One result of this is red sunsets as the dust scatters light from the Sun, but a much more subtle effect is that of Global Cooling. The dust high up in the atmosphere reflects a lot of the light from the Sun so that it never reaches the Earth's surface. This causes the Earth to cool down. Volcanoes also cause these problems as they throw up dust into the atmosphere too. In fact, it is thought that the sudden fall in Global Warming - i.e. the reversal of the gradual increase in temperature that had been seen up to then - at the end of the 1980s was due to the eruption of Mount Pinatubo in the Philippines. However, the bomb that landed on Hiroshima was small compared to today's nuclear bombs and probably did not have much affect on the weather at all.