Northern Prairie Wildlife Research Center
Weather is the main controlling agent of fire behavior, smoke behavior, fuel condition and flammability, and fire containment; all of these affect the success and safety of the burn.
The weather variables most applicable to prescription burns are air temperature, relative humidity, wind direction, wind speed, precipitation, and air mass stability (Sando 1969). Because most of the NGP has low topographic relief, general weather patterns are usually not affected by topographic features.
A combination of wind speed, relative humidity, temperature, and solar insulation largely determines fuel condition which, in turn, affects fire behavior.
Seasonal wind direction is important when burning near areas having restrictions or smoke regulations. Daily and seasonal precipitation patterns often determine when burns can be conducted, whereas days since measurable precipitation (greater than 1 mm, or 0.01 in) determine the severity of the fire or completeness of a burn in terms of fuel consumption.
Air temperatures higher than 20 degrees C (68 F) are recommended when prescriptions call for total fuel consumption. These include initial reclamation burns or burning to reduce undesirable plant species and medium to heavy coarse fuels like brush or trees.
Hot (high intensity) fires also produce higher risks than cool (low intensity) fires, thus requiring more emphasis on control measures.
Day length in the NGP varies from 8 hr in December to 16 hr in June. Hottest parts of a day during the main burning season (March-November) generally occur between 1100 and 1600 hr. Temperatures generally drop after 1600 hr and are usually coolest within 2 hr of daybreak.
Fire behavior and fuel conditions are most unpredictable when temperatures are rising during morning hours. This should be considered in any burn action plan. Temperatures between 21 and 32 degrees C (70 to 90 F) would be optimum for complete burns in the NGP.
Relative humidity is an expression of the actual amount of moisture in the air compared to the total amount the air is capable of holding at that temperature and pressure.
A temperature rise of 11 degrees C (20 F) from sunrise to midafternoon reduces the relative humidity by about one half. A similar temperature drop in late afternoon or early evening can cause relative humidity to rise by twofold.
However, when a cold front passes over an area, the temperature drop is usually accompanied by a drop in humidity. The lower humidity is a result of a change in air mass from warm and moist to cold and dry.
Preferred relative humidity for prescribed burning varies from 25 to 50%. Under certain conditions, a wider range of relative humidities - as low as 20% and as high as 80% - can produce satisfactory burns.
When relative humidity is as low as 20%, prescribed burning is dangerous because fires are more intense and spotting is more likely. When the relative humidity is higher than 50%, fires may not burn an area completely or may not burn hot enough to accomplish the desired result.
Relative humidity changes can quickly affect the moisture content and flammability of grassland fuels Increases in air temperature and solar radiation cause relative humidity to drop, and falling temperatures and cloudiness or darkness cause relative humidity to rise.
Because relative humidity is so dependent on temperatures, sunlight, and precipitation, it is not a good weather variable to use for predicting fire behavior. Grassland fuels can be burned under certain conditions at any level of relative humidity (0 to 100%); however, we recommend that most prescription burns be done when relative humidity is between 20 and 80%.
Prescribed fires behave in a more predictable manner if some wind movement is present.
The most desirable wind speeds for burning in the region are fairly steady winds between 8 and 29 km/h (5 and 18 mph), but specific conditions may tolerate higher speeds.
Persistent winds from a constant direction before, during, and after a burn provide the safest conditions for burning. Gusty or variable winds are indicators of unstable atmospheric conditions. Immediate changes in wind direction can cause instant fire control and smoke management problems.
Probable wind directions for any particular burn should be obtained just prior to burn time and from your best weather forecast source. Placement of firebreaks and other fire containment measures and smoke management are largely dependent on wind direction.
In this region, changes in wind direction are most likely to occur following calm or low wind speeds, and when winds are from east or northeast directions. Northwester winds are the most persistent.
Examples of some average prevailing wind direction information are shown for a sample of U.S. Weather Bureau reporting stations (Fig 34). This type of information is good for planning, but only local experience and forecasts should be relied upon for specific burns.
Calm or low wind speed (less than 3 km/h, or 2 mph) days are not necessarily good burning days. Fire spread will be slow; the fire will take longer to complete and result in higher containment and labor costs; the fire will create its own wind and may change its own direction; and heat will dissipate more slowly, sometimes resulting in damage to non-target plant species.
