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Reading a windgram

A windgram aligns surface forcing with the atmosphere above one model grid cell. Each vertical slice is a forecast sounding; the horizontal axis connects those soundings through the day. Read the chart from the flight band outward: usable height, stability and moisture, wind through that height, then the surface fields driving the change.

Check lift, climb limits, wind, surface forcing, and pressure

1. When does usable lift reach launch? Follow the solid usable-lift line across the horizontal launch marker. A valley thermal forecast below launch does not establish a launch cycle.
2. What stops the climb? Compare usable lift with the dashed boundary-layer top and dotted cloud base. Their order identifies an energy limit, a stable cap, a moisture limit, or a sounding ceiling.
3. What occupies the flight band? Read wind from launch to usable-lift top. Cross-hatching puts near-saturated model layers inside that band; the 0 °C contour locates the freezing level.
4. Does the surface support the profile? Thermal velocity w* should grow with boundary-layer depth. Cloud and precipitation can interrupt the surface heat that feeds it.
5. Is the synoptic pattern changing? Pressure tendency can flag an approaching system or a building ridge. It cannot select a launch hour by itself.

Pressure, precipitation, cloud, and w* use separate scales

The four strips share the time axis but use separate numeric scales:

Each vertical slice of the main panel is one atmospheric column at the local time printed below it. The left axis gives metres above sea level and the right axis gives feet above sea level. The launch marker, winds, temperature contours, cloud layers, and derived heights therefore share one vertical coordinate. The coloured field interpolates lapse rate between the model’s pressure levels; it is not a stack of observed layers.

In the fixed forecast sample, pressure spans 101.0–101.4 kPa, no precipitation reaches the grid cell, cloud fraction ranges from 0% to 49%, and w* peaks at 2.8 m/s at 14:00. That peak identifies the hour with the greatest modelled convective energy. Height, moisture, and wind still decide what that energy produces.

Read pressure as a tendency

The pressure strip auto-scales to the day. A four-hectopascal change can fill the strip even though it is a modest synoptic tendency; the drawn amplitude is not a pressure-gradient map.

Falling pressure often accompanies an approaching low, trough, or front. Read it with thickening cloud, precipitation, and strengthening or backing winds before inferring deterioration. Rising pressure often follows a frontal passage or marks a building ridge. Clearing can follow, while subsidence under the ridge can also strengthen an inversion and trap valley haze or smoke. A flat line says that mean sea-level pressure changes little at that grid cell during the displayed hours.

Wind responds to the pressure gradient between places, not the pressure value at one launch. Two days can both read 101.2 kPa and carry different wind because the surrounding isobars differ. Treat the strip as a regime-change clue, then check forecast maps, winds aloft, and observations.

Read the stability scale against a rising parcel

The chart reports local environmental lapse rate in degrees Celsius per 1,000 ft. Its sign matters: negative values mean the environment cools with height; positive values mean temperature increases with height, which is an inversion.

An unsaturated rising parcel cools at about 3 °C per 1,000 ft. When the environment cools at least that quickly, the parcel can remain warmer than its surroundings and continue upward. The red and orange classes at or below −2.5 °C per 1,000 ft therefore mark the least resistance to dry convection. Between about −2.5 and −1.5, a dry parcel loses buoyancy more readily, while a saturated parcel may continue because condensation slows its cooling. The exact moist-adiabatic rate changes with temperature and moisture, so the chart’s fixed “conditional” bins are a reading aid rather than a parcel calculation. The −1.5 to −1.2 class sits near the moist-adiabatic range and remains condition-dependent. Values from −1.2 to 0 are stable. Positive grey classes are inversions: temperature warms with height and caps vertical exchange.

Read the shape before the colour name. A shallow unstable layer under a stable band supports low, short-lived mixing. A deep unstable column with positive w* supports a deeper boundary layer and can mix stronger winds from aloft toward launch. An unstable column ending in a stable lid can produce a defined top and organized climbs below it. An inversion crossing launch height must erode before surface-driven air can connect the valley with launch. A red column at night or under zero surface heat still has no modelled thermal engine; lapse rate describes susceptibility, while w* supplies forcing.

