The KT boundary when the dinosaurs died was caused by a meteor which hit off the Yucatan peninsula in the Caribbean about 64.5 million years ago. When this happened a large amount of sulfuric acid was thrown into the atmosphere. This acid reflected the sunlight trying to get to the earth. As a result the temperature of the earth went from about 20 degrees C to -5 degrees C in about a week. We also know that this occurred in June because of leaf fossils which show the effect of the quick freeze.

(The Great Courses – The Physics of History)