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 email)

HUMIDITY, CONDENSATION & CLOUD FORMATION

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

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° DP=28° RH=100% Desert Air Temp=95° DP=41° RH=16%

Summertime Relative Humidity

SLING PSYCHROMETER Gives Wet Bulb Temperature 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 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.

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.

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

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

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

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

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

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

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

ADVECTION FOG

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

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

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

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)

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, 63-72. (The causes of wind, clouds, and fronts). 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 )