It is a difficult question to answer. There are a lot of possible things that could cause a drought, however one of the most interesting is the effect of atmospheric pressure.
If you listen to weather forecasts you would have heard of a low and high pressure areas. You might have also noticed a pattern. When the weather forecast talks about low pressure it is normally accompanied by rain while high pressure is accompanied by sunny periods and dryness. Dryness over a long period of time could mean the loss of a lot of ground water and therefore create a drought. You get long periods of high pressure over deserts,
So why does this happen?
When you have high pressure dry air descends from the colder air higher in the atmosphere (cold air is more dense and heavier than warm air. In low pressure the movement of the air is in the opposite direction with the warmer, ground level air rising carrying with it water vapour which eventually forms clouds as it gets colder.
Lisa, there are some difficult ideas here. Think about them and then ask another question.That’s the way science works.