It's not that they didn't take into account the effect of heat, it's that they didn't take into account the effect of fire. There is a difference: the heat output of an atomic bomb is relatively easy to calculate. Whether that heat will start fires, however, requires a lot of messy guessing about the combustible material in the city in question, and how those fires will spread is very hard to calculate. The argument is the planners focused mostly on the prompt, easy-to-calculate effects (blast and radiation), which led them to not consider how the fire would spread, which meant that they underestimated the effects of the bombs, leading them to think it would take many more bombs to destroy a city than it probably would (overkill).

The book for this thesis is Lynn Eden's Whole World on Fire Organizations, Knowledge, and Nuclear Weapons Devastation. It is an argument as much about how organizations deal with troublesome knowledge as it is about nuclear war. It is not without its controversy.

There wasn't an 'anti-nuclear lobby' in WWII, it was the very infancy of the development of nuclear weapons. As such, the CND did not come into being until 1957 and more generally it wasn't until the beginning of counter-culture as we understand it, in the 60s, that the public started to become critical of Western nuclear weapons programmes.

None of which is to say with certainty whether the military planners were aware of the effects of heat, though it's worth considering the very first prototypes were tested as part of the Trinity programme and observed in person at various distances. This was just prior to the attacks on Japan which used the same design as those tested under Trinity.

Regarding the narrative of nuclear survivability, for the duration of the Cold War, the US and UK governments have sought both to reassure in terms of civil defence planning and advice, and to manufacture the assent of the people in terms of maintaining and developing nuclear defence programmes which often span several political lifespans.

Geographical differences come in to play; the 'problem of geography' is primarily a UK problem in which it would take only two or three such thermonuclear explosions to send us back to the stone age. As such, the tone of civil defence advice is quite grim ('Protect & Survive') versus the American notion of 'Duck & Cover', although it's worth noting the latter is from the 50s and the former from the 80s. Both countries promoted a sense of survivability in the narratives of the 60s - until the arms race heated up and it became clear the Soviets could launch somewhat more than a handful.

During this time survivability was promoted in the UK - user 'Pandemian' on Youtube has a great selection of UK nuclear-themed civil defence films and documentaries. To get a feeling for how things have changed, watch the 'Sound an Alarm' playlist, which is basically like a mini-movie selling the UK civil defence infrastructure to the public. It was produced by the government with the co-operation of the Royal Observer Corp, and shows how the work of the former UK Weapons Monitoring Organisation (UKWMO) was thought to have been able to mitigate the effects of a nuclear attack on the UK. It even ends with the 'all-clear' sound of the sirens (was this before or after our discovery of EMP?), and the protagonists reflecting on 'getting back to work' (this is from memory).

The public film campaigns of the government were not without opposition though; 'The War Game' (1965) by Peter Watkins examines closely how much destruction the UK would have received and de-constructs the notion that WWIII would be just like the blitz in WWII but with bigger explosions, exploring the lack of awareness the public had regarding the dangers of types of radiation and the effects of fallout and heat as well as blast. The prevailing plan at the time was to evacuate the public 'to safer areas' - women and children to the coastal regions of the UK, in complete denial as to the way in which fallout will travel.

The most damning assessment of the likely decimation of the British way of life comes with the more familiar BBC docu-drama, 'Threads' (1984), shortly after a most thorough and unprecedented examination offered by Australian journalist John Pilger in 'The Truth Game' (1983). It is this latter film I urge you to watch, no journalist will ever again get the level of access he was granted to film and interview the personnel on the front line of the cold war, and does so without pandering to any military narrative.

The thing is if that calculation was important or needed at all in the Cold War scenario. In that period the MAD (Mutual Assured Destruction) came into existance, this meant that once the Nuclear War was initiated civilization as we know it was likely to be wiped out, this is the case of strategic ballistic missiles. Many of the targets were other missiles silos, production centers, military bases, etc, but with the MAD scenario the possibilities of overkilling kind of became irrelevant.