II. Weather and Climate (Chapter 10)
Hydrologic cycle (p. 251): The path water takes through the Earth system. Involves evaporation of sea water, condensation into clouds, precipitation, and flow of water back to the sea.
The hydrologic cycle is driven by heat energy originating from the sun, (external energy source).
Evaporation of water involves the transition of liquid water to water vapor, a gas. Water vapor dissolves in the atmosphere, analogous to salt dissolving in water. Water can only dissolve so much salt. At some point, water can dissolve no more salt, and the solution is saturated. Likewise, air can only dissolve so much water. When air is saturated with water, its relative humidity is 100%. If air has absorbed only 50% of the water it can hold, its relative humidity is 50%. Thus relative humidity is a measure of the amount of water is contained in air as a percentage of the maximum amount of water that can be dissolved in air (p. 251-252).
Relative humidity and the amount of water air can hold is related to temperature. If the temperature rises, air can hold more water (analogous to being able to dissolve more salt in hot water than in cold water). If hot salt water is allowed to cool, salt crystals will form at some temperature where the water is saturated in salt. If warm air cools, at some point water will condense as clouds and rain when the air becomes saturated in water and relative humidity is 100%. This temperature is known as the dew point.
Air temperature and relative humidity are two of the most important controls on weather (p. 252-254).
Examples:
Warm air at the surface is less dense than cold air and will rise. As warm air rises, it expands and cools. At some temperature, the relative humidity in the rising air mass reaches 100% and water will condense as clouds.
As winds move moist air from west to east over the Sierra Nevada mountains, air is forced upwards (orographic uplift). As the air rises, it expands and cools to a point where the relative humidity reaches 100%. Clouds form and it rains on the windward side in the mountain range. On the opposite side of the range, the now undersaturated air sinks, compresses and heats up. As the air heats, the relative humidity decreases. Since the air is dry and can absorb more water, it dries the landscape it flows across. This we have deserts on the leeward side of the mountains.
Low pressure systems (p. 258-260):
Rising air decreases pressure in the surface. Low pressure systems are large areas of rising air. Condensation of water in the rising airmass produces storminess associated with low pressure systems. Winds blow into low pressure systems, and in the northern hemisphere, the coriolis effect (p. 254-256) results in counter clockwise rotation about the low pressure system. Cold air flowing into the low pressure system from the north produces cold fronts. Warm air flowing from the south results in warm fronts.
Weather fronts occur where one airmass moves faster than another airmass. At cold fronts, a rapidly moving cold airmass under rides a warm airmass. The warm air is rapidly pushed upwards and expands. Clouds form in the dynamically uplifting warm air, resulting in thunderstorms.
At warm fronts, a rapidly moving warm airmass over rides a slower cold airmass. The warm air gently uplifts, resulting of cloudiness and steady rain over a wide area.
High pressure systems
Sinking air increases pressure at the Earth's surface. High pressure systems are large areas of dense, sinking air. As air sinks, it compresses and heats, thus lowering the relative humidity of the airmass. High pressure systems are characterized by dry weather and a lack of precipitation. Air flows out of a high pressure system, and the coriolis effect results in a clockwise rotation of the system.
Global circulation patterns (p. 256-258):
Hot air at the equator rises, resulting in a permanent low pressure belt known as the intertropical convergence zone.
Warm air rising at the intertropical convergence zone sinks at permanent belts of high pressure known as the subtropical high at 30 degrees north and south latitudes. These permanent high pressure belts give rise to the deserts at 30 degrees north and south latitudes. The subtropical jet occurs at this boundary. Air circulating from the intertropical convergence zone to the subtropical highs form the Hadley cells.
In the northern hemisphere, sinking air in the subtropical high flows northwards and rises at the polar front. The polar front is marked by the polar front jet. The polar front can be thought of as a permanent cold front where cold polar air is pushed below relatively warm air in the Ferrel cell. The coriolis effect results in opposed winds converging at the polar front, and the formation on the low pressure systems that dominate weather in North America (p. 258-259).
Most northern hemisphere storms form as low pressure systems along the polar front. At the polar front, a combination of sinking air ant the coriolis effect results in winds blowing in opposite directions (westerly in the mid-latitudes, and easterly in polar regions. These opposing winds result in shearing along the polar front, and counter-clockwise (cyclonic) rotating air masses. These are low pressure systems. Cold, polar air flows into the low from the north, while warm, moist air flows into the low pressure system from the south. This results in formation of cold and warm fronts.
Low pressure systems pull cold air in from the north and warm air in from the south. This causes the polar front, and the polar jet stream, to bow southwards, forming a trough in the jet stream. Most low pressure systems are located within these troughs (e.g., low pressure troughs). Conversely, high pressure systems are located in ridges
Thunderstorms develop along a cold front through the rapid and dynamic uplift of moist air over advancing cold air. When the temperature difference between warm and cold air masses is large, and when strong jet stream winds blow over the tops of thunderstorms, rotating thunderstorms that generate tornadoes may develop. Many tornadoes form when strong winds blowing over the storm initiated strong horizontal rotations. Powerful updrafts of rising air tip these rotational cells to a vertical position, thus generating a tornado.
Climate and Climate change (p. 245-251):
Seasonal changes: distinct seasons occur because of the Earth's tilt (23.5 degrees) relative to its orbit. This causes variations in the angle that sunlight hits the earth (angle of incidence) and thus variation in the intensity of sunlight.
Over the past 2 million years, the earth has experiences numerous “ice ages” characterized by glaciers advancing over the northern latitudes. Glaciation is associated with cyclical periods of global cooling. Cool periods are interspersed with cyclical warm periods. Cyclical warming and cooling appear to be related to three superimposed long term cycles (see pages 249-251, and figure 10-4):
Precession: Wobble of the earth's rotational axis.
Change in the earth's angle of tilt
Changes in the shape of the earth's orbit about the sun.
Global warming:
The earth is currently experiencing a period of global warming. The current increase in global temperatures coincides with an increase in CO2 concentration in the atmosphere. CO2 is a greenhouse gas, in that it traps solar heat in the atmosphere. CO2 has increased steadily since the beginning of the industrial revolution, at about 1800, as a result of combustion of coal, oil, etc. However, samples of air entrapped in glacial ice show that past warming periods have been associated with increases in CO2, and that CO2 increased most dramatically just prior to a glacial advance. The current period of global warming in increasing CO2 may be a combination of natural and human caused factors.