Chapter 4 - Moisture

Water

- Consists of a bond between an oxygen atom and two hydrogen atoms

- Angle between bond is 105°, which leaves the oxygen atom partially negative and the hydrogen atoms partially positive

Hydrogen Bonds

Positive part of molecule that attracts the negative part of another molecule

- Hydrogen bond between molecules must be broken for water to go into the vapor phase

Sublimation

Ice to water

Deposition

Vapor to ice

Evaporation

Liquid to water

Condensation

Vapor to liquid

Saturation

When equal numbers of molecules are evaporating and condensing

Transporation

Living organisms adding water vapor to the air

Precipitation Cycle: Stage 1 - Hydrogen cycle

- Heat of the surface allows water to evaporate and enter the atmosphere

- Transpiration happens

- Rising air cools and leads to condensation into water droplets

Precipitation Cycle: Stage 2

- When water droplets become large enough, they fall from the sky as precipitation

- Precipitation at the surface can seep into the ground and become ground water

- Additional precipitation acts as runoff and will collect in lakes and oceans

Absolute Humidity

Mass of water per volume of air (not a good measure because air pressure changes)

A.H. = Mass H₂O/volume of air

Specific Humidity

Mass of water per mass of air (units of g/kg)

S.H. = Mass H₂O/Mass of air

Mixing Ratio

Mass of water per mass of dry air (units of g/per kg.)

- Used on thermodynamic charts of the atmosphere

w = Mass of H₂O/Mass of dry air

Vapor Pressure

Portion of atmospheric pressure provided by water vapor

- If 1% of the molecules in the air are water vapor, then 1% of the total pressure is the vapor pressure

Saturation Vapor Pressure (eₛ)

The pressure provided by water vapor if the air were saturated

- Saturation vapor pressure increases with temperature because more molecules have the velocity necessary to evaporate from the liquid water's surface

Relative Humidity (RH)

Ratio of water vapor in the air compared to the max water vapor content at saturation

- Can be calculated from the vapor pressure (e) and the saturation vapor pressure (eₛ)

- RH is high when DP and air temp are close together

Trends in Humidity

- RH is highest in morning when air temp is lowest

- RH is lowest in the afternoon when air temp is highest

Heat Index (HI)

How hot the air feels (apparent temperature)

- HI is determined by air temp and RH

- Prolonged exposure to large HI may result in heat exhaustion, heat cramps, and sunstroke.

- Wet bulb temp is a good measure of how cool the skin can become from evaporation

Dew Point

The temperature at which the RH becomes 100% w.r.t. a flat surface of water

Frost Point

The temperature at which the RH becomes 100% w.r.t. a flat surface of ice

Density of Humid Air

- H₂O has a molecular weight (MW) of 18

- N₂ has a MW of 28

- O₂ has a MW of 32

- Dry air has a MW of 29

The larger the percentage of water vapor in the air, the less density the air will have

Psychrometer

Measures wet bulb and dry bulb temperatures, which can be converted into a dew point temperature

Hair Hygrometer

Measures humidity based on the principle that hair is longer when the air is more humid