When They Collide

The consequences will depend on the size and structure of the comet nucleus. A comet nucleus more than 1 km in size may be travelling at a speed between 20 and 70 km/s.
Barringer Crater Arizona, USA	When it collides, its enormous kinetic energy will be converted into a destructive force equal to several million hydrogen bombs. If it is a hard rock, it will leave an enormous crater, form a huge cloud of debris consisting of hot molten rock, that will rain back on the earth. In fact in the long term, this debris could affect the climate of the entire earth. The shockwaves generated will trigger earthquakes and tidal waves.
If on the other hand, the comet nucleus is not hard and it explodes before reaching the surface, it will still generate shock waves that will set green forests on fire, shatter structures, cause earth quakes and tidal waves. Both will cause enormous short term and long term destruction through out the world.

Smaller sized objects may cause damage over a restricted area. The famous Barringer crater in Arizona, U. S. A and the Lonar Lake in Maharashtra India are two examples of craters left by impacts of objects much smaller than 1 km in size and rocky in composition.

Lonar Lake
Maharashtra

Comet Swift-Tuttle

Comet Swift - Tuttle, a periodic comet that was visible in 1992, was predicted at that time, to be in collision course with the earth during its next passage in 2116. Since that time however new observations have shown that it will miss the earth comfortably ! But the earth residents of that time will see a spectacular display by it as it will be very close to the earth then, by astronomical standards! Since 1980 we have increasing evidence to show that a 10 km rocky object, probably an asteroid, hit the earth 65 million years ago and may have been responsible for the extinction of dinosaurs. The crater has been located in Chicxulub, Mexico.

In June, 1908 a much smaller object exploded 8 km above the earth, causing wide spread damage in the Tunguska river basin in Siberia.