Unit 2 topic 3

• Moisture in the atmosphere


  Gas, liquid, solid
• Processes that alter moisture states


  1. Evaporation

  2. Condensation

  3. Freezing

  4. Melting

  5. Deposition

  6. Sublimation
• Causes of precipitation


  1. Convection

  2. Frontal uplift

  3. Orographic uplift
• Cooling of air


  Radiation cooling
• Relative humidity


  • Saturation
  • Dew point
  • Condensation level
  • Condensation nuclei
• Types of precipitation


  • Clouds
  • Rain
  • Hail
  • Snow
  • Dew
  • Fog

  
• Evaporation


  • Heat energy from solar radiation transfers to surface water, changing it to water vapour
  • Affected by: amount of solar energy, availability of water, humidity of air, air temperature
• Condensation nuclei


  Small particles (e.g. smoke, salt, dust) that water vapour condenses on
• Condensation


  • Direct cause of all forms of precipitation
  • Occurs when air temperature is reduced to dew point
  • Occurs when air volume increases with no heat addition (e.g. air rising over mountains, air masses meeting)
• Freezing, melting, deposition, sublimation


  1. Water vapour condenses directly into ice crystals below 0°C

  2. Dew forms as condensation and water droplets are deposited on cold surfaces

  3. Hoar frost forms when water vapour deposits directly onto freezing surfaces
• Frontal rainfall


  Occurs when warm and cold air masses meet, with the warm air forced to rise over the cold air, causing condensation and precipitation

  
• Relief (orographic) rainfall


  Occurs when moist air is forced to rise over mountains, causing condensation and precipitation on the windward side, with a rain shadow on the leeward side

  
• Convectional rainfall


  Occurs when the Earth's surface is heated, causing the air above to rise, cool, and condense, forming towering cumulonimbus clouds that produce heavy rain and possible thunderstorms

  
• The graph shows annual precipitation totals across a mountain range
• Adiabatic processes refer to the rising and sinking of air, which causes cooling and warming respectively
• Lapse rate


  • The rate of temperature decrease with altitude, averaging 6°C per 1000m (Environmental lapse rate)
  • Dry adiabatic lapse rate is higher at around 10°C per 1000m
  • Saturated adiabatic lapse rate is lower at around 4-9°C per 1000m
• Atmospheric stability


  • Stable conditions exist when rising air cools more quickly than surrounding air, causing it to sink
  • Unstable conditions exist when rising air cools more slowly than surrounding air, causing it to continue rising and form clouds/precipitation
  • Conditional instability exists when air is stable when dry but unstable when saturated
• The diagram shows a cross-section of relief and average annual precipitation amounts in the western USA

  
• Clouds form when rising air cools to the point where it can no longer hold its water vapour, causing condensation
• Table if it is dry and would sink to the ground, but if it becomes saturated, it is forced to rise and may become unstable
• How Clouds Form


  1. Sun heats up the ground

  2. Ground warms up the air

  3. Hot air rises

  4. Air cools

  5. Water vapour condenses, forming clouds

  6. Water droplets get too big, they fall out as rain
• Main cloud types


  • Cirrus (wispy)
  • Cumulus (heaped)
  • Stratus (in layers)
• Nimbus


  Added to cloud type means the cloud is a rain cloud
• Principal cloud forms


  • High clouds - cirrus, cirrostratus, cirrocumulus
  • Middle clouds - altostratus, altocumulus
  • Low clouds - nimbostratus, stratocumulus, stratus
  • Clouds with vertical development - cumulus, cumulonimbus
• Cumulus


  Fluffy clouds that can bring a lot of rain
• Stratus


  Big blankets of cloud that can cover the sky but don't bring too much rain
• Cirrus


  Thin, wispy, high clouds
• Cloud formation is not evenly distributed across the Earth's surface due to global atmospheric circulation
• At the equator, high temperatures result in high rates of evaporation. The warm moist air rises, cools, and condenses to form towering banks of cloud and heavy rainfall in a low-pressure zone known as the ITCZ (Inter-tropical Convergence Zone)
• In the mid-latitudes, cloud formation is driven by the convergence of warm Tropical air and cold Arctic air. The boundary of these two air masses creates the Polar Front. This results in rising air and cloud formation

  
• Strong upper-level winds in the Jet Stream drive these unstable weather systems across the mid-latitudes, resulting in largely changeable conditions like those experienced in the UK
• Cloud formation can occur on a localised scale, e.g., convectional thunderstorms. However, this is very "hit and miss" but demonstrates that variations in water cycle processes can occur locally

  
• Absolute humidity


  The amount of water vapour in the atmosphere, e.g., 8 grams of water in a cubic metre of air
• Relative humidity


  The amount of water vapour present in the atmosphere expressed as a percentage of the maximum amount of water the air can hold at that temperature, e.g., 50%. Completely saturated air will have a humidity of 100%
• Why Relative Humidity Changes


  As air is warmed, the amount of moisture it can hold increases, so if no new moisture is added to a pocket of air as it warms, its relative humidity will decrease
• Radiation Cooling


  Occurs in low-lying areas during calm weather, especially during spring and autumn. The ground's surface, cooled rapidly at night by radiation, cools the air immediately above it. This air then flows into hollows by gravity and is cooled to dew point, causing condensation
• How Hail Forms


  1. Raindrops are carried up and down in vertical air currents in large cumulonimbus clouds

  2. Raindrops freeze and partially melt several times before the pellet is large enough to escape the cloud

  3. As the hailstone falls, the outer layer may melt but freeze again with further uplift

  4. The process can occur many times before the hail finally falls to the ground
• Snow


  Frozen precipitation that forms when the temperature is below freezing point and water vapour is converted into a solid

  
• Dew


  The direct deposition of water droplets onto the surface and vegetation that occurs in clear, calm, anticyclonic conditions where there is rapid radiation cooling by night
• How Fog Forms


  1. Radiation fog occurs when the Earth's surface cools the air above it to dew point, causing condensation

  2. Advection fog is formed when warm, moist air flows horizontally over cooler land or sea