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 2024-01-13.

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 2 4 4 3 4 4 5% 7% 62%	74778
	Night	2 3 2 4 4 2 4 5% 6% 8% 61%	74961
Transparency	Day	19% 7% 11% 20% 24% 18%	74485
	Night	20% 8% 13% 22% 23% 14%	74775
Seeing	Day	8% 16% 27% 26% 18% 5%	13907
	Night	5% 9% 23% 35% 24% 5%	23377

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 4 1 5% 5% 4 5% 6% 8% 11% 48%	1519
	11th to 20th	Day	1 4 2 5% 4 2 6% 6% 7% 8% 54%	1488
	21st to end	Day	3 6% 2 6% 5% 3 6% 7% 9% 10% 44%	1782
February	1st to 10th	Day	2 4 3 4 5% 3 5% 5% 7% 10% 53%	1630
	11th to 20th	Day	1 4 3 6% 4 3 5% 6% 7% 9% 51%	1750
	21st to end	Day	1 6% 1 6% 5% 3 5% 5% 8% 10% 50%	1480
March	1st to 10th	Day	3 5% 3 6% 6% 3 7% 7% 8% 10% 42%	1800
	11th to 20th	Day	1 5% 2 6% 5% 3 6% 6% 8% 9% 47%	1938
	21st to end	Day	1 3 2 5% 5% 4 5% 6% 8% 9% 51%	2323
April	1st to 10th	Day	2 4 3 6% 5% 3 6% 5% 7% 9% 52%	2160
	11th to 20th	Day	2 4 2 4 5% 2 4 5% 6% 9% 57%	2129
	21st to end	Day	2 4 2 3 4 3 4 4 5% 7% 62%	2160
May	1st to 10th	Day	3 4 2 4 4 2 4 4 5% 7% 62%	2272
	11th to 20th	Day	3 2 2 4 5% 3 4 4 5% 7% 59%	2423
	21st to end	Day	4 5% 3 5% 5% 4 5% 5% 6% 6% 53%	2723
June	1st to 10th	Day	3 3 2 4 4 3 4 4 5% 8% 62%	2520
	11th to 20th	Day	2 2 1 2 2 2 2 3 4 5% 76%	2520
	21st to end	Day	2 1 1 2 3 2 2 3 3 5% 77%	2520
July	1st to 10th	Day	1 1 1 1 2 1 3 2 3 4 81%	2520
	11th to 20th	Day	1 2 1 1 2 1 3 2 4 4 80%	2520
	21st to end	Day	2 2 1 2 3 2 3 3 3 5% 73%	2690
August	1st to 10th	Day	2 2 1 2 3 2 3 3 4 5% 74%	2298
	11th to 20th	Day	1 1 2 2 1 3 3 3 4 80%	2122
	21st to end	Day	1 1 1 1 1 1 2 2 2 3 85%	2376
September	1st to 10th	Day	1 1 1 1 1 2 2 4 5% 81%	2160
	11th to 20th	Day	1 1 1 2 3 2 3 3 3 5% 75%	2124
	21st to end	Day	1 2 1 2 2 2 3 3 3 4 77%	1980
October	1st to 10th	Day	3 4 1 3 4 2 3 3 4 6% 68%	1841
	11th to 20th	Day	2 2 1 4 3 2 2 4 4 6% 71%	1800
	21st to end	Day	1 2 2 3 3 2 3 3 5% 6% 70%	1926
November	1st to 10th	Day	2 4 1 3 5% 4 5% 4 6% 7% 59%	1674
	11th to 20th	Day	2 2 1 2 3 3 5% 6% 7% 10% 59%	1710
	21st to end	Day	1 3 2 5% 4 3 4 6% 6% 10% 54%	1658
December	1st to 10th	Day	2 4 2 4 5% 3 5% 7% 7% 10% 50%	1520
	11th to 20th	Day	2 9% 5% 7% 7% 4 7% 7% 8% 9% 35%	1928
	21st to end	Day	1 6% 4 6% 6% 4 6% 6% 8% 13% 39%	2794
January	1st to 10th	Night	2 5% 3 6% 5% 3 5% 6% 8% 13% 45%	2646
	11th to 20th	Night	1 4 2 5% 6% 3 5% 5% 8% 10% 50%	2512
	21st to end	Night	3 5% 3 6% 6% 3 5% 5% 7% 9% 48%	2574
February	1st to 10th	Night	2 4 2 5% 4 3 5% 5% 6% 8% 56%	2328
	11th to 20th	Night	1 4 2 4 4 2 4 5% 8% 10% 55%	2098
	21st to end	Night	1 4 3 5% 5% 3 4 5% 7% 9% 53%	1776
March	1st to 10th	Night	1 5% 3 5% 4 3 5% 6% 8% 11% 49%	2160
	11th to 20th	Night	1 4 2 5% 4 3 5% 7% 9% 10% 51%	2020
	21st to end	Night	1 4 1 4 4 2 5% 5% 7% 9% 56%	2031
