Skyview Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Skyview Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	6% 4 1 4 3 3 3 3 5% 6% 61%	152106
	Night	5% 3 1 4 3 3 3 3 5% 7% 63%	147251
Transparency	Day	23% 4 8% 21% 28% 17%	147199
	Night	21% 4 8% 21% 28% 19%	143168
Seeing	Day	7% 5% 23% 33% 24% 7%	17579
	Night	7% 6% 32% 40% 15% 1	27468

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	7% 5% 2 5% 3 3 4 3 4 6% 58%	5287
	11th to 20th	Day	2 3 1 3 2 2 2 2 4 5% 75%	5587
	21st to end	Day	8% 5% 2 5% 3 4 4 3 5% 6% 54%	6468
February	1st to 10th	Day	5% 6% 2 4 2 3 3 2 4 6% 64%	2100
	11th to 20th	Day	6% 5% 2 4 4 3 3 3 5% 6% 58%	2049
	21st to end	Day	5% 6% 2 4 3 2 2 3 4 5% 63%	1730
March	1st to 10th	Day	5% 5% 2 4 3 3 2 3 4 6% 64%	4600
	11th to 20th	Day	9% 4 2 4 2 3 3 3 5% 6% 59%	4861
	21st to end	Day	3 3 2 5% 3 3 4 4 5% 8% 61%	5866
April	1st to 10th	Day	5% 4 1 4 2 2 3 3 4 6% 66%	2520
	11th to 20th	Day	8% 5% 2 5% 3 3 2 3 4 6% 60%	2491
	21st to end	Day	4 4 1 4 3 3 3 3 5% 6% 65%	2520
May	1st to 10th	Day	9% 5% 2 5% 3 3 3 3 4 6% 57%	7511
	11th to 20th	Day	8% 4 1 3 2 2 2 2 4 5% 66%	7627
	21st to end	Day	10% 5% 2 5% 3 4 4 3 5% 7% 53%	8404
June	1st to 10th	Day	4 4 2 4 3 3 4 3 6% 10% 57%	2730
	11th to 20th	Day	4 4 1 3 3 3 3 3 5% 7% 64%	2730
	21st to end	Day	4 2 1 3 4 4 5% 4 6% 6% 61%	2730
July	1st to 10th	Day	7% 4 2 6% 4 4 5% 4 6% 7% 49%	7679
	11th to 20th	Day	4 3 1 5% 3 4 4 4 6% 8% 57%	7680
	21st to end	Day	2 2 1 3 3 3 3 3 5% 8% 66%	8109
August	1st to 10th	Day	2 2 1 3 2 3 3 3 6% 8% 68%	5468
	11th to 20th	Day	3 2 1 4 3 4 5% 4 7% 10% 58%	5415
	21st to end	Day	5% 3 1 4 3 4 4 4 6% 7% 59%	5984
September	1st to 10th	Day	8% 5% 2 5% 4 4 3 3 5% 8% 52%	2382
	11th to 20th	Day	9% 5% 1 5% 4 4 5% 4 5% 8% 50%	2200
	21st to end	Day	9% 3 1 4 3 3 3 3 4 5% 64%	2200
October	1st to 10th	Day	4 3 1 2 2 2 2 2 2 3 77%	3590
	11th to 20th	Day	12% 4 1 3 2 2 2 2 3 4 64%	3553
	21st to end	Day	5% 3 1 2 2 2 2 2 4 4 74%	3304
November	1st to 10th	Day	6% 3 1 3 2 2 3 2 4 5% 69%	1853
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 6% 69%	1890
	21st to end	Day	9% 5% 1 3 2 2 3 3 5% 5% 62%	1890
December	1st to 10th	Day	6% 5% 2 5% 3 4 3 2 5% 6% 59%	3030
	11th to 20th	Day	4 3 1 4 4 3 3 3 4 5% 66%	3481
	21st to end	Day	5% 5% 2 4 2 2 3 2 3 4 69%	4587
January	1st to 10th	Night	4 5% 2 4 3 3 3 3 5% 7% 60%	7634
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 6% 77%	7458
	21st to end	Night	7% 6% 2 5% 3 3 4 3 5% 7% 54%	7746
February	1st to 10th	Night	6% 5% 2 4 3 2 3 3 4 5% 64%	2520
	11th to 20th	Night	6% 4 2 5% 3 3 3 3 5% 7% 59%	2458
	21st to end	Night	5% 4 1 3 3 3 4 3 5% 6% 63%	2076
March	1st to 10th	Night	3 5% 1 4 3 3 3 3 4 6% 66%	5494
	11th to 20th	Night	5% 4 1 3 3 2 3 2 4 7% 65%	5327
