Unit 2.3: Humidity, clouds, and precipitation

Question 1

What are the 3 processes that control water? Define them.

Answer 1

1. Evaporation: When water changes from liquid to gas. It is a cooling process, it absorbs the heat then carries it into the atmosphere.
2. Condensation: When water changes from gas to liquid. It is a warming process, water vapour releases heat and becomes liquid.
3. Precipitation: When solid or liquid falls form atmosphere to ground.

Question 2

What is latent heat?

Answer 2

Heat that is taken up and stored when a substance changes states and cannot be measured by a thermometer. It is taken from environment during evaporation and released during condensation.

Question 3

What changes the state of water?

Answer 3

Latent heat

Question 4

What is saturation?

Answer 4

The point that the air’s water vapour content is greater than or equal to the air’s water vapour capacity.

Question 5

What is humidity?

Answer 5

A measure of water vapour content in the atmosphere. It is controlled by temperature because warm air can hold more water vapour.

Question 6

What is vapour pressure?

Answer 6

Portion of air pressure exerted only by molecules of water vapour.

Question 7

What is saturated vapour pressure?

Answer 7

The maximum amount of water vapour the air can hold at a given temperature. It is the vapour pressure where saturation occurs. It varies by temperature because warm air can hold more.

Question 8

What are the two kinds of humidity?

Answer 8

Specific humidity: Actual water vapour content. Water vapour (g) in a given mass of air (kg).

Relative humidity: Ratio of water content to saturated vapour pressure. Vapour pressure/saturated vapour pressure.

Question 9

What are some characteristics of relative humidity?

Answer 9

• Highest in early morning when the temperature lowest.
• When air temperature goes up, so does saturated vapour pressure, therefore RH goes down.
• Indicates how close saturation is.

Question 10

What is the dew point temperature?

Answer 10

Temperature when saturation occurs and relative humidity is 100%. It depends on water content, not the actual temperature.
*Dry air has a low dew point.

Question 11

What is an air parcel?

Answer 11

Body of air with uniform humidity and temperature.

Question 12

What are adiabatic processes?

Answer 12

They describe how temperature of air parcel changes as it rises and falls in the atmosphere.

• when air rises, volume expands, air parcel cools
• when air falls, volume compresses, air parcel warms

Question 13

What is the difference between dry and moist adiabatic rates?

Answer 13

Dry: air is unsaturated, air parcel warms/cools at constant rate of 10oC/1km.
Moist: air is saturated, air parcel warms/cools at a rate of 4-9oC/1km.

Question 14

What is atmospheric stability?

Answer 14

The measure of the tendency for vertical motion in the atmosphere.

Question 15

What are 3 stability scenarios in the atmosphere?

Answer 15

1. Stable atmosphere: Air resists vertical motion. Air parcel that begins to rise will not continue because it is cooler than atmosphere. *results in clear skies
2. Unstable atmosphere: Parcel continues to rise because it is warmer than the atmosphere and therefore less dense. *promotes clouds & precipitation formation
3. conditionally unstable atmosphere: parcel stability is determined by saturation. It is stable while unsaturated near the ground; and unstable when saturated at high altitudes.

Question 16

What makes air parcels lift?

Answer 16

1. Convective uplift: Surface heated unequally; it forms a bubble of air and it rises cause it is less dense.
2. Orographic uplift: air is forced to rise when it contacts a barrier.
3. Frontal uplift: interaction between 2 masses, warm and cold air. Cold air is denser than warm air therefore the warm air rises and cools at the dry adiabatic rate.
4. Convergent uplift: air rises when surface winds converge in a cyclone.

Question 17

What are clouds?

Answer 17

They are a dense concentration of suspended minute liquid droplets or tiny ice crystals. Each particle is formed around a tiny centre of mass known as the condensation nuclei.

Question 18

What is fog?

Answer 18

A cloud on the earths surface. It is formed when surface is saturated, often trapped below a low-level temperature inversion layer.

Question 19

What 3 conditions are used to classify clouds?

Answer 19

Degree of vertical development, height of cloud base, and precipitation producing.

Question 20

In the condition of “Degree of vertical development”, what are the two types of clouds?

Answer 20

• Stratus-type: Horizontal sheets, they cover much or all of the sky. They are often featureless and are created by low to moderate atmospheric instability.
• Cumulus-type: Vertically developed, billowy clouds. They resemble ever-changing cotton balls in the sky and are formed by high atmospheric instability.

Question 21

In the condition of “Height of base”, what are the 3 types of clouds?

Answer 21

• Cirrus-type: high clouds, base above 6km from the surface.
• Alto type: mid-level cloud, base is between 2-6km from surface.
• Stratus/Cumulus: Low clouds, base is below 2 km from surface.

Question 22

In the condition of “Precipitation producing”, what are the 2 types of clouds?

Answer 22

• Nimbostratus: Dark grey overcast skies, light to moderate rainfall occurs over a period of greater than 6 hours.
• Cumulonimbus: towering thunder clouds, heavy rainfall for periods less than 6 hours.

Question 23

What are the different types of fog?

Answer 23

• Radiation fog: forms of clear nights with intense radiation cooling. Tends to collect in low lying areas such as river valleys.
• Evaporation fog: Forms when colder air advected over a warmer body of water, which will lose heat to the air. Forms thin, smoke like wisps on surface and most common in autumn or early morning.
• Advective fog: Forms when relatively warm, moist air is advective over a colder surface. Warm air loses heat, temperature will drop below dew point and temperature will occur.

Question 24

What are the 2 formation mechanisms of precipitation?

Answer 24

1. Collision&coalescence process: occurs in tropical regions and clouds are warm with a temperature of greater than -10oC. As water droplets grow in Atmosphere, they fall through clouds and collide with other droplets and combine.
2. Ice-crystal growth process: Mid-high altitudes, cold clouds with temperature between -10 to -40oC. Supercooled water droplets evaporate and deposited as ice crystals on freezing nuclei. Precipitation leaves cloud as snow, even on hot days.

Question 25

What is the lifting condensation level?

Answer 25

Altitude where the temperature of the air parcel reaches dew point temperature and becomes saturated. It is also the point where clouds and precipitation will form.

Question 26

What are the 5 types of precipitation?

Answer 26

1. Rain: snowflakes that fall form clouds through a warm atmosphere and melts before reaching surface.
2. snow: snowflakes form in clouds and falls through cold atmosphere.
3. sleet: requires unusual vertical temperature profile. Needs a shallow warm layer above a colder sub-freezing layer directly above surface. Goes from cold to warm to cold and forms ice pellets.
4. Freezing rain: when air immediately above surface is at or very near freezing. It melts and freezes just as it hits something cold on the surface.
5. Hail: Forms in a cumulonimbus cloud and goes through very strong vertical up and downdrafts. Creates large ice pellets that melt and freeze multiple times.