Review
1. Air Temperature
(1) A measure of the average speed of air molecules (kinetic
molecular theory).
(2) Faster average speeds of air molecules correspond to higher
air temperatures.
2. Kelvin Temperature Scale (K)
(1) 0 K = -273 oC (absolute zero temperature).
(2) K = 273 + oC
3. Normal Temperature
The average temperature for a 30-years period. The Normal maximu/
minimum temperature on a given day is the averaging of the past
30-year maximum/minimum temperatures on that same day.
4. Air pressure
(1) The weight of an air column with a cross section area of one
square cm above a given height.
(2) The air pressure units: mb (millibar), inch, hPa (hectopascal)
1 mb = 1 hPa = 0.0295 inch mercury height (the weight of the
mercury of 0.0295 inch height in a glass tube of one square
cm cross section area).
(3) Air pressure always decreases with the increasing elevation at
a logarithmic rate.
The logarithmic rate: The air pressure decreases more rapidly
with the increasing height (at a larger rate) in the lower
atmosphere than in the upper atmosphere
(4) Standard sea level: The average air pressure at the sea
level: 1013 mb or 29.92 inches
(5) Weather station observed air pressures are converted to
sea-level pressures using hypsometric equation for the
purpose of the spatial comparison.
(6) The mid-atmosphere: 500 mb or 18000 feet or 5.5 km.
About 50% of the atmosphere masses (molecules) is above or below
500 mb level.
About 70% of the atmosphere masses is above the 700-mb level (30%
below this level)
5. Wind (advection)
(1) The horizontal movement of an air current from the high to
the low pressure areas.
(2) Wind speed units: mph, knot, m/s.
1 knot =1.15 mph = 0.5 m/s
(3) Wind direction: The direction from which the air blows
(source of wind or come from direction)
A. Compass direction: N, NE, E, SE, S, SW, W, NW.
B. Azimuth Angle: The clockwise angle between the north and the
wind direction
0 : calm (wind speed < 1 mph) 180: S wind
45 : NE wind 270: W wind
90 : E wind 360: N wind
6. Relative Humidity (RH expressed in %)
(1) The ratio of the existing moisture (water vapor) in an air
parcel to the maximum amount of moisture that air parcel can
hold at a given air temperature.
RH = evp/svp
evp: existing vapor pressure, determined by the dew point
temperature.
svp: saturation vapor pressure, determined by the air
temperature. Warm air can hold more water vapor than
cold air of the same volume.
(2) The relative humidity at a given location is lower during the
daytime than at night provided no significant changes in synoptic
weather patterns. At night, the svp decreases due to the
decrease in air temperature.
7. Dew point temperature (or dew point)
(1) The air temperature at which saturation or condensation occurs.
(2) Saturation: evp = svp or RH = 100%.
(3) Condensation: water vapor becomes liquid water (cloud or fog droplet)
(4) Psychrometer:
A. Dry-bulb temperature = air temperature.
B. Wet-bulb temperature: Evaporation from the wet cloth reduces
temperature.
C. Wet-bulb depression: (Dry-bulb temperature) -(Wet-bulb
temperature).
8. The first Law of Thermodynamics
A. Adiabatic process: When an air parcel rises or sinks (subsides),
there is no mixing between the air parcel and its surrounding air
(environmental air).
B. Rising air parcel: expansion cooling.
(A). The volume of the air parcel increases (expands) due to the
decreased air pressure surrounding the air aarcel.
(B). The mean molecular speed of the air parcel slows down to
release the thermal energy to do the work of the volume
expansion of the air parcel.
C. Sinking (subsiding) air parcel: compressional heating.
(A). The volume of the air parcel shrinks due to the increase in
the air pressure of the surrounding air.
(B). The mean molecular speed of the air parcel increases due to
the compression of the volume.
9. Forces Making Air Rise
(1). Convection: caused by solar heating during the daytime.
(2). Orographic uplifting: Air parcel is forced to rise by a
mountain (topography).
(3). Surface Low: Surface air converges to a low pressure area and
rises.
(4). Frontal uplifting: Warm air overrides the cold air along fronts.
10. Cloud Types
Cloud: liquid and/or ice droplets suspended high above the ground.
Fog: liquid and/or ice droplets suspended immediately over the
ground.
(1) High clouds: Ice crystal only, silver white, cloud base >
18000 feet. A storm is nearby.
A. Cirrus (Ci): mare's tail, feather- or hook-shapes.
B. Cirrocumulus (Cc): mackerel cloud. lumps, cotton-balls,
fish scales.
C. Cirrostratus (Cs): halo (A bright ring around the sun or
the moon).
(2) Middle Clouds: ice and water, gray, cloud base between 6000
and 18000 feet. Within a storm.
A. Altocumulus (Ac): linear or roll shape.
B. Altostratus (As): the sun's image is visible.
(3) Low clouds: Water droplets only, dark gray, within a storm,
cloud base < 6000 feet.
