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Meteorology
5.03 MoistureReferences:
FTGU pages 135-138, 146-147
5.03 Moisture
• MTPs:– Humidity– Changes of State– Dew and Frost– Cloud Formation– Precipitation– ICAO Lapse Rates– Inversion and Isothermals
Humidity
• Relative Humidity – Ratio of water present in
air compared to the amount the same volume could hold if it were saturated
• Dew Point – Temperature to which
air must be cooled at constant pressure to become saturated
Humidity
• The warmer the air, the more water vapour it can hold...Why?
1 kg air at 0 degrees = 5g water
1 kg air at30 degrees = 30g water
Humidity
• Example– If we took a parcel of air at 10oC at 100%
relative humidity and warmed it up, how would the relative humidity change?
Confirmation
• Define Relative Humidity
• Define Dew Point
Changes of State
• Freezing• Evaporation• Melting• Sublimation• Condensation
Changes of State
Confirmation
1. What do you call a change of state from liquid to solid?
2. From solid to gas?
3. From vapour to liquid?
Dew and Frost
• Dew and Frost form on clear, still nights• Vegetation and other objects cool by radiation below the
dewpoint• If the dewpoint is above freezing, dew will form by
condensation• If the dewpoint is below freezing, frost will form by
sublimation
Cloud Formation
• Invisible water vapour becomes visible as water droplets or ice
• Condensation of water vapour
Cloud Formation
• What is required:– High relative
humidity– Condensation
nuclei– Cooling of the
air• Cold surface• Adiabatic
cooling
Cloud Formation
• Steps– Air is heated
and rises– Air cools to
point of saturation
– Air condenses onto condensation nuclei
Confirmation
• What are the three things required for cloud formation?
Precipitation
• Water droplets grow in size and weight and fall due to gravity– Can also occur below freezing (water vapour and ice
crystals)
The average rain drop is a million times larger than a cloud water droplet
Precipitation
If the cloud is…..
• Below freezing – joining of ice crystals
• Above freezing = rain• If temp below is cold
enough to allow crystals to fall to ground = snow
Precipitation
• Regions of a cloud
• Snow
• Rain and/or snow
• Rain
• Large drops and heavy rain = strong vertical motion
Precipitation
Types of precipitation
• Drizzle – very small drops of
water which appears to float
• Rain– Large water
droplets
Precipitation
• Hail– Hard transparent
layer of ice covering soft white core
• Snow Grains– Tiny snow
crystals that have acquired a coating of rime
Precipitation
• Snow Pellets– Soft white ice
(hail without hard transparent layer
• Snow– Agglomeration of
ice crystals hexagonal/star shaped
Precipitation
• Ice Prisms– Tiny ice crystals in the
form of needles
• Ice Pellets– Formed by freezing of
raindrops
Confirmation
• What are the 8 different types or precipitation?
ICAO Lapse Rates
• Lapse rate – Rate of decrease in temperature with
altitude
ICAO Lapse Rate
• Lapse rates:
• Recall ICAO Standard Atmosphere:– Air is perfectly dry gas– Mean sea level pressure of 29.92– Mean sea level temp of 15°C
ICAO Lapse Rates• Can determine base of clouds:
– Temperature on ground 10 degrees– Dew point 7 degrees– Lapse Rate Dry Adiabatic (3°C/1000 ft)– Cloud base = 1000 feet
Inversion and Isothermals
• There are exceptions to standard lapse rates
• Inversion– Increase in temperature with altitude
• Isothermal Layer– Layer in which temperature remains the
same• Both these conditions produce stability.
Confirmation
1. What is the dry adiabatic lapse rate?– _____ degrees per _________feet
2. What is hail?
3. A parcel of air has a relative humidity of 50%. If the temperature were to decrease how would the relative humidity change?
Rain on the Beach