Page 239 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 7 Water and Atmospheric Moisture 203
concepts review
key leARNiNG
■ Describe the heat properties of water, and identify the traits of its three phases: solid, liquid, and gas.
Water is the most common compound on the surface of Earth, and it possesses unusual solvent and heat char­ acteristics. Owing to Earth’s temperate position rela­ tive to the Sun, water exists here naturally in all three states—solid, liquid, and gas. A change from one state to another is a phase change. The change from liquid to solid is freezing; from solid to liquid, melting; from vapour to liquid, condensation; from liquid to vapour, vaporization or evaporation; from vapour to solid, depo­ sition; and from solid to vapour, sublimation.
The heat energy required for water to change phase is latent heat because, once absorbed, it is hidden within the structure of the water, ice, or water vapour. For 1 g of water to become 1 g of water vapour by boil­ ing requires the addition of 540 cal, or the latent heat of vaporization. When this 1 g of water vapour con­ denses, the same amount of heat energy, 540 calories, is liberated and is the latent heat of condensation. The latent heat of sublimation is the energy exchanged in the phase change from ice to vapour and vapour to ice. Weather is powered by the tremendous amount of latent heat energy involved in the phase changes among the three states of water.
phase change (p. 183)
sublimation (p. 183)
latent heat (p. 184)
latent heat of vaporization (p. 185) latent heat of condensation (p. 185) latent heat of sublimation (p. 185)
1. Describe the three states of matter as they apply to ice, water, and water vapour.
2. What happens to the physical structure of water as it cools below 4°C? What are some visible indications of these physical changes?
3. What is latent heat? How is it involved in the phase changes of water?
4. Take 1 g of water at 0°C and follow the changes it undergoes to become 1 g of water vapour at 100°C, describing what happens along the way. What amounts of energy are involved in the changes that take place?
■ Define humidity and relative humidity, and explain dew-point temperature and saturated conditions in the atmosphere.
The amount of water vapour in the atmosphere is humidity. The maximum water vapour possible in air is principally a function of the temperature of the air and of the water vapour (usually these temperatures are the same). Warmer air produces higher net evaporation rates and maximum possible water vapour, whereas cooler air can produce net condensation and lower the possible water vapour.
Relative humidity is a ratio of the amount of water va­ pour actually in the air to the maximum amount possible at a given temperature. Relative humidity tells us how near the air is to saturation. Relatively dry air has a lower rela­ tive humidity value; relatively moist air has a higher rela­ tive humidity percentage. Air is said to be at saturation when the rate of evaporation and the rate of condensation reach equilibrium; any further addition of water vapour or temperature lowering will result in active condensation (100% relative humidity). The temperature at which air achieves saturation is the dew-point temperature.
Among the various ways to express humidity and rela­ tive humidity are vapour pressure and specific humidity. Vapour pressure is that portion of the atmospheric pressure produced by the presence of water vapour. A comparison of vapour pressure with the saturation vapour pressure at any moment yields a relative humidity percentage. Specific humidity is the mass of water vapour (in grams) per mass of air (in kilograms) at any specified temperature. Because it is measured as a mass, specific humidity does not change as temperature or pressure changes, making it a valuable measurement in weather forecasting. A comparison of spe­ cific humidity with the maximum specific humidity at any moment produces a relative humidity percentage.
Two instruments measure relative humidity, and in­ directly the actual humidity content of the air; they are the hair hygrometer and the sling psychrometer.
humidity (p. 186)
relative humidity (p. 186) saturation (p. 187)
dew-point temperature (p. 187) vapour pressure (p. 188) specific humidity (p. 189)
hair hygrometer (p. 189)
sling psychrometer (p. 189)
5. What is humidity? How is it related to the energy present in the atmosphere? To our personal comfort and how we perceive apparent temperatures?
6. Define relative humidity. What does the concept rep­ resent? What is meant by the terms saturation and dew-point temperature?
7. Using Figures 7.10 and 7.11, derive relative humidity values (vapour pressure/saturation vapour pressure; specific humidity/maximum specific humidity) for levels of humidity in the air different from the ones presented as examples in the chapter discussion.
8. How do the two instruments described in this chap­ ter measure relative humidity?
9. How does the daily trend in relative humidity values compare with the daily trend in air temperature?
■ Define atmospheric stability, and relate it to a parcel of air that is ascending or descending.
In meteorology, a parcel of air is a volume (on the order of 300 m in diameter) that is homogenous in tempera­ ture and humidity. The temperature of the volume of air determines the density of the air parcel. Warm air has a lower density in a given volume of air; cold air has a higher density.