Air Dryness is the most important factor for great visibility conditions, a necessary condition, but not enough to guarantee perfect results.
For our experience, values lower than 40% RH (Relative Humidity) start to produce good results, with the best pictures taken with 25% RH or less:
|50%||Poor but visible|
|40%||Good with processing|
|30%||Naked eye visibility|
|0%||Excellent but impossible|
The humidity we use for calculations is Relative to air conditions, so warmer air will usualy have lower RH values, leading to best visibility than colder air.
For this reason, warmer-than-usual air conditions is a common indicator of good visibility, as it happens with warm fronts or “heat waves” during winter time.
Calculating RH for visibility conditions is not easy, as line of sight crosses different layers in the atmosphere at different heights, as we saw in this chapter of this guide.
Several webpages on internet can provide this information, as for example: