2.2 Weather processes and phenomena

What is evaporation?

A change of state from liquid to gas - for example water to water vapour.

What is sublimation?

A change of state from solid to gas without passing through the liquid phase

Explain how orographic uplift occurs.

Moist air is forced to rise when it encounters a physical barrier such as a mountain. As the air is pushed up the windward side, it expands due to lower atmospheric pressure at higher altitudes. Once the air passes over the peak and descends down the leeward side, it compresses and warms, reducing relative humidity and creating drier conditions (rain shadow effect).

Explain how frontal uplift works

A warm air mass meets a colder, denser air mass. The colder air stays close to the ground due to its higher density, forcing the warmer, less dense air to rise above it. As the warm air rises, it expands due to decreasing pressure at higher altitudes, which causes it to cool (adiabatic cooling). As the temperature decreases, the air becomes more stable at higher altitude.

What is melting?

 A change of state from solid to liquid - for example ice to water.

What is freezing?

A change of state from liquid to solid - for example water to ice.

What is condensation?

A change of state from gas to liquid - for example water vapour to water.

What are the 3 causes of precipitation.

Convection, frontal and orographic uplift of air, radiation cooling.

Explain how convectional uplifting works.

 

 

The earth's surface is heated by solar radiation (short wave) and converted to long wave radiation which warms the air in contact with it. The air pocket becomes less dense (expansion of molecules) and warmer than surrounding air. Thus it rises until such time as it is at the same density/temperature as surrounding air.

Explain how radiation cooling causes uplift

The ground loses heat through longwave radiation, especially at night when there is little or no incoming solar radiation. As the surface cools, it cools the air directly in contact with it through conduction. This air becomes colder and denser than the air above, so it remains near the surface rather than rising. As cooling continues, the temperature of the air may reach a stable point close to the ground, with colder air trapped beneath slightly warmer air above (temperature inversion).

Explain why an air parcel cools as it expands.

Air parcel rises (orographic, convectional, frontal). A rising parcel of air expands because the air pressure falls with elevation. The energy lost during the expansion of the air parcel causes the temperature to decrease.

How do clouds form?

Clouds form when moist air is forced to rise and undergoes adiabatic cooling as pressure decreases with altitude. As the air cools, its relative humidity increases until it reaches 100% (the dew point). At this stage, water vapour condenses onto condensation nuclei such as dust or salt particles, forming microscopic water droplets or ice crystals. These remain suspended in the atmosphere, creating clouds, with their type depending on atmospheric stability and the uplift mechanism.

How does rain form?

Air rises due to it being hotter, and so less dense than the surrounding air. It then cools adiabatically as it rises. Water held within the air parcel is cooled and condensed to the point where relative humidity reaches 100%. When the updraft becomes not strong enough to hold up the water held it falls as rain.

How does hail form?

Hail forms within large cumulonimbus clouds during thunderstorms. Strong convection creates powerful updraughts that carry water droplets high into the cloud, where temperatures are below freezing. The droplets freeze into ice and are repeatedly lifted and dropped by the updraughts, gaining layers of ice each time. When the hailstones become too heavy for the updraughts to hold up, they fall to the ground as hail.

How does snow form?

Snow forms in cold atmospheric conditions where temperatures within the cloud are below 0°C. Instead of condensing into liquid water, water vapour freezes directly into ice crystals. These ice crystals grow and join together to form snowflakes. As they increase in size and weight, they fall to the ground as snow, remaining frozen due to cold air below the cloud.

How does dew form?

Condensation of moisture on a surface, as the air is cooled to dew point, by contact with a surface losing heat by radiation.

How does radiation fog form?

Air at ground level cools down rapidly, due to the loss of longwave radiation at night due to lack of insulation from clouds (more likely during high pressure). Cooling of air by conduction causes dew point to be met and condensation to occur.