Synoptic charts provide a summary of weather conditions over large areas, covering entire continents or ocean basins. Surface synoptic weather charts map mean sea level pressure using contours called isobars. Isobars connect areas of equal pressure, and their distribution reveals the location of high and low pressure systems provides an indication of
wind speed, and direction. Isobars that are close together mean that horizontal pressure gradient is changing rapidly, indicating strong wind. Conversely, well-spaced isobars reflect fairly stable horizontal pressure gradients and light winds. Wind travels parallel to isobars, and flows clockwise around high pressure and counter clockwise around low pressure in the northern hemisphere (these directions are reversed in the southern hemisphere).
Unlike weather forecasts from model data, synoptic charts represent a forecast that has been interpreted by a meteorologist who has referred to an extensive number of observational data sources. For example, radar, satellite imagery, radiosondes, weather stations, weather buoys, and ship and aircraft observations are common data sources used by meteorologists which are then assimilated to create a synoptic chart. Using these data sources, forecasters can identify the location of fronts, waves, ridges, and troughs. These features reflect the likely position of heavy rainfall and convection, or settled weather conditions.
Surface synoptic charts are powerful tools because they convey so much information on a single low resolution chart. Offshore sailors, or those in remote locations where access to data is limited, commonly rely on grib files (such as Predict Wind) for their weather. While grib files are a valuable resource, they are visualizations of model data that has not been interpreted by a forecaster. As such, the position of features such as fronts, troughs, waves, and ridges will be difficult or impossible to discern from a grib file. Surface charts are easily accessible when data is limited and can be requested and emailed via satellite when on passage. They are a great supplement to grib files when passage making
Upper level charts
Meteorologists also produce and refer to charts of the upper levels of the atmosphere. Upper level charts, also known as isobaric charts, differ from surface charts because they represent a pressure level in the atmosphere, and their contours reflect the height of that pressure level above the surface. For a given pressure level, a low height contour represents an area of low pressure (a trough or depression), and a high height contour represents an area of high pressure (a ridge). Similar to surface charts, the upper level flow is parallel to the height contours and the closer the contours are to each other, the stronger the wind. Due to the rotation of the Earth, upper level flow almost always flows from west to east.
Upper level charts generally have less complexity compared to surface level charts, and this complexity decreases with height. While surface charts contain many closed features, such as circular high and low pressure systems, upper level charts tend to have wavy contours that reflect ridges and troughs in the upper atmosphere. Because the tropopause is higher at the equator than at the poles, waves that point towards the equator are troughs, and those that point towards the pols are ridges.
Upper level charts provide information on general weather conditions and can also indicate the expected trajectory of surface level weather features. For example, upper level troughs indicate cooler conditions and ridges indicate warmer conditions. In terms of steering surface weather, the location of ridges and troughs and the distance between height contours will determine the expected trajectory of low pressure systems. If the amplitude of upper level ridges and troughs is large, they can create blocks which will stall weather features at the surface resulting in days, and sometimes even weeks of consistent weather.
Upper level charts at different heights provide information on various atmospheric features. For example, 200 and 300 hPa charts can provide a good summary of jet stream features, 500 hPa is the best level for understanding steering ridges and troughs in the middle atmosphere, and 850 hPa charts can be used to more easily determine the location of frontal systems and features such as low level jets.