But a lawn fire is not directly temperature related, is it? I suspect the grass has dried from lack of rain, such that some small spark that would ordinarily have fizzled has taken hold.
In the Western US, construction, mining, forestry, etc. industries are well aware of the fire risk from routine work (construction equipment, off road vehicle traffic, chain saw work, etc.). In "red flag" conditions, there are fire watch policies. For example in forestry work, my crew would have to cease chain saw work late in the morning and stand a fire watch for 30 minutes before leaving the work site.
So as I watch the news about these fires, my first thought is ignitions: routine activities (parking a vehicle on grass, driving construction equipment, discarding a cigarette) that would be unlikely to cause a fire in normal (backward looking) UK weather conditions. My next thought is that once a fire has started, it is likely to burn hot and fast. Hotter and faster than I suspect fire fighters in London are accustomed to.
As an aside, I have been trying to figure out a good source for weather data. I see plenty of sources for temperature, but I am curious about the wind speeds and relative humidity. From the photos I am seeing, it looks like wind speeds are over 10 mph.
https://www.metoffice.gov.uk/weather/observations/gcpvj0v07
Humidity was about 15-20% during the afternoon, winds 13 gusting 26mph.
The weather over the last couple of days has felt qualitatively different - and we've had nothing like the temperatures further south.
I recall not buying a diesel mazda car because it had a disclaimer about not parking on long grass because of the danger of starting a fire. Apparently, mazda finally made it to recall status:
https://www.abc.net.au/news/rural/2017-12-29/ford-rangers-ma...
From the owner's manual⋮
> Parking over or near anything flammable, such as dry grass, is dangerous.
https://owners-manual.mazda.com/gen/en/cx-5/cx-5_8gj1ee18b/c...
The rate of evaporation is directly related to the temperature. Even ignoring additional factors affecting evaporation like wind and topographic variance in dryness, the vapor pressure of water (the biggest contributor to evaporation rate) at 40C is an order of magnitude higher than it is at 20C:
https://en.wikipedia.org/wiki/Evaporation#/media/File:Water_...
Sufficient subsequent days of high heat and low humidity will dry out a lawn very quickly, as grass is a plant that is not adapted to retain moisture very well.
With hot/dry enough conditions for a sufficient period of time and a bit of kinetic energy in the form of wind, grass that isn't watered quickly becomes flammable, which is why it is usually where large fires first take hold.
Together with the heat the compost pile spontaneously ignited. They went to get a hose to douse the smolder, but by the time they got back wind blew embers into dry grass, and woosh.
A rare combination of events can cause a compost pile to catch fire. These all must be met before the occasion arises.
- The first is dry, unattended material with pockets of debris mixed throughout that aren’t uniform.
- Next, the pile must be large and insulated with limited airflow.
- And, finally, improper moisture distribution throughout the pile.
Key to preventing any issues is proper maintenance of your organic matter to prevent hot compost bins or piles.
https://www.gardeningknowhow.com/composting/basics/overheate...