7500 Foot Desert Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 7500 Foot Desert Observatory clear sky chart from 2005-11-06 to 2025-03-27.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	2 3 1 4 3 4 5% 4 7% 9% 58%	112900
	Night	2 4 2 5% 3 4 4 4 6% 8% 58%	115286
Transparency	Day	11% 44% 7% 16% 15% 6%	112653
	Night	13% 43% 7% 16% 16% 5%	115123
Seeing	Day	6% 11% 24% 28% 21% 9%	15757
	Night	4 6% 28% 41% 18% 2	25998

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	4 2 6% 4 4 6% 5% 7% 8% 55%	3727
	11th to 20th	Day	2 2 7% 4 4 5% 4 6% 8% 60%	4100
	21st to end	Day	1 4 3 6% 4 5% 5% 4 6% 9% 53%	4510
February	1st to 10th	Day	4 3 4 3 3 4 3 6% 10% 58%	1810
	11th to 20th	Day	1 3 1 4 4 3 4 4 7% 10% 59%	1949
	21st to end	Day	1 5% 2 5% 3 3 4 3 5% 7% 62%	1650
March	1st to 10th	Day	1 3 2 3 3 3 4 3 6% 8% 66%	3315
	11th to 20th	Day	2 6% 2 6% 5% 5% 5% 5% 6% 9% 47%	3600
	21st to end	Day	1 5% 2 5% 3 4 5% 4 6% 8% 57%	4067
April	1st to 10th	Day	1 3 1 4 4 4 3 4 6% 8% 62%	2280
	11th to 20th	Day	2 4 1 4 2 4 4 4 6% 8% 60%	2251
	21st to end	Day	3 6% 1 4 4 3 4 3 5% 9% 59%	2280
May	1st to 10th	Day	3 4 1 4 3 3 3 3 5% 6% 66%	3800
	11th to 20th	Day	2 3 2 6% 4 5% 6% 5% 8% 9% 49%	4189
	21st to end	Day	2 2 1 3 2 4 4 4 6% 8% 65%	4825
June	1st to 10th	Day	2 3 1 3 3 3 4 5% 7% 9% 59%	2660
	11th to 20th	Day	1 2 1 3 2 3 4 3 6% 7% 68%	2660
	21st to end	Day	3 3 1 4 3 3 5% 6% 7% 10% 55%	2660
July	1st to 10th	Day	2 2 1 4 4 4 6% 5% 7% 9% 56%	4430
	11th to 20th	Day	2 2 1 4 3 5% 6% 6% 10% 13% 48%	4316
	21st to end	Day	3 4 1 5% 4 5% 6% 6% 9% 11% 45%	4454
August	1st to 10th	Day	3 3 1 5% 5% 7% 8% 7% 10% 12% 39%	4014
	11th to 20th	Day	2 3 1 4 4 5% 6% 5% 8% 11% 51%	3726
	21st to end	Day	3 3 1 5% 5% 6% 7% 6% 10% 12% 41%	4180
September	1st to 10th	Day	3 4 1 6% 4 5% 4 4 7% 9% 54%	2280
	11th to 20th	Day	4 3 1 5% 3 4 4 4 5% 9% 59%	2280
	21st to end	Day	2 3 1 3 3 3 3 3 5% 7% 68%	2077
October	1st to 10th	Day	2 2 2 2 1 2 3 5% 7% 74%	3170
	11th to 20th	Day	3 3 1 4 3 3 4 3 6% 8% 61%	3150
	21st to end	Day	2 2 1 3 2 3 2 2 4 6% 72%	3438
November	1st to 10th	Day	1 3 1 3 2 2 3 2 5% 7% 71%	1811
	11th to 20th	Day	1 2 1 3 2 3 3 2 5% 7% 72%	1800
	21st to end	Day	2 4 2 4 3 4 4 3 5% 7% 63%	1791
December	1st to 10th	Day	2 1 2 2 2 2 2 4 10% 71%	2620
	11th to 20th	Day	1 2 1 3 2 4 6% 6% 7% 10% 58%	3026
	21st to end	Day	3 2 5% 4 4 4 4 7% 11% 56%	4004
January	1st to 10th	Night	6% 3 8% 4 3 4 4 6% 8% 53%	6281
	11th to 20th	Night	1 2 1 5% 4 4 5% 3 4 8% 63%	5880
	21st to end	Night	1 5% 2 6% 4 5% 5% 4 7% 10% 51%	6468
February	1st to 10th	Night	1 5% 2 6% 3 3 3 2 5% 9% 62%	2587
	11th to 20th	Night	1 5% 2 5% 3 4 4 3 6% 8% 60%	2338
	21st to end	Night	1 6% 2 5% 2 3 3 3 5% 7% 64%	1980
March	1st to 10th	Night	1 6% 2 4 3 2 3 3 4 6% 66%	3768
