Meteorology Exam 2 Review: Atmospheric Moisture, Pressure, Clouds, and Precipitation

Vocabulary review flashcards covering atmospheric thermodynamics, stability, cloud classification, and precipitation mechanisms for Meteorology Exam 2.

Air Pressure

The force exerted per unit area by air molecules on a surface ($P = F/A$), acting equally in all directions.

Barometer

An instrument used to measure atmospheric pressure, commonly expressed in millibars ($\text{mb}$), pascals ($\text{Pa}$), kilopascals ($\text{kPa}$), or inches of mercury.

Ideal Gas Law

The physical law relating pressure ($P$), volume ($V$), temperature ($T$), and molar amount ($n$): $PV = nR^*T$, where $R^*$ is the molar gas constant.

Dalton's Law

A physical law stating that the total atmospheric pressure is equal to the sum of the partial pressures exerted by each individual constituent gas.

Atmospheric Nitrogen Partial Pressure

The primary component of air pressure, accounting for approximately $78\%$ of total pressure ($\approx 780\,\text{mb}$ in a $1{,}000\,\text{mb}$ parcel).

Compressibility of Air

The physical property of air that causes atmospheric density and vertical pressure to decrease nonlinearly with increasing altitude.

Sea Level Pressure

Surface pressure mathematically adjusted to sea level altitude ($\approx 1013\,\text{mb}$ standard) to compare horizontal pressure variations without elevation distortion.

Evapotranspiration

The combined flux of water vapor released into the atmosphere from vegetated surfaces and soil.

Water Vapor Transport

The movement of atmospheric moisture from ocean source regions toward continental land masses.

Water Vapor Pressure

The partial pressure exerted strictly by water vapor molecules in the air, typically contributing between $3\,\text{mb}$ and $30\,\text{mb}$ to total atmospheric pressure.

Saturation Vapor Pressure

The vapor pressure at which the rate of evaporation precisely balances the rate of condensation, increasing nonlinearly with higher air temperatures.

Relative Humidity (RH)

The ratio of the air's actual vapor pressure to its saturation vapor pressure, expressed as: $\text{RH} = \left(\frac{\text{actual vapor pressure}}{\text{saturation vapor pressure}}\right) \times 100\%$

Dew Point

The temperature to which air must be cooled at constant pressure to reach saturation ($\text{RH} = 100\%$).

Extreme Humidity Dew Point Threshold

A dew point of $75^\circ\text{F}$ or higher, characterized as gross and very uncomfortable because slowed sweat evaporation prevents bodily cooling.

Diurnal Relative Humidity Cycle

The daily pattern where relative humidity reaches a maximum in the morning when temperatures are near the dew point, and falls to a minimum in the afternoon as solar heating elevates saturation vapor pressure.

Overnight Low Forecasting Rule

The meteorological principle stating that on clear nights, ambient air temperature cools toward the dew point, which acts as a practical floor for minimum overnight temperatures.

Pathways to Air Saturation

The three physical mechanisms by which air achieves $100\%$ relative humidity: adding water vapor, mixing cold air with warm moist air, or cooling air to its dew point.

Adiabatic Process

A thermodynamic change in parcel temperature that occurs without the addition or withdrawal of heat, such as expansion cooling during ascent.

Diabatic Process

A temperature change driven by the direct transfer of heat into or out of a parcel, such as nocturnal radiation cooling producing radiation fog.

Dry Adiabatic Lapse Rate (DALR)

The constant rate at which an unsaturated air parcel cools when rising or warms when sinking, equal to approximately $10^\circ\text{C/km}$.

Saturated Adiabatic Lapse Rate (SALR)

The rate at which a saturated parcel cools as it rises, approximately $5^\circ\text{C/km}$, reduced relative to the DALR due to the release of latent heat of condensation.

Fog

A suspension of microscopic condensed water droplets located at ground level.

Sling Psychrometer

A meteorological device pairing a dry-bulb thermometer and a wet-bulb thermometer to measure relative humidity and dew point via evaporative cooling.

Wet-Bulb Depression

The numerical temperature difference between the dry-bulb thermometer and the wet-bulb thermometer ($\text{Dry-Bulb} - \text{Wet-Bulb}$).

Orographic Lifting

The upward deflection of moving air parcels forced by encountering elevated topography such as mountain ranges.

Frontal Lifting

The atmospheric lifting mechanism occurring along weather fronts where dense cold air undercuts warm air, or warm air ascends over a colder air wedge.

Atmospheric Convergence

The horizontal inflow of air streams toward a surface low-pressure center, forcing the accumulated air to ascend.

Localized Convection

The buoyant rising of warm air parcels initiated when solar radiation heats a localized portion of Earth's surface, often producing fair-weather cumulus clouds.

