Atmospheric Moisture

Atmospheric Moisture This chapter discusses: 1. The role of water in the atmosphere 2. Terms and definitions for atmospheric moisture 3. The impact of moisture on comfort

Structure of Water Figure 5.1 Figure 5.2 Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere. Sublimation & deposition describe the non-incremental changes between solid and vapor phases.

Evaporating Water into Air Liquid water experiences dynamic departures of water molecules from its surface, called evaporation, together with arrivals of molecules from adjacent vapor, called condensation. When air is saturated, evaporation and condensation are in equilibrium. Figure 5.3A

Condensation onto Nuclei Condensation of water vapor into liquid water is more likely to occur when the vapor cools and slows, and attaches to nuclei. Heated vapor moves so fast that it bounces away from condensation nuclei Figure 5.4A

Closing the Hydrologic Cycle Figure 5.5 Evaporation, and transpiration by plants, transports liquid water into atmospheric vapor. Condensation converts this gas back to a liquid droplet, which may then fall as precipitation to ground or surface water supplies.

Descriping Atmospheric Moisture Atmospheric water vapor has been defined several different ways. These terms include absolute humidity, specific humidity, mixing ratio, vapor pressure, and relative humidity. Figure 5.6

Absolute & Specific Humidity For a given mass of water vapor in an air parcel, the absolute humidity changes as the parcel volume changes (e.g., lifts or descends). Figure 5.7 Specific humidity is concerned with the mass of vapor to mass of air, and is not affected by changes in parcel volume. Figure 5.8

Specific Humidity vs. Saturation Warm air can absorb more vapor than cold air, so for a given parcel of air, specific humidity declines from its highest in the tropics to its lowest in the colder poles. Desert air, at 30°, however, is not more saturated than polar air. Figure 5.9

Determining Vapor Pressure Average atmospheric pressure of 1013 mb is comprised in part by the weight of vapor molecules. Warmer air can absorb more vapor than cooler air before it saturates, and can have higher saturated vapor pressures. Figure 5.10

Relative Humidity Trends Figure 5.11 Relative humidity (RH) indicates air parcel proximity to saturation. Saturation can be achieved, or RH increased, by adding more water or dropping the air temperature. Dew point is the temperature at which saturation occurs.

Seasonal Dew Point Maps Figure 5.12B Figure 5.12A January, July, dew point

Dew Point vs. Relative Humidity Dew point is the temperature for saturation, and used with a vapor pressure curve reveals the mass of vapor in the air. While relative humidity may be higher in polar air, more water is actually absorbed in desert air. Figure 5.13A

Relative and Specific Humidity Figure 5.14 Relative humidity (RH) as an indicator of saturation reveals that desert air is far from saturated, and that cold polar air nears saturation. Graphs of RH contrast with specific humidity in the deserts and poles. Figure 5.9

Sources of Moisture Figure 5.15 Patterns of US humidity are strongly governed by wind direction and ocean temperatures. Cooler Pacific waters create lower humidities in the west, while warmer Gulf waters generate high humidity along the southeast and east coast.

Relative Humidity and Comfort Unsaturated air may absorb more water from the evaporation of human sweat. The departure of fast moving, and by definition higher temperature, water molecules into the vapor phase cools the human skin. In winter, this process can make a dry house extra chilly.

Heat Index & Safety Human perception of temperature is distinct from measured air temperature, and is particularly different at higher humidities when the human body is less efficient at sweating and self-cooling. On hot days, fans that move saturated air away from the skin help humans avoid unwanted heat syndromes. Figure 5.16

Sling Psychrometer Figure 5.17 Wet bulb temperature indicates how cool a surface will become by evaporating water into the air, and when compared with the dry bulb, or regular, air temperature it indicates relative humidity. These two temperatures are measured by this instrument.

Hair & Other Hygrometers Figure 5.18 Human and horse hair becomes roughly 2.5% shorter as relative humidity drops from 100% to 0%, which is the principle operating the hair hygrometer. Other hygrometers are based on electrical resistance, infrared absorption, and dew point condensation.

Atmospheric Moisture