METEOROLOGY
GEL-1370
Chapter Four
Humidity, Condensation & Clouds
Goal for this Chapter
We are going to learn answers to the following questions:
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Why eyeglasses often fog up after coming indoors on a cold day?
On a cold winter, why do you feel your skin to be very dry, lips crack, and have itching
sensation on the skin?
You often itchy throat during winter than summer?
Why you have different types, colors and shapes of cloud during various days and times of the
year?
Learn various terms (relative humidity, vapor pressure, dew, frost, haze, fog, cirrus clouds,
cumulus clouds, stratocumulus clouds, etc.) used by Meteorologists
Water Circulation in the Atmosphere
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Evaporation: Conversion of liquid water into vapor
Condensation: Conversion of water vapor into liquid
Precipitation: Stuff that falls to the surface from the growth of cloud particles (rain, snow or hail)
Transpiration: Release of moisture from plants/trees
Hydrologic Cycle: Cycling of water molecules from liquid to vapor and back to liquid- ocean –
atmosphere – land – ocean
Some Facts: 1.5 billion billion gallons of water vapor evaporate into the atmosphere in a year; 85%
from ocean (70% of the earth’s surface area) & 15% from evaporation and transpiration
Total Mass of Water Vapor ~ 7 days of precipitation for the globe (Residence time ~ 7 days)
Hydrologic Cycle
Saturation & Condensation
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Saturation: When number of molecules escaping a parcel of air = number of returning
molecules
Wind can push away the molecules from a water body and hence will enhance evaporation
Warm temperature will aid the molecules to move faster leading to evaporation – warmer the
water, greater the rate by evaporation
Condensation Nuclei: Microscopic particles such as dust, smoke, salt from ocean spray, etc –
serve as surfaces on which water vapor can condense
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Condensation occurs when air is cooled – speed of the vapor molecules decrease leading to
more condensation
Saturation & Humidity
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Saturation is more likely to occur in cool air than warm air – Warm air can hold more water
vapor molecules before getting saturated than cold air – Warm air has greater capacity to hold
water vapor
Absolute Humidity or Water vapor density in a parcel of air = Weight of water vapor (g) /
volume of air (m3)
Specific Humidity in a parcel of air = Weight of water vapor (g) / Total weight of all the air
(kg)
Mixing Ratio in a parcel of air = Weight of water vapor (g) / Weight of the dry air (kg)
Relative Humidity = Amount of water vapor in the air / Maximum amount of water required
for saturation at that temperature
Water molecules at the surface of the water – more molecules are evaporating than
condensing – net evaporation is taking place
Number of water molecules escaping = number returning – air is saturated with the water
vapor
Water vapor content inside the parcel of air (Humidity)
Humidity contd.
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Relative Humidity (RH) tells us how close the air is to being saturated
RH = (vapor pressure / saturation vapor pressure) x 100
Air RH >100%, is said to be supersaturated
Vapor Pressure: Total pressure inside a parcel of air = Sum of the pressures of individual
gases
Total pressure (1000 mb) = Partial pressure (PP) exerted by N2 (78% or 780 mb) + PP exerted
by O2 (21% or 210 mb) + PP exerted by water vapor (1% or 10 mb)
The partial pressure of water vapor (Actual vapor pressure) ~ 10 mb – only a small fraction of
the total air pressure
Vapor Pressure
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High actual vapor pressure indicates large numbers of water vapor molecules
Saturation Vapor Pressure: Pressure that the water vapor molecules would exert if the air were
saturated with vapor at a given temperature
At higher air temperatures, it takes more water vapor to saturate the air (increase in temp leads
to increased in the velocity – number of molecules escaping per second increases)
Saturation vapor pressure increases with increasing temperature
Relative Humidity – contd.
Change in RH can come from
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Change in Water pressure Content
Change in the air temperature (previous view graph)
For a constant air temperature, RH increases with addition of water vapor
For a constant water vapor, increase in air temperature lowers the RH
Highest RH occurs in the early morning (coolest time corresponds to the highest RH); lowest
RH occurs during the warmest part of the afternoon
Rapid evaporation of moisture human body causes skin to crack, dry skin, itching, etc; itchy
throat, etc.