A. Stratus (St): blanket, sheet, dark gray, chance of rain.
B. Nimbostratus (Ns or Nb): flat and dark cloud base.
rainy weather.
Virga: falling rain drops off the bases of Ns, Ac, Ci that
evaporated before reaching the ground level (showing
as dark bands or lines off the cloud base)
scud: small dark cloud patches associated with the Ns.
C. Stratocumulus (Sc): Clouds spread horizontally with roll or
linear shapes.
(4) Vertical clouds:
A. Cumulus humilis: fair weather cumulus.
B. Cumulus congestus: cauliflower-shape. a precursor of
cumulonimbus.
C. Cumulonimbus (Cb): Towering cloud reaching the
tropopause with an anvil top.
Anvil: A rising air parcel is unable to penetrate
into the stratosphere and therefore it spreads
horizontally, forming a flat cloud top near the tropopause
or the lower stratosphere (warmer air above).
D. Castellanus: castle-like clouds.
(5) Special types of clouds:
A. Lenticularis: UFO-like, Ac, lee-wave cloud.
B. Mammatus: ball-shape clouds.
(A). Evaporation of falling rain drops causes the
saturation and condensation of the air immediately
below the cloud base.
(B). may be associated with the occurrence of tornado.
11. Precipitation Theories
(1). Bergeron's Theory (Rain and snow originate from ice crystals in
clouds)
A. The SVP over a supercooled water surface is greater than the
SVP over an ice surface.
(A). At -10 oC, SVP is 2.86 mb over the supercooled water
surface and 2.60 mb over the ice surface.
(B). Supercooled water is the liquid water with temperature
O oC or lower (can exist at -40 oC).
B. Deposition (vapor becomes ice) over ice crystals while
evaporation over water droplets.
C. Ice crystals grow at the expense of water droplets
(shrinks).
D. When ice crystals grow to a certain size, they fall off the
cloud base as snow.
(A). Cloud base height: Snow on mountains (close to cloud
base).
Rain on valley floor (snow melts into rain because of
sufficient altitude).
(B). Temperature below cloud base: Rain when air
temperature is greater than 0 oC.
Snow when air temperature is lower than 0 oC.
E. Cloud extends beyond the freezing level (temperature = 0
oC).
F. Cold cloud: ice crystals and supercooled water droplets in
the cloud.
G. Cold rain: Rain originates from cold cloud.
(2). Coalescence Theory
A. Different sizes of water droplets with different terminal
velocity (constant velocity of a falling object, buoyance and
friction forces equal gravity) collide with each other to
form large ones.
B. Small droplets fall into the wake of large droplets.
C. Warm rain: Rain originates from warm cloud (no ice crystals
and supercooled water in the cloud).
12. Stability:
The tendency of an air parcel (any given volume of air) to resist
against the vertical displacement.
(1). Unstable air:
A. The air parcel does not resist against the vertical
displacement.
B. At a given elevation, the air parcel is warmer than the
surrounding air (the environment air).
C. The air parcel tends to rise even without the external
force to uplift the air parcel.
D. Large environmental lapse rate (observed lapse rate): > 5.5
oF/ 1000 ft or 1 oC/ 100 m.
E. Cloudy (cumulus types of clouds), shower rains
(2). Stable air:
A. The air parcel tends to resist against the vertical
displacement when uplifted by external forces.
B. At a given elevation, the air parcel is colder than the
environmental air.
C. The air parcel will not rise without an external force to
uplift it.
D. Small environmental lapse rate: < 5.5 oF/1000 ft or
1 oC/100m.
E. Stratus types of cloud, drizzles, foggy.
(3). Neutral stability:
A. The air parcel stays at the elevation it is uplifted to.
B. The environmental lapse rate = the dry adiabatic rate
(unsaturated air).
C. The environmental lapse rate = the wet adiabatic lapse rate
(within cloud or fog where the air is saturated).
(4). Conditional stability:
A. The environmental lapse rate is between dry- and wet-adiabaitc
lapse rate.
B. Unsaturated air: stable.
C. Saturated air: unstable.
(5). Dry or unsaturated adiabatic lapse rate (Air parcel's lapse rate)
A. When an unsaturated air parcel rises, its temperature
decreases by 1 oC for every 100 m increase in elevation.
B. A theoretical lapse rate calculated from the gas laws and the
First Law of Thermodynamics.
(6). Wet or saturated adiabatic
lapse rate (within clouds or fogs): Within clouds or fogs where
the air is saturated, the stability is determined by comparing
environmental lapse rate with wet adiabatic lapse rate
(0.6 oC/100 m).
(7). Wet adiabatic lapse rate is smaller than dry adiabatic lapse
rate because of the release of latent heat (heat required
for phase change from vapor to liquid) that warms up the air. It
varies according to moisture amount condensed. It is about 0.6 oC/
100 m in the lower atmosphere.