“When a cold front moves through the area, it pulls air pollution, dust, and haze with it, often providing excellent viewing conditions with very stable air. Periods immediately after a heavy rain are often best of all; the rain does a wonderful job of clearing dust from the air, which improves atmospheric transparency, stabilizes the air, and reduces the effects of light pollution. (It doesn’t matter how much light escapes into the air if there’s nothing for it to reflect from.)”