Atmospheric Science, Energy Balance, and Hydrologic Processes

Flashcards covering key vocabulary, physical principles, and governing equations across atmospheric thermodynamics, general circulation, solar radiation, energy balances, and catchment hydrology.

Condensation

The phase change of water from vapour to liquid.

Sublimation

The phase change of water from ice directly to vapour.

Dew-point Temperature

The temperature to which air must cool (at constant pressure and moisture) to become saturated; higher values indicate more humid air.

Saturated Air

An equilibrium state where the evaporation rate equals the condensation rate, actual vapour pressure equals saturation vapour pressure ($e = e_{\text{sat}}$), and relative humidity is $100\%$.

Adiabatic Process

A process where a parcel of air rises or falls with no energy exchange with the surrounding air.

Dry Adiabatic Lapse Rate (DALR)

The rate at which an unsaturated rising parcel of air cools, equal to $-0.98\,^\circ\text{C}\text{ per }100\,\text{m}$.

Saturated Adiabatic Lapse Rate (WALR)

The rate at which a saturated rising parcel of air at its dew point cools, equal to $-0.5\,^\circ\text{C}\text{ per }100\,\text{m}$; it is slower than the dry rate because condensation releases latent heat.

Condensation Level

The height at which a rising air parcel reaches its dew point and its cooling rate transitions from the dry adiabatic rate to the saturated adiabatic rate.

Environmental Lapse Rate (ELR)

The actual measured temperature profile of the atmosphere, averaging about $-6.5\,^\circ\text{C}\text{ per km}$ in the troposphere.

Stable Atmosphere

An atmospheric state where a rising parcel cools at an adiabatic rate that makes it colder and denser than the surrounding environment, suppressing further uplift.

Unstable Atmosphere

An atmospheric state where a rising parcel stays warmer and less dense than the surrounding environment, causing uplift to continue.

Troposphere

The lowest atmospheric layer where most weather occurs, containing about $80\%$ of the atmosphere's mass and spanning $8\text{ to }16\,\text{km}$ in thickness.

Coriolis Effect

The apparent deflection of moving air or water due to Earth's rotation, directing movement to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; it is zero at the equator, strongest at the poles, and acts only over large scales.

Hadley Cell

A general circulation cell located between $0^\circ$ and $30^\circ$ latitude where air rises at the ITCZ, moves poleward aloft, descends at the subtropical high (about $30^\circ$), and returns toward the equator along the surface as trade winds.

Ferrel Cell

A middle-latitude atmospheric mixing cell between $30^\circ$ and $60^\circ$ with prevailing surface westerlies, driven dynamically by the Hadley and Polar cells.

Polar Cell

A circulation cell located between $60^\circ$ and $90^\circ$ latitude where cold air descends at the poles, flows equatorward along the surface as polar easterlies, and rises at approximately $60^\circ$.

Markov Chain Property

A statistical dependency property wherein today's state depends exclusively on yesterday's state, used to model the clustering of wet and dry days.

Stefan-Boltzmann Law

A physical law stating that the total radiant energy emitted by a body is proportional to the fourth power of its absolute temperature, defined for a black body as $E = \sigma T^4$, where $\sigma = 5.672 \times 10^{-8}\,\text{W/m}^2/\text{K}^4$.

Black Body

An ideal physical emitter and absorber that absorbs and emits radiation perfectly at all wavelengths, having an emissivity ($\epsilon$) of $1$.

Grey Body

An object that emits radiation less effectively than a black body and reflects some radiation, having an emissivity ($\epsilon$) of less than $1$ ($E = \epsilon \sigma T^4$).

Solar Constant

The solar radiation per unit area received at the top of Earth's atmosphere on a plane perpendicular to the rays at the mean Earth-Sun distance, valued at approximately $1361\,\text{W/m}^2$.

Albedo

The fraction of incoming solar radiation reflected by a surface, expressed on a scale from $0$ to $1$.

Natural Greenhouse Effect

The warming of Earth's surface by approximately $33\,\text{K}$ (from $-18\,^\circ\text{C}$ to $+15\,^\circ\text{C}$) due to naturally occurring greenhouse gases absorbing outgoing longwave radiation and re-emitting it toward space and back to the surface.

Solar Declination

The angle at solar noon between the sun and the vertical at the equator, defined as positive when the sun is in the northern sky ($0^\circ$ at equinoxes, $-23.5^\circ$ at the December solstice, and $+23.5^\circ$ at the June solstice).

Milankovitch Cycles

Three collective orbital variations influencing external solar radiation over long timeframes: obliquity (tilt shifts between $22.2^\circ$ and $24.5^\circ$ over $\approx 41{,}000\,\text{years}$), eccentricity (orbit shape cycling over $\approx 95{,}000\text{ to }400{,}000\,\text{years}$), and precession (date of perihelion shifting over $\approx 23{,}000\,\text{years}$).

Catchment

A topographically defined area of land where all surface water drains to a common single outlet on a stream or river; also called a drainage basin or watershed.

Porosity

The proportion of total soil volume occupied by void space, calculated as $\text{pore volume} / \text{total soil volume}\;(\text{m}^3/\text{m}^3)$.

Field Capacity

The soil moisture content remaining after gravity drainage has ceased, where water is retained in micropores by capillary forces.

Wilting Point

The lower threshold of soil moisture below which vegetation can no longer exert enough suction to extract water.

Evapotranspiration (ET)

The combined transfer of water into the atmosphere by evaporation (from soil, open water, and wet leaves) and transpiration (via leaf stomata following plant root uptake).

Vapour Pressure Deficit (VPD)

The atmospheric drying demand defined by the difference between saturation vapour pressure and actual vapour pressure ($\text{VPD} = e_{\text{sat}} - e$).

Potential Evapotranspiration (PET)

The theoretical maximum rate of evapotranspiration that would occur if soil water were freely available, governed primarily by available energy and atmospheric demand.

Actual Evapotranspiration (AET)

The true volume of water lost to the atmosphere through evapotranspiration, bounded by PET and regulated by soil moisture availability.

Priestley-Taylor Equation

An energy-based radiation formulation for potential evapotranspiration: $\text{PET} = 1.26 \times \left[\frac{\Delta}{\Delta + \gamma}\right] \times R_n$, where $\gamma = 0.665 \times 10^{-3} \times P$.

Darcy's Law

A principle establishing that groundwater discharge velocity through saturated porous media is proportional to the hydraulic gradient: $V = k_{\text{sat}} \times \left(\frac{\Delta H}{l}\right)$.

Infiltration Excess Runoff

Surface runoff generated when rainfall intensity exceeds the rate at which soil can absorb water (infiltration capacity), common on compacted ground, impervious surfaces, or in arid catchments.

Saturation Excess Runoff

Surface runoff occurring when the entire soil column becomes saturated from below, preventing any additional precipitation from infiltrating.

Subsurface Stormflow

Rapid lateral flow of water through preferred pathways (such as macropores, soil pipes, and burrows) on sloped terrain when moisture exceeds field capacity.

Baseflow

The sustained, delayed component of stream discharge fed by groundwater drainage between precipitation events.

Rating Curve

A calibrated empirical relationship or curve relating river stage (water level height) to total volumetric discharge ($Q$).

Flow Duration Curve

A cumulative frequency plot displaying the proportion or percentage of time that a specific stream discharge is equalled or exceeded.