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 2026-09-29.

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%	148876
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	142538
Transparency	Day	43% 6% 12% 23% 15% 2	152411
	Night	43% 5% 11% 22% 16% 2	147092
Seeing	Day	22% 17% 33% 19% 7% 2	17507
	Night	21% 14% 34% 24% 6%	26317

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	10% 8% 2 8% 5% 5% 5% 4 5% 6% 42%	4661
	11th to 20th	Day	14% 8% 3 6% 3 3 4 3 4 5% 47%	4650
	21st to end	Day	14% 8% 2 4 3 3 3 3 5% 6% 46%	5124
February	1st to 10th	Day	18% 7% 2 5% 5% 5% 5% 4 6% 7% 37%	1962
	11th to 20th	Day	13% 8% 3 6% 4 4 4 5% 6% 7% 40%	2048
	21st to end	Day	18% 9% 2 6% 4 5% 4 4 5% 5% 38%	1730
March	1st to 10th	Day	21% 10% 3 6% 4 4 4 3 4 5% 36%	4576
	11th to 20th	Day	15% 8% 2 6% 4 4 4 4 6% 7% 38%	4892
	21st to end	Day	26% 8% 2 6% 4 4 4 4 6% 6% 30%	5884
April	1st to 10th	Day	20% 8% 3 7% 5% 4 4 3 4 6% 36%	2520
	11th to 20th	Day	19% 7% 2 4 4 4 5% 4 5% 7% 38%	2515
	21st to end	Day	20% 7% 2 6% 4 4 4 4 5% 6% 37%	2730
May	1st to 10th	Day	26% 10% 3 7% 4 4 4 3 6% 6% 28%	7639
	11th to 20th	Day	19% 8% 3 7% 4 4 4 4 4 7% 35%	7735
	21st to end	Day	21% 7% 2 7% 5% 4 5% 4 6% 8% 31%	9060
June	1st to 10th	Day	19% 6% 2 6% 4 4 5% 5% 7% 8% 35%	2940
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 4 6% 8% 38%	2940
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 6% 9% 36%	2940
July	1st to 10th	Day	19% 6% 2 5% 5% 4 5% 4 6% 9% 35%	8280
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 45%	8280
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 7% 44%	8993
August	1st to 10th	Day	12% 6% 3 7% 5% 5% 6% 5% 7% 10% 34%	6067
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	5888
	21st to end	Day	12% 4 2 5% 4 4 5% 4 7% 9% 45%	6080
September	1st to 10th	Day	14% 6% 1 6% 4 4 5% 4 5% 7% 46%	2510
	11th to 20th	Day	15% 6% 2 4 3 3 3 2 4 6% 51%	2310
	21st to end	Day	19% 5% 2 6% 4 3 4 2 5% 6% 43%	2288
October	1st to 10th	Day	22% 5% 2 4 4 4 3 3 4 6% 43%	2200
	11th to 20th	Day	15% 6% 1 5% 4 3 3 3 5% 7% 47%	1956
	21st to end	Day	20% 6% 2 6% 4 3 4 3 5% 6% 41%	1980
November	1st to 10th	Day	16% 6% 2 4 4 3 4 2 4 5% 50%	1851
	11th to 20th	Day	18% 6% 2 4 3 4 3 2 4 5% 48%	1890
	21st to end	Day	18% 7% 2 5% 4 3 3 4 4 6% 43%	1890
December	1st to 10th	Day	15% 8% 2 4 3 3 4 3 4 5% 49%	2753
	11th to 20th	Day	20% 6% 2 4 3 3 3 2 4 5% 48%	3066
	21st to end	Day	20% 10% 3 6% 4 4 4 3 4 5% 38%	4048
January	1st to 10th	Night	14% 7% 2 6% 3 4 4 3 4 6% 47%	8234
	11th to 20th	Night	15% 10% 3 8% 5% 4 4 3 4 5% 39%	8235
	21st to end	Night	14% 10% 3 6% 5% 5% 4 3 4 5% 41%	9057
February	1st to 10th	Night	16% 10% 2 6% 4 4 4 4 5% 7% 39%	2653
	11th to 20th	Night	14% 9% 3 6% 4 5% 4 4 5% 7% 40%	2458
	21st to end	Night	18% 9% 2 7% 3 4 4 3 4 6% 41%	2076
March	1st to 10th	Night	24% 8% 2 5% 3 4 4 3 5% 5% 38%	5484
	11th to 20th	Night	15% 10% 3 7% 4 4 4 3 5% 6% 38%	5286
	21st to end	Night	25% 10% 2 6% 4 4 4 4 4 5% 32%	5274