	11th to 20th	Night	4 9% 3 6% 4 4 4 3 6% 8% 50%	3600
	21st to end	Night	2 8% 3 7% 4 4 4 3 5% 6% 54%	3717
April	1st to 10th	Night	3 5% 2 4 3 2 3 3 5% 8% 62%	1900
	11th to 20th	Night	2 5% 2 4 3 3 4 4 6% 8% 59%	1877
	21st to end	Night	5% 6% 2 5% 2 3 4 4 6% 7% 56%	1900
May	1st to 10th	Night	5% 4 1 3 2 3 3 3 4 5% 67%	3170
	11th to 20th	Night	4 5% 2 6% 4 5% 5% 4 7% 9% 48%	2695
	21st to end	Night	3 2 1 3 3 3 3 3 5% 7% 67%	2762
June	1st to 10th	Night	1 3 1 3 3 4 4 4 7% 10% 60%	1520
	11th to 20th	Night	2 2 1 3 3 2 3 3 6% 7% 71%	1520
	21st to end	Night	3 3 2 4 4 4 5% 5% 8% 9% 53%	1520
July	1st to 10th	Night	3 3 1 5% 5% 5% 7% 6% 8% 10% 48%	2540
	11th to 20th	Night	2 3 1 5% 4 6% 7% 7% 10% 13% 42%	2635
	21st to end	Night	4 5% 2 6% 4 6% 7% 5% 9% 12% 39%	3109
August	1st to 10th	Night	5% 5% 2 8% 6% 7% 7% 7% 9% 11% 31%	2925
	11th to 20th	Night	3 3 2 5% 5% 5% 7% 7% 11% 12% 41%	3113
	21st to end	Night	3 4 2 7% 6% 7% 7% 6% 10% 12% 34%	3487
September	1st to 10th	Night	5% 4 2 5% 4 4 5% 4 7% 9% 52%	1900
	11th to 20th	Night	3 3 1 3 4 3 3 3 5% 9% 62%	1900
	21st to end	Night	4 3 1 3 3 2 3 3 4 7% 68%	2098
October	1st to 10th	Night	2 2 1 3 2 2 3 3 5% 6% 70%	3800
	11th to 20th	Night	3 3 1 4 3 3 4 4 5% 7% 63%	3778
	21st to end	Night	3 2 1 2 2 2 2 3 5% 5% 74%	4120
November	1st to 10th	Night	1 2 1 3 2 2 3 3 4 7% 74%	2468
	11th to 20th	Night	1 2 1 2 2 2 3 2 4 7% 75%	2600
	21st to end	Night	2 5% 2 4 2 3 2 3 6% 7% 65%	2605
December	1st to 10th	Night	1 3 2 4 2 2 2 2 7% 10% 66%	4570
	11th to 20th	Night	1 3 2 3 2 3 4 4 6% 8% 64%	5181
	21st to end	Night	1 4 2 5% 4 5% 5% 4 7% 10% 52%	6974

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	18% 28% 9% 24% 19% 2	3728
	11th to 20th	Day	15% 29% 9% 19% 22% 6%	4100
	21st to end	Day	17% 31% 5% 15% 25% 6%	4510
February	1st to 10th	Day	6% 62% 5% 14% 8% 5%	1810
	11th to 20th	Day	7% 62% 4 13% 11% 3	1959
	21st to end	Day	9% 61% 2 8% 14% 6%	1650
March	1st to 10th	Day	6% 40% 4 13% 31% 5%	3315
	11th to 20th	Day	19% 43% 8% 18% 11% 2	3600
	21st to end	Day	11% 42% 8% 26% 11% 2	4067
April	1st to 10th	Day	7% 65% 3 13% 10% 2	2280
	11th to 20th	Day	7% 65% 3 9% 11% 4	2254
	21st to end	Day	9% 65% 4 13% 9% 1	2280
May	1st to 10th	Day	11% 41% 7% 11% 16% 15%	3800
	11th to 20th	Day	12% 45% 9% 15% 11% 7%	4193
	21st to end	Day	6% 40% 7% 11% 21% 15%	4825
June	1st to 10th	Day	6% 65% 5% 9% 9% 6%	2660
	11th to 20th	Day	5% 64% 3 9% 9% 10%	2660
	21st to end	Day	7% 64% 4 10% 9% 6%	2660
July	1st to 10th	Day	11% 38% 5% 11% 17% 18%	4361
	11th to 20th	Day	14% 38% 13% 24% 9% 3	4241
	21st to end	Day	14% 37% 10% 22% 11% 4	4314
August	1st to 10th	Day	20% 41% 18% 14% 5% 2	4000
	11th to 20th	Day	14% 39% 14% 13% 13% 6%	3762
	21st to end	Day	22% 41% 14% 13% 9% 2	4179
September	1st to 10th	Day	10% 61% 5% 11% 9% 5%	2280
