Fire Weather - NWCG
FIREWEATHERAGRICULTURE HANDBOOK 360U.S. Department of Agriculture Forest ServiceNFES 1174PMS 425 – I
FIRE WEATHERA GUIDE FOR APPLICATION OF METEOROLOGICALINFORMATION TO FOREST FIRE CONTROL OPERATIONSMark J. SchroederWeather Bureau, Environmental Science Services AdministrationU.S. Department of CommerceandCharles C. BuckForest Service, U.S. Department of AgricultureMAY 1970U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE AGRICULTURE HANDBOOK 360
Table of ContentsPREFACE.1INTRODUCTION.2Chapter 1: BASIC PRINCIPLES.3Chapter 2: TEMPERATURE.16Chapter 3: ATMOSPHERIC MOISTURE.28Chapter 4: ATMOSPHERIC STABILITY.41Chapter 5: GENERAL CIRCULATION.56Chapter 6: GENERAL WINDS.70Chapter 7: CONVECTIVE WINDS.86Chapter 8 AIR MASSES AND FRONTS.101Chapter 9: CLOUDS AND PRECIPITATION.115Chapter 10: THUNDERSTORMS.128Chapter 11: WEATHER AND FUEL MOISTURE.138Chapter 12: FIRE CLIMATE REGIONS.150Index.167
PREFACEWeather is never static. It is always dynamic. Itsinterpretation is an art. The art of applying complexinformation about weather to the equally complextask of wildland fire control cannot be acquired easilyespecially not by the mere reading of a book.The illustrations are designed to help you “see” theweather from many different locations. Sometimesyou will need a view of the entire North AmericanContinent-other times you will look at a small areacovering only a few square miles or even a fewsquar e yards. The illustrations should help you toevaluate fire weather in all of its dimensions, andsimultaneously to keep track of its continually changing character.The environment is in control in wildland firefighting.Free-burning fires are literally nourished by weatherelements, atmospheric components, and atmospheric motion. Out- guessing Mother Nature in order towin control is an extremely difficult task. We need tosoothe her with understanding.In the illustrations, red represents heat, and bluerepresents moisture . Watch for changes in thesetwo most important factors and how they causechanges in all other elements influencing fire behavior.We have attempted to present information in sucha way that your daily and seasonal awareness offire weather can begin with reliable basic knowledge. We have kept the use of technical terms toa minimum, but where it was necessary for clearand accurate presentation, we have introduced anddefined the proper terms. Growing awareness of fireweather, when combined with related experience onfires, can develop into increasingly intuitive, rapid,and accurate applications. Toward this end, we havepreceded each chapter with a paragraph or two onimportant points to look for in relating weather factors to fire control planning and action.Assistance in the form of original written material,reviews, and suggestions was received from sucha large number of people that it is not practical toacknowledge the contribution of each individual.They are all members of two agencies:U.S. Department of Commerce, Environmental Science Services Administration,Weather Bureau and U.S. Department of Agriculture,Forest Service.Their help is deeply appreciated, for without it thispublication would not have been possible.1
INTRODUCTIONWhat is WEATHER? Simply defined, it is the state ofthe atmosphere surrounding the earth. But the atmosphere is not static-it is constantly changing. So wecan say that weather is concerned with the changingnature of the atmosphere. Familiar terms used todescribe weather are: A farmer needs to understand only that part of theshifting weather pattern affecting the earth’s surface-and the crop he grows.The launcher of a space missile must know, fromhour to hour, the interrelated changes in weather inthe total height of the atmosphere, as far out as itis known to exist, in order to make his decisions foraction.TemperaturePressureWind speedWind directionHumidityVisibilityCloudsPrecipitationBut the man whose interest is wildland fire is neitherlimited to the surface nor concerned with the wholeof the earth’s atmosphere. T he action he takes isguided by understanding and interpreting weathervariations in the air layer up to 5 or 10 miles abovethe land. These variations, when described in waysrelated to their influences on wildland fire, constituteFIRE WEATHER. When fire weather is combinedwith the two other factors influencing fire behavior-topography and fuel - a basis for judgment isformed.The atmosphere is a gaseous mantle encasing theearth and rotating with it in space. Heat from thesun causes continual changes in each of the aboveelements. These variations are interdependent;affecting all elements in such a manner that weatheris ever changing in both time and space.Because weather is the state of the atmosphere, itfollows that if