Transcript Document

Chapter 4: Humidity,
Condensation and Clouds
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Circulation of water in the atmosphere
Evaporation, condensation and saturation
Humidity
Dew and frost
Fog
Foggy weather
Clouds
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Circulation of Water in the Atmosphere
Hydrologic cycle
evaporation and transpiration (from vegetation)
condensation
precipitation (rain, snow, hail) – when cloud droplets
grow large enough to fall to surface
runoff
• The total amount of water vapor stored in the
atmosphere amounts to only one week’s supply of
precipitation for the planet.
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Q: The earth’s hydrological cycle is exceedingly efficient
in circulating water in the atmosphere.
a) true; b) false
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Evaporation, Condensation and Saturation
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Saturation: water molecules moving from liquid to vapor
(evaporation) equal those moving from vapor to liquid
(condensation)
• Saturation is more likely to occur in cool air
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Evaporation is
increased by
stronger wind;
higher T
Q: Water surface
evaporation is stronger
when air is
a) moist;
b) dry
c) not too moist or dry
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Evaporation, Condensation and Saturation
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condensation needs condensation nuclei (microscopic bits of
dust, smoke, and salt from ocean spray)
• In very clean air, about 10,000 condensation nuclei
are typically found in one cubic centimeter of air,
a volume approximately the size of your fingertip.
• Condensation occurs
primarily when the air
is cooled
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Q: The favorable condition for ocean surface evaporation
includes: a) lower ocean temperature, b) stronger wind,
c) humid atmosphere
Q: As the air temperature decreases, the maximum amount
of water vapor that can exist in the air
a) increases, b) decreases, c) does not change
Q: What is condensation?
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Humidity
Humidity describes the amount of water vapor in the air.
•Water vapor density (absolute
humidity): mv/V (kg/m3)
•Specific humidity
mv/(mv+md) (kg/kg)
•Mixing ratio: mv/md (kg/kg)
•Vapor pressure: pv (mb)
p = pv + pd
•Relative humidity (%)
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Vapor Pressure
Saturated vapor pressure
increases exponentially
with T, and also
depends on air pressure
It is also provided in
Table B.1 on p. 460.
T= 7C, Pv = 10.2 mb
T =10C, Pv = 12.3mb
T =21C, Pv = 25.0 mb
T = 24C, Pv = 29.6 mb
(29.6-25.0) > (12.3-10.2)
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Relative Humidity
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definition of relativ humidity:
actual vapor pressure divided
by saturation vapor pressure
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(RH = e/es * 100%)
Supersaturation (RH > 100%)
How to increase RH?
Increase e
decrease es (by
decreasing T)
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Relative Humidity and Dew Point
dew point temperature (Td)
• It is the T to which air would have to be cooled (with
no change in air pressure and moisture content) for
saturation to occur
• Higher Td indicates higher actual water
vapor content
• Actual vapor pressure = saturated
pressure at Td
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dew point depression (T – Td) versus
relative humidity
Higher (T – Td) indicates lower RH
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Ta = -2C
Td = -2C
Ta – Td = 0C
RH = 100%
Ta = 35C
Td = 10C
(> -2C)
Ta – Td = 25C
RH = 21% (<100%)
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Q: For Ta = 30C,
Td = 10C, what is actual
vapor pressure?
a) 12 mb, b) 42 mb,
c) 50 mb
Q: For Ta = 30C,
Td = 10C, what is RH?
a) 12/42, b) 10/30,
c) 50/86
Q: For the actual vapor
pressure of 42 mb, Td
is
a) 10C, b) 20C, 30C
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Q: Which would decrease with the increase of T?
a) water vapor density, b) specific humidity, c) mixing ratio
Q: Which would decrease significantly with the increase of T?
a) vapor pressure, b) relative humidity, c) mixing ratio
Q: For water vapor in the hot, `dry’ air in the Sahara desert
versus that in the cold, `damp’ polar air, which is true?
a) the former is higher in mixing ratio,
b) the former is higher in relative humidity
Q: The ratio of vapor pressure over dry air pressure near
surface is around: a) 0.01, b) 0.1, c) 1.0
Q: The actual water vapor amount is NOT represented by
a) vapor pressure, b) relatively humidity, c) mixing ratio
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Average Surface dew-point temperature (oF) in July
Monsoon onset
in Tucson is
defined as the
first day when
the daily Td is
greater than
55oF for three
consecutive
days.
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Relative Humidity and Human Discomfort
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wet bulb temperature
Tw: lowest T attained by evaporating water into the air;
a good measure of how cool the skin can become
Td: reached by cooling the air to saturation (without change
of water vapor);
a good measure of actual
vapor content
 Heat index
Q: Why do both temperature and
relative humidity contribute to
warm-weather discomfort?
