<P> As they blow across tropical regions, air masses heat up over lower latitudes due to more direct sunlight . Those that develop over land (continental) are drier and hotter than those that develop over oceans (maritime), and travel northward on the western periphery of the subtropical ridge . Maritime tropical air masses are sometimes referred to as trade air masses . The one region of the Earth which has an absence of trade winds is the north Indian ocean . </P> <P> Clouds which form above regions within trade wind regimes are typically composed of cumulus which extend no more than 4 kilometres (13,000 ft) in height, and are capped from being taller by the trade wind inversion . Trade winds originate more from the direction of the poles (northeast in the Northern Hemisphere, southeast in the Southern Hemisphere) during the cold season, and are stronger in the winter than the summer . As an example, the windy season in the Guianas, which lie at low latitudes in South America, occurs between January and April . When the phase of the Arctic oscillation (AO) is warm, trade winds are stronger within the tropics . The cold phase of the AO leads to weaker trade winds . When the trade winds are weaker, more extensive areas of rain fall upon landmasses within the tropics, such as Central America . </P> <P> During mid-summer in the Northern Hemisphere (July), the westward - moving trade winds south of the northward - moving subtropical ridge expand northwestward from the Caribbean sea into southeastern North America (Florida and Gulf Coast). When dust from the Sahara moving around the southern periphery of the ridge travels over land, rainfall is suppressed and the sky changes from a blue to a white appearance which leads to an increase in red sunsets . Its presence negatively impacts air quality by adding to the count of airborne particulates . Although the Southeast USA has some of the cleanest air in North America, much of the African dust that reaches the United States affects Florida . Since 1970, dust outbreaks have worsened due to periods of drought in Africa . There is a large variability in the dust transport to the Caribbean and Florida from year to year . Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s . </P>

What does the movement of air masses cause