Download Humidity and Clouds

Clouds and Humidity
The Water Cycle
Three Requirements for a Cloud:
• A cooling mechanism
• Moisture
• Condensation nuclei
Clouds form by moist air cooling. Warm air can
dissolve more water than cold. We cannot see
dissolved water in air.
Causes of cooling:
• By rising: Adiabatic Cooling
a decrease in pressure (in our lab)
• By contact with a cold surface
There are three main types of clouds:
Stratus
Cumulous
Cirrus
Stratus
• Horizontal Layers, Sheet-like clouds
(condensation is low to the ground)
• Stratus clouds are grayish clouds
that usually cover the entire sky.
• Looks like fog.
Nimbostratus: Indicates light RAIN
Formation of Fog
With some wind, the layer of cold air at the
surface is mixed into the lowest few meters
of the atmosphere. When the temperature
drops below the dew point, the excess water
vapor condenses on the ground as dew and
also in the air as fog (tiny droplets of water).
Cirrus Clouds
•Occur at high altitudes
•Mostly ice crystals
Cirrus Clouds Cause a
Halo around the moon.
Folklore has it that a ring around the moon indicates bad weather is coming, and in many
cases this may be true. The ice crystals that cover the halo signify high altitude, thin cirrus
clouds that normally precede a warm front by one or two days. Typically, a warm front will be
associated with a low pressure system which is commonly referred to as a storm.
Cumulous Clouds
•Fair weather clouds
•Created by vertical movement
•Fluffy, flat-bottomed clouds
Cumulonimbus Clouds
Indicates Heavy Rains
Hail can cause the most
damage, especially if wind
blow. It can damage crops or
structures
Hail forms in cumulonimbus
clouds during a thunderstorm
when water freezes in layers
around a small nucleus of
ice. Hailstones grow larger as
they're tossed up and down by
rising and falling air.
Thunderstorms can produce:
http://upload.wikimedia.org/wikipedia/commons/d/d6/LightningCNP.ogg
• HUMIDITY: Amount of moisture in the air
• RELATIVE HUMIDITY: Ratio of water dissolved IN
air to the amount that COULD BE dissolved in the
air
• SATURATION: Air has all the water vapor it can
possibly “hold” or dissolve.
As temperature decreases relative humidity
increases when water vapor is constant.
Warm air can dissolve more water!
Dry Bulb Temperature
The dry bulb temperature is the air temperature measured using a standard
thermometer. It is the temperature reported in daily weather forecasts and is
sometimes referred to as the ambient air temperature.
Wet Bulb Temperature
The wet bulb temperature with a standard thermometer and a wet piece of cloth
covers the bulb of the thermometer. As air passes over the wet cloth, the water in
the cloth evaporates, drawing heat out of the thermometer.
Relative humidity using wet bulb
and dry bulb temperatures.
• If the air is very humid (moist), only a small amount of
moisture will evaporate from the cloth. This means the
wet bulb temperature will only be a little lower than
the dry bulb temperature.
Conversely, if the humidity of the air is low (dry), the
moisture will evaporate from the cloth quickly. This
means that the wet bulb temperature will be much
lower than the dry bulb temperature.
If it is raining or there is heavy fog, the air is saturated,
and the dry bulb temperature will be equal to the wet
bulb temperature.
Relative Humidity Charts
Dew forms when the air in contact with the ground cools and can no longer “hold”
all the water it had dissolved in it at a higher temperature. The air becomes
saturated. This excess water vapor condenses and this liquid water appears as dew.
The temperature at which this happens is the dew point.
If the dew point temperature is below the freezing point, excess water vapor
changes from the gas to the solid phase (deposition) forming frost.
Dew and frost typically form overnight as temperatures decrease.
Adiabatic Cooling (from rising air)
is different for dry vs wet air.
Why does air cool slower in wet air?
Dry versus Moist-Adiabatic Process
The moist adiabatic lapse rate is less than the dry adiabatic lapse rate
because as vapor condenses into water (or water freezes into ice) for a
saturated parcel, latent heat is released during condensation.
Dry and Moist Adiabatic Cooling
For Chinook Winds the moist air cools at a
slower rate on the way up then it warms on
the way down on the other side.
Fronts
• Boundary between contrasting air masses
• Stationary front: boundary is stationary on the
landscape
• Cold front: cold air moves into a region occupied
by warmer air
• Warm front: warm air moves into a region
occupied by colder air
• occluded front: a cold front "catches up" with a
warm front forcing all warm air off the ground
WARM FRONT
Cold Front
Source Areas for Air Masses
In the Flathead Valley we
are influenced by cold air
masses from the North,
Pacific air masses from
the west and Continental
tropic air masses from
the Southwest.
Phase Changes