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

The Hydrologic Cycle

• The hydrologic cycle is a process in which water (in all 3 phases) is redistributed along the earth's surface and atmosphere.

• Evaporation over landmasses and transpiration by plants change liquid water into gaseous water vapors. These processes account for 15% of the total water vapors found in atmosphere. Evaporation over the ocean makes up for the rest of the water vapors in the atmosphere (85%).

• The total mass amount of water vapor stored in the atmosphere at any moment accounts for approximately over a week’s supply of the world's precipitation. This value only varies slightly daily. (The hydrological cycle is very efficient in circulating water in the atmosphere)

15 % 85 %

Structure of Water

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

• During a phase change, water changes form but not its properties.

• The positive charge portion of the water molecule attaches to a negative charge region of another water molecule along with the 105° angle between the hydrogen atoms forming a hexagonal shaped ice crystals.

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

Water Phase and Molecular Speed

• The image above show how water molecules look in all three phases from a microscopic point of view. Notice the speed of the molecules in each of the phases.

• Recall that the temperature and molecular speed are related through kinetic energy.

Saturation versus Unsaturation

• When liquid water experiences dynamic departures of water vapor molecules from its surface, this process is called evaporation.

• Condensation is the process when water vapor molecules turn into a liquid state.

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

• Temperature of water affects evaporation rate.

Unsaturated Saturated

Condensation onto Nuclei

• Condensation of water vapor into liquid water is more likely to occur when the vapor cools and slows, and attaches to condensation nuclei such as microscopic particles of dust, smoke, soil, and salt from ocean spray.

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

• Condensation occurs primarily when air is cooled.

Atmospheric Moisture• Air parcel – an imaginary

small body of air that is used to explain air behaviors and characteristics that is independent of the outside environment.

• Atmospheric water vapor has been defined several different ways.

• These terms include: 1. absolute humidity2. specific humidity3. mixing ratio4. vapor pressure5. relative humidity6. dew point temp

Humidity – refers to any one of a number of ways of specifying the amount of water vapor in the air.

• Absolute humidity (g/m3) – the mass of water vapor in a given volume of air. It can be thought of as density of water vapor in the air.

• Specific humidity (g/kg) – the ratio of the mass of water vapor in a given parcel to the total mass of air in the parcel.

• Mixing ratio (g/kg) – the ratio of the mass of water vapor in a given volume of air to the mass of dry air.

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vaporwaterofmassHumiditySpecific

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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).

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

• The specific humidity and mixing ratio of an air parcel remains constant as long as water vapor is not added to or removed from the parcel.

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vaporwaterofmassHumidityAbsolute

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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°N, however, is not more saturated than polar air (10 g/kg vs. 5 /kg).

Vapor PressureSuppose pressure within the an air parcel is 1000 mb:

• The total air pressure inside is due to collisions of the molecules against the wall of the parcel.

• The total air pressure inside is equal to the sum of the pressures of the individual gases (78% nitrogen, 21% oxygen, and 1% water vapor)

• So inside the parcel, nitrogen = 780 mb, oxygen = 210 mb, and water vapor = 10 mb.

• The partial pressure of water vapor is called the actual vapor pressure (e).

• The actual vapor pressure is normally a small fraction of the total air pressure (e.g. 10 mb / 1000 mb).

• The more air molecules inside a parcel, the greater the total air pressure will be, therefore actual vapor pressure is a good measure of the water vapor content in the atmosphere.

Determining Vapor Pressure• Average sea-level atmospheric

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

• Saturation vapor pressure (es) describes how much water vapor is necessary to make the air saturated at any given temperature.

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

Vapor pressure, Elevation, and Boiling Point• Boiling point occurs when saturation

vapor pressure of the escaping bubbles is equal to the total atmospheric pressure (otherwise the bubbles would collapse)

• Higher water temperatures produce higher vapor pressures due to the direct relationship between es and temperature.

• Any change in the pressure will change the boiling of water.

• So in Denver (~ 834 mb), our boiling point of water is at about 94 °C.

• Water temperature remains constant once the boil point is reached because the energy supplied to the water is used to convert liquid into gas.

• Therefore, food takes longer to cook at higher altitudes.

Relative Humidity• Relative humidity (RH) is most often used but commonly misunderstood.

• RH is the ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at that particular pressure and temperature.

or

• 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.

• Dew point is a good indicator of the actual water vapor content in the air at the earth’s surface since

pressure only varies slightly at the ground.

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Relative Humidity

• When water vapor is added to the air (temperature remaining constant), the RH increases.

• If water vapor content is held constant and the temperature is raised, the RH lowers.

• An increase/decrease in temperature without a change in water vapor content will decrease/increase the RH.

Water Vapor and Temperature Control

• Dew point temperature plays a significant in controlling local and regional temperature.

• Since higher dew point values equate to more water vapors in the atmosphere therefore the higher the dew point, the more longwave radiation the atmosphere will absorb.

• The image above shows how more outgoing IR energy escapes back into space with a lower dew point temperature value.

Seasonal Dew Point Maps

January July

• Brown star and arrows depict low dew point source regions.

• Green arrows indicate high dew point source regions.

Dew Point vs. Relative Humidity

• Dew point is the temperature at which saturation occurs, 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. RH has a dependency on temperature and pressure.

Relative and Specific Humidity

• 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.

Sources of Moisture

• Patterns of US humidity are strongly governed by wind direction and ocean temperatures.

• Cooler Pacific waters create lower humilities 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 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

• Heat Index is an index where it combines the effects of air temperature and RH to determine an apparent temperature.

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

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

Heat Index

Measuring Humidity

• 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 a sling psychrometer.

• The greater the depression value between the two temperatures, the greater the evaporation rate within the environment.

Why is dry air “heavier” than moist air?

H2O Weight # of Atoms Molecular Weight

Hydrogen: 1 x 2 = 2

Oxygen: 16 x 1 = 16

18