RH highest in the cool morning; lowest in warmest time
Humidity
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Assume: Air temperature in the morning is 10°C; the saturation vapor pressure is 12 mb; if the
temp warms to 30 °C (water vapor content remains constant), actual vapor pressure remains
the same – 12 mb, but the saturation vapor pressure has changed (temp effect) to 42 mb
RH = (12 mb/42 mb) x 100 = 29% - RH has decreased from 100% to 29% because of change
in temp.
Dew Point: The temperature to which air would have to be cooled for saturation to occur
(keeping air pressure or moisture content constant)
Humidity – contd.
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High dew point --- high water vapor content
Low dew point – low water vapor content
[Air temp – Dew Point] --- high values indicate RH is low; close to zero, RH is high; Zero
implies air is saturated and the RH is ~100%
Polar Air is ‘dry’ when the RH is ~100% (Air temp and dew point are close together; low dew
point temp means little water vapor in the air)
Wet-bulb Temperature: Lowest temperature that can be reached by evaporating water into
the air
During summer, when wet-bulb temp is low, fast evaporation takes place from the skin
Polar Air temp = -2°C; dew point = -2°C; RH=100%
Desert Air temp = 35°C; dew point = -5°C; RH=16%
Human body adjustment
When the human body temp rises, the hypothalamus gland (gland in the brain that regulates body
temp) activates the body’s heat regulating mechanism, and evaporation increases – we feel
thirsty to keep up the fluid level in the body
Heat Index: It is the apparent temperature of what the temperature ‘feels like’ – It takes air temp
and RH into account (air temp of 100°F & RH ~60% - Heat index ~130 °F)
Measurement of dew point and RH is done using Psychrometer
Heat Index
Sling Psychrometer
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Two bulbs, one end has a piece of cloth (wick) covering the bulb – called the wet bulb is
dipped in clean water – Other end is kept dry; both bulbs ventilated with by whirling the
instrument or drawing air past it with an electric fan; water evaporates and thermometer cools;
drier the air, greater the amount of evaporation & cooling – dry thermo gives air temp
Diff between dry and wet bulbs = wet bulk depression
Humidity measurements
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Large depression --- more water can evaporate into the air – RH is very low
Small depression --- little evaporation of water vapor is possible
Humidity are commonly called Hygrometers – Human humidity increases, the length of hair
increases and the RH decreases, so does the hair length
Electrical Hygrometer: Plate coated with a film of carbon; electrical current sent across the
plate; water vapor absorbed changes the electrical resistance of the carbon coating
Hair hygrometer – changes in the length of a human or horse hair
Dew, Frost, Fog
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When air cools below the dew point, water vapor begins to condense upon surfaces forming
tiny visible specks of water called ‘dew’
If the air temp falls below freezing, the dew will freeze, becoming tiny beads of ice called
frozen dew
When dew is seen, it will not rain; if grass is dry, rain may come
When air temp is much below freezing, water vapor can directly become ice – Deposition
(opposite is sublimation) – White crystal is called frost
When RH >75%, some of the water vapor may condense onto condensation nuclei – size
increases and can scatter light --- becoming haze
Haze, Fog
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Fog: Visibility < 1 km; air is wet with millions of tiny liquid droplets (or ice crystals), the haze
becomes cloud with earth’s surface as the base
Radiation Fog or Ground Fog: Produced by the earth’s radiational cooling; most common
over land in late fall & winter; ground cools so does the air directly above it, and a surface
inversion forms --- moist lower layer quickly becomes saturated, and fog forms
Valley Fog: Radiation fog in a valley; heavy air drains downhill and collects in valley bottoms
How fog disappears when sun rises: Sunlight penetrates and warms the ground; temp of air
increases; warm rises and mixes w/foggy air above; droplets evaporate
Formation of dew; if the water freezes, frozen dew
High RH of the cold air above the lake is causing the Formation of a layer of haze on a
still winter morning
Radiation fog nestled in a valley
Fog – contd.
