It! Which Weather Measurements Are Related to the Occurrence of Wildland Fire?

Glossary complexity (käm plek suh te): The state of being complicated or having many related parts. predict (pre dikt): To tell what one thinks will happen in the future. (wild fir): An uncontrolled wildland fire started naturally or by careless human action. associated (uh so she a ted): Closely connected with another. relative (rel uh tiv hu mid uh te): The percentage of vapor in the air relative to the total amount of the air can hold at that . weather stations (weh thür sta shuns): Places where instruments measure and record weather conditions. saturated (sah chür at ed): Soaked completely through. intensity (in ten si te): The quality of being very strong.

Pronunciation Guide a as in ape ô as in for ä as in car u as in use e as in me ü as in fur i as in ice oo as in tool o as in go ng as in sing

Accented syllables are in bold.

47 conditions into separate mea- with fire during those days. surements. The daily weather The scientist in this study is not really a lot of separate wanted to identify the most conditions. Instead, it is a important daily weather con- related set of conditions. ditions that were associated However, by separating the with a dangerous or a large weather into different kinds of wildfire. To identify these con- measurements, the scientist ditions, the scientist measured made the problem easier to things like air temperature, study and to understand. relative humidity, and speed. Thinking About the Introduction Environment Although scientists know Meet Dr. Potter: can that many weeks of low rain- I like being a scientist be a threat to fall increase the chances of because as a child I never the health and wildfires, they do not know stopped asking why. As an safety of people and animals which daily weather condi- adult, I’m still asking why. (figure 1). Wildfires might be tions are the best for deter- started by mistake, as when mining the danger of fire. Thinking About people are not careful with Scientists believe that when Science campfires or with matches. certain kinds of weather con- The world is a Wildfires may also be started ditions occur, they can better complicated by or by other natur- predict wildfires. place. When al means during dry weather. Unfortunately, they have you look closely Most people know that weeks not checked to see if these at things in of dry weather will increase same conditions occur on days nature, you will find that the danger of a wildfire. with no wildfires. If scientists many things are connected in Whether a wildfire spreads can determine which daily one way or another. Scientists may also depend on the weather conditions are the study this complexity in weather that occurs each day. best for identifying the risk of nature, but they also search If the weather on some days is wildfires, they can more easily for simpler ways to under- more likely to help a wildfire determine which days wild- stand what they observe. to spread, people should be fires might occur. The scientist One of the ways that they extra careful with matches or in this study wanted to deter- try to simplify their research is mine which daily by studying things piece by weather conditions piece. Instead of observing are associated with everything all at once, they large or dangerous observe and measure separate wildfires. things. Then, they examine how the separate things are related. In this study, the sci- entist wanted to know which daily weather conditions are most related to forest wild- fires. To answer his question, he divided the daily weather Figure 1. Wildfire. 48 Reflection The scientist collected Section weather measurements • What is the recorded at 20 weather sta- question the sci- tions on the dates the fires had entist is trying burned (figure 3). For each to answer? wildfire, he used information • Is it important to be able to from the closest weather sta- predict on which days a tion. He then divided the wildfire might burn? Why weather information from or why not? each weather station into four groups, based on the season Methods the fire had burned. This The scientist collected infor- meant that the scientist had mation from large wildfires information from each weath- that had burned in areas er station for the days that across the United States. To wildfires burned in the spring, make sure that he was collect- summer, fall, and winter (fig- ing weather information only ure 4). The scientist also - for large wildfires, the scientist lected weather measurements decided that wildfires burning for days in which wildfires less than 400 hectares would had not burned. In this way, not be included. (To find out the scientist was able to com- how many acres this is, multi- pare the weather measure- ply 400 times 2.47.) He found ments made on days when information on 459 large wildfires did not burn with wildfires that had burned measurements made on days between 1971 and 1984 in which wildfires had burned. Figure 3. Weather station. (figure 2).

17 8 14 11 8 17 1 2 26 4 0 18 21 0 1 10 0 5 4 1 3 29 0 11 25 85 3 0 64 23 6 0 0 1 1 0 5 7 0 0 0 17 19 51 49 7 0 0 23 15 25 5 1 5 01 0 5 0 0