April	1st to 10th	Night	2 3 2 4 5% 2 5% 6% 8% 10% 53%	1800
	11th to 20th	Night	2 3 1 4 4 2 4 6% 7% 7% 60%	1779
	21st to end	Night	3 3 1 4 3 2 3 4 6% 7% 64%	1800
May	1st to 10th	Night	3 3 2 3 4 2 4 5% 5% 6% 62%	1687
	11th to 20th	Night	1 3 1 4 3 2 4 4 6% 8% 63%	1466
	21st to end	Night	4 4 2 4 4 2 4 4 5% 8% 58%	1552
June	1st to 10th	Night	4 4 2 4 3 2 3 4 4 6% 65%	1440
	11th to 20th	Night	1 1 1 2 2 2 2 3 3 5% 79%	1440
	21st to end	Night	3 2 1 2 2 1 2 3 3 3 78%	1440
July	1st to 10th	Night	1 1 1 1 1 1 3 2 3 5% 82%	1440
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 4 79%	1440
	21st to end	Night	2 2 1 2 3 1 3 3 4 5% 73%	1623
August	1st to 10th	Night	3 2 1 2 3 2 3 4 4 5% 71%	1641
	11th to 20th	Night	2 1 1 1 2 2 2 3 3 5% 78%	1780
	21st to end	Night	2 1 1 1 2 1 1 2 2 3 84%	1980
September	1st to 10th	Night	1 1 1 1 1 1 2 2 4 85%	1800
	11th to 20th	Night	1 2 1 2 2 1 3 2 4 5% 78%	1834
	21st to end	Night	1 2 1 2 2 2 3 3 3 5% 76%	1980
October	1st to 10th	Night	3 2 1 3 3 2 3 3 4 5% 71%	2116
	11th to 20th	Night	2 2 1 2 2 2 2 3 4 5% 76%	2160
	21st to end	Night	2 3 1 2 3 1 2 3 5% 6% 72%	2428
November	1st to 10th	Night	2 3 2 3 4 2 4 5% 6% 8% 61%	2405
	11th to 20th	Night	1 3 1 4 3 2 4 3 6% 10% 63%	2470
	21st to end	Night	2 4 2 3 4 3 4 5% 6% 10% 58%	2517
December	1st to 10th	Night	2 6% 2 4 5% 3 4 6% 6% 8% 53%	2660
	11th to 20th	Night	2 8% 3 7% 6% 4 6% 7% 9% 10% 38%	3276
	21st to end	Night	1 6% 3 8% 8% 4 7% 9% 8% 9% 36%	4862

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	27% 13% 19% 27% 13% 1	1519
	11th to 20th	Day	25% 10% 18% 24% 17% 7%	1488
	21st to end	Day	31% 9% 15% 23% 19% 3	1782
February	1st to 10th	Day	24% 9% 16% 27% 18% 6%	1630
	11th to 20th	Day	26% 7% 16% 28% 18% 5%	1760
	21st to end	Day	24% 11% 20% 25% 16% 4	1480
March	1st to 10th	Day	32% 13% 18% 22% 14% 2	1800
	11th to 20th	Day	28% 11% 17% 19% 20% 5%	1938
	21st to end	Day	25% 12% 23% 25% 11% 3	2323
April	1st to 10th	Day	27% 10% 17% 21% 19% 6%	2160
	11th to 20th	Day	22% 9% 15% 26% 22% 6%	2132
	21st to end	Day	19% 8% 15% 29% 25% 4	2160
May	1st to 10th	Day	20% 7% 10% 29% 29% 5%	2272
	11th to 20th	Day	23% 6% 10% 26% 27% 7%	2426
	21st to end	Day	28% 9% 12% 23% 22% 6%	2724
June	1st to 10th	Day	19% 6% 10% 22% 31% 11%	2520
	11th to 20th	Day	11% 4 7% 21% 34% 23%	2520
	21st to end	Day	12% 4 5% 11% 31% 38%	2520
July	1st to 10th	Day	8% 2 4 10% 25% 50%	2451
	11th to 20th	Day	9% 3 3 14% 22% 48%	2445
	21st to end	Day	14% 4 5% 8% 23% 46%	2540
August	1st to 10th	Day	13% 4 5% 11% 23% 44%	2282
	11th to 20th	Day	9% 3 4 8% 29% 47%	2122
	21st to end	Day	7% 2 3 8% 34% 47%	2376
September	1st to 10th	Day	7% 3 4 9% 31% 46%	2160
	11th to 20th	Day	11% 4 6% 18% 35% 25%	2124
	21st to end	Day	12% 3 5% 18% 36% 25%	1980
October	1st to 10th	Day	19% 3 9% 18% 30% 21%	1841
