We commonly understand weather to refer to temperature, moisture (humidity, rain or snow) and wind. However, to understand the effect of weather on fires it’s necessary to have a more technical understanding – to understand what causes particular weather.

Weather is basically a result of the uneven heating of the earth by the sun. The heating is uneven due to several factors. The earth tilts on its axis as it orbits the sun, causing the seasons and causing the Northern Hemisphere to be cooler in December while the Southern Hemisphere is warmer in December, for example.

Example of the Southern Hemisphere's exposure to sunlight during the summer months

Heating also varies according to the time of day – nights are generally cooler than days. Heating also varies over land masses and bodies of water. Bodies of water have much less variation in their temperature while land has a greater variation in temperature.

Land and Sea Surface temperature variations from 2003 to 2016 

All of this uneven heating causes temperature differences, which in turn causes air currents to develop. The atmosphere, in effect, becomes a giant heat engine causing high and low pressure areas, wind, clouds, and rain systems. We can describe the atmosphere as a chaotic system; a small change in one part of the system can cause major changes in other parts of the system.

This chapter will examine aspects of weather to build your understanding of how it affects fires, including:

  • Temperature

  • Dew point

  • Relative humidity

  • Cloud cover

  • Weather maps

  • Pressure systems

  • Frontal systems

  • Atmospheric stability and the Haines index

  • Inversion layers

  • Specific fire weather

At the end of this chapter you will learn about taking weather measurements in the field.