Evaporation, Condensation and Saturation

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Transcript Evaporation, Condensation and Saturation

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
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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Q1: The earth’s hydrological cycle is exceedingly
efficient in circulating water in the atmosphere.
a) true; b) false
Fig. 4-1, p. 80
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)
• Evaporation is increased by
stronger wind;
higher T
Q2: Water surface
evaporation is stronger
when air is
a) moist;
b) dry
c) not too moist or dry
Evaporation, Condensation and Saturation
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condensation needs condensation nuclei (microscopic bits of
dust 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
• Saturation is more likely
to occur in cool air
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 (%)
Vapor Pressure
Saturated vapor pressure
increases exponentially
with T
It is also provided in
Table B.1 on p. 440.
T= 7C, Pv = 10.2 mb
T =10C, Pv = 12.3mb
T =21C, Pv = 25.0 mb
T = 24C, Pv = 29.6 mb
Fig. 4-5, p. 83
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)
Fig. 4-7, p. 85
Q3: Which would decrease with the increase of T?
a) water vapor density
b) specific humidity
c) mixing ratio
Q4: Which would decrease significantly with the increase of T?
a) vapor pressure
b) relative humidity
Q5: 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
Relative Humidity and Dew Point
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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) and relative humidity
Higher (T – Td) indicates lower RH
Ta = -2C
Td = -2C
Ta – Td = 0C
RH = 100%
Ta = 35C
Td = 10C
Ta – Td = 25C
RH = 21%
Fig. 4-9a, p. 87
Q6: For Ta = 30C,
Td = 10C, what is actual
vapor pressure?
a) 12 mb,
b) 42 mb,
c) 50 mb
Q7: For Ta = 30C,
Td = 10C, what is RH?
a) 12/42,
b) 10/30,
c) 50/86
Fig. 4-5, p. 83
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;
a good measure of actual vapor content
Graphically demonstrate Td < Tw < T
Q8: Under what conditions, Td = Tw = T?
a) saturation, b) very dry, c) not too dry nor wet
 Heat index
Q9: Why do both temperature and relative humidity contribute to
warm-weather discomfort?
A: higher RH;
less body moisture evaporation; less cooling; feel warmer
Q10: Which has a higher heat index?
a) Ta = 100F, RH = 20% (AZ), b) Ta = 95F, RH = 40% (FL)
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. 443-446) for
Ta = 20C, Ta-Tw = 5C:
Td = 12C, RH = 58%
Ta = 90F, Ta-Tw = 10F: Td = 76F, RH = 65%
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Hygrometers
hair hygrometer and electrical hygrometer: RH
infrared hygrometer: moisture content;
dew cell: vapor pressure
dew-point hygrometer (for ASOS)
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
Q11: `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:
when moist air from your mouth or
nose meets the cold air and mixes
with it
haze: hygroscopic (`water seeking’)
condensation nuclei allows water
vapor to condense when RH < 100%;
haze has a better visibility than fog
Q12: The London fog is caused by
a) radiation, b) advection, c) mixing
Foggy Weather
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coastal or water body: radiation
Interior: radiation and upslope
hazard to aircraft: Some airports use fog-dispersal equipment
Annual
number
of fog
days
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.
Table 4-2, p. 98
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
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
cumulus congestus (Tcu): line of
towering Cu
Cumulonimbus (Cb):
thunderstorm with anvils
• Not all cumulus clouds grow to be
thunderstorms, but all thunderstorms
start out as cumulus clouds.
Fig. 4-32, p. 104
Some Unusual Clouds
lenticular clouds:
lens-like; UFO
 Pileus: `cap’
 Mammatus clouds: baglike
 Contrails: condensation trail
from engine exhaust
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Q13: This is: a) Cc, b) Ac,
c) Sc, d) Cu
Q14: This is: a) Cs, b) As,
c) St, d) Ns
Q15: This is: a) Cu, b) Tcu, c) Cb