Thermally Direct & Indirect Circulations

Thermally direct circulations are when warm air rises and cold air sinks, converting potential energy into kinetic energy by lowering the center of mass (i.e. Hadley and Polar cells).

Thermally indirect circulations are when cold air rises and warm air sinks, converting kinetic energy into potential energy by raising the center of mass (i.e. Ferrel cell).

Sea Breeze / Land Breeze Circulations

Sea/Lake Breeze Front is a boundary that is usually small and temporary but usually causes an abrupt drop in temperature as it passes (a distinct boundary between the cooler maritime air and the continental air it displaces). Whereas, just a sea/lake breeze is heating over the inland area which causes air to expand upward and diverge at higher altitudes. This creates a surface low-pressure area and the sea breeze flows inland from the sea.

Pressure starts out equal over the land and sea/lake but due to unequal surface heating. The land is warmer due to the ground absorbing heat faster than the sea/lake, resulting in the ground heating the air by conduction. The less dense, warmer air is rising and expanding thus increasing the upper-level pressure and creating divergence above while, decreasing the pressure over the land. Whereas, over the sea/lake, the cooler and denser air which has a relatively higher pressure resulting in low-level convergence due to the fact that air flows from high to low pressure (pressure gradient force).

The strength of the breeze depends on the strength of the land-sea temperature difference (gradient). Sea/lake breezes occur mid to late afternoon when the land-sea temperature difference (gradient) is greatest and tend to be more intense than land breezes. Although, thunderstorms may develop if atmospheric instability is enhanced by surface heating.

Land breezes, on the other hand, are a reverse circulations that tend to occur during the pre-dawn hours. At night, the land surface cools more rapidly than the sea thus becoming denser resulting in a higher surface pressure and an offshore flow.

Jetstreaks and Associated Circulations

            A jetstream is a narrow band of strong winds that encircles the Earth in the mid-latitudes, containing regions where locally strong pressure gradients produce exceptionally strong winds, called jetstreaks, which migrate through curved flow patterns. A jetstreak is located in a region of strong pressure gradient and is indicated by the large values of isotachs (lines of constant wind speeds) and the close spacing of the pressure or height contours. As air moves through the jetstreak, air parcels are displaced northward in the entrance region and southward in the exit region. Divergence occurs in the right entrance region (looking in the direction of the flow) while convergence in the left entrance region displaces air from the right (south) to the left (north) side of the jet. Divergence aloft will result in lower surface pressure, whereas convergence will result in higher surface pressure.

            Although the jetstream is not always a single ribbon of fast-moving air encircling the pole. In nature, a single jetstream can split into two branches and then merge again at a downstream location. In fact, the most extreme low pressures associated with cyclones in the middle latitudes usually occur when two (or even three) jetstreaks, each embedded in a different branch, interact with one another as their parent jetstreams merge.

Physical Principles: Maintaining General Circulation

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt