Chapter I
Introduction
A.
Abstract
What is Rain and How is it Created?
This question is a common
question but it requires a clear and detailed
explanation and correspond to the
natural sciences. Rain occurs in every region worldwide periodically.
Rain is a form of precipitation
in the form of liquid. Precipitation itself can
be either solid (eg, snow and hail) or
aerosol (such as dew
and fog). Rain
forms when separate
drops fell to earth
from the clouds. Not all rain water to the
surface of the earth because some
evaporates while falling
through dry air.
Rain type is
called virga.
Rain plays an
important role in the
hydrological cycle. Lembaban from
the sea evaporated, turned into
the clouds, gathered into clouds, then
fall back to earth, and finally returned to
the sea through rivers and
streams to repeat the original recycling.
B.
Problem Objective
The Problem Objective of this essay are
1.
Formation of Rain
2.
Causes of Rain
3.
Characteristics
4.
Measurement
5.
Forecasting
6.
Impact of Rain
7.
Global Climatology
C.
Purpose
The
purpose of this essay are
1.
For writer,
this essay is a assigment from lecturer as one of Assigment Mark in the Final
Mark
2.
For reader,
this essay can give many new knowledge about Rain and how it is created.
Chapter II
Rain
What is
Rain and How is it Created?
A rain shaft at the base of a thunderstorm
Rain is
liquid precipitation, as opposed to non-liquid
kinds of precipitation such as snow, hail
and sleet. Rain requires the presence of a thick
layer of the atmosphere to have temperatures above the melting point of water
near and above the Earth's surface. On Earth, it is the condensation
of atmospheric water vapor into drops
of water
heavy enough to fall, often making it to the surface. Two processes, possibly
acting together, can lead to air becoming saturated leading to rainfall:
cooling the air or adding water vapor to the air. Virga is precipitation
that begins falling to the earth but evaporates before reaching the surface; it
is one of the ways air can become saturated. Precipitation forms via collision
with other rain drops or ice crystals within a cloud. Rain drops range in
size from oblate, pancake-like shapes for larger drops, to small spheres for
smaller drops.
Moisture moving along
three-dimensional zones of temperature and moisture contrasts known as weather
fronts is the major method of rain production. If enough moisture
and upward motion is present, precipitation falls from convective clouds (those
with strong upward vertical motion) such as cumulonimbus
(thunderstorms) which can organize into narrow rainbands.
In mountainous areas, heavy precipitation is possible where upslope flow
is maximized within windward sides of the terrain
at elevation which forces moist air to condense and fall out as rainfall along
the sides of mountains. On the leeward side of mountains, desert climates can
exist due to the dry air caused by downslope flow which causes heating and
drying of the air mass.
The movement of the monsoon trough, or intertropical convergence zone,
brings rainy seasons
to savannah
climes.
Rain is the primary source of freshwater for most areas of the world, providing
suitable conditions for diverse ecosystems, as well as water for hydroelectric
power plants and crop irrigation.
Rainfall is measured through the use of rain gauges.
Rainfall amounts are estimated actively by weather radar
and passively by weather satellites.
The urban heat
island effect leads to increased rainfall, both in amounts and
intensity, downwind of cities. Global
warming is also causing changes in the precipitation pattern
globally, including wetter conditions across eastern North America
and drier conditions in the tropics. Precipitation is a major component of the water cycle,
and is responsible for depositing most of the fresh water
on the planet.
The globally averaged annual precipitation is 990 millimetres (39 in). Climate
classification systems such as the Köppen climate classification system use
average annual rainfall to help differentiate between differing climate
regimes. Antarctica is the Earth's driest continent. Rain is also known or
suspected on other worlds, composed of methane,
iron, neon, and sulfuric acid
rather than water.
Hydrologic cycle
Water
is cycled between the oceans and the atmosphere. As always, the cycle is driven
by the Sun. Most of the water vapour in the atmosphere (84 %) comes from the oceans.
Transpiration from plant leaves accounts for most of the rest. The Sun heats the
water in the oceans (and on the land surface), giving evaporation. The warm, moist
air rises, expands (under reduced pressure higher in the atmosphere) and cools.
