Weather guru Michael Carr inaugurates a multi-part series on basic weather with this discussion of how the jet stream influences localized weather.
An approaching squall-line signals energy transfer and strong winds.
What is weather? Weather is the process of the earth's atmosphere moving energy. Energy in motion is most noticeable to us as wind, and strong wind results from large transfers of energy. If the earth's atmosphere were the same temperature everywhere, we would have no weather since there would be no need or impetus for energy transfer, but the fact that polar regions are very cold and the equator is very hot invites energy transfer.
A temperature difference across a geographical area also represents a pressure and density difference. This difference causes energy movement, i.e. wind! Thus large temperature differences imply strong winds.
We experience strong winds, for example, with the passage of cold fronts, where cold air from northerly regions displaces warmer air from southerly regions. Strong winds are also experienced with weather features such as thunderstorms, squall lines, approaching high-pressure systems, and hurricanes. These are all weather events where a significant temperature difference exists across a geographical area.
So, always take note of temperature and stay aware of any changes, as this is a direct indication of weather and wind strength.
Jet Stream: A Driving Force We sail on the ocean's surface and so are naturally interested in wind speed and direction at sea level. However, it is air movement a few miles above the ocean's surface that governs surface winds and needs to be understood by every sailor attempting to predict surface-weather conditions.
Circling the earth, moving from west to east are currents of air that, since their discovery during World War II, have been called jet-stream winds. These winds, not slowed by surface friction, move at speeds averaging 50 to 150 knots.
Jet stream winds are a result of two influences: (1) the atmosphere's continual effort to reach thermal equilibrium by moving hot air from equatorial regions to polar regions and (2) deflection of airflow (to the right in the Northern Hemisphere) caused by the earth's rotation, known as the Coriolis effect.
There's a method to the madness: the atmosphere is seeking thermal equillibrium.
Jet-stream winds, by both their flow direction and speed, act as steering currents for surface weather, influencing the formation of lows, highs, troughs, ridges, and fronts. Marine Prediction Center (MPC) meteorologists analyze jet-stream activity twice each day and then produce surface weather analyses and forecasts.
When marine meteorologists examine jet-stream activity they look at its bottom layer, which lies at an altitude of approximately 18,000 feet, or more precisely at a pressure level of 500 millibars (mb). This level is approximately halfway up in the earth's atmosphere, since normal sea-level pressure is 1,013 mb.
Meteorologists plot jet stream wind speed and direction on a chart referred to as the 500-mb chart. This picture of jet-stream activity provides tremendous insight on present and future surface weather conditions.
Jet stream flow generally conforms with the earth's west-to-east motion, but it also meanders north and south, with this meandering described in two ways: as either meridional or zonal. Meridional flow refers to a significant north-south or south-north undulation. It mixes cold air from the northern latitudes with warm air from the southern latitudes and is a harbinger of unsettled and possibly stormy weather.
Zonal flow refers to a predominate west-to-east motion without the undulating, wavy motion found in meridional flow. Zonal flow does not, generally, produce as much mixing of cold and warm air, but it does move weather systems from west to east quickly. So, when a 500-mb chart is examined, we should first determine if it reflects meridional or zonal flow, since flow pattern is an excellent long-term indicator of surface weather.
If a 500-mb chart shows meridional flow, then troughs and ridges can be identified since surface low-pressure systems form in the vicinity of troughs and high-pressure systems form in the vicinity of ridges. Lows form under the influence of jet stream troughs because here airflow is counterclockwise, the same as wind flow around surface lows. Surface highs form under 500-mb ridges since airflow is clockwise.
Troughs and ridges vary in amplitude and length, illustrating high and low-pressure areas.
How are 500-mb troughs and ridges identified? Troughs are U
shaped and ridges resemble an inverted U
. Troughs and ridges show variation in both amplitude and length, with both troughs and ridges providing indications of surface low and high-pressure characteristics. Generally the larger the amplitude and stronger the upper air winds, the stronger the surface system, since large amplitude indicates a high degree of cold dry air mixing with warm, moist air.
Troughs and ridges at the 500-mb level change slowly, often taking days, weeks, and occasionally months to change their flow pattern. This is why they are good indicators of surface weather.
Troughs with sufficient wind strength and development to bring surface low development are drawn on 500-mb charts using a bold dashed line. Two rules of thumb can be used in estimating surface-feature strength and movement when examining a 500-mb chart:
- Lows move across the earth's surface at a speed equal to 1/3 to 1/2 of the 500-mb wind speed directly above that feature
- The 5,640-meter height contour on the 500-mb chart, which is highlighted in bold, is a general indicator of storm track direction and southern extent of Beaufort force 7 (28-33 knot) or greater surface winds in winter, and Force 6 (22-27 knot) winds in summer
To accompany 500-mb analyses and forecasts, MPC meteorologists produce four detailed surface analyses each day, each one depicting isobars, winds, frontal systems (occluded, stationary, cold, and warm), low and high-pressure center positions, and central pressures. Warnings for gale and storm-force systems are labeled and spelled out in bold, capital letters.
Each surface analysis depicts the 24-hour forecast position for each low and high-pressure system. Pressure decreases of 24 mb for any given 24-hour period are noted in bold, capital letters as RAPIDLY INTENSIFYING. If a low-pressure system is forecast to become a gale or storm then DVLPG GALE OR DVLPG STORM will be written near the L which denotes the low-pressure system.
Surface analyses are generated at 00Z, 06Z, 12Z, and 18Z (Zulu, or Greenwich Mean Time) each day for both the Atlantic and Pacific Oceans. The isobar spacing in each analysis is done at 4-mb intervals and labeled every 8 mb. The central pressure of highs and lows are labeled with three or four digits, placed near the appropriate L or H, and underlined. Surface charts for Atlantic and Pacific Oceans are issued in two parts, which overlap by 10 degrees of longitude, between 165W and 175W in the Pacific Ocean and between 40W and 50W in the Atlantic Ocean. Both parts show low and high-pressure location by drawing an arrow to the forecast, 24-hour position, labeled by an X for lows and a circle with X inside for highs.
Surface analyses are produced in two parts and issued in a larger format than other weather charts to allow sailors to use them as an actual plotting sheet and to make them easier to read. Surface charts and 500 mg charts read together enable sailors to track both the motion and intensity of surface lows, highs, and fronts.
Surface analysis charts show the atmosphere at just one moment in time though and thus should be compared with other sources of weather, such as satellite imagery and long-range predictions, to detect and verify changes after an analysis is completed.
To assist sailors in accessing marine weather charts, the Marine Prediction Center has created a homepage on the World Wide Web (www) where all weather charts produced and transmitted via weatherfax are now available.
Using a laptop computer and phone, either land line or cellular, all weather service charts are available on demand. The World Wide Web address for the Marine Prediction Center charts is: http://www.nws.noaa.gov/om/marine/home.htm