Atmospheric pressure is the force per unit area exerted on the surface by the weight of air above that surface. Atmospheric pressure varies widely on Earth, and these pressure changes are important in terms of local weather. For business aircraft operators, analyzing and understanding weather pressure systems are always important elements in pre-trip planning.

The following is an overview of what you need to know about weather pressure systems before your next mission in your business aircraft:

1. Weather pressure systems

The tilt, rotation, and differential isolation of the Earth create unequal heating of the surface. This results in unequal distribution of pressure across the surface. Temperature and pressure differences create air circulation, leading to larger-scale air movements. High-pressure centers are areas of higher pressure where, in the Northern Hemisphere, the wind circulation is clockwise. Low-pressure centers are areas of lower pressure where, in the Northern Hemisphere, the wind circulation is counterclockwise. In addition to high- and low-pressure systems, there are also ridges (high pressure) and troughs (low pressure) to consider.

2. Pressure systems affect flight

Flight operations are affected by weather pressure systems. The degree of impact is based on type of pressure system, size, strength, and vertical structure of the system, as well as its geographical location and season/time of year.

3. These systems are predictable

Surface analysis and constant pressure charts depict location and strength of pressure systems. Numerical computer models assist aviation meteorologists in forecasting strength/movement of these pressure systems. Two examples of numerical models are the North American Mesoscale model, which forecasts pressure systems up to 84 hours in advance, and the Global Forecast Model, which forecasts pressure systems 384 hours out. Other models exist, such as ECMWF and UKMET, which also forecast strength/movement of pressure systems. Be aware that each model has its own tendency and biases when accurately forecasting pressure systems.

4. Patterns exist with pressure systems

Semi-permanent pressure systems are common in different regions of the world. Examples of large low-pressure systems include the Aleutian and Icelandic low-pressure systems. These are typically strong in the winter season but weak, or non- existent, during summer months. Examples of semi-permanent high-pressure systems include the Azores/Bermuda high and North Pacific high. These systems are stronger in the summer season and weak, or non-existent, in winter.

5. Some regions exhibit little variation in weather pressure systems

There are areas where atmospheric pressure is generally constant. The regions around both the north and south poles are characterized by areas of constant high pressure. Low-pressure systems exist in areas of the Inter-Tropical Convergence Zone (ITCZ) – along the equator – with subtropical high-pressure areas located at both 30N and 30S latitude.

6. Know how these systems are depicted on charts

Surface- and constant-pressure charts indicate location of highs, lows, ridges and troughs. High-pressure centers are indicated with a blue “H,” with the center pressure in millibars. Low-pressure centers are indicated with a red “L,” with the center pressure in millibars. Lines of equal or constant pressure called “isobars” are analyzed on surface charts.

7. Know how isobars are indicated on a chart

Isobars are lines of equal or constant pressure, indicated as black lines (contours) on a surface analysis. Using isobars (contours) makes it easier to determine locations or areas of high and low pressure. Constant-pressure charts will depict ISO heights or lines of equal heights.

8. Pressure systems are measured in millibars

Millibars are most often used on surface charts to measure pressure systems; this provides a uniform or standard form of measurement. Operators should have conversion tables or smart phone apps handy and available in case conversions are necessary.

9. Coriolis Effect plays a role in weather pressure systems

The Coriolis Effect is defined as the deflection of moving objects when viewed in a rotating reference frame. This effect deflects air circulation to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

10. Pressure systems vary by height

Depending on the type of system and season of the year, height of a pressure system can be shallow – extending to less than 850 millibars (mb) – or it can be fairly strong – extending to levels above 500 mb. Pressure always decreases with height, at a lapse rate of about 1 inch of mercury per 1,000 feet. Temperature advection will either strengthen or weaken a pressure system. Altitude of the isobaric surface (constant pressure) above sea level depends on temperature. In regions where the air in that column is cold and dense, altitude of the isobaric surface (constant pressure) stands lower than over a region where air is warmer and less dense.

11. Know how to extrapolate speed of air pressure changes

Pressure increases or decreases depending on the degree of temperature advection on the affected system. In general the more rapid the temperature advection into a pressure system is, the more rapid the change in the pressure system will be.

12. Additional information

Pilots using constant-pressure charts should use the contour patterns to gain better insight into weather conditions. On Northern Hemisphere charts where contours deviate far to the north (ridges), weather can usually be associated with warm- and dry-surface conditions. Conversely, where the contours deviate far to the south (troughs), weather is generally associated with cold- and wet-surface conditions. Operators should also use satellite imagery, additional weather depictions, and radar charts to gain fuller insight into weather conditions. A good source for constant-pressure charts and surface analysis in the U.S. and North America is the AWC website. Another good source is NWS fax charts.

Conclusion

Surface analysis and constant-pressure charts are effective tools in gaining insight into how the atmosphere is acting. These tools help aircrews understand the strength, weakness, and weather associated with weather pressure systems and fronts.

Questions?

If you have any questions about this article or anything else related to aviation weather meteorology, contact me at stevearbogast@univ-wea.com.