Chapter 6
Atmospheric Moisture, Precipitation and Weather Systems
Water-Vapor-Precipitation System Powered entirely by heat from solar radiation. In order for this matter to become rain/snow, it must first be converted to a liquid or solid form.
1) Land Evaporation/Ocean Evaporation
2) Condensation: requires cooling, first visible evidence, being clouds. Vapor to liquid phase. Occurs when the air is cooled below its dew-point temperature where air would be saturated and contains all the water vapor it can hold.
3) Precipitation
Clouds are made up of minute water particles, both liquid and solid, most of which never produce precipitation. In the clouds that yield precipitation, condensation must advance and add enough water to cloud particles that they become too heavy for the atmosphere to support.
Water Vapor is the most variable gas in the lower atmosphere (0-4% of air). Vapor is also one of the three natural phases of water on Earth and each phase contains heat energy and when water changes phase, energy is exchanged. 25% of solar energy delivered to Earth is used in evaporation.
The maximum amount of water vapor air can hold is dependent on air temperature. The warmer air is, the greater amount of water vapor it can hold. Water vapor is part of atmospheric circulation system and is subject to transfer by wind systems across the planet. Vapor is moved from tropical oceans to midlatitude land areas, for example, huge amounts of energy are also moved. This energy is released as heat when the vapor condenses to form cloud and precipitation particles.
Measures of Humidity
1) Absolute Humidity is a measure of weight in grams of water vapor in a parcel of air with a volume of one cubic meter. It is useful by expressing water content of a large body of air, such as the great air masses that migrate across the midlatitudes. (Saturation absolute humidity: max. capacity of vapor held)
2) Specific Humidity is a measure of weight in water vapor in grams to the weight of the air holding the vapor (including vapor). It is most commonly used in weather analysis as the moisture content of air can be evaluated regardless of changes in its volume.
3) Relative Humidity is overall the most commonly used measure as it expresses vapor intent in grams relative to the amount of vapor that can be held in the same air when it is fully saturated with vapor. (Parcel of air contains 5 grams, but its saturation is at 10 grams, then the relative humidity is 50 percent) Since air’s vapor-holding capacity changes with its temperature, it is necessary to specify the temperature at which a humidity reading is taken.
Dew point: the temperature at which it reaches saturation. If temperature dips below dew point, fog/dew can be expected.
Relative Humidity can exceed 100% under certain conditions (supersaturated) and involves dramatic cooling of air, which is resistant to condensation.
30degrees can hold 6x more water vapor than 0degrees
Air Movement and Moisture Condensation Processes
Advection: warm air, cooling as it falls into contact with a cool surface. (Coastal/snow-covered areas crossed by warm winds) Advection fog often originates when warm, moist air from the Pacific Ocean drifts westward over cold water driven southward by currents.
Atmospheric Instability can be illustrated using a model called the parcel method. This model takes a large bubble of air, called a parcel and measures its changes when it moves upward into the atmosphere.
The vertical motion of a parcel depends on its mass relative to that of the air around it. If it’s lighter, it rises (unstable). If stays (equal) or sinks (heavier), it is considered stable.
1) The rate of temperature change inside a moving air parcel (adiabatic lapse rate).
2) The rate of temperature change at corresponding elevations in the air through which the parcel is moving (ambient atmospheric lapse rate)
Dry Adiabatic Cooling: the rate of cooling in rising, cloudless air (DA lapse rate)
