Earth Science Lecture 21 Moisture, Clouds and Precipitation

Main Ideas

1. Water, Phase Changes, and Latent Heat

Water occurs as solid ice, liquid water, and invisible water vapor. Fog is visible, but water vapor itself is not.

Phase changes include:

• Melting (fusion): ice to liquid
• Evaporation: liquid to vapor
• Condensation: vapor to liquid
• Freezing: liquid to ice
• Sublimation: solid directly to vapor
• Deposition: vapor directly to solid

Latent heat is energy exchanged as water molecules’ hydrogen bonds break or form:

• Breaking bonds during evaporation or melting absorbs energy and cools the surroundings.
• Forming bonds during condensation or freezing releases energy and warms the surroundings.

Examples include sweat cooling the body through evaporation and farmers using water during a freeze because freezing water releases heat.

2. Humidity and Dew Point

• Specific humidity is the actual amount of water vapor in the air, expressed as grams of vapor per kilogram of air.
• Relative humidity compares the amount of vapor present with how much the air could hold at its current temperature, expressed as a percentage.
• Warm air can hold more water vapor than cold air. Therefore, relative humidity alone can be misleading when comparing how much moisture the air contains.
• The dew point is the temperature at which air with a given specific humidity becomes saturated and condensation begins. The lecturer recommends it as a more direct indicator of moisture in the air than relative humidity.
• Dew forms when a surface cools to the dew point. Fog and clouds form when the air itself reaches the dew point. Fog is essentially a cloud at ground level.

3. How Rising Air Cools

The lecture’s central mechanism for cloud formation is cooling air until it reaches its dew point.

As a parcel of air rises, the surrounding pressure decreases. The air expands and cools; as it sinks, pressure increases and it warms. These are adiabatic temperature changes.

The lecturer demonstrates this with a bicycle pump and temperature probe: increasing pressure warms the air, while releasing pressure cools it.

A rising parcel cools at one of two adiabatic lapse rates:

• Dry adiabatic lapse rate (DALR): About 10°C per 1,000 meters when condensation is not occurring.
• Wet adiabatic lapse rate (WALR): About 5–10°C per 1,000 meters after condensation begins. Condensation releases latent heat, which offsets some of the cooling. The rate varies with the amount of moisture and condensation.

4. Environmental Lapse Rate and Atmospheric Stability

The environmental lapse rate (ELR) describes how the temperature of the surrounding atmosphere changes with altitude. Its average is about 6.5°C per 1,000 meters, but it varies with weather conditions.

Stability depends on how the temperature of a rising air parcel compares with the surrounding air:

• Absolute stability: ELR is less than the wet adiabatic lapse rate. The rising parcel becomes colder and denser than its surroundings, discouraging further rise. Stable conditions tend to produce gentler weather, such as layered clouds and drizzle.
• Absolute instability: ELR is greater than the dry adiabatic lapse rate. The parcel remains warmer and more buoyant than its surroundings, encouraging strong vertical movement and potentially severe storms.
• Conditional instability: ELR falls between the wet and dry adiabatic lapse rates. The parcel’s behavior depends on whether it becomes saturated and begins condensing.

The lecture contrasts stable, layered cloud conditions with unstable conditions that can produce towering clouds, thunderstorms, hail, and other intense weather.

5. Cloud Formation and Cloud Types

A simplified sequence for cloud formation is:

1. Air rises.
2. Falling pressure causes it to expand and cool adiabatically.
3. It reaches its dew point.
4. Water vapor condenses, typically onto condensation nuclei.
5. Tiny droplets or ice crystals form a cloud.

Condensation nuclei are small airborne particles—such as dust, smoke, pollution, or sea salt—that provide surfaces for condensation. The lecturer notes that cloud seeding introduces particles to encourage cloud formation.

Cloud names are associated with altitude and form:

• Cirrus: High, thin, wispy clouds, largely composed of ice crystals.
• Alto clouds: Middle-level clouds.
• Stratus clouds: Low, layered clouds, often associated with overcast skies.
• Nimbostratus: Layered clouds associated with steady precipitation.
• Cumulonimbus: Towering thunderstorm clouds associated with heavy rain, lightning, and hail.

6. Condensation Versus Precipitation

Clouds consist of very small water droplets or ice crystals. They remain suspended because they are not yet large enough to overcome air resistance.

Precipitation occurs when droplets or ice particles grow large enough to fall to the ground. It may be liquid or solid, including rain and snow.

Two precipitation-forming processes are described:

• Collision and coalescence: Small cloud droplets collide and combine into larger drops. The lecturer says this is especially common in the tropics.
• Bergeron process: Ice crystals grow as water vapor deposits directly onto them. The lecture identifies this as the dominant process in mid-latitudes. In a relatively warm place such as Corpus Christi, falling snow or ice may melt and reach the ground as rain.

Hail forms in strong thunderstorm updrafts: ice particles are repeatedly lifted and fall through the cloud, accumulating growth until they become heavy enough to fall through the updraft.

The lecturer asks students to consult the textbook for precipitation types, particularly freezing rain.

Speakers and Sources Featured

• Speaker: The lecturer, identified in the video context as Dr. Hurt; he also narrates the bicycle-pump demonstration.
• Sources and materials referenced: The course textbook, Chapter 19; Lecture Assignment 21; and a Weather Underground weather report used as an example when discussing Corpus Christi humidity.