Atmospheric Moisture - Water, Latent Heat, Evaporation, Humidity, Condensation, Adiabatic Processes

The Hydrologic Cycle

Circulation of Earth’s water supply

between atmosphere and surface.

Parts of the Hydrologic Cycle (5)

1. Evaporation & transpiration: liquid → gas

2. Condensation: gas → liquid

3. Advection: air flow from one location to another

4. Precipitation: liquid water droplets in cloud get bigger → air can’t hold it anymore → falls to surface

5. Surface runoff/subsurface flow: water flowing from one place to another (above and below surface)

Atom

The fundamental building blocks of matter

• Nucleus of protons and neutrons

• Surrounded by “shells” of electrons

Molecule

two or more atoms

The Water Molecule

• H₂O (two hydrogen atoms + one oxygen atom)

• Held together by covalent bonds

• Weak electrical polarity

• Negative charged oxygen + positive charged hydrogen = hydrogen bond (weaker than covalent bonds)

• Tend to “stick”

Properties of Water (6)

1. Liquidity

2. Ice expansion

3. Surface tension

4. Capillarity

5. Solvent ability

6. Specific heat capacity

Property of water: Liquidity

Water exists as a liquid at temperatures found at places on Earth

Property of water: Ice expansion

• Water expands when it gets colder

• Freezing water/ice is less dense than liquid water

  • (ice floats on liquid, lakes freeze top down)

• Important for weathering: expansion of ice can break apart rocks

Property of water: Surface tension

• Because of water’s electrical polarity, liquid water molecules stick together (cohesion) → high surface tension

Property of water: Capillarity

• Water moves upward when confined

• Adhesion + surface tension → water “sticks” to other substances & climbs up

• Enables water to circulate upward through rock cracks, soil, stems, etc.

• For water, adhesion > cohesion

  • Water molecules are more attracted to other substances than each other

Property of water: Solvent ability

• Water can dissolve almost any substance → water in nature is nearly always impure (usually contains other chemicals + minerals)

• Water molecules are attracted to other “polar” chemical compounds → can break their bonds and dissolve the materials

Property of water: Specific heat capacity

• Specific heat: the amount of energy required to raise the temperature of 1 gram by 1℃

• Water has HIGH specific heat capacity (5x greater than land)

• Can absorb lots of energy with small increase in temperature

Phase Changes of Water

Sublimation: solid → gas

Deposition: gas → solid

Melting: solid → liquid

Freezing: liquid → solid

Condensation: gas → liquid

Evaporation/Vaporization: liquid → gas

Latent heat

• The energy used to break or form bonds (in rearranging molecular structure between phases)

• When undergoing phase, the temperature of water will NOT increase above 100℃

• Adding energy DOES NOT increase temperature, but increases internal structural energy

• Latent heat is exchanged in each phase change!!!

Latent heat in melting vs freezing

• Latent heat of melting: the energy required to melt ice

• Latent heat of fusion: the energy released as water freezes

Latent heat in evaporation vs condensation

• Latent heat of evaporation: the energy required to convert liquid water to water vapor

• Latent heat of condensation: the energy released when water condenses to liquid

Latent heat in evaporation vs vaporization

• Boiling: when vaporization occurs beneath the liquid surface

• Most water vapor is added to atmosphere as surface evaporation

• Latent heat of evaporation is greater than that required to boil water

Why is latent heat important for the atmosphere?

• Energy transferred by air masses and wind affects the stability of atmosphere and power of many storms

• Evaporation = a cooling process

  • Latent heat energy is “stored” in water vapor

• Condensation = a warming process

  • Latent heat energy is released back into the atmosphere

Evaporation

• Process that puts water vapor into the atmosphere

• Water vapor is added to air when the rate of evaporation exceeds the rate of condensation

Rate of evaporation depends on (3):

• Temperature (of air and water)

  • Warm water and warm air promote evaporation

• Amount of water vapor in the air

  • Vapor pressure: pressure exerted by water vapor

  • Higher temperature = higher maximum vapor pressure

    • Warm air can hold more water vapor than cold air can

  • Saturation: when air reaches its maximum vapor pressure at its given temperature (balance between evaporation and condensation)

    • Evaporation takes place more rapidly with less water vapor in air → gets slower as air gets closer to saturation

    • Humid day air is lighter than dry air but feels heavy because of sweat evaporating slowly

• Wind (air movement)

  • If air is in motion (ex: wind), water vapor is dispersed more widely → rate of evaporation increases

