Watch Observatory 2
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watch Observatory 2 clear sky chart from 2005-11-06 to 2026-07-01.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	138125
	Night	16% 12% 4 9% 5% 5% 5% 4 5% 6% 29%	134217
Transparency	Day	35% 35% 8% 15% 6%	141557
	Night	38% 35% 7% 13% 7% 1	138861
Seeing	Day	25% 16% 30% 19% 8% 2	16586
	Night	24% 14% 29% 23% 8% 1	25611

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	15% 18% 6% 13% 6% 5% 4 3 4 5% 20%	4661
	11th to 20th	Day	12% 16% 6% 13% 7% 6% 5% 4 6% 4 20%	4650
	21st to end	Day	10% 15% 6% 14% 9% 7% 6% 4 5% 5% 20%	5142
February	1st to 10th	Day	15% 15% 4 12% 7% 5% 4 4 6% 5% 24%	1890
	11th to 20th	Day	13% 14% 4 10% 8% 6% 5% 4 5% 7% 25%	1917
	21st to end	Day	19% 13% 5% 10% 6% 5% 4 3 5% 5% 25%	1730
March	1st to 10th	Day	18% 12% 4 12% 7% 5% 4 3 4 5% 25%	4576
	11th to 20th	Day	12% 14% 4 10% 6% 5% 4 4 6% 6% 30%	4901
	21st to end	Day	19% 9% 3 9% 6% 4 4 3 4 5% 34%	6026
April	1st to 10th	Day	21% 12% 3 8% 5% 5% 4 3 4 5% 32%	2520
	11th to 20th	Day	21% 9% 3 7% 5% 4 4 3 5% 5% 34%	2494
	21st to end	Day	22% 9% 3 8% 5% 4 4 3 5% 5% 32%	2645
May	1st to 10th	Day	23% 9% 4 9% 5% 5% 5% 4 6% 7% 23%	8237
	11th to 20th	Day	18% 9% 3 8% 5% 4 5% 4 5% 7% 32%	8221
	21st to end	Day	20% 9% 3 6% 4 4 5% 3 6% 7% 34%	9060
June	1st to 10th	Day	22% 7% 3 7% 5% 4 5% 4 5% 8% 30%	2940
	11th to 20th	Day	14% 6% 3 7% 6% 5% 5% 4 6% 8% 36%	2940
	21st to end	Day	18% 7% 3 7% 4 5% 5% 5% 6% 8% 31%	2940
July	1st to 10th	Day	12% 7% 3 8% 6% 6% 6% 5% 6% 8% 33%	6360
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 4 7% 8% 41%	6360
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 10% 46%	6926
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 9% 35%	4563
	11th to 20th	Day	13% 6% 3 7% 5% 4 5% 4 6% 8% 39%	4082
	21st to end	Day	15% 5% 2 6% 4 5% 5% 4 7% 9% 38%	4312
September	1st to 10th	Day	12% 6% 3 5% 4 4 4 4 6% 8% 44%	2400
	11th to 20th	Day	12% 4 2 6% 3 4 4 3 4 7% 50%	2346
	21st to end	Day	16% 7% 3 6% 4 4 5% 4 5% 7% 41%	2200
October	1st to 10th	Day	18% 8% 2 6% 4 5% 4 3 5% 6% 38%	2017
	11th to 20th	Day	19% 9% 3 9% 5% 6% 4 3 5% 5% 31%	1995
	21st to end	Day	23% 11% 4 8% 4 4 5% 3 5% 5% 29%	1980
November	1st to 10th	Day	16% 9% 4 8% 5% 5% 5% 4 4 8% 32%	1851
	11th to 20th	Day	16% 11% 4 8% 6% 7% 5% 4 4 7% 29%	1769
	21st to end	Day	21% 14% 4 9% 7% 6% 4 3 7% 5% 21%	1680
December	1st to 10th	Day	19% 19% 6% 10% 6% 5% 4 3 6% 6% 16%	2680
	11th to 20th	Day	19% 15% 5% 10% 5% 5% 5% 4 4 5% 24%	3066
	21st to end	Day	23% 15% 4 10% 5% 5% 5% 3 4 5% 21%	4048
January	1st to 10th	Night	13% 20% 8% 14% 5% 5% 4 3 4 5% 20%	8234
	11th to 20th	Night	12% 17% 7% 18% 8% 6% 5% 4 4 5% 15%	8235
	21st to end	Night	11% 15% 5% 12% 7% 6% 5% 4 5% 6% 24%	9020
February	1st to 10th	Night	14% 13% 5% 11% 7% 6% 6% 4 5% 6% 22%	2730
