Why is it so bad when 2 fires join together

The news people keep reporting how bad it will be when two fires join together. Why is this?

I would think that 2 fires coming together would either be just as bad as before without any increase in total badness (I know not a word, but it is fun to use). But I thought it could be a good thing since the amount of fuel should be decreased since the fires have individually burned up alot prior to joining.

What am I missing?

Wouldn’t you also have a lot less fire perimiter to cover, increasing the coverage you have around the fire?

What are we missing

Well, a larger fire will have a larger convection column, which can increase fire intensity. This could also make the fire more unpredictable in terms of rate and directions of spread. It could also increase the potential for spot fires, as well making it more difficult to contain. That said, it’s not always a bad thing. Last year the Rodeo and Chediski fires in Arizona were allowed to burn together.

By the way, if you’re talking about the Grand Prix Fire and the Old Fire, they’ve already merged:

St. Urho

I had heard the concerns before the Old and Grand Prix fires merged (they did that yesterday). But today there was concerns on the news about 2 of the fires in San Diego merging and they were about to when I left for work this morn.

Well, when two fires come together, it’s especially bad for anything between them. It all gets cooked from every direction, and anything trying to escape can find all courses of exit blocked.

I believe that’s what they’re referring to.

In the western US forests, there are two types of forest fires, the regular surface kind and “crown fires”. In the latter, the fire is hot enough to set the crowns of the trees on fire and can travel from crown to crown, rather than along the ground. A much worse fire, harder to control, does more damage to trees (pines generally die after their crowns are killed). The Tillamook Burn was a crown fire.

Since two about-to-merge fires seriously dry out the vegetation inbetween them there can be a hotter than usual fire zone created. This can setup the conditions necessary for a crown fire.

Even in scrub vegetation zones, such conditions give the opportunity for a fire to become much worse, e.g., a “ground fire” vs a “surface fire”. The former kills roots and damages the soil.

Large fires can create their own weather. When you have two joining, it creates an unpredictable weather system, I suppose.