METEOROLOGY
GEL-1370
Chapter Four
Humidity, Condensation & Clouds
Goal for this Chapter
We are going to learn answers to the following
questions:
• 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
• 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
• 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
• Condensation occurs when air is cooled – speed of the
vapor molecules decrease leading to more condensation
Saturation & Humidity
• 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.
• 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
• 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
– 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
• 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.
• 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
• 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
• 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
• 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
• 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.
• 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
• 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)
• Approxi. Height of cloud bases above surface
Cloud
Group
Tropical
Region
High (Ci, 6000 –
Cs, Cc)
18000 m
Mid-lati.
region
Polar
Region
5000 –
3000 –
13,000 m 8000 m
Middle
(As, Ac)
2000 –
8000 m
2000 –
7000 m
2000 –
4000 m
Low (St,
Sc, Ns)
0 – 2000
m
0 – 2000
m
0 – 2000
m
Classification of cloud
• First classification (Luke Howard) based on they appear
to a ground observer:
– 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.
• 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
• 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.
• 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
• 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