Visual Reconstructions Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Visual Reconstructions Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	13% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	125949
	Night	14% 11% 4 8% 5% 4 4 4 5% 6% 35%	123842
Transparency	Day	47% 7% 12% 21% 12% 1	129533
	Night	47% 6% 12% 21% 12% 1	128329
Seeing	Day	16% 11% 29% 24% 14% 5%	16646
	Night	18% 9% 29% 30% 12% 1	26242

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	12% 15% 6% 16% 6% 4 4 3 4 5% 25%	3617
	11th to 20th	Day	14% 15% 6% 16% 8% 10% 7% 4 4 4 11%	3636
	21st to end	Day	22% 16% 6% 11% 6% 5% 5% 3 5% 5% 15%	4466
February	1st to 10th	Day	14% 16% 4 12% 7% 6% 5% 4 5% 5% 21%	1800
	11th to 20th	Day	13% 13% 6% 12% 7% 5% 5% 4 5% 5% 24%	1866
	21st to end	Day	13% 15% 4 12% 7% 4 4 4 5% 6% 26%	1650
March	1st to 10th	Day	16% 13% 5% 9% 6% 4 4 3 4 6% 30%	4632
	11th to 20th	Day	12% 12% 4 8% 6% 5% 5% 3 5% 7% 33%	4980
	21st to end	Day	19% 14% 3 9% 5% 5% 4 3 4 5% 29%	5465
April	1st to 10th	Day	15% 14% 3 7% 5% 4 4 4 5% 7% 30%	2300
	11th to 20th	Day	16% 11% 3 7% 5% 5% 5% 4 5% 6% 34%	2557
	21st to end	Day	18% 9% 3 8% 5% 4 4 3 5% 7% 35%	2600
May	1st to 10th	Day	18% 10% 3 8% 5% 4 4 3 6% 6% 34%	5862
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 8% 33%	5861
	21st to end	Day	14% 6% 2 7% 4 4 4 4 5% 7% 41%	6433
June	1st to 10th	Day	10% 6% 3 7% 6% 5% 5% 4 6% 9% 40%	2916
	11th to 20th	Day	10% 5% 3 7% 5% 4 5% 5% 6% 9% 41%	3000
	21st to end	Day	9% 6% 3 7% 5% 5% 6% 6% 8% 9% 37%	2926
July	1st to 10th	Day	9% 5% 2 7% 6% 5% 6% 5% 8% 10% 38%	6360
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 5% 7% 10% 44%	6360
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 6% 9% 44%	6470
August	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 40%	4234
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 5% 9% 10% 37%	4174
	21st to end	Day	9% 6% 2 7% 6% 5% 6% 5% 7% 9% 38%	4675
September	1st to 10th	Day	11% 5% 3 7% 5% 5% 5% 4 5% 8% 42%	2250
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 7% 8% 48%	2200
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	2200
October	1st to 10th	Day	12% 6% 2 6% 4 4 5% 3 6% 7% 44%	2125
	11th to 20th	Day	16% 10% 3 6% 5% 5% 5% 4 4 7% 35%	1980
	21st to end	Day	19% 11% 3 8% 4 5% 4 4 5% 6% 30%	1957
November	1st to 10th	Day	10% 10% 3 8% 4 5% 5% 4 6% 6% 38%	1762
	11th to 20th	Day	15% 12% 3 7% 6% 6% 4 4 5% 6% 31%	1800
	21st to end	Day	17% 11% 6% 10% 5% 4 4 3 5% 6% 28%	1636
December	1st to 10th	Day	13% 13% 7% 12% 5% 5% 5% 4 5% 7% 24%	2600
	11th to 20th	Day	20% 20% 4 9% 4 4 4 4 5% 5% 21%	2820
	21st to end	Day	26% 20% 7% 11% 6% 4 4 2 4 4 13%	3779
January	1st to 10th	Night	13% 14% 5% 11% 6% 5% 5% 3 5% 6% 27%	6348
	11th to 20th	Night	11% 16% 6% 15% 8% 5% 6% 4 5% 6% 17%	6340
	21st to end	Night	17% 17% 7% 10% 5% 5% 5% 4 4 4 23%	6490
February	1st to 10th	Night	14% 16% 4 11% 7% 5% 5% 4 5% 5% 23%	2600
