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The infamous Phoenix “Brown Cloud”

Post on Dec-27-2016 at 20:47:00

A common sight in the Valley of the Sun during the winter months is the “brown cloud” across the horizon.

If you haven’t seen it, this cloud is a distinctive brown layer that is visible near the surface, especially during the morning hours. So what causes it?

Typically, in the atmosphere, as you go up in elevation the temperature will become cooler. With an inversion, the reverse occurs, cold air becomes trapped beneath a layer of warm air. These inversions typically occur at night, when skies are clear and calm winds are present.

As the sun sets in the evening, the ground quickly cools which in turn cools the air near the ground. As this continues overnight, the inversion becomes stronger.  

The cold layer of air near the ground is capped (think of a lid on a jar) with a warm layer of air right above it. As a result, any pollution in the Valley gets trapped within the cold layer of air. If winds are calm for an extended period, the air will become even more polluted as there is no ventilation.

When strong inversions are in the forecast “No Burn Days” will be issued. These alerts are normally in effect during the holidays as during these periods; conditions are typically perfect for strong inversions, and many people are using their wood burning fireplaces. This combination can result in very high levels of pollution across the Valley.

If windy conditions are present, the inversion does not develop as the cool air that forms near the surface mixes with the warm air above it. Windy conditions were present Christmas Eve and Christmas Day this year which is why no pollution advisories were needed.