1. Humidity Under what conditions do you see the above?
Showers, cool drinks on a hot day, parking.
Under what conditions do you see the above?

2. 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

3. Measuring relative humidity
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)

4. Saturation quantity: maximum capacity of water vapor the air can hold
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
Numbers are idealized
Air temperature

5. 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

6. 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

7. What are our diurnal patterns of humidity?
When are our maximum and minimum relative humidities? [Lexington weather conditions]

8. 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. Explain why and where in their homes people use these appliances and how they work
Humidifier
Dehumidifier

11. Where would you expect to use an evaporative cooling system (also known as a swamp cooler?

13. 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

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

15. Frost Radiation frost Advection 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

16. Clouds:
Air temp cooled to dew point
Liquid water condensates out of water vapor RH = 100%

17. But most of the water vapor is at the surface
But most of the water vapor is at the surface. A mechanism is needed to get water vapor to cool, condensate and form clouds. What might that mechanism be?
Large amount of water vapor in red. The y-axis is
denotes altitude above the surface.

18. LIFTING: If you can get the surface air to rise it will cool, reach dew point, and form clouds
Cooler aloft
Surface
Large amount of water vapor in red. The y-axis is
denotes altitude above the surface.

19. Sources of lifting

20. 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.

21. Adiabatic processes – changing temperature by changing air pressure
Adiabatic warming
Adiabatic cooling

23. Atmospheric stability
Stability is a measure of the probability of cloud formation

24. The troposphere is the layer of the atmosphere in which most of our relevant weather processes take place.

25. Skew-t plot Red line shows air temp and blue line shows dew point temp
At what pressure level
are clouds likely to
form ?
Tracking the temperature and dew point of a parcel of air as it is lifted through the atmosphere…..

26. 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

27. 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

28. 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.

29. Stable atmospheric conditions
Clear skies; any cloud development is high and lacking a
strong vertical dimension. Vertical motions resisted.

30. Unstable atmospheric conditions
Cumulonimbus
A thunderstorm, a hallmark of unstable atmospheric
Conditions. Atmosphere promotes vertical motion.

31. 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?

32. Sources of lifting

33. Convectional surface heating
Surface heating and wind convergence lead to cloud formation on the Florida peninsula
Convectional surface heating

34. Wind convergence and surface heating (along the ITCZ)
ITCZ, zone of wind convergence and surface heating leading to low pressure and cloud formation

35. Orographic lifting

36. Rainshadow effect, a product of orographic lifting
Rainshadow effects in Washington State.

37. Topography of Washington state

38. Rainshadow deserts
North American rainshadow deserts: Death Valley (Sierra Nevada), eastern Washington and eastern Oregon (Cascades)

39. Orographic lifting
Rainshadow effects on Hawaii

40. Orographic lifting

41. Frontal lifting
Cold front