The water vapour condenses to form clouds. The winds then carry the clouds
across the Earth’s surface until the water is released as precipitation (rain,
hail or snow) to fall on the Earth for further recycling. Most of the
precipitation falls into the oceans (75 % of the surface of the planet being
ocean). The rate of circulation of water within the hydrologic cycle is very
rapid. Since the total mass of water in the atmosphere is constant,
precipitation is balanced by evaporation. Thus, comparing throughputs shows
that the average residence time of water molecules in the atmosphere is about 10
days.
A.
Formation
a.
Water-saturated air
Air contains water vapor and the
amount of water in a given mass of dry air, known as the Mixing Ratio, is
measured in grams of water per kilogram of dry air (g/kg). The amount of
moisture in air is also commonly reported as relative
humidity; which is the percentage of the total water vapor air can
hold at a particular air temperature. How much water
vapor a parcel of air can contain before it becomes saturated
(100% relative humidity) and forms into a cloud (a group of visible
and tiny water and ice particles suspended above the Earth's surface) depends on its
temperature. Warmer air can contain more water vapor than cooler air before
becoming saturated. Therefore, one way to saturate a parcel of air is to cool
it. The dew point
is the temperature to which a parcel must be cooled in order to become
saturated.
There are four main mechanisms for
cooling the air to its dew point: adiabatic cooling, conductive cooling,
radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises
and expands.
The air can rise due to convection, large-scale atmospheric motions, or a physical
barrier such as a mountain (orographic
lift). Conductive cooling occurs when the air comes into contact
with a colder surface, usually by
being blown from one surface to another, for example from a liquid water
surface to colder land. Radiational cooling occurs due to the emission of infrared
radiation, either by the air or by the surface underneath. Evaporative
cooling occurs when moisture is added to the air through evaporation, which
forces the air temperature to cool to its wet-bulb temperature, or until it reaches
saturation.
The main ways water vapor is added
to the air are: wind convergence into areas of upward motion,
precipitation or virga falling from above, daytime
heating evaporating water from the surface of oceans, water bodies or wet land, transpiration
from plants,
cool or dry air moving over warmer water, and lifting
air over mountains. Water vapor
normally begins to condense on condensation nuclei such as dust, ice, and
salt in order to form clouds. Elevated portions of weather fronts (which are
three-dimensional in nature) force broad
areas of upward motion within the Earth's atmosphere which form clouds decks
such as altostratus
or cirrostratus. Stratus
is a stable cloud deck which tends to form when a cool, stable air mass is
trapped underneath a warm air mass. It can also form due to the lifting of advection fog
during breezy conditions.
b.
Coalescence
The shape of rain drops depend
upon their size
Coalescence occurs when water droplets
fuse to create larger water droplets, or when water droplets freeze onto an ice
crystal, which is known as the Bergeron
process. Air resistance typically causes the water droplets in a
cloud to remain stationary. When air turbulence occurs, water droplets collide,
producing larger droplets. As these larger water droplets descend, coalescence
continues, so that drops become heavy enough to overcome air resistance and
fall as rain. Coalescence generally happens most often in clouds above
freezing, and is also known as the warm rain process. In clouds
below freezing, when ice crystals gain enough mass they begin to fall. This
generally requires more mass than coalescence when occurring between the
crystal and neighboring water droplets. This process is temperature dependent,
as supercooled water droplets only exist in a cloud that is below freezing. In
addition, because of the great temperature difference between cloud and ground
level, these ice crystals may melt as they fall and become rain.
Raindrops have sizes ranging from
0.1 millimetres (0.0039 in) to 9 millimetres (0.35 in) mean diameter,
above which they tend to break up. Smaller drops are called cloud droplets, and
their shape is spherical. As a raindrop increases in size, its shape becomes
more oblate, with its largest cross-section facing the oncoming airflow. Large
rain drops become increasingly flattened on the bottom, like hamburger
buns; very large ones are shaped like parachutes. Contrary to
popular belief, their shape does not resemble a teardrop. The biggest
raindrops on Earth were recorded over Brazil and the Marshall
Islands in 2004 — some of them were as large as 10 millimetres
(0.39 in). The large size is explained by condensation on large smoke particles or by
collisions between drops in small regions with particularly high content of
liquid water.
Intensity and duration of rainfall
are usually inversely related, i.e., high intensity storms are likely to be of
short duration and low intensity storms can have a long duration. Rain drops
associated with melting hail tend to be larger than other rain drops. Raindrops
impact at their terminal velocity, which is greater for larger
drops due to their larger mass to drag ratio. At sea level and without wind,
0.5 millimetres (0.020 in) drizzle impacts at 2 metres per second (4.5 mph), while
large 5 millimetres (0.20 in) drops impact at around 9 metres per second
(20 mph).