Separate cloud cover, cloud base, and cross-hatching

Cloud percentage is the model’s total column coverage. It does not say where the cloud sits. The dotted cloud-base line is a lifted-condensation-level estimate for a parcel starting at model terrain. It asks where that surface parcel would first saturate.

Cross-hatching asks a different question. The renderer marks interpolated regions where pressure-level dew-point depression is below 0.5 °C. Those layers are already near saturation in the model sounding. A hatched layer above a high surface cloud base can therefore represent mid-level or upper cloud that the surface parcel did not create. Hatching through the flight band means the model places cloud in the air pilots would occupy.

The three moisture signals can disagree without contradicting each other. Total cloud may be high while none of the sparse pressure levels lands inside a thin saturated layer. A low surface cloud base may cap usable lift while total coverage remains small. Hatching can appear with zero precipitation because cloud need not produce precipitation at the ground.

Precipitation is not automatically rain

The precipitation strip reports liquid-equivalent depth for each model step. An hourly model column contains an hourly total; a three-hourly model column contains a three-hour total. Comparing their bar heights as rates would exaggerate the three-hourly model.

The strip does not classify phase. The 0 °C contour shows the dry-bulb freezing level, and the hatched layers show where the sampled column is near saturation. Precipitation reaching a launch in air well above 0 °C is likely rain. A subfreezing column supports snow. A warm layer above a subfreezing launch can melt snow and permit refreezing, so a precise phase call needs the full temperature and wet-bulb profile; the freezing contour alone cannot supply it.

In the wet stable column, GDPS publishes 10.5 mm for a three-hour step and 80% cloud cover. The launch-level air is about 12 °C, the freezing level is near 3,490 m, and two sampled layers are cross-hatched. This is rain at launch with cloud through parts of the column, not “10.5 mm of rain everywhere below cloud base.” Thermal velocity is only 0.5 m/s and the usable-lift top remains below launch.

Boundary-layer top, usable-lift top, and cloud base answer different questions

• Boundary-layer top — dashed amber. A dry parcel lifted from the model surface becomes no warmer than the environment at this elevation. It describes thermodynamic depth.
• Usable-lift top — solid blue. canadarasp’s strongest-core profile falls to a 1 m/s sink threshold here, unless cloud base stops it first. It describes a modelled climb limit.
• Cloud base — dotted slate. A surface parcel reaches saturation here. When this line pins the solid line, moisture limits usable height.

Thin isotherms show the elevation of fixed temperatures. The emphasized 0 °C contour is the freezing level. Cross-hatching and cloud base remain separate because existing model cloud and a lifted surface parcel are separate pieces of evidence.

The boundary-layer and usable-lift lines can cross because they do not estimate the same quantity. Why usable lift can sit above the boundary layer explains the updraft coefficient and sink threshold behind the solid line.

Wind barbs: read the flight band

The shaft points toward the direction the wind comes from. A half tick counts 5 units, a full tick 10, and a pennant 50; a circle means calm. The unit is a chart convention, not part of the symbol. Aviation charts commonly count knots; canadarasp and this project count km/h. Read the printed unit before decoding the feathers.

Scan vertically at the hour of interest. Start near launch, continue through the usable-lift band, and read the wind just above its top. Speed increasing with height implies stronger drift and can bring faster air toward launch as mixing deepens. A directional change through the band marks shear and can place part of the climb in lee flow even when the launch-level barb looks acceptable. Then scan through time for the arrival of valley circulation, strengthening synoptic flow, or a reversal.

What the chart cannot decide

A windgram is a forecast at a grid cell, not an observation or a go/no-go tool. It cannot resolve rotor behind a spur, a launch cycle inside the grid cell, smoke suppressing surface heat, precipitation phase without a fuller thermodynamic profile, or an early valley-wind arrival. Compare it with current observations, aviation forecasts, radar or satellite, forecast maps, and local site knowledge.

The stability interpretation follows the FAA Glider Flying Handbook, and pressure tendency follows the National Weather Service observing handbook. The rendering lineage lives in the canadarasp windgram source and acrophobia.ca’s windgram component.