	11th to 20th	Night	13% 13% 4 10% 7% 5% 5% 4 5% 6% 27%	2415
	21st to end	Night	18% 12% 3 9% 5% 4 4 4 5% 6% 29%	1980
March	1st to 10th	Night	15% 13% 3 8% 4 5% 4 3 5% 6% 34%	5220
	11th to 20th	Night	9% 8% 3 7% 5% 5% 5% 4 6% 7% 41%	4980
	21st to end	Night	15% 9% 3 9% 5% 4 4 3 4 5% 38%	5467
April	1st to 10th	Night	17% 10% 3 6% 5% 4 4 4 6% 7% 34%	2116
	11th to 20th	Night	18% 8% 3 5% 4 4 3 4 6% 6% 38%	1792
	21st to end	Night	16% 7% 2 7% 5% 4 4 4 5% 7% 40%	1800
May	1st to 10th	Night	20% 9% 3 8% 5% 4 4 3 5% 6% 33%	4033
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 3 5% 7% 39%	4032
	21st to end	Night	14% 5% 2 6% 4 4 4 4 5% 7% 44%	4421
June	1st to 10th	Night	12% 5% 2 6% 3 3 4 3 6% 7% 47%	1474
	11th to 20th	Night	10% 5% 2 6% 5% 5% 4 4 7% 8% 45%	1400
	21st to end	Night	9% 6% 2 5% 4 3 4 4 6% 8% 49%	1467
July	1st to 10th	Night	7% 5% 2 5% 4 5% 5% 4 7% 10% 47%	3618
	11th to 20th	Night	9% 4 2 5% 4 4 4 4 7% 8% 48%	3617
	21st to end	Night	7% 5% 2 5% 4 4 5% 5% 6% 7% 49%	4380
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 5% 8% 47%	2915
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 4 6% 7% 47%	2875
	21st to end	Night	8% 5% 2 6% 5% 4 5% 3 6% 7% 48%	3223
September	1st to 10th	Night	10% 5% 2 6% 4 4 5% 3 5% 7% 49%	2126
	11th to 20th	Night	9% 5% 1 5% 3 3 3 3 4 7% 57%	2200
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	2200
October	1st to 10th	Night	13% 6% 3 6% 3 3 3 3 5% 7% 48%	2273
	11th to 20th	Night	18% 8% 3 7% 4 5% 4 4 4 5% 38%	2377
	21st to end	Night	21% 9% 2 7% 5% 4 5% 4 5% 6% 32%	2638
November	1st to 10th	Night	12% 8% 3 7% 4 4 4 4 5% 7% 43%	2536
	11th to 20th	Night	15% 11% 3 8% 6% 5% 5% 4 6% 6% 31%	2600
	21st to end	Night	19% 13% 4 8% 5% 4 4 4 6% 6% 28%	2760
December	1st to 10th	Night	13% 13% 6% 12% 6% 5% 5% 4 6% 6% 24%	4580
	11th to 20th	Night	15% 19% 6% 10% 6% 5% 4 3 5% 6% 20%	5369
	21st to end	Night	26% 20% 6% 10% 5% 4 3 3 4 4 16%	7180

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	60% 6% 11% 15% 7% 1	3618
	11th to 20th	Day	72% 7% 9% 7% 5%	3636
	21st to end	Day	68% 8% 10% 8% 6%	4466
February	1st to 10th	Day	62% 8% 9% 14% 7%	1800
	11th to 20th	Day	60% 5% 10% 16% 7% 1	1875
	21st to end	Day	57% 7% 11% 17% 7% 1	1650
March	1st to 10th	Day	55% 5% 14% 17% 8% 2	4632
	11th to 20th	Day	48% 6% 15% 21% 8% 1	4980
	21st to end	Day	57% 6% 11% 15% 10% 1	5465
April	1st to 10th	Day	51% 6% 12% 19% 11% 1	2300
	11th to 20th	Day	48% 6% 11% 20% 13% 2	2557
	21st to end	Day	48% 6% 13% 18% 13% 1	2600
May	1st to 10th	Day	48% 7% 10% 19% 12% 4	5897
	11th to 20th	Day	47% 8% 12% 20% 12% 1	5865
	21st to end	Day	41% 6% 9% 20% 19% 5%	6447
June	1st to 10th	Day	38% 7% 13% 26% 14% 2	2916
	11th to 20th	Day	37% 9% 13% 23% 15% 2	3000
	21st to end	Day	39% 8% 14% 22% 16% 2	2926
July	1st to 10th	Day	38% 7% 13% 24% 16% 2	6291
	11th to 20th	Day	32% 7% 15% 28% 17% 1	6285
	21st to end	Day	34% 8% 13% 22% 22% 1	6286
