2.3 Weather processes and phenomena

Question 1

What is sublimation?

Answer 1

Change of a solid into a gas

Question 2

What is deposition?

Answer 2

Change of a gas into a solid

Question 3

What is absolute humidity?

Answer 3

The actual amount of water vapour in the air

Question 4

What is humidity?

Answer 4

Refers to how moist the air is because of the water vapour it contains

Question 5

What is relative humidity?

Answer 5

Ratio between the amount of water vapour and the maximum amount of water the air can hold at saturation.

Question 6

What is needed for condensation to occur?

Answer 6

Microscopic particles of dust, smoke or salt = condensation nuclei. These particles are hygroscopic, so they attract water and aid condensation.

Question 7

What is rain?

Answer 7

One type of liquid precipitation that is the result of water vapour condensing and precipitating.

Question 8

What is the collision theory?

Answer 8

Droplets collide with each other and join together to form a larger drop. 3 different processes:

1) coalescence = two water droplets collide and join
2) aggregation = two ice crystals collide and join to form snow
3) accretion = an ice crystal collects a water droplet, leading to hail

Question 9

What is the Bergeron-Findeisen Theory?

Answer 9

Describes growth of cloud droplets that occurs when the air is between ice and water saturation.

Question 10

What is dew point?

Answer 10

Point at which cloud droplets form

Question 11

What is frontal rainfall?

Answer 11

Cold polar air meets warm tropical maritime air and because the warm air is less dense, it can’t push the cold air out of the way, so rises over the cold air at the warm front, and is pushed up rapidly by the advancing air at the cold front.
A long period of low-intensity rainfall occurs at the warm front, and short, high-intensity rainfall occurs at the cold front.
Eventually, the cold front catches up with the warm front because the cold air in the rear moves faster. The warm air is completely lifted off the ground by the cold air, forming an occluded front with long periods of rain.

Question 12

What are depressions and how to they form?

Answer 12

They are low-pressure systems that form when a warm air mass meets a cool air mass
They are surface pressure systems linked to upper air movements that are part of the global atmospheric circulation and transfers of energy. They form a trough in upper westerlies and are driven by the jet stream.

Question 13

What is convection rainfall?

Answer 13

Occurs over a hot land surface or over a structure that emits heat

1) The air is warmed by contact with the heat source
2) The heated air expands, becomes less dense, rises and cools
3) Clouds form as moisture in the air condenses and the latent heat released adds further warmth to speed the ascent, which can reach the tropopause, forming cumulonimbus clouds.

Question 14

What is relief/orographic rainfall?

Answer 14

1) Warm, moist air is forced to rise over high areas
2) Air cools and condenses, forming clouds
3) Rain falls
4) Descending air is compressed so warms and its relative humidity reduces

Question 15

What is fog?

Answer 15

Tiny water droplets suspended in the air. Droplets usually grow on pollution particles. It is formed by the cooling of warm moist air, leading to condensation of water vapour.

Question 16

What is radiation fog?

Answer 16

Usually occurs in winter, aided by clear skies and calm conditions.
Forms only over land - the cooling of land overnight by thermal radiation cools the air close to the surface (vertical movement).
This reduces the ability of the air to hold moisture, allowing condensation and fog to occur.
Usually disappears in the morning when the sun warms the air.

Question 17

What is advection fog?

Answer 17

Occurs when moist air passes over a cool surface and is cooled (horizontal movement)
Common example of this is when moist tropical air moves over cooler waters
Another common example is when a warm front passes over an area with snow cover.

Question 18

How does hail form?

Answer 18

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

Question 19

What are the three main types of cloud?

Answer 19

Stratus - layer clouds that form when there is little vertical uplift but it is over a wide area
Cumulus - occur when more vertical but localised uplift results in heaped clouds with flat bases and globular upper surfaces
Cirrus - form where condensation occurs at very high levels, forming wispy clouds made of ice.

Question 20

Which clouds produce precipitation?

Answer 20

Clouds that prevent sunlight penetrating and are black from below.
Nimbostratus
Cumulonimbus - high intensity short duration
Stratus - low intensity long duration