there were no atmosphere there wouldbe no weather. Such is the case on the moon. Athigh altitudes, where the earth’s atmosphere becomes extremely thin, the type of weather familiar tous, with its clouds and precipitation, does not exist.The varying moods of the ever-changing weatherfound in the lower, denser atmosphere affect allof us. Sometimes it is violent, causing death anddestruction in hurricanes, tornadoes, and blizzards.Sometimes it becomes balmy with sunny days andmild temperatures. And sometimes it is oppressive with high humidities and high temperatures.As the weather changes, we change our activities,sometimes taking advantage of it and at other timesprotecting ourselves and our property from it.2
Chapter 1: BASIC PRINCIPLESWildland fires occur in and are affected by the condition of the lower atmosphere at any one moment and by itschanges from one moment to the next. At times, fires may be affected only by the changes in a small area at ornear the surface; at other times, the region of influence may involve many square miles horizontally and severalmiles vertically in the atmosphere. All these conditions and changes result from the physical nature of the atmosphere and its reactions to the energy it receives directly or indirectly from the sun.This chapter presents basic atmospheric properties and energy considerations that are essential to understandwhy weather and its component elements behave as they do. We can see or feel some of these componentelements, whereas others are only subtly perceptible to our senses. But these elements are measurable, and themeasured values change according to basic physical processes in the atmosphere. These changes in values ofweather elements influence the ignition, spread, and intensity of wildland fires.Layers Of The AtmosphereIt is convenient for our purposes to divide theatmosphere into several layers based primarily ontheir temperature characteristics. The lowest layeris the troposphere. Temperature in the tropospheredecreases with height, except for occasional shallowlayers. This temperature structure allows verticalmotion and resultant mixing. Hence, this is a generally mixed, some- times turbulent layer. Here occurpractically all clouds and storms and other changesthat affect fire. In this layer, horizontal winds usuallyincrease with height.space. It is characterized by a steadily increasingtemperature with height.Let us now return to our principal interest - the troposphere - and examine it a little more Closely. Thetroposphere is a region of change – able weather. Itcontains about three-quarters of the earth’s atmosphere in weight, and nearly all of its water vaporand carbon dioxide.Composition of the TroposphereAir in the troposphere is composed mostly of twogases. Dry air consists of about 78 percent nitrogenby volume and about 21 percent oxygen. Of theremainder, argon comprises about 0.93 percent andcarbon dioxide about 0.03 percent. Traces of severalother gases account for less than 0.01 percent.The depth of the troposphere varies from about5 miles over the North and South Poles to about10 miles over the Equator. In temperate and PolarRegions, the depth increases somewhat in the summer and decreases somewhat in the winter. In thetemperate regions, the depth will vary even withinseasons as warm or cold air invades these regions.In addition to these gases, the troposphere containsa highly variable amount of water vapor-from nearzero to 4 or 5 percent. Water vapor tends to actas an independent gas mixed with the air. It has aprofound effect on weather processes, for withoutit there would be no clouds and no rain. Variationsin the amount of water vapor influence the moisturecontent and flammability of surface fuels.The troposphere is capped by the tropopause - thetransition zone between the troposphere and thestratosphere. The tropopause is usually marked by atemperature minimum. It indicates the approximatetop of convective activity.Through most of the stratosphere, the temperatureeither increases with height or decreases slowly. Itis a stable region with relatively little turbulence, extending to about 15 miles above the earth’s surface.The troposphere also contains salt and dust particles, smoke, and other industrial pollutants. Theseimpurities affect the visibility through the atmosphereand also may serve as nuclei for the condensation ofwater vapor in cloud formation.Above the stratosphere is the mesosphere, extending to about 50 miles. It is characterized by anincrease in temperature from the top of the stratosphere to about 30 miles above the earth’s surface,and then by a decrease in temperature to about 50miles above the surface.Air, although not heavy compared with other familiar substances, does have measurable mass andresponds accordingly to the force of gravity. At theouter limits of the atmosphere, the air is extremelyrarefied, each cubic footThe thermosphere is the outermost layer, extendingfrom the top of the mesosphere to the threshold of3
