LEC 07: ATMOAPHERIC MOISTURE

Water is a Polar Molecule

Why?

Molecule with uneven distribution of electrical charge.

Hydrogen atoms are slightly positive (+)

Oxygen atom is slightly negative (-).

Water is a Good Solvent

Why?

Its positive (+) and negative (-) ends allow it to dissolve many substances.

3 Phases of Matter

Solid: Ice, snow, and glaciers.

Liquid: Oceans, lakes, rivers, and rain.

Gas: Water vapor in the atmosphere.

3 Phases of Matter

Relevance

Water changes between these 3 phases.

Through evaporation, condensation, melting, and freezing.

Helps transfer energy throughout Earth's atmosphere.

Melting (Solid → Liquid)

Water Cycle

Ice absorbs heat and changes into liquid water.

→ Ice melting into a puddle.

Evaporation (Liquid → Gas)

Water Cycle

Liquid water absorbs heat and changes into water vapor.

→ Water evaporating from an ocean into the atmosphere.

Condensation (Gas → Liquid)

Water Cycle

Water vapor cools and changes into tiny liquid water droplets.

→ Clouds forming in the atmosphere.

Freezing (Liquid → Solid)

Water Cycle

Liquid water loses heat and changes into ice.

→ A lake freezing in winter.

Ice Is Less Dense Than Liquid Water

Why?

Same mass of ice occupies more space

→ v.s. the same mass of liquid water.

Ice Is Less Dense Than Liquid Water

Significance for Life

Floating ice forms an insulating layer on the surface of lakes and oceans.

Helps the water underneath remain liquid

→ Allows aquatic life to survive in cold conditions.

Water Vapor Is Less Dense Than Air

Why?

Water Vapor less dense than dry air at the same temperature and pressure

→ less mass in the same volume.

Water Vapor Is Less Dense Than Air

Effect on Atmosphere

Water Vapor is added to air, it decreases the air's average density.

→ Makes the air more buoyant and more likely to rise.

Saturation Capacity Increases Exponentially with Temperature

Why?

As air temperature increases, its ability to hold water vapor increases rapidly.

Rising Air Switches from Dry → Moist Adiabatic Rate

When?

When it cools to its dew point and becomes saturated

(100% relative humidity).

Adiabatic Rate

The rate an air parcel changes temperature as it rises or sinks

→ without exchanging heat with its surroundings.

Clouds Form when Dew point is Passed

Why?

When air cools to its dew point and becomes saturated

(100% relative humidity)

→ Water vapor condenses into tiny liquid water droplets.

Relative Humidity

% of water vapor amount in the air

→ compared to maximum amount it can hold at that temperature.

Specific Humidity

The actual mass of water vapor in the air

→ compared to total mass of air (g of water/kg of air).

Clausius-Clapeyron Relationship

Why 80% relative humidity feels different in the summer and winter?

Summer: 80% humidity means more actual water vapor in the air, making it feel more humid.

Winter: 80% humidity means less actual water vapor in the air, making it feel less humid.

Lapse Rate

Rate that air temperature decreases

→ as altitude increases.