1. WEEK 4: 19 SEP 2017 Weather Review
Concept of Temperature (supp. Notes);
Humidity and condensation and cloud formation;
Scientific Review – Definition; (supp. Notes)
Hurricanes – an introduction:
Names, numbers, season, examples (supp. Notes)
HOMEWORK #1 ISSUED (click) (instructions via )

3. HUMIDITY, CONDENSATION &
CLOUD FORMATION

4. Earth’s Water Cycle
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

5. Review: PHASE TRANSITIONS
Water is the only substance that can be found in the atmosphere in the gaseous, liquid and solid phases

6. SATURATION
When Rate of Evaporation = Rate of Condensation

7. SATURATION
Saturation means that the rates of evaporation and condensation are in equilibrium and any additional water vapor that is added will throw this balance off.
Rate of Evaporation = Rate of Condensation.
To compensate there will be condensation (i.e., cloud formation and/or precipitation).
At higher temperatures the rate of evaporation is higher, so more water vapor is needed to achieve equilibrium. Therefore saturation is temperature dependent.

8. PARTIAL PRESSURES Total Pressure = PN2+ PO2 + PAr + PH20 + PCO2 + …
Vapor Pressure:
The pressure due to
water molecules alone.
If the total pressure
were ~ 1000 mb,
PN2 ~ 780 mb %
PO2 ~ 210 mb %
PH20 ~ 10 mb %

9. JANUARY WATER VAPOR (mb)

10. JULY WATER VAPOR (mb)

11. Summary of Humidity Definitons
Vapor Pressure
Partial pressure of water vapor molecules (hPa)
Saturation Vapor Pressure
Partial pressure of water vapor in saturated air
Absolute Humidity
Mass of water vapor in a fixed volume of air
Specific Humidity
Mass of water vapor in a fixed total (wet & dry) mass of air (g/kg)
Mixing Ratio
Mass of water vapor in a fixed mass of dry air (g/kg)

12. RELATIVE HUMIDITY Relative Humidity = water vapor content
water vapor capacity
RH = 100 X actual vapor pressure
saturation vapor pressure
Measure of how close air is to becoming saturated, NOT how much water vapor is in the air.
If RH = 100%, then the air is saturated and condensation occurs.

13. RELATIVE HUMIDITY BASICS
When water vapor content or temperature changes, so does relative humidity.
With a constant water vapor content, cooling the air raises the RH and heating the air lowers it.
The Dew Point Temperature is a good measure of the water vapor content in the atmosphere.

14. TEMPERATURE vs. RH Highest RH normally early morning
Lowest in the afternoon.
The change in temperature, changes the saturation vapor pressure.

15. TEMPERATURE vs. RH

16. POLAR AIR vs. DESERT AIR Polar Air Temp=28° DP=28° RH=100%
Desert Air Temp=95°
DP=41° RH=16%

17. Summertime Relative Humidity

18. SLING PSYCHROMETER Gives Wet Bulb Temperature Wick
Wet Bulb Thermometer
Dry Bulb Thermometer
Gives Wet Bulb Temperature

19. HAIR HYGROMETER
As RH increase Hair length Increases
“Bad hair day”

20. ELECTRONIC MEASUREMENTS
Measuring Dew-Point Temperature
Cool a mirror until condensation appears on mirror
Use optical device to detect water and measure temperature
Measuring Humidity
Electric current through ceramic material
Change in Resistance = Change in Humidity

21. THE FORMATION OF DEW & FROST
Dew forms on objects when they cool below the dew point temperature.
Most likely on clear, calm nights due to increased radiative cooling
Frost forms when dew point is below 32°F
Frozen Dew when dew initially forms above 32° F and then the temperature drops to below freezing. Looks “pebbly”.

22. HEAT INDEX
Figure 4.12 Air temperature (°F) and relative humidity are combined to determine an apparent temperature or heat
index (HI). An air temperature of 96°F with a relative humidity of 55 percent produces an apparent temperature (HI)
of 112°F.

23. Dew
Frost
Frozen Dew
Dew forms on clear nights when objects on the surface cool to a temperature below the dew point. If these beads of water should freeze, they would become frozen dew.

24. CLOUD CONDENSATION NUCLEI
Good CCNS are hygroscopic (“like” water)
Natural CCNs
Sea salt particles
Natural sulfur emissions
Vegetation burning
CCNs from human activity
Pollutants from fossil fuels
Sulfur dioxide > particulate sulfuric acid and ammonium sulfate salts
Nitrogen oxides > gaseous nitric acid which can combine with ammonia to form ammonium nitrate particles

25. TYPICAL DROPLET SIZES

26. CLOUD DROPLET FORMATION
Below the Dew Point water vapor will tend to condense and form cloud/fog drops
Formation on cloud condensation nuclei (CCN)
Most effective CCN are water soluble.
Without particles clouds would not form in the atmosphere

27. STEPS IN CLOUD FORMATION
Air cools causing RH to increase
Radiative cooling at surface or
Expansion in rising parcel
CCN take up water vapor as RH increases
Dependent on particle size and composition
If RH exceeds critical value, drops are activated and grow readily into cloud drops

28. Cloud in Contact with the Ground Five Types
FOG
Cloud in Contact with the Ground
Five Types
Radiation Fog
Advection Fog
Upslope Fog
Evaporation Fog
Precipitation Fog

29. RADIATION FOG Moist ground Clear Skies Calm Wind
Surface cooling via radiation
Lowest air near the ground cools to dew point
Fog deepens from the ground up
Ideal Conditions
Moist ground
Clear Skies
Calm Wind
“Tule” Fog

30. RADIATION FOG Radiation fog nestled in a valley in central Oregon.
Fig. 4.4, p. 98

31. RADIATION FOG
Radiation fog nestled in a valley in central Oregon.

32. ADVECTION FOG Cold upwelled water
Warm air advects (moves) over cold surface
Cold surface cools air
Saturation = fog formation
Common on West Coast
Cold upwelled water

33. ADVECTION FOG
“Fog Drip”
20-40% of a Redwood’s water

34. ADVECTION FOG

35. UPSLOPE FOG Moist air flows up along slope
Expansion of rising air > cooling and RH increases

36. EVAPORATION FOG Warm, moist air mixes with colder air > saturation
Evaporation (steam) fog
Warm, moist air mixes with colder air > saturation
Examples
Exhale on a cold day
Evaporation from a relatively warm lake and mixing with colder air above.
Smokestack plume

37. PRECIPITATION FOG
Evaporation of falling rain cools air and leads to saturation

38. FOG and VISIBILITY Traffic fatalities Airport accidents and closures
Light scattering by fog drops degrades visibility
Traffic fatalities
Airport accidents and closures
Mitigation
Fog monitoring and warning (optical sensors)
Fog dispersal (expensive and of limited utility)

40. READING ASSIGNMENT Tue, Sep. 26th & Oct. 10th
Pielke and Pielke; Chapter 2 (Point of view:societal vulnerability to disasters associated with hurrcianes)Simmons and Sutter: Foreword and Ch. 1 (Point of view: societal impacts should partially dictate the direction of scientific research and governmental response)
Simmons and Sutter: Chapter 2; (Point of View; How do tornado frequencies suggest the societal vulneratbility tornado disaster?)
Williams: Chapter 3 (all); Chapter 4 (Fronts), Chapter 5, (The causes of wind, clouds, and fronts).
Zebrowski: Chapter 3, pp ; 77-83; ; Chapter 5, pp ; Chapter 8, (The interrelationship of human settlement patterns and natural disaster, how winds affect the surface of the sea, hurricane-related flood and wind disasters )