VSOR Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the VSOR Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	113464
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	115458
Transparency	Day	43% 6% 11% 22% 16% 3	117045
	Night	43% 5% 11% 22% 16% 3	120041
Seeing	Day	22% 17% 33% 19% 7% 2	15658
	Night	21% 14% 33% 24% 7%	24668

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	11% 7% 2 8% 5% 5% 4 4 5% 7% 43%	3597
	11th to 20th	Day	14% 7% 3 6% 3 3 4 3 5% 5% 48%	3570
	21st to end	Day	17% 8% 2 5% 4 3 3 3 6% 5% 43%	3960
February	1st to 10th	Day	19% 7% 2 5% 5% 5% 5% 4 6% 7% 36%	1868
	11th to 20th	Day	13% 8% 3 6% 4 4 5% 5% 6% 6% 39%	1948
	21st to end	Day	18% 9% 2 6% 5% 5% 4 4 5% 5% 38%	1650
March	1st to 10th	Day	21% 10% 3 6% 4 4 4 3 4 5% 36%	4476
	11th to 20th	Day	15% 8% 2 6% 4 4 4 4 6% 7% 39%	4786
	21st to end	Day	27% 8% 2 6% 4 4 4 4 6% 6% 30%	5757
April	1st to 10th	Day	20% 8% 3 7% 5% 4 4 3 4 6% 36%	2400
	11th to 20th	Day	19% 7% 2 4 5% 4 5% 4 5% 7% 38%	2394
	21st to end	Day	20% 6% 2 6% 5% 4 5% 4 5% 6% 37%	2600
May	1st to 10th	Day	24% 12% 3 8% 4 4 4 3 5% 6% 27%	4236
	11th to 20th	Day	23% 8% 3 6% 4 4 4 4 5% 7% 31%	4135
	21st to end	Day	14% 6% 2 5% 4 4 4 3 5% 9% 44%	4837
June	1st to 10th	Day	20% 6% 2 5% 4 4 5% 4 7% 8% 34%	2660
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 4 6% 8% 38%	2660
	21st to end	Day	17% 6% 3 7% 5% 5% 5% 4 6% 9% 34%	2660
July	1st to 10th	Day	13% 7% 3 6% 5% 5% 6% 5% 6% 9% 35%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 6% 10% 40%	4440
	21st to end	Day	18% 7% 3 7% 5% 5% 4 4 6% 7% 35%	4814
August	1st to 10th	Day	15% 6% 3 7% 6% 5% 6% 5% 7% 9% 32%	4142
	11th to 20th	Day	14% 7% 3 6% 5% 5% 6% 4 6% 7% 39%	3995
	21st to end	Day	8% 4 2 6% 4 5% 5% 4 7% 9% 47%	4180
September	1st to 10th	Day	14% 6% 1 5% 4 4 5% 4 5% 7% 46%	2271
	11th to 20th	Day	15% 6% 2 4 2 3 3 2 4 6% 51%	2090
	21st to end	Day	18% 5% 2 6% 3 3 4 2 5% 6% 44%	2090
October	1st to 10th	Day	23% 5% 2 5% 4 4 3 3 4 6% 41%	2090
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 5% 7% 48%	1859
	21st to end	Day	20% 6% 2 6% 4 3 4 3 5% 6% 41%	1881
November	1st to 10th	Day	16% 6% 2 4 4 3 4 2 4 5% 50%	1761
	11th to 20th	Day	18% 6% 2 5% 3 4 3 2 4 5% 48%	1800
	21st to end	Day	18% 7% 2 5% 4 3 4 4 4 6% 43%	1800
December	1st to 10th	Day	15% 8% 2 4 3 3 4 3 4 5% 49%	2671
	11th to 20th	Day	20% 6% 2 4 3 3 3 2 4 5% 48%	2986
	21st to end	Day	20% 10% 3 6% 4 4 3 3 4 5% 38%	3960
January	1st to 10th	Night	14% 7% 2 5% 3 3 3 3 4 7% 50%	6349
	11th to 20th	Night	13% 9% 3 8% 5% 4 4 3 5% 5% 40%	6315
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 4 5% 38%	6996
February	1st to 10th	Night	16% 10% 2 6% 4 4 4 4 5% 6% 38%	2527
	11th to 20th	Night	15% 10% 3 6% 4 5% 4 4 5% 6% 40%	2338
	21st to end	Night	17% 9% 2 6% 3 4 4 3 4 6% 41%	1980
