The GRAMET (GRaphical METeorological) chart shows a vertical cross-section of weather along your flight route. The X-axis is distance (NM), the Y-axis is altitude (Flight Level). Here's what each layer means:
Cloud Layers
| Symbol | Type | Description |
|---|
| Dark grey | Low clouds (ST/SC/CU) | Stratus, Stratocumulus, Cumulus. Surface to ~FL065. Affects visibility and VFR flight. |
| Medium grey | Mid clouds (AS/AC) | Altostratus, Altocumulus. FL065 to FL200. Can produce icing. |
| Light grey | High clouds (CI/CS/CC) | Cirrus, Cirrostratus, Cirrocumulus. Above FL200. Thin ice crystal clouds. |
| Rose/red | CB / TCU | Cumulonimbus / Towering Cumulus. Convective — severe turbulence, lightning, hail. AVOID. |
Cloud types are inferred from GFS coverage, thickness, and altitude data. Labels like "TCU 99%" show the dominant type and average coverage.
Icing
| Symbol | Severity | Description |
|---|
| Light cyan | Moderate icing (IENG 30-80) | Ice accumulation on aircraft surfaces. De-icing equipment required. Consider altitude change. |
| Dark cyan | Severe icing (IENG 80+) | Rapid ice accumulation. Immediate altitude change required. Dangerous to all aircraft. |
Icing ≠ Snow. Icing means ice is building up ON your aircraft (wings, propeller, pitot tubes). It occurs when you fly through clouds where temperature is between -14°C and 0°C. Snow is frozen precipitation falling through the air — a different phenomenon.
IENG Formula
The IENG (Icing ENGine) index is computed from temperature and cloud coverage. It peaks at -7°C. IENG ≥ 30 = moderate, ≥ 80 = severe. Convective icing adds buoyancy-driven accretion from CB/TCU clouds.
Precipitation
| Symbol | Type | Description |
|---|
| - - - - Blue dashed lines | Rain | Liquid precipitation falling from cloud base to the ground. Exists below the freezing level. |
| ✧ White/blue stars | Snow | Frozen precipitation falling within clouds. Exists above the freezing level up to the cloud top. |
Rain vs Snow vs Icing — what's the difference?
• Rain = liquid water drops falling below the 0°C level. Gets you wet, reduces visibility.
• Snow = frozen precipitation (ice crystals) falling above the 0°C level. Reduces visibility, accumulates on ground.
• Icing = ice forming ON your aircraft when flying through supercooled water droplets in clouds. This is a flight safety hazard.
They often overlap in the same altitude band but mean completely different things to a pilot.
Temperature
| Symbol | Description |
|---|
| ▬▬▬ Bold red line | Freezing level (0°C). The altitude where temperature crosses zero. Rain above this becomes snow. Critical for icing assessment. |
| - - - - Green dashed | Isotherms. Lines of equal temperature, every 10°C. Shows the thermal structure of the atmosphere. |
Wind
| Symbol | Description |
|---|
| Wind barbs | Wind speed and direction. The staff points into the wind (FROM direction). Half barb = 5 kt, full barb = 10 kt, pennant flag = 50 kt. |
| -24 | Temperature label (magenta). Temperature in °C at each wind barb position. |
| ○ / ● | Sky coverage (oktas). WMO circles at each barb: empty = clear, full = overcast. Shows how much sky is covered by cloud at that altitude. |
| - - - - Brown dashed | Isotachs. Lines of equal wind speed (50 kt, 75 kt, etc.). |
Turbulence
| Symbol | Severity | Description |
|---|
| Amber | Moderate (E 80-160) | Unsecured objects may move. Variations in airspeed. Passengers should be seated. |
| Dark brown | Severe (E ≥ 160) | Large airspeed variations. Aircraft may be momentarily out of control. Avoid. |
SLD (Supercooled Large Droplets)
Red overlay — Areas where supercooled large water droplets exist. Extremely dangerous icing conditions that can overwhelm de-icing systems. All three criteria must be met: temperature -20 to 0°C, liquid water present, predominantly liquid (not ice).
Other Elements
| Symbol | Description |
|---|
| Brown/green fill | Terrain. Darker = directly under route (centerline). Lighter = highest terrain within ±5 NM (buffer). Green = lowlands below FL020. |
| - - - FL085 | Cruise FL line. Your planned flight level. |
| ▬▬ Orange line | Sunrise curve. FL at which the sun rises (geometric horizon). |
| ▬▬ Purple line | Sunset curve. FL at which the sun sets. |
| ⚡ | Thunderstorm symbol. Based on Lifted Index. Larger = more severe. |
Data Source
All weather data comes from the NOAA GFS (Global Forecast System) at 0.25° resolution. Updated 4 times daily (00Z, 06Z, 12Z, 18Z). Forecast coverage up to 16 days (209 forecast hours). Terrain from ETOPO 2022 at 60 arc-second resolution.
Disclaimer: This tool is for flight planning reference only. Always cross-check with official METARs, TAFs, SIGMETs, and AIRMETs before flight. Not approved for operational dispatch.