	11th to 20th	Night	13% 13% 4 11% 6% 6% 6% 4 5% 5% 27%	2587
	21st to end	Night	15% 14% 4 10% 6% 4 4 4 4 5% 30%	2076
March	1st to 10th	Night	20% 13% 4 8% 6% 4 4 3 4 5% 29%	5484
	11th to 20th	Night	14% 13% 4 8% 5% 5% 5% 4 5% 7% 29%	5263
	21st to end	Night	16% 11% 4 9% 5% 5% 5% 4 4 5% 31%	5148
April	1st to 10th	Night	23% 10% 3 8% 4 4 4 3 4 6% 31%	2100
	11th to 20th	Night	22% 9% 3 8% 5% 3 4 3 5% 5% 33%	2075
	21st to end	Night	21% 8% 3 7% 5% 4 4 3 5% 6% 34%	1974
May	1st to 10th	Night	20% 10% 3 9% 6% 5% 6% 4 6% 7% 23%	4702
	11th to 20th	Night	17% 9% 4 8% 4 4 4 4 5% 7% 33%	4639
	21st to end	Night	17% 10% 3 8% 5% 4 4 3 6% 7% 32%	5114
June	1st to 10th	Night	19% 7% 3 6% 6% 4 5% 5% 7% 9% 31%	1680
	11th to 20th	Night	16% 7% 3 5% 4 4 5% 3 6% 7% 40%	1680
	21st to end	Night	17% 7% 3 8% 5% 4 5% 5% 5% 8% 34%	1680
July	1st to 10th	Night	12% 7% 3 8% 5% 5% 6% 5% 7% 9% 33%	3600
	11th to 20th	Night	12% 6% 2 7% 5% 5% 4 3 5% 8% 42%	3600
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 8% 46%	3926
August	1st to 10th	Night	13% 7% 3 7% 4 4 4 3 6% 9% 39%	2586
	11th to 20th	Night	14% 7% 2 6% 5% 4 5% 4 5% 7% 41%	2929
	21st to end	Night	13% 7% 2 7% 5% 4 5% 4 6% 8% 40%	3586
September	1st to 10th	Night	10% 6% 2 8% 5% 5% 4 3 6% 8% 44%	2000
	11th to 20th	Night	14% 6% 2 4 4 4 3 2 5% 7% 50%	2052
	21st to end	Night	16% 6% 2 5% 4 5% 4 3 5% 7% 42%	2200
October	1st to 10th	Night	18% 6% 2 6% 4 4 4 3 5% 6% 39%	2380
	11th to 20th	Night	18% 8% 3 7% 5% 5% 5% 4 4 6% 34%	2403
	21st to end	Night	22% 9% 3 7% 4 5% 5% 4 5% 5% 30%	2860
November	1st to 10th	Night	19% 9% 3 8% 5% 4 4 3 5% 6% 33%	2667
	11th to 20th	Night	17% 10% 3 8% 5% 5% 5% 4 5% 5% 32%	2849
	21st to end	Night	20% 12% 4 10% 6% 5% 4 4 5% 6% 24%	2940
December	1st to 10th	Night	17% 17% 6% 12% 7% 6% 5% 4 5% 5% 18%	4720
	11th to 20th	Night	22% 16% 4 10% 5% 4 4 3 4 4 24%	5348
	21st to end	Night	25% 17% 4 9% 4 4 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	54% 21% 9% 12% 3	4662
	11th to 20th	Day	54% 21% 6% 10% 7% 1	4680
	21st to end	Day	55% 23% 8% 7% 7% 1	5142
February	1st to 10th	Day	26% 59% 3 10% 2 1	1890
	11th to 20th	Day	25% 58% 4 9% 5%	1926
	21st to end	Day	26% 59% 3 6% 5% 1	1730
March	1st to 10th	Day	47% 28% 6% 9% 8% 3	4579
	11th to 20th	Day	40% 28% 11% 11% 8% 1	4901
	21st to end	Day	41% 27% 3 13% 14% 1	6026
April	1st to 10th	Day	23% 59% 3 8% 6% 1	2520
	11th to 20th	Day	23% 59% 2 9% 6% 1	2494
	21st to end	Day	22% 59% 3 9% 6%	2645
May	1st to 10th	Day	49% 24% 5% 16% 6%	8249
	11th to 20th	Day	43% 23% 11% 18% 5%	8225
	21st to end	Day	42% 22% 7% 21% 7%	9061
June	1st to 10th	Day	21% 59% 3 10% 6% 1	2940
	11th to 20th	Day	19% 59% 5% 10% 6% 1	2940
	21st to end	Day	20% 58% 6% 11% 5% 1	2940
July	1st to 10th	Day	37% 28% 10% 19% 6%	6290
	11th to 20th	Day	29% 28% 10% 24% 10%	6285
	21st to end	Day	24% 27% 11% 32% 6%	6728
