High Pressure Systems- An anticyclone is an area of high pressure on a weather chart that is completely encircled by an isobar. A High that is not completely encircled by an isobar is known as a Ridge. A cyclone is an area on a weather chart that is completely encircled by an isobar. As the observer virtually moves toward the center of the Low they would find the atmospheric pressure decreasing. An isobar is plotted at every 4-millibar change in pressure.
Winds tend to move in counter-clockwise motion around an area of Low pressure.
A Low that is not completely encircled by an isobar is known as a trough . During the summer season, continents warm more than the oceans to, which explains why it the North American Thermal Low is found on the continent in the warm season and the Pacific High is found offshore. On the other hand, during winter, the oceans are warmer than the continents, with high pressure over North America and Asia. In the summer, wind speeds are of higher values later in the day because the temperature of the land mass is increasing.