Humidity, Condensation,
Clouds, and Fog
or
Water in the Atmosphere
The Hydrologic Cycle
Where the Water Exists on Earth
Evaporation From the
Oceans and Land
________________
The Source of Water Vapor
for the Atmosphere
! Water molecules move in both
directions between liquid and
gas.
! More molecules move from
liquid to gas than from gas to
liquid, and thus there is a net
loss of liquid – evaporation.
! Eventually liquid water will dry
up, unless replaced by a
source such as precipitation.
! Some water molecules that
leave the liquid move away
from the surface because the
container is open.
Only water molecules are
shown in the air.
OPEN TOP
What affects evaporation rate?
! Wind speed
- The stronger the wind, the more rapid the
evaporation.
- Why? - Stronger wind over surface blows
away vapor molecules from near
surface, so they will not re-enter liquid
surface.
- Example - Clothes dry out faster in the
wind, we feel colder in the wind with
wet clothes
! Temperature of the water
– The higher the water temperature, the
more rapid the evaporation.
- Why? - The higher temperature means that
water molecules in the liquid are moving
faster, and will escape to form vapor.
- Example - wet clothes dry faster in the sun
! The humidity of the air (humidity = amount
of water vapor in the air)
– The higher the humidity, the slower the
evaporation.
- Why? - The more water molecules in the
air, the more that go back to the liquid.
- Example - clothes dry faster and your
perspiration evaporates quickly in dry
climates
Now Saturate the Air
! Put a lid on container so no
water-vapor molecules blow
away
! Water molecules evaporate from
the liquid until enough
accumulate in the air so that
they move back to the liquid at
the same rate.
! At this point we say that the air
is saturated with water vapor – it
has reached its capacity.
The Amount of Water Vapor the
Air Can Hold Depends on
Temperature
! The amount of water vapor that may be
evaporated into air is directly proportional to
the air temperature. That is, the higher the
temperature, the more water vapor the air
can hold.
For Example
! Allow liquid water in a closed container to
evaporate until the air is saturated.
! Warm the air by 20 F, and evaporation will begin
again because the air is no longer saturated. At
the new saturation, when evaporation stops, twice
the original amount of water will be in the air.
! Warm the air another 20 F, and the water in the air
will double again when saturation is reached and
evaporation stops.
So
! For about every 20 F change in
temperature, the capacity of the air
for water vapor changes by a factor
of two
Condensation to Form Fog,
Clouds, Dew – How it Happens in
the Atmosphere
! Condensation occurs mainly when air is cooled
and its capacity for water vapor is decreased.
! In the atmosphere, water vapor condenses on
particles of solid material rather than in the open
air.
! These particles may be natural or man-made, and
are called condensation nuclei.
! Thus, the condensation that begins each cloud
droplet or fog droplet takes place on a solid
particle.
Humidity –
Any way of specifying the
amount of water vapor in air
1. Vapor Pressure
Concept - Each gas in the atmosphere contributes
its own partial pressure (part of the total). Say the
total pressure is 1000 millibars, and 1% of the air
is water vapor.
nitrogen (78%) – partial pressure = 780 mb
oxygen (21%) – partial pressure = 210 mb
water vapor (1%) – partial pressure = 10 mb
(Saturation vapor pressure = vapor pressure if the
air is saturated)
How saturation
vapor pressure
depends on
temperature – it
takes more water
vapor to saturate
the air at higher
temperatures!!!!
2. Relative Humidity
Relative humidity tells us how close the air is to
being saturated
RH =
water vapor content
water vapor capacity
RH =
actual vapor pressure
saturation vapor pressure
x 100
or
x 100
Relative Humidity (cont.)
! If air contains ½ the amount required for
saturation the RH = 50%
! If air is saturated, RH= 100%
! How does RH change? – two ways
– Change the temperature (capacity for water
vapor)
– Change the amount of water vapor in the air
Change of RH that results from daily
temperature change
Relative humidity and human comfort
! The body cools itself through
- evaporation from the skin
- evaporation from the respiratory track
- radiation
- conduction and convection
! When the body is under heat stress (large sources of heat
from metabolism or external sources), evaporation of
perspiration from the skin is most important cooling
mechanism.
! But, evaporation slows down when air is close to saturation
(high RH).
! Thus, we are uncomfortably hot when the RH is high.
3. Dewpoint
The temperature to which you have to cool the
air to produce saturation
Fog
! Air becomes saturated and droplets form on
condensation nuclei
! There is no physical difference between a
fog and a cloud, except that we tend to think
of a fog as being near the ground
! There are different names for a fog,
depending on how it forms (that is, what
causes the saturation)
Fog types
Radiation fog
! The surface of Earth cools at night because
infrared radiation is emitted.
! The cool surface cools the air near it.
! If the air temperature cools to the dewpoint
temperature, a radiation fog forms.
Radiation fog (continued)
! These fogs tend to form
- in low areas (e.g., valleys) because the cool air
near the ground drains downhill.
- over moist surfaces (e.g., river valleys) because
the dew point is higher.
- when the sky is clear and air is dry, because more
infrared energy escapes to space rather than
being absorbed and radiated back to Earth
- when the overall airmass is moist, and thus the
dewpoint is high
- when nights are long in the winter, allowing more
cooling
- when the winds near the surface are relatively light
Advection fog
! When air moves horizontally (advects) over
a cold surface, the air near the surface cools
! If the air cools to the dewpoint,
condensation takes place and an advection
fog forms.
Advection fog (continued)
! These fogs tend to form
- when air moves over a cold water surface
- when the air has a high relative humidity
to begin with
Mixing fog
! Say two volumes of air are unsaturated but
have a high relative humidity.
! When they mix, the resulting mixture may be
saturated.
Mixing Fog (examples)
!
!
!
!
!
Your breath on a cold day
Condensation trails from jets
Exhaust from automobiles
“Steam fog” over a warm water surface
Fog forming over snow that is melting on the
highway
Fog climatology – average number of days a year
with heavy fog
Cloud Types
Cirrus
Cirrocumulus
Cirrostratus
Altocumulus
Altostratus
Nimbostratus
Stratocumulus
Stratus
“Fair-weather” cumulus
Cumulus congestus
Cumulonimbus
Lenticular or wave clouds