A few years ago, I wrote in this topic an article on low pressure systems in general ("The Highs and Lows of Weather: Part 2 - The Low"). It covered surface low pressure regions in general. But often on the weather reports we hear of tropical lows, subtropical lows, extratropical lows, polar lows, and thermal lows, (not to mention Colorado lows, Hatteras lows, etc.) In this installment of Science of the Sky, I look at some of the more common flavours of low pressure systems. (Does Ben and Jerry's have a weather-named flavour of ice cream?)
To start, I'll recap what a surface Low or low pressure cell is. (I will use Low - capitalized - herein to distinguish between an organized region of low pressure and low pressure in general.) A Low is characterized by converging air flow at the surface, ascending air currents at its core where the converging currents meet, and regions of divergent air flow above where air currents flow away from the rising core. When the combination of these processes removes air faster from the surface than it can be replaced by the inflow, a temporary mass deficit in the air column lowers its weight, and thereby reducing its surface pressure.
We have no strict value for what the surface pressure should be in a Low. Usually, Lows have pressures less than the mean surface pressure of 1013 mb, but that is rather arbitrary as an area can be considered a Low as long as its pressure is less than the air surrounding it. So a Low can have its pressure above the mean surface pressure if it is sandwiched between areas of higher pressure, an analogy is a high-elevation valley between two mountain ridges. The figure here shows some extremes of pressure measured on the planet.
When the pressure gradient across the region of lowest pressure is strong enough, winds will flow around the Low in a counterclockwise manner, a condition meteorologists term cyclonic. Hence the alternate name for Lows: cyclone.
The five main flavours for Lows are: Tropical Lows, Subtropical Lows, Extratropical Lows, Polar Lows and Thermal Lows. In part, the qualifying adjective, excluding the last on the list, describes where the Low formed: the tropics, subtropics, mid-latitudes, or polar regions, respectively. However, there are important differences in their structure and method of formation. Thermal Lows generally have a preferred region of formation as well.
Tropical Lows are born over the warm waters and land in the equatorial zone. To form a rotating low pressure system, the Low must be born away from the equator itself, but a general region of low pressure, known as the intertropical convergence zone, is a permanent feature of the atmosphere around the Equator. In the tropics when a region of intense thunderstorms builds over the warm surface below, conditions are ripe for forming a tropical Low. With surface convergence and light winds aloft, the thunderstorms can organize into a rotating system that spins away from the tropics. Such tropical lows may become the juvenile stage for tropical storms and hurricanes/typhoons, or live a short, stormy life before fading away. Heat released as water vapour condenses to form clouds is the major source of their energy. Tropical Lows are sometimes called warm-core systems because they form due to warm air rising with no help from cold air; tropical Lows do not have fronts.