	11th to 20th	Day	14% 3 9% 24% 31% 19%	1800
	21st to end	Day	14% 5% 10% 27% 29% 15%	1926
November	1st to 10th	Day	22% 8% 13% 22% 26% 9%	1674
	11th to 20th	Day	17% 8% 19% 24% 27% 5%	1710
	21st to end	Day	23% 9% 14% 24% 25% 6%	1658
December	1st to 10th	Day	26% 11% 15% 25% 19% 5%	1520
	11th to 20th	Day	38% 14% 13% 18% 13% 4	1928
	21st to end	Day	31% 12% 20% 22% 14% 2	2794
January	1st to 10th	Night	28% 11% 19% 26% 15% 1	2646
	11th to 20th	Night	26% 11% 18% 24% 14% 7%	2512
	21st to end	Night	30% 8% 17% 21% 20% 3	2574
February	1st to 10th	Night	25% 10% 15% 25% 19% 6%	2328
	11th to 20th	Night	22% 9% 18% 29% 19% 4	2099
	21st to end	Night	24% 9% 20% 27% 15% 5%	1776
March	1st to 10th	Night	25% 12% 19% 23% 16% 4	2160
	11th to 20th	Night	21% 14% 18% 22% 20% 5%	2020
	21st to end	Night	21% 13% 24% 25% 13% 3	2031
April	1st to 10th	Night	23% 12% 18% 26% 16% 5%	1800
	11th to 20th	Night	19% 10% 18% 29% 18% 7%	1779
	21st to end	Night	18% 9% 17% 32% 20% 3	1800
May	1st to 10th	Night	19% 9% 12% 26% 28% 6%	1687
	11th to 20th	Night	18% 8% 14% 29% 24% 7%	1466
	21st to end	Night	23% 9% 14% 27% 22% 4	1552
June	1st to 10th	Night	20% 6% 10% 24% 29% 10%	1440
	11th to 20th	Night	9% 5% 6% 22% 36% 22%	1440
	21st to end	Night	11% 3 6% 15% 33% 33%	1440
July	1st to 10th	Night	7% 2 3 10% 31% 46%	1400
	11th to 20th	Night	9% 3 5% 12% 26% 45%	1392
	21st to end	Night	14% 3 5% 8% 25% 44%	1533
August	1st to 10th	Night	15% 4 6% 13% 23% 38%	1632
	11th to 20th	Night	10% 3 4 10% 31% 42%	1780
	21st to end	Night	8% 1 3 10% 34% 43%	1980
September	1st to 10th	Night	6% 2 4 8% 36% 44%	1800
	11th to 20th	Night	10% 4 7% 20% 34% 25%	1834
	21st to end	Night	12% 3 6% 20% 35% 24%	1980
October	1st to 10th	Night	17% 5% 8% 20% 28% 21%	2116
	11th to 20th	Night	11% 4 10% 22% 35% 18%	2160
	21st to end	Night	12% 5% 11% 25% 34% 14%	2428
November	1st to 10th	Night	18% 9% 14% 24% 23% 11%	2405
	11th to 20th	Night	17% 8% 14% 27% 27% 6%	2470
	21st to end	Night	21% 9% 13% 27% 24% 7%	2517
December	1st to 10th	Night	27% 11% 14% 24% 19% 5%	2660
	11th to 20th	Night	32% 15% 15% 19% 16% 3	3276
	21st to end	Night	33% 13% 20% 18% 13% 3	4862

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	18% 15% 30% 21% 13% 2	180
	11th to 20th	Day	9% 16% 31% 22% 17% 5%	196
	21st to end	Day	8% 13% 30% 26% 18% 4	374
February	1st to 10th	Day	11% 13% 29% 27% 17% 4	345
	11th to 20th	Day	10% 20% 26% 31% 12% 1	415
	21st to end	Day	9% 21% 26% 30% 14%	346
March	1st to 10th	Day	10% 22% 33% 22% 10% 2	420
	11th to 20th	Day	10% 22% 31% 20% 15% 4	420
	21st to end	Day	12% 24% 29% 24% 11%	462
April	1st to 10th	Day	11% 26% 29% 20% 11% 4	420
	11th to 20th	Day	15% 26% 26% 19% 11% 3	412
	21st to end	Day	11% 22% 29% 22% 13% 4	420
May	1st to 10th	Day	13% 21% 27% 25% 11% 3	423
