2.3 Atmosphere and weather: weather processes and phenomena Flashcards
What are the factors affecting evaporation
Initial humidity of the air – if air is very dry then strong evaporation occurs; if it is saturated then very little occurs
Supply of heat – the hotter the air, the more evaporation that takes place
Wind strength – under the calm conditions the air becomes saturated rapidly
Describe convectional rainfall
When the land becomes very hot, it heats the air above it. This air expands and rises. As it rises, cooling and condensation takes place. If it continues to rise, rain will fall. It is very common in tropical areas and is associated with the permanence of the ITCZ. In temperate areas, convectional rain is more common in summer.
Describe frontal rainfall
Frontal rain occurs when 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 most common in the middle and high latitudes where warm tropical air and cold polar air converge
Describe orographic rainfall
Air may be forced to rise over a barrier such as a mountain. As it rises, it cools, condenses and forms rain. There is often a rain shadow effect, whereby the leeward slope receives a relatively small amount of rain. Altitude is important especially on a local scale. In general, there are increases of precipitation up to about 2km. Above this level, rainfall decreases because the air temperature is so low.
Describe radiation cooling
This is a common process on calm evening when there is a clear sky. It may occur when the land surface rapidly loses heat as the sun goes down and long wave earth radiation is quickly lost to space. The air in contact with the ground is then cooled by conduction and condensation of the water vapour near the ground surface may take place and form fog – called radiation fog.
Describe clouds
Clouds form when air is cooled until it reached dew point and the water vapour condenses either into water droplets or ice crystals. At some point the water vapour condenses either into water droplets or ice crystals become large enough to fall as precipitation – rain, snow, and hail. There are 20+ types of clouds and they are classified according to their height and shape.
Describe rain
Rain is formed when the condensation of water vapour takes place around a particle of dust – a condensation, or hygroscopic nuclei. This initially produces tiny water droplets less than 0.05mm in diameter. Being so tiny, they are easily kept up in the air by rising air currents and form clouds. As further condensation takes place, they become larger and heavier droplets which are too heavy to be kept up by rising air currents so they fall as rain.
Describe hail
Hail: When raindrops are cooled below freezing, hail is formed. Hail is frozen raindrops that are larger than 5mm in diameter. They are commonly found in towering cumulonimbus clouds which have rapidly rising air within them, that can reach heights in excess of 10kkm – well above the height where temperatures are below freezing. Hail is most common in warmer climates where the land is heated sufficiently to produce a rapid uplift of air.
Describe snow
Snow forms in the same way as air, except that the water vapour goes directly into a frozen state, as an ice crystal – a process called sublimation. The ice crystals are quite complicated hexagonal shaped structures and tend to interlock and become attached to each other to form a larger snowflake. For snow to fall, the temperatures at the ground level must be below freezing. As it falls, it will encounter warm temperatures near the ground surface, so it may partially melt; this is called sleet.
Describe dew
Dew is water droplets that form when moist air comes into contact with a cold surface such as grass, leaves, windows and car roofs in the early morning or evening. It is formed most easily on surfaces which are not warmed by conducted heat from the land surface.
Describe fog
Fog: there are two types of fog – radiation and advection fog. Radiation fog is commonly formed on calm evenings when there is a clear sky. When the land surface rapidly loses heat as the sun goes down, long wave earth radiation is quickly lost to space. The air in contact with the ground is then cooled by conduction and this may cause condensation to take place. This tends to be most common where cold dense air accumulates – often in valley bottoms at dawn and sunrise. Advection fog is formed when warm moist air moves over either a cooler land or sea surface. The air is then cooled and condensation takes place to form fog.
