Transcript HumidityStability

Define and calculate the humidity of an air parcel
Explain the relationship between air temperature, saturation mixing ratio, and
relative humidity
Explain how relative humidity varies diurnally
Explain how dew and frost form
Define stability
Explain how clouds form
State the values for the saturated (or moist) adiabatic rate (SALR) and the dry
adiabatic lapse rate (DALR)
Contrast the environmental lapse rate (ELR), SALR, and DALR for stable,
unstable, and conditionally instability
Explain how air can be heated or cooled by adiabatic processes
Explain why the DALR is larger than the SALR
Detail the processes that occur at the lifting condensation level (LCL)
From a plot showing ELR, DALR, and SALR determine if the parcel is stable,
unstable, or conditionally unstable for a given altitude.
Explain orographic lifting and the rainshadow effect.
Identify locations where the rainshadow effect is pronounced
Name and explain the four ways in which air can be lifted to form clouds
Humidity
Under what conditions do you see the above?
Humidity
• Measure of the
quantity of water
vapor in the air
• Overall
atmosphere is 78%
nitrogen, 21%
oxygen, and only
1% water vapor.
Idealized representation of the atm: a
“parcel” of air
Measuring relative humidity (RH)
• For a parcel of air, RH is the ratio of the
amount of actual water vapor in the air
(AMR) divided by the maximum amount of
water vapor the air can hold at the given
temp (SMR)
Saturation quantity: maximum capacity
of water vapor the air can hold (SMR)
A plot of saturated mixing ratio
(left) shows that as
temperature increases, the
maximum amount of water the
air can hold increases. Warm
air can hold more water
vapor than cold air
The curve indicates where RH
= 100%, or where the actual
amount of water vapor in the
air is at the maximum for that
given temperature.
Air temperature
Temperature along the line is
called the dew point
temperature.
When RH = 100%:
AMR = SMR
Air is saturated with water vapor
Air temp = dew point
Water condensates out of air
Suspended liquid water droplets form in the air
To reach RH = 100%
Add more water vapor to the air
Cool the air down to its dew point temperature
Lower the air pressure of the parcel
What are our diurnal patterns of
humidity?
When are our maximum and minimum relative humidities?
[Lexington weather conditions]
Heat (or comfort) index: the
human element in humidity
When RH is high, air is
near dew point
temperature and is close
to the maximum amount
of water vapor it can hold
Inefficient evaporative
cooling for humans
Temp experienced by
humans is higher than air
temp
Risk of heat exhaustion
or stroke
Explain why and where in their homes
people use these appliances and how
they work
Humidifier
Dehumidifier
Where would you expect to use an
evaporative cooling system (also
known as a swamp cooler?
Measuring humidity
• Automated sensors most
commonly used today, but
basic principle of sling
psychrometer (right) still
applies
• A sling psychrometer measures
difference between the dry bulb
temperature and the wet bulb
temperature
Dew
• High humidities
during the day
• Clear night with
radiative cooling
• Air temp cools to dew
point
• Condensation forms
on surface (dew)
Frost
• Radiation frost
– Occurs on days with high humidities
and clear calm nights
– Dew point temperature goes below
freezing a few inches above the
surface
• Advection frost
– More common on cloudy, windy
nights with strong cold air advection
– Dew point temps go below freezing
– Deeper freezing layer
Clouds like those below are liquid water droplets (or ice if they
are higher).
…but most of the water vapor is at the surface How can we
get this surface water vapor to cool, condensate, and form
clouds?
Large amount of water vapor in red. The y-axis is
denotes altitude above the surface as a function of air pressure.
A lifting mechanism is needed: If you can get the surface air to
rise it will cool, reach dew point, and form clouds
Cooler aloft
Surface
Sources of lifting
The secret of weather prediction is knowing whether or not air
is rising, or if it is moving downward.
An atmosphere with a propensity for rising air is unstable.
This air may cool to form clouds, and potentially, rainfall.
Stable denotes an atmosphere with air that is not rising or
being lifted. It is not likely to form clouds or rainfall.
Adiabatic processes – changing
temperature by changing air pressure
• Adiabatic warming
• Adiabatic cooling
Atmospheric stability
• Stability is a measure of
the probability of cloud
formation
The troposphere is the layer of the atmosphere in which most of our
relevant weather processes take place.
Adiabatic processes
• Changing the temperature of a parcel of air by
changing air pressure
– Adiabatic cooling: parcel is lifted, surrounding air
pressure decreases, and air parcel expands, parcel
temp decreases
– Adiabatic warming: parcel descends, surrounding air
pressure increases, parcel contracts, parcel temp
increases
Skew-t plot
Red line shows
air temp and blue
line shows dew
point temp
At what pressure level
are clouds likely to
form ?
Lapse rates
A lapse rate is a rate of temperature
decrease per change in altitude.
Three relevant lapse rates for the
basics of understanding stability:
ELR – environmental lapse rate;
applies to environment
surrounding parcel
DALR (dry adiabatic lapse rate) and
SALR (saturated adiabatic lapse
rate); apply to parcel
Lifting condensation level (LCL) –
where parcel DALR switches to
SALR
Stability determined by comparing
lapse rates
• Lapse rate – rate of
change of temperature
• The steeper the slope of
a lapse rate line, the
slower the rate of cooling
• Levels of atmospheric
stability: stable, unstable,
conditional instability
• Level of stability
determined by looking at
all three lapse rates.
Stable atmospheric conditions
Clear skies; any cloud development is high and lacking a
strong vertical dimension. Vertical motions resisted.
Unstable atmospheric conditions
A thunderstorm, a hallmark of unstable atmospheric
Conditions. Atmosphere promotes vertical motion.
Conditional instability
• Conditional instability is the most common
state of the atmosphere.
• A lift is needed to make a stable parcel go
unstable.
• What are the sources of lifting?
Convectional surface heating
Wind convergence and surface heating (along the
ITCZ)
Orographic lifting
Rainshadow effect, a product of orographic lifting
Rainshadow deserts
• North American rainshadow deserts:
Death Valley (Sierra Nevada), eastern
Washington and eastern Oregon
(Cascades)
Orographic lifting
Orographic lifting
Frontal lifting