How to read lapse rate diagram

Also known as SkewT logP diagram, lapse rate diagram is used to display air temperature and dew point temperature in function of pressure altitude.

What we can find out from this diagram:

  • thermals (if they kick off, how high they get)
  • convective cloud base and cloud top altitude
  • top and bottom altitude of any other clouds

Sources of this diagram:
http://www.ready.noaa.gov/ready/cmet.html

Glider Forecast:

What the above diagram contains

Fixed lines (not changing in function of the daily forecast):

Grey horizontal lines: pressure altitude (in forecasting, altitude is not expressed in meters or feet above sea level, but air pressure). The altitude (m or ft) for a given pressure value varies in function of the daily air pressure, but when hovering (desktop) or tapping (mobile) on the chart, our diagram will tell you the altitude in meters.

Skewed (sloped) red lines: temperature values. The temperature axis is sloped 45 degrees to the left, so each of these thin red lines on the chart marks a given temperature value (0°C in the figure below.)

Hint: The diagram has two axis: the vertical pressure axis (=> horizontal isobars) and the temperature axis sloped 45° to the left (=> isotherms sloped 45° to the right). If all this seems too much to understand, don’t worry just read on. You’ll get what’s important below 😉

  • Green horizontal line (LOC) (if any): altitude of flying site
  • Orange horizontal line (GND): ground level

The above lines are fixed, so they don’t vary in function of the daily forecast.

Lines changing in function of daily forecast:

  • Thick red line: air temperature at different altitudes
  • Thick blue line: dew point temperature at different altitudes

These are the values forecasted for the day for different altitudes

If you hover the mouse on our diagram or tap on it in the mobile app (coming soon) you can get the air temp, dew temp and lapse rate values for a given altitude. In the above example at 1431 m ASL we have 6.4°C air temp, 5.4°C dew temp and 0.9°C/100m lapse rate. What a day!

Hint: when the red and blue lines are close together anywhere in the graph, we have condensation (clouds), because air temperature is equal to dew point temperature. So the diagram shows clouds at all altitude levels, not just convective clouds.

Thermal prediction lines:

This is the fun part: how the thermal is modelled in the diagram.

First some background information: The thermal bubble starts from the ground level. Its temperature is usually around 2°C higher than the temperature of the surrounding air (trigger temperature). The thermal bubble has a certain (constant) dewpoint temperature. As the thermal rises, the pressure around it gets lower. So the thermal bubble expands and it cools down. The cooling rate is 1°C/100m or 5 1/2°F per 1,000feet of lift (dry adiabatic lapse rate).

As the bubble rises and cools down, eventually it reaches its dewpoint temperature and it condenses to a cloud. The condensed (saturated) thermal (cloud) keeps rising further but it cools at a different rate: 0.5 ˚C/100m (moist adiabatic lapse rate).

The thermal is shown in the diagram by a tent-like structure with the dry adiabat (thermal cooling rate) in red, the constant dew temp in blue and the moist adiabat (cloud cooling rate) grey line starting from the point of condensation (top of the tent).

Now lets put everything together (merge the thermal and the surrounding air) and read the chart.

Reading the chart

  • The dry adiabat (thermal cooling rate) is to the right of the air temperature curve so the thermal bubble is warmer than the surrounding air => the thermal will kick off.
  • The dry adiabat (thermal cooling) meets the dew temp constant (blue line) before crossing the air temperature curve so the thermal will reach condensation level where the red and blue lines meet (top of tent) (here cloud base is at 1162 m at a temperature of 8.8°C).

  • The moist adiabat (cloud cooling rate) will continue to rise (the cloud rises) until it reaches the temperature of the surrounding air (red curve). The point where the grey curve intersects with the red curve is the cloud top altitude (1872m ASL).

Other cases (when the day is less perfect)

  • When the dry adiabat (thermal cooling rate) is to the left of the air temperature (red curve), the temp at the ground is lower than the average temp of the air (ground cooling effect) so thermals will not kick off (morning, evening, night hours):

  • When the dry adiabat (thermal cooling rate) crosses the air temp curve before reaching the dew point temp, we have blue thermals (thermals will stop at 916 m ASL below)

  • If the thermal kicks off and a cloud is formed and the moist adiabat (cloud cooling rate) does not cross the air temperature curve or it crosses very high, we have overdevelopment.