Atmospheric Water Vapour The concentration of the invisible gas, water vapour varies greatly from place to place and from time to time. In warm tropical areas it may be 4% of the atmospheric gases while in cold polar latitudes it may dwindle to a mere trace.
Different states of water vapour Equilibrium undeveloped, i.e. air is unsaturated & water evaporates from the surface State of Equilibrium i.e. water molecules leave & return in equal numbers Super-saturation, i.e. air is saturated & water molecules return to liquid state
Vapour Pressure n the total air pressure = the sum of the partial pressures for nitrogen + oxygen + water vapour + others
Saturation vapour pressure n The amount of water vapour that can be held in the air depends upon the temperature
Humidity n Water vapour exists in the atmosphere as an invisible gas. n The measure of the air’s water vapour content is referred to as it’s humidity and is measured in different ways.
Humidity n Water vapour stores latent heat which is released during condensation and is one of the most important atmospheric sources of energy. n Condensation results in water droplet formation and hence cloud, fog and mist which obscure visibility.
Humidity n The main source of atmospheric humidity is evaporation and as the earth’s surface is mostly covered by oceans the amount of evaporation is huge.
Absolute Humidity n is a measure of the actual amount of water vapour present in a given volume of air. n It is expressed as g/m 3 dry air.
Humidity Mixing Ratio (HMR) n Humidity can also be expressed in ways that are not influenced by changes in volume. n The humidity mixing ratio is the ratio of the mass of water vapour present relative to the mass of dry air in the air parcel. n It is expressed as g/kg of dry air.
HMR, cont’d n The HMR remains constant as long as the moisture content remains the same n warmer air has a greater capacity to hold water vapour than cold air
HMR (cont.) n At the higher temperatures the water vapour molecules have a greater kinetic energy (average speed). Thus at high temperatures the molecules will have sufficient energy to remain as vapour. n As the temperature lowers, the average speed of the molecules decreases and hence fewer molecules have the energy to remain as vapour.
Saturated Vapour Content G/M 3 Saturation Line Dew Point
Saturation n When air at a particular temperature contains the maximum amount of water vapour possible, the air is said to be saturated. n Referring to the Vapour Capacity graph, it can be seen that at 30°C, the maximum vapour capacity is 30gm/m3. n The air is saturated.
Saturated Humidity Mixing Ratio n Any cooling below the temperature and the air would be supersaturated and condensation would occur to establish a vapour capacity equilibrium at the lower temperature. n The air would be at its saturated humidity mixing ratio (SHMR). n The SHMR is the value of the humidity mixing ratio at a given temperature and pressure when the air is saturated.
Dew Point n Is the temperature at which a sample of air would just become saturated with respect to a plane surface of water if cooled at constant pressure. n The water still exists as water vapour at saturation.
Dew Point n Cooling below the dew point will initiate condensation in the presence of condensation nuclei. n If no nuclei are present the water content will remain as water vapour. n The air is then described as being “supersaturated.”
Relative Humidity n Indicates the relative degree of saturation of the air. n At saturation RH = 100% Relative Humidity = Absolute Humidity Saturated Vapour Concentration
RH Calculation n Referring to the Vapour Capacity curve: n Absolute Humidity at 20°C = 10 gm/m 3 n Saturated Vapour Concentration at 20°C = 15 gm/m 3 10 15 x 100% = 67%RH =
Wet Bulb Temperature n Is the lowest temperature to which air can be cooled by the evaporation of water. n Note: Wet-bulb temperature must not be confused with Dew-point temperature. n Wet-bulb temperature is always between the Dew-point and Dry-bulb temperatures
Wet Bulb (Cont.) n If air is cooled towards it’s dew-point; n The Relative Humidity will increase; n Dry Bulb and Wet Bulb temperatures will decrease until; n the Dew-point is reached at which time: u Dew-point = Wet-bulb = Dry-bulb Temperature.
Points to note: n Dew-point is the best indicator as to the actual water vapour content of an air mass. n Dew points change very little during the day unless: u there is evaporation from wet ground or; u precipitation occurs or; u there is a change of air mass.
Hygrometer n Used to measure humidity u Electrical F Used in radiosondes u Infrared
Change of State n Water exists in three states, namely: u ice(solid), u water(liquid) or u water vapour(gas). n Every time a substance changes state, latent heat is involved and it is important to understand its role in weather processes.
Heat Input/Output n Heat input/output appears in 2 ways: u Sensible Heat which involves: F An observable change in temperature. u Latent Heat which involves F no observable change in temperature:
Heat Input/Output. Cont’d n Latent Heat involves a change of state. u Solid Liquid. u Liquid Gas. n Latent heat is also involved when change of state is directly from: u Solid Gas. n In which case the process is known as u ‘Sublimation.’
Heating/Cooling of a Liquid n Heat input/output is usually expressed in u Joules, F 1 joule is the amount of heat necessary to raise the temperature of 1 kg of water 1°C, or; u Calories, F 1 calorie is the amount of heat necessary to raise the temperature of 1 gram of water 1°C.
Deposition/Sublimation n Hoar frost comes from a deposition process n Airframe ice may reduce slowly in sub-zero air by sublimation
Latent Heat (cont.) n Most of the energy in the atmosphere is stored in the form of latent heat of evaporation. n This is released during atmospheric cooling as as latent heat of condensation. n Latent heat release is thus the major energy source in all weather systems especially thunderstorms and tropical revolving storms.