81 33 30 6 5

Figure 2. Number of wildfires in each Figure 4. Location of weather stations and State. number of days there were fires recorded at each weather station. 49 Weather measurements used include air temperature, wind Fire Facts ate its own wind, thus helping speed, relative humidity, dew to spread itself. When the air point depression, and wind For wildland fires, fuels con- above a flame gets heated, it shear. depression is sist of burnable materials, rises. When it rises, fresh air the air temperature minus the such as trees, shrubs, and rushes in to fill the vacuum. dew point temperature. The grasses. Besides the availabili- The fresh air provides a new dew point temperature is the ty of fuel, the type of weather source of oxygen for the fire. air temperature at which the occurring at that time can Thus, if fuels are available air is saturated with water. help a wildfire spread. Air and there is a lack of moisture occurs when temperature, humidity, and in the air, a wildfire can con- at different heights wind affect the spread of a tinue to spread in part by cre- blow in different directions or wildfire. A wildfire can gener- ating its own wind. at different speeds. Reflection Section • Why do you think that the scientist had to use weather data from fires that had high, there is little moisture in relative humidity, dew point already occurred? the air and a wildfire is more depression, and wind shear • Which of the five measure- likely to become large or dan- were the weather measure- ments do you think were gerous. ments most associated with more closely associated large or dangerous wildfires. with large wildfires? Why? Reflection This research suggests that Section dew point depression is the Findings • Even without most important measurement. The scientist found that air weather instru- On days when large wildfires temperature, relative humidi- ments, humans burned between 1971 and ty, and dew point depression can generally 1984, the dew point depres- were the three weather mea- tell when the dew point sion was high. When people surements most associated depression is low. Even try to predict wildfires based with wildfires. When air tem- though you will perspire if on weather conditions, they perature is high and the the temperature is high, should pay the closest atten- amount of water in the air is what happens to your per- tion to dew point depression. low, large or dangerous wild- spiration when the dew fires are more likely to burn. point depression is low? Reflection Of these three measurements, • Why do you think that your Section dew point depression is the perspiration does not evap- • How did this single best measurement to orate off of your skin when research simpli- use when trying to predict the dew point depression is fy what was wildfires. When dew point low? known about depression is low, there is a lot the association between of water in the air, and wild- Implications wildfires and daily weather fires are not as likely to burn. In the past, scientists conditions? When dew point depression is thought that air temperature, 50 Fire Safety Tips from Smokey and His Friends at the Texas Forest Service!

The Keetch-Byram (kech bi 1. 0-200: Soil and fuel 4. 600-800: Fires will burn rum) Drought Index, or moisture content are high. all of the fuels off of the KBDI, is a mathematical sys- Most fuels will not readily ground. Fires will burn tem developed to help people ignite. There is not much throughout the night and understand how likely a danger of wildfire. heavier fuels will actively wildfire is to occur. The 2. 200-400: Fires will more burn, increasing the intensity KBDI rates current and readily burn, but heavier of the fire. expected weather conditions fuels will not ignite readily. and places them on a num- 3. 400-600: Fires will read- bered scale, from 0 to 800. ily burn in all directions. In Here are what the numbers some places, all of the fuel on mean: the ground will burn away. Larger fuels may smolder for many days, possibly creating problems with smoke.

Discovery FACTivity perature in the shade and is low. Weather reports often The best time to do this record the air temperature. give the air’s dew point tem- FACTivity is when the tem- Hold the thermometer against perature. Knowing the dew perature is high. It is best if the outside of the can so you point will help you to deter- the temperature is over 85 °F can measure the temperature mine whether dew or is (or 29 ° C). In this FACTivity, of the air immediately outside likely to occur. you will determine the air’s of the can. Put some ice into dew point temperature. Dew the water and stir. The dew Activity from: Bosak, S. V. (2000). Science is...: A source book of fascinating facts, point is the point at which the point of the air surrounding projects, and activities, Ontario, Canada: air, at a given temperature, the can is the temperature reg- Scholastic Canada, Ltd., p. 446. can hold no more moisture. istered on the thermometer From Potter, Brian E. (1996). The question you will answer when the first sign of moisture Atmospheric properties associated with is: What happens when the air appears on the outside surface large wildfires. International Journal of can hold no more moisture? of the can. Record the temper- Wildland Fire, 6(2): 71-76. For this activity, you will need ature at dew point. What has a cleaned-out vegetable can, happened? The ice has caused filled three-quarters high with the air immediately surround- water, a thermometer, a ing the can to cool. As the air spoon, ice, paper, and a pencil. cools, it absorbs moisture The method you will use to which you cannot see until it answer this question is: Let can hold no more moisture. the vegetable can filled with Now calculate the dew point water sit for a few hours out- depression. (See “Methods” side in the shade. It should to learn how to do this.) If the reach air temperature before air temperature and the dew you continue. Using the ther- point are far apart, the air is mometer, measure the air tem- dry and the relative humidity 51