Evapotranspiration

Process of water vapor entering the air from land sources

2 sources of evapotranspiration (evaporation from land)

1. Soil and other inanimate surfaces

2. Plants (majority)

• Transpiration: how plants give up moisture through their leaves

Humidity

The amount of water vapor in the air

3 ways to measure actual water vapor content (how much water vapor is actually in the air)

1. Absolute humidity

2. Specific humidity

3. Vapor pressure

Absolute humidity

• The mass of water vapor in a given volume of air

  • Usually expressed in grams of water vapor per cubic meter of air

• Affected by: changes in air volume (if air expands or compresses as it moves vertically)

  • Total amount of water vapor DOES NOT change

• Absolute humidity is NOT used to describe moisture in air that is rising or descending

Specific humidity

• The mass of water vapor in a given mass of air

  • Usually expressed in grams of water vapor per kilogram of air

• Affected by: changes in quantity of water vapor

  • NOT affected by variations in air volume

• Maximum specific humidity (saturation specific humidity) is determined by temperature

  • Cold air = small maximum specific humidity

  • Warm air = large maximum specific humidity

Vapor pressure

• Contribution of water vapor to total atmospheric pressure

• Saturation vapor pressure: maximum vapor pressure at a given temperature

Relative humidity

• Describes how close the air is to saturation with water vapor (compares actual amount of vapor to the water vapor capacity of the air)

(check equation)

Water vapor capacity

• Water vapor capacity: the maximum amount of water vapor that air can contain at a given temperature

  • Determined by temperature, which determines the rate of vaporization of water

Does cold air have high/low water vapor capacity? Warm air?

• Cold air = low water vapor capacity

• Warm air = high water vapor capacity

Factors changing relative humidity (3)

• Water vapor content

  • Less water vapor = relative humidity decreases (vice versa)

• Water vapor capacity

  • depends on temperature

• Temperature

  • Temperature increase → water vapor capacity increases → relative humidity decreases (vice versa)

  • Temperature decrease alone can bring air to saturation (100% relative humidity)

Temperature-relative humidity relationship

• inverse relationship

• Temperature ⬆  Relative humidity ⬇

• Temperature ⬇ Relative humidity ⬆

Dew Point Temperature

• The temperature to which air must cool in order to saturate

• Affected by: moisture content of air

• Can also describe the actual water vapor content of air

Sensible temperature

The temperature that a person’s body feels (relative humidity, dew point, and wind influence our perception of warmth and cold)

What is high and low sensible temperature?

High sensible temperature = when a warm, humid day seems hotter than it is

• Because air is near saturation

• Sweat on our skin does not evaporate rapidly

Low sensible temperature = when a warm, dry day seems cooler than it is

Condensation

• gas (water vapor) → liquid

• air must be saturated for condensation to take place

Why can condensation intensify storms?

When condensation happens (water vapor changes to water), energy is released which heats up clouds and surrounding air → temperature increase, air spreads, lower pressure → higher wind turbulence → stronger storms

Supersaturated air

relative humidity greater than 100%

Condensation nuclei

• “Surfaces”/collection points for water vapor molecules during condensation

  • Ex: tiny particles of dust, smoke, salt, pollen, bacteria, etc.

• If no surface where condensation can take place is available, condensation can only occur under extreme conditions

Supercooled Water

Promote the growth of ice particles in cold clouds by:

1. Freezing around the particles

2. Evaporating into vapor → water molecules are added to the ice crystals

Adiabatic Processes

When the temperature of a parcel of air changes without any heat being exchanged with its surroundings (only changed by the expansion/compression of air)

Adiabatic cooling

• The process where air cools as it expands without any heat exchange with its surroundings

• Air rises → pressure decreases → air expands and temperature drops

Lifting condensation level (LCL)

• If air mass rise high enough, it reaches dew point, saturates, condensation begins, clouds form

• The altitude at which this occurs is LCL

Dry Adiabatic Rate

As unsaturated air rises, it cools at the relatively steady rate of 10℃ per 1000 meters

Saturated adiabatic rate

Diminished rate of cooling when release of latent heat during condensation counteracts adiabatic cooling

• Average rate = 6℃ per 1000m

When does adiabatic warming happen?

• Adiabatic warming occurs when air descends

• Increasing temp of descending air → increases water vapor capacity → saturated air becomes unsaturated

• This is why descending air can’t make clouds

Environmental lapse rate (different from adiabatic process because…)

Environmental lapse rate describes temperature of still air at different altitudes

Why does ocean have higher evaporation than precipitation?

Water vapor