weather processes and phenomena

• orographic rain:

  1. evaporation from the ocean, as water absorbs UV from the sun
  2. wind moves moist air up the mountain, which acts as a barrier
  3. air rises and cools due to adiabatic cooling
  4. water vapour condenses when dew point is reached, forming nimbostratus clouds
  5. further cooling leads to precipitation, which releases latent heat warming the air
  6. warm air moves down the other side of the mountain
• adiabatic cooling - pressure of atmosphere decreases, so air expands, causing temperature to decrease as heat is distributed over a larger volume
• convectional rainfall:

  1. UV rays are absorbed by low albedo ground
  2. warmed land heats air next to it via conduction
  3. warm air rises by convection as it is less dense
  4. as air rises, it cools adiabatically and reaches dew point, clouds form
  5. still warm moist air continues rising, forming a cumulonimbus
  6. as more water condenses, more latent heat is released, so air rises, so more water condenses
  7. cumulonimbus reaches tropopause and oves sideways, making powerful thunderstorms with hail
  8. water turns to ice as it rises, then falls when it's too heavy, but is brought back up by air rising and repeats
• frontal rainfall:

  1. interaction of warm and cold air masses
  2. uplift is caused by undercutting of warm air by cold air, at the fronts
  3. moderate to light showers at warm front, and heavy showers at cold front
  4. mechanism of condensation at dew point, and formation of rain drops around condensation nuclei which provides a surface for condensation
  5. precipitation occurs when water droplets increase sufficiently in size
  6. occluded fronts
• At a warm front, warm air rises over colder, dense air, and the cold air acts a mountain, so orographic rainfall occurs. There is rain at the warm front, then warmer temperatures as the front passes. There are initially high altitude clouds, followed by mid altitude clouds, and finally stratus and nimbostratus.
• At a cold front, cold air forces warm air upwards, so the air acts like convectional rainfall. Storms and extreme weather occur at the cold front, then clearer skies with cooler temperatures.
• Occluded fronts are where a cold front overtakes a warm front, causing short bursts of rainfall.
• Warm moist air from Atlantic Ocean go over south western France, and over the very steep Pyrenees, causing orographic rain. There is a clear change, visible by the temperate vegetation becoming mediterranean vegetation.
• The Spanish plain is heated intensely by the sun, as it has lots of bare earth. The wind carries moist air from the Atlantic Ocean, causing convectional rainfall.
• absolute humidity - actual amount of water in the atmosphere, measured as grams of water per m3 of air
• 100% relative humidity - air is holding the most water it can at that temperature
• dew point - air is saturated, so it is the point at which excess water vapour starts to condense to form liquid water, due to a reduction in temperature or increase in water vapour
• relative humidity - how much water vapour there is compared to the maximum amount of water vapour that the air can hold before dew point is reached
• Water must have a condensation nuclei or contact in order to condense.
• mist - visibility between 1000m and 5000m, at 93% relative humidity
• fog - visibility below 1000m
• heavy fog - visibility below 200m
• Mist and fog need condensation nuclei, so they are most common in urban areas (dust), and coastal areas (salt).
• Advection fog forms when warm moist air from the oceans blows over cold land, causes a decrease in temperature, which increases relative humidity and dew point is reached. This is helped by the presence of salt crystals to act as condensation nuclei e.g. South West England where the Gulf Stream is warmer than land during the winter, and air is moved by south-west prevailing winds.
• Radiation fog occurs when the sun heats the land during the day, so the air holds lots of heat. At night, heat is radiated from the land, cooling air by conduction, so relative humidity increases to dew point and fog forms e.g. Pyrenees where air is moist due to Atlantic Ocean.
• Up-slope fog forms when wind blows moist air towards a slope, and air rises with the terrain and cools due to adiabatic cooling, so relative humidity increases, so dew point is reached, so fog forms.
• Evaporation fog occurs when water is warmer than the air, so the water evaporates and warms the air which rises. The rising air mixes with the cold air, and cools, so condenses into fog.
• Valley fog forms when air cools at higher elevations and descends downslope into the valley, reducing the temperature in the valley below dew point, so air condenses making fog.
• Factors affecting evaporation:

  • high temperatures
  • dry air (unsaturated)
  • calm conditions
  • water source
• Factors affecting condensation:

  • cooler temperatures below dew point
  • condensation nuclei
• 3 types of cooling:

  1. conduction cooling
  2. radiation cooling
  3. expansion cooling
• conduction (contact) cooling - moist air comes into contact with a cold object that has a temperature below dew point
• radiation cooling - air loses heat to space by long wave radiation from clouds and gases in the atmosphere
• expansion cooling - air rises, and expands, so temperature falls so air cools below dew point
• Frost forms due to deposition (desublimation) where water vapour in the air comes into contact with a surface that is below freezing and is instantly turned to ice.
• Monsoons occur when the ITCZ is over India. This means that the land is heated so air rises, and air from the south replaces it. Inflowing winds cross warm sea from Southern Hadley cell, bringing moisture. The wind releases moisture over the land.
• Dry season in India occurs because the land is cold so there is high pressure. The wind is outblowing from the Himalayas, and is cold and dry from the Northern Hadley cell.
• jet stream - narrow belt of fast winds that move at a high altitude
• Rossby wave - wide areas of high altitude air moving around mid-latitudes
• Jet stream variations:

  1. jet stream starts to undulate
  2. Rossby waves begin to form
  3. waves are strongly developed, cold air occupies troughs of low pressure
  4. when waves are cinched off, cyclones of cold air are formed
• When the polar jet stream meanders, it changes where high and low pressures are located. As it meanders southwards, it brings cold air that descends in a clockwise direction, causing high pressure. When it turns to head north, it pulls up warm, moist air which rises and causes low pressure.
• Jet streams are created by the pressure gradient between warmer air and colder air. The wind blows from the warm air to the cold air, to balance out the pressure gradient. It is then bent by the Coriolis effect, so it blows towards the east.