Watch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watch Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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	16% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	144364
	Night	16% 11% 4 9% 5% 5% 5% 4 5% 6% 30%	137858
Transparency	Day	34% 34% 9% 16% 7% 1	147810
	Night	37% 34% 8% 14% 7% 1	142488
Seeing	Day	20% 10% 28% 25% 13% 4	16838
	Night	22% 9% 28% 29% 10% 1	25750

By Month

Percentage Cloud Cover NE (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	14% 18% 6% 12% 6% 5% 4 4 5% 6% 20%	4661
	11th to 20th	Day	12% 14% 6% 12% 7% 6% 6% 4 5% 6% 22%	4650
	21st to end	Day	9% 13% 5% 12% 10% 7% 7% 4 6% 5% 22%	5124
February	1st to 10th	Day	14% 15% 4 11% 6% 5% 5% 4 5% 5% 25%	1874
	11th to 20th	Day	14% 12% 4 9% 7% 6% 5% 4 6% 6% 27%	1932
	21st to end	Day	17% 14% 4 10% 6% 5% 4 3 5% 4 25%	1730
March	1st to 10th	Day	17% 12% 5% 10% 7% 6% 5% 4 5% 5% 26%	4576
	11th to 20th	Day	10% 13% 4 11% 6% 5% 4 4 6% 7% 30%	4855
	21st to end	Day	18% 8% 4 9% 5% 4 4 3 5% 5% 34%	6053
April	1st to 10th	Day	20% 11% 3 7% 4 5% 5% 3 5% 6% 31%	2520
	11th to 20th	Day	20% 9% 3 7% 5% 4 4 3 5% 5% 34%	2494
	21st to end	Day	21% 9% 3 8% 5% 4 4 3 5% 5% 32%	2662
May	1st to 10th	Day	22% 9% 4 9% 5% 5% 5% 4 6% 7% 23%	8194
	11th to 20th	Day	18% 9% 3 8% 4 4 4 4 6% 7% 33%	8221
	21st to end	Day	20% 9% 2 7% 4 4 4 4 6% 6% 34%	9060
June	1st to 10th	Day	20% 8% 3 7% 4 4 5% 4 5% 7% 31%	2940
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 5% 7% 8% 37%	2940
	21st to end	Day	18% 8% 3 6% 5% 5% 5% 4 7% 8% 32%	2940
July	1st to 10th	Day	10% 6% 3 8% 5% 5% 6% 5% 7% 9% 37%	8280
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 4 6% 9% 46%	8280
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 7% 10% 42%	8993
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 34%	4703
	11th to 20th	Day	13% 6% 3 7% 5% 5% 4 4 6% 9% 40%	4221
	21st to end	Day	15% 5% 2 6% 4 4 5% 5% 7% 9% 40%	4420
September	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 44%	2400
	11th to 20th	Day	12% 4 2 6% 3 4 4 4 5% 7% 50%	2334
	21st to end	Day	16% 7% 2 5% 4 4 5% 3 5% 7% 41%	2200
October	1st to 10th	Day	19% 7% 3 6% 5% 3 4 4 5% 6% 38%	2037
	11th to 20th	Day	17% 8% 3 8% 6% 5% 6% 4 4 6% 32%	1980
	21st to end	Day	23% 11% 3 7% 4 3 4 4 6% 6% 29%	1980
November	1st to 10th	Day	16% 10% 3 8% 4 5% 5% 4 6% 7% 32%	1851
	11th to 20th	Day	17% 10% 3 9% 5% 5% 5% 4 6% 6% 29%	1785
	21st to end	Day	21% 12% 3 9% 6% 6% 6% 4 5% 6% 22%	1680
December	1st to 10th	Day	19% 20% 5% 10% 5% 4 5% 3 6% 6% 16%	2680
	11th to 20th	Day	18% 12% 4 10% 6% 5% 5% 4 5% 4 26%	3066
	21st to end	Day	23% 14% 4 10% 5% 4 5% 4 5% 6% 20%	4048
January	1st to 10th	Night	13% 20% 7% 14% 5% 4 4 3 4 5% 20%	8234
	11th to 20th	Night	12% 16% 7% 17% 8% 6% 5% 3 5% 5% 16%	8235
	21st to end	Night	11% 15% 5% 12% 8% 6% 5% 3 4 6% 25%	9057
