During the winter in the far north, the long polar nights produce strong radiative cooling over the frozen Arctic Ocean and the typically snow-covered reaches of Alaska and Canada. This results in very cold surface temperatures and a temperature inversion that acts to inhibit cloud development, creating a positive feedback on the radiational cooling process. In summertime, the large amount of insolation received over the very long days in the northern reaches of North America acts to raise temperatures there so that these areas are more in line with much of the rest of the continent, thus decreasing the north-south temperature gradient. Occasionally, extreme summer heat waves spread over much of the central United States and parts of central and eastern Canada. The hot air can be extremely humid because of low-level southerly airflow off warm Gulf of Mexico waters.
Annual precipitation amounts over North America show large spatial variations.
The other main continental maximum in annual precipitation is located in the southeastern United States. Mean annual precipitation amounts along the central Gulf coast exceed 150 cm. This feature is associated with convection that often is fueled by a strong low-level southerly jet stream bringing abundant moisture from the Gulf of Mexico.