Wind flow in tropical regions-20N to 20S-is shown on weather charts using a symbol called streamlines. Streamlines are continuous lines showing wind direction resulting from trade wind convergence near the equator-also known as the Intertropical Convergence Zone (ITCZ).
Flow in tropical areas differs from that found in higher latitudes, where winds are a result of the exchange of energy that takes place when warm and cold air masses collide and then mix with each other.
Outside the tropics winds blow parallel to isobar lines, turning slightly inward around low-pressure systems (app. 20 degrees) and outward around high-pressure systems. Wind speed is related to spacing of these isobar lines, with tight lines indicating significant pressure differences (gradients) and strong winds.
Since tropical regions do not generally have distinctive air masses and thus no mixing of warm and cold air masses, isobar lines are not appropriate for representing tropical wind flow so Meteorologists use instead streamlines.
As mentioned above, tropical winds result from advection (horizontal movement), convergence and subsequent air rise into the upper atmosphere brought about by an abundance of heat being continually pumped into tropical regions by our Sun.
This horizontal convergence and ascending air results in cumulus and cumulonimbus cloud development, which is tracked by weather satellites. These tropical clouds have individual diameters of one to fifty miles and when clustered may have diameters from fifty to 500 miles. Cloud cells and clusters, however, are not consistent or continuous in their development, a result of uneven equatorial heating due to seasonal, geographic and climatological variables.
There are, therefore, noticeable fluctuations in tropical winds and weather, with transitions from calm to squalls, rain and showers, followed by clear skies and Sun often occurring in short periods of time.
Twice daily, at 12Z (0700 EST) and 18Z (1300 EST), the National Weather Services Tropical Prediction Center, located at the National Hurricane Center in Miami FL, issues streamline analysis charts for Pacific and Atlantic Oceans. Streamlines are bold, curved lines plotted parallel to surface airflow, using arrows to show wind direction from actual, or derived data received from ships and satellites.
In addition these charts show expected movement of surface synoptic (large-scale) features; highs, lows, fronts, troughs, and ridges. Internationally recognized symbols are used to designate tropical systems:
Circle with dot inside represents a tropical disturbance Circle with an X inside represents a tropical depression Circle with center open and spiraling arms represents a tropical storm Circle with center filled and spiraling arms represents hurricane
Two types of wind data are plotted on a streamline analysis; ship reports and boundary layer winds (winds with 2000 feet of earths surface). Boundary layer winds are determined from model forecasts, and are included on charts to indicate wind flow in areas with sparse ship report data. Regions of significant thunderstorm or rain activity are enclosed with scalloped lines-resembling a hand drawn cloud, and are labeled according to coverage and intensity:
SCT: scattered showers and rain ISOLD: isolated showers and rain MOD: moderate intensity STG: strong intensity
Tropical waves and cyclones are noted with their name and latest advisory position. Cyclones are designated using a 6-digit code, which provides general information about that feature.
For example: the code 150806 placed adjacent to a cyclone would indicate the following: 15 is the day of the month when this system became developed and consistent, 08 is 8th tropical storm of that season, and 06 is number of days this system has been under analysis.
During Hurricane season (June to November) streamline analysis in an important tool in detecting and tracking tropical waves, areas of low pressure which are breeding grounds for development of full-fledged hurricanes.
Mariners heading into tropical regions should become familiar with tropical surface analysis charts, since, similar to using a 500 Mb upper air, wind-wave, or satellite image chart, a tropical analysis streamline chart is a useful tool to use in route planning and weather strategy.