Observatory weather charts are posted on this page for this month and last. Two 12-hour charts span one day. Each 12-hour chart has an upper graph plotting the difference between sky temperature and ground temperature. More negative differences are clearer; more positive differences are for increasing cloudiness. The plot is also color-coded for ease of interpretation: white is mostly clear; yellow is moving into cloudy conditions; red is very cloudy; and blue indicates rain or snow. The lower chart is wind speed. Speeds below 7.5 mph are OK for observing; speeds between 7.5 and 20 mph are rated as windy; and above 20 mph, very windy.