Flying in the Blue

I got some positive feedback from my article on flying under overcast areas so I thought a good progression would be a similar article on flying in blue areas. There is not a lot written on the subject but it is something we occasionally must do - either to get home across a large cloudless area or to reach a good cloud field or street on course. And, on some very dry days, we must fly without clouds just to fly XC at all. The common wisdom is to avoid blue areas and stay connected to the clouds.  But sometimes you just can’t do that. And, there are places in the world where pilots regularly fly 500 km (and bigger) tasks entirely in the blue. So here are some ideas and tricks I’ve learned on “What to do when it’s blue?”.

Why is it blue?

First, try to figure out why it’s gone blue in a particular area. This can be very helpful in making the decision to press on into the blue or to take a big diversion to go around it.  If there are clouds before and after the blue area then there is likely some characteristic of the ground that’s contributing to the absence of cloud. It might be the residual effect of a recent localized rainstorm that prevents the same surface heating that you have previously experienced on that flight (look for puddles and standing water in fields), or it may be a cooler swampy area, or an area where soil conditions change, or a change in vegetation that is preventing high surface heating. Sometimes you will see blue areas that correspond with non-agricultural areas - the farmers know which soils are worth planting and which are not.   There will still be dry thermals in most of these blue areas, but they will be weaker and not extend quite as high (which is why they are not forming clouds).

The time of day is important too.  It’s much dicier to press into a blue area early in the day. There is a strong possibility that the ground area has not yet heated enough to trigger thermals worth working.  A blue hole later in the day is more likely to be the result of a drying air mass in the blue area with a higher dew point that the thermals can’t quite reach. I have more confidence crossing blue areas in the afternoon than early in the day.

To blue? Or not to blue?

Other factors also impact the decision to fly into the blue area.  If the previous cloud field area you are leaving had a high cloud base you can assume that the thermals in the blue area will also be reasonably high (although not reaching the dew point). The wind is also a factor because the same boundary/shear ratio thermal breakup issues that impact cloud forming thermals apply even more so to the weaker thermals you are likely to find in the blue. The worst-case situation is leaving low cloud bases for a blue area in a strong wind - there is a lot working against you. And remember that if you are navigating to a cloud field across the blue (and not to a fixed land point) your target cloud or cloud field is moving with the wind.

As you approach the blue area you want to climb as high as possible under the clouds before setting off into the blue - mindful of the old adage “the last cloud never works.” Both your speed to fly and your height band strategies should be adjusted toward the conservative end of the spectrum. You want to reach the far cloud field as high as possible - or at least cover as much terrain in the blue as possible.  This means flying a bit slower than normal XC cruise speed.  Similarly, you will want to use all available lift (no matter how weak) at least until you get a good sense of how consistent the blue thermals are working.  Both dry and cloud forming thermals have a larger diameter as they get higher so stay at the very top of your height band where you are more likely to encounter the edge of a thermal. Remain conservative on both height band and speed until you have a good reason to change.  Few pilots have won contests by being brave and fast in the blue. That’s a good way to meet farmers.