March	1st to 10th	Night	24% 8% 2 5% 3 4 4 3 5% 5% 37%	5364
	11th to 20th	Night	15% 10% 3 7% 5% 4 4 3 5% 6% 38%	5172
	21st to end	Night	25% 10% 2 6% 4 4 4 4 4 5% 32%	5159
April	1st to 10th	Night	19% 9% 3 6% 4 4 5% 3 6% 6% 36%	2000
	11th to 20th	Night	20% 8% 2 6% 4 3 4 4 4 5% 39%	1955
	21st to end	Night	21% 7% 2 6% 4 4 4 3 5% 5% 39%	1800
May	1st to 10th	Night	24% 11% 3 8% 5% 4 4 3 5% 7% 27%	2924
	11th to 20th	Night	22% 9% 2 7% 5% 5% 5% 4 6% 7% 26%	2793
	21st to end	Night	16% 6% 3 6% 4 3 4 4 6% 8% 40%	2738
June	1st to 10th	Night	17% 7% 3 7% 3 4 4 4 5% 7% 37%	1520
	11th to 20th	Night	18% 7% 2 6% 5% 4 4 3 6% 8% 38%	1520
	21st to end	Night	16% 5% 2 5% 4 4 5% 5% 6% 10% 38%	1520
July	1st to 10th	Night	12% 6% 2 7% 5% 5% 5% 5% 8% 10% 35%	2520
	11th to 20th	Night	11% 7% 3 8% 6% 4 4 3 6% 9% 39%	2520
	21st to end	Night	16% 9% 3 7% 5% 5% 5% 3 6% 7% 36%	2739
August	1st to 10th	Night	20% 6% 3 7% 5% 5% 5% 4 6% 8% 31%	2757
	11th to 20th	Night	9% 7% 3 8% 5% 6% 6% 5% 7% 6% 38%	2814
	21st to end	Night	9% 4 1 4 3 3 4 4 5% 8% 54%	3476
September	1st to 10th	Night	13% 6% 2 6% 5% 3 4 4 5% 7% 46%	1909
	11th to 20th	Night	14% 5% 2 3 3 4 3 2 4 6% 53%	2090
	21st to end	Night	18% 6% 2 6% 5% 3 3 4 5% 5% 43%	2090
October	1st to 10th	Night	20% 6% 2 6% 4 4 4 3 4 6% 42%	2090
	11th to 20th	Night	15% 5% 2 6% 3 3 4 3 5% 6% 48%	2318
	21st to end	Night	21% 7% 2 6% 4 4 3 3 5% 6% 39%	2717
November	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 5% 49%	2537
	11th to 20th	Night	18% 6% 2 5% 3 4 3 3 4 5% 47%	2600
	21st to end	Night	16% 7% 2 5% 4 3 4 3 5% 7% 44%	2600
December	1st to 10th	Night	18% 7% 2 5% 4 4 3 3 4 6% 43%	4507
	11th to 20th	Night	22% 8% 2 6% 3 3 3 3 4 5% 41%	5208
	21st to end	Night	17% 10% 3 7% 4 3 3 3 4 5% 40%	6996

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	39% 4 8% 29% 18% 2	3598
	11th to 20th	Day	39% 4 10% 22% 20% 4	3600
	21st to end	Day	42% 4 11% 22% 19% 2	3960
February	1st to 10th	Day	47% 5% 13% 19% 14% 3	1868
	11th to 20th	Day	42% 7% 11% 22% 15% 3	1957
	21st to end	Day	46% 5% 10% 21% 15% 3	1650
March	1st to 10th	Day	50% 3 10% 16% 11% 9%	4479
	11th to 20th	Day	42% 7% 12% 26% 11% 2	4786
	21st to end	Day	54% 4 6% 15% 14% 7%	5757
April	1st to 10th	Day	49% 5% 8% 18% 17% 4	2400
	11th to 20th	Day	44% 6% 10% 21% 17% 3	2394
	21st to end	Day	46% 6% 8% 20% 17% 4	2600
May	1st to 10th	Day	57% 7% 11% 14% 10% 1	4250
	11th to 20th	Day	51% 8% 13% 16% 10% 2	4139
	21st to end	Day	36% 7% 11% 25% 19% 2	4837
June	1st to 10th	Day	45% 7% 12% 24% 11% 1	2660
	11th to 20th	Day	43% 7% 13% 20% 15% 2	2660
	21st to end	Day	45% 7% 13% 21% 14% 1	2660
July	1st to 10th	Day	44% 8% 11% 21% 15% 2	4370
	11th to 20th	Day	39% 7% 15% 24% 14% 2	4365
	21st to end	Day	46% 7% 13% 19% 13% 2	4661
August	1st to 10th	Day	45% 9% 16% 21% 8% 1	4128
	11th to 20th	Day	42% 8% 14% 26% 10% 1	4034