August	1st to 10th	Day	30% 37% 11% 16% 5%	4549
	11th to 20th	Day	30% 38% 10% 20% 3	4123
	21st to end	Day	27% 37% 12% 16% 7%	4312
September	1st to 10th	Day	14% 59% 7% 14% 6%	2400
	11th to 20th	Day	13% 57% 6% 16% 8% 1	2346
	21st to end	Day	16% 58% 6% 13% 7% 1	2200
October	1st to 10th	Day	27% 36% 9% 22% 5%	3277
	11th to 20th	Day	25% 36% 12% 21% 5%	3223
	21st to end	Day	28% 36% 15% 17% 4	3180
November	1st to 10th	Day	28% 54% 6% 9% 3 1	1851
	11th to 20th	Day	23% 56% 5% 10% 5% 1	1769
	21st to end	Day	30% 57% 6% 5% 3	1680
December	1st to 10th	Day	46% 37% 8% 9% 1	2680
	11th to 20th	Day	46% 32% 6% 10% 6%	3066
	21st to end	Day	49% 26% 6% 14% 5%	4048
January	1st to 10th	Night	56% 21% 9% 10% 4	8234
	11th to 20th	Night	60% 20% 6% 9% 5%	8280
	21st to end	Night	50% 22% 6% 11% 9% 1	9020
February	1st to 10th	Night	26% 59% 4 8% 2	2730
	11th to 20th	Night	24% 58% 3 10% 4	2589
	21st to end	Night	22% 59% 3 8% 7%	2076
March	1st to 10th	Night	43% 28% 4 12% 10% 3	5494
	11th to 20th	Night	40% 28% 11% 11% 7% 3	5264
	21st to end	Night	43% 27% 3 16% 9% 1	5148
April	1st to 10th	Night	24% 59% 3 8% 6%	2100
	11th to 20th	Night	24% 59% 2 8% 6%	2075
	21st to end	Night	22% 59% 3 7% 8% 1	1974
May	1st to 10th	Night	48% 24% 9% 15% 4	4708
	11th to 20th	Night	42% 23% 9% 18% 8%	4640
	21st to end	Night	43% 22% 8% 16% 11%	5115
June	1st to 10th	Night	21% 59% 3 10% 7% 1	1680
	11th to 20th	Night	18% 59% 5% 9% 8% 1	1680
	21st to end	Night	21% 58% 4 10% 6% 1	1680
July	1st to 10th	Night	36% 28% 13% 15% 7%	3560
	11th to 20th	Night	31% 27% 11% 24% 7%	3552
	21st to end	Night	26% 27% 10% 25% 12%	3809
August	1st to 10th	Night	28% 37% 10% 19% 5%	2576
	11th to 20th	Night	31% 37% 11% 17% 3	2954
	21st to end	Night	27% 38% 10% 15% 9% 1	3586
September	1st to 10th	Night	15% 57% 6% 15% 7%	2000
	11th to 20th	Night	14% 57% 6% 17% 6% 1	2052
	21st to end	Night	16% 57% 6% 13% 8%	2200
October	1st to 10th	Night	25% 37% 11% 21% 6% 1	3887
	11th to 20th	Night	25% 36% 10% 22% 6%	3914
	21st to end	Night	29% 37% 13% 14% 7%	4610
November	1st to 10th	Night	27% 54% 7% 8% 4 1	2667
	11th to 20th	Night	25% 55% 5% 10% 5%	2849
	21st to end	Night	28% 55% 6% 8% 3	2940
December	1st to 10th	Night	46% 36% 9% 8% 2	4720
	11th to 20th	Night	48% 31% 3 10% 8%	5348
	21st to end	Night	50% 25% 4 11% 9%	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	30% 18% 34% 16% 3	220
	11th to 20th	Day	34% 25% 27% 9% 4 1	220
	21st to end	Day	21% 22% 37% 16% 4	245
February	1st to 10th	Day	31% 29% 27% 11% 2	330
	11th to 20th	Day	28% 27% 36% 8%	404
	21st to end	Day	30% 20% 35% 13% 1	446
March	1st to 10th	Day	30% 22% 30% 14% 4	540
	11th to 20th	Day	31% 22% 31% 13% 2 1	540
	21st to end	Day	33% 23% 25% 13% 6% 1	594
April	1st to 10th	Day	34% 20% 29% 13% 4	540
	11th to 20th	Day	36% 18% 25% 13% 6% 1	532
	21st to end	Day	32% 20% 29% 12% 7% 1	540
