The sun is the primary mechanism that warms the earth's surface. If this is the case then why can sunny days be very cold?

1. The sun angle is important to how much the sun can warm the air. If the sun is low on the horizon, the energy coming from the sun will be spread over a larger area. In winter, the sun angle is lowest. Low winter sun angles are not efficient in warming the air. Also, low sun angles are more likely to have energy that is bounced off the atmosphere or the earth's surface rather than being absorbed by the surface. In addition, lower sun angles have to travel a much further distance through the atmosphere before reaching the surface, increasing the amount of energy that does not reach the surface.

2. The length of daylight is also important. The energy that warms the air is cumulative. If the sun is out longer, the earth's surface via sunlight absorption will be able to warm the air more than when the sun is out for a shorter time period. The daylight length is shorter in the winter, especially between the higher mid-latitudes and the poles.

3. An air mass takes time to modify. An air mass modifies by moving over a region that has different surface temperature and moisture characteristic. The modification process of an air mass takes several days after it leaves the source region. This "lag" prevents the sun from being able to warm very cold air instantaneously. The air can be very cold on a sunny day due to the sun not having enough time to significantly modify the air yet. In winter, cold air is more difficult for the sun to modify (especially between high mid-latitudes and the poles) since the sun angle is low and days are short.