	11th to 20th	Day	12% 23% 25% 21% 12% 6%	448
	21st to end	Day	11% 18% 21% 24% 20% 7%	485
June	1st to 10th	Day	8% 18% 28% 21% 19% 6%	450
	11th to 20th	Day	9% 18% 30% 23% 14% 5%	450
	21st to end	Day	3 12% 27% 30% 22% 6%	450
July	1st to 10th	Day	1 13% 31% 28% 21% 6%	446
	11th to 20th	Day	2 10% 27% 31% 22% 7%	444
	21st to end	Day	3 7% 27% 30% 25% 7%	487
August	1st to 10th	Day	4 11% 26% 27% 25% 7%	447
	11th to 20th	Day	1 9% 25% 28% 26% 12%	437
	21st to end	Day	2 8% 29% 31% 25% 5%	495
September	1st to 10th	Day	3 7% 29% 28% 25% 8%	450
	11th to 20th	Day	5% 14% 24% 28% 23% 6%	450
	21st to end	Day	4 13% 28% 35% 16% 4	450
October	1st to 10th	Day	5% 14% 27% 25% 22% 6%	450
	11th to 20th	Day	7% 12% 22% 29% 24% 6%	450
	21st to end	Day	6% 14% 24% 27% 23% 5%	441
November	1st to 10th	Day	11% 12% 23% 22% 21% 9%	270
	11th to 20th	Day	4 14% 21% 30% 21% 9%	270
	21st to end	Day	5% 13% 27% 24% 23% 7%	246
December	1st to 10th	Day	10% 14% 23% 23% 22% 8%	180
	11th to 20th	Day	6% 18% 32% 22% 15% 8%	180
	21st to end	Day	16% 16% 21% 28% 16% 4	198
January	1st to 10th	Night	6% 13% 29% 26% 19% 8%	760
	11th to 20th	Night	5% 12% 24% 37% 17% 5%	728
	21st to end	Night	5% 7% 29% 34% 19% 6%	792
February	1st to 10th	Night	7% 10% 25% 35% 21% 4	720
	11th to 20th	Night	4 11% 27% 34% 19% 5%	704
	21st to end	Night	7% 15% 26% 35% 14% 3	592
March	1st to 10th	Night	6% 14% 34% 28% 15% 4	720
	11th to 20th	Night	4 15% 26% 26% 26% 3	720
	21st to end	Night	4 19% 30% 31% 15% 2	792
April	1st to 10th	Night	5% 13% 29% 32% 20% 1	720
	11th to 20th	Night	8% 15% 25% 32% 17% 4	711
	21st to end	Night	6% 12% 26% 32% 20% 5%	720
May	1st to 10th	Night	6% 13% 25% 32% 21% 4	607
	11th to 20th	Night	3 8% 24% 40% 22% 3	404
	21st to end	Night	6% 6% 17% 38% 29% 4	389
June	1st to 10th	Night	6% 4 22% 37% 27% 4	360
	11th to 20th	Night	1 6% 22% 41% 29% 2	360
	21st to end	Night	2 3 18% 43% 31% 3	360
July	1st to 10th	Night	1 1 12% 50% 33% 3	350
	11th to 20th	Night	1 16% 54% 27% 1	348
	21st to end	Night	4 1 10% 44% 36% 5%	441
August	1st to 10th	Night	4 1 13% 42% 37% 4	567
	11th to 20th	Night	3 3 14% 45% 30% 5%	712
	21st to end	Night	2 2 17% 46% 30% 4	792
September	1st to 10th	Night	2 2 16% 38% 37% 6%	720
	11th to 20th	Night	2 5% 18% 31% 37% 7%	720
	21st to end	Night	3 5% 21% 32% 30% 8%	720
October	1st to 10th	Night	5% 6% 20% 36% 28% 6%	720
	11th to 20th	Night	5% 8% 13% 39% 28% 7%	720
	21st to end	Night	4 7% 19% 31% 29% 11%	792
November	1st to 10th	Night	4 12% 20% 28% 29% 7%	740
	11th to 20th	Night	4 10% 26% 36% 19% 5%	760
	21st to end	Night	6% 9% 26% 31% 23% 5%	760
December	1st to 10th	Night	8% 11% 24% 34% 16% 6%	760
	11th to 20th	Night	6% 13% 26% 33% 16% 5%	760
	21st to end	Night	8% 12% 32% 32% 14% 3	836

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