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AS GEOGRAPHY - ATMOSPHERE AND WEATHER - WEATHER PHENOMENA
Atmospheric stability refers to air that does not rise, which can lead to fog, mist or frost formation. Instability occurs when air rises, forming clouds and rain. Clouds are classified by form, height, and whether convection or fronts caused them. Convectional rainfall happens when warm land heats the air above, causing it to rise and condense. Frontal rainfall occurs at boundaries of warm and cold air masses. Orographic rainfall results when air is forced to rise over mountain barriers, cooling and condensing. Hail forms through a process of freezing and melting within cumulonimbus clouds. Dew and radiation fog form on clear nights through surface cooling and condensation. Advection fog
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Introduction to the theme of geography focusing on atmosphere and weather phenomena.
Explains atmospheric stability (fog, frost) vs instability (clouds, rain).
Describes cloud composition, types (stratiform, cumuliform), and conditions for formation.
Details the process of convectional rainfall, common in tropical regions, caused by heat.
Describes frontal rainfall when warm air rises over cold air, leading to rain in mid/high latitudes.
Describes how air rises over mountains causing orographic rain, including rain shadow effects.
Explains hail formation within cumulonimbus clouds through updrafts, freezing, and melting cycles.
Describes snow formation from frozen precipitation, influenced by temperature and moisture.
Details the process of dew formation under stable conditions and cooling of surfaces at night.
Differentiates between fog (dense cloud at ground level) and mist (less dense with more visibility).
Explains radiation fog, formed by rapid cooling at night in low-lying areas, clearing by sunrise.
Describes advection fog formed by warm moist air moving over cooler surfaces (land/sea).
Clarifies common student misconceptions about fog occurrence, coastal temperatures, and hydrograph terms.
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AS GEOGRAPHY - ATMOSPHERE AND WEATHER - WEATHER PHENOMENA
- 1.
- 2. STABILITYINSTABILITY Atmospheric stability andinstability are closely linked to weather phenomena. Stability means that air does not rise. Stability can lead to the formation of fog, mist and frost. Under clear skies, temperatures may drop enough to form frost. Instability produces unstable or rising air forming clouds and rain.
- 3. CLOUDS They are formedof millions of tiny water droplets held in suspension. They are classified in a number of ways: - Form or shape: stratiform (layered) and cumuliform (heaped) - Height: low (less than 2000m), medium or alto (2000-7000m) and high (7000-13000m) The important facts to keep in mind are: - In unstable conditions the dominant form of uplift is convection and this may form cumulus clouds - Where fronts are involved a variety of clouds exist - Relief or topography causes stratiform (layered) or cumuliform clouds, depending on the stability of air
- 5. CONVECTIONALRAINFALL When theland becomes very hot it heats the air above it. This air expands and rises. As it rises, cooling and condensation take place. If it continues to rise, rain will fall. This is very common in tropical areas. In temperate areas, convectional rain is more common in summer.
- 6. FRONTALORCYCLONICRAINFALL Frontal rain occurswhen warm air meets cold air. The warm air, being lighter and less dense, is forced to rise over the cold, denser air. As it rises it cools, condenses and forms rain. It is very common in mid and high latitudes, where warm tropical air and cold polar air converge.
- 8. RELIEFOROROGRAPHICRAINFALL Air may beforced to rise over a barrier (such as a mountain). As it rises it cools (adiabatic process), condenses and forms rain. There is often a rain shadow effect in which the downward slope receives a small amount of rain. Altitude is important on a local scale. There are increases of precipitation up to 2 km. Above this level rainfall decreases because of the air temperature being so low.
- 10. HAIL Hail is madeup of alternate shells of clear and opaque ice, formed by raindrops being carried up and down in vertical air currents in large cumulonimbus clouds. Freezing and partial melting may occur several times before the pellet is large enough to escape from the cloud. As a raindrop is carried high up in the cloud it freezes. As the hailstone falls, the outer layer may be melted but can freeze again with further uplift. The process can occur many times before the hail finally falls to the ground, when its weight is great enough to overcome the strong portions of air.
- 13. SNOW Snow isfrozen precipitation. Snow crystals form when the temperature is below freezing point and water vapour is converted into a solid. Very cold air contains a limited amount of moisture, so the heaviest snowfalls tend to occur when warm moist air is forced over very high mountains or when warm moist air comes into contact with very cold air at a front.
- 14. DEW Dew is thedirect deposition of water droplets onto the surface and vegetation. It occurs in clear, calm anticyclonic conditions (stability) where there is rapid radiation cooling by night. The temperature reaches dew point, and further cooling causes condensation and direct precipitation onto the ground and vegetation.
- 15. FOGMIST Fog is cloudat ground level. Fog is a very dense cloud at ground level in which you can see at maximum 1km. Mist is a rarefied cloud at ground level in which you can see in between 1 and 5 km distance.
- 16. RADIATIONFOG Radiation fog isformed in low-lying areas during calm weather, especially during spring and autumn. The surface of the ground, cooled fast 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. Ideal conditions include a surface layer of moist air and clear skies to allow maximum radiation cooling to occur quickly. As the sun rises, radiation fog clears away. Under cold anticyclonic conditions in late autumn and winter, fog may be thicker and more persistent, and around large towns smog may develop under an inversion layer.
- 18. ADVECTIONFOG Advection fog isformed when warm moist air flows horizontally over cooler land or sea.
- 19. TIPS Some students thinkthat fog is common all year round. It is more common in temperate areas in spring and autumn. In summer, the sea is cooler than the land so air is not cooled when it blows onto the land, while in winter there are more low pressure systems, causing higher winds and mixing the air. Some students state that all coastal areas have mild temperatures – it all depends on the temperature of the ocean current. Abbreviations are fine – for example LP for low pressure, SH for southern hemisphere – but when you first mention the term, write it out in full and add the abbreviation in brackets.