Thanks for the info, cranky. I've been using w.o.w.'s for about 10 years with great success. The weather around here can be so wacky in April: 70-80F during the day, 25-35F at night. But the winds up to 50mph can be the worst, drying out or knocking over delicate seedlings.
Many gardeners around here, including me, use white plastic or fabric to cover the soil in containers - the sun at 7000F above sea level is so powerful, you need to reflect the heat rather than absorb it into the soil. Right now, the air temp is 69F, it is mostly cloudy, and my IR laser gun is reading surface temp of 93F on the ground.