February	1st to 10th	Night	13% 14% 5% 12% 8% 5% 5% 4 5% 6% 22%	2741
	11th to 20th	Night	14% 11% 5% 10% 6% 5% 5% 4 5% 6% 28%	2573
	21st to end	Night	14% 15% 4 10% 6% 5% 3 3 4 5% 31%	2076
March	1st to 10th	Night	20% 13% 4 9% 5% 4 5% 3 4 5% 29%	5484
	11th to 20th	Night	16% 13% 3 8% 5% 5% 5% 4 6% 6% 29%	5309
	21st to end	Night	16% 11% 4 8% 5% 5% 5% 3 5% 5% 32%	5119
April	1st to 10th	Night	24% 10% 2 7% 5% 5% 4 3 5% 6% 30%	2100
	11th to 20th	Night	22% 9% 4 7% 4 3 4 3 5% 5% 33%	2075
	21st to end	Night	21% 9% 3 8% 5% 3 4 3 5% 6% 35%	1957
May	1st to 10th	Night	22% 9% 3 8% 5% 5% 5% 4 5% 8% 25%	4747
	11th to 20th	Night	19% 8% 3 8% 5% 5% 5% 4 5% 7% 32%	4639
	21st to end	Night	18% 9% 3 8% 5% 5% 5% 3 5% 8% 32%	5114
June	1st to 10th	Night	19% 7% 2 7% 5% 4 6% 4 6% 8% 30%	1680
	11th to 20th	Night	17% 7% 3 6% 4 3 5% 4 6% 7% 39%	1680
	21st to end	Night	18% 8% 2 7% 5% 5% 4 4 6% 6% 35%	1680
July	1st to 10th	Night	11% 6% 2 7% 5% 5% 6% 5% 8% 10% 34%	4680
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 6% 8% 45%	4680
	21st to end	Night	12% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	5084
August	1st to 10th	Night	14% 7% 3 8% 4 4 4 4 6% 7% 40%	2666
	11th to 20th	Night	15% 7% 3 7% 4 4 4 3 6% 6% 40%	3008
	21st to end	Night	14% 6% 2 7% 5% 4 4 4 6% 7% 41%	3677
September	1st to 10th	Night	11% 6% 3 7% 5% 4 5% 3 6% 7% 44%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	2065
	21st to end	Night	16% 6% 2 5% 4 4 4 3 5% 8% 42%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 4 5% 3 5% 6% 38%	2360
	11th to 20th	Night	19% 8% 2 8% 5% 5% 4 4 5% 6% 35%	2420
	21st to end	Night	22% 8% 3 8% 5% 5% 4 3 5% 5% 31%	2860
November	1st to 10th	Night	19% 9% 3 7% 5% 5% 5% 3 5% 7% 32%	2667
	11th to 20th	Night	18% 11% 4 8% 5% 5% 5% 4 5% 5% 32%	2833
	21st to end	Night	21% 13% 4 9% 6% 5% 5% 3 5% 5% 25%	2940
December	1st to 10th	Night	18% 17% 6% 12% 6% 5% 6% 3 5% 5% 18%	4720
	11th to 20th	Night	23% 14% 4 10% 5% 4 4 3 5% 4 23%	5348
	21st to end	Night	25% 16% 4 9% 4 3 3 3 5% 6% 21%	7150

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	51% 21% 11% 13% 3	4662
	11th to 20th	Day	49% 22% 8% 12% 8% 1	4680
	21st to end	Day	50% 23% 9% 9% 8% 1	5124
February	1st to 10th	Day	24% 59% 3 10% 3 1	1874
	11th to 20th	Day	24% 57% 4 9% 5%	1941
	21st to end	Day	25% 58% 3 6% 6% 1	1730
March	1st to 10th	Day	45% 29% 5% 10% 8% 3	4579
	11th to 20th	Day	38% 29% 13% 11% 9% 1	4855
	21st to end	Day	39% 28% 4 13% 14% 1	6053
April	1st to 10th	Day	22% 59% 3 8% 6% 1	2520
	11th to 20th	Day	22% 59% 3 10% 6% 1	2494
	21st to end	Day	22% 58% 4 9% 7%	2662
May	1st to 10th	Day	47% 24% 7% 16% 6%	8206
	11th to 20th	Day	41% 23% 11% 18% 6%	8225
	21st to end	Day	42% 22% 9% 19% 8%	9061
June	1st to 10th	Day	21% 58% 3 10% 7% 1	2940
	11th to 20th	Day	19% 59% 5% 10% 6% 1	2940
	21st to end	Day	20% 58% 4 12% 5% 1	2940
July	1st to 10th	Day	34% 22% 12% 22% 10%	8210
	11th to 20th	Day	26% 21% 12% 29% 12%	8205
