Depressions

A depression, a low, and a cyclone describe the same weather system from different angles. The National Weather Service defines a depression as a region of low atmospheric pressure, usually accompanied by low clouds and precipitation. Its glossary defines a low pressure system more mechanically: an area of relative pressure minimum with converging winds, rotating in the same direction as the earth — counterclockwise in the Northern Hemisphere — and "also known as a cyclone." Cyclone describes the rotation. Low describes the position on a chart. Depression names the region of lowered pressure and the weather gathered around it.

That distinction matters more than it sounds, because the word carries a measurement rather than a shape. And the measurement is relative, not absolute.

Relative to what

Standard pressure at sea level is 1013.25 millibars, equivalently 29.92 inches of mercury. Values used in meteorology run from roughly 100 to 1050 millibars. The NWS definition does not hinge on one numerical cutoff. What defines the system is that the pressure at the centre is lower than the pressure around it, which is why the glossary phrase is relative pressure minimum.

Station readings are all converted to a sea-level equivalent before anyone compares them, because pressure falls steeply with height — half of all the air molecules in the atmosphere sit below 18,000 feet. Without that correction, every mountain observatory would look like a permanent storm.

The mature version of this system in the mid-latitudes is the extratropical cyclone: often 2,000 kilometres across, and usually containing a cold front trailing toward the equator for hundreds of kilometres. These are the migratory frontal cyclones of the middle and higher latitudes.

Where the dip comes from

The Norwegian meteorologists who worked this out in the 1910s and 1920s described a life cycle, and the sequence still holds. It begins with a boundary — warm air on one side, cold on the other, often stationary and going nowhere. Then an upper-level low embedded in the jet stream moves over that front, and the front develops a kink. The stationary boundary splits into a cold front and a warm front, and the air masses begin to rotate around the developing wave.

The forcing comes from above. A shortwave trough in the mid or upper atmosphere induces upward motion ahead of itself. On a 500 millibar chart — a level that sits between roughly 16,000 and 20,000 feet — forecasters track this by looking at vorticity, the spin in the air about a vertical axis. Downstream of a vorticity maximum is where vorticity is increasing fastest, and that implies convergence at low levels, air rising, and possible precipitation. Upstream, air sinks and the weather improves.

The tilt of the trough is a second tell. A trough leaning northeast to southwest — positive tilt — generally signals a weakening system and the least severe weather. When shortwave energy races east and distorts the axis to a northwest-to-southeast orientation, the tilt goes negative, which typically means a strong surface low is developing. That configuration produces a large change in wind direction between the surface and the upper atmosphere, and that shear is what supercell thunderstorms need.

What deepening actually does

Deepening means the central pressure is falling. The consequence is geometric: as the centre drops while the surroundings hold, the isobars around it crowd together. Wind speed is directly proportional to the pressure gradient, so tighter spacing means faster wind, full stop. This is the reason a forecaster reading a chart looks at isobar spacing before anything else.

Three forces set the flow. The pressure gradient force pushes air from high pressure toward low. The Coriolis force, described mathematically by Gustav-Gaspard Coriolis in 1835, deflects moving air to the right in the Northern Hemisphere; those two together would have the wind running parallel to straight isobars with high pressure on the right. Friction at the surface breaks that balance and turns the wind slightly inward, which is why air spirals into a low rather than orbiting it.

That inward spiral is the mechanism underneath everything the depression does to you. Mass continuity says air cannot pile up at a point. Converging air has to go somewhere, and the only direction left is up. Rising air cools, condensation begins to exceed evaporation, and the invisible vapour becomes cloud and then rain. A deepening low is not raining because it is a low; it is raining because friction and convergence are forcing air upward faster.

Reading one from your own barometer

The Reverend Dr. Brewer, writing in England in 1848, catalogued the rules that still work: a sudden falling of the barometer denotes high wind; in very hot weather, a falling barometer denotes thunder; the barometer sinks lowest of all for wind and rain together. In wet weather, a falling barometer means much more wet to come.

The trap is the daily tide. Pressure rises and falls twice a day under solar heating alone, reaching its lowest around 4 a.m. and 4 p.m. and its highest around 10 a.m. and 10 p.m. A modest afternoon fall may be nothing but the clock. What distinguishes an approaching depression is a fall that continues through the times pressure should be climbing. The daily swing is largest near the equator and smallest toward the poles.

Geography also limits Brewer's rules. The largest pressure changes with storms occur in Alaska and the northern half of the continental United States. In the tropics, outside tropical cyclones, there is very little day-to-day pressure change at all, and none of the rules apply.

Ending, and refusing to end

The cold front moves faster than the warm front and eventually overtakes it, forming an occluded front. As occlusion increases, the warm moist air is progressively lifted away from the centre, and without that warm air mass to sustain it, the low gradually dissipates.

Unless it doesn't. When a strong shortwave dives south on the west side of an equator-reaching trough, its momentum can pull the trough clean out of the main airflow, leaving a closed circulation isolated from the steering winds. These cut-off lows drift for days over the same ground, with unsettled weather over their eastern half and precipitation sometimes wrapping around the north end into the northwest quadrant. When a cut-off low is locked beneath a blocking high in a Rex pattern, the persistence itself becomes the hazard — flooding under the low, short-term drought under the high, and neither budging until one of the two centres changes intensity.