Page 240 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

204 part II The Water, Weather, and Climate Systems
Stability refers to the tendency of an air parcel, with its water­vapour cargo, either to remain in place or to change vertical position by ascending (rising) or descend­ ing (falling). An air parcel is stable if it resists displace­ ment upward or, when disturbed, it 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 (air temperature) similar to its own.
stability (p. 190)
10. Differentiate between stability and instability of a parcel of air lifted vertically in the atmosphere.
11. Whataretheforcesactingonaverticallymovingpar­ cel of air? How are they affected by the density of the air parcel?
■ Illustrate three atmospheric conditions—unstable, con- ditionally unstable, and stable—with a simple graph that relates the environmental lapse rate (ELR) to the dry adiabatic rate (DAR) and moist adiabatic rate (MAR).
An ascending (rising) parcel of air cools by expansion, re­ sponding to the reduced air pressure at higher altitudes. A descending (falling) parcel heats by compression. Temperature changes in ascending and descending air parcels are adiabatic, meaning they occur as a result of expansion or compression, without any significant heat exchange between the surrounding environment and the vertically moving parcel of air.
The dry adiabatic rate (DAR) is the rate at which “dry” air cools by expansion (if ascending) or heats by compres­ sion (if descending). The term dry is used when air is less than saturated (relative humidity is less than 100%). The DAR is 10 C°·1000 m−1. The moist adiabatic rate (MAR) is the average rate at which moist (saturated) air cools by ex­ pansion on ascent or warms by compression on descent. The average MAR is 6 C°·1000 m−1. This is roughly 4 C° less than the dry rate. The MAR, however, varies with moisture content and temperature and can range from 4 to 10 C° per 1000 m.
A simple comparison of the DAR and MAR in a verti­ cally moving parcel of air with the environmental lapse rate (ELR) in the surrounding air reveals the atmosphere’s stability—whether it is unstable (air parcel continues lift­ ing), stable (air parcel resists vertical displacement), or conditionally unstable (air parcel behaves as though un­ stable if the MAR is in operation and stable otherwise).
adiabatic (p. 191)
dry adiabatic rate (DAR) (p. 191) moist adiabatic rate (MAR) (p. 191)
12. How do the adiabatic rates of heating or cooling in a vertically displaced air parcel differ from the normal lapse rate and environmental lapse rate?
13. Why is there a difference between the dry adiabatic rate (DAR) and the moist adiabatic rate (MAR)?
14. Whatatmospherictemperatureandmoisturecondi­ tions would you expect on a day when the weather is unstable? When it is stable? Relate in your answer what you would experience if you were outside watching.
15. Use the “Atmospheric Stability” animation in the Mastering GeographyTM Study Area. Try different temperature settings on the sliders to produce stable and unstable conditions.
■ Identify the requirements for cloud formation, and explain the major cloud classes and types, including fog.
A cloud is an aggregation of tiny moisture droplets and ice crystals suspended in the air. Clouds are a constant reminder of the powerful heat­exchange system in the en­ vironment. Moisture droplets in a cloud form when satu­ ration and the presence of cloud-condensation nuclei in air combine to cause condensation. Raindrops are formed from moisture droplets through either the collision–coalescence process or the Bergeron ice-crystal process.
Low clouds, ranging from surface levels up to 2000 m in the middle latitudes, are stratus (flat clouds, in layers) or cumulus (puffy clouds, in heaps). When stratus clouds yield precipitation, they are nimbostratus. Sometimes near the end of the day, lumpy, grayish, low­level clouds called stratocumulus may fill the sky in patches. Middle­ level clouds are denoted by the prefix alto-. Altocumu- lus clouds, in particular, represent a broad category that includes many different types. Clouds at high altitude, principally composed of ice crystals, are called cirrus. A cumulus cloud can develop into a towering giant cumu- lonimbus cloud (-nimbus in Latin denotes “rain storm” or “thundercloud”). Such clouds are called thunderheads be­ cause of their shape and their associated lightning, thun­ der, surface wind gusts, updrafts and downdrafts, heavy rain, and hail.
Fog is a cloud that occurs at ground level. Radiative cooling of a surface that chills the air layer directly above the surface to the dew­point temperature creates saturated conditions and a radiation fog. Advection fog forms when air in one place migrates to another place where conditions exist that can cause saturation—for example, when warm, moist air moves over cooler ocean currents. Upslope fog is produced when moist air is forced to higher elevations along a hill or mountain. Another fog caused by topogra­ phy is valley fog, formed because cool, denser air settles in low­lying areas, producing fog in the chilled, saturated layer near the ground. Another type of fog resulting from evaporation and advection forms when cold air flows over the warm water of a lake, ocean surface, or swimming pool. This evaporation fog, or steam fog, may form as the water molecules evaporate from the water surface into the cold overlying air.
cloud (p. 194)
moisture droplet (p. 194) cloud-condensation nuclei (p. 194) stratus (p. 196)
nimbostratus (p. 196)
cumulus (p. 196)
stratocumulus (p. 196) altocumulus (p. 196)
cirrus (p. 196)
cumulonimbus (p. 197)
fog (p. 197)
radiation fog (p. 197)
rime fog (p. 197)
ice-crystal fog (p. 197)
advection fog (p. 197)
upslope fog (p. 198)
valley fog (p. 198)
evaporation fog (p. 198)