The sound of
raindrops hitting water is caused by bubbles of air oscillating
underwater. The METAR code for rain is RA,
while the coding for rain showers is SHRA.
B.
Causes
a.
Frontal activity
Stratiform (a broad shield of
precipitation with a relatively similar intensity) and dynamic precipitation
(convective precipitation which is showery in nature with large changes in
intensity over short distances) occur as a consequence of slow ascent of air in
synoptic systems (on the order of cm/s),
such as in the vicinity of cold fronts and near and poleward of surface warm fronts.
Similar ascent is seen around tropical cyclones
outside of the eyewall, and in comma-head precipitation
patterns around mid-latitude cyclones. A wide
variety of weather can be found along an occluded front, with thunderstorms
possible, but usually their passage is associated with a drying of the air mass.
Occluded fronts usually form around mature low-pressure areas. What
separates rainfall from other precipitation types, such as ice pellets
and snow,
is the presence of a thick layer of air aloft which is above the melting point
of water, which melts the frozen precipitation well before it reaches the ground.
If there is a shallow near surface layer that is below freezing, freezing rain
(rain which freezes on contact with surfaces in subfreezing environments) will
result.
Hail becomes an
increasingly infrequent occurrence when the freezing level within the
atmosphere exceeds 11,000 feet (3,400 m) above ground level.
b.
Convection
Convective precipitation
Convective
rain, or showery precipitation, occurs from convective clouds, e.g.,
cumulonimbus
or cumulus congestus. It falls as showers with
rapidly changing intensity. Convective precipitation falls over a certain area
for a relatively short time, as convective clouds have limited horizontal
extent. Most precipitation in the tropics appears to be convective; however, it has been
suggested that stratiform precipitation also occurs. Graupel
and hail
indicate convection. In
mid-latitudes, convective precipitation is intermittent and often associated
with baroclinic boundaries such as cold fronts,
squall lines,
and warm fronts.
c.
Orographic effects
Main articles: Orographic
lift, Precipitation types (meteorology),
and United States rainfall climatology
Orographic precipitation
Orographic precipitation occurs on
the windward
side of mountains and is caused by the rising air motion of a large-scale flow
of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In
mountainous parts of the world subjected to relatively consistent winds (for
example, the trade winds), a more moist climate
usually prevails on the windward side of a mountain than on the leeward
or downwind side. Moisture is removed by orographic lift, leaving drier air
(see katabatic wind) on the descending and generally
warming, leeward side where a rain shadow is observed.
In Hawaii, Mount Waiʻaleʻale, on the island of Kauai, is
notable for its extreme rainfall, as it has the second highest average annual
rainfall on Earth, with 460 inches (12,000 mm). Systems known
as Kona storms
affect the state with heavy rains between October and April. Local
climates vary considerably on each island due to their topography, divisible
into windward (Koʻolau) and leeward (Kona)
regions based upon location relative to the higher mountains. Windward sides
face the east to northeast trade winds and receive much more rainfall;
leeward sides are drier and sunnier, with less rain and less cloud cover.
In South America, the Andes mountain range
blocks Pacific moisture that arrives in that
continent, resulting in a desertlike climate just downwind across western
Argentina.
The Sierra Nevada range creates the same
effect in North America forming the Great Basin
and Mojave
Deserts.
d.
Within the tropics
Rainfall
distribution by month in Cairns showing the extent of the wet season at that location
The wet, or rainy, season is the
time of year, covering one or more months, when most of the average annual
rainfall in a region falls. The term green
season is also sometimes used as a euphemism
by tourist authorities. Areas with wet
seasons are dispersed across portions of the tropics
and subtropics. Savanna
climates and areas with monsoon regimes have wet summers and dry winters. Tropical
rainforests technically do not have dry or wet seasons, since their rainfall is
equally distributed through the year. Some
areas with pronounced rainy seasons will see a break in rainfall mid-season
when the intertropical convergence zone
or monsoon
trough move poleward of their location during the middle of the warm
season.
When the wet season occurs during the warm season, or summer, rain
falls mainly during the late afternoon and early evening hours. The wet season
is a time when air quality improves, freshwater
quality improves, and vegetation
grows significantly.
