Cyclones are caused due to low pressure. Low pressure systems, also known as cyclones, occur when the air pressure at the surface is lower than the surrounding air. This low pressure draws in surrounding air, causing it to rise and creating a vertical circulation. As the air rises, it cools and condenses, forming clouds and precipitation.

High pressure systems, on the other hand, are the opposite of cyclones. In high pressure systems, the air pressure at the surface is higher than the surrounding air, resulting in sinking air and clearer skies. High pressure systems are associated with calm weather conditions.

Low temperature and high density are not direct causes of cyclones. While low temperature and high density of air can contribute to changes in pressure, they are not the primary factors that cause the formation of cyclones.

In summary, cyclones are caused by low pressure systems, which lead to the rise of air and the formation of clouds and precipitation.