Glossary
Glossary
| Term | Definition |
| Acid Rain | Precipitation with high acidity caused by atmospheric pollution. It is primarily produced by sulphuric acid (from SO2 emitted by power plants) and nitric acid (from nitrogen oxides in car exhausts), and it causes damage to lakes and forests. |
| Anticyclone | A large high-pressure area where air is sinking. Sinking air is warmed by compression, which can form an elevated warm layer (inversion) that traps pollutants in a thin mixed layer below, leading to conditions of particularly bad air pollution. |
| Atmospheric Dispersion | The process by which a contaminant in the atmosphere is mixed and spread out. It depends on the mean wind speed and the characteristics of atmospheric turbulence (eddies). |
| Atmospheric Turbulence | The presence of horizontal and vertical eddies in the atmosphere that mix contaminated air with clean air. It is formed by two main mechanisms: heating from below (convection) and wind shear (mechanical turbulence). |
| Chlorofluoromethanes (CFM) | Stable chemical compounds used in sprays, foams, and refrigeration that can seep into the stratosphere. In the high stratosphere, UV radiation dissociates CFMs, producing chlorine which catalytically destroys ozone. |
| City Models | Comprehensive simulations that calculate the total air pollution from all sources within a city for different meteorological conditions. They are used to understand contaminant distribution, devise economical control strategies, and plan future urban growth. |
| Convection | A form of atmospheric turbulence produced by heating from below. It occurs when the temperature decreases rapidly with height (lapse rate exceeds 1°C/100m) and results in strong vertical motion. |
| Diffusion Equation | A mathematical equation used to calculate the concentration of a contaminant. Its application in atmospheric problems has not been very successful due to the complexity and variability of atmospheric diffusion coefficients (K’s). |
| Downwash | A phenomenon where the effluent rise from a stack is negative due to a low efflux velocity and certain wind velocities, causing the plume to be drawn downward. |
| Effective Source Height (H) | The height used for dispersion calculations, determined by adding the physical stack height to the additional rise of the effluent (plume rise). |
| Fumigation | A condition of strong pollution near the ground that occurs when an elevated polluted layer is mixed with ground air. This typically happens in the morning after sunrise when the growing mixed layer reaches the height of an elevated plume that was trapped above a nighttime inversion. |
| Greenhouse Effect | The warming of the Earth’s surface caused by atmospheric gases that transmit short-wave solar radiation but absorb a portion of the outgoing long-wave infrared radiation. CO2 is a primary greenhouse gas. |
| Inversion Layer | A layer in the atmosphere where temperature increases with height. These stable layers limit the vertical rise of effluent and trap pollutants, preventing their upward dispersion. |
| Lapse Rate | The rate at which atmospheric temperature decreases with an increase in altitude. A rapid decrease (exceeding 1°C/100m) promotes convection. |
| Mechanical Turbulence | A type of atmospheric turbulence that occurs when the wind changes with height (wind shear). It increases with wind speed and terrain roughness. |
| Mesometeorology | The branch of meteorology dealing with atmospheric phenomena on a scale of several kilometers to 100 kilometers. It is crucial for understanding local wind patterns influenced by surface features like hills, lakes, and cities. |
| Mixed Layer | The layer of air near the surface where pollutants are mixed by turbulence. Its depth (D) is a key factor in the “ventilation factor” and is typically deeper during the day and shallower at night. |
| Normal (Gaussian) Distribution | A statistical distribution often assumed to describe the concentration of effluent from a continuous source in both the vertical and horizontal directions relative to the plume’s centerline. |
| Ozone Hole | A phenomenon observed over the South Pole during the Southern Spring where the total column ozone falls to about half its normal value for a month or so. |
| Pasquill–Gifford Method | An empirical method for estimating the vertical and lateral spread (standard deviations σy and σz) of a plume as a function of downwind distance and meteorological “stability categories” (A through F). |
| Richardson Number (Ri) | A dimensionless number that characterizes the relative importance of heat convection versus mechanical turbulence. Negative values indicate convection is significant or dominant, a value near zero indicates mechanical turbulence only, and positive values indicate stable stratification that damps turbulence. |
| Roughness Length (z0) | A parameter that characterizes the roughness of a terrain, varying from about 0.1 cm over smooth sand to a few meters over cities. It is proportional to the size of eddies that can exist among roughness elements and influences the intensity of mechanical turbulence. |
| Tetroon | A tetrahedral balloon that drifts horizontally and is tracked by radar. It is used in local wind studies to map air trajectories on the mesoscale. |
| Turbidity | An indicator of the reduction of light intensity due to haze, smoke, and other airborne particles. It is typically larger over cities than in the country. |
| Ventilation Factor | The product of the mixing depth (D) and the average wind speed in the mixed layer (V). Air pollution concentration is inversely proportional to this factor; a small ventilation factor leads to high pollution levels. |