Tropical
cyclones, a source of very heavy rainfall, consist of large air
masses several hundred miles across with low pressure at the centre and with
winds blowing inward towards the centre in either a clockwise direction (southern
hemisphere) or counter clockwise (northern hemisphere). Although cyclones
can take an enormous toll in lives and personal property, they may be important
factors in the precipitation regimes of places they impact, as they may bring
much-needed precipitation to otherwise dry regions.
Areas in their path can receive a year's worth of rainfall from a tropical
cyclone passage.
e.
Human influence
Image of Atlanta,
Georgia showing temperature distribution, with blue showing cool
temperatures, red warm, and hot areas appear white.
The fine particulate matter
produced by car exhaust and other human sources of pollution forms cloud condensation nuclei, leads to the
production of clouds and increases the likelihood of rain. As commuters and
commercial traffic cause pollution to build up over the course of the week, the
likelihood of rain increases: it peaks by Saturday, after five days of weekday
pollution has been built up. In heavily populated areas that are near the
coast, such as the United States' Eastern
Seaboard, the effect can be dramatic: there is a 22% higher chance
of rain on Saturdays than on Mondays. The urban heat
island effect warms cities 0.6 °C (1.1 °F) to 5.6 °C (10.1 °F) above
surrounding suburbs and rural areas. This extra heat leads to greater upward
motion, which can induce additional shower and thunderstorm activity. Rainfall
rates downwind of cities are increased between 48% and 116%. Partly as a result
of this warming, monthly rainfall is about 28% greater between 20 miles
(32 km) to 40 miles (64 km) downwind of cities, compared with upwind. Some cities
induce a total precipitation increase of 51%.
Mean surface
temperature anomalies during the period 1999 to 2008 with respect to the
average temperatures from 1940 to 1980
Increasing temperatures tend to
increase evaporation which can lead to more precipitation. Precipitation
generally increased over land north of 30°N from 1900 through 2005 but has
declined over the tropics since the 1970s. Globally there has been no
statistically significant overall trend in precipitation over the past century,
although trends have varied widely by region and over time. Eastern portions of
North and South America, northern Europe, and northern and central Asia have
become wetter. The Sahel, the Mediterranean, southern Africa and parts of
southern Asia have become drier. There has been an increase in the number of
heavy precipitation events over many areas during the past century, as well as
an increase since the 1970s in the prevalence of droughts—especially in the
tropics and subtropics. Changes in precipitation and evaporation over the
oceans are suggested by the decreased salinity of mid- and high-latitude waters
(implying more precipitation), along with increased salinity in lower latitudes
(implying less precipitation and/or more evaporation). Over the contiguous
United States, total annual precipitation increased at an average rate of
6.1 percent since 1900, with the greatest increases within the East North
Central climate region (11.6 percent per century) and the South
(11.1 percent). Hawaii was the only region to show a decrease
(-9.25 percent).
The most successful attempts at
influencing weather
involve cloud seeding which include techniques used to
increase winter
precipitation over mountains and suppress hail.
C.
Characteristics
a.
Patterns
Rainbands
are cloud
and precipitation areas which are significantly elongated. Rainbands can be stratiform
or convective, and are
generated by differences in temperature. When noted on weather radar
imagery, this precipitation elongation is referred to as banded structure. Rainbands in
advance of warm occluded fronts and warm fronts
are associated with weak upward motion, and tend to be
wide and stratiform in nature.
Rainbands spawned near and ahead
of cold fronts
can be squall lines
which are able to produce tornadoes. Rainbands
associated with cold fronts can be warped by mountain barriers perpendicular to
the front's orientation due to the formation of a low-level barrier jet. Bands of
thunderstorms can form with sea breeze and land breeze
boundaries, if enough moisture is present. If sea breeze rainbands become
active enough just ahead of a cold front, they can mask the location of the
cold front itself.
