Jack C. Davis Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jack C. Davis Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	1 3 1 4 3 3 3 3 5% 6% 67%	135600
	Night	1 3 2 4 3 3 4 3 5% 7% 64%	132478
Transparency	Day	16% 6% 10% 19% 27% 22%	130572
	Night	17% 8% 12% 22% 25% 16%	128486
Seeing	Day	8% 16% 26% 26% 19% 6%	16435
	Night	5% 9% 23% 36% 23% 4	25790

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	1 3 2 5% 4 4 5% 5% 7% 10% 56%	3639
	11th to 20th	Day	1 4 2 5% 4 4 5% 4 6% 6% 61%	3714
	21st to end	Day	2 7% 4 8% 5% 4 5% 4 7% 7% 47%	4510
February	1st to 10th	Day	2 4 3 4 5% 4 5% 4 8% 9% 52%	1811
	11th to 20th	Day	1 5% 2 6% 4 5% 5% 5% 8% 9% 49%	1950
	21st to end	Day	1 5% 1 6% 4 4 4 5% 9% 10% 52%	1650
March	1st to 10th	Day	2 8% 4 10% 4 4 6% 6% 8% 10% 38%	4518
	11th to 20th	Day	3 1 4 3 3 4 4 6% 9% 63%	4906
	21st to end	Day	1 6% 3 6% 4 5% 4 5% 7% 9% 53%	5851
April	1st to 10th	Day	2 4 2 6% 4 5% 5% 4 7% 8% 53%	2400
	11th to 20th	Day	2 5% 1 4 4 3 4 5% 6% 9% 57%	2369
	21st to end	Day	2 5% 1 4 4 4 4 3 6% 7% 60%	2400
May	1st to 10th	Day	2 4 1 4 2 2 2 2 3 5% 71%	5721
	11th to 20th	Day	1 2 1 3 3 3 3 3 4 6% 71%	6145
	21st to end	Day	2 2 1 3 2 3 3 3 4 5% 73%	6902
June	1st to 10th	Day	2 3 1 4 3 3 3 4 6% 8% 63%	2800
	11th to 20th	Day	1 2 1 2 1 2 3 2 4 5% 77%	2800
	21st to end	Day	1 2 1 2 2 2 2 2 3 5% 78%	2800
July	1st to 10th	Day	1 1 1 2 1 2 2 88%	6319
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 3 81%	6320
	21st to end	Day	1 1 1 2 2 2 3 3 5% 7% 74%	6754
August	1st to 10th	Day	2 1 1 2 2 1 2 1 2 3 83%	5697
	11th to 20th	Day	1 1 1 1 1 1 3 90%	5364
	21st to end	Day	1 2 1 3 2 2 3 2 3 4 77%	5984
September	1st to 10th	Day	1 1 1 1 2 2 2 4 5% 80%	2400
	11th to 20th	Day	1 1 1 3 3 3 3 2 3 5% 74%	2360
	21st to end	Day	1 2 1 3 2 3 2 2 3 4 78%	2200
October	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 75%	3349
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 5% 76%	3261
	21st to end	Day	1 1 1 2 3 3 4 3 6% 8% 69%	3462
November	1st to 10th	Day	2 4 1 4 4 5% 4 3 7% 7% 60%	1854
	11th to 20th	Day	2 3 1 3 3 4 4 5% 8% 9% 58%	1890
	21st to end	Day	1 3 2 5% 4 4 4 5% 6% 9% 56%	1832
December	1st to 10th	Day	1 3 1 4 3 3 4 4 5% 8% 64%	2636
	11th to 20th	Day	2 7% 3 7% 5% 5% 6% 5% 9% 10% 43%	3028
	21st to end	Day	1 5% 3 7% 4 4 5% 5% 9% 15% 41%	4004
January	1st to 10th	Night	1 3 2 5% 3 3 3 4 8% 12% 56%	6306
	11th to 20th	Night	4 2 6% 5% 5% 5% 4 6% 7% 57%	6232
	21st to end	Night	2 7% 3 7% 4 5% 5% 3 5% 6% 50%	6468
February	1st to 10th	Night	2 4 2 6% 4 5% 5% 4 7% 8% 54%	2587
	11th to 20th	Night	1 5% 2 5% 3 4 4 5% 8% 10% 53%	2338
	21st to end	Night	1 4 2 6% 4 4 4 4 6% 9% 54%	1980
March	1st to 10th	Night	1 6% 4 7% 5% 5% 5% 5% 8% 9% 44%	5376
	11th to 20th	Night	3 1 6% 3 4 5% 5% 7% 9% 58%	5092
