Topic 5B: WEATHER RELATIONSHIPS KEY WEATHER VARIABLES Topic 5: METEOROLOGY

There are four key weather variables used to track and record weather conditions. Associated with each of the four are particular instruments used to measure the variable and proper units used in recording the measurements. 1) Temperature :THERMOMETER º FAHRENHEIT (USA) or º CELSIUS

2) Pressure :BAROMETER MILLIBARS ( mb ) or INCHES of MERCURY ( in Hg ) Note: As Temperature INCREASES, Pressure DECREASES, LOW Pressure = WARM Air so LOW Pressure = WARM Air

3) R elative Humidity : HYGROMETER (The most accurate type is the S SS SLING PSYCHROMETER ) PERCENT ( % )

4) Wind Velocity : ANEMOMETER ANEMOMETER - SPEED ( mi /hr or km /hr) WIND VANE WIND VANE - DIRECTION ( FROM which the wind blows) ANEMOMETER WIND VANE

ATMOSPHERIC MOISTURE: HUMIDITY: Amount of moisture ( WATER V APOR ) in air WARM air is able to hold more M OISTURE than C OLD air, so T EMPERATURE indicates air’s M OISTURE CAPACITY ( the amount of M OISTURE the air C AN hold) SATURATED Air = Air holding the most M OISTURE possible at the air’s current T EMPERATURE SATURATED = FULL

DEW POINT : DEW POINT : The TEMPERATURE at which a mass of air would become SATURATED Indicates air’s MOISTURE CONTENT (the amount of MOISTURE the air DOES hold)

RELATIVE HUMIDITY : MOISTURE actually in the air compared with the MOISTURE that the air COULD hold at the current TEMPERATURE, given as a PERCENT Found by comparing DEW POINT (indicating Moisture CONTENT ) to TEMPERATURE (indicating Moisture CAPACITY ) Example: Temperature = 20ºCTemperature = 20ºC Dew Point = 5ºCDew Point = 18ºC LOWHIGH LOW RH HIGH RH

Which basic variable is the “key” variable for tracking weather systems and predicting weather? AIR PRESSURE

Moisture enters the ATMOSPHERE by 1) EVAPORATION (mainly from the OCEANS ) 2) TRANSPIRATION : Release of WATER VAPOR from the LEAVES of PLANTS

One acre of Corn releases 4,000 gallons of water vapor into the atmosphere EVERY DAY.

Factors affecting EVAPORATION RATE : 1) TEMPERATURE  2) SURFACE AREA  3) MOISTURE CONTENT of the Air  4) WIND SPEED  5) SALT CONTENT of the Water  DIRECT INVERSE DIRECT DIRECT INVERSE

WATER VAPOR leaves the Atmosphere by: CONDENSATION ( GAS to LIQUID ) or by SUBLIMATION ( GAS directly to SOLID ) SUBLIMATION of WATER VAPOR produces ICE CRYSTALS ( FROST and SNOW )

For CONDENSATION or SUBLIMATION to occur: 1) Air must be SATURATED, usually when TEMPERATURE falls to the DEW POINT 2) CONDENSATION NUCLEI must be present to provide a SURFACE (provided in the Atmosphere mainly by AEROSOLS )

Average rain drop size = 2 millimeters

Evaporation T ranspiration Condensation Sublimation

As CONDENSATION or SUBLIMATION occurs in RISING Air, Masses of tiny WATER DROPLETS or ICE CRYSTALS form, held up by the RISING air CLOUDS: form at the ALTITUDE where TEMPERATURE has dropped to the DEW POINT, called the CLOUD BASE

If the CONDENSATION Rate is HIGH, WATER DROPLETS or ICE CRYSTALS join, becoming HEAVY enough to fall through the RISING air as PRECIPITATION

As Concentration of AEROSOLS in the Atmosphere INCREASES, Atmospheric TRANSPARENCY (Absence of SOLID Particles) and PROBABILITY of PRECIPITATION INCREASES due to the increased number of CONDENSATION NUCLEI available High Tranparency LowTranparency Low Tranparency DECREASES

Note that Atmospheric TRANSPARENCY does NOT refer to CLEAR Skies (the Absence of CLOUD COVER), BUT to the Absence of AEROSOLS As PRECIPITATION Rate INCREASES, Atmospheric TRANSPARENCY since PRECIPITATION Removes AEROSOLS from the Atmosphere INCREASES,

As Temperature In creases, (1) Air (BAROMETRIC) PRESSURE De creases Key Weather Relationships

As Temperature In creases, (1) Air (BAROMETRIC) PRESSURE De creases (2) Air’s MOISTURE CAPACITY In creases (3) RELATIVE HUMIDITY De creases IF Air’s MOISTURE CONTENT Remains the Same Key Weather Relationships ( DEW POINT )

MAJOR GASES in the ATMOSPHERE Key Atomic Masses: H = 1C = 12N = 14O = 16 NITROGEN  N 2 = OXYGEN  O 2 = CARBON DIOXIDE  CO 2 = WATER VAPOR  H 2 O = N O CO 2 H2OH2O

Since WATER VAPOR is LESS DENSE than other gases in Air (which is made up largely of NITROGEN and OXYGEN) As Air’s MOISTURE CONTENT In creases, Air’s DENSITY De creases, so Air PRESSURE De creases N O N N N N N O O N N N N N N N O N N N N O N N N N N

Air Temp Dwpt Temp Big difference Low chance of precipitation Small difference High chance of precipitation LOW RHHIGH RH As the DIFFERENCE between TEMPERATURE and DEW POINT De creases, RELATIVE HUMIDITY In creases, so PROBABILITY of PRECIPITATION In creases

As TEMPERATURE De creases or as DEW POINT (MOISTURE CONTENT) In creases, TEMPERATURE ― DEW POINT DIFFERENCE De creases, so both RELATIVE HUMIDITY and PROBABILITY of PRECIPITATION In crease TEMPERATURE DEW POINT TEMPERATURE DEW POINT

As Concentration of AEROSOLS In creases, Atmospheric TRANSPARENCY De creases, and PROBABILITY of PRECIPITATION In creases (due to Increased CONDENSATION NUCLEI)

As PRESSURE GRADIENT In creases, WIND VELOCITY In creases

If the first factor in each pair increases, does the second factor increase or decrease ? 1) Temperature Pressure 2) Wind Speed Evaporation Rate 3) Temperature-Dew Point Difference Relative Humidity 4) Temperature Evaporation Rate 5) Temperature Relative Humidity 6) Relative Humidity Evaporation Rate Increases Decreases Increases Decreases