Flash and Fireball 

- An intense flash of light, as quick as lightning but a thousand times brighter, is the first effect of a nuclear bomb exploded in the air.
- Heat radiation is then released that is strong enough to set fire to material 14 kilometers (8.68 miles) from the explosion.
- The bomb's intense X-ray pulses are lethal up to 3 kilometers (1.86 miles) from the explosion.
- After the initial flash, a fireball forms and rises, releasing blindingly bright flashes and radiating heat. People up to 80 kilometers (49.6 miles) from the explosion can be temporarily or permanently blinded by looking at the fireball.
- People within 10 kilometers (6.2 miles) of the explosion will be deeply burned. Lighter burns will occur at greater distances such as 15 kilometers (9.3 miles).
- The spreading range of heat and light rays depends on the weather. 

Blast 

- Begins at the same time as the flash and fireball but moves more slowly.
- Causes major damage to houses up to 14 kilometers (8.68 miles) from the explosion and breaks windows 20 to 30 kilometers (12.4 to. 18.6 miles) from the explosion.
- Almost everyone would be killed within 3 kilometers (1.8 miles) and 50 percent of the people 8 kilometers (4.96 miles) away would be killed.
- Causes of death range from blast radiation or collapsing and flying masonry.
- Hurricane-strength winds follow the blast. 

Firestorm 

- The fires started by the initial flash intensify as gas mains break, flames spread and wind increases, resulting in a firestorm.
- The fires burn up all available oxygen.
- Temperatures everywhere, even in basements and bomb shelters, rise above lethal levels.
- Strong winds blowing into the fires make it nearly impossible for survivors to run outwards from the fire. 

Delayed Radiation Fallout 

- A nuclear explosion leaves everything in the surrounding area radioactive.
- Most radioactive products are in a gas form and would rise with the fireball and come down slowly.
- A large fraction, a minimum of one-third, of a bomb's fissile material is not destroyed, thus resulting in widespread contamination and increasing the late risk of cancer in survivors.
- Left-over plutonium and uranium have no immediate toxic effects.  

Source: Nuclear Weapons by Dr. Alan Phillips — www.geocities.com/Area51/Vault/5862/Nuclear.html