Section 7: Atmospheric Water & The Hydrologic Cycle

GES 108

Water

A compound (H₂O) essential to life and Earth’s systems, existing naturally in all three physical states

Sublimation

The process in which a solid changes directly into a gas without passing through the liquid state. Example

Deposition

The process in which a gas changes directly into a solid without becoming a liquid first. Example

Cloud

A visible mass of tiny water droplets and ice crystals suspended in the atmosphere. Clouds are visible only when many droplets cluster together in large numbers.

Precipitation

Any form of water, liquid or solid, that falls from the atmosphere to Earth’s surface. Examples include rain, snow, sleet, and hail.

Hydrologic Cycle

The continuous circulation of water through the atmosphere, biosphere, lithosphere, and hydrosphere. The cycle is powered entirely by solar energy and would not be possible without plate tectonics. About 85% of atmospheric water vapor comes from ocean evaporation.

Evaporation

The process by which liquid water changes into water vapor and enters the atmosphere. Most evaporation occurs from the surface of the oceans.

Transpiration

The release of water vapor from plant leaves into the atmosphere.

Evapotranspiration

The combined process of evaporation from land and water surfaces and transpiration from plants. It accounts for about 15% of the water vapor in the atmosphere.

Polar Molecule

A molecule with an uneven distribution of electrical charge, such as water, giving it a positive and negative side.

Cohesion

The attraction between water molecules caused by hydrogen bonding, which allows water to stick to itself.

Adhesion

The attraction between water molecules and other surfaces or materials. This property allows water to climb upward through small spaces, such as plant stems or soil.

Universal Solvent

A substance, like water, that can dissolve many other substances due to its polarity.

Density (of Water)

Water is most dense as a liquid and less dense as a solid. Ice floats because it has a lower density than liquid water.

Specific Heat

The amount of heat energy required to raise the temperature of a substance. Water has a high specific heat, meaning it can absorb or release large amounts of heat with minimal temperature change.

Latent Heat

The energy absorbed or released by water during a phase change (for example, melting or evaporation) without changing temperature. This energy transfer plays a critical role in weather and climate.

Latent Heat of Water

The ability of water to absorb, transport, and release solar energy as it changes state. This process redistributes energy across Earth, balancing temperatures between the tropics and poles and fueling weather systems such as hurricanes.

Humidity

The amount of water vapor present in the air. It is an important factor in determining apparent temperature and weather conditions.

Apparent Temperature

The perceived temperature felt by humans, which accounts for humidity and other atmospheric conditions.

Dew Point Temperature (Td)

The temperature at which air must cool to become saturated with water vapor, leading to condensation.

Relative Humidity (RH)

The ratio of the current amount of water vapor in the air to the maximum amount the air can hold at that temperature, expressed as a percentage.

Vapor Pressure (e)

The pressure exerted by water vapor in the atmosphere.

Saturation Vapor Pressure (es)

The maximum vapor pressure that can exist at a given temperature before condensation occurs.

Sling Psychrometer

An instrument used to measure relative humidity by comparing temperatures from wet and dry thermometers swung through the air.

Weather Balloon

A device that carries instruments into the atmosphere to measure humidity, temperature, and pressure at various altitudes.

Atmospheric Instability

A condition in which warm air rises and cool air sinks, promoting cloud formation and vertical air movement.

Adiabatic Cooling

The process by which rising air expands and cools without losing heat to its surroundings. As air rises, it cools toward its dew point, leading to cloud formation.

Adiabatic Lapse Rate

The rate at which temperature decreases with increasing altitude in a rising air parcel.

Lifting Condensation Level (LCL)

The altitude at which rising air cools to its dew point and condensation begins, forming clouds.

Adiabatic Warming

The process in which a descending air parcel compresses and warms as pressure increases.

Windward Side

The side of a mountain that faces incoming wind, receives moist air, and experiences higher rainfall.

Leeward Side

The side of a mountain sheltered from wind, which receives less precipitation and has drier conditions.

Rain Shadow

A dry region on the leeward side of a mountain range caused by the blocking of moist air and precipitation on the windward side.

Orographic Uplift

The upward movement of air that occurs when wind encounters a mountain range, forcing air to rise and cool.

Frontal Uplift

The upward movement of warm air when it meets a cooler air mass along a front. Warm air rises because it is less dense.

Convergent Uplift

The upward movement of air that occurs when winds from different directions meet and are forced upward.

Convective Uplift

The upward movement of air caused by surface heating, which makes air parcels rise. This process often leads to thunderstorm development.

Fog

A stratus cloud that forms on or near the ground, reducing visibility to less than one kilometer (0.62 miles). Fog has a high albedo, meaning it reflects most sunlight.

Radiation Fog

Fog that forms when the ground cools rapidly at night by radiating heat, cooling the air above it to the dew point.

Advection Fog

Fog formed when warm, moist air moves horizontally (advection) over a cooler surface, such as ocean water.

Ice-Crystal Process

The formation of snow in clouds where the temperature is 0°C or lower, as water vapor deposits directly onto ice crystals.