	21st to end	Day	30% 21% 14% 29% 6%	8795
August	1st to 10th	Day	30% 36% 13% 15% 6%	4689
	11th to 20th	Day	31% 35% 11% 20% 3	4263
	21st to end	Day	26% 36% 12% 16% 9%	4420
September	1st to 10th	Day	14% 58% 7% 16% 6%	2400
	11th to 20th	Day	13% 57% 6% 16% 7% 1	2334
	21st to end	Day	16% 57% 6% 13% 7% 1	2200
October	1st to 10th	Day	28% 36% 9% 21% 6%	3310
	11th to 20th	Day	23% 36% 14% 22% 5%	3208
	21st to end	Day	27% 36% 16% 17% 5%	3180
November	1st to 10th	Day	27% 54% 6% 9% 4	1851
	11th to 20th	Day	23% 54% 5% 12% 5% 1	1785
	21st to end	Day	29% 55% 6% 7% 3	1680
December	1st to 10th	Day	47% 35% 6% 11% 1	2680
	11th to 20th	Day	43% 32% 6% 11% 7%	3066
	21st to end	Day	48% 26% 8% 13% 5%	4048
January	1st to 10th	Night	54% 20% 10% 11% 4	8234
	11th to 20th	Night	56% 22% 8% 10% 4	8280
	21st to end	Night	48% 22% 7% 12% 10%	9057
February	1st to 10th	Night	26% 58% 4 9% 3	2741
	11th to 20th	Night	23% 58% 3 10% 5%	2575
	21st to end	Night	21% 58% 3 9% 8%	2076
March	1st to 10th	Night	42% 28% 5% 11% 10% 4	5494
	11th to 20th	Night	38% 29% 10% 12% 9% 2	5310
	21st to end	Night	41% 28% 4 16% 9% 1	5119
April	1st to 10th	Night	24% 58% 3 8% 6%	2100
	11th to 20th	Night	25% 59% 2 8% 6% 1	2075
	21st to end	Night	22% 59% 3 7% 8% 1	1957
May	1st to 10th	Night	46% 24% 11% 14% 5%	4753
	11th to 20th	Night	42% 23% 9% 19% 6%	4640
	21st to end	Night	42% 23% 8% 15% 12%	5115
June	1st to 10th	Night	20% 59% 3 10% 8% 1	1680
	11th to 20th	Night	18% 59% 4 10% 8% 1	1680
	21st to end	Night	21% 57% 4 11% 6% 1	1680
July	1st to 10th	Night	36% 22% 15% 16% 10% 1	4640
	11th to 20th	Night	29% 22% 13% 28% 9%	4632
	21st to end	Night	33% 21% 13% 24% 9%	4967
August	1st to 10th	Night	30% 36% 10% 18% 5%	2656
	11th to 20th	Night	33% 36% 12% 15% 4	3032
	21st to end	Night	29% 37% 10% 14% 10% 1	3677
September	1st to 10th	Night	14% 57% 6% 15% 8%	2000
	11th to 20th	Night	14% 56% 7% 16% 6% 1	2065
	21st to end	Night	17% 57% 5% 13% 9%	2200
October	1st to 10th	Night	26% 36% 11% 20% 6% 1	3854
	11th to 20th	Night	25% 36% 9% 23% 7%	3931
	21st to end	Night	29% 36% 13% 14% 9%	4610
November	1st to 10th	Night	27% 53% 6% 10% 4 1	2667
	11th to 20th	Night	25% 54% 5% 10% 5%	2833
	21st to end	Night	28% 55% 6% 7% 3	2940
December	1st to 10th	Night	45% 35% 8% 9% 3	4720
	11th to 20th	Night	45% 31% 5% 9% 9%	5348
	21st to end	Night	48% 26% 6% 12% 8%	7150

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	24% 13% 31% 22% 10%	220
	11th to 20th	Day	28% 15% 32% 19% 6%	220
	21st to end	Day	19% 15% 34% 23% 9% 1	242
February	1st to 10th	Day	26% 16% 36% 17% 5%	322
	11th to 20th	Day	22% 12% 42% 21% 3	419
	21st to end	Day	26% 11% 31% 28% 3	446
March	1st to 10th	Day	23% 12% 35% 22% 7%	540
	11th to 20th	Day	22% 13% 35% 22% 6% 2	540
	21st to end	Day	24% 13% 31% 18% 11% 2	594
April	1st to 10th	Day	28% 13% 27% 21% 9% 1	540
	11th to 20th	Day	26% 14% 27% 20% 11% 3	532
