Chapter Three CHAPTER THREE Humidity and Condensation Water vapour is water in the vapour state and although quite invisible, is always present in the atmosphere The amount varies in both time and place It is supplied by evaporation from the sea surface and to much lesser degree from lakes, rivers, snow, ice and other moist surfaces on land Cloud and fog should not be confused with water vapour-they are visible water droplets in the liquid state Saturated air The maximum quantity of water vapour which can be contained in a given volume of air is limited by temperature The higher the temperature the greater the quantity of water vapour the air can hold When the maximum possible amount is present the air is said to be saturated Air which contains less water vapour than the maximum possible amount is said to be unsaturated (Air which is unsaturated is often called dry air although it contains some water vapour) If unsaturated air is cooled sufficiently it will become saturated Further cooling will result in the excess water vapour being condensed into the liquid or solid state (visible water droplets or ice crystals) Dew-point is the temperature to which unsaturated air must be cooled to become saturated So long as air remains unsaturated and the amount of water vapour remains constant, the dew-point will also remain constant although the air temperature changes If air is saturated its temperature is its dew point Absolute humidity is the water vapour content of the air at anyone time and place expressed in mass per unit volume-usually in grammes per cubic metre (gm/m3) Relative humidity is the ratio of the amount of water vapour actually present to the maximum amount possible at the same temperature It is expressed as a percentage Refer now to Figure 3.1 which is a graph showing the approximate saturation values of absolute humidity against air temperature Exact values 10 Chapter Three are of little or no concern to the mariner but the shape of the curve is of immense importance and this will become apparent in later chapters The following should be noted: At a temperature of, say, 20oC the maximum possible amount of water vapour which the air can contain is about 20 g/m3, whereas at 40oC it can hold about 50g/m3 Thus, the higher the air temperature the greater the saturation value of absolute humidity Suppose a value of air at a temperature of 40oC contains 20 g/m3 of water vapour (a) Relative humidity = 20 = 40% 50 (b) If the air is cooled sufficiently it will become saturated at about 20°C The dew-point is then 20°C The dew-point is constant through any changes of temperature above 20°C (c) Should the temperature fall below the original dew-point (20°C) to, say l0oC, then the new dew-point would be l0oC, the absolute humidity 11 g/m3 and a total of (20 - 11) = g/m3 of water vapour would have been condensed into visible droplets (eg fog, mist, cloud or dew) During such cooling below 20°C, the dew-point will at all stages, be equal to the existing air temperature The curve shows that a fall in temperature of saturated air results in the condensation of the excess water vapour The higher the initial dew-point the greater the amount of water vapour condensed HYGROSCOPIC NUCLEI Within the troposphere there is always present a varying quantity of very minute solid particles, such as dust,* salt from sea spray, soot and smoke from domestic and factory fires, plus other sources of pollution Most of these particles are hygroscopic - that is they tend to attract and absorb moisture They are the nuclei onto which water vapour condenses, forming visible water droplets (cloud or fog) Without their presence any condensation would be highly improbable They are most abundant in the levels near the surface from whence they are carried upwards to higher levels by convection In those industrial areas where there is a very high degree of smoke pollution, fog can sometimes form in falling temperatures a little before the dew-point has been reached *Dust is not generally considered to be hygroscopic 11 Chapter Three 12 Chapter Three QUESTIONS: REMEMBER TO USETRE GLOSSARY (Appendix One) TEST QUESTIONS Chapters to (inc) Describe briefly the characteristics and extent of the tropopause What is the average lapse rate of temperature within the troposphere? In spite of the relatively high surface temperatures within the tropics, the temperature at the tropopause over the equator is much lower than over the polar caps Explain why this is so Name and describe the processes whereby heat is transferred from the Earth's surface to the troposphere Write brief notes on the diurnal, variation of temperature lapse rate in the lower levels of the troposphere Explain why atmospheric pressure decreases with height above sea level (a) What is the approximate average rate of fall of atmospheric pressure within the lower levels' (b) On what does the actual rate depend? Explain why Solar radiation and temperature Describe the greenhouse effect on the Earth's atmosphere Define specific heat 10 (a) Describe briefly each of the main factors on which the maximum temperature attained by a land surface, for a given amount of solar radiation, depends (b) Account fully for the relatively very small daily changes in the temperature of the sea surface 13