A column of air from sea level to the top of the atmosphere weighs about the same as a 30-inch columnof mercury of the same diameter.Energy can be, and constantly is being, transformedfrom one form to another, but energy is alwaysconserved in the process. It cannot be created nordestroyed, although a transformation between energy and mass does occur in atomic reactions.vcontaining only a few molecules and weighing virtually nothing. At sea level, however, a cubic foot of air,compressed by all the air above it, contains manymolecules and weighs 0.08 pounds at 32 F. The total weight of a 1-inchsquare column of air extendingfrom sea level to the top of the atmosphere averages14.7 pounds. This is the normal pressure exerted bythe atmosphere at sea level and is referred to as thestandard atmospheric pressure.Kinetic energy is energy of motion, whereas potential energy is energy due to position, usually with respect to the earth’s gravitational field. The motion ofa pendulum is a good example of the interchange ofpotential and kinetic energy. At the end of its swing,a pendulum has potential energy that is expendedin the down stroke and converted to kinetic energy.This kinetic energy lifts the pendulum against theforce of gravity on the upstroke, and the transformation back to the potential energy occurs. Lossescaused by friction of the system appear in the formof heat energy. The sun is the earth’s source of heatand other forms of energy.A common method of measuring pressure is that ofcomparing the weight of the atmosphere with theweight of a column of mercury. The atmosphericpressure then may be expressed in terms of theheight of the column of mercury. The normal valueat sea level is 29.92 inches. A more common unit ofpressure measurement used in meteorology is themillibar (mb.). A pressure, or barometer, reading of29.92 inches of mercury is equivalent to 1013.25mb. While this is the standard atmospheric pressureat sea level, the actual pressure can vary from 980mb. or less in low-pressure systems to 1050 mb. ormore in high-pressure systems.The common storage battery in charged conditionpossesses chemical energy. When the battery terminals are connected to a suitable conductor, chemicalreaction produces electrical energy. When a batteryis connected to a motor, the electrical energy isconverted to mechanical energy in the rotation of therotor and shaft. When the terminals are connectedto a resistor, the electrical energy is converted tothermal energy. When lightning starts forest fires, asimilar conversion takes place.Atmospheric pressure decreases with in- creasingaltitude. Measured at successive heights, the weightof a column of air decreases with increasing altitude.The rate of decrease is about I inch of mercury, or34 mb., for each 1,000 feet of altitude up to about7,000 feet. Above about 7,000 feet, the rate ofdecrease becomes steadily less. In midlatitudes the500 mb. level is reached at an average altitude ofabout 18,000 feet. Thus, nearly half the weight of theatmosphere is below this altitude, or within about 31/2 miles of the surface.ENERGY IN THE TROPOSHPERETremendous quantities of energy are fed into thetroposphere, setting it in motion and making it workin many ways to create our ever- changing weather.At any time and place, the energy may be in any oneform or a combination of several forms. All energy,however, comes either directly or indirectly from thesun.Chemical energy can be transformed into electrical energy,which in turn can be transformed into mechanical energy orthermal energy.Energy is present in these various forms in theatmosphere. They are never in balance, however,and are constantly undergoing conversion fromone form to another, as in the case of the pendulum or the storage battery. Their common source isthe radiant energy from the sun. Absorption of thisenergy warms the surface of the earth, and heatis exchanged between the earth’s surface and thelower troposphere.Simply defined, energy is the capacity to do work.Its more common forms are heat or thermal energy,radiant energy, mechanical energy (which may beeither potential or kinetic), chemical energy, andelectrical energy. There are also atomic, molecular,and nuclear energy.4
All forms of energy in the atmosphere stem originally from the radiant energy of the sun that warms the surface of the earth. Energychanges from one to another in the atmosphere; so does energy in a swinging pendulum.5