Once a cyclone occludes, a trough of warm air aloft, or "trowal" for
short, will be caused by strong southerly winds on its eastern periphery
rotating aloft around its northeast, and ultimately northwestern, periphery
(also known as the warm conveyor belt), forcing a surface trough to continue into
the cold sector on a similar curve to the occluded front. The trowal creates
the portion of an occluded cyclone known as its comma head, due to the comma-like shape of the mid-tropospheric
cloudiness that accompanies the feature. It can also be the focus of locally
heavy precipitation, with thunderstorms possible if the atmosphere along the
trowal is unstable enough for convection. Banding
within the comma head precipitation pattern of an extratropical cyclone can yield
significant amounts of rain. Behind
extratropical cyclones during fall and winter, rainbands can form downwind of
relative warm bodies of water such as the Great Lakes.
Downwind of islands, bands of showers and thunderstorms can develop due to low
level wind convergence downwind of the island edges. Offshore California,
this has been noted in the wake of cold fronts.
Rainbands within tropical cyclones
are curved in orientation. Tropical cyclone rainbands contain showers and
thunderstorms that, together with the eyewall and the eye, constitute a hurricane or
tropical storm. The extent of rainbands around a tropical cyclone
can help determine the cyclone's intensity.
b.
Acidity
The pH of rain varies, especially
due to its origin. On Americas East Coast, rain that is derived from the Atlantic
Ocean typically has a pH of 5.0-5.6; rain that comes across the continental
from the west has a pH of 3.8-4.8; and local thunderstorms can have a pH as low
as 2.0.
Rain becomes acidic primarily due to the presence of two strong acids, sulfuric acid
(H2SO4) and nitric acid
(HNO3). Sulfuric acid is derived from natural sources such as
volcanoes, and wetlands (sulfate reducing bacteria); and anthropogenic sources
such as the combustion of fossil fuels, and mining where H2S is
present. Nitric acid is produced by natural sources such as lightning, soil
bacteria, and natural fires; while also produced anthropogenically by the
combustion of fossil fuels and from power plants. In the past 20 years the
concentrations of nitric and sulfuric acid has decreased in presence of
rainwater, which may be due to the significant increase in ammonium (most
likely as ammonia from livestock production), which acts as a buffer in acid
rain and raising the pH.
D.
Measurement
a.
Gauges
The standard way of measuring
rainfall or snowfall is the standard rain gauge, which can be found in 100-mm
(4-in) plastic and 200-mm (8-in) metal varieties. The inner
cylinder is filled by 25 mm (0.98 in) of rain, with overflow flowing
into the outer cylinder. Plastic gauges have markings on the inner cylinder
down to 0.25 mm (0.0098 in) resolution, while metal gauges require
use of a stick designed with the appropriate 0.25 mm (0.0098 in)
markings. After the inner cylinder is filled, the amount inside it is
discarded, then filled with the remaining rainfall in the outer cylinder until
all the fluid in the outer cylinder is gone, adding to the overall total until
the outer cylinder is empty. Other types of
gauges include the popular wedge gauge (the cheapest rain gauge and most
fragile), the tipping bucket rain gauge, and the weighing rain gauge. For those
looking to measure rainfall the most inexpensively, a can that is cylindrical
with straight sides will act as a rain gauge if left out in the open, but its
accuracy will depend on what ruler you use to measure the rain with. Any of the
above rain gauges can be made at home, with enough know-how.
When a precipitation measurement
is made, various networks exist across the United States and elsewhere where
rainfall measurements can be submitted through the Internet, such as CoCoRAHS
or GLOBE. If a network
is not available in the area where one lives, the nearest local weather or met
office will likely be interested in the measurement.
One millimeter of rainfall is the
equivalent of one liter of water per square meter. This makes computing the
water requirements of crops simple.
b.
Remote sensing
Twenty-four
hour rainfall accumulation on the Val d'Irène radar in Eastern Canada. Zones
without data in the east and southwest are caused by beam blocking from
mountains. (Source: Environment Canada)
One of the main uses of weather
radar is to be able to assess the amount of precipitations fallen over large
basins for hydrological
purposes.
For instance, river flood control, sewer management and dam construction are
all areas where planners use rainfall accumulation data. Radar-derived rainfall
estimates compliment surface station data which can be used for calibration. To
produce radar accumulations, rain rates over a point are estimated by using the
value of reflectivity data at individual grid points. A radar equation is then
used, which is,
Z = ARb,
where Z represents the radar
reflectivity, R represents the rainfall rate, and A and b are constants. Satellite
derived rainfall estimates use passive microwave
instruments aboard polar orbiting as well as geostationary weather
satellites to indirectly measure rainfall rates. If one wants
an accumulated rainfall over a time period, one has to add up all the
accumulations from each grid box within the images during that time.
c.
