Watson Starview Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watson Starview Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	10% 5% 2 5% 4 4 4 3 5% 7% 50%	131262
	Night	11% 5% 2 5% 3 3 3 3 5% 7% 54%	129920
Transparency	Day	32% 5% 10% 21% 21% 11%	135030
	Night	31% 5% 10% 20% 21% 13%	134216
Seeing	Day	12% 7% 23% 30% 22% 6%	16572
	Night	12% 7% 29% 37% 14% 1	26602

By Month

Percentage Cloud Cover SE (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	10% 8% 3 5% 3 3 3 2 4 6% 54%	5289
	11th to 20th	Day	12% 5% 2 4 2 2 3 2 5% 6% 58%	5310
	21st to end	Day	20% 7% 3 7% 4 3 3 2 3 4 43%	6111
February	1st to 10th	Day	16% 7% 2 5% 3 2 3 2 3 5% 51%	2100
	11th to 20th	Day	14% 8% 2 5% 4 4 4 3 5% 5% 47%	1999
	21st to end	Day	17% 6% 2 5% 3 3 3 2 4 5% 51%	1650
March	1st to 10th	Day	11% 6% 2 6% 4 3 3 3 4 5% 52%	4882
	11th to 20th	Day	13% 5% 2 4 4 3 4 3 4 6% 52%	4980
	21st to end	Day	10% 6% 3 6% 3 4 5% 4 5% 6% 48%	5462
April	1st to 10th	Day	11% 8% 2 7% 4 4 3 4 5% 6% 45%	2200
	11th to 20th	Day	14% 6% 2 6% 5% 4 4 3 4 6% 46%	2188
	21st to end	Day	12% 5% 1 5% 5% 4 5% 3 5% 6% 49%	2580
May	1st to 10th	Day	15% 6% 3 7% 4 4 5% 4 6% 8% 37%	5862
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 6% 8% 45%	5862
	21st to end	Day	12% 7% 2 6% 5% 5% 5% 5% 7% 8% 37%	6434
June	1st to 10th	Day	5% 4 1 4 4 4 5% 4 6% 9% 54%	2600
	11th to 20th	Day	6% 3 2 5% 4 3 4 5% 7% 9% 54%	2600
	21st to end	Day	6% 4 1 4 5% 5% 5% 5% 7% 10% 49%	2600
July	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 6% 9% 41%	5900
	11th to 20th	Day	4 3 1 4 4 4 5% 4 7% 10% 55%	5900
	21st to end	Day	6% 4 2 5% 4 4 5% 5% 7% 11% 49%	6422
August	1st to 10th	Day	1 1 3 2 3 3 3 5% 7% 71%	4234
	11th to 20th	Day	3 2 1 3 3 3 3 4 7% 8% 63%	3917
	21st to end	Day	4 3 1 4 4 5% 5% 5% 7% 9% 53%	4147
September	1st to 10th	Day	10% 4 2 5% 4 5% 5% 4 6% 6% 50%	2200
	11th to 20th	Day	10% 4 2 7% 5% 4 4 3 6% 8% 47%	2200
	21st to end	Day	7% 3 2 4 3 4 4 3 6% 9% 55%	2200
October	1st to 10th	Day	5% 4 1 4 3 4 4 3 4 7% 62%	2200
	11th to 20th	Day	9% 4 1 4 3 3 3 2 3 5% 60%	2050
	21st to end	Day	11% 4 1 4 2 3 3 3 3 5% 61%	2200
November	1st to 10th	Day	13% 6% 1 5% 4 3 3 2 4 6% 53%	2058
	11th to 20th	Day	7% 5% 3 5% 4 3 3 3 5% 6% 56%	1937
	21st to end	Day	15% 6% 2 5% 4 4 4 2 4 6% 49%	1890
December	1st to 10th	Day	18% 7% 2 7% 3 3 3 2 4 4 46%	3030
	11th to 20th	Day	13% 7% 2 6% 4 4 4 3 5% 5% 47%	3481
	21st to end	Day	11% 6% 2 4 3 2 3 2 3 4 60%	4587
January	1st to 10th	Night	12% 6% 2 4 3 2 2 2 3 4 59%	7634
	11th to 20th	Night	8% 3 1 4 2 2 2 2 4 6% 66%	7680
	21st to end	Night	23% 7% 3 6% 3 3 3 2 3 5% 41%	8145
February	1st to 10th	Night	12% 6% 2 6% 3 3 3 2 4 5% 53%	2520
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 4 6% 50%	2398
	21st to end	Night	16% 5% 2 5% 3 3 3 3 5% 5% 51%	1980
