Atmospheric Moisture, Lapse Rates, and Precipitation Processes

Vocabulary flashcards covering humidity metrics, adiabatic processes, atmospheric lifting mechanisms, stability categories, and precipitation formation processes.

Hydrologic cycle

The perennial movement of water in its three phases among the atmosphere, land, and oceans.

Saturation

The maximum amount of water that can exist in the air as vapor, reached when evaporation rate equals condensation rate.

Evaporation

The phase change where liquid water changes into water vapor.

Condensation

The phase change where water vapor changes into liquid water.

Vapor pressure

The part of the total atmospheric pressure due to water vapor.

Absolute humidity

The mass of water vapor per unit volume of air ($g/m^3$), which changes as air expands.

Specific humidity

The mass of water vapor per unit mass of all air, which does not change as air expands or contracts.

Mixing ratio

The mass of water vapor per unit mass of dry air, which does not change as air expands and usually differs from specific humidity by about $2\%$.

Dew point temperature

The temperature to which air would have to be chilled to become saturated.

Relative Humidity (RH)

The amount of water vapor in the air relative to the maximum amount possible at a given temperature, expressed as a percentage.

Frost point

The term used when air reaches saturation at temperatures below freezing.

Diabatic cooling

A process of cooling air that involves energy transfer, such as through conduction.

Adiabatic process

A process in which the temperature of an air parcel changes in response to a change in pressure within the air parcel due to vertical movement.

Dry Adiabatic Lapse Rate (DALR)

The rate at which rising air cools or descending air warms when no condensation is occurring in unsaturated air, equal to $10\,^{\circ}\text{C}/1000\,\text{m}$.

Lifting Condensation Level (LCL)

The altitude where a rising, cooling air parcel becomes saturated and condensation begins.

Saturated Adiabatic Lapse Rate (SALR)

The rate of cooling or warming when air is saturated and condensation is occurring ($4\,^{\circ}\text{C} - 9\,^{\circ}\text{C}/1000\,\text{m}$), which is less than the DALR because latent heat is released as water condenses.

Orographic lifting

The forced ascent of moist oceanic air over a mountain barrier, leading to adiabatic cooling, clouds, precipitation, and subsequent warming on descent.

Rain shadow

A dry area on the leeward side of a mountain formed when descending air compresses and warms after passing the mountain summit.

Isohyets

Lines on a map connecting points of equal rainfall.

Cold front

A boundary where colder, denser air lifts warm air along a vigorous steep slope, developing showers and sometimes severe thunderstorms.

Warm front

A boundary where warmer, less dense air rises up and over cooler air in a gradual rising motion, developing steady and more widespread precipitation.

Convergent lifting

Lifting that occurs when air horizontally moves and converges along the Earth's surface into a low-pressure cell and is forced upward.

Convectional lifting

Lifting initiated by unequal heating over land surfaces, creating a convection cell of warm, rising air.

Static stability

The atmospheric condition that inhibits vertical displacement of air parcels due to buoyancy effects.

Buoyancy

The tendency for less dense air to rise upward through denser, colder air.

Environmental Lapse Rate (ELR)

The rate of temperature change with altitude of still air, averaging $6.5\,^{\circ}\text{C}/1000\,\text{m}$ in the troposphere.

Absolutely unstable air

A condition where rising air cools more slowly than surrounding air ($ELR > DALR$ or $SALR$), causing the parcel to become warmer and more buoyant and rise at increasing speed.

Absolutely stable air

A condition where rising air cools more rapidly than surrounding air ($ELR < DALR$ or $SALR$), causing it to become cooler, less buoyant, and sink back if the lifting mechanism stops.

Conditionally unstable air

A condition where the ELR lies between the DALR and SALR, causing an air parcel to cool more rapidly initially but then cool more slowly and rise freely if lifted high enough.

Entrainment

The mixing of nearby unsaturated air into the margin of a rising air parcel by turbulence, which cools the parcel and limits continued rising.

Condensation nuclei

Tiny centers of solid matter (such as salt and dust) upon which individual cloud droplets form.

Collision-Coalescence process

A warm-cloud precipitation process ($> 0\,^{\circ}\text{C}$) where fine cloud droplets collide and merge, beginning with falling collector drops.

Bergeron process

A cold-cloud process ($< 0\,^{\circ}\text{C}$) where ice crystals grow at the expense of supercooled water droplets because condensation equals evaporation on droplets while deposition exceeds sublimation on ice.

Riming

The rapid growth of falling ice crystals when they collide with supercooled water droplets.

Aggregation

The joining together of ice crystals to form a snowflake.

Cloud seeding

The attempt to stimulate precipitation by introducing materials into clouds to convert supercooled droplets into ice crystals.

Silver Iodide

A cloud seeding material often mixed with smoke from ground burners that initiates the Bergeron process by acting directly as freezing nuclei.

Stratocumulus clouds

Low, patchy, sheet-like clouds covering more of the Earth's surface than any other cloud type, playing an important role in climate, global weather patterns, and reflection.