ATMOSPHERIC MOISTURE, AIR MASSES & FRONTS BY: NEETU SINGH Atmospheric Moisture The fourth element of weather and climate is moisture. This might seem a familiar feature because everyone knows what water is. In actuality, however, most atmospheric moisture occurs not as liquid water but rather as water vapour, which is much less conspicuous and much less familiar. One of the most distinctive attributes of water is that it occurs in the atmosphere in three physical states – solid (snow, hail, sleet, and ice), liquid (rain, water droplets) and gas (water vapour). Of the three states, the gas state is the most important insofar as the dynamics of the atmosphere are concerned. The impact of Atmospheric Moisture on the Landscape When the atmosphere contains enough moisture, water vapour may condense to form haze, fog, cloud, rain, sleet, hail, or snow, producing a skyscape that is both visible and tangible. Precipitation produces dramatic short-run changes in the landscape whenever rain puddles form, streams and rivers flood, or snow and ice blanket the ground. The long-term effect of atmospheric moisture is even more fundamental. Water vapour stores energy that can galvanize the atmosphere into action, as, for example, in the way rainfall and snowmelt in soil and rock are an integral part of weathering and erosion. In addition, the presence or absence of precipitation is critical to the survival of almost all forms of terrestrial vegetation. Water Vapour and the Hydrologic Cycle Water vapour is a colourless, odourless, tasteless, invisible gas that mixes freely with the other gases of the atmosphere. We are likely to become aware of water vapour when humidity is high because the air feels sticky, clothes feel damp, and our skin feels clammy, or when humidity is low because our lips chap and our hair will not behave. Water vapour is a minor constituent of the atmosphere, with the amount present being quite variable from place to place and from time to time. It is virtually absent in some places but constitutes as much as 4 percent of the total atmospheric volume in others. Essentially, water vapour is restricted to the lower troposphere. More than half of all water vapour is found within 1.6 km of Earth’s surface, and only a tiny fraction exists above 6.4 km. The erratic distribution of water vapour in the atmosphere reflects the ease with which moisture can change from one state or another at the pressures and temperatures found in the lower 1 | DIRECTION IAS troposphere. Moisture can leave Earth’s surface as a gas and return as a liquid or a solid. Indeed, there is a continuous interchange of moisture between Earth and the atmosphere. This unending circulation of our planet’s water supply is referred to as the hydrologic cycle, and its essential feature is that liquid water (primarily from the oceans) evaporates into the air, condenses to the liquid (or solid) state, and returns to Earth as some form of precipitation. The movement of moisture through the cycle is intricately related to many atmospheric phenomena and is an important determinant of climate because of its role in rainfall distribution and temperature modification. Evaporation The conversion of moisture from liquid to gas – in other words, from water to water vapour – is called evaporation. This process involves molecular escape: molecules of water escape from the liquid surface into the surrounding air. The molecules of liquid water are continuously in motion and frequently collide with one another. Evaporation can take place at any temperature, but higher temperatures cause molecules to move faster and collide more forcefully. The impact of such collisions near the water surface may provide sufficient energy to allow the molecules to break free from the water and enter the air. The energy absorbed by the escaping molecules is stored as latent heat of vaporization and is released as latent heat of condensation when the vapour changes back to a liquid. Because heat leaves the water as latent heat in the evaporating molecules, the remaining water is cooled, and this process is called evaporative cooling. The effect of evaporative cooling is experienced when a swimmer leaves a swimming pool on a dry, warm day. The dripping wet body immediately loses moisture through evaporation to the surrounding air, and the skin feels the consequent drop in temperature. The amount and rate of evaporation from a water surface depend on three factors: the temperature (of both air and water), the amount of water vapour already in the air, and whether the air is still or moving. 