Two types of smog are recognized: London and Los Angeles types. London smog usually occurs during the winter season when indoor heating is used (from the turn of October to March). This type of smog results from a concentration of sulfur oxides (IV), nitrogen oxides, carbon oxides, soot and slowly falling dust. When accompanied by a specific atmospheric phenomenon, known as temperature inversion, the combination of pollutants leads to very poor air quality. Although typically temperatures go down when altitude increases, with inversion, air temperatures actually rise as well. The second known type of smog is known as Los Angeles smog, which is also called photochemical smog. It occurs mainly in the summer months, in subtropical regions. Los Angeles smog consists mainly of gases: carbon oxides, nitrogen oxides and hydrocarbons.