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.

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