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Cloud formation require high levels of humidity and a condensation nuclei. In North America, anti-cyclones develop due to the low pressure systems that move from west to east across the continent. These are commonly a result of the collision of both cold and warm air masses. Cold air sinks and warm air rises, therefore when cold air catches up to a warm air mass and this lifts the warm air up this is called uplift. This leads to the production of cloudy conditions and rain. Due to the weather conditions being similar this means that there is no front or trough or trowel etc, present. The formation of cloudy weather is due to human activities in the city causing an abundance of condensation nuclei. One of the products of combustion is water vapour this will increase the humidity in the city. This is an example of local effects, and therefore could be why there is cloud over the large city and when nearby small towns are clear.