Forecasting Tutorial: Air Masses
Forecasting Tutorials is a series of in-depth articles where we teach you the techniques of surf and marine forecasting.
Meteorology Basics: Pressure systems
By Marine Forecaster Katie Jackson
Our main concern in this article has to do with the relationship of a pressure system to its surroundings. Air pressure can be measured (atm, hectopascal, millibar) but the relative difference between pressure systems is what counts for weather and wind behaviour. In forecasting, it is best to focus on the interplay between pressure systems. It is the way pressure systems interact that matters.
Basic weather charts used in forecasts are barometric. They outline air masses by differentiating areas of surface pressure. At the surface, pressure is the measured force the air mass above us exerts down. Each area of equal atmospheric pressure on a chart has an isobar, which outlines the pressure gradient and encircles areas of high and low pressures. So what do isobars have to do with waves? Isobars paint a picture of the wind forcing that effects wave generation. Difference in pressure leads air to move as winds and this ultimately leads to waves when it occurs over the ocean.
High pressure/Low pressure
There isn't a particular number that distinguishes a high pressure from a low pressure. As well, there is not a set temperature range associated with either one. A high-pressure system occurs where air mass is denser than in surrounding areas, and therefore exerts a higher force or pressure. The opposite is true for low pressure. If the air mass above the Earth is less dense than in surrounding areas, less force or pressure exerted, and therefore an area of relatively low pressure occurs.
- High Pressure = air condensing sinking to surface
- Low Pressure = air expansion rising up to atmosphere.
If the air is entering an area of low pressure at the surface, winds curve inward and move up, known at convergence. If air is exiting an area of high pressure, winds are forcing down and curve out, known as divergence. Not only are horizontal changes in wind occurring during convergence and divergence but vertical changes as well. The movement of air vertically in the atmosphere is a strong indication of atmospheric stability and great changes in pressure, temperature, and winds differ at each layer. Atmospheric charts are made at varying heights in the atmosphere to distinguish these changes in vertical profiles of air masses.
The pressure gradient force is the main force acting on air to make it move. When air moves it manifests as wind. The pressure gradient is simply the change of air pressure over distance and relates directly to wind speed. For ocean winds, the pressure gradient at mean sea level is the most important driving factor for wave generation.
Pressure gradient = pressure difference / distance
If a large change in atmospheric pressure occurs over a small distance, a strong pressure gradient occurs. This leads to significant force on air to move from high to low pressure and results in high wind speeds. Large pressure changes in close proximity can be referred to have a “tight” pressure gradient due to closely places isobars shown on a MSLP chart. The opposite will occur if a pressure gradient is “loose” with less change experienced between pressure systems or the pressure systems are far apart.
Air flows clockwise around low-pressure systems and anticlockwise around high-pressure systems in the southern hemisphere (opposite in the northern hemisphere). This directional change in wind occurs due to the rotation of the Earth around its axis, known at the Coriolis effect. Initial air movement begins with the pressure gradient, but the Coriolis effect may change wind direction once in motion. This secondary force is dependent on location, with strongest effect at the poles and zero at the equator.
Last mentioned but certainly not least are frictional forces. Frictional force will result in the decay of wind speeds or change in direction due to shearing effect. For surface winds over the ocean, frictional drag is considered low compared to surface winds over land. It is the friction that occurs over the ocean surface that is responsible for wave formation. Moving air mass transfers energy from the atmosphere to the sea through winds and the friction helps to form waves. It all begins with changes in atmospheric pressure that bring us to the point of initial swell development.
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