Fire Weather, PMS 425-1

A Guide for Application of Meteorological Information to Forest Fire Control Operations

U.S. Department of Agriculture Forest Service * Handbook 360 - May 1970

Mark J. Schroeder, Weather Bureau, Environmental Science Services Administration, U.S. Department of Commerce, Charles C. Buck, Forest Service, U.S. Department of Agriculture

Preface

Weather is never static. It is always dynamic. Its interpretation is an art. The art of applying complex information about weather to the equally complex task of wildland fire control cannot be acquired easily especially not by the mere reading of a book.

The environment is in control in wildland firefighting. Free-burning fires are literally nourished by weather elements, atmospheric components, and atmospheric motion. Outguessing Mother Nature in order to win control is an extremely difficult task. We need to soothe her with understanding.

We have attempted to present information in such a way that your daily and seasonal awareness of fire weather can begin with reliable basic knowledge. We have kept the use of technical terms to a minimum, but where it was necessary for clear and accurate presentation, we have introduced and defined the proper terms. Growing awareness of fire weather, when combined with related experience on fires, can develop into increasingly intuitive, rapid, and accurate applications. Toward this end, we have preceded each chapter with a paragraph or two on important points to look for in relating weather factors to fire control planning and action.

The illustrations are designed to help you "see" the weather from many different locations. Sometimes you will need a view of the entire North American Continent-other times you will look at a small area covering only a few square miles or even a few square yards. The illustrations should help you to evaluate fire weather in all of its dimensions, and simultaneously to keep track of its continually changing character.

In the illustrations, red represents heat, and blue represents moisture. Watch for changes in these two most important factors and how they cause changes in all other elements influencing fire behavior.

Assistance in the form of original written material, reviews, and suggestions was received from such a large number of people that it is not practical to acknowledge the contribution of each individual.

They are all members of two agencies:

• U.S. Department of Commerce, Environmental Science Services Administration, Weather Bureau
• U.S. Department of Agriculture, Forest Service

Their help is deeply appreciated, for without it this publication would not have been possible.

Introduction

What is WEATHER? Simply defined, it is the state of the atmosphere surrounding the earth. But the atmosphere is not static-it is constantly changing. So we can say that weather is concerned with the changing nature of the atmosphere. Familiar terms used to describe weather are

• Temperature
• Pressure
• Wind speed
• Wind direction
• Humidity
• Visibility Clouds
• Precipitation

The atmosphere is a gaseous mantle encasing the earth and rotating with it in space. Heat from the sun causes continual changes in each of the above elements. These variations are interdependent; affecting all elements in such a manner that weather is ever changing in both time and space.

Because weather is the state of the atmosphere, it follows that if there were no atmosphere there would be no weather. Such is the case on the moon. At high altitudes, where the earth's atmosphere becomes extremely thin, the type of weather familiar to us, with its clouds and precipitation, does not exist.

The varying moods of the ever-changing weather found in the lower, denser atmosphere affect all of us. Sometimes it is violent, causing death and destruction in hurricanes, tornadoes, and blizzards. Sometimes it becomes balmy with sunny days and mild 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 times protecting ourselves and our property from it.

A farmer needs to understand only that part of the shifting weather pattern affecting the earth's surface-and the crop he grows.

The launcher of a space missile must know, from hour to hour, the interrelated changes in weather in the total height of the atmosphere, as far out as it is known to exist, in order to make his decisions for action.

But the man whose interest is wildland fire is neither limited to the surface nor concerned with the whole of the earth's atmosphere. The action he takes is guided by understanding and interpreting weather variations in the air layer up to 5 or 10 miles above the land. These variations, when described in ways related to their influences on wildland fire, constitute FIRE WEATHER. When fire weather is combined with the two other factors influencing fire behavior-topography and fuel - a basis for judgment is formed.