Meteorology
5.03 Moisture
References:
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:
Dry Adiabatic
3.0 °C / 1000 ft
Saturated Adiabatic
1.5° C / 1000 ft
Standard
2.0 °C / 1000 ft
ICAO
1.98 °C / 1000 ft
• 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.
More on this in 5.05
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