However, if there is fog or haze or any kind of opaque material or structure between people and the oncoming fireball, the effects of the thermal pulse can be greatly reduced. With medium haze it can be cut by 50% and with heavy fog down to even just 10%. Smog in the big cities could actually be partly protective for once.
Also, while it delivers most of its energy within the first second, the larger the bomb the longer it'll take to deliver its full compliment of thermal energy -- up to several seconds for some megaton bombs. Quickly diving behind anything creating a shadow could be lifesaving.
Besides fog, smog, haze or clouds, there are buildings, trees, hills and other objects that would also block and reduce some portion of the thermal pulse. In fact, the more densely built-up an area is, the less likely the inhabitants would be to suffer the full impact of the thermal pulse. Of course, they may still have to deal with the resultant fires, as well as any blast damage.