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Chapter 3
AGRICULTURAL METEOROLOGY
3.1. Introduction
Meteorology is the whole science of the atmosphere. It includes not only the physics, chemistry
and dynamics of the atmosphere, but also many of its direct effect upon the earth's surface, the
oceans and life in general.
What is understood today about the nature of atmosphere, weather and climate is the
culmination of centuries of painstaking inquiry from scientists from many disciplines. Physicists,
chemists, astronomers, and other applied basic principles in unlocking the mysteries of the
atmosphere. The roots of modern meteorology in fact, extend back to the 4th century B.C. and
Aristotle's "Meteorological", the first treatise on atmospheric science.
Although much progress has been made, many questions remain regarding the workings of the
atmosphere, weather and climate.
Modern atmospheric scientists (meteorologists and
climatologists) continue the efforts of their predecessors and although armed with sophisticated
tools such as satellites and electronic computers, they still rely on the scientific method of
investigation.
Weather is the state of the atmosphere at some place and time, described in terms of such
variables as temperature, cloudiness, precipitation and wind. On the other, climate is a weather
condition at some locality averaged over a specified time period, but climate encompasses more
than this. Departures from averages and extremes in weather are also important aspects of climate.
Climate can be considered the ultimate environmental control in that climate determines, for
example, what crops to be cultivated, the long-term water supply, and the average heating and
cooling requirements of homes.
The atmosphere, where weather takes place, encircles the globe as a relatively thin envelope of
gases and tiny suspended particles. In fact, 99% of the atmosphere's mass is confined to a layer
having a thickness that is only about 0.25% of the earth's diameter. And yet, the thin atmospheric
skin is essential for life and the orderly functioning of physical and biological processes on earth.
The atmosphere shield organisms from exposure to hazardous levels of ultraviolet radiation, it
contains the gases required for the life-sustaining processes of cellular respiration and
photosynthesis; and it supplies the water needed by all life.
The Origin of the Atmosphere
The origin of the atmosphere on logical grounds was not a unique event. It was inextricably
associated with evolutionary processes that brought the earth out of seeming cosmic chaos.
According to one popular view of how this event occurred, an accretion of primordial gases and
particles somehow discharged or otherwise separated from the sun, resulted in an initially molten
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earth. A contrary argument suggests that the earth condensed out of gases and dusts representing the
debris of earlier stellar cataclysms. Subsequent gravitational heating, coupled with radioactive
disintegration at a rate of about 15 times the present one, melted the cosmic dust pile into the
spheroidal earth. In either case, the primordial gases became so hot that individual molecular
energies carried this original atmosphere away from earth's gravity to be lost in space forever.
Subsequently as the earth grow colder, a solid crust began to form over a molten core. Gases,
such as carbon dioxide, nitrogen, and water vapor, previously dissolved in the molten magma, were
slowly released. These gases began to form a new atmosphere. Perhaps the original composition
was similar to the present emanations from volcanoes; 60-70 percent water vapor, 10-15 percent
carbon dioxide, 8-10 percent nitrogen, with sulfur compounds making up the remainder.
As cooling proceeded, liquid water existed along with the vapor and clouds formed. Then came
the rains, but only in the upper atmosphere. As falling raindrops neared the red-hot crust, they
evaporated again and the cycle was repeated.
For hundreds of thousands of years, a dark, dank cloud cover hung over the earth. When the
crust finally cooled water reached the earth in the form of torrential rains lasting more than 40,000
years! The precipitation carried away the gaseous carbon dioxide that ultimately combined with the
eroding earth to form enormous beds of carbonate rocks.
Oxygen seems to have been a late comer. According to one theory, as water vapor rose to the
uppermost layers of the atmosphere, by convective transport perhaps, the water molecules were
dissociated into atomic oxygen and atomic hydrogen by the action of solar ultraviolet (UV)
radiation. The hydrogen diffused away and escaped from the atmosphere; the oxygen combined to
form molecular gas. On the other hand, perhaps plant life liberated the oxygen during the process of
photosynthesis, although where the plants originally obtained the necessary oxygen with which to
live is uncertain.
