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Clear Skies Ahead

Why is there no haze in winter?

Clear Skies Ahead The summit of Mount Diablo is a great place for wintertime views, sometimes all the way to the Sierra Nevada. Photo by Sanbeiji. Used under Creative Commons

by Kate Brittain — published December 04, 2009

Dig out the binoculars; it's clear skies ahead.

In the Bay Area, the winter months are your best bet for a clear view of, well, anything you're trying to spy--be it bird, Big Dipper or sweeping vista.

A year lived in the region (let alone two or three or a lifetime) will tell you as much. But observance of the trend leaves its cause a curiosity. Why are winter skies the least hazy?

Distribution of pollutants in the atmosphere is the primary factor determining how obscured your view will be. In the winter, winds tend to be stronger. This mixes the air of the Bay Area more thoroughly, and consequently disperses pollution that would otherwise stagnate. The air sweeping down from Canada following a cold front also tends to contain less salt and pollution.

The lower angle of the sun in winter also contributes to greater visibility. It prevents a significant buildup of photochemical pollutants, to which the region's inland valleys are prone. It's a combination of sunlight and warm temperatures (both, of course, harder to come by this time of year) that cause hydrocarbons and oxides of nitrogen to react, forming secondary photochemical pollutants like ozone.

In sum: it's the right time to bundle up and make the trek you've been contemplating. The view from the summit should be at its best.

This article is part of our "Signs of the Season" series, focusing on a particularly characteristic species, group of species, or phenomenon for the season.

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