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 - PowerPoint PPT Presentation

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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

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Title: Slide 1 Author: Shawn Lewers Last modified by: JAS Created Date: 7/6/2006 8:44:01 AM Document presentation format: On-screen Show (4:3) Company – PowerPoint PPT presentation

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Title: 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


1
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
formDefine 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 SALRDetail 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
2
Humidity
Under what conditions do you see the above?
3
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
4
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)

5
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. Temperature
along the line is called the dew point
temperature.
Saturation quantity maximum capacity of water
vapor the air can hold (SMR)
Air temperature
6
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
7
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
8
What are our diurnal patterns of humidity?
When are our maximum and minimum relative
humidities? Lexington weather conditions
9
Heat (or comfort) index the human element in
humidity
When RH is high, air is near dew point and holds
near maximum amount of water vapor Inefficient
evaporative cooling for humans Temp experienced
by humans is higher than air temp Risk of heat
exhaustion or stroke
10
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11
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12
Explain why and where in their homes people use
these appliances and how they work
Humidifier
Dehumidifier
13
Where would you expect to use an evaporative
cooling system (also known as a swamp cooler?
14
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15
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

16
Dew
  • High humidities during the day
  • Clear night with radiative cooling
  • Air temp cools to dew point
  • Condensation forms on surface (dew)

17
Frost
  • Radiation frost
  • Occurs on days with high humidities and clear
    calm nights
  • Dew point temperature goes below freezing
  • Advection frost
  • More common on cloudy, windy nights with strong
    cold air advection
  • Dew point temps go below freezing

18
Clouds Air temp cooled to dew point Liquid water
condensates out of water vaporRH 100
19
A mechanism is needed to get water vapor to cool,
condensate and form clouds. Most of the water
vapor is at the surface. What might that
mechanism be?
Large amount of water vapor in red. The y-axis
is denotes altitude above the surface as a
function of air pressure.
20
LIFTING If you can get the surface air to rise
it will cool, reach dew point, and form clouds
Cooler aloft
Surface
21
Sources of lifting
22
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.
23
Adiabatic processes changing temperature by
changing air pressure
  • Adiabatic warming
  • Adiabatic cooling

24
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25
Atmospheric stability
  • Stability is a measure of the probability of
    cloud formation

26
The troposphere is the layer of the atmosphere in
which most of our relevant weather processes take
place.
27
Skew-t plot Red line shows air temp and blue
line shows dew point temp At what pressure
level are clouds likely to form ?
28
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

29
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
30
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.

31
Stable atmospheric conditions
Clear skies any cloud development is high and
lacking a strong vertical dimension. Vertical
motions resisted.
32
Unstable atmospheric conditions
A thunderstorm, a hallmark of unstable
atmospheric Conditions. Atmosphere promotes
vertical motion.
33
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?

34
Sources of lifting
35
Convectional surface heating
36
Wind convergence and surface heating (along
the ITCZ)
37
Orographic lifting
38
Rainshadow effect, a product of orographic lifting
39
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40
Rainshadow deserts
  • North American rainshadow deserts Death Valley
    (Sierra Nevada), eastern Washington and eastern
    Oregon (Cascades)

41
Orographic lifting
42
Orographic lifting
43
Frontal lifting
44
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