Atmospheric Moisture

0.001% of all water on Earth is stored in the atmosphere

You drink this

Net Source of Water

Net Sink of Water

Rainfall’s Origins: Salty or Fresh?

The percentage of annual precipitation over land that originally vaporized from the ocean, averaged over 15 years.

Measures of Humidity

• Humidity: measure of how much water vapor is in the air

• Four ways to express humidity– Vapor Pressure (mb)

– Specific (g/kg)

– Relative (%)

– Dew Point (Temperature)

Maximum Humidity

• Saturation Vapor Pressure: maximum amount of water vapor a specific body of air can hold at a given temperature

Warmer air can hold more H2O• Air at 20C can hold 3x as much H2O

as air at 0C.• Saturation requires less H2O for ice

than water

Vapor Pressure: What is measured

Vapor Pressure Deficit: Saturation Vapor Pressure minus Vapor Pressure

Specific Humidity

• Specific humidity: (actual) measurable amount of water vapor in a parcel of air (g/kg). Typical value is 10-15 g/kg– Similar to “vapor pressure”

Relative Humidity

• Relative humidity: ratio between specific and maximum humidity

• Expressed as a percentage relative to saturation, at 100% relative humidity, air is saturated

• Relative humidity =

Specific humidity (or vapor pressure)

Maximum humidity (or saturation vapor pressure)

X 100

Relative Humidity

Unsaturated Saturated

30° C 15° C 5° C

The vapor pressure generally varies only slightly through the day; so the relative humidity varies inversely with temperature.

How Humid is it?

Dew-point temperature:Temperature at which air with a given amount of moisture reaches saturation

Maximum Humidity tells us at what temperature this will occur

Dew Point Temperature

Average surface dewpoint for July.

How Humid is it?

Dew point temperature: temperature when air is saturated w/respect to water vapor for a given amount of moisture in the air

Precipitable WaterAKA “Total Column Water Vapor”

Depth of liquid water if all moisture in the vertical column of the atmosphere was condensed into liquidGlobal average: 2.5cm

• Air is cooled– Saturation vapor pressure decreases while actual vapor

pressure remains constant– How do we cool air?

• Water vapor added at a constant temperature– Vapor pressure increases while saturation vapor pressure

remains constant– How do we add water vapor?

How Air Becomes Saturated

Monitoring Humidity

• Hygrometer– Measures the water vapor

concentration of air

• Wet bulb temperature

• Hair hygrometer– Relates changes in length

of a humid hair to humidity – Hair lengthens as relative

humidity increases

Wet Bulb Temperature

• Temperature at which at wet object will cool to after evaporative cooling

• Typically less than ambient temperature, warmer than dew-point temperature

• Wet bulb depression: Air temperature (dry-bulb) minus wet bulb temperature

Atmospheric Lifting and Precipitation

To form precipitation, air must move upward and be cooled by adiabatic processes.

Four ways for air to move upward:

1. Convection

2.Orographic

3. Convergence

4.Frontal

Atmospheric Lifting

Convectional lifting: bubble of warm air rises within an unstable body of air– Environmental lapse

rate exceed dry/moist adiabatic lapse rate

Atmospheric Lifting

• Orographic lifting: occurs when a flowing body of air encounters a mountain range

Rain Shadowed Deserts of central Washington

Atmospheric Rivers

– Narrow filaments (typically 150-300 miles wide) of enhanced water vapor in the atmosphere that exist in the warm sector of extratropical cyclones

– Characterized by warm temperature, high moisture content and strong low level winds

– Most of the water vapor is contained below 700mb

– 30-50% of the average annual precipitation for the West Coast

Atmospheric Lifting

• Convergent lifting: occurs when large bodies of air meet

Atmospheric Lifting

• Frontal lifting: occurs along the boundary of contrasting air masses