	11th to 20th	Day	8% 60% 4 10% 11% 6%	2280
	21st to end	Day	7% 60% 3 10% 14% 6%	2077
October	1st to 10th	Day	5% 36% 2 13% 29% 16%	3170
	11th to 20th	Day	15% 39% 6% 20% 15% 4	3150
	21st to end	Day	9% 37% 3 16% 27% 8%	3438
November	1st to 10th	Day	7% 55% 2 12% 18% 5%	1811
	11th to 20th	Day	7% 57% 4 13% 17% 3	1800
	21st to end	Day	11% 57% 4 12% 14% 3	1791
December	1st to 10th	Day	6% 35% 3 30% 22% 4	2620
	11th to 20th	Day	9% 32% 7% 33% 16% 2	3024
	21st to end	Day	13% 28% 7% 23% 26% 4	4004
January	1st to 10th	Night	22% 27% 6% 25% 17% 3	6281
	11th to 20th	Night	12% 31% 9% 20% 19% 8%	5880
	21st to end	Night	15% 32% 8% 18% 22% 5%	6468
February	1st to 10th	Night	7% 61% 5% 13% 10% 4	2587
	11th to 20th	Night	8% 61% 6% 12% 11% 3	2338
	21st to end	Night	8% 62% 2 7% 16% 5%	1980
March	1st to 10th	Night	11% 40% 3 10% 31% 5%	3768
	11th to 20th	Night	18% 42% 9% 17% 12% 2	3600
	21st to end	Night	18% 41% 6% 25% 9% 1	3717
April	1st to 10th	Night	9% 65% 4 12% 9% 2	1900
	11th to 20th	Night	8% 66% 3 11% 9% 4	1877
	21st to end	Night	11% 66% 5% 10% 8% 1	1900
May	1st to 10th	Night	11% 41% 5% 11% 19% 13%	3170
	11th to 20th	Night	13% 46% 10% 16% 9% 6%	2696
	21st to end	Night	7% 42% 7% 10% 21% 13%	2762
June	1st to 10th	Night	7% 66% 6% 8% 7% 6%	1520
	11th to 20th	Night	4 65% 4 7% 9% 10%	1520
	21st to end	Night	9% 64% 5% 8% 8% 6%	1520
July	1st to 10th	Night	17% 39% 6% 14% 8% 17%	2500
	11th to 20th	Night	20% 38% 21% 16% 3 3	2587
	21st to end	Night	21% 38% 13% 15% 9% 3	3011
August	1st to 10th	Night	32% 40% 13% 8% 4 2	2916
	11th to 20th	Night	18% 42% 16% 8% 9% 6%	3143
	21st to end	Night	26% 41% 14% 12% 7% 1	3487
September	1st to 10th	Night	11% 60% 6% 11% 8% 5%	1900
	11th to 20th	Night	7% 61% 4 9% 12% 7%	1900
	21st to end	Night	9% 59% 3 8% 15% 5%	2098
October	1st to 10th	Night	9% 37% 3 15% 24% 13%	3800
	11th to 20th	Night	16% 37% 7% 17% 18% 5%	3778
	21st to end	Night	9% 38% 2 19% 26% 7%	4120
November	1st to 10th	Night	7% 55% 3 12% 19% 5%	2468
	11th to 20th	Night	5% 57% 2 15% 17% 4	2600
	21st to end	Night	8% 58% 4 13% 14% 3	2605
December	1st to 10th	Night	8% 36% 4 34% 17% 1	4570
	11th to 20th	Night	8% 32% 6% 26% 25% 2	5182
	21st to end	Night	17% 28% 8% 21% 23% 2	6974

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	4 14% 24% 38% 16% 3	300
	11th to 20th	Day	6% 10% 26% 39% 16% 3	300
	21st to end	Day	2 8% 22% 44% 17% 6%	330
February	1st to 10th	Day	2 21% 34% 30% 11% 2	300
	11th to 20th	Day	6% 17% 36% 31% 9% 1	300
	21st to end	Day	16% 18% 25% 31% 8% 2	249
March	1st to 10th	Day	11% 18% 33% 23% 12% 3	346
	11th to 20th	Day	11% 20% 30% 23% 12% 4	500
	21st to end	Day	13% 26% 34% 19% 7% 2	531
April	1st to 10th	Day	10% 21% 37% 20% 10% 2	460
	11th to 20th	Day	13% 24% 35% 17% 9% 1	452
	21st to end	Day	11% 21% 38% 20% 9% 1	460
