Download Week 4 Class PPT Notes

WEEK 4: 15 FEB 2016
• Concept of Temperature;
• Humidity and condensation and cloud formation;
• Scientific Review – Definition;
• Hurricanes – an introduction:
• Names, numbers, season, examples
• HOMEWORK #1 ISSUED (click) (instructions via email)
HUMIDITY,
CONDENSATION &
CLOUD FORMATION
Earth’s Water Cycle
Review: PHASE TRANSITIONS
Water is the only substance that can be found
in the atmosphere in the gaseous, liquid and
solid phases
SATURATION
When Rate of Evaporation = Rate of Condensation
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.
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
78%
PO2 ~ 210 mb
21%
PH20 ~ 10 mb
1%
JANUARY WATER VAPOR (mb)
JULY WATER VAPOR (mb)
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)
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.
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.
TEMPERATURE vs. RH
• Highest RH normally early morning
• Lowest in the afternoon.
• The change in temperature, changes the saturation vapor
pressure.
TEMPERATURE vs. RH
POLAR AIR vs. DESERT AIR
Polar Air Temp=28°
Desert Air Temp=95°
DP=28°
DP=41°
RH=100%
RH=16%
Summertime Relative Humidity
SLING PSYCHROMETER
Wick
Wet Bulb Thermometer
Dry Bulb Thermometer
Gives Wet Bulb Temperature
HAIR HYGROMETER
• As RH increase Hair length Increases
• “Bad hair day”
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
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”.
HEAT INDEX
Dew
Frost
Frozen Dew
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
TYPICAL DROPLET SIZES
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
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
FOG
• Cloud in Contact with the Ground
• Five Types
– Radiation Fog
– Advection Fog
– Upslope Fog
– Evaporation Fog
– Precipitation Fog
RADIATION FOG
•
•
•
•
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
RADIATION FOG
Fig. 4.4, p. 98
RADIATION FOG
ADVECTION FOG
• Warm air advects (moves)
over cold surface
• Cold surface cools air
• Saturation = fog formation
• Common on West Coast
– Cold upwelled
water
ADVECTION FOG
“Fog Drip”
•20-40% of a
Redwood’s
water
ADVECTION FOG
UPSLOPE FOG
• Upslope fog
– Moist air flows
up along slope
– Expansion of
rising air >
cooling and RH
increases
EVAPORATION FOG
• 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
PRECIPITATION FOG
• Evaporation of falling rain cools air and leads to saturation
FOG and VISIBILITY
• 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)
READING ASSIGNMENT
• Tue, Feb. 22th & Mar. 8th
1.
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)
2.
Simmons and Sutter: Chapter 2; (Point of View; How do tornado
frequencies suggest the societal vulneratbility tornado
disaster?)
3.
Williams: Chapter 3 (all); Chapter 4 (Fronts), Chapter 5, 63-72. (The
causes of wind, clouds, and fronts).
4.
Zebrowski: Chapter 3, pp. 53-62; 77-83; 96-101; Chapter 5, pp.
143-145; 157-163 Chapter 8, 229-251 (The interrelationship of
human settlement patterns and natural disaster, how winds
affect the surface of the sea, hurricane-related flood and wind
disasters )