High Clouds

Clouds situated at altitudes exceeding $6\,\text{km}$ ($19{,}000\,\text{ft}$) composed predominantly of ice crystals, including cirrus, cirrostratus, and cirrocumulus.

Cirrostratus

Thin, sheet-like high ice clouds that refract sunlight to produce optical halos and sun dogs.

Middle Clouds

Clouds positioned between $2\,\text{km}$ and $6\,\text{km}$ ($6{,}500\text{--}19{,}000\,\text{ft}$) composed mostly of liquid water droplets, including altocumulus and altostratus.

Altostratus

Grayish or bluish mid-level layered cloud sheets that scatter shortwave solar radiation sufficiently to obscure cast shadows on the ground.

Environmental Lapse Rate (ELR)

The observed vertical rate of temperature decrease in the ambient surrounding atmosphere ($\text{temperature decrease} \div \text{height increase}$).

Absolutely Unstable Atmosphere

An atmospheric column condition where $\text{ELR} > \text{DALR} > \text{SALR}$, making any lifted parcel warmer and less dense than surrounding ambient air.

Absolutely Stable Atmosphere

An atmospheric state where $\text{DALR} > \text{SALR} > \text{ELR}$, causing any displaced parcel to be colder and denser than surrounding air and sink back down.

Conditionally Unstable Atmosphere

An atmospheric state where $\text{DALR} > \text{ELR} > \text{SALR}$, in which an unsaturated parcel is stable until lifted past its level of free convection, where it becomes buoyant.

Lifting Condensation Level (LCL)

The altitude at which a lifted unsaturated parcel cools to its dew point, reaches saturation, and initiates droplet condensation at cloud base.

Level of Free Convection (LFC)

The height in a conditionally unstable atmosphere where an artificially lifted parcel's temperature becomes greater than that of the ambient environment, allowing free vertical ascent.

Temperature Inversion

An extremely stable vertical layer where air temperature increases with height, capping vertical convection and trapping pollutants.

Entrainment

The turbulent mixing of colder, drier ambient environmental air into an ascending convective cloud parcel, which cools the parcel and suppresses vertical growth.

Terminal Velocity

The steady falling speed achieved when upward aerodynamic drag exactly balances the gravitational weight of a falling drop ($\approx 1\,\text{cm/s}$ for a $10\,\mu\text{m}$ droplet; $\approx 650\,\text{cm/s}$ for a $1{,}000\,\mu\text{m}$ raindrop).

Heterogeneous Nucleation

The initial condensation of water vapor onto microscopic hygroscopic particles known as cloud condensation nuclei.

Collision-Coalescence Process

The primary droplet growth mechanism in warm tropical clouds where large collector droplets fall, collide with smaller drops, and merge with near $100\%$ coalescence efficiency.

Supercooled Water

Liquid water droplets maintained in a liquid state at ambient temperatures below the standard freezing point ($< 0^\circ\text{C}$).

Bergeron Process

The growth of ice crystals in mixed-phase cold clouds occurring because saturation vapor pressure is lower over ice than over liquid water, causing droplets to evaporate while vapor deposits directly onto ice.

Riming

The rapid freezing of supercooled cloud droplets upon colliding with a falling ice crystal, accelerating solid particle growth.

Aggregation

The clumping together of falling ice crystals aided by a thin liquid film, forming multi-crystal snowflakes.

Lake-Effect Snow

Localized heavy snow produced when cold, dry continental air masses absorb heat and moisture over a warm lake before rising via shoreline convergence.

Sleet

Precipitation consisting of translucent ice pellets formed when snowflakes melt entirely in a warm layer aloft, then refreeze within a deep near-surface subfreezing layer.

Freezing Rain

Precipitation that melts completely in a warm layer aloft, becomes supercooled while traversing a shallow subfreezing surface layer, and freezes instantly on contact with ground infrastructure.

Precipitation Temperature Profiles

The four characteristic vertical atmospheric soundings relative to the $0^\circ\text{C}$ isotherm governing whether hydrometeors reach the surface as rain, sleet, snow, or freezing rain.

Graupel

Spongy, heavily rimed ice pellets resembling soft snow that serve as embryos for hailstone development.

Hail

Layered ice precipitation formed when supercooled water continually accretes onto ice embryos suspended by violent updrafts within cumulonimbus clouds.

Tipping Bucket Rain Gauge

An automated mechanical rain gauge that accumulates water in a two-compartment see-saw mechanism, recording a calibrated increment with each physical tip.

CoCoRaHS

Community Collaborative Rain, Hail & Snow Network; a grassroots organization of volunteer observers taking standardized daily ground precipitation readings.

NEXRAD

Next Generation Radar; a nationwide network of high-resolution Doppler radars measuring precipitation reflectance, precipitation distance, and radial wind velocity.

Doppler Shift

The frequency change of electromagnetic radar pulses scattered back from hydrometeors, used to measure horizontal motion toward or away from the radar antenna.