Met 4 - Humidity a n d Stability

What is absolute humidity?

Total amount of water vapour present in the atmosphere (g/m³).

How does temperature affect air's water-holding capacity?

Warm air can hold more water vapour than cold air.

What is saturation in terms of humidity?

Point at which air holds the maximum water vapour possible at its temperature.

What happens when saturated air is cooled?

Water vapour condenses into cloud, mist, or fog.

What is dew point temperature?

The temperature at which air becomes saturated and condensation begins.

What does a wide temperature/dew-point spread indicate?

Relatively dry air and a high cloud base or no cloud.

What does a narrow temperature/dew-point spread indicate?

Moist air, low cloud base, or fog.

What is relative humidity?

Percentage of water vapour in the air compared to the maximum it could hold at that temperature.

What happens at 100% relative humidity?

Air is saturated; cloud, mist, or fog may form.

What is humidity mixing ratio?

Grams of water vapour per kilogram of dry air.

What is the difference between absolute and specific humidity?

Absolute is per unit volume; specific is per unit mass.

How does humidity affect air density?

Moist air is less dense than dry air.

What is latent heat?

Energy absorbed or released during a change of state without temperature change.

What is sublimation?

Change of state from solid to gas.

What is deposition?

Change of state from gas to solid.

What is the Dry Adiabatic Lapse Rate (DALR)?

3°C per 1000 ft.

What is the Saturated Adiabatic Lapse Rate (SALR)?

Approximately 1.8°C per 1000 ft.

What is the Environmental Lapse Rate (ELR)?

Actual rate of temperature decrease with height in the atmosphere.

When is the atmosphere absolutely stable?

When ELR < SALR.

When is the atmosphere absolutely unstable?

When ELR > DALR.

When is the atmosphere conditionally unstable?

When ELR is between DALR and SALR.

What does atmospheric stability describe?

Air’s resistance to vertical motion.

What happens to a parcel of air in a stable atmosphere?

It returns to its original level if displaced vertically.

What happens to a parcel of air in an unstable atmosphere?

It continues to rise if displaced vertically.

What clouds form in a stable atmosphere?

Stratus (layered) clouds.

What clouds form in an unstable atmosphere?

Cumulus (heaped) clouds.

What is a temperature inversion?

An atmospheric layer where temperature increases with height.

What are four causes of temperature inversions?

1. Surface cooling during cold nights. 2. Warm air settling over cold air at a front. 3. Inversion turbulence. 4. Subsidence in stable air masses.