2.3 Weather processes and phenomena

Atmospheric moisture processes: evaporation, condensation, freezing, melting, deposition, and sublimation. Causes of precipitation: convection, frontal and orographic uplift of air, and radiation cooling. Types of precipitation: clouds, rain, hail, snow, dew, and fog.

Causes of precipitation:

Convection

Frontal

Orographic uplift of air

Radiation cooling

What is frontal rainfall?

Two air masses meet together, the boundary between the two = weather front

don’t mix due to differences in temperature and density.

Fronted are areas where rainfall take place

Describe how frontal rainfall occur

cold dense air try squeeze through and lifts warmer less dense air up. Air being lifted means a low pressure so cold front through warm front - low pressure system. Cause severe rainfall or thunderstorms

As warm air is forced to ride it cools and moisture condenses. Also warm air in contact with cold front so cools. Clouds for, - continued condensation increase water droplet growth and when heavy enough falls.

Uk as an example of fronts

Warm tropical air meets cold polar air

Cold fronts

Move from west to east.

Move faster than warm fronts because cold air denser.

Boundary of advancing cold air mass

What is a squall line

Line fo severe thunderstorms that forms along a cold front

Warm fronts

Slowly north to south.

Bring more moderate and long lasting weather patterns as less dense/

High pressure systems = calm clear weather.

Warm front - warm air Slide Over cold air mass.

Exam question: explain how convection and frontal uplift of air can lead to precipitation

1. They are both mechanisms whereby air is forced to rise to reach few point so condensation can occur causing precipitation

2. If I keep on rising sufficient condensation forPrecipitation to occur

3. Convection requires the heating of the land surface to produce that is capable of rising

4. Frontal precipitation requires the warm air to rise above cold air at a front.

5. In both situations that I need to keep on rising leading to heavy intense rainfal.

6. For convection needs to be warmer than the general environmental.

7. Front to uplift will depend on the type of front

Mark scheme answer for frontal rainfall occurring (4 marks)

1. Two different air masses meet

2. At warm fronts, warm air rises over cold air

3. At cold fronts, warm air is forced to rise because of underb]cutting by the cold dense air

4. Cooling and condensation occur leading to rainfall

Describe how orographic rainfall leads to the formation of precipitation (3)

Barrier forces air to rise (mountains)

Air expands and cools

Dew point is reached leading to condensation around hygroscopic nuclei.

Once saturated, air cooks at a lower rate causing instability

Collision/ cohesion of water droplets creates larger ones => falls= precipitation.

Describe how convectional rain leads to precipitation

1. Suns energy heat, surface, three conduction

2. Water evaporate to form water vapour

3. Warm air rises and expands three convection

4. Is it cool is it condenses to reach dew point?

5. Condensation forms clouds

6. Does lead to heart intense rainfall:

   • Large water droplets made from cohesion of water

   • Water droplets heavy as they needed to overcome up taught

     = Type of rainfall between the tropics, and during warm, sunny spells

Types of rainfall: describe how fog is formed?

1. Cooling of warm, moist air

2. Leading to condensation of the water vapour

3. Cooling may be a result of either radiationFrom the ground surface at night? The movement of warm, moist air over cold surface.

Fog is tiny water droplets suspended in the air near the ground reducing visibility to less than 1km

What does Adiabatic processes mean

Adiabatic means rising and sinking of air. - anything that occurs as air rises or sinks

Causes of precipitation: what is rain?

Formed when falling water droplets collide and coalesce.

Some may be melted snowflakes or hailstones

precipitation: hail

Formed when ice particles fall and rise in cumulonimbus clouds and super-cooled water droplets collide with and freeze around them (accretion)

Radiation fog: explain

Forms as a result of radiation cooling.

Sun rises, fog lifts, smog to appear under inversion layer i cold air trapped by warm air above it)

Steam fog: explain

Localised when cold air blows over warmer water - air become saturated due to evaporation resulting condensation causes steam

Advection fog: Explain

Winds move towards poles over cold sea - so chilled to below dew point forming advection fog.

Winds blow over cold ocean current - air passes from sea onto cold land in winter.

Define the terms atmospheric stability and atmospheric instability (4)

stability - where, if a parcel of air is displaced upwards it will return to its original position (because it remains cooler and heavier than the surrounding air). (2)

instability - where, if a parcel of air rises, it will continue to rise as it remains warmer than the surrounding air even though being cooled adiabatically. (2)

What is temperature inversion? (2)

When temperature rises (1) rather than falls with altitude (1).

What is frontal inversion?

colder, denser air mass descends, forming warmer air above. Barrier created where the two meet that prevents warm air parcels from rising through to the warm air.

What is subsidence inversion?

Subsidence inversion: air moving upwards experiences adiabatic cooling, due to pressure decrease.

This air falls, becomes denser and warms, warm air reaches a cooler layer of unstable air, and a temperature inversion is created.

A temperature inversion will dissipate once sun has heated the ground long enough to cause cool air above to warm by conduction processes - allowing warm air to rise.

Explain the precipitation process

1. Requires hygroscopic condensation nuclei

2. Collision theory: droplets in clouds collide together to form a larger droplet:

3. Causes coalescence: two droplets combine to form rain

4. Aggregation: two ice crystals collide to form snow

5. Accretion: ice crystals collect water droplet forming hail.

How does snow form?

When tiny ice crystals in cloud stick together - (accretion_ to become snowflakes.

If enough rustías stick together, they’ll become heavy ene ought to fall to the ground.

Snowflakes that descend through moist air that is slightly warmer than 0 degrees will melt around edges and stick together to produce big flakes.

snowflakes that fall through cold dry air produce powdery snow that doesn’t stick together.

Snow is formed when temperatures are low and there is moisture in the atmosphere in the form of tiny ice crystals