	21st to end	Day	31% 8% 13% 30% 18% 1	4180
September	1st to 10th	Day	37% 6% 10% 24% 21% 2	2271
	11th to 20th	Day	35% 5% 7% 28% 22% 3	2090
	21st to end	Day	42% 4 9% 21% 22% 2	2090
October	1st to 10th	Day	40% 5% 9% 24% 18% 3	3480
	11th to 20th	Day	31% 4 7% 29% 26% 4	3062
	21st to end	Day	33% 5% 10% 29% 20% 3	3081
November	1st to 10th	Day	38% 4 12% 26% 17% 2	1761
	11th to 20th	Day	39% 4 8% 24% 22% 3	1800
	21st to end	Day	42% 5% 13% 25% 13% 2	1800
December	1st to 10th	Day	39% 4 14% 28% 14% 1	2671
	11th to 20th	Day	40% 3 14% 18% 20% 5%	2986
	21st to end	Day	49% 4 8% 21% 14% 4	3960
January	1st to 10th	Night	35% 3 10% 33% 16% 2	6349
	11th to 20th	Night	45% 5% 11% 21% 14% 4	6360
	21st to end	Night	49% 4 9% 16% 17% 4	6996
February	1st to 10th	Night	46% 4 13% 21% 14% 2	2527
	11th to 20th	Night	45% 4 9% 21% 16% 4	2340
	21st to end	Night	44% 4 8% 23% 16% 4	1980
March	1st to 10th	Night	49% 3 8% 16% 16% 7%	5374
	11th to 20th	Night	47% 6% 15% 18% 13% 2	5173
	21st to end	Night	54% 3 5% 12% 20% 6%	5159
April	1st to 10th	Night	48% 4 9% 19% 16% 3	2000
	11th to 20th	Night	46% 5% 8% 20% 18% 3	1955
	21st to end	Night	46% 5% 9% 18% 18% 4	1800
May	1st to 10th	Night	56% 7% 12% 16% 8% 2	2930
	11th to 20th	Night	54% 8% 13% 14% 8% 2	2794
	21st to end	Night	41% 7% 15% 21% 15% 1	2739
June	1st to 10th	Night	46% 7% 13% 22% 11% 1	1520
	11th to 20th	Night	44% 6% 13% 22% 14% 1	1520
	21st to end	Night	40% 8% 14% 24% 13% 1	1520
July	1st to 10th	Night	42% 8% 14% 21% 15%	2480
	11th to 20th	Night	40% 8% 15% 23% 13% 1	2472
	21st to end	Night	47% 7% 11% 19% 14% 1	2651
August	1st to 10th	Night	50% 7% 14% 18% 10% 1	2747
	11th to 20th	Night	42% 10% 14% 24% 10%	2841
	21st to end	Night	27% 6% 16% 28% 23% 1	3476
September	1st to 10th	Night	37% 5% 12% 24% 19% 2	1909
	11th to 20th	Night	33% 5% 10% 23% 26% 2	2090
	21st to end	Night	41% 7% 8% 22% 20% 2	2090
October	1st to 10th	Night	39% 5% 14% 24% 15% 2	3480
	11th to 20th	Night	27% 3 9% 27% 30% 3	3854
	21st to end	Night	34% 4 11% 30% 19% 2	4467
November	1st to 10th	Night	37% 5% 11% 23% 20% 4	2537
	11th to 20th	Night	41% 3 9% 22% 21% 4	2600
	21st to end	Night	40% 5% 13% 26% 14% 2	2600
December	1st to 10th	Night	43% 4 12% 28% 13% 1	4507
	11th to 20th	Night	46% 4 14% 15% 20% 1	5208
	21st to end	Night	46% 4 7% 23% 17% 2	6996

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	27% 25% 31% 14% 5%	200
	11th to 20th	Day	22% 29% 37% 10% 3 1	200
	21st to end	Day	22% 25% 37% 11% 5%	220
February	1st to 10th	Day	28% 20% 34% 16% 2	410
	11th to 20th	Day	26% 23% 38% 12% 2	496
	21st to end	Day	25% 19% 35% 17% 3	414
March	1st to 10th	Day	25% 24% 33% 15% 2	500
	11th to 20th	Day	25% 24% 32% 13% 4 1	500
	21st to end	Day	31% 20% 33% 8% 7% 1	550
April	1st to 10th	Day	32% 24% 24% 14% 5% 1	500
	11th to 20th	Day	29% 26% 30% 11% 4 1	492
	21st to end	Day	26% 20% 34% 16% 3 1	500