March	1st to 10th	Night	10% 6% 2 6% 4 3 4 3 5% 7% 49%	4973
	11th to 20th	Night	11% 5% 2 4 3 4 4 3 5% 6% 52%	4980
	21st to end	Night	10% 8% 2 6% 4 4 3 3 4 6% 50%	5467
April	1st to 10th	Night	9% 6% 3 4 4 5% 4 3 5% 7% 51%	2200
	11th to 20th	Night	12% 5% 2 6% 4 4 4 3 6% 6% 47%	2155
	21st to end	Night	11% 7% 2 5% 4 3 4 3 4 6% 51%	1820
May	1st to 10th	Night	19% 6% 2 5% 3 3 4 3 5% 7% 42%	4053
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 4 5% 8% 49%	4053
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 7% 9% 42%	4443
June	1st to 10th	Night	3 2 1 4 3 3 4 4 7% 10% 59%	1800
	11th to 20th	Night	4 3 1 3 3 3 3 4 5% 10% 60%	1800
	21st to end	Night	4 4 1 3 3 4 5% 3 6% 9% 58%	1800
July	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 7% 9% 46%	4078
	11th to 20th	Night	3 1 1 4 2 3 4 4 6% 11% 62%	4077
	21st to end	Night	3 4 1 5% 3 4 5% 4 7% 11% 53%	4449
August	1st to 10th	Night	1 1 1 1 2 2 2 4 7% 78%	2926
	11th to 20th	Night	3 2 1 3 2 3 3 3 6% 10% 64%	3137
	21st to end	Night	4 3 1 4 3 3 4 3 5% 11% 59%	3755
September	1st to 10th	Night	9% 4 2 5% 4 4 4 4 5% 6% 53%	2200
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 5% 8% 49%	2200
	21st to end	Night	7% 2 1 4 3 2 4 4 5% 8% 59%	2200
October	1st to 10th	Night	5% 3 1 2 3 3 3 3 5% 6% 68%	2200
	11th to 20th	Night	10% 3 1 3 3 2 3 3 4 6% 62%	2330
	21st to end	Night	11% 4 1 4 2 3 3 2 4 5% 62%	2640
November	1st to 10th	Night	12% 4 2 4 3 3 2 3 4 5% 59%	2460
	11th to 20th	Night	6% 5% 1 5% 3 3 4 2 3 6% 61%	2678
	21st to end	Night	14% 6% 2 5% 3 3 4 3 5% 6% 50%	2730
December	1st to 10th	Night	18% 6% 3 5% 3 3 3 3 5% 6% 46%	4380
	11th to 20th	Night	15% 9% 2 5% 3 3 3 2 4 5% 49%	4946
	21st to end	Night	12% 6% 2 4 2 3 2 2 3 4 59%	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	34% 3 6% 24% 24% 9%	5290
	11th to 20th	Day	29% 2 5% 16% 29% 18%	5310
	21st to end	Day	48% 2 4 13% 22% 11%	6111
February	1st to 10th	Day	38% 4 7% 13% 21% 17%	2100
	11th to 20th	Day	39% 4 7% 19% 19% 12%	2009
	21st to end	Day	37% 4 5% 15% 21% 17%	1650
March	1st to 10th	Day	34% 4 5% 16% 19% 21%	4882
	11th to 20th	Day	34% 6% 8% 8% 14% 31%	4980
	21st to end	Day	36% 5% 9% 18% 23% 8%	5464
April	1st to 10th	Day	39% 6% 8% 16% 15% 15%	2200
	11th to 20th	Day	40% 5% 9% 16% 19% 12%	2188
	21st to end	Day	37% 6% 7% 20% 22% 9%	2580
May	1st to 10th	Day	44% 7% 11% 17% 17% 4	5900
	11th to 20th	Day	36% 6% 10% 15% 24% 8%	5866
	21st to end	Day	41% 8% 12% 22% 13% 5%	6447
June	1st to 10th	Day	26% 8% 14% 27% 21% 4	2600
	11th to 20th	Day	26% 7% 13% 28% 20% 6%	2600
	21st to end	Day	30% 10% 13% 23% 21% 3	2600
July	1st to 10th	Day	39% 8% 18% 28% 7% 1	5836
	11th to 20th	Day	23% 7% 16% 34% 17% 3	5831
	21st to end	Day	28% 7% 17% 28% 17% 2	6240
August	1st to 10th	Day	12% 4 12% 35% 35% 1	4220
	11th to 20th	Day	18% 6% 19% 39% 15% 2	3955
	21st to end	Day	25% 9% 18% 27% 19% 2	4146
