11. lows, counterclockwise around highs). Cells are usually in mid-ocean or mid-continent, not at the boundary between. Sub-polar oceans strengthen lows in winter due to the higher temperature of water relative to near-by land masses (ex: Alaska). Strong highs occur over sub-tropic oceans in summer for the reverse reason. Over continents there is a reversal of pressures in the change of seasons. Sub-tropical highs exist over large ocean areas throughout the year. The Aleutian low in the north Pacific and the Icelandic low in the north Atlantic are the major lows of the northern hemisphere. Types of fronts 1. Secondary fronts are formed orographically when a mountain barrier is lying across a strong wind. This creates a ridge of slightly higher pressure on the windward side, a slightly lower pressure on the lee side. This is a relatively unimportantant type of front. It may be important in the amount of precipitation that falls, but changes in pressure is not the cause (ex: east of Rockies, east of Appalachians, east of Sierra Nevada). No real front present. 2. Secondary front with a true front - a low pressure trough and frontal system tend to persist for some time (ex: east of Rockies in winter). It is due to strong maritime polar air from the Pacific. The temperature contrast is quite sharp - it is a warm, dry front. When this air collides with maritime (Gulf of Mexico) air, squalls and thundestorms result. This type is not too important. 3. This type of front results from the close proximity of two air masses of quite different temperatures. It usually results when continental polar air (cold) from the Canadian shield comes into contact with maritime Caribbean air (warm)