We bought a package for a week in Peru (which we will probably extend to at least 10 days) to visit Lima, Cusco, and Machu Picchu), and are trying to decide when the climate there would be best. We are thinking of taking an airplane excursion to fly over the Nasca lines, so the weather for the flight is also a consideration. I'd be grateful for any advice.

The longer you wait during that period, the greater the chance for rain, at least in Cusco and MP. I'd go in September. August is still pretty busy.

Thanks--I was leaning toward September but found some of the weather info confusing.
This is just what I needed!

Regarding the differences between August
(crowded) and September.....I have a big birthday in early August and was hoping to spend it at Machu Picchu. I would be spending several nights in the Sacred Valley(Ollantaytambo/ Hotel Pakaritampu), one night at the ruins of Machu Picchu( M.P. Sanctuary Lodge) and then three nights in Cusco(Monasterio). Will the experience in August be appreciably less satisfactory due to the crowds.....and likewise, would the experience be appreciably better in September?

First, whenever you can go, go. The trip will be wonderful no matter when you take it, and being at MP seems a pretty disticntive way to spend a milestone birthday. But the June-August period sees a lot of visitors. If you're staying at the Sanctuary Lodge, you will have access to the ruins after all the day trippers have gone back down the hill, or the next morning before the next group begins to arrive. Just realize that you won't have the place to yourself.

If you have your heart set on specific lodgings during that time, I don't think it's too soon to look into making reservations.

How early is "early August"? Peru's Independence Day is observed July 28-29. I wouldn't even attempt to travel at the very end of July. I'd try to arrive after that time.