September	1st to 10th	Day	34% 6% 10% 22% 18% 10%	2200
	11th to 20th	Day	35% 7% 12% 19% 20% 5%	2200
	21st to end	Day	26% 7% 10% 21% 21% 14%	2200
October	1st to 10th	Day	15% 3 6% 13% 25% 38%	3590
	11th to 20th	Day	18% 4 7% 19% 30% 23%	3316
	21st to end	Day	23% 2 4 18% 38% 14%	3538
November	1st to 10th	Day	34% 4 8% 18% 25% 11%	2058
	11th to 20th	Day	30% 5% 8% 18% 22% 17%	1937
	21st to end	Day	38% 5% 6% 17% 20% 14%	1890
December	1st to 10th	Day	43% 3 6% 16% 23% 9%	3030
	11th to 20th	Day	39% 6% 6% 12% 24% 13%	3479
	21st to end	Day	30% 3 5% 17% 28% 18%	4587
January	1st to 10th	Night	32% 2 5% 20% 27% 13%	7635
	11th to 20th	Night	22% 2 4 18% 30% 24%	7680
	21st to end	Night	48% 2 4 10% 22% 14%	8145
February	1st to 10th	Night	35% 4 6% 12% 23% 19%	2520
	11th to 20th	Night	37% 4 7% 18% 20% 15%	2398
	21st to end	Night	38% 4 6% 13% 20% 19%	1980
March	1st to 10th	Night	34% 5% 7% 15% 23% 15%	4973
	11th to 20th	Night	33% 6% 9% 10% 15% 27%	4980
	21st to end	Night	38% 5% 7% 17% 23% 10%	5467
April	1st to 10th	Night	34% 6% 9% 19% 17% 16%	2200
	11th to 20th	Night	37% 6% 10% 14% 18% 15%	2155
	21st to end	Night	35% 6% 10% 19% 20% 10%	1820
May	1st to 10th	Night	43% 6% 9% 20% 15% 7%	4080
	11th to 20th	Night	33% 7% 11% 19% 18% 12%	4054
	21st to end	Night	38% 7% 14% 26% 11% 4	4452
June	1st to 10th	Night	20% 8% 17% 30% 22% 4	1800
	11th to 20th	Night	20% 6% 16% 30% 21% 6%	1800
	21st to end	Night	23% 9% 14% 29% 20% 4	1800
July	1st to 10th	Night	34% 8% 21% 30% 6% 1	4035
	11th to 20th	Night	17% 7% 22% 37% 16% 1	4026
	21st to end	Night	25% 8% 21% 29% 14% 4	4320
August	1st to 10th	Night	8% 4 12% 39% 36% 2	2916
	11th to 20th	Night	17% 5% 27% 34% 15% 2	3165
	21st to end	Night	21% 9% 21% 29% 18% 3	3755
September	1st to 10th	Night	31% 7% 12% 22% 18% 10%	2200
	11th to 20th	Night	33% 6% 15% 23% 17% 7%	2200
	21st to end	Night	23% 6% 13% 26% 18% 14%	2200
October	1st to 10th	Night	12% 2 7% 15% 22% 42%	3590
	11th to 20th	Night	17% 4 9% 17% 31% 23%	3790
	21st to end	Night	22% 3 5% 20% 33% 17%	4252
November	1st to 10th	Night	31% 4 7% 19% 25% 14%	2460
	11th to 20th	Night	26% 4 8% 20% 23% 18%	2678
	21st to end	Night	38% 4 7% 16% 21% 15%	2730
December	1st to 10th	Night	40% 4 6% 18% 21% 11%	4380
	11th to 20th	Night	41% 3 6% 9% 24% 17%	4947
	21st to end	Night	32% 3 4 17% 27% 18%	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	18% 14% 40% 22% 6%	330
	11th to 20th	Day	21% 13% 35% 23% 8% 1	330
	21st to end	Day	17% 16% 40% 22% 5%	363
February	1st to 10th	Day	23% 12% 38% 21% 6%	330
	11th to 20th	Day	22% 17% 39% 20% 3	315
	21st to end	Day	23% 17% 39% 20% 1	249
March	1st to 10th	Day	21% 16% 38% 22% 4	300
	11th to 20th	Day	19% 18% 39% 19% 4	300
	21st to end	Day	17% 16% 40% 22% 5%	330
April	1st to 10th	Day	19% 20% 41% 16% 3	300
	11th to 20th	Day	18% 16% 40% 19% 6%	306
