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Unit 4: Temperature-Moisture Relationship

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Temperature & Heat  ::   Temperature & Altitude   ::   Atmospheric Moisture   ::   Relative Humidity   ::   Clouds   ::   Exercises

Objectives:

Upon completion of this unit you will be expected to:

  • Compare the effects of daytime solar radiation, and nighttime heat losses from various topographic surfaces.
  • Explain temperature lapse rates in the atmosphere, and give the theoretical values for dry, wet, and normal lapse rates.
  • Explain the relationship between temperature, dew point, and relative humidity.
  • Give the dry bulb and wet bulb temperatures, determine relative humidity and dew point using appropriate psychrometric tables.
  • Determine relative humidities at various temperatures within a fixed or stationary air mass when given one temperature and corresponding relative humidity.
  • Give four groups of clouds, their approximate base height, and explain how these clouds are formed.
  • Given six indicator cloud types, describe their probable effect on fire weather.

Introduction:

Weather is the most variable and often the most critical determinant of fire behavior. This is the first of several units that will deal with weather and its relationship to fire behavior. This unit will discuss atmospheric temperatures, moisture, and the relationship between these two elements.
You should be familiar with the atmosphere that surrounds the earth and its life-supporting elements of oxygen, moisture, and other gases that affect our activities and well-being. This atmosphere is very dynamic, with conditions changing from moment to moment, that can impact on our activities on very short notice.
These short term atmospheric variations are what we call weather. To most people, weather is thought of in terms of temperature, humidity, precipitation, cloudiness, sunshine, visibility, and wind. The fire manager is concerned with all of these factors, since his successes and failures are often dependent on his keeping current with the weather. An understanding of weather processes and the ability to observe and interpret atmospheric conditions are of great advantage to the fire manager.

Temperature & Heat

Temperature : The degree of hotness or coldness of a substance; a measure of its molecular activity, it is measured by a thermometer on a designated scale, i.e. Fahrenheit or Celsius.

This unit will concentrate on several weather factors, especially those related to temperature and moisture in the atmosphere. Here we begin our discussion with temperature and heat. The two are not synonymous, although often used in the same sense. Temperature is defined as the degree of hotness or coldness of a substance; a measurement of its molecular activity. It is measured by a thermometer on a designated scale such as Fahrenheit or Celsius. In this course, all temperatures will be given in degrees Fahrenheit.

Where does heat come from? For the most part, all of the heating of the earth's surface and its atmosphere comes from the sun through solar radiation. On a very small scale, heat may be generated by a large, active forest fire or some other energy-releasing activity, but the sphere of influence by these heat sources, worldwide, are relatively small. This discussion will concentrate on solar radiation.

Fortunately, our planet basks in the radiant heat of "Ol' Sol" each day and provides a climate favorable to all living things. It's also fortunate that our day-night cycle is only 24 hours in length; if longer, we might have much greater temperature differences between day and night. Much of the heat incoming during the day is lost at night. The atmosphere and the earth retain a certain amount of heat from day to day which reduces the wide temperature variations that might otherwise exist.

Solar Radiation
Approximate distribution of incoming solar radiation.

The distribution of incoming solar radiation varies from day to day, depending on atmospheric conditions. (See figure above) Perhaps 43 percent of the sun's energy reaches the earth's surface. Some of, the rays are reflected back into space, and some are absorbed by the atmosphere.

Solar Heating Received and Surface Temperatures

The primary concern relating to this daytime solar heating is the temperature it will create on the earth's surface and in the surrounding air. These temperatures vary on a daily basis.

The amount of solar heating received and surface temperatures are dependent on:

  1. The amount of moisture and pollutants in the air which can reflect and/or absorb incoming radiation. The presence and thickness of clouds, water vapor, haze, and smoke are factors.
  2. The angle and duration of solar rays striking the surface as affected by time of day and year, and by topography. Latitude, slope, aspect, elevation, and shape of the country are factors.
  3. The surface properties of terrain and vegetation which reflect and/or absorb incoming radiation. Color, texture, transparency, conductivity, and specific heat are factors.

 

Heat Gain and Loss at the Earth's Surface

Heat gain and loss at the Earth's surface depends on:

  1. Color and Texture : Affect the capability of substances to absorb or reflect radiation, e.g., black mostly absorbs, while white mostly reflects. rough absorbs while smooth reflects.
  2. Transparency : The distribution of incoming heat throughout a substance, e.g., water is transparent, while soil is not.
  3. Conductivity : The ability to transfer heat through the substance, e.g., rock has much better conductivity than wood.
  4. Specific Heat : The capacity of various materials for containing, holding, or absorbing heat, compared to that of water.
  5. Surface Moisture : Heat is gained or lost during changes in moisture state, e.g., evaporation cools, while condensation gives off heat.

 

Air Near the Earth's Surface

more

Air near the earth's surface is heated and cooled by conduction, radiation and convection.

Copyright 2008, Michael Jenkins. Cite/attribute Resource . admin. (2005, October 25). Unit 4: Temperature-Moisture Relationship. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_4__Temperature-Moisture_Relationship_1.html. This work is licensed under a Creative Commons License Creative Commons License