In any event, all evidence suggests that little or no major change in the composition of the
atmosphere, except for perhaps a slow but steady increase in the amount of oxygen, has occurred
during the most recent 2 billion years of the earth's 4-6 billion years of existence.
Temperature Profile of the Atmosphere
For convenience of study, the atmosphere is usually subdivided into concentric layers according
to the vertical profile of the average air temperature:
a. Troposphere - extends from the earths surface to an average of altitude of ranging from
20 km at the equator down to 8 km at the poles. This were most weather
occurs. Normally, but not always, the temperature within the troposphere
decreases, with altitude. Hence, air temperatures on mountain topes are
usually lower than in surrounding lowlands.
b. Tropopause - extends zone between troposphere and the next layer, the stratosphere.
c. Stratosphere - extends from the tropopause up to about 50 km. On average, in the lower
portion of the stratosphere, the temperature does not change with altitude.
This level is ideal for jet aircraft travel since it is above the weather with
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d.
e.
f.
g.
excellent visibility and generally smooth flying conditions.
Stratopause - about 20 m above the stratosphere, the temperature increases with altitude.
This is the transition zone between the stratosphere and the next layer, the
mesosphere.
Mesosphere - the temperature once again, decreases with increasing altitude.
Mesopause - about 80 km above the earth's surface, features the lowest average
temperature in the atmosphere, -90oC or -130oF.
Thermosphere - temperatures at first are constant and then increase rapidly with altitude.
Within this layer, temperature is more variable with time than in any
other region of the atmosphere.
Relative Proportion of Gases Composing Dry Air in the Lower Atmosphere (Below 80 km)
Gas
% by Volume
Nitrogen
Oxygen
Argon
Carbon dioxide
Neon
Helium
Gas
78.08
20.95
0.93
0.035
0.0018
0.00052
Methane
Krypton
Nitrous oxide
Hydrogen
Ozone
Xenon
% by Volume
0.00014
0.00010
0.00005
0.00005
0.000007
0.000009
Some Typical Atmospheric Phenomena
1. Clouds - Water vapor is an invisible gas, but the condensation and deposition products of
water vapor are visible. Clouds are the visible manifestations of the condensation
and deposition of water vapor with in the atmosphere. They are composed of tiny
water droplets or ice crystals or a mixture of both.
Cloud Classification
---------------------------------------------------------------------------------------------------------------Genus
Altitude above
Shape and
the ground (km)
appearance
---------------------------------------------------------------------------------------------------------------High clouds
Cirrus (Ci)
6-18
Delicate streaks or patches
Cirrostratus (Cs)
6-18
Transparent thin white sheet or veil
Cirrocumulus (Cc)
6-18
Layer of small white puffs or ripples
Middle clouds
Altostratus (As)
Altocumulus (Ac)
2-6
2-6
Uniform white or gray sheet or layer
White or gray puffs or waves in patches or
layers
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Low clouds
Stratocumulus (Sc)
9-2
Stratus (St)
Nimbostratus (Ns)
0-2
0-4
Patches or layer of large rolls or merged
puffs
Uniform gray layer
Uniform gray layer from which precipitation
is falling
Clouds with vertical development
Cumulus (Cu)
0-3
Detached heaps or puffs with sharp outlines
and flat base, and slight or moderate vertical
extent.
Cumulonimbus (Cb)
0-3
Large puffy clouds of great vertical extent
with smooth or flattened tops, frequently
anvil-shaped from which showers fall with
thunder
---------------------------------------------------------------------------------------------------------------2. Fog - a visibility-restricting suspension of tiny water droplets or ice crystals in an air layer
next to the earth's surface. Simplify, fog is a cloud in contact with the ground. By
international convention, fog is defined as restricting visibility to 1000 m (1 km) or
less (for aviation purposes, the criterion for fog is a visibility restriction of 10 km or
less); otherwise the suspension is called "mist". (The popular definition of mist is
light drizzle). Fog may develop when air becomes saturated through radiational
cooling, advective cooling, the addition of water vapor, or expansional cooling.
