This lower region is characterised by a fall in air temperature with height averaging about 0.6oC per 100 m 1°F per 300 feet, a very appreciable quantity of water vapour, vertical curren
Trang 1CHAPTER ONE The Atmosphere INTRODUCTION
The Earth with its atmosphere making their daily revolution together could
he likened to an enormous grapefruit having a skin which is thinner than nee paper The difference in this analogy is that the "Skin" around the Earth is an invisible gas termed the atmosphere and held to the Earth by gravitational force Its upper boundary has not yet been positively defined In meteorology
we are concerned almost entirely with the lower region of the atmosphere called the troposphere which extends from the surface to a maximum height
of about 10 miles (compared with the Earth's diameter of about 6.900 miles) Because of its gaseous state, internal motions and physical effects it is mainly responsible for all our "weather" (state of sky, clouds, precipitation, fog, mist and other meteorological phenomena)
The composition of the atmosphere Dry air is composed of a mixture of
gases: within about 10 miles of the Earth's surface which is the zone in which
we are interested the principal ones are Nitrogen (about 78 per cent) and Oxygen (about 21 per cent): there are also small quantities of other gases such
as Argon Carbon Dioxide Helium and Ozone Finally there is a variable amount of water vapour in the atmosphere (see below)
The importance of water vapour The above gases are all, except carbon
dioxide, more or less constant in proportional composition and are essential to life but meteorological interest is centred chiefly on the amount of moisture
(water vapour) in the atmosphere The amount of water vapour present at any
time is very varied because of changes in temperature and in the amount of evaporation from water surfaces and in condensation and precipitation The changing quantities of dust and salt particles in the atmosphere are also of great meteorological importance
VERTICAL SECTION OFTHE ATMOSPHERE
Figure 1 I is a schematic diagram showing a vertical section of the lower part
of our atmosphere which is termed the troposphere and, from our earthbound
viewpoint, is really the "effective atmosphere" This lower region is characterised by a fall in air temperature with height averaging about 0.6oC per 100 m (1°F per 300 feet), a very appreciable quantity of water vapour, vertical currents of air, turbulent eddies and hence formation of cloud
Trang 2layer called the Tropopause, immediately above which we find the
Stratosphere in which temperature change with height is small and a layer of Ozone is found which protects the Earth from harmful effects of ultra violet
radiation Above this comes the Ionosphere which plays such an important
part in the world of radio transmission and reception
Pressure of the atmosphere Our atmosphere comes under the gravitational
force of the earth and although all gases are light they do have weight; the nearer to the Earth the greater the amount of atmosphere pressing down and the greater the weight or atmospheric pressure per square unit area of Earth's surface At sea level the average atmospheric pressure is about 1013.2 hPa: at
a height of 3.000 m this will have fallen to about 670hPa It should be borne
in mind that atmospheric pressure at any point is a force which acts horizontally in all directions as well as upwards and downwards
HEATING OF THE TROPOSPHERE
The atmosphere is transparent to the short-wave radiation from the Sun and receives little or no appreciable heat from this source The Earth, however is heated directly by the Sun's rays and the surface layer of air is warmed by contact with the Earth This warmth is then spread upwards by convection, turbulence and conduction The latter process is, by itself, very slow Thus air temperature in the lower levels tends to follow that of the underlying surface
VARIATION OF TEMPERATURE WITH HEIGHT
(See Lapse Rate in Appendix 1)
Under normal conditions atmospheric temperature decreases with height from the surface up to the tropopause because the heating element (the Earth) has maximum effect at close quarters Above the tropopause air temperature is no longer governed by upward air currents which transfer heat from surface levels The reasons for this will become apparent in later chapters The average lapse rate of temperature within the troposphere is about O.6cC per
100 metres (1oF per 100 feet) The actual lapse rate varies appreciably from day to day and from place to place, especially in levels near the surface where considerable changes often occur within a few hours
Environmental Lapse Rate (ET.R) within the troposphere Figures 5.2 (a) to (d)
show four characteristic graphs of Air temperature v Height within the troposphere The actual values for temperature and height have been omitted
on purpose The SHAPE of the curve is one of the most important factors in the development of clouds, rain, hail, thunder and weather systems
Trang 3The diurnal variation of lapse rate in the lower levels of the troposphere is often very marked over a land surface, especially in fine dry weather with clear skies In the mornings when the Earth is cool a little before sunrise, the lapse rate is small
and inversions (i.e increase of temperature with height see Figure 5.2 (b))
are common After sunrise the land warms rapidly causing an increase in the
temperature lapse rate, and this may become steep (i.e large) by mid or late
afternoon As darkness approaches, the Earth cools once more and its temperature continues to fall throughout the night thereafter the cycle is repeated These effects may be modified or masked at times by the direction and force of wind
Trang 4VARIATION OF PRESSURE WITH HEIGHT
Atmospheric pressure at any level is the weight of the air above that level It follows therefore that the pressure must always decrease with height In the lower levels the average rate at which pressure falls is approximately 1 hPa
per 27.7 m of height, but the actual rate at any given time is governed by
temperature
In Figure 1.2 A & B are two columns of air having the same
cross-sectional area and the same mean sea level pressure, but they have different
mean temperatures
The cold air at B is denser and heavier per unit volume than the warm air at
A but the pressure difference between the top and bottom of each column is
the same Thus column A exerts exactly the same force as column B and the rate at which pressure falls with height must be greater in the cold column
Throught this book the authors have adopted the use of hPa, hectopascals, which is the SI preferred unit rather than mb, millibars Altbough the latter is still commonly used by the media it bas been thought to be sufficient important
to use the preferred unit Fortunately, they are the same numerically