Intensity
Rainfall intensity is classified according to the rate of
precipitation:
·
Light rain — when the precipitation rate is < 2.5
millimetres (0.098 in) per hour
·
Moderate rain — when the precipitation rate is between
2.5 millimetres (0.098 in) - 7.6 millimetres (0.30 in) or 10
millimetres (0.39 in) per hour
·
Heavy rain — when the precipitation rate is > 7.6
millimetres (0.30 in) per hour, or
between 10 millimetres (0.39 in) and 50 millimetres (2.0 in) per hour
d.
Return period
The likelihood or probability of
an event with a specified intensity and duration, is called the return period
or frequency.
The intensity of a storm can be predicted for any return period and storm
duration, from charts based on historic data for the location. The term 1
in 10 year storm describes a rainfall event which is rare and is only
likely to occur once every 10 years, so it has a 10 percent
likelihood any given year. The rainfall will be greater and the flooding will
be worse than the worst storm expected in any single year. The term 1 in
100 year storm describes a rainfall event which is extremely rare and
which will occur with a likelihood of only once in a century, so has a 1 percent
likelihood in any given year. The rainfall will be extreme and flooding to be
worse than a 1 in 10 year event. As with all probability events, it is
possible, though improbable, to have multiple "1 in 100 Year
Storms" in a single year.
E. Forecasting
The Quantitative Precipitation Forecast (abbreviated QPF)
is the expected amount of liquid precipitation accumulated over a specified
time period over a specified area. A QPF
will be specified when a measurable precipitation type reaching a minimum
threshold is forecast for any hour during a QPF valid period. Precipitation
forecasts tend to be bound by synoptic hours such as 0000, 0600, 1200 and
1800 GMT.
Terrain is considered in QPFs by use of topography or based upon climatological
precipitation patterns from observations with fine detail. Starting in
the mid to late 1990s, QPFs were used within hydrologic forecast models to
simulate impact to rivers throughout the United States. Forecast models show significant
sensitivity to humidity levels within the planetary boundary layer, or in the lowest
levels of the atmosphere, which decreases with height. QPF can be
generated on a quantitative, forecasting amounts, or a qualitative, forecasting
the probability of a specific amount, basis. Radar imagery
forecasting techniques show higher skill
than model forecasts within 6 to 7 hours of the time of the radar image.
The forecasts can be verified through use of rain gauge measurements, weather
radar estimates, or a combination of both. Various skill scores can be
determined to measure the value of the rainfall forecast.
F.
Impact
a.
Effect on agriculture
Rainfall estimates for southern Japan and the surrounding
region from July 20–27, 2009.
Precipitation, especially rain, has a dramatic effect on agriculture.
All plants
need at least some water to survive, therefore rain (being the most effective
means of watering) is important to agriculture. While a regular rain pattern is
usually vital to healthy plants, too much or too little rainfall can be harmful, even
devastating to crops. Drought
can kill crops and increase erosion, while overly
wet weather can cause harmful fungus growth. Plants need
varying amounts of rainfall to survive. For example, certain cacti require
small amounts of water, while
tropical plants may need up to hundreds of inches of rain per year to survive.
In areas with wet and dry seasons, soil nutrients diminish
and erosion increases during the wet season. Animals
have adaptation and survival strategies for the wetter regime. The previous dry
season leads to food shortages into the wet season, as the crops have yet to
mature.
Developing countries have noted that their populations show seasonal weight
fluctuations due to food shortages seen before the first harvest, which occurs
late in the wet season. Rain may be harvested through the use of rainwater
tanks; treated to potable use or for non-potable use indoors or for
irrigation,.
Excessive rain during short periods of time can cause flash floods.
b.
In culture
Cultural attitudes towards rain
differ across the world. In temperate climates, people tend to be more stressed when the weather
is unstable or cloudy, with its impact greater on men than women. Rain can also
bring joy, as some consider it to be soothing or enjoy the aesthetic appeal of
it. In dry places, such as India, or during
periods of drought, rain lifts
people's moods. In Botswana, the Setswana word for rain, "pula", is used as the name of
the national currency, in recognition of the economic importance of
rain in this desert country. Several
cultures have developed means of dealing with rain and have developed numerous
protection devices such as umbrellas and raincoats, and diversion devices such as gutters
and storm drains
that lead rains to sewers. Many people
find the scent during and immediately after rain pleasant or distinctive. The
source of this scent is petrichor, an oil produced by plants, then absorbed by rocks
and soil, and later released into the air during rainfall.