May	1st to 10th	Day	27% 17% 33% 16% 6% 1	543
	11th to 20th	Day	21% 19% 32% 19% 8% 2	567
	21st to end	Day	23% 18% 31% 21% 7% 1	617
June	1st to 10th	Day	27% 11% 32% 22% 6% 2	570
	11th to 20th	Day	18% 16% 32% 24% 7% 2	570
	21st to end	Day	23% 12% 35% 19% 10% 2	570
July	1st to 10th	Day	16% 9% 33% 26% 13% 2	525
	11th to 20th	Day	14% 11% 33% 27% 12% 2	524
	21st to end	Day	16% 9% 34% 28% 11% 2	576
August	1st to 10th	Day	15% 8% 27% 32% 14% 3	526
	11th to 20th	Day	15% 7% 27% 31% 14% 7%	519
	21st to end	Day	17% 10% 32% 23% 13% 5%	583
September	1st to 10th	Day	18% 10% 30% 21% 16% 6%	530
	11th to 20th	Day	14% 7% 25% 23% 21% 10%	530
	21st to end	Day	19% 14% 31% 19% 14% 4	530
October	1st to 10th	Day	22% 15% 27% 21% 12% 3	530
	11th to 20th	Day	24% 20% 28% 20% 6% 1	527
	21st to end	Day	33% 14% 30% 18% 6%	462
November	1st to 10th	Day	31% 18% 27% 17% 6% 1	425
	11th to 20th	Day	27% 22% 30% 16% 5% 1	309
	21st to end	Day	32% 21% 27% 14% 5%	220
December	1st to 10th	Day	30% 18% 34% 15% 4	220
	11th to 20th	Day	30% 27% 31% 10% 1 1	220
	21st to end	Day	33% 22% 29% 14% 2	242
January	1st to 10th	Night	27% 19% 36% 14% 4	840
	11th to 20th	Night	30% 20% 26% 19% 4 1	840
	21st to end	Night	21% 18% 34% 21% 5%	924
February	1st to 10th	Night	22% 19% 33% 22% 4	840
	11th to 20th	Night	23% 22% 34% 17% 3	823
	21st to end	Night	25% 18% 32% 23% 2	692
March	1st to 10th	Night	26% 19% 30% 21% 4	840
	11th to 20th	Night	28% 18% 32% 18% 3	840
	21st to end	Night	26% 17% 27% 23% 6% 1	924
April	1st to 10th	Night	32% 16% 28% 20% 4	840
	11th to 20th	Night	27% 18% 29% 20% 5%	831
	21st to end	Night	29% 15% 29% 20% 7%	714
May	1st to 10th	Night	26% 12% 31% 23% 6% 2	436
	11th to 20th	Night	24% 11% 30% 25% 9% 1	414
	21st to end	Night	17% 11% 38% 25% 8%	455
June	1st to 10th	Night	21% 8% 28% 32% 10%	420
	11th to 20th	Night	22% 9% 27% 30% 11% 1	420
	21st to end	Night	18% 9% 30% 27% 15% 1	420
July	1st to 10th	Night	13% 7% 30% 34% 15% 1	390
	11th to 20th	Night	14% 5% 29% 37% 15%	388
	21st to end	Night	13% 4 31% 37% 13% 1	438
August	1st to 10th	Night	14% 6% 23% 39% 18% 1	398
	11th to 20th	Night	14% 2 26% 34% 22% 2	620
	21st to end	Night	17% 6% 28% 31% 17% 2	880
September	1st to 10th	Night	14% 6% 31% 33% 15% 1	800
	11th to 20th	Night	18% 6% 21% 31% 22% 3	800
	21st to end	Night	20% 9% 28% 24% 18% 2	800
October	1st to 10th	Night	23% 10% 27% 28% 12% 1	800
	11th to 20th	Night	24% 12% 31% 24% 10%	800
	21st to end	Night	28% 15% 28% 25% 4	880
November	1st to 10th	Night	29% 17% 24% 23% 7% 1	820
	11th to 20th	Night	23% 19% 33% 16% 8%	840
	21st to end	Night	28% 19% 27% 20% 7%	840
December	1st to 10th	Night	31% 18% 28% 19% 4	840
	11th to 20th	Night	31% 23% 26% 16% 5% 1	840
	21st to end	Night	37% 18% 26% 15% 4	924

Page generated at 2026-06-28 02:10:45. Version ac07

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