Heat Energy and TemperatureHeat energy represents the total molecular energy ofa substance and is therefore dependent upon boththe number of molecules and the degree of molecular activity. Temperature, although related to heat, isdefined as the degree of the hotness or coldness ofa substance, determined by the degree of its molecular activity. Temperature reflects the average molecular activity and is measured by a thermometer ona designated scale, such as the Fahrenheit scale orthe Celsius scale.of the cooler substance will be different from theresulting decrease in temperature of the warmersubstance. For example, if 1 pound of water at 70 F.is mixed with 1 pound of gasoline, specific heat 0.5,at 60 F., the exchange of heat will cause the temperature of the gasoline to rise twice as much as thisexchange causes the water temperature to lower.Thus, when 3 1/3 B.t.u. has been exchanged, thepound of water will have decreased 3 1/3 F. and thepound of gasoline will have increased 6 2/3 F. Thetemperature of the mixture will then be 66 2/3 F.If heat is applied to a substance, and there is nochange in physical structure (such as lee to wateror water to vapor), the molecular activity increasesand the temperature rises. If a substance loses heat,again without a change in physical structure, themolecular activity decreases and the temperaturedrops.With minor exceptions, solids and liquids expandwhen their molecular activity is increased by heating.They contract as the temperature falls, and the molecular activity decreases. The amount of expansionor contraction depends on the size, the amount oftemperature change, and the kind of substance. Theexpansion and contraction of liquid, for example,is used in a thermometer to measure temperaturechange. Thus, volume changes with temperature,but at any given tempera, lure the volume is fixed.Heat and temperature differ in that heat can beconverted to other forms of energy and can be transferred from one substance to another, while temperature has neither capability. Temperature, however, determines the direction of net heat transfer fromone substance to another. Heat always flows fromthe substance with the higher temperature to theone with the lower temperature, and stops flowingwhen the temperatures are equal. In this exchangeof heat, the energy gained by the cooler substanceequals that given up by the warmer substance, butthe temperature changes of the two are not necessarily equal.Since different substances have different molecularstructures, the same amount of heat applied to equalmasses of different substances will cause one substance to get hotter than the other. In other words,they have different heat capacities. A unit of heat capacity used in the English system of measures is theBritish thermal unit (B.t.u.). One B.t.u. is the amountof heat required to raise the temperature of 1 poundof water 1 F.If the volume of a gas is held constant, the pressure increases as the temperature rises, and decreases as the temperature falls.The reaction of gases to temperature changes issomewhat more complex than that of liquids or solids. A change in temperature may change either thevolume or pressure of the gas, or both. If the volume is held constant, the pressure increases as thetemperature rises and decreases as the temperaturefalls.The ratio of the heat capacity of a substance to thatof water is defined as the specific heat of the substance. Thus, the specific heat of water is 1.0-muchhigher than the specific heat of other common substances at atmospheric temperatures. For example,most woods have specific heats between 0.45 and0.65; ice, 0.49; dry air, 0.24; and dry soil and rock,about 0.20. Thus, large bodies of water can storelarge quantities of heat and therefore are great moderators of temperature.Since the atmosphere is not confined, at- mospheric processes do not occur under constant volume.Either the pressure is constant and the volumechanges, or both pressure and volume change. Ifthe pressure remains constant, the volume increases as the temperature rises, and decreases as thetemperatures falls. The change in volume for equaltemperature changes is much greater in gasesIf heat flows between two substances of differentspecific heats, the resulting rise in temperature6
under constant pressure than it is in liquids andsolids. Consequently, cha
WEATHER AGRICULTURE HANDBOOK 360 U.S. Department of Agriculture Forest Service NFES 1174 PMS 425 – I. FIRE WEATHER A GUIDE FOR APPLICATION OF METEOROLOGICAL INFORMATION TO FOREST FIRE CONTROL OPERATIONS Mark J. Schroeder Weather Bureau, Environmental Science Services Administration U.S. Department of Commerce and Charles C. Buck Forest Service, U.S. Department of Agriculture MAY 1970 U.S .
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