Chapter 5 Atmospheric Moisture and Precipitation

• The hydrologic cycle, p. 195

Evaporation depends on:

• Temperature

• Wind

• Relative Humidity

Measures of HumidityPrinciple: the warmer an air mass, the higher its water

vapor capacity• specific humidity is the mass of water vapor (g) per mass

of air (kg)• dewpoint is the temperature that the air would have to be

cooled off to in order to induce condensation.• vapor pressure: the portion of total air pressure

contributed by water vapor molecules, in millibars (mb) or inches of mercury. Higher amounts of humidity mean higher vapor pressure. Higher dwpts: higher vap. press.

• relative humidity

RH% = 100*actual water vapor pressure (determined by dewpt.)saturation vapor pressure (determined by air temp.)

Saturation vapor pressure & temperature

Air Temperature (°F) Saturation Vapor Pressure (“ Hg)

20 0.11

30 0.17

40 0.25

50 0.36

60 0.52

70 0.74

80 1.03

90 1.42

Air Temperature and Saturation Vapor Pressure

0

0.5

1

1.5

0 50 100

temperature (degrees F)

S.V

.P. (

in. H

g)

CONDENSATION

• condensation: when air reaches saturation (~100% relative humidity). Process: either cool off air to saturate, or add enough water vapor

• condensation nuclei: include salt, dust, smoke, others

• case of dew & frost:• clouds consist of water droplets and/or

ice crystals

Processes of growth of droplets into drops that can fall:

• ice crystal process - ice crystals behave as condensation nuclei: vapor droplets sublimate onto ice crystals

• coalescence process - large droplets fall faster than small droplets and collide/ coalesce with them

Atmospheric Stability & Lapse Rates• stability condition of the atmosphere when rising air becomes

cooler and denser than the surrounding air and is forced to subside.

• instability when rising air becomes warmer and less dense than the surrounding air and continues to rise.

• Environmental Lapse Rate: observed rate of temperature change in the atm. (average: 0.65 °C/ 100m)

• Dry Adiabatic Rate (DAR): rate at which unsaturated air cools as it is forced upward and expands. (1 °C/ 100m)

• Saturated Adiabatic Rate (SAR): rate at which saturated air cools as it is forced upward and expands. (~0.5 °C/100m) Also known as moist adiabatic rate.

• Example: chart with average Env. Lapse Rate and unsaturated air parcel.

• Stable conditions do not favor precipitation, unstable conditions do.

Lifting mechanisms & precipitation

basic principle: air must be lifted and cooled to the dewpoint in order to induce condensation

1. orographic precipitation: air forced upslope cools2. convectional: surface heating induces air to rise and cool

example: thunderstorms, which have 3 stages: developing, mature, dissipating

3. Low pressure systems: mid latitude cyclones, tropical weather systems and Inter Tropical Convergence Zone

4. frontal -front: boundary between air masses of different temperature and humidity, mid latitude cyclone example

Global precipitation map: (see folder)

GLOBAL PRECIPITATION PATTERNS

• general pattern: high pressure favors aridity (espec. W. Coasts), low pressure favors precipitation

• Areas of great precipitation

ITCZ (equatorial low) and related Monsoon lands, zone of sub polar lows, the littorals: (trade wind coasts & westerly coasts)

West side of Sub Tropical Highs: greater instability and precipitation than E side

FOG

• Advection

• Radiation

• Upslope

Areas of low precipitation

• Stable east side of sub tropical highs (W. Coasts) coastal deserts: Atacama (SA), Baja Cal., Namibian coast (Africa)

• rain shadow deserts (eg. Mohave and Sonoran) • Polar deserts: dominated by high pressure and low temps. ensure low moisture content of air and

modest annual precip.

Seasonality of precipitation: most regions have their low precipitation season in the low sun season, with the important exception of mid-latitude west coasts (such as the west coast of the U.S., W. Australia)