	21st to end	Night	1 6% 2 6% 4 5% 5% 5% 8% 9% 49%	5122
April	1st to 10th	Night	2 3 1 4 5% 3 4 6% 8% 9% 54%	2000
	11th to 20th	Night	2 4 1 4 4 3 3 6% 7% 7% 60%	1979
	21st to end	Night	3 3 1 4 2 3 3 4 6% 7% 63%	2000
May	1st to 10th	Night	2 3 1 3 2 2 2 3 4 5% 73%	4251
	11th to 20th	Night	1 2 1 3 2 2 3 2 5% 6% 74%	3736
	21st to end	Night	2 2 1 3 2 2 2 2 4 6% 75%	3972
June	1st to 10th	Night	4 4 1 4 3 3 3 3 4 7% 65%	1600
	11th to 20th	Night	1 1 2 1 2 2 2 3 4 80%	1600
	21st to end	Night	2 2 2 2 2 2 3 3 3 80%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 3 90%	3640
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 3 79%	3640
	21st to end	Night	2 2 1 2 2 2 4 3 5% 6% 71%	4098
August	1st to 10th	Night	2 1 1 2 1 2 2 1 3 4 81%	4119
	11th to 20th	Night	1 1 1 1 1 2 1 2 4 85%	4506
	21st to end	Night	2 2 1 3 2 2 3 2 3 3 76%	5015
September	1st to 10th	Night	2 1 1 1 1 2 1 2 4 84%	2000
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 5% 76%	2038
	21st to end	Night	1 2 1 3 2 2 3 2 3 5% 77%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 79%	3848
	11th to 20th	Night	1 2 1 2 1 2 2 2 3 4 81%	3897
	21st to end	Night	1 2 1 3 3 2 3 3 6% 7% 69%	4344
November	1st to 10th	Night	2 3 1 3 3 3 4 4 6% 8% 63%	2665
	11th to 20th	Night	1 3 1 4 3 3 4 3 6% 10% 62%	2730
	21st to end	Night	1 4 1 4 3 3 4 4 7% 10% 59%	2783
December	1st to 10th	Night	1 4 1 3 3 3 4 4 5% 7% 65%	4588
	11th to 20th	Night	2 5% 2 5% 4 4 6% 6% 10% 12% 44%	5176
	21st to end	Night	1 5% 3 8% 6% 6% 7% 7% 9% 11% 38%	6952

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	20% 12% 26% 25% 17%	3639
	11th to 20th	Day	22% 9% 12% 23% 28% 6%	3714
	21st to end	Day	32% 10% 11% 18% 23% 7%	4510
February	1st to 10th	Day	25% 10% 15% 25% 17% 7%	1811
	11th to 20th	Day	26% 7% 15% 28% 19% 4	1960
	21st to end	Day	22% 11% 20% 26% 16% 6%	1650
March	1st to 10th	Day	30% 13% 16% 19% 17% 5%	4518
	11th to 20th	Day	14% 8% 15% 35% 24% 3	4906
	21st to end	Day	23% 11% 18% 24% 22% 2	5851
April	1st to 10th	Day	26% 10% 17% 23% 20% 5%	2400
	11th to 20th	Day	21% 8% 15% 27% 22% 7%	2372
	21st to end	Day	19% 9% 14% 28% 26% 4	2400
May	1st to 10th	Day	15% 6% 7% 26% 38% 8%	5721
	11th to 20th	Day	13% 5% 9% 22% 40% 11%	6148
	21st to end	Day	13% 6% 7% 19% 42% 14%	6904
June	1st to 10th	Day	18% 6% 10% 23% 32% 11%	2800
	11th to 20th	Day	10% 4 7% 20% 34% 26%	2800
	21st to end	Day	11% 3 4 10% 32% 40%	2800
July	1st to 10th	Day	5% 2 2 5% 18% 68%	6251
	11th to 20th	Day	9% 3 2 7% 12% 67%	6245
	21st to end	Day	9% 4 5% 12% 28% 42%	6557
August	1st to 10th	Day	11% 3 3 7% 23% 52%	4148
	11th to 20th	Day	6% 2 3 5% 21% 64%	3880
	21st to end	Day	6% 2 2 8% 38% 44%	4311
September	1st to 10th	Day	8% 3 4 9% 33% 44%	2400
	11th to 20th	Day	12% 4 6% 18% 36% 24%	2360
	21st to end	Day	12% 3 5% 17% 37% 26%	2200