August	1st to 10th	Day	37% 8% 14% 25% 15% 1	4221
	11th to 20th	Day	37% 9% 13% 30% 10% 1	4213
	21st to end	Day	38% 7% 13% 27% 13% 1	4674
September	1st to 10th	Day	38% 8% 14% 25% 13% 2	2250
	11th to 20th	Day	31% 7% 13% 33% 15% 1	2200
	21st to end	Day	43% 7% 14% 23% 12%	2200
October	1st to 10th	Day	30% 6% 12% 32% 20% 2	3463
	11th to 20th	Day	39% 5% 11% 26% 17%	3220
	21st to end	Day	45% 5% 13% 23% 12% 1	3207
November	1st to 10th	Day	43% 8% 12% 24% 12% 1	1762
	11th to 20th	Day	52% 5% 14% 18% 10% 1	1800
	21st to end	Day	58% 6% 12% 17% 6% 1	1636
December	1st to 10th	Day	60% 5% 17% 14% 4	2600
	11th to 20th	Day	65% 5% 11% 12% 6%	2816
	21st to end	Day	74% 5% 8% 8% 5%	3779
January	1st to 10th	Night	56% 5% 11% 19% 6% 2	6348
	11th to 20th	Night	66% 6% 11% 11% 5%	6340
	21st to end	Night	63% 6% 10% 13% 7%	6490
February	1st to 10th	Night	60% 8% 11% 14% 7% 1	2600
	11th to 20th	Night	56% 7% 11% 17% 8% 1	2417
	21st to end	Night	54% 6% 13% 18% 8% 1	1980
March	1st to 10th	Night	50% 5% 14% 20% 10% 1	5220
	11th to 20th	Night	40% 6% 16% 23% 14% 1	4980
	21st to end	Night	48% 5% 11% 20% 15% 1	5467
April	1st to 10th	Night	49% 6% 11% 20% 12% 2	2116
	11th to 20th	Night	44% 7% 11% 18% 18% 2	1792
	21st to end	Night	43% 6% 11% 20% 17% 3	1800
May	1st to 10th	Night	51% 6% 10% 16% 14% 4	4060
	11th to 20th	Night	44% 7% 11% 25% 11% 2	4033
	21st to end	Night	38% 7% 12% 23% 17% 5%	4429
June	1st to 10th	Night	35% 6% 11% 27% 18% 3	1474
	11th to 20th	Night	35% 8% 13% 26% 17% 2	1400
	21st to end	Night	32% 7% 13% 25% 22% 1	1467
July	1st to 10th	Night	31% 6% 14% 29% 16% 4	3580
	11th to 20th	Night	31% 6% 15% 29% 16% 2	3572
	21st to end	Night	31% 8% 14% 22% 23% 2	4252
August	1st to 10th	Night	35% 6% 14% 27% 16% 1	2904
	11th to 20th	Night	34% 8% 11% 34% 12% 2	2902
	21st to end	Night	33% 8% 12% 33% 14% 1	3223
September	1st to 10th	Night	34% 7% 13% 28% 16% 2	2126
	11th to 20th	Night	27% 6% 13% 33% 20% 2	2200
	21st to end	Night	40% 6% 12% 29% 12% 1	2200
October	1st to 10th	Night	26% 4 10% 32% 27% 1	3715
	11th to 20th	Night	42% 5% 12% 23% 18%	3876
	21st to end	Night	42% 4 12% 28% 13% 1	4338
November	1st to 10th	Night	41% 6% 13% 26% 14% 1	2536
	11th to 20th	Night	50% 6% 14% 20% 9% 1	2600
	21st to end	Night	56% 6% 13% 19% 6%	2760
December	1st to 10th	Night	59% 8% 17% 14% 3	4580
	11th to 20th	Night	63% 6% 12% 13% 6% 1	5372
	21st to end	Night	72% 5% 9% 8% 6%	7180

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	21% 19% 36% 17% 7%	300
	11th to 20th	Day	20% 19% 42% 15% 3 1	300
	21st to end	Day	26% 14% 37% 17% 5%	330
February	1st to 10th	Day	22% 19% 39% 16% 3 1	300
	11th to 20th	Day	22% 20% 36% 20% 2	300
	21st to end	Day	19% 24% 42% 12% 3	249
March	1st to 10th	Day	24% 18% 37% 17% 4	361
	11th to 20th	Day	21% 16% 32% 24% 6% 1	500
	21st to end	Day	26% 15% 32% 21% 5% 1	550
April	1st to 10th	Day	17% 17% 37% 19% 8% 1	500
	11th to 20th	Day	24% 17% 29% 15% 9% 5%	492
