Chapter One CHAPTER 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 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 precipitation and various atmospheric disturbances Then comes a transition Chapter One layer 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 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 Chapter One The 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 Chapter One VARIATION 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 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 crosssectional 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