	21st to end	Night	3 3 1 4 3 3 4 3 5% 8% 63%	5302
April	1st to 10th	Night	5% 3 1 4 3 3 3 3 4 6% 65%	2100
	11th to 20th	Night	6% 3 1 5% 3 3 2 4 5% 7% 61%	2077
	21st to end	Night	5% 3 1 4 3 3 4 3 4 7% 64%	2100
May	1st to 10th	Night	9% 3 1 3 3 3 3 3 4 5% 63%	5419
	11th to 20th	Night	5% 2 1 2 2 3 3 2 4 5% 69%	5265
	21st to end	Night	8% 5% 2 4 3 4 4 4 5% 6% 54%	5803
June	1st to 10th	Night	4 3 2 4 3 3 4 4 6% 11% 56%	1890
	11th to 20th	Night	2 2 1 2 2 2 4 4 7% 9% 63%	1890
	21st to end	Night	3 2 1 4 3 2 3 4 7% 9% 61%	1890
July	1st to 10th	Night	5% 5% 2 6% 5% 4 6% 5% 8% 10% 45%	5310
	11th to 20th	Night	3 3 1 4 4 4 4 4 7% 11% 56%	5310
	21st to end	Night	2 2 1 2 2 3 3 3 6% 10% 65%	5982
August	1st to 10th	Night	2 2 1 3 2 3 3 3 6% 11% 65%	4564
	11th to 20th	Night	3 2 1 5% 4 5% 6% 4 7% 10% 52%	4523
	21st to end	Night	5% 3 1 4 3 3 4 3 5% 10% 60%	4993
September	1st to 10th	Night	7% 3 2 5% 4 3 4 4 5% 8% 56%	2016
	11th to 20th	Night	9% 4 2 5% 3 4 4 3 6% 8% 54%	2200
	21st to end	Night	6% 3 1 4 3 3 3 3 4 6% 65%	2200
October	1st to 10th	Night	3 2 1 2 2 1 2 2 3 4 78%	3590
	11th to 20th	Night	11% 3 1 3 2 2 3 2 3 5% 64%	3570
	21st to end	Night	5% 2 1 2 2 2 2 2 3 5% 75%	4460
November	1st to 10th	Night	5% 3 1 2 2 2 2 3 4 5% 71%	2665
	11th to 20th	Night	2 3 1 3 2 2 3 2 5% 6% 71%	2730
	21st to end	Night	7% 4 1 3 3 3 3 3 5% 7% 63%	2730
December	1st to 10th	Night	6% 4 1 4 3 3 4 3 4 6% 61%	4380
	11th to 20th	Night	4 4 1 4 2 2 3 3 4 6% 68%	4946
	21st to end	Night	4 4 2 4 3 3 3 2 4 5% 67%	6633

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	28% 3 5% 19% 32% 14%	5290
	11th to 20th	Day	15% 2 5% 18% 37% 24%	5587
	21st to end	Day	31% 3 5% 15% 32% 14%	6468
February	1st to 10th	Day	23% 3 5% 19% 30% 20%	2100
	11th to 20th	Day	27% 3 5% 21% 26% 18%	2059
	21st to end	Day	25% 2 3 13% 34% 23%	1730
March	1st to 10th	Day	22% 2 3 9% 28% 35%	4600
	11th to 20th	Day	25% 3 5% 15% 27% 25%	4861
	21st to end	Day	21% 4 6% 22% 28% 19%	5866
April	1st to 10th	Day	21% 3 5% 16% 31% 24%	2520
	11th to 20th	Day	26% 4 6% 16% 25% 24%	2494
	21st to end	Day	20% 4 7% 21% 30% 19%	2520
May	1st to 10th	Day	29% 4 7% 19% 25% 17%	7511
	11th to 20th	Day	21% 3 5% 14% 24% 33%	7631
	21st to end	Day	31% 5% 10% 19% 13% 21%	8405
June	1st to 10th	Day	22% 5% 15% 28% 20% 9%	2730
	11th to 20th	Day	20% 4 9% 21% 28% 17%	2730
	21st to end	Day	21% 6% 10% 27% 24% 12%	2730
July	1st to 10th	Day	32% 6% 10% 24% 23% 4	7616
	11th to 20th	Day	23% 6% 15% 32% 20% 4	7611
	21st to end	Day	16% 5% 15% 33% 29% 3	7928
August	1st to 10th	Day	13% 4 11% 28% 32% 11%	4012
	11th to 20th	Day	17% 5% 10% 28% 31% 10%	3931
	21st to end	Day	23% 5% 12% 28% 26% 7%	4311
September	1st to 10th	Day	30% 5% 11% 20% 22% 12%	2382
	11th to 20th	Day	32% 5% 9% 24% 24% 7%	2200