G.
Global Climatology
Approximately 505,000 cubic
kilometres (121,000 cu mi) of water falls as precipitation each year
across the globe with 398,000 cubic kilometres (95,000 cu mi) of it over
the oceans. Given
the Earth's surface area, that means the globally averaged annual precipitation
is 990 millimetres (39 in). Deserts are defined as areas with an average
annual precipitation of less than 250 millimetres (10 in) per year, or as areas
where more water is lost by evapotranspiration than falls as precipitation.
a. Deserts
Largest deserts
The northern half of Africa is
primarily desert
or arid,
containing the Sahara.
Across Asia, a large annual rainfall minimum, composed primarily of deserts,
stretches from the Gobi desert in Mongolia west-southwest through
western Pakistan (Balochistan) and Iran into the Arabian
desert in Saudi Arabia. Most of Australia is semi-arid or desert, making it the
world's driest inhabited continent. In South America, the Andes mountain range
blocks Pacific moisture that arrives in that
continent, resulting in a desertlike climate just downwind across western
Argentina.
The drier areas of the United States are regions where the Sonoran
desert overspreads the Desert Southwest, the Great Basin and central
Wyoming.
Polar Desert
Since rain only falls as liquid,
in frozen temperatures, rain can not fall. As a result, very cold climates see
very little rainfall and are often known as polar deserts. A common
biome in this area is the tundra which has a short summer thaw and a long frozen winter.
Ice caps
see no rain at all, making Antarctica the world's driest continent.
b.
Rainforests
Rainforests are areas of the world
with very high rainfall. Both tropical and temperate rainforests exist. Tropical
rainforests occupy a large band of the planet mostly along the equator.
Most temperate rainforests are located on mountainous west coasts between 45
and 55 degrees latitude, but they are often found in other areas.
Around 40-75% of all biotic life
is found in rainforests. Rainforests are also responsible for 28% of the
world's oxygen turnover.
c.
Monsoons
The equatorial region near the Intertropical Convergence Zone
(ITCZ), or monsoon trough, is the wettest portion of the world's continents.
Annually, the rain belt within the tropics marches northward by August, then
moves back southward into the Southern Hemisphere by February and March. Within Asia,
rainfall is favored across its southern portion from India east and northeast
across the Philippines and southern China into Japan due to the monsoon advecting
moisture primarily from the Indian Ocean into the region. The monsoon
trough can reach as far north as the 40th parallel in East Asia during August before
moving southward thereafter. Its poleward progression is accelerated by the
onset of the summer monsoon which is characterized by the development of lower
air pressure (a thermal low) over the warmest part of Asia. Similar, but
weaker, monsoon circulations are present over North America
and Australia. During the
summer, the Southwest monsoon combined with Gulf of California and Gulf of
Mexico moisture moving around the subtropical
ridge in the Atlantic ocean bring the promise of afternoon and
evening thunderstorms to the southern tier of the United States as well as the Great Plains. The
eastern half of the contiguous United States east of the 98th meridian, the mountains of the Pacific
Northwest, and the Sierra Nevada range are the wetter
portions of the nation, with average rainfall exceeding 30 inches (760 mm)
per year.
Tropical
cyclones enhance precipitation across southern sections of the
United States,
as well as Puerto Rico, the United States Virgin Islands, the Northern Mariana Islands, Guam, and American
Samoa.
Chapter III
Closing
A.
Conclusion
Rain is liquid precipitation, as opposed to non-liquid
kinds of precipitation such as snow, hail
and sleet. Rain is one of phenomenon in
Water cycle. In this phenomenon we can find saveral term such as rainfall, intensity and forecasting.
Each term has different meaning.
B.
Suggestion
Should
any readers have any case histories that they feel are of particular
significance, I should be most interested to receive them for possible
inclusion at a later date. Also, any comments or corrections that readers might
have would also be gratefully received.
REFERENCE
Dželalija.2004.Environmental
Physics.Split : University of Molise, University of Split,
Valahia University of Targoviste
Idziak, Adam F,. ____. Geophysics
in Mining And Environmental Protection.
Berlin : Springer
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