Seasonal affective disorder, or SAD, also known as winter depression is an affective, or mood disorder. Most SAD sufferers experience normal mental health throughout most of the year, but experience depressive symptoms in the winter or summer. SAD is rare, if existent at all, in the tropics, but is measurably present at latitudes of 30 degrees N (or S) and higher. Seasonal mood variations are believed to be related mostly to daylight, not temperature. For this reason, SAD is prevalent even in mid-latitude places with mild winters, such as Seattle and Vancouver. People who live in the Arctic region are especially susceptible due to the effects of polar nights. Prolonged periods of overcast weather can also exacerbate SAD. SAD is a serious disorder, sometimes triggering dysthymia or clinical depression. A number of treatments exist for SAD. These include light therapies, medication, ionized-air reception and cognitive therapy.