Given how unpredictable the weather can be, with it being warm enough to barbecue one day and cold enough to send people scrambling to buy snow tires the next, it's no wonder early people created an entity whose whims controlled it all. Her name was Mother Nature, and she alternately supported and punished in a seemingly arbitrary fashion, and she still does to this day, though we know her to actually be a complex system of interdependent variables.
Though Mother Nature is still referenced in expressions today, very few people in the modern world actually believe she is a real entity anymore. When a dentist can replace your teeth and a rocket can go to the moon, it's hard not to turn to science to explain things. However, early people really did believe Mother Nature was a sort of deity - also called Gaia - who exerted control over nature. Believing this helped them to cope and feel like they had some control over the weather by praying to this deity.
So what sorts of things are incorporated into the Mother Nature umbrella? Pretty much any natural process (i.e. not created or caused by man) was said to be the work of Mother Nature. This included subtle natural processes like the changing of the seasons and even the physical maturing of a child, as well as more overt natural events such as rainstorms and tornadoes, which had an added element of wild randomness that scares many people.
Since those early days, we have made a lot of progress in deciphering what exactly is behind the processes "Mother Nature" uses, especially the predictable changes. There are books available on how hormones are responsible for changes in the body and scientific evidence that proves the seasons are the result of a tilt in the Earth's axis. This is the result of humanity's predilection for pattern recognition and the progression of technological measuring devices.
But what about the random things? Can we predict a thunderstorm? A hurricane? A volcanic eruption? We understand how these things happen and to some extent we can predict them by watching out for the common warning signs, but we are still unable to prevent them from destroying our communities. That's why we need disaster management teams and insurance, which are often our only defense against "Mother Nature" when she "decides" to wreak havoc.