A: higher RH; less body
moisture evaporation;
less cooling; feel warmer
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Relative Humidity and Human Discomfort
Q: How to demonstrate
Td < Tw < T
Q: Under what conditions,
Td = Tw = T?
a) saturation, b) very dry,
c) not too dry nor wet
Q: Which is heavier?
a) dry air, b) moist air, c) same
(Note that the molecular mass of
dry air is 29 g/mol)
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Q: Which has a higher heat index?
a) Ta = 100F, RH = 20% (AZ), b) Ta = 95F, RH = 40% (FL)
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Measuring Humidity
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Psychrometers
Wet-bulb T (Tw); dry-bulb T (Ta);
wet-bulb depression (Ta-Tw);
Find Td and RH (based on
Table D on p. 463-466) for
Ta = 20C, Ta-Tw = 5C:
Td = 12C, RH = 58%
Ta = 90F, Ta-Tw = 10F:
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Td = 76F, RH = 65%
Hygrometers
hair hygrometer and electrical hygrometer: RH
infrared hygrometer: moisture content;
dew cell: vapor pressure
dew-point hygrometer (for ASOS)
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Dew and Frost
dew: condensation at Td > 0C (spherical beads of water)
 frozen dew: dew forms and freezes (spherical beads)
 frost: deposition (vapor to solid) at Tf < 0C (tree-like branch)
 freeze (black frost): Ta drops below 0C without reaching Tf
Q: `Freeze’ condition is colder than frost, because
a) frost would release latent heat;
b) `freeze’ condition would release latent heat
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Fog
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radiation fog: cooling from ground
advection fog:
warm, moist air over cold surface
upslope fog: cooling
evaporation (mixing) fog:
haze: hygroscopic (`water seeking’)
condensation nuclei allows water
vapor to condense when RH < 100%;
higher RH increases droplet size and
concentration, leading to fog;
haze has a better visibility than fog
Q: The tiny cloud from your exhaled
breach in winter is related to
a) radiation fog, b) advection fog,
c) mixing fog, d) haze
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Foggy Weather
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coastal or water body:
advection
Interior:
radiation and upslope
hazard to aircraft:
Some airports use
fog-dispersal equipment
Annual number of fog days
Q: The London fog is
caused by: a) radiation,
b) advection, c) mixing
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Classification of Clouds (chart at end of book)
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major cloud types
low, middle, high, vertical
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cloud appearance
sheetlike, puffy, wispy, rain cloud
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cloud base
0-2 Km, 2-6 km, 6-10km
• It’s easy to identify clouds, but it takes practice.
The ability to identify clouds allows you to forecast
many aspects of the weather using nothing but your
eyes.
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High Clouds
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All high clouds: thin, high
Cirrus (Ci): wispy
Cirrocumulus (Cc): small, white puffs, rippling
Cirrostratus (Cs): usually thin, often producing
a halo
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Middle Clouds
All middle clouds: thicker than high clouds, cloud base > 2 km
 Altocumulus (Ac): gray, puffy (larger, darker than Cc)
 Altostratus (As): gray layer cloud with `watery sun’
(difference from Cs: darker, dimly visible, no ground shallows)
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Low Clouds
All low clouds: base < 2km,
thicker than middle/high clouds
 Nimbostratus (Ns)
dark gray with light rain
 Stratocumulus (Sc):
larger cloud elements with lower
cloud base than Ac
 Stratus (St)
uniform grayish cloud;
has a more uniform base than Ns;
has a lower base and dark gray
than As;
resembles a fog that does not reach
the ground
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Clouds with Vertical
Development
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Cumulus (Cu): puffy, floating
`cotton’ with flat base (difference
from Sc: larger fraction of blue
sky, tower-shaped top)
cumulus congestus (Tcu): line of
towering Cu, rain shower
Cumulonimbus (Cb): with anvils,
thunderstorm, lightning, and
possibly large hail
• Not all cumulus clouds grow to be
thunderstorms, but all thunderstorms
start out as cumulus clouds.
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Some Unusual Clouds
lenticular clouds:
lens-like; UFO; caused by
mountain waves
 Pileus: `cap’
 Mammatus clouds: baglike
 Contrails: condensation trail
from engine (“moist”)
exhaust
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Q: This is: a) Cc, b) Ac,
c) Sc, d) Cu
Q: This is: a) Cs, b) As,
c) St, d) Ns
Q: This is: a) Cu,
b) Tcu, c) Cb
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