December 18, 2010

Calcuating Trigger Time

The last post covered the process of calculate trigger temperature and the mechanics of the atmosphere that are underway as the day heats up and thermals start. Today's post will cover how to estimate what time the surface temperature will reach trigger temperature and will also cover a few other tips related to trigger temperature.

To find the estimated trigger time first recall this plot from the last post:
It turns out that the heat required to eliminate the ground inversion is proportional to the area of the triangle formed by the temperature profile (shown above as the area shaded with dashed lines). So, first calculate the area under the temperature profile, this can be done by copying the plot onto graph paper and counting the squares or by simple geometry; the area of a triangle is 1/2 * base * height. For plots of this type the y axis might be in milibars, you will want to convert milibars into feet. The x axis is temperature. That means the area calculation you have done will result in units of (degree*ft). Which is handy because the chart below, which shows the available heating from the sun just happens to be in degree*ft!

This chart is for a latitude of 45deg, if your latitude is greater/less move/down up 1/2 hour for every 5deg difference in latitude. This chart allows us to predict when the air will have been heated enough to over power the ground inversion. Note that the above chart is for clear skies, cirrus, fog, smoke or any other sun blocker can reduce the about of energy reaching the ground and should be accounted for.

The other method, which is much more user friendly is to rely on your local weather station predictions for the temperature as throughout the day. You can always supplement this information with temperature readings of you own. I glued a thermometer to my wing wheel so when i'm waiting to launch I can monitor the rise in temperature and know if it is rising faster or slower than predicted and adjust my launch time accordingly.

All of these methods help you become more aware of how the soaring day starts and how to position yourself to start your flight on time and get the most of the soaring day for those long tasks. Until next time...

Keep soaring,

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