	21st to end	Day	26% 12% 29% 21% 10% 2	540
May	1st to 10th	Day	23% 10% 29% 26% 9% 3	543
	11th to 20th	Day	19% 9% 30% 23% 15% 4	567
	21st to end	Day	20% 10% 30% 25% 12% 4	617
June	1st to 10th	Day	20% 9% 25% 29% 13% 4	570
	11th to 20th	Day	16% 9% 27% 30% 16% 2	570
	21st to end	Day	21% 7% 28% 26% 15% 4	570
July	1st to 10th	Day	10% 8% 27% 31% 18% 6%	565
	11th to 20th	Day	12% 8% 28% 29% 17% 6%	564
	21st to end	Day	13% 8% 22% 34% 19% 5%	620
August	1st to 10th	Day	14% 5% 23% 30% 21% 7%	566
	11th to 20th	Day	11% 5% 22% 32% 21% 9%	559
	21st to end	Day	14% 7% 26% 25% 20% 8%	619
September	1st to 10th	Day	15% 7% 24% 25% 19% 10%	530
	11th to 20th	Day	12% 6% 21% 25% 23% 13%	530
	21st to end	Day	18% 8% 25% 23% 19% 6%	530
October	1st to 10th	Day	20% 9% 26% 26% 14% 6%	530
	11th to 20th	Day	20% 12% 31% 24% 12% 2	519
	21st to end	Day	28% 12% 25% 25% 10%	462
November	1st to 10th	Day	27% 15% 23% 25% 9% 1	425
	11th to 20th	Day	23% 14% 34% 21% 8% 1	325
	21st to end	Day	29% 15% 31% 18% 6%	220
December	1st to 10th	Day	30% 10% 28% 23% 7% 1	220
	11th to 20th	Day	25% 14% 40% 16% 5% 1	220
	21st to end	Day	29% 12% 32% 19% 5% 2	242
January	1st to 10th	Night	24% 12% 32% 27% 5%	840
	11th to 20th	Night	28% 10% 31% 25% 6%	840
	21st to end	Night	19% 10% 35% 28% 8% 1	924
February	1st to 10th	Night	19% 11% 36% 27% 7% 1	840
	11th to 20th	Night	20% 11% 36% 27% 5%	823
	21st to end	Night	22% 9% 34% 30% 5%	692
March	1st to 10th	Night	24% 12% 31% 27% 6%	840
	11th to 20th	Night	25% 11% 30% 27% 6%	840
	21st to end	Night	23% 11% 28% 29% 9%	924
April	1st to 10th	Night	28% 11% 27% 29% 5%	840
	11th to 20th	Night	25% 11% 30% 27% 6%	831
	21st to end	Night	26% 9% 27% 28% 9%	697
May	1st to 10th	Night	25% 6% 29% 29% 9% 2	455
	11th to 20th	Night	21% 8% 27% 33% 10% 1	414
	21st to end	Night	17% 6% 31% 36% 9%	455
June	1st to 10th	Night	24% 4 27% 35% 11%	420
	11th to 20th	Night	18% 8% 25% 34% 15% 1	420
	21st to end	Night	19% 6% 26% 31% 17% 1	420
July	1st to 10th	Night	12% 4 25% 40% 19% 1	410
	11th to 20th	Night	11% 4 25% 42% 16% 1	408
	21st to end	Night	14% 5% 21% 42% 17% 1	460
August	1st to 10th	Night	15% 4 17% 43% 21% 1	418
	11th to 20th	Night	14% 3 21% 36% 24% 3	638
	21st to end	Night	15% 4 23% 36% 21% 2	917
September	1st to 10th	Night	13% 5% 23% 39% 19% 1	800
	11th to 20th	Night	17% 5% 17% 35% 24% 3	800
	21st to end	Night	19% 6% 23% 31% 18% 2	800
October	1st to 10th	Night	21% 6% 27% 30% 15% 1	800
	11th to 20th	Night	22% 9% 29% 29% 11% 1	800
	21st to end	Night	26% 10% 30% 28% 6%	880
November	1st to 10th	Night	25% 12% 28% 27% 8%	820
	11th to 20th	Night	23% 13% 33% 22% 9% 1	840
	21st to end	Night	29% 13% 30% 20% 7% 1	840
December	1st to 10th	Night	29% 11% 30% 24% 5%	840
	11th to 20th	Night	28% 15% 30% 22% 5% 1	840
	21st to end	Night	35% 11% 27% 22% 5%	924

Page generated at 2026-08-27 04:07:12. Version ac07

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