Atmospheric MoistureAtmospheric Moisture

This chapter discusses:This chapter discusses:

1. The role of water in the atmosphere1. The role of water in the atmosphere

2. Terms and definitions for atmospheric 2. Terms and definitions for atmospheric moisturemoisture

3. The impact of moisture on comfort3. The impact of moisture on comfort

Structure of WaterStructure of Water

Water's unique molecular structure and hydrogen bonds enable Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.all 3 phases to exist in earth's atmosphere.

Sublimation & deposition describe the non-incremental changes Sublimation & deposition describe the non-incremental changes between solid and vapor phases.between solid and vapor phases.

Figure 5.1Figure 5.1

Figure 5.2Figure 5.2

Evaporating Water into AirEvaporating Water into Air

Figure 5.3AFigure 5.3A

Liquid water experiences Liquid water experiences dynamic departures of water dynamic departures of water molecules from its surface, molecules from its surface, called evaporation, together called evaporation, together with arrivals of molecules with arrivals of molecules from adjacent vapor, called from adjacent vapor, called condensation.condensation.

When air is saturated, When air is saturated, evaporation and evaporation and condensation are in condensation are in equilibrium.equilibrium.

Condensation onto NucleiCondensation onto Nuclei

Condensation of Condensation of water vapor into water vapor into liquid water is more liquid water is more likely to occur when likely to occur when the vapor cools and the vapor cools and slows, and attaches to slows, and attaches to nuclei.nuclei.

Heated vapor moves Heated vapor moves so fast that it bounces so fast that it bounces away from away from condensation nucleicondensation nuclei

Figure 5.4AFigure 5.4A

Closing the Hydrologic CycleClosing the Hydrologic Cycle

Figure 5.5Figure 5.5

Evaporation, and transpiration by plants, transports liquid water Evaporation, and transpiration by plants, transports liquid water into atmospheric vapor.into atmospheric vapor.

Condensation converts this gas back to a liquid droplet, which may Condensation converts this gas back to a liquid droplet, which may then fall as precipitation to ground or surface water supplies.then fall as precipitation to ground or surface water supplies.

Descriping Atmospheric MoistureDescriping Atmospheric Moisture

Figure 5.6Figure 5.6

Atmospheric water Atmospheric water vapor has been vapor has been defined several defined several different ways.different ways.

These terms include These terms include absolute humidity, absolute humidity, specific humidity, specific humidity, mixing ratio, vapor mixing ratio, vapor pressure, and relative pressure, and relative humidity.humidity.

Absolute & Specific HumidityAbsolute & Specific Humidity

Figure 5.7Figure 5.7

For a given mass of water vapor For a given mass of water vapor in an air parcel, the absolute in an air parcel, the absolute humidity changes as the parcel humidity changes as the parcel volume changes (e.g., lifts or volume changes (e.g., lifts or descends).descends).

Specific humidity is concerned Specific humidity is concerned with the mass of vapor to mass of with the mass of vapor to mass of air, and is not affected by air, and is not affected by changes in parcel volume.changes in parcel volume.

Figure 5.8Figure 5.8

Specific Humidity vs. SaturationSpecific Humidity vs. Saturation

Figure 5.9Figure 5.9

Warm air can absorb more Warm air can absorb more vapor than cold air, so for a vapor than cold air, so for a given parcel of air, specific given parcel of air, specific humidity declines from its humidity declines from its highest in the tropics to its highest in the tropics to its lowest in the colder poles.lowest in the colder poles.

Desert air, at 30°, however, Desert air, at 30°, however, is not more saturated than is not more saturated than polar air.polar air.

Determining Vapor PressureDetermining Vapor Pressure

Average atmospheric pressure Average atmospheric pressure of 1013 mb is comprised in part of 1013 mb is comprised in part by the weight of vapor by the weight of vapor molecules.molecules.

Warmer air can absorb more Warmer air can absorb more vapor than cooler air before it vapor than cooler air before it saturates, and can have higher saturates, and can have higher saturated vapor pressures.saturated vapor pressures.

Figure 5.10Figure 5.10

Relative Humidity TrendsRelative Humidity Trends

Relative humidity (RH) indicates air parcel proximity to saturation.Relative humidity (RH) indicates air parcel proximity to saturation.

Saturation can be achieved, or RH increased, by adding more Saturation can be achieved, or RH increased, by adding more water or dropping the air temperature.water or dropping the air temperature.

Dew point is the temperature at which saturation occurs.Dew point is the temperature at which saturation occurs.

Figure 5.11Figure 5.11

Seasonal Dew Point MapsSeasonal Dew Point Maps

Figure 5.12AFigure 5.12A

January, July, dew pointJanuary, July, dew point

Figure 5.12BFigure 5.12B

Dew Point vs. Relative HumidityDew Point vs. Relative Humidity

Figure 5.13AFigure 5.13A

Dew point is the temperature for Dew point is the temperature for saturation, and used with a vapor saturation, and used with a vapor pressure curve reveals the mass of pressure curve reveals the mass of vapor in the air.vapor in the air.

While relative humidity may be While relative humidity may be higher in polar air, more water is higher in polar air, more water is actually absorbed in desert air.actually absorbed in desert air.

Relative and Specific HumidityRelative and Specific Humidity

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

Figure 5.14Figure 5.14

Figure 5.9Figure 5.9

Sources of MoistureSources of Moisture

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

Figure 5.15Figure 5.15

Relative Humidity and ComfortRelative Humidity and Comfort

Unsaturated air may absorb more water from the Unsaturated air may absorb more water from the evaporation of human sweat.evaporation of human sweat.

The departure of fast moving, and by definition higher The departure of fast moving, and by definition higher temperature, water molecules into the vapor phase cools temperature, water molecules into the vapor phase cools the human skin.the human skin.

In winter, this process can make a dry house extra In winter, this process can make a dry house extra chilly.chilly.

Heat Index & SafetyHeat Index & Safety

Figure 5.16Figure 5.16

Human perception of Human perception of temperature is distinct temperature is distinct from measured air from measured air temperature, and is temperature, and is particularly different particularly different at higher humidities at higher humidities when the human body when the human body is less efficient at is less efficient at sweating and self-sweating and self-cooling.cooling.

On hot days, fans that On hot days, fans that move saturated air move saturated air away from the skin away from the skin help humans avoid help humans avoid unwanted heat unwanted heat syndromes.syndromes.

Sling PsychrometerSling Psychrometer

Figure 5.17Figure 5.17

Wet bulb temperature indicates how cool a surface will become by Wet bulb temperature indicates how cool a surface will become by evaporating water into the air, and when compared with the dry evaporating water into the air, and when compared with the dry bulb, or regular, air temperature it indicates relative humidity.bulb, or regular, air temperature it indicates relative humidity.

These two temperatures are measured by this instrument.These two temperatures are measured by this instrument.

Hair & Other HygrometersHair & Other Hygrometers

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

Figure 5.18Figure 5.18