	21st to end	Day	23% 13% 29% 23% 9% 2	500
May	1st to 10th	Day	22% 13% 29% 23% 11% 3	503
	11th to 20th	Day	19% 10% 27% 26% 12% 6%	527
	21st to end	Day	14% 8% 24% 27% 20% 8%	573
June	1st to 10th	Day	12% 9% 32% 30% 15% 3	686
	11th to 20th	Day	10% 7% 33% 31% 17% 3	730
	21st to end	Day	10% 7% 30% 31% 17% 5%	730
July	1st to 10th	Day	8% 5% 24% 29% 23% 11%	721
	11th to 20th	Day	10% 5% 25% 33% 19% 8%	718
	21st to end	Day	8% 4 22% 31% 24% 11%	596
August	1st to 10th	Day	9% 3 21% 27% 25% 14%	526
	11th to 20th	Day	11% 3 19% 32% 22% 13%	519
	21st to end	Day	11% 4 21% 30% 22% 12%	583
September	1st to 10th	Day	12% 6% 23% 24% 23% 12%	530
	11th to 20th	Day	8% 4 18% 25% 29% 16%	530
	21st to end	Day	18% 7% 24% 23% 20% 8%	530
October	1st to 10th	Day	14% 10% 27% 22% 18% 9%	530
	11th to 20th	Day	20% 14% 32% 24% 8% 2	522
	21st to end	Day	24% 16% 35% 19% 5%	406
November	1st to 10th	Day	15% 19% 34% 22% 8% 2	300
	11th to 20th	Day	23% 19% 33% 17% 7% 1	300
	21st to end	Day	22% 19% 37% 18% 4	217
December	1st to 10th	Day	22% 18% 39% 14% 7% 1	200
	11th to 20th	Day	29% 18% 37% 14% 3 1	200
	21st to end	Day	28% 21% 32% 16% 2	287
January	1st to 10th	Night	22% 14% 37% 22% 5% 1	1000
	11th to 20th	Night	23% 14% 34% 24% 4 1	993
	21st to end	Night	20% 12% 34% 28% 5%	880
February	1st to 10th	Night	22% 12% 39% 24% 4	800
	11th to 20th	Night	20% 15% 35% 26% 5%	783
	21st to end	Night	24% 13% 41% 20% 2	660
March	1st to 10th	Night	20% 12% 36% 27% 5%	800
	11th to 20th	Night	17% 14% 38% 25% 5% 1	800
	21st to end	Night	22% 11% 32% 27% 8% 1	880
April	1st to 10th	Night	21% 14% 32% 28% 4 1	716
	11th to 20th	Night	20% 11% 32% 27% 8% 1	593
	21st to end	Night	18% 9% 31% 32% 9%	600
May	1st to 10th	Night	19% 7% 30% 34% 10% 1	600
	11th to 20th	Night	20% 5% 26% 32% 15% 1	591
	21st to end	Night	11% 3 24% 41% 20% 2	650
June	1st to 10th	Night	13% 5% 26% 41% 15%	435
	11th to 20th	Night	10% 4 24% 47% 15%	400
	21st to end	Night	10% 4 21% 45% 19% 1	467
July	1st to 10th	Night	7% 3 18% 41% 28% 4	585
	11th to 20th	Night	9% 3 18% 41% 25% 3	582
	21st to end	Night	5% 3 18% 48% 24% 2	657
August	1st to 10th	Night	8% 3 17% 37% 33% 3	597
	11th to 20th	Night	7% 2 18% 37% 30% 5%	594
	21st to end	Night	9% 3 17% 37% 31% 3	660
September	1st to 10th	Night	11% 6% 19% 34% 25% 5%	735
	11th to 20th	Night	10% 3 16% 34% 32% 5%	800
	21st to end	Night	18% 6% 20% 31% 20% 5%	800
October	1st to 10th	Night	15% 8% 26% 31% 19% 3	800
	11th to 20th	Night	22% 12% 28% 27% 9% 2	793
	21st to end	Night	25% 11% 31% 28% 6%	836
November	1st to 10th	Night	18% 10% 33% 25% 13% 1	780
	11th to 20th	Night	21% 16% 32% 24% 7% 1	800
	21st to end	Night	25% 13% 35% 22% 6%	800
December	1st to 10th	Night	25% 12% 36% 20% 6%	800
	11th to 20th	Night	23% 14% 37% 23% 3	875
	21st to end	Night	29% 12% 33% 22% 4	1100

Page generated at 2025-10-16 01:50:29. Version ac07

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