My grandfather used to say that when...

Although this is a common saying, scientists have not found this to be true, at least directly. However, there is research that suggests that cows tend to lie down when they are cold to conserve body heat, and to stand when they are hot in order to allow body heat to disperse more rapidly. When a rain system approaches, often the air temperature drops, so you might see cows lying down to help keep themselves warm. However, cows lie down for other reasons, too, such as if they are just plain tired!