2.3 Weather Processes and Phenomena Flashcards
What is humidity?
The moisture content within the air. Absolute humidity is the actual amount in the air, whereas relative humidity measures how near the air is to saturation, indicating how much water vapour the air is holding compared to the maximum amount it could hold at that pressure and temperature.
Relative humidity = absolute humidity / saturation moisture content x 100
What is evaporation/condensation and freezing?
Evaporation occurs when vapour pressure of water exceeds the atmosphere, causing it to become heated and the air is unsaturated.
The rates will rise when: humidity of the air is low, there is more heat, and there is less wind.
Condensation occurs when water vapor is cooled below the dew point, condensing into water droplets, provided there are nuclei in the air such as dust, salt or smoke which allow condensation.
It may also occur when there is enough water vapour evaporated for it to become saturated.
Cooling may occur in 3 ways - conduction cooling (contact with cooler objects below the dew point), radiation cooling when air loses heat to space, and expansion cooling when air rises it cools as the temperature cools
Freezing is obvious - sublimation occurs when water goes straight from solid to a gaseous state
how do clouds form?
When water is condensed, they float in the atmosphere as clouds. If they coalesce the droplets can become heavy enough to overcome gravity and fall as rain. For rain to form, water and ice exists in clouds at temperatures below 0 degrees.
At these temperatures, water droplets and ice droplets form and ice crystals grow by condensation which can overcome turbulence and form cloud updrafts and fall. As they fall, they coalesce and form snowflakes which melt and become rain in the warmer layers.
What are the 4 main causes of precipitation?
Convectional rainfall: When land becomes hot, heats the air causing it to expand and rise. As it rises, it cools leading to precipitation. Tropical and ITCZ areas common
Frontal uplift: At the ITCZ moist, tropical air masses of the same temperature meet and rise due to winds moving into the low-pressure system, making dense cumulonimbus clouds
Warm tropical air masses and cold polar air masses met along the polar fronts, forming low pressure systems called depressions. These have warm and cold fronts. As the warm air is lighter and less dense it is forced to rise over the cold, denser air and cools, condenses and forms rain. The warm air cannot push the cold air out of the depression and so is forced to rise.
Orographic rainfall occurs when the pressure force is strong enough to cause air to rise over a hill or mountain. As it rises, it cools and reaches dew point and condenses, forming clouds. Sometimes the conditions will cause clouds to be too thin to have rain or snow. This often gives heavy rainfall but ascent over a hill or mountain could cause thin cloud or fog.
What is radiation cooling?
On cloudless nights land surface loses heat rapidly by radiation and there is little wind, so the air is in constant contact with the cold surface. Therefore, dew, radiation fog and ice form on the bottom of valleys.
How do thunderstorms occur?
Where there is instability to great heights as large updraft is caused by uplift and thus energy is released when condensation occurs. This makes the air unstable, causing rainfall to occur however the uplift prevents the precipitation from falling.
As it matures, the onset of heavy precipitation occurs and rainfall drags down the cold air with it, spreading the cloud and giving it an anvil shape.
The cloud finally dissipates when downdrafts prevent instability, which also triggers the rise of warm air in between. This is common in tropical and warm areas where there is a lot of moisture
How does lightning occur?
Tensions between charged areas like the cloud and ground or within the cloud. The upper parts of the cloud are positive and the lower is negative causing tension and lightning to occur between the two.
What are the types of cloud? How do they form?
2km ASL:
Stratus: layer clouds formed by little vertical uplift over wide areas
Cumulus: more vertical but localised uplift, heaped clouds with flat bases (normal)
Stratocumulus: mixture of both
2-6km ASL:
Altostratus (middle stratus)
Altocumulus (Middle cumulus)
6-16km ASL:
Cirrus: condensation occurs at high levels, forming wispy clouds made of ice.
Cirrostratus: (high stratus)
Cirrocumulus: (high cumulus)
What are the clouds that produce precipitation?
Nimbostratus clouds - thick, dark grey clouds which produce steady rain.
The towering cumulonimbus is the largest cloud, composed of ice crystals at the top and water at the bottom. They are dense, dark grey clouds growing from near sea level to up to 16km, where it spreads out to form an anvil top as the air cannot rise through the tropopause.
Precipitation occurs in clouds when water droplets coalesce together, ice crystals aggregate and form snow, or ice crystals collect water vapour to form hair (accretion)
How does snow form?
Frozen precipitation occurs when water vapour condenses at freezing temperature. This causes crystals and snowflakes. If the lower atmosphere is cold enough, the snowflakes will reach the ground without melting.
The cloud needs lots of moisture and low temperatures to form snow - very cold air however has little moisture so the heaviest snow occurs when warm, moist air is forced over cold areas like mountains or fronts
How does hail form?
Hail is concentric rings of ice formed when raindrops are carried up and down in air currents in cumulonimbus clouds. The freezing and partial melting occurs several times until the ice pellet is large enough to escape from the cloud.
As the raindrops are carried up by the cumulonimbus they freeze. Water droplets form lower in the cloud are carried up and freeze around a nucleus - may be an ice crystal, snow pellet or frozen raindrop. The hailstones collide with droplets of cooled water, which freeze on impact forming a layer of ice around the hailstone.
As it falls due to the increased weight, the outer layer may be melted as there is less freezing but may freeze again when there is further uplift, where it reaches the top again and is covered by more coatings of opaque ice. The process repeats until the hail finally grows large enough to fall to the ground as it overcomes the uplift.
How is dew formed?
- Dew is formed on cold cloudless nights where air is chilled by contact with the cold land, where it becomes colder and condenses
- There must be little wind and the ground should be above 0 degrees but below dew point.
- When there is a light breeze, it is possible for condensation to occur in the air, forming radiation fog at low levels
- The surface is cooled rapidly at night by radiation which flows into the air and is cooled to the dew point, creating moist air and fog.
- As the sun rises, the radiation fog disperses.
- In cold anticyclonic conditions the fog may be thicker and more persistent, creating smog. It usually occurs on cold winter nights.
What is fog and the different types?
Advection fog is formed when warm moist air flows horizontally over a cold land or sea surface, causing it to be chilled below its dew point. This may occur over cold ocean currents. The air is cooled to below its dew point.
steam fog may occur when cold air blows over warmer water, as evaporation saturated the air and leads to steaming.
Frontal fog forms at a front where warm air is cooled at a cool front.
Hill fog forms when wind is forced up a mountain or hill and cools.