3. Precipitation - water in solid or liquid form that falls to the earth's surface.
4. Halo - a whitish ring of light surrounding the sun, or sometimes, the moon. It is formed
when the sun's rays are refracted by tiny ice crystals that compose high, thin clouds as
cirrostratus. Refraction is the bending of light as it passes from one transparent
medium (such as air) into another transparent medium (such as ice or water). The
light rays bend because the speed of light is greater in air than in ice or water.
5. Sundogs - bright spots appearing on either side of the sun are caused by refraction of
sunlight by ice crystals suspended in the atmosphere. They are called sundogs
because they appear to follow the sun around the sky.
6. Rainbow - caused by a combination of refraction and reflection of sunlight (or rarely, of
moonlight) by raindrops. Sunlight striking a shaft of falling raindrops is
refracted and internally reflected by each drop of rain.
7. Corona - a series of alternating light and dark rings that surround the moon, or less after,
the sun. Typically, a corona is far smaller than a halo. It is caused by diffraction
of light around water droplets that compose a thin, translucent veil of
altocumulus, altocumulus lenticularis, or cirrocumulus. Diffraction is the slight
bending of a light wave as it moves along the boundary of an object such as a
water droplet.
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8. Glory - concentric rings of color around a shadow on the clouds below. To see a glory, an
observer must be in bright sunshine. Above a cloud or fog layer, and the sun must
be situated so as to cast the observer's shadow on the clouds below.
9. Thunderstorm - the blackening of the sky and abrupt freshening of wind followed by
bursts of torrential rain, flashes of lightning and rumbles of thunder. It is a
small scale system and thus affects a relatively small area and is shortlived. It is the product of very strong convection that extends deep into the
troposphere, sometimes reaching to the tropopause or higher.
10. Lightning - a brilliant flash of light produced by an electrical discharge of about 100
million volts. On a clear day, the earth's surface is negatively charged and the
upper troposphere is positively charged. As a cumulonimbus cloud develops,
however, charges separate within the cloud such that the upper region of the
could becomes positively charged and the cloud base becomes negatively
charged. The negatively-charged cloud base then induces a positive charge
on the portion of the ground directly under the cloud. Air is a very good
electrical insulator, and so, as a thunderstorm cells forms and electrical
charges build, a tremendous potential soon develops for an electrical
discharge, that is, lightning. When the thunderstorm cell reaches its mature
stage, the electrical resistance of the air breaks down and lightning occurs,
thereby neutralizing the electrical charges. Lightning may take a path between
the positive and the negative portions of a cloud, or between clouds, or
between a cloud and the ground.
11. Thunder - lightning heats the air along the narrow conducting path to temperatures that
may exceed 25,000oC (45,000oF). Such intense heating expands the air
violently and initiates a sound wave that we hear as thunder.
12. Tornado - a small mass of air that whirls rapidly about an almost vertical axis. It is made
visible by clouds and by dust and debris sucked into the system. Tornados are
approximately funnel-shaped although a variety of forms have been observed,
ranging from cylindrical masses of nearly uniform lateral dimensions to long,
slender, rope-like pendants. By convention, when the circulation remains a left,
it is termed a "funnel cloud", but when it touches down on the ground, it is
called a tornado.
13. Waterspout - a tornado-like disturbance that occurs over the ocean or over a large inland
lake. It is so named because it consists of a whirling mass of water that
appears to stream out of the base of the parent cumulonimbus cloud. A
waterspout is usually considerably less energetic, smaller and shorter lived
than a tornado. The rare intense waterspout may well be a tornado that
formed over land and then moved over a body of water.
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14. Hurricane - violet oceanic cyclones that originate in tropical latitudes, usually in the late
summer and fall (autumn) of the year, has a maximum wind speed greater
than 119 km/hour.
a. Tropical depression - a developing tropical disturbance with maximum wind speeds
topping 37 km/hour.
b. Tropical storm - tropical disturbance with maximum wind speeds reaching 63
km/hour.
15. Typhoon - hurricane or intense tropical storm that develop over the Pacific Ocean.
16. Cyclone - a weather system characterized by relatively low surface air pressure compared
with the surrounding air.
Reference:
MORAN, J.M. and M.D. MORGAN. 1989. Meteorology: The Atmosphere and the Science of
Weather. Second Edition. Macmillan Publishing Company, New York.