Transcript Atmospheric Moisture and Precipitation

Chapter 5 Atmospheric Moisture and Precipitation
• The hydrologic cycle
Evaporation depends on:
• Temperature
• Wind
• Relative Humidity
Measures of Humidity
Principle: 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)
20
30
40
50
60
70
80
90
Saturation Vapor
Pressure (“ Hg)
0.11
0.17
0.25
0.36
0.52
0.74
1.03
1.42
S.V.P. (in. Hg)
Air Temperature
and Saturation
Vapor Pressure
1.5
1
0.5
0
0
50
temperature (degrees F)
100
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 (ELR): 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)
• 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.
2.
3.
4.
orographic precipitation: air forced upslope by winds cools
convectional: surface heating induces air to rise and cool
example: thunderstorms, which have 3 stages: developing,
mature, dissipating
Low pressure systems: mid latitude cyclones, tropical weather
systems and Inter Tropical Convergence Zone
frontal -front: boundary between air masses of different
temperature and humidity, mid latitude cyclone example
Global precipitation map: (see folder)
GLOBAL PRECIPITATION PATTERNS
• view precip. map in folder
• 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 exception: greater
instability and precipitation than E side
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 exception of mid-latitude
west coasts (such as the west coast of the U.S., W. Australia)
FOG
• Advection
• Radiation
• Upslope