	21st to end	Day	16% 14% 40% 22% 8%	680
May	1st to 10th	Day	12% 12% 40% 28% 7%	703
	11th to 20th	Day	15% 9% 31% 33% 11% 2	725
	21st to end	Day	14% 6% 30% 34% 14% 2	789
June	1st to 10th	Day	6% 3 19% 43% 24% 5%	730
	11th to 20th	Day	5% 2 18% 49% 23% 3	730
	21st to end	Day	5% 2 12% 39% 34% 9%	730
July	1st to 10th	Day	9% 2 9% 35% 33% 11%	721
	11th to 20th	Day	3 1 5% 37% 45% 9%	718
	21st to end	Day	4 6% 35% 44% 11%	794
August	1st to 10th	Day	2 4 33% 46% 15%	726
	11th to 20th	Day	5% 1 4 36% 40% 15%	564
	21st to end	Day	6% 2 3 26% 37% 25%	483
September	1st to 10th	Day	11% 1 5% 25% 39% 19%	330
	11th to 20th	Day	7% 1 4 28% 43% 18%	330
	21st to end	Day	9% 2 7% 32% 36% 14%	330
October	1st to 10th	Day	5% 2 12% 35% 36% 11%	330
	11th to 20th	Day	8% 5% 24% 33% 25% 5%	330
	21st to end	Day	18% 10% 24% 27% 20% 1	363
November	1st to 10th	Day	21% 7% 28% 28% 11% 5%	330
	11th to 20th	Day	11% 10% 32% 32% 14% 2	330
	21st to end	Day	18% 15% 36% 17% 12% 1	330
December	1st to 10th	Day	17% 8% 35% 28% 9% 3	330
	11th to 20th	Day	15% 16% 35% 24% 8% 2	330
	21st to end	Day	15% 13% 38% 26% 7% 1	363
January	1st to 10th	Night	17% 10% 36% 30% 8%	1050
	11th to 20th	Night	15% 11% 36% 31% 6%	1050
	21st to end	Night	18% 12% 38% 27% 5%	1066
February	1st to 10th	Night	16% 12% 38% 29% 4	840
	11th to 20th	Night	20% 14% 41% 24% 2	804
	21st to end	Night	22% 12% 44% 21% 2	660
March	1st to 10th	Night	17% 14% 38% 29% 2	630
	11th to 20th	Night	14% 12% 40% 29% 5%	600
	21st to end	Night	11% 14% 46% 27% 3	660
April	1st to 10th	Night	12% 12% 46% 28% 2	600
	11th to 20th	Night	15% 13% 44% 25% 4	593
	21st to end	Night	12% 11% 44% 25% 7%	600
May	1st to 10th	Night	12% 11% 43% 28% 5%	600
	11th to 20th	Night	12% 6% 36% 36% 10% 1	591
	21st to end	Night	8% 7% 36% 36% 12%	650
June	1st to 10th	Night	3 3 23% 46% 23% 1	600
	11th to 20th	Night	5% 1 27% 53% 13% 1	600
	21st to end	Night	4 2 16% 52% 24% 3	600
July	1st to 10th	Night	7% 2 12% 48% 28% 3	585
	11th to 20th	Night	4 1 8% 56% 29% 2	582
	21st to end	Night	2 1 8% 54% 32% 4	657
August	1st to 10th	Night	2 5% 55% 34% 4	597
	11th to 20th	Night	5% 1 5% 53% 31% 4	594
	21st to end	Night	5% 4 45% 40% 5%	660
September	1st to 10th	Night	10% 1 9% 42% 34% 4	600
	11th to 20th	Night	10% 2 8% 41% 36% 3	600
	21st to end	Night	8% 1 12% 43% 34% 4	600
October	1st to 10th	Night	6% 2 13% 49% 27% 4	600
	11th to 20th	Night	12% 3 25% 41% 18% 2	730
	21st to end	Night	13% 9% 28% 33% 17% 1	880
November	1st to 10th	Night	13% 4 24% 43% 14% 1	820
	11th to 20th	Night	9% 7% 31% 38% 13% 1	998
	21st to end	Night	17% 11% 30% 30% 11%	1050
December	1st to 10th	Night	14% 8% 32% 36% 8% 1	1050
	11th to 20th	Night	19% 9% 32% 35% 6%	1050
	21st to end	Night	16% 10% 34% 33% 8%	1155

Page generated at 2026-02-13 00:42:15. Version ac07

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