2 | DIRECTION IAS Temperature The water molecules in warm water are more agitated than those in cool water; thus, there is more evaporation from the former. Warm air also promotes evaporation. Just as high water temperature produces more agitation in the molecules of liquid water, so high air temperature produces more agitation in the molecules of all the gases making up the air. The more energetic molecules in warm air bounce around more than do the molecules in cool air. As these bouncing molecules in warm air above a body of water hit the liquid surface, they give some of their energy to the liquid molecules. The energized liquid molecules then evaporate into the warm air. Water molecules cannot keep vapourizing and entering the air without limit, however. Each gas in the atmosphere exerts a pressure, and the sum of all the pressures exerted by the individual gases is what we call atmospheric pressure. The pressure exerted by water vapour in the air is called vapour pressure. At any given air temperature, there is a maximum vapour pressure that water vapour molecules can exert. When there are enough water vapour molecules in air to exert the maximum vapour pressure at any given temperature, we say that the air is saturated with water vapour. When this maximum vapour pressure is exceeded, some water molecules must leave the air and becomes liquid. More and more vapour molecules condense to the liquid state until the vapour pressure exerted by the remaining vapour molecules is right at the maximum value again. The higher the air temperature, the higher the maximum vapour pressure. In other words, the warmer the air, the more water vapour it can hold before becoming saturated. Still versus Moving Air If the air overlying a water surface is almost saturated with water vapour, very little further evaporation can take place. Under these conditions, the rate at which water molecules go from the liquid state to the gas state is about the same as the rate at which they go from the gas state to the liquid state. If the air remains calm and the temperature does not change, there is no net evaporation. If the air is in motion, however, through windiness and/or turbulence, the water vapour (and all other) molecules in it are dispersed more widely. This dispersing of vapour molecules originally in the air at the air-water interface means that that air is now farther from being saturated than it was when it was still. Because the air is now farther from being saturated, the rate of evaporation increases. To summarize, the rate of evaporation from a water surface is determined by the temperature of the water, the temperature of the air, and the degree of windiness. Higher temperatures and greater windiness cause more evaporation. 3 | DIRECTION IAS Evapotranspiration Although most of the water that evaporates into the air comes from bodies of water, a relatively small amount comes from the land. This evaporation from land has two sources: (1) soil and other inanimate surfaces and (2) plants. The amount of moisture that evaporates from soil is relatively minor, and thus most of the land-derived moisture present in the air comes from plants. The process whereby plants give up moisture through their leaves is called transpiration, and so the combined process of water vapour entering the air from land sources is called evapotranspiration. Thus, the water vapour in the atmosphere was put there through evaporation from bodies of water and evapotranspiration from land surfaces. Whether a given land location is wet or dry, depends on the rates of evapotranspiration and precipitation. To analyze these rates, we need to know about a concept called potential evapotranspiration. This is the amount of evapotranspiration that would occur if the ground at the location in question were sopping wet all the time. To determine a value for the potential evapotranspiration at any location, data on temperature, vegetation, and soil characteristics at that location are added to the actual evapotranspiration value in a formula that results in an estimate of the maximum evapotranspiration that could result under local environmental conditions if the moisture were available. In locations where the precipitation rate exceeds the potential evapotranspiration rate, a water surplus accumulates in the ground. In many parts of the world, however, there is no groundwater surplus, except locally and/or temporarily, because the potential evapotranspiration rate is higher than the precipitation rate. Where potential evapotranspiration exceeds actual precipitation, there is no water available for storage in soil and in plants; dry soil and brown vegetation are the result. Measures of Humidity The amount of water vapour in the air is referred to as humidity. It can be measured and expressed in a number of ways, each useful for certain purposes. Absolute Humidity A direct measure of the water vapour content of air is absolute humidity, which is the amount of water vapour in a given volume of air. Absolute humidity is normally expressed in grams of vapour per cubic meter of air (1 gram is approximately 0.035 ounces, and 1 cubic meter is about 4 | DIRECTION IAS 35 cubic feet) and so changes as the volume of air being considered changes.