Moisture in the Atmosphere Flashcards
Water Vapour
water as gas. amount of water vapour in atmosphere = humidity. Temp of air affects amount of water vapour
Rain
Liquid water found atmosphere when air temperature cool for vapour condense. Water vapour is initially very light and small, and remain suspend in
the air as cloud droplets. They will join together, become heavy and
eventually fall into the ground. Liquid found in atmosphere as clouds or rain
Ice
temp of air below 0, water freeze. This how hailstones fall. If water sublimes - becomes snow
Coalescence
water droplets bump into each other becoming
bigger as a result of air movement within the cloud.
Types of precipitation:
rain snow hail frost sleet
What refers to water in the air
Dew point
Condensation level
Humidity
Relative humidity
Specific humidity
The mass of the water vapour (in grams) contained in a kilogram of dry air.
Absolute humidity
The mass of water vapour (in grams) contained in a cubic metre of dry air.
Relative humidity
The proportion of water vapour in an
air parcel compared to the amount of water vapour it
could hold at a given temperature.
Saturated air parcel
Relative humidity = 100%. Lots of moisture in air
What affects humidity
Temp of air and water present in atmosphere. Warm air holds more moisture
Relative Humidity calculations
(Actual amount/Saturation amount) *100 = x%
dew point
Temp where air condenses. Higher altitudes = lower dew point temp. Height at which dew pt is reached - condensation level
Why does it rain in the Highveld during the
afternoon?
Heating will raise the temperatures and increase evaporation
(also cause convection currents)
Heated air holds more moisture ( expanding air -> more
space to hold moisture) and increases as the day goes on.
By late afternoon the air may be full of moisture and can be
seen as clouds.
Leads to afternoon rainstorms, sometimes have hail too.
(Why? Think convection currents).
When air temperature drop the water condenses and can
cause rain
Cumulous cloud shape
Patchy looking
Stratus cloud
Layered clouds can form at any height and are the most
common type
Nimbus(rain) cloud
Nimbostratus and cumilonimbus
Cloud formation
Condensation and water vapor needed.
Rising currents help to suspend water droplets in the sky
Water droplets will fall when they get to heavy.
Solid surfaces in the sky act as condensation nuclei and help
promote the condensation process
Coalescence
Air movements in clouds cause tiny droplets to collide
Rain formation
Coalescence occurs. More turbulent the air, bigger the drops until become too heavy and fall to earth. Avg rain drop 2mm in diameter.
How rainfall show on map
Colour shading and isohyets - lines on map joining places of equal rainfall.
trigger action
natural mechanism that causes air to rise
Types of rain
Convection(heating), Frontal/cyclonic(Sea) and Relief/orographic(Mountain)
Convection rain
Convection rain is a thunderstorm. Surface heating is trigger action.
Steps of convection rain
Cumulous stage - hot air rises causing cumulous clouds grow higher. Mature stage - Cumulous clouds reach 7km high. Air gets cold and starts to sink. Both up and down currents are now present. Is now a cumulonimbus cloud. Friction between currents causes electrical imbalance results lightning. Very strong currents result water drops freezing causing hail. Dissipating stage is once everything gets too heavy and falls.
Relief rain
Mountains force air to rise causing it to condense and result in rain. The mountain is the trigger action
Steps of relief rain
Moist air blows off an ocean and rises up the mountain
range.
The windward side of the mountain is the side the wind
strikes first and the leeward side is the side the wind is
blowing away from
Air rises, cools and condenses on the windward side.
Turbulence causes coalescence and start to fall as rain.
Most moisture is lost on the wind ward side and little is left
on the leeward side.
The drier side of the mountain is called the rain shadow.
Orographic rain is usually gentle rain that lasts as long as the air
coming in from the ocean is being forced up the mountain.
The cloud responsible is normally Nimbostratus but cumulonimbus
can form if the air rises very quickly.
Drakensberg and Cape fold mountains
front
boundary between two different kinds of air - usually hot+cold. Fronts develop between warm, moist air masses and dry, cold air masses.
frontal rain trigger action
trigger action is the front
Frontal rain steps
When the air masses meet the warm air will rise over the cold air as warm air is less dense and lighter.
Warm air will rise, cool and condense.
A warm front has a very gentle slope
With a variety of clouds. The rain falls
from the Nimbostratus.
A cold front has a steeper slope which
makes warm air rise more quickly.
Cumulonimbus clouds
Hail formation
Strong convection currents cause up and down currents in the
cumulonimbus clouds.
Water drops carried above freezing level
Begins to fall and collect more water
After each updraft forms a new layer
Continues until hail too heavy and falls. Melts slightly while falling
Formation of snow:
Water vapour condensing onto condensation nuclei to form ice
crystals. Turning from water vapour to snow without condensing is
called sublimation
Snow crystals float and freeze onto each other getting bigger
They are six sided and fall from nimbostratus clouds.
Increase in size by further sublimation or by collisions (known as
snow flakes)
Dew
Condensed water droplets seen on grass/plants
Dew formation
At night land loses heat to terrestrial radiation. The cold land cools the
air above it.
IF the temperature drops below dew point water vapor on the
ground condenses to from dew. Dew most common in spring+autumn in highveld
Frost
If temperature drops below freezing, instead of water vapor condensing, it sublimates . Bad for certain plants but good for citrus. Protect plants with fans/artificial heating get frost resistant crops.
Frost formation in valleys
Clear calm and windless nights (wind mixes air)
Land on side of the valley cools and begins to sink to valley floor
Warm air at the bottom of valley rises above the cold sinking air to
form the inversion layer.
Cold air on the valley floor cools and pools in the frost pocket. This is
where fog will develop once dew point temperature is reached.
The water droplets will freeze if the temperature hits freezing point.
