What we are looking for - the perfect day.
1. Sufficient temperature differential to cause good convection (10-16C)
a. Cold front having passed through recently.
b. Solar heating of the earth surface a long enough day with sun high enough to get perpendicular(ish) heating rays.
c. Source Met Office/BBC
2. Not too much water vapour to over develop
a. No layers of warm trapped moist air
b. Cold sector
c. Overdevelopment cuts off the sun and reduces solar heating. No thermals.
d. Source Soundings, satellite pictures
3. Enough water vapour to develop cloud
a. A function of dew point against daytime temperatures. Cold air holds little moisture but raised to a high enough level will eventually cool to the dew point given a sufficiently unstable airmass.
b. Source Soundings.
4. Enough wind to trigger thermals.
a. A warm air pocket will form at the surface and will remain relatively stagnant there unless something triggers it to rise. The slightest of breezes to take the warm air over a cooler area (lake) or pushed onto a hill will cause it to rise and then render the temperature differential sufficient to generate buoyancy.
b. Source Soundings, Metars, Tafs, Synoptic Charts.
5. Not too much wind to slow us down.
a. Fly at 60kph average into a 30 knot headwind and we get nowhere. We do not make up all lost time with a down wind leg on a closed circuit task!! Streeting can avoid turning and then increase cross country speed significantly.
b. Strong wind coupled with suitable inversions cause wave. This can be interesting, fun and rewarding but it can ruin an otherwise good cross country day.
c. Sources as above.
6. Anticyclonic curvature in the isobars.
a. Helps to keep the instability within acceptable limits. This stops convection going too much into the freezing level and causing precipitation and spreadout.
b. Continues to pile air into the system and generates and inversion to prevent precipitation. As time goes by this lowers the level of convection and reduces thermal heights and may prevent convection to the level needed for cloud to form and cause a blue day.
c. Source Synoptic Charts