Amounts of rain (or rain itself) are difficult to predict. Knowledge of the precipitation date and amount prior to a burn will be helpful in predicting fire behavior and intensity and will help you decide what control measures are necessary. Expect more smoke from moist than from dry fuels.
Grasslands have been burned successfully just 9 hr after 1.2 cm (0.5 in) of rain. Cool burns are achievable when 1.2 cm or more o rain has fallen 24 to 36 hr prior h the burn. Hotter fires usually require 5 or more dry days prior burning, but the drying time is dependent on solar radiation, air temperature, and wind speed. Average rainfall is highest in June as a result, vegetative growth is greatest in June and early July. Consequently, burning conditions during June and early July are regulated by precipitation events more than in other months.
Although we did not record sunshine duration or solar radiation during prescribed burns, we did notice some obvious differences in flammability of grassland fuels on sunny, partly sunny, and 100% overcast days. Fires spread faster and could be set sooner after rain on days with full or partial sunshine than on heavily overcast days.
Mobley et al (1977) reported that wide differences existed between air and fuel temperatures when fuels were exposed to sunshine and that moisture moved readily from the warm fuel to air, even though the relative humidity was high.
Atmospheric stability is the resistance of the atmosphere to vertical motion. A prescribed fire generates vertical motion by heating the air, but the strength of convective activity over a fire is affected by the stability of the air mass.
Strong convective activity will increase the drafts into the fire and can result in erratic fire behavior.
When the atmosphere is stable, a small decrease in temperature occurs with an increase in altitude. Under stable conditions, inversions can develop in which temperature actually increases with height.
Stable air tends to restrict convection-column development and produces more uniform burning conditions. However, combustion products are held in the lower layers of the atmosphere, especially under temperature inversions, and visibility may be reduced because of smoke accumulation. Temperature inversions can also be a problem at night.
When the atmosphere is unstable, there is a large decrease in temperature with height. Once air starts to rise, it will continue to rise, and strong convective activity may develop over the fire. Strong indrafts will help confine a fire to its prescribed area.
In extreme cases, the effect of air mass instability on fire behavior results in erratic spread rates and spotting. The burn no longer meets the prescription and might have to be extinguished.
Forecasts of low-level stability, inversion layers, or unstable conditions can be obtained from fire-weather forecasters. Local indicators at the fire site should also be observed. Indicators of stability are steady winds, clouds in layers, and poor visibility due to haze and smoke hanging near the ground. Unstable conditions are indicated by dust devils, gusty winds, good visibility, and clouds with vertical growth.
Three sources of weather information are available. Use at least one of these before starting prescription fires and during burning. The sources are (1) National Weather Service (NOAA), (2) Fire Damage Rating System (not available in all states), and (3) local observations.
Field observations of weather should be made at or near the prescribed burn area before and during burning. Such observations serve as a check on the weather forecast and keep the burning crew up-to-date on any changes or effects of local influences.
Compact belt weather kits containing a psychrometer and windspeed measuring instrument are available. With this kit, and by observing cloud conditions and other weather indicators, a competent observer can obtain a fairly complete picture of current weather.
Successful grassland burning and smoke management is based largely on adequate weather knowledge. Before a fire is set, the weather forecast should be known for at least the next 24 hr and when possible for the next 48 hr. A weather forecast for the next 4 to 5 days might be necessary on a large fire with high risk or potentially bad smoke management problems.
The National Weather Service (NOAA) is usually the best source of local weather forecasts and information, particularly for forecasts of several days in advance. Most states have at least one toll-free telephone number available to cooperators or agencies. Ask for a spot weather forecast. Permanently staffed airport terminals are another good source of daily NOAA forecasts. Some National Weather Service offices will also furnish daily fire danger forecasts.
Secondary sources of weather forecasts and outlooks are the meteorologists at local television or radio stations, who should be able to provide reliable 48-hr advance forecasts. Many local radio stations or weather radios provide an early morning daily agricultural forecast from 0600-0900 hr that gives relative humidity, wind, temperature, etc.
Current weather information may be checked via two-way radio communication with an automatic or instant indoor-outdoor weather station at a local headquarters, or with readings from instruments from a belt weather kit.