	21st to end	Day	24% 3 7% 19% 34% 14%	2200
October	1st to 10th	Day	15% 3 6% 13% 30% 34%	3590
	11th to 20th	Day	25% 2 7% 17% 25% 24%	3553
	21st to end	Day	16% 2 3 22% 36% 21%	3304
November	1st to 10th	Day	19% 3 6% 17% 32% 23%	1853
	11th to 20th	Day	18% 3 4 20% 36% 19%	1890
	21st to end	Day	24% 3 5% 17% 31% 21%	1890
December	1st to 10th	Day	27% 2 5% 26% 28% 13%	3030
	11th to 20th	Day	21% 2 5% 15% 36% 20%	3479
	21st to end	Day	21% 2 4 21% 35% 16%	4587
January	1st to 10th	Night	25% 3 4 26% 33% 10%	7635
	11th to 20th	Night	11% 2 7% 22% 39% 19%	7458
	21st to end	Night	30% 3 6% 16% 29% 16%	7746
February	1st to 10th	Night	24% 2 5% 15% 30% 23%	2520
	11th to 20th	Night	26% 3 7% 19% 25% 20%	2458
	21st to end	Night	22% 3 4 14% 27% 29%	2076
March	1st to 10th	Night	20% 2 4 11% 25% 37%	5494
	11th to 20th	Night	20% 3 7% 16% 17% 38%	5327
	21st to end	Night	19% 4 8% 20% 28% 21%	5302
April	1st to 10th	Night	21% 3 7% 17% 27% 25%	2100
	11th to 20th	Night	23% 3 9% 17% 25% 23%	2077
	21st to end	Night	21% 4 9% 21% 26% 19%	2100
May	1st to 10th	Night	24% 4 9% 25% 21% 17%	5418
	11th to 20th	Night	17% 5% 7% 13% 28% 31%	5266
	21st to end	Night	29% 7% 13% 15% 10% 26%	5805
June	1st to 10th	Night	21% 7% 16% 29% 17% 10%	1890
	11th to 20th	Night	16% 5% 13% 23% 27% 17%	1890
	21st to end	Night	18% 6% 13% 28% 21% 14%	1890
July	1st to 10th	Night	32% 6% 12% 29% 19% 2	5265
	11th to 20th	Night	20% 6% 16% 31% 22% 4	5256
	21st to end	Night	14% 5% 13% 30% 35% 3	5850
August	1st to 10th	Night	14% 4 9% 28% 33% 11%	3344
	11th to 20th	Night	20% 6% 9% 25% 31% 9%	3283
	21st to end	Night	18% 4 16% 28% 28% 6%	3597
September	1st to 10th	Night	26% 7% 11% 21% 25% 11%	2016
	11th to 20th	Night	28% 5% 13% 20% 26% 7%	2200
	21st to end	Night	22% 3 7% 21% 30% 17%	2200
October	1st to 10th	Night	13% 3 6% 12% 25% 40%	3590
	11th to 20th	Night	24% 3 5% 18% 26% 23%	3570
	21st to end	Night	14% 2 4 20% 36% 25%	4460
November	1st to 10th	Night	16% 2 6% 20% 33% 22%	2665
	11th to 20th	Night	15% 2 5% 19% 38% 21%	2730
	21st to end	Night	22% 3 5% 17% 34% 19%	2730
December	1st to 10th	Night	25% 3 6% 21% 31% 14%	4380
	11th to 20th	Night	19% 2 3 14% 39% 22%	4947
	21st to end	Night	21% 3 4 21% 36% 16%	6633

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	12% 10% 31% 33% 13% 2	330
	11th to 20th	Day	7% 12% 34% 32% 13% 2	330
	21st to end	Day	8% 7% 35% 34% 15% 1	363
February	1st to 10th	Day	8% 8% 36% 35% 11% 1	330
	11th to 20th	Day	8% 11% 41% 29% 11% 1	330
	21st to end	Day	7% 12% 41% 29% 9% 1	273
March	1st to 10th	Day	8% 8% 39% 32% 12%	330
	11th to 20th	Day	15% 8% 31% 32% 12% 2	330
	21st to end	Day	6% 11% 39% 31% 11% 2	489
April	1st to 10th	Day	8% 13% 43% 27% 9% 1	540
	11th to 20th	Day	11% 11% 46% 26% 5% 1	532
	21st to end	Day	7% 11% 39% 33% 9% 1	540
