Warm water that decreases the level of dissolved oxygen in the water can harm aquatic animals such as fish and other marine animals. It can also increase the metabolic rate of aquatic animals, resulting in food source shortages as well as a huge decrease in the population. Coal-burning power plants are well known producers of thermal pollution in nearby bodies of water that they use as cooling ponds. Cities trap and radiate more heat that forests and fields. The warmer air makes thermal currents in the air which can act as a barrier to the normal wind patterns. This may also affect rainfall.