MOISTURE, FOG AND CLOUDS Formation

Basic facts on water Water (H20) is the most abundant and important material on the planet (explaining why a lot of the met office are oceanologists) Water molecules are free to move around completely as a gas, are closer together in a liquid and locked into an orderly pattern as a solid. The ice crystal structure is hexagonal (which explains the shape of snowflakes) In freezing air, if there is enough energy available, ice can go straight from a solid to a gas. This is called sublimation. Equally, when water vapour goes straight to being a solid, this is called deposition. Applying warmth warmth to frozen water molecules make them move more quickly, the bonds break and molecules become less dense

BASIC FACTS ON WATER (2) Hydrological system – water vapour is carried by air currents, topography etc cause it to condense and flow back into bodies of water to be evaporated. Ponds, lakes, slow moving rivers have evaporation occur on its surface. Energy is stolen from the surface, making the surface deprived of energy. In a closed system, equilibrium will occur – equal amounts of evaporation and condensation to balance the system. Air pressure at sea level is a result of pressure from all gas molecules (including oxygen and nitrogen), but you can separate these out. The total pressure of these, is equal to the sum of all known gases – known as Dalton’s law of partial pressure If an area is at ‘dew point’ temperature and then becomes even cooler, it forms frost

FORMATION OF FOG Needs dew point Needs a floating surface (E.g. Particulates that are air-born – condensation nuclei) As relative humidity reaches 75-100% (saturation) water condenses onto condensation nuclei As air cools, condensation droplets get larger In big cities, fog is typically bigger – this is because there is more condensation nuclei for the vapour to condensate on. E.g. London and San Francisco suffered from very heavy fog as a result of pollution up until the 80s/90s when stricter car rules for fuel efficiency and stack chimneys were created. Look at radiation fog and advection fog Fog is dense (water and air mixed) and it is subject to gravity. This is why fog often forms in valleys. “burn off” – This is a fallacy, fog cannot “burn off”. When fog dissipates, for temperature does not have to rise, only the ground needs to heat up which makes the air temperature above it rise.

Formation of clouds Cloud formation is the same as fog, it still needs to reach dew point and condensation nuclei, but in the air In clouds, adiabatic heating and cooling occur to form them Some clouds have ice particles within them, and they’re kept up by latent heat from vapour, until it properly condenses and begins to fall Could formation may be convectional, orographic and frontal.

Convection clouds Moist air is carried upwards through vertical convection and then cools to the dew point (convectional condensation level) These have distinctive flat bases and fluffy tops because of the ice crystals at the top Often associated with hailstorms

Orographic clouds Triggered around mountains. Moist air flow (sometimes from the coast) is forced over a mountain range – as it does, it gets colder and reaches the dew point. When reaching the other side of the mountain range, it is left with dry air as it has dropped the moisture. This is named the Foehn/föhn wind. Rain shadow

Frontal clouds Two air masses, one warm and one cold, collide. The cool, dry air works like a mountain, and pushes the warm, moist air up because the warm air is less dense. Warm air cools and meets the dew point after being forced upwards. This cold body of air has to be sufficiently big enough to force the warm air upwards

GLOSSARY Sublimation – When ice crystals go directly from being a solid to a gas with a high amount of energy Deposition – When water vapour goes straight from being a gas to a solid Open System – Is a cycle which has external input (E.g. wind in the hydrological system) Closed System – A cycle where there are no external inputs or factors and it is self contained. These do not generally occur in the natural environment and are mostly created in labs Equilibrium – When the amount of input and output are even and balanced (E.g. equal amounts of evaporation and condensation happening at the same time Vapour pressure – The expression given to the air moisture content when measuring the pressure exerted by the water molecules within it Saturation vapour pressure – The threshold at which the water pressure in the air is so high it begins to condense at any given temperature Dew point – Temperature at which air must be cooled for the formation of dew (with no change in air pressure or moisture content) Dalton’s law of partial pressure – states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases. (With partial pressures referring to the hypothetical pressure that a gas might exert if it was singular and at the same temperature) Condensation nuclei – Small particles/ impurities which fill the air (E.g. pollution, skin cells, pollen) and may create a surface for water to condense and fog to form (radius of <0.2µm – Aitken nuclei/0.2-1µm – large nuclei/>1µm – Giant nuclei) µm – Measure of Micrometres Relative Humidity – Refers to water vapour in the air relative to the highest point for that temperature (expressed in percentage) Radiation fog – (Common in the UK and Europe) occurs typically on clear nights when radiation leaves rapidly, cooling the ground and the air around it (Can happen near football pitches) Advection fog – Occurs where hot, humid air moves across a cold surface and cools to form fog. Adiabatic heating and cooling – When air warms up it rises, and it becomes less dense and expands because of decreased air pressure and vice versa. (AIIPGATPE) Föhn wind/Chinook – A dry, warm wind which descends down a lee slope (AIIPGATPE)