In a previous post about collecting data, we mentioned how sunlight can make ozone damage easier to see as the light shines through the leaf. If you carefully turn the leaf vertically, more light tends to shine through, making chlorosis and stippling more visible.
Another reason to wait for a sunny day to collect data is because if rain starts to fall, the papers we collect data on begin to get wet and illegible. A few times, we were collecting data in the garden when it began to drizzle or even pour rain, and had to leave before we had data for all the leaves we were tracking. Then, when we looked at the data at the end of last season, there were missing points.
Collecting data earlier in the day can also be helpful. One time we came in the evening to collect data, and the sun became too low in the sky to clearly see the ozone damage before we were finished. Also, it can get super hot and uncomfortable in the afternoon. Collecting data, or other garden work, is easier on a cool morning than going outside during a summer heat wave in the afternoon.
While collecting data on the same day of the week is good for consistency and can be practical when figuring out who will work in the garden on a particular day, if it starts to rain or becomes cloudy, it is better to collect data a day late than not see some damage due to lack of sunlight.