Actually we have a good idea.

Hiroshima turned into a firestorm, but the Nagasaki didn't despite being hit with a bomb with almost twice the yield.

There is an interesting reason why Hiroshima turned into a firestorm but Nagasaki didn't, and it has implications for modern nuclear war.

First, firestorms require several different things to happen:

1. An available fuel load of at least 40 kg/m2.

2. Calm weather, with winds below 13 km/hr.

3. Thousands of roughly simultaneous ignition sources.

4. An area of at least 1.3 square kilometers.

Hiroshima and Nagasaki were both predominantly of wooden construction, and Hiroshima especially had a "built up" area downtown that was largely 2 story wooden structures with narrow streets. Here is a picture of a Hiroshima street prior to the bombing:

You can see the streets are pretty narrow, and the buildings are 2 stories, and made mostly of wood.

But the thing that caused Hiroshima to experience a firestorm is breakfast.

Let me explain.

The bomb detonated at 8:15 AM local time. Back then, most Japanese cooked on a charcoal brazier made usually of diatomaceous earth called a "shichirin". The charcoal from cooking breakfast would have still been lit at the bottom of those shichirin, you can't just turn it off like a stove.

So you've got a very large number of buildings constructed of wood, often with internal walls made of paper and thin strips of wood. These collapse in on the shichirin, causing them to break and/or spill their still lit coals on to flammable material.

The firestorm itself didn't break out until about 15 minutes after the blast, and the majority of surviving witnesses agreed that it was broken shichirin that caused most of the fires according to the US Strategic Bombing Survey conducted after the war. .

The Nagasaki bomb detonated at 11:00 AM local time. This is long after the breakfast fires had burnt out, but before any lunch or dinner fires were started. There was no firestorm.

Now, fast forward to modern times, and a typical 500 kiloton strategic warhead detonated over a modern city is unlikely to cause a firestorm, simply because there isn't enough of a fuel load to sustain it. Modern cities have buildings made of concrete, steel, and glass (and in lower income areas brick), with relatively wide streets. Modern US cities have a fuel load of between 14 and 21 kg/m2, at most about half that necessary for a firestorm to occur.

That's independent of the yield of the bomb: Even the Tsar Bomba couldn't cause a firestorm if the fuel load won't support it.

Also, there is something that people don't intuitively understand about explosions, nuclear or conventional: Doubling the yield does not double the destruction. It's not a linear relationship. Damage goes up with the inverse cube of yield. So for example a 10 kiloton device detonated at optimum airburst height will cause damage over 53.4 square kilometers, whereas a 100 kiloton device, despite 10 times the yield, will only cause damage over 160 square kilometers, just 3 times the area.