The area of effectively complete destruction extended to 25 km, and ordinary houses would be subjected to severe damage out to 35 km. The destruction and damage of buildings at much greater ranges than this which occurred was due to the effects of atmospheric focusing, an unpredictable but unavoidable phenomenon with very large atmospheric explosions that is capable of generating localized regions of destructive blast pressure at great distances (even exceeding 1000 km).

A 100 Mt weapon can level urban areas in a zone 60 km wide, cause heavy damage in a zone 100 km across, and cause 3rd degree burns in a region 170 km across (only a bit smaller than the width of West Germany). Such a weapon can only be used as a means of destroying an entire urban region - a major urban complex including suburbs and even neighboring cities. This scale of destruction is much larger than any discrete urban area in Western Europe. With its dense settlement, use of such a weapon in Europe is equivalent to an attack on a major portion of an entire nation and its population. Fallout from a low altitude or surface burst in central England could produce lethal exposures extending into the Warsaw Pact nations; a similar explosion in West Germany could create lethal fallout as far as the Soviet border. And in the United States there were only three urban regions at that time large enough to conceivably merit attack with such a weapon - New York, Chicago, and Los Angeles. On any smaller target it would be simple overkill. Even if the Tu-95 were able to reach Chicago, the closest plausible US target, (which is doubtful given the enormous payload, far in excess of normal for long-range missions, the added drag from the belly bulge required to house the bomb) it would have been detected crossing the North American early warning line and then been over US and Canadian territory for 8 hours - ample time for jet fighters to intercept and shoot it down [Zaloga 1993].

http://nuclearweaponarchive.org/Russia/TsarBomba.html