Page 226 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

190 part II The Water, Weather, and Climate Systems
Rotating drum and graph paper on which relative humidity readings are traced
Inked needle
Bundled strands of human hair are held tightly between two brackets
Mechanism to sense tension from hair strands
(a) The principle of a hair hygrometer.
(b) Sling psychrometer with wet and dry bulbs.
▲Figure 7.12 instruments that measure relative humidity. [Bobbé Christopherson.]
atmospheric Stability
Meteorologists use the term parcel to describe a body of air that has specific temperature and humidity charac­ teristics. Think of an air parcel as a volume of air that is about 300 m in diameter or more.
Two opposing forces decide the vertical position of a parcel of air: an upward buoyant force and a down­ ward gravitational force. A parcel of lower density than the surrounding air is buoyant, so it rises; a rising par­ cel expands as external pressure decreases. In contrast, a
▲Figure 7.13 Principles of air stability and balloon launches. Hot-air balloons being launched in southern Utah illustrate the principles of stability. Why do you think these balloon launches are taking place in the early morning? [Steven k. Huhtala.]
Animation
Atmospheric Stability
Buoyancy force
Lower air pressure
Higher air pressure
3. The rising balloon is like an unstable air parcel, continuing to rise until its temperature becomes cooler than the surrounding air.
Gravitational force
4. When the air in the balloon cools, it increases in density and the balloon sinks with gravity.
2. After the balloon fills with hot air (which is less dense), the balloon rises with the buoyancy force.
parcel of higher density descends under the force of grav­ ity because it is not buoyant; a falling parcel compresses as external pressure increases. The temperature of the volume of air determines the density of the air parcel— warm air has lower density; cold air has higher density. Therefore, buoyancy depends on density, and density depends on temperature. (Look ahead to Geosystems in Action, Figure GIA 7, for an illustration.)
Stability refers to the tendency of an air parcel either to remain in place or to change vertical position by ascend­ ing (rising) or descending (falling). An air parcel is stable if it resists displacement upward or, when disturbed, tends to return to its starting place. An air parcel is unstable if it continues to rise until it reaches an altitude where the surrounding air has a density and temperature similar to its own. The behaviour of a hot­air balloon illustrates these concepts (Figure 7.13). The relative stability of air parcels in the atmosphere is an indicator of weather conditions.
Adiabatic Processes
The stability or instability of an air parcel depends on two temperatures: the temperature inside the parcel and the temperature of the air surrounding the parcel. The
1. The air-filled balloon sits on the ground with the same air temperature inside as in the surrounding environment, like a stable air parcel.