Question

if the density of air decreases with altitude, then the air should be lighter and low pressure should be created , then why there is high pressure ( forgetting about the coldair)

Answer 1

Pressure with Height: pressure decreases with increasing altitude. The pressure at any level in the atmosphere may be interpreted as the total weight of the air above a unit area at any elevation. At higher elevations, there are fewer air molecules above a given surface than a similar surface at lower levels.