October	1st to 10th	Day	12% 3 6% 13% 35% 32%	3349
	11th to 20th	Day	12% 2 6% 20% 46% 13%	3261
	21st to end	Day	12% 5% 9% 29% 34% 12%	3462
November	1st to 10th	Day	20% 7% 14% 25% 26% 9%	1854
	11th to 20th	Day	19% 9% 18% 25% 25% 5%	1890
	21st to end	Day	21% 8% 14% 25% 26% 6%	1832
December	1st to 10th	Day	17% 7% 13% 30% 28% 5%	2636
	11th to 20th	Day	30% 12% 17% 21% 16% 3	3028
	21st to end	Day	25% 14% 25% 23% 11% 1	4004
January	1st to 10th	Night	18% 10% 26% 28% 16% 1	6306
	11th to 20th	Night	23% 9% 15% 22% 25% 5%	6232
	21st to end	Night	30% 9% 12% 17% 26% 5%	6468
February	1st to 10th	Night	25% 10% 16% 24% 19% 7%	2587
	11th to 20th	Night	22% 9% 17% 29% 20% 3	2339
	21st to end	Night	23% 9% 19% 27% 16% 7%	1980
March	1st to 10th	Night	27% 12% 15% 24% 17% 5%	5376
	11th to 20th	Night	17% 11% 14% 41% 14% 2	5092
	21st to end	Night	24% 13% 24% 28% 9% 2	5122
April	1st to 10th	Night	21% 12% 18% 28% 17% 4	2000
	11th to 20th	Night	19% 9% 16% 30% 17% 7%	1979
	21st to end	Night	19% 9% 16% 32% 20% 3	2000
May	1st to 10th	Night	12% 6% 9% 31% 37% 5%	4248
	11th to 20th	Night	11% 5% 11% 25% 38% 10%	3736
	21st to end	Night	10% 5% 8% 24% 44% 8%	3972
June	1st to 10th	Night	19% 6% 10% 25% 30% 10%	1600
	11th to 20th	Night	8% 5% 6% 21% 36% 24%	1600
	21st to end	Night	10% 2 6% 14% 34% 34%	1600
July	1st to 10th	Night	4 1 2 5% 22% 67%	3600
	11th to 20th	Night	11% 2 3 6% 14% 64%	3592
	21st to end	Night	12% 4 5% 15% 24% 40%	3983
August	1st to 10th	Night	12% 4 4 9% 29% 43%	2989
	11th to 20th	Night	6% 2 2 6% 25% 59%	3256
	21st to end	Night	8% 1 2 10% 40% 39%	3608
September	1st to 10th	Night	6% 2 4 9% 38% 41%	2000
	11th to 20th	Night	11% 4 7% 20% 34% 23%	2038
	21st to end	Night	12% 3 5% 20% 35% 25%	2200
October	1st to 10th	Night	11% 3 5% 15% 33% 34%	3848
	11th to 20th	Night	8% 3 6% 21% 48% 13%	3897
	21st to end	Night	12% 6% 8% 28% 35% 11%	4344
November	1st to 10th	Night	17% 8% 15% 26% 23% 11%	2665
	11th to 20th	Night	18% 8% 14% 28% 26% 6%	2730
	21st to end	Night	19% 9% 13% 28% 24% 7%	2783
December	1st to 10th	Night	19% 7% 11% 28% 26% 9%	4588
	11th to 20th	Night	24% 14% 18% 26% 16% 2	5176
	21st to end	Night	30% 14% 21% 21% 12% 2	6952

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	17% 16% 30% 22% 14% 3	200
	11th to 20th	Day	9% 14% 31% 25% 16% 4	235
	21st to end	Day	8% 13% 30% 28% 17% 4	440
February	1st to 10th	Day	14% 14% 28% 25% 15% 4	406
	11th to 20th	Day	10% 21% 26% 30% 12% 1	495
	21st to end	Day	9% 19% 26% 32% 14%	414
March	1st to 10th	Day	12% 22% 32% 22% 10% 2	500
	11th to 20th	Day	12% 22% 27% 21% 14% 3	500
	21st to end	Day	14% 23% 28% 23% 11% 1	550
April	1st to 10th	Day	11% 24% 29% 20% 12% 4	500
	11th to 20th	Day	13% 24% 27% 19% 13% 3	492
	21st to end	Day	11% 21% 29% 21% 14% 4	500
May	1st to 10th	Day	12% 22% 28% 23% 11% 3	503
