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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

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Humidity

•  The warmer the air, the more water vapour it can hold...Why?

1kgairat0degrees=5gwater

1kgairat30degrees=30gwater

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

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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?

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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

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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?

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Precipitation •  Water droplets grow in size and weight and fall due

to gravity –  Can also occur below freezing (water vapour and ice

crystals)

Theaverageraindropisamillion1meslargerthanacloudwaterdroplet

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

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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

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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?

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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

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

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

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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