May	1st to 10th	Day	9% 21% 38% 20% 10% 2	463
	11th to 20th	Day	6% 11% 29% 28% 21% 5%	637
	21st to end	Day	7% 14% 27% 30% 17% 5%	734
June	1st to 10th	Day	4 8% 19% 34% 29% 7%	680
	11th to 20th	Day	3 7% 27% 33% 22% 8%	680
	21st to end	Day	5% 5% 23% 30% 29% 9%	680
July	1st to 10th	Day	4 3 14% 28% 37% 14%	671
	11th to 20th	Day	4 3 7% 24% 40% 22%	606
	21st to end	Day	6% 3 9% 24% 34% 25%	531
August	1st to 10th	Day	5% 3 11% 29% 32% 20%	487
	11th to 20th	Day	6% 3 13% 32% 32% 15%	477
	21st to end	Day	6% 3 13% 33% 26% 20%	539
September	1st to 10th	Day	4 4 20% 24% 28% 20%	490
	11th to 20th	Day	4 5% 24% 32% 25% 10%	490
	21st to end	Day	3 8% 16% 25% 31% 17%	304
October	1st to 10th	Day	5% 10% 21% 19% 25% 19%	300
	11th to 20th	Day	6% 13% 21% 25% 24% 11%	300
	21st to end	Day	10% 12% 23% 24% 23% 9%	330
November	1st to 10th	Day	5% 14% 29% 25% 22% 5%	300
	11th to 20th	Day	3 11% 23% 35% 24% 5%	300
	21st to end	Day	5% 14% 26% 29% 18% 7%	300
December	1st to 10th	Day	6% 11% 25% 38% 13% 7%	300
	11th to 20th	Day	5% 12% 21% 44% 16% 2	300
	21st to end	Day	10% 16% 26% 37% 9% 2	330
January	1st to 10th	Night	2 7% 31% 44% 14% 1	980
	11th to 20th	Night	4 6% 30% 46% 13% 1	800
	21st to end	Night	3 6% 28% 50% 12% 1	880
February	1st to 10th	Night	3 5% 43% 42% 8% 1	800
	11th to 20th	Night	3 8% 43% 38% 7%	784
	21st to end	Night	7% 8% 43% 38% 4	660
March	1st to 10th	Night	4 9% 40% 39% 8% 1	800
	11th to 20th	Night	6% 7% 32% 42% 13%	800
	21st to end	Night	4 10% 41% 36% 8%	757
April	1st to 10th	Night	7% 10% 43% 35% 4	570
	11th to 20th	Night	5% 13% 38% 34% 9%	563
	21st to end	Night	9% 7% 39% 37% 8%	570
May	1st to 10th	Night	5% 8% 35% 40% 11%	570
	11th to 20th	Night	8% 5% 24% 44% 18% 2	561
	21st to end	Night	5% 7% 32% 38% 16% 1	617
June	1st to 10th	Night	2 2 24% 46% 25% 1	570
	11th to 20th	Night	2 3 28% 49% 17%	570
	21st to end	Night	3 3 17% 44% 32% 1	570
July	1st to 10th	Night	6% 3 11% 40% 37% 3	555
	11th to 20th	Night	4 2 7% 33% 50% 4	552
	21st to end	Night	4 3 5% 29% 50% 9%	624
August	1st to 10th	Night	7% 2 5% 33% 43% 9%	567
	11th to 20th	Night	6% 3 8% 41% 38% 4	564
	21st to end	Night	4 3 7% 41% 40% 5%	627
September	1st to 10th	Night	7% 2 11% 43% 34% 3	570
	11th to 20th	Night	4 2 16% 47% 29% 3	570
	21st to end	Night	4 4 20% 45% 25% 2	583
October	1st to 10th	Night	3 6% 26% 40% 22% 3	760
	11th to 20th	Night	3 3 29% 40% 22% 2	760
	21st to end	Night	7% 5% 25% 42% 19% 3	836
November	1st to 10th	Night	4 6% 30% 43% 17% 1	908
	11th to 20th	Night	2 7% 31% 43% 15% 2	1000
	21st to end	Night	5% 9% 30% 40% 15% 1	1000
December	1st to 10th	Night	3 8% 34% 42% 11% 1	1000
	11th to 20th	Night	5% 9% 34% 42% 11%	1000
	21st to end	Night	4 12% 34% 42% 8% 1	1100

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