May	1st to 10th	Day	31% 24% 28% 13% 3 1	503
	11th to 20th	Day	23% 19% 37% 15% 6% 1	487
	21st to end	Day	18% 14% 38% 22% 7% 1	529
June	1st to 10th	Day	20% 14% 37% 20% 7% 2	490
	11th to 20th	Day	15% 16% 39% 22% 7% 1	490
	21st to end	Day	16% 15% 38% 22% 8% 1	490
July	1st to 10th	Day	18% 8% 33% 28% 11% 1	485
	11th to 20th	Day	16% 7% 34% 30% 11% 3	484
	21st to end	Day	14% 8% 37% 32% 8% 1	532
August	1st to 10th	Day	11% 10% 33% 33% 11% 2	486
	11th to 20th	Day	16% 8% 32% 28% 13% 3	479
	21st to end	Day	13% 9% 33% 27% 14% 4	539
September	1st to 10th	Day	18% 10% 29% 26% 14% 3	490
	11th to 20th	Day	20% 11% 28% 19% 14% 8%	490
	21st to end	Day	21% 13% 32% 21% 9% 4	490
October	1st to 10th	Day	27% 13% 26% 21% 10% 4	490
	11th to 20th	Day	17% 19% 32% 21% 9% 2	433
	21st to end	Day	25% 14% 33% 18% 8% 1	429
November	1st to 10th	Day	19% 17% 35% 17% 10% 2	395
	11th to 20th	Day	25% 22% 33% 15% 5% 1	400
	21st to end	Day	25% 22% 29% 18% 6% 1	400
December	1st to 10th	Day	23% 21% 37% 17% 2	245
	11th to 20th	Day	31% 26% 32% 10% 3	200
	21st to end	Day	24% 21% 36% 15% 3 2	220
January	1st to 10th	Night	22% 20% 39% 17% 3	800
	11th to 20th	Night	26% 18% 35% 18% 3	800
	21st to end	Night	20% 19% 37% 20% 4	880
February	1st to 10th	Night	22% 18% 37% 22% 1	800
	11th to 20th	Night	24% 18% 40% 17% 1	783
	21st to end	Night	24% 19% 34% 20% 3	660
March	1st to 10th	Night	24% 18% 36% 19% 4	800
	11th to 20th	Night	23% 17% 38% 17% 4	800
	21st to end	Night	25% 17% 35% 18% 4	880
April	1st to 10th	Night	25% 19% 31% 20% 4	800
	11th to 20th	Night	24% 18% 33% 19% 5%	771
	21st to end	Night	24% 16% 37% 22% 2	600
May	1st to 10th	Night	29% 14% 32% 20% 4	600
	11th to 20th	Night	20% 15% 40% 21% 4	551
	21st to end	Night	19% 9% 35% 31% 5% 1	411
June	1st to 10th	Night	21% 7% 36% 29% 7% 1	380
	11th to 20th	Night	16% 10% 35% 33% 6% 1	380
	21st to end	Night	17% 6% 37% 33% 6%	380
July	1st to 10th	Night	13% 4 33% 40% 10%	370
	11th to 20th	Night	13% 4 33% 38% 12%	368
	21st to end	Night	13% 4 31% 42% 10% 1	416
August	1st to 10th	Night	12% 5% 33% 39% 10% 1	525
	11th to 20th	Night	11% 7% 30% 40% 12%	581
	21st to end	Night	14% 5% 24% 40% 16% 2	836
September	1st to 10th	Night	15% 6% 31% 34% 13%	760
	11th to 20th	Night	17% 8% 27% 29% 18% 2	760
	21st to end	Night	21% 8% 27% 30% 13% 2	760
October	1st to 10th	Night	23% 10% 27% 26% 12% 2	760
	11th to 20th	Night	18% 13% 31% 26% 11% 1	760
	21st to end	Night	25% 13% 33% 22% 7%	836
November	1st to 10th	Night	17% 15% 33% 26% 8% 1	780
	11th to 20th	Night	22% 19% 32% 19% 7%	800
	21st to end	Night	21% 21% 33% 20% 5% 1	800
December	1st to 10th	Night	25% 17% 35% 18% 4	800
	11th to 20th	Night	24% 19% 32% 22% 4	800
	21st to end	Night	24% 17% 36% 21% 3	880

Page generated at 2025-05-08 09:34:01. Version ac07

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