earth, and sometime funnel to the ground in an erratic fashion. Due to the topography and the
location of the Gulf the south-central U.S. has the greatest frequency on Earth.
Ell Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific as opposed to La
Niña which characterized by unusually cold ocean temperatures in the Equatorial Pacific. El Niño is an oscillation
of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the
Among these consequences are increased rainfall across the southern tier of the US and in Peru which has
caused destructive flooding and drought in the West Pacific sometimes associated with devastating brush
fires in Australia Observations of conditions in the tropical Pacific are considered essential for the prediction of
short term (a few months to 1 year) climate variations.
To provide necessary data, NOAA operates a network of buoys which measure temperature currents and
winds in the equatorial band These buoys daily transmit data which are available to researchers and
forecasters around the world in real time.
In normal, non-El Niño conditions, the trade winds blow towards the west across the tropical Pacific These
winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at
Indonesia than at Ecuador.
The sea surface temperature is about 8 °C higher in the west, with cool temperatures off South America due
to an upwelling of cold water from deeper levels. This cold water is nutrient- rich supporting high levels of
primary productivity diverse marine ecosystems and major fisheries Rainfall is found in rising air over the
warmest water, and the east Pacific is relatively dry.
During El Niño, the trade winds relax in the central and western Pacific leading to a depression of the
thermo cline in the eastern Pacific, and an elevation of the thermocline in the west This reduces the efficiency of
upwelling to cool the surface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The
result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which
adversely affected higher trophic levels of the food chain, including commercial fisheries in this region. The
weakening of easterly tradewinds during El Niño is evident in this figure as well. Rainfall follows the warm water
eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement
of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric
circulation which in turn force changes in weather in regions far removed from the tropical Pacific. ...
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- Spring '08
- Precipitation, warm air, Warm front, tropical Pacific