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Advection fog: Warm, moist air moving over a colder air --- warm air cools to its saturation
point, forming advection fog
Ice Fog: Marine air moves over an ice or snow surface, ice crystals form instead of water
droplets – produce ice fog
Upslope fog: Fog that forms as moist air flows up along an elevated plain, hill or mountain
Evaporation (mixing) fog: Fog formed from the mixing of two unsaturated masses of air –
e.g., fog produced from our breathing
Steam fog: Fog forming over lakes on autumn mornings, as cold air settles over water still
warm from the summer
Fog – contd. & Clouds
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Precipitation (or frontal) fog: A warm rain falling through a layer of cold, moist air can
produce fog
Heavy fog is more prevalent in coastal margins than in the center of the continent
In US, foggiest spot is Cape Disappointment, Washington (at the mouth of the Columbia
River)
Clouds: A visible aggregate of tiny water droplets or ice crystals suspended in the air
Classification of Clouds: 10 Basic types
Average annual # of days with heavy fogs in USA
High Clouds (Cirrus, Ci, Cirrostratrus, Cs); Middle (Altostratus, As, Altocumulus, Ac); Low
(Stratus, St, Stratocumulus, Sc, Nimbostratus, Ns); & Clouds w/vertical development (cumulus,
Cu, & Cumulonimbus, Cb)
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Approxi. Height of cloud bases above surface
Classification of cloud
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First classification (Luke Howard) based on they appear to a ground observer:
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Sheetlike cloud – stratus (in Latin means ‘layer’)
Puffy cloud – cumulus (means ‘heap’)
Cirrus (means curl of hair’
Nimbus (means ‘violent rain’)
Other cloud description is based on the combination of these four basic cloud forms – e.g.,
nimbostratus
Next classification based on the height of the cloud’s base above the surface: high clouds, middle
clouds, low clouds and clouds that show vertical rather than horizontal development
Altitudes separating high and middle cloud groups overlaps and varies with latitude – cirrus clouds at
4,000 m above Alaska will not be seen above Miami at the same height
Cirrus clouds-6-18 km in tropics; 5-13 km in midlatitude; 3-8 km in polar region
Cloud classification – contd.
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High Clouds: Clouds composed of ice crystals and are thin;
– Cirrus clouds usually move across the sky from west to east
– cirrocumulus less seen frequently than cirrus
– cirrostratus form ahead of an advancing storm; appearance of cirrostratus used to predict rain or
snow within 12-24 hrs
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Middle Clouds: Have bases 2-7 km;
– Altocumulus: One part of the cloud is darker than the other
– Altostratus: Gray or blue-gray cloud often covering entire sky or over an area that extends over
many hundreds of square km; often forms ahead of storms having widespread and continuous
precipitation; if precipitation takes place from this cloud, the base height lowers
Cirrocumulus clouds: Less frequently seen, appear as small, rounded, white puffs
Cirrostratus clouds with a halo: form ahead (12-24 hrs) of an advancing
storm
Altocumulus cloud: Presence in warm, humid summer morning – thunderstorms by late
afternoon
Altostratus cloud: Often form ahead of storms having widespread and relatively
continuous precipitation
Cloud classification – contd.
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Low Clouds: Bases lie below 2000 m; composed of water droplets (in cold weather may
contain ice crystals and snow)
– Nimbostratus: Dark gray, ‘wet’-looking cloud layer; associated with continuously falling rain or
snow; intensity of rain is light or moderate; never heavy, showery variety
– Stratocumulus: Rain or snow rarely fall from stratocumulus
– Stratus: When a thick fog ‘lifts,’ the resulting cloud is low stratus; generally no precipitation falls
from the stratus
Clouds with Vertical Development: Takes a variety of shapes, most often looks like a piece of
floating cotton with sharp outlines and flat base; precipitation from cumulus congestus is always
showery
Nimbostratus cloud: associated with continuously falling rain or snow; Intensity: light to
moderate
Stratocumulus clouds: Rarely rain or snow fall from this cloud
A layer of low-lying stratus clouds: no precipitation falls from the stratus clouds
Cumulus clouds
Cumulus congestus: Precipitation is always showery
Cumulonimbus: Thunderstorm cloud
Clouds with vertical development
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Cumulonimbus is a thunderstorm cloud; condensation of water vapor --- release of energy –
violent up- and down-drafts – lightning, thunder, violent tornadoes associated with the
cumulonimbus
Other Clouds:
Lenticular clouds: formed in the wave crest ; wave is formed by the moist air crossing a
mountain barrier 0rkednmoist air crossing
Mammatus clouds: Formed in sinking air
Condensation trail air (or contrail): Trail of Condensed vapor produced by a jet aircraft
Generalized Classification of cloud type based on height
Lenticular clouds (forms in crest) formed on the eastern side of the Sierra
Nevada
Mammatus clouds: forms in sinking air
Contrail forming behind a jet aircraft – 10 km from ground
chapter –4- Summary