#### Transcript Humidity and Atmospheric Stability

```Define relative humidity
Calculate the humidity of an air parcel
Explain the relationship between air temperature and humidity
Explain how relative humidity varies diurnally
Explain how dew and frost form
Define stability
Explain how clouds form
Explain how air can be heated or cooled by adiabatic processes
State the values for the saturated (or moist) adiabatic rate (SALR) and the dry
Describe how the environmental lapse rate (ELR) differs from the SALR and
DALR
Explain why the DALR is larger than the SALR
Explain what happens 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
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)
• AMR: Actual mixing ratio
• SMR: Saturated mixing ratio
A plot of SMR (left) shows that
warm air can hold more
water vapor than cold air
SMR
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.
Temperature along the line is
called the dew point
temperature.
Air 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 (adiabatic processes)
What are our diurnal patterns of
humidity?
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 heat stroke
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
– The greater the difference in
temperature between wet and dry
bulb the lower the relative
humidity
Dew
• High humidities
during the day
• Clear night with
• Air temp cools to dew
point
• Condensation forms
on surface (dew)
Frost
– Occurs on days with high humidities
and clear calm nights
– Dew point temperature goes below
freezing a few inches above the
surface
– More common on cloudy, windy
nights with strong cold air advection
– Dew point temps go below freezing
– Deeper freezing layer coats ground
and trees
Clouds: liquid water droplets formed from the condensation of
water vapor when air is cooled to its dew point
…but most of the water vapor is at the surface How can we
get this surface water vapor to cool, condensate, and form
clouds up high?
Upper
troposphere
Surface
Large amount of water vapor shown in red. The y-axis
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
Upper
troposphere
Cooler aloft
Surface
Surface
Sources of lifting
The troposphere is the layer of the atmosphere in which
most of our relevant weather processes take place.
Upper
troposphere
Air pressure in millibars (mb)
Skew-t plot
Surface
Red line shows
air temp and blue
line shows dew
point temp
At what pressure level
are clouds likely to
form ?
Atmospheric stability
• A measure of the probability
of cloud formation
• The secret of weather
prediction is knowing lifting is
promoted or resisted
– Unstable atmosphere – lifting is
promoted, air rises, cools,
condensates and forms clouds, and
potentially, rainfall.
– Stable atmosphere – lifting is
resisted, even if lifted, air will not
continue to rise but will sink. No
clouds, clear conditions.
• 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
surrounding air pressure increases, parcel
contracts, parcel temp increases
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:
DALR (dry adiabatic lapse rate) and
rate); apply to parcel
Lifting condensation level (LCL) –
where parcel DALR switches to
SALR
ELR – environmental lapse rate;
applies to environment
surrounding parcel
Stability determined by comparing
lapse rates
• The more vertical 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 rising motion is resisted. Low to no cloud
development. If clouds develop, high and thin..
Unstable atmospheric conditions
A thunderstorm, a hallmark of unstable atmospheric
Conditions. Atmosphere promotes vertical motion and
cloud formation.
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?
Lifting mechanisms
Surface heating
Lifting mechanisms
Wind convergence and surface heating (along the
ITCZ)
Lifting mechanisms
Orographic lifting
Direction that
predominant
winds are
blowing
Orographic lifting
Rainshadow effect, a product of orographic lifting
• Rain falls on one side of mountain range
but not the other
• Air is rising and cooling on windward side
of mountain to produce rain
• Air is sinking and warming on leeward side
– Eastern Oregon and Western Washington