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GLOSSARY OF WEATHER TERMS
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| Atmospheric Stability (stable VS unstable): | |
| Atmospheric stability is the resistance of the atmosphere to vertical motion. The disruption of temperature vertically in the troposphere influences vertical motion. A large decrease of temperature with height indicates an unstable condition which promotes up & down currents. A small decrease with height indicates a stable condition which inhibits vertical motion. Where the temperature increases with height, through an inversion, the atmosphere is extremely stable. For example, when the atmosphere is unstable, the winds tend to be turbulent and gusty, thereby causing a fire to behave erratically. Thunderstorm with strong updrafts and downdrafts develop in an unstable atmosphere which contain sufficient moisture. | |
| Barometer: | |
| A barometer is an instrument used to measure atmospheric pressure by using water, air, or mercury. Pressure tendency can forecast short term changes in weather in that high pressure in a region indicates fair weather while low pressure indicates storms are more likely. | |
| Chandler Burn Index (CBI): | |
| The CBI is calculated using only
air temperature and relative humidity to create a numerical index of
fire danger. It is based solely on weather conditions, either current or
forecasted, with no adjustment for fuel moisture. The index uses a scale
of less than 50 to greater than 97.5.
CBI of < 50 = Low Danger The formula is: CBI = (((110 - 1.373 * RH) - 0.54 * (10.20 - T)) * (124 * 10**(-0.0142*RH)))/60 (USFS) [Where T is temperature in Celsius and RH is relative humidity] |
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| Dew Point: | |
| The temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. Dew point is associated with relative humidity in that a high relative humidity indicates the dew point is closer to the current air temp. Relative humidity of 100% indicates that the dew point is equal to the current temperature (and the air is completely saturated with water). When the dew point stays constant and temperature increases, relative humidity decreases | |
| Fire Weather Area Breakdown: | |
| El Paso County North: Area includes Monument, Wescott, Air Force Academy, Black Forest, Falcon (excluding Dist. 4) and areas north of Hwy 94 El Paso County South: |
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| Haines Index: | |
| Also called the Lower Atmosphere
Stability Index (LASI), the Haines Index is calculated from a stability
term and a moisture term. The stability term is derived from the
temperature difference at two atmosphere levels. The moisture term is
derived from the dew point depression of the lower atmosphere level. The
index is calculated over three ranges: low elevation (at or near sea
level), mid elevation (1000-3000 foot elevation) and high elevation
(> 3000 elevation). The Index ranges from 2 to 6 for indicating
potential for large fire growth.
6 = a high potential for large fire growth Drawbacks of the Index are that it does not take into account wind velocities, fuel moisture, terrain, or fuel continuity. The index is is calculated every day during wild fire season at 5 am and 5 pm. This index has been shown to be correlated with large fire growth on initiating and existing fires where surface winds do not dominate fire behavior. |
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| Mixing Height: | |
| Mixing heights represent the top of the layer through which relatively vigorous mixing will take place. It is the height at which smoke will lose its buoyancy and stop rising. | |
| Relative Humidity (RH): | |
| Term used to describe the amount of water vapor that exists in a gaseous mixture of air and water. | |
| Transport Winds: | |
| Defined as the average wind speed and direction of all winds within the layer bounded by the surface and the mixing heights. Knowledge of transport winds is a crucial factor in effective management of smoke management programs. Transport winds provide land managers with information about the horizontal dispersion (location and distance downwind from the source) of suspended particulates from prescribed fires and slash burns. | |
| Ventilation Rate: | |
| The ventilation rate is defined as the product of the transport wind speed and the mixing height. This is used for forecasting smoke behavior and trajectories. | |
| Colorado Fire Weather Zones: | |
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