5.03 Moisture References: FTGU pages 135-138, 146-147 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 Dew Point 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? -Warm air can hold much more moisture (water vapour) than cold air. For example, one kilogram of dry air at 30c can hold 30 grams of water vapour before it becomes saturated. One kg of dry air at 0c can hold only 5gms of water vapour before it is saturated. This is because one kg of warm air has a much greater volume than one kg of cold air, thus allowing it to hold a greater quantity of water vapour. 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 The ratio of water present in the air compared to the amount the same volume could hold if it were saturated Temperature to which air must be cooled at constant pressure to become saturated

Changes of State Freezing Evaporation Melting Sublimation Condensation

Changes of State

Confirmation What do you call a change of state from liquid to solid? From solid to gas? From vapour to liquid? Freezing Sublimation Condensation

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? High relative humidity condensation nuclei cooling of the air

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 Above freezing = rain 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 Rain very small drops of water which appears to float Rain Large water droplets

Precipitation Hail Snow Grains Hard transparent layer of ice covering soft white core Snow Grains Tiny snow crystals that have acquired a coating of rime

Precipitation Snow Pellets Snow Soft white ice (hail without hard transparent layer Snow Agglomeration of ice crystals hexagonal/star shaped

Precipitation Ice Prisms Ice Pellets 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 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

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? 2. What is hail? _____ 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? Hard transparent layer of ice covering soft white core

Rain on the Beach