Aviation weather charts are essential tools that pilots and flight dispatchers use to ensure safety and efficiency during flight planning. These specialized meteorological forecasts provide a standardized, visual summary of atmospheric conditions that could affect aircraft operations. The charts employ a universal system of symbols and colors to quickly communicate complex weather information, allowing aviation professionals to make informed decisions about routing and altitude.
The Purpose of High-Level Significant Weather Charts
The High-Level Significant Weather, or SIGWX High, chart provides a forecast of weather phenomena specifically relevant to aircraft operating at high cruising altitudes. This chart typically covers the airspace from Flight Level 250 (FL250), which is 25,000 feet, up to Flight Level 630 (FL630). Its primary function is to map out atmospheric hazards that are significant for jet aircraft traveling long distances in the upper troposphere and lower stratosphere.
The phenomena depicted include the location and intensity of jet streams, areas of clear-air turbulence, the expected height of the tropopause, cumulonimbus clouds, volcanic ash clouds, and tropical cyclones. By providing a snapshot of these conditions at a specified time, the SIGWX High chart helps flight crews avoid weather that could cause discomfort or damage.
The Indication of the Bold Green Line
On a High-Level SIGWX chart, the bold green line is the standardized symbol used to identify the axis of a jet stream. A jet stream is a fast-flowing, narrow air current in the atmosphere that typically moves from west to east. The bold green line is drawn only where the wind speed along this core is forecast to be 80 knots (approximately 92 miles per hour) or greater.
This line is important for flight planning because its location allows pilots to use powerful tailwinds or avoid strong head-winds. The line includes small arrowheads indicating the direction of the wind flow. Additionally, wind barbs (pennants and feathers) are placed along the green line to denote the actual wind speed in knots at various points along the jet stream axis.
While the bold green line itself marks the jet stream, the region immediately surrounding this core is often where severe weather, particularly clear-air turbulence (CAT), is most likely to form. The strong horizontal and vertical wind shear created by the jet stream’s rapid flow causes the air to become highly turbulent. Therefore, the green line acts as a strong indicator of a high-risk area, even though it is not the turbulence boundary.
Interpreting Turbulence Severity and Vertical Extent
Forecast areas of moderate or severe turbulence are not marked by the green line but are enclosed by bold, dashed lines (yellow or black). This boundary outlines the horizontal extent of the forecast hazard, which can be associated with mountain waves or wind shear near the jet stream.
Within the enclosed dashed area, specific notation is used to convey the severity and the vertical limits of the turbulence. The intensity is indicated by simple abbreviations, such as “MOD” for moderate or “SEV” for severe. The vertical extent of the turbulent layer is provided by a pair of three-digit numbers separated by a slant, representing the top and base flight levels in hundreds of feet.
For example, a notation reading “FL350/280” indicates that the turbulence is forecast to extend from Flight Level 350 (35,000 feet) down to Flight Level 280 (28,000 feet). If the base extends below the chart’s lower limit of FL250, the base altitude is encoded with “XXX,” such as in “310/XXX,” indicating the hazard continues below 25,000 feet. This detailed information helps pilots choose an alternative flight path above, below, or around the hazardous zone.