Weather Charts

The last lesson gave us the eyes: satellite, radar, the reporting aircraft. This lesson is where all of it lands on paper: the charts a crew actually opens on the morning of a flight, and the gridded forecast world that stands behind every one of them.

The chart family

Three charts carry most of the pre-flight weather story:

The significant weather chart — forecasts the hazards: where the cumulonimbus, turbulence, icing and jets will be.
The surface chart — shows the pressure pattern: highs, lows, fronts and the wind they drive.
The upper-air charts — give wind and temperature at cruising levels.

The surface and upper-air charts each come in two flavours: an analysis drawn from observations (the atmosphere as it was) and a prognosis, the forecast chart, the atmosphere as it is expected to be. The significant weather chart has no analysis flavour at all: it is by nature a forecast only.

The significant weather chart

Significant weather charts come in three altitude bands:

high level (SWH) — FL 250 to 630;
medium level (SWM) — FL 100 to 250;
low level — the airspace below, the low-level versions usually national products for general aviation.

Each is valid at a fixed time, not over a period: you interpolate between consecutive charts for times in between. The symbology is compact and worth knowing cold: a scalloped line encloses an area of significant cloud or weather; inside it, cumulonimbus entries give amount and character (ISOL for isolated, OCNL for occasional, FRQ for frequent, EMBD for embedded in other cloud), with the vertical extent written as top over base in flight levels, XXX standing for a level outside the chart's coverage. A CB entry is deliberately economical: CB implies its own hazards (thunderstorm, moderate or severe icing and turbulence, and hail), so these are not drawn separately inside a CB area. Elsewhere the chart marks:

moderate and severe turbulence areas — dashed enclosures and the caret-shaped turbulence symbol;
icing areas and mountain waves;
the jet axis — a bold arrow carrying its flight level, maximum speed and vertical extent;
spot tropopause levels — in small boxes, with H and L marking the local maxima and minima;
fronts — with their speed of movement;
named symbols for volcanic eruptions and tropical cyclones.

Reading a route across the SWC

The exam's favourite use of the chart is a route question: given a track and a flight level, say what the flight will meet. The method is mechanical. Follow the route across the chart; at every scalloped or dashed area it crosses, compare your flight level with the printed vertical extent: the area concerns you only if your level lies between base and top. An EMBD CB area with tops FL 450 concerns a flight at FL 340 very much; a clear-air-turbulence area lying between FL 390 and FL 450 does not. Check where the route crosses the jet arrow and at what angle, remembering from the jet lesson that the cold side and the sloping boundary below the core hold the clear-air turbulence, and read the boxed tropopause levels to judge how close the cruise sits beneath the lid.

The surface chart

The surface chart is the isobar map from the pressure lessons, drawn operationally. On it you should recognise at sight:

closed isobars around highs and lows;

a ridge — an elongated tongue of high pressure with anticyclonic curvature;

a trough — an elongated extension of low pressure, often frontal;

a col — the slack saddle between two highs and two lows where gradients die and fog lingers.

Fronts wear their standard symbols: semicircles on the warm front's leading side, triangles on the cold front's, both together for an occlusion, and symbols alternating on opposite sides for a quasi-stationary front. Around a mid-latitude frontal low the chart splits into the three weather regimes of the frontal lessons: the frontal side ahead of the warm front, the warm sector between the fronts, and the rear side behind the cold front. Read the low's anatomy off the chart and the module 5 weather sequence follows.

Wind from the isobars

The surface chart also hands you the wind. Wind blows nearly parallel to the isobars with low pressure on the left in the northern hemisphere (Buys Ballot, as ever), and its strength comes from the isobar spacing: tight isobars, strong wind, read against the geostrophic scale printed on the chart. Two corrections from the wind lessons still apply. Near the surface, friction backs the wind across the isobars towards low pressure and slows it: about 30 degrees and half the geostrophic speed over land, about 10 degrees and seventy per cent over the sea. And the same spacing means a stronger geostrophic wind at lower latitude, because the Coriolis parameter shrinks towards the equator.

Upper-air charts

Upper-air charts are drawn on pressure surfaces, not at fixed altitudes. A constant-pressure chart is a chart of one pressure level (850, 700, 500, 300, 250 or 200 hPa, corresponding roughly to FL 050, 100, 180, 300, 340 and 390), showing the varying height of that surface. Its lines have their own names, all built the same way:

An isohypse, or contour line — joins points of equal geopotential height of the pressure surface: the isobaric surfaces of module 1, drawn as the contour lines the wind lesson first named. Contours aloft play exactly the role isobars play at the surface: wind parallel to the contours, low heights on the left in the northern hemisphere, spacing giving speed.

An isotherm — joins points of equal temperature.

An isotach — joins points of equal wind speed, and is the line that draws the jet cores on maximum-wind charts.

The forecast wind and temperature chart

For flight planning the workhorse is the forecast upper-wind and temperature chart, issued for a set of flight levels from the world area forecast system's gridded data. At each grid point an arrow flies with the wind, its tail feathered with the speed: a half feather is 5 kt, a full feather 10 kt, a pennant 50 kt — so a pennant, a feather and a half feather read 65 kt. Beside each arrow sits the spot temperature, and by convention at cruising levels temperatures are negative unless explicitly prefixed PS for plus.

A worked interpolation

Route values rarely sit on a grid point, so the exam asks you to interpolate: a distance-weighted average, and for a point midway, the plain mean. Take the figure's leg at FL 340. Wind direction: midway between 270 and 300 degrees is 285 degrees (watching, on other problems, for the wrap through north, where 350 and 010 average to 360, not 180). Speed: midway between 65 and 40 kt is 52.5, call it 53 kt. Temperature: midway between minus 45 and minus 51 is minus 48 degrees Celsius. The deviation follows from the ISA lesson: standard temperature is 15 minus 2 degrees per thousand feet, so at FL 340 it is 15 minus 68, which is minus 53; and an outside-air temperature of minus 48 against a standard of minus 53 is ISA plus 5.

The gridded world behind the charts

Every chart in this lesson is a picture drawn from the same underlying product. Numerical weather prediction computes the forecast on a three-dimensional grid: latitude and longitude horizontally, height or pressure levels vertically. The world area forecast centres prepare global sets of these gridded forecasts for flight planning: upper wind, temperature and humidity. The same centres also produce gridded datasets for the flight level and temperature of the tropopause, the direction and speed of the maximum wind, and cumulonimbus cloud, icing and turbulence: the raw material from which the significant weather chart's scalloped areas and jet arrows are drawn.

From grid to cockpit

Increasingly the grid reaches the crew directly, not only through the drawn chart. The CB and turbulence datasets can be rendered as flight-hazard visualisations: hazard volumes painted along the planned route on a planning tool or electronic flight bag. And information-processing systems can merge the gridded forecasts with data relayed from aircraft (the routine downlinks and pilot reports of the previous lesson): a turbulence forecast overlaid with the reports of crews who just flew through it is worth more than either alone, and that merged picture is the direction meteorological briefing is moving.