May	1st to 10th	Day	7% 8% 37% 35% 11% 1	700
	11th to 20th	Day	12% 7% 31% 30% 17% 4	775
	21st to end	Day	8% 6% 29% 34% 18% 3	844
June	1st to 10th	Day	5% 3 17% 44% 27% 4	780
	11th to 20th	Day	5% 2 23% 41% 24% 5%	780
	21st to end	Day	3 2 14% 41% 33% 8%	780
July	1st to 10th	Day	6% 2 6% 33% 37% 17%	771
	11th to 20th	Day	3 2 3 31% 45% 16%	768
	21st to end	Day	2 3 27% 47% 20%	772
August	1st to 10th	Day	2 1 5% 33% 42% 17%	567
	11th to 20th	Day	5% 1 5% 29% 40% 20%	534
	21st to end	Day	5% 1 4 26% 40% 24%	583
September	1st to 10th	Day	8% 2 5% 33% 36% 16%	512
	11th to 20th	Day	1 2 5% 37% 40% 15%	330
	21st to end	Day	11% 2 9% 26% 38% 13%	330
October	1st to 10th	Day	9% 2 15% 35% 30% 9%	330
	11th to 20th	Day	7% 4 22% 37% 24% 6%	330
	21st to end	Day	12% 6% 25% 26% 26% 6%	363
November	1st to 10th	Day	12% 5% 26% 32% 22% 3	330
	11th to 20th	Day	5% 8% 23% 35% 22% 6%	330
	21st to end	Day	14% 9% 34% 26% 13% 3	330
December	1st to 10th	Day	9% 5% 37% 31% 16% 1	330
	11th to 20th	Day	6% 7% 28% 43% 15% 2	330
	21st to end	Day	9% 9% 41% 27% 12% 2	363
January	1st to 10th	Night	7% 9% 36% 40% 9%	1050
	11th to 20th	Night	6% 7% 37% 40% 9% 1	971
	21st to end	Night	7% 7% 40% 38% 8%	924
February	1st to 10th	Night	9% 7% 42% 33% 9% 1	840
	11th to 20th	Night	8% 9% 45% 32% 5%	824
	21st to end	Night	7% 11% 46% 33% 3	692
March	1st to 10th	Night	7% 11% 42% 35% 5%	840
	11th to 20th	Night	10% 11% 42% 31% 6%	737
	21st to end	Night	5% 12% 47% 32% 5%	693
April	1st to 10th	Night	6% 12% 46% 33% 3	630
	11th to 20th	Night	6% 10% 54% 28% 3	623
	21st to end	Night	7% 9% 51% 30% 4	630
May	1st to 10th	Night	7% 5% 47% 37% 3	630
	11th to 20th	Night	7% 6% 37% 38% 12%	621
	21st to end	Night	9% 3 39% 39% 9%	683
June	1st to 10th	Night	4 3 31% 51% 11%	630
	11th to 20th	Night	3 3 38% 46% 10%	630
	21st to end	Night	3 1 24% 50% 21%	630
July	1st to 10th	Night	5% 1 11% 47% 33% 2	615
	11th to 20th	Night	3 1 6% 50% 39% 1	612
	21st to end	Night	1 1 5% 45% 46% 2	690
August	1st to 10th	Night	2 1 7% 49% 40% 2	627
	11th to 20th	Night	4 1 6% 48% 38% 2	606
	21st to end	Night	4 1 5% 45% 43% 2	660
September	1st to 10th	Night	8% 2 9% 47% 33% 2	600
	11th to 20th	Night	10% 1 11% 49% 27% 1	600
	21st to end	Night	8% 2 17% 42% 29% 2	600
October	1st to 10th	Night	6% 2 25% 47% 20% 1	600
	11th to 20th	Night	10% 4 29% 44% 13% 1	600
	21st to end	Night	9% 5% 27% 37% 20% 2	1000
November	1st to 10th	Night	7% 4 28% 44% 15% 1	1025
	11th to 20th	Night	4 6% 31% 42% 15% 2	1050
	21st to end	Night	9% 7% 32% 38% 13% 1	1050
December	1st to 10th	Night	9% 7% 34% 41% 9% 1	1050
	11th to 20th	Night	9% 7% 39% 36% 9%	1050
	21st to end	Night	9% 8% 37% 37% 9% 1	1155

Page generated at 2026-08-12 02:37:19. Version ac07

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