	11th to 20th	Day	12% 21% 25% 22% 13% 7%	528
	21st to end	Day	9% 17% 22% 25% 20% 7%	573
June	1st to 10th	Day	7% 16% 26% 24% 21% 7%	530
	11th to 20th	Day	9% 20% 28% 24% 15% 5%	530
	21st to end	Day	3 12% 26% 30% 22% 7%	530
July	1st to 10th	Day	2 12% 29% 29% 22% 6%	526
	11th to 20th	Day	2 9% 26% 32% 22% 8%	524
	21st to end	Day	3 7% 26% 30% 25% 8%	575
August	1st to 10th	Day	4 10% 26% 27% 25% 7%	527
	11th to 20th	Day	2 11% 25% 26% 25% 12%	517
	21st to end	Day	3 8% 27% 30% 25% 7%	583
September	1st to 10th	Day	3 7% 29% 29% 24% 8%	530
	11th to 20th	Day	5% 14% 24% 28% 22% 7%	530
	21st to end	Day	4 13% 25% 34% 18% 5%	530
October	1st to 10th	Day	5% 14% 27% 26% 21% 7%	530
	11th to 20th	Day	7% 12% 23% 26% 25% 6%	530
	21st to end	Day	7% 14% 23% 28% 23% 5%	523
November	1st to 10th	Day	10% 12% 24% 22% 23% 9%	330
	11th to 20th	Day	6% 14% 22% 31% 19% 8%	330
	21st to end	Day	6% 13% 25% 25% 24% 6%	300
December	1st to 10th	Day	9% 13% 22% 22% 22% 13%	204
	11th to 20th	Day	7% 18% 30% 22% 17% 7%	200
	21st to end	Day	18% 16% 20% 29% 14% 3	220
January	1st to 10th	Night	6% 12% 31% 26% 18% 7%	800
	11th to 20th	Night	5% 12% 25% 38% 16% 5%	800
	21st to end	Night	6% 7% 29% 35% 18% 5%	880
February	1st to 10th	Night	8% 11% 26% 33% 19% 3	800
	11th to 20th	Night	4 12% 27% 35% 17% 5%	784
	21st to end	Night	6% 15% 26% 38% 13% 2	660
March	1st to 10th	Night	7% 13% 34% 28% 14% 3	800
	11th to 20th	Night	4 16% 26% 26% 25% 3	800
	21st to end	Night	6% 19% 30% 30% 14% 1	880
April	1st to 10th	Night	5% 13% 30% 32% 19% 1	800
	11th to 20th	Night	8% 14% 25% 33% 16% 4	791
	21st to end	Night	7% 11% 25% 33% 20% 4	800
May	1st to 10th	Night	6% 13% 26% 32% 20% 3	673
	11th to 20th	Night	4 8% 24% 41% 21% 2	449
	21st to end	Night	6% 6% 18% 41% 27% 3	433
June	1st to 10th	Night	6% 4 21% 39% 27% 4	400
	11th to 20th	Night	1 5% 23% 43% 26% 2	400
	21st to end	Night	2 3 18% 45% 30% 2	400
July	1st to 10th	Night	1 1 13% 52% 31% 2	390
	11th to 20th	Night	1 15% 54% 28% 1	388
	21st to end	Night	3 1 11% 44% 37% 5%	491
August	1st to 10th	Night	4 1 12% 45% 35% 3	631
	11th to 20th	Night	2 3 16% 45% 29% 4	792
	21st to end	Night	2 3 17% 45% 30% 3	880
September	1st to 10th	Night	2 2 18% 39% 35% 5%	800
	11th to 20th	Night	2 5% 19% 33% 35% 7%	800
	21st to end	Night	3 5% 21% 36% 29% 7%	800
October	1st to 10th	Night	4 5% 20% 37% 28% 6%	800
	11th to 20th	Night	5% 8% 15% 40% 27% 6%	800
	21st to end	Night	4 8% 20% 32% 27% 10%	880
November	1st to 10th	Night	4 11% 21% 28% 28% 7%	820
	11th to 20th	Night	4 10% 27% 36% 18% 5%	840
	21st to end	Night	6% 10% 25% 32% 23% 5%	840
December	1st to 10th	Night	8% 11% 24% 34% 17% 6%	808
	11th to 20th	Night	6% 13% 26% 33% 16% 5%	800
	21st to end	Night	7% 13% 33% 31% 13% 3	880

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