Thermal Effects of Nuclear Explosions:

As it can be seen nuclear explosions generate a tremendous amount of heat and fire:

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PHYSICS OF NUCLEAR BLAST:

Blast Figure from Encarta

 

What is most interesting about nuclear blasts is that the blast is like a huge flash of light.  Basically the blast causes thermal radiation.  For this brief period depending on the size, maybe a couple seconds, the blast can posses intensity of 1000W/cm^2, in comparison the sunís intensity is 0.14 W/cm^2.  The intense thermal radiation is produced by the fireball of the blast.  The radiation is similar to that given off by the sun in that it is absorbed by the opaque surface layer of the material it is hitting.  So the same rules apply, basically more thermal radiation will be absorbed by a darker color.  For an example, people with darker skin are more prone to burns than those who have lighter skin. Table 3 below is based on a person with medium skin color, it shows the calories per centimeter squared that a certain magnitude of explosion will cause at a certain radial distance away from the blast. 

SEVERITY 20 Kilotons 1 Megaton 20 Megatons
1st Degree 2.5 cal/cm^2 (4.3 km) 3.2 cal/cm^2 (18 km) 5 cal/cm^2 (52 km)
2nd Degree 5 cal/cm^2 (3.2 km) 6 cal/cm^2 (14.4 km) 8.5 cal/cm^2 (45 km)
3rd Degree 8 cal/cm^2 (2.7 km) 10 cal/cm^2 (12 km) 12 cal/cm^2 (39 km)

table 3 [5]

TYPES OF BURNS CLASSIFIED [5]:

First degree flash burns:

Second degree burns:

Third degree burns:

As you can see a nuclear weapon has two main prongs of causing damage, it causes tremendous damage from radiation effects and it also causes great damage with its powerful thermal blast.

References