Page 223 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 7 Water and Atmospheric Moisture 187
Maximum water vapour possible
Water vapour
50% relative humidity
11 A.M.
Warmer air— greater maximum water vapour possible
Cooler air— lesser maximum water vapour possible
Water vapour
100% relative humidity
5 A.M.
Water vapour
20% relative humidity
5 P.M.
Saturation
▲Figure 7.7 Water vapour, temperature, and relative humidity. The maximum water vapour possible in warm air is greater (net evaporation more likely) than that possible in cold air (net condensation more likely), so relative humidity changes with temperature, even though in this example the actual water vapour present in the air stays the same during the day.
decrease in temperature that reduces the evaporation rate results in active condensation (forming clouds, fog, or precipitation).
The temperature at which a given sample of vapour­containing air be­ comes saturated and net condensa­ tion begins to form water droplets is the dew-point temperature. The air is saturated when the dew­point temperature and the air temperature are the same. When temperatures are below freezing, the frost point is the temperature at which the air becomes saturated, leading to the formation of frost (ice) on exposed surfaces.
A cold drink in a glass provides a familiar example of these conditions (Figure 7.8a). The air near the glass chills to the dew­point temperature
Dew (active condensation)
places actual water vapour in the air as the numerator and water vapour possible in the air at that temperature as the denominator:
Actual water vapour in the air
Relative humidity 5 Maximum water vapour possible 3 100 in the air at that temperature
Warmer air increases the evaporation rate from water surfaces, whereas cooler air tends to increase the conden­ sation rate of water vapour onto water surfaces. Because there is a maximum amount of water vapour that can exist in a volume of air at a given temperature, the rates of evaporation and condensation can reach equilibrium at some point; the air is then saturated, and the balance is saturation equilibrium.
Figure 7.7 shows changes in relative humidity throughout a typical day. At 5 a.m., in the cool morning air, saturation equilibrium exists, and any further cooling or addition of water vapour produces net condensation. When the air is saturated with maximum water vapour for its temperature, the relative humidity is 100%. At 11 a.m., the air temperature is rising, so the evaporation rate exceeds the condensation rate; as a result, the same volume of water vapour now occupies only 50% of the maximum possible capacity. At 5 p.m., the air tempera­ ture is just past its daily peak, so the evaporation rate exceeds condensation by an even greater amount, and relative humidity is at 20%.
Saturation and Dew Point Relative humidity tells us how near the air is to saturation and is an expression of an ongoing process of water molecules moving between air and moist surfaces. At saturation, or 100% relative humidity, any further addition of water vapour or any
Cold glass chills the surrounding air layer to the dew-point temperature
(a) When the air reaches the dew-point temperature, water vapour condenses out of the air and onto the glass as dew.
(b) Cold air above the rain-soaked rocks is at the dew point and is saturated. Water evaporates from the rock into the air and condenses in a changing veil of clouds.
▲Figure 7.8 Dew-point temperature examples. [Robert Christopherson.]