Weather charts

Significant Weather Charts (SIGWX)

Significant Weather Charts (SIGWX) provide a forecast of significant meteorological phenomena for aviation.

Front Symbols

• Warm Front: Red semicircles.
• Cold Front: Blue triangles.
• Occluded Front: Alternating semicircles and triangles (purple).
  • Cold Occlusion: The occlusion line follows the cold front line.
• Stationary Front: Semicircles and triangles on opposite sides of the line.

Significant Phenomena

• Fog: Horizontal lines symbol.
• Smoke: Specific symbol.
• Haze: Infinity symbol (∞).
• Tropical Cyclone: Spiral symbol with two arms.
• Volcano: Trapezoid symbol with smoke/ash.

Jet Streams

Represented as thick lines with arrows.

• Core: Wind speed at the core is indicated with barbs/flags (e.g., 120 kt).
• Altitude: Flight level of the core is indicated (e.g., FL370).
• Depth: Lower and upper levels between which the wind exceeds 80 kt are indicated (e.g., FL230 - FL440).
• Wind Change: Dashed lines parallel to the Jet indicate areas of Clear Air Turbulence (CAT).

Tropopause

• Height: Indicated in hundreds of feet inside rectangles (e.g., 350 = FL350).
• High/Low Centers: Enclosed in polygons with an "H" (High) or "L" (Low).
  • The tropopause is higher at the equator and lower at the poles.
  • In a Jet Stream, the tropopause is lower on the cold (polar) side and higher on the warm side.

Clouds and Associated Hazards

On SIGWX charts, the mention of CB (Cumulonimbus) automatically implies the presence of:

• Thunderstorms (TS).
• Hail (GR).
• Moderate or Severe Turbulence.
• Moderate or Severe Icing.

Coverage:

• ISOL (Isolated): < 50% of the area (individual).
• OCNL (Occasional): 50-75% of the area (well separated).
• FRQ (Frequent): > 75% of the area (little or no separation).
• EMBD (Embedded): Within other cloud layers.

Numerical Weather Prediction (NWP)

3D Grid System

Meteorological models divide the atmosphere into a three-dimensional grid.

• Horizontal Data: Latitude and Longitude.
• Vertical Data: Height or Pressure.
• Time is not used as a grid axis itself, but time steps are calculated.

WAFC (World Area Forecast Centre)

There are two centres (London and Washington) designated to prepare and issue global forecasts.

• They generate gridded forecasts of wind, temperature, humidity, tropopause, CB, icing, and turbulence.
• These data are integrated into flight planning systems.

Pilot Reports

Aircraft data (AIREP/PIREP) can be merged into processing systems to improve situational awareness and refine future prediction models.

Surface Charts

Show pressure at sea level using isobars.

• Low Pressure (Cyclone/Depression): Air rotates counter-clockwise (Northern Hemisphere). Winds converge and ascend -> Bad weather.
• High Pressure (Anticyclone): Air rotates clockwise (Northern Hemisphere). Winds diverge and descend (subsidence) -> Good weather, but possible fog/haze due to inversion.
• Col: Area between two highs and two lows. Calm winds. In summer conducive to air mass thunderstorms; in winter to fog.
• Ridge: Extension of high pressure. Good weather.
• Trough: Extension of low pressure. Bad weather.

Upper Wind and Temperature Charts

Show wind direction and intensity and temperature at specific flight levels.

Interpretation

• Wind Barbs:
  • Short line: 5 kt.
  • Long line: 10 kt.
  • Triangle/Flag: 50 kt.
  • Direction is from where the wind blows (compare with local meridians for True North).
• Temperature: Indicated next to the station or on the grid.
  • ISA Deviation Calculation:
    1. Calculate Standard ISA Temp: $15 - (2 \times \text{Altitude in thousands of feet})$.
    2. Compare with Actual Temp (OAT).
    3. Example at FL180 with OAT -30°C:
       • ISA FL180 = $15 - (2 \times 18) = 15 - 36 = -21^\circ\text{C}$.
       • Difference: $-30$ is 9 degrees colder than $-21$.
       • Deviation: ISA -9°C.