A high pressure area (anticyclone) is associated with sinking air. A low pressure (cycone) is associated with rising air. Rains are generally associated with rising air because as the air rises it cools. Cool air cannot hold as much moisture as warmer air, and so the moisture condenses and can fall as rain. Although a high means sinking air, it can result in turbulence, producing cumulous and nimbo-cumulous clouds, and possible showers or thundershowers if it is warm enough, because, although the air mass as a whole is pressing down (high pressure), pockets of rising air can occur if there is instability (mixing of air layers). Again, as these pockets rise, the moisture is condensed out resulting in rain if the moisture content is sufficient. In the northern hemisphere, winds blow clockwise around a high and counterclockwise (anticlockwise to you across the pond) around a low. This is why mountains “squeeze out” the moisture, and it rains on the upslope, but is dry on the downslope (air rises as it goes up the mountain and the moisture content is decreased, but also it warms up on the downslope).
Pressure differences create winds. The greater the gradient, the stronger the winds. Air, rising up in a warm area diverges out of that area. Since massis leaving the relatively warmer regon, there is less mass and the surface pressure drops. The warmer air moves toward the colder region, adding mass to that region and causing the surface pressure to rise there. This tilts the normal sea level pressure (of 1000 millibars), leaving us with relatively higher pressure where it is colder near the surface and relatively lower pressure where it is warmer near the surface. This creates the circulation.
This is the same phenomenon that causes the sea breeze effect. The ocean absorbs more solar radiation during the day than the sand does because it is darker. The ocean isn’t hotter than the sand because it has an internal circulation and because liquid water has significant thermal inertia. Warm air rises over the heated land and cool air sinks over the cooler ocean. The surface sea breeze blows inland from the cooler sea to the warmer air. If the land surface becomes colder at night, the circulation will reverse, and a land breeze will develop.
On the beach, the land surface heats up during the day, and the thickness of the layer between two isobars increases. Air rises over the land and spreads toward the sea, creating a pressure difference between the land air and the sea air. The cool sea air starts moving inland. When the cooler air from the sea collides with the warmerair over the land, a front is formed. The sea air burrows beneath the less dense air over land, forcing the land air to rise over the sea breeze front. If the land air is sufficiently moist and if the frontal lifting is strong and deep enough, the land air can be lifted to its lifting condensation level. If the land air reaches its level of free convection (the level at which a rising air parcel is slightly warmer than its surrounding environment), and thus becomes buoyant. That’s why thunderstorms may occur 10-15 miles landward from the sea during summer. Ultimately, the rising parcel will exhaust its supply of water vapor and cool down to the same temp as its surrounding.
Winds swirl counterclockwise around a cyclone, but most winds are angled across the isobars toward the low. This creates surface convergence, leading to ascent, clouds, and storms.
A cold front is the surface position of a dome of dense air, pushing against the warmer, less dense air ahead of it. The colder air subtends the warmer air, forcing the less dense air to rise and causing clouds and precipitation. The pressure is low all along the front, but a kink can sometimes develop along the front and launch a more localized circulation. Spin makesor enhances low pressure, so the lowest pressure along the front is in the center of the spin, producing a cyclone. The developing cyclone’s counterclockwise circulation starts pushing he air masses more firmly against each other and the cold front starts to swing around the south end of the cyclone. As the fast moving cold front sweeps around the cyclone, it catches upwith the slow moving warm front, first near the cyclone’s center. This forms an occluded front. As the occlusion advances, the fronts zip up and precipitation may be heaviest now. Heaviest rains can also occur ahead of the cold front in prefrontal squall lines.