April	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 6% 6% 37%	2100
	11th to 20th	Night	19% 8% 2 6% 4 3 4 4 4 5% 39%	2054
	21st to end	Night	20% 7% 2 6% 4 3 4 3 4 5% 39%	1890
May	1st to 10th	Night	27% 9% 3 7% 4 4 4 3 5% 6% 29%	5259
	11th to 20th	Night	19% 7% 2 7% 5% 4 5% 4 7% 7% 33%	5180
	21st to end	Night	21% 8% 4 8% 5% 4 5% 4 5% 7% 31%	5114
June	1st to 10th	Night	17% 7% 3 7% 3 4 4 4 5% 7% 38%	1680
	11th to 20th	Night	17% 7% 2 6% 5% 4 4 3 7% 8% 37%	1680
	21st to end	Night	16% 5% 2 6% 4 4 5% 4 6% 9% 39%	1680
July	1st to 10th	Night	15% 7% 2 7% 5% 4 4 4 6% 9% 36%	4680
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 3 6% 9% 46%	4680
	21st to end	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	5084
August	1st to 10th	Night	15% 6% 3 7% 5% 5% 6% 4 7% 9% 35%	4037
	11th to 20th	Night	11% 6% 3 7% 4 5% 5% 4 7% 7% 39%	4139
	21st to end	Night	12% 5% 2 5% 4 4 4 4 6% 8% 47%	5045
September	1st to 10th	Night	13% 6% 2 6% 5% 4 4 4 5% 6% 46%	2110
	11th to 20th	Night	14% 5% 2 4 3 4 3 3 4 6% 53%	2310
	21st to end	Night	20% 6% 2 6% 5% 3 3 4 5% 5% 42%	2289
October	1st to 10th	Night	20% 6% 2 5% 4 3 4 3 4 6% 44%	2200
	11th to 20th	Night	15% 5% 2 6% 3 3 4 3 5% 6% 47%	2440
	21st to end	Night	21% 7% 2 6% 4 4 3 3 5% 6% 40%	2860
November	1st to 10th	Night	14% 5% 2 5% 4 3 4 3 5% 5% 49%	2667
	11th to 20th	Night	17% 6% 2 5% 3 4 3 3 4 5% 48%	2730
	21st to end	Night	16% 7% 2 5% 4 3 4 4 5% 7% 44%	2730
December	1st to 10th	Night	18% 8% 2 5% 4 4 3 3 4 6% 43%	4645
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 4 5% 42%	5348
	21st to end	Night	17% 10% 3 7% 4 3 3 3 5% 6% 40%	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	41% 4 10% 26% 17% 2	4662
	11th to 20th	Day	41% 4 9% 21% 20% 5%	4680
	21st to end	Day	38% 3 11% 25% 21% 2	5124
February	1st to 10th	Day	46% 4 13% 19% 15% 2	1962
	11th to 20th	Day	41% 7% 11% 23% 15% 3	2057
	21st to end	Day	46% 5% 10% 21% 15% 3	1730
March	1st to 10th	Day	50% 3 10% 16% 11% 9%	4579
	11th to 20th	Day	42% 7% 13% 26% 11% 2	4892
	21st to end	Day	54% 4 7% 15% 14% 7%	5884
April	1st to 10th	Day	49% 5% 8% 18% 17% 3	2520
	11th to 20th	Day	44% 6% 10% 20% 17% 3	2515
	21st to end	Day	46% 6% 7% 20% 17% 4	2730
May	1st to 10th	Day	55% 6% 12% 15% 11% 1	7651
	11th to 20th	Day	48% 7% 13% 19% 12% 1	7739
	21st to end	Day	48% 7% 10% 22% 12% 1	9061
June	1st to 10th	Day	44% 7% 12% 24% 12% 1	2940
	11th to 20th	Day	42% 6% 13% 21% 15% 3	2940
	21st to end	Day	44% 7% 13% 21% 14% 1	2940
July	1st to 10th	Day	45% 7% 13% 21% 13% 1	8210
	11th to 20th	Day	32% 7% 16% 29% 15% 1	8205
	21st to end	Day	39% 6% 11% 25% 18% 2	8795
August	1st to 10th	Day	43% 9% 18% 19% 9% 1	6053
	11th to 20th	Day	38% 7% 14% 31% 9%	5927
	21st to end	Day	33% 7% 14% 29% 16% 2	6080
September	1st to 10th	Day	37% 6% 10% 25% 20% 2	2510
	11th to 20th	Day	34% 5% 8% 28% 22% 4	2310
	21st to end	Day	42% 4 9% 21% 21% 2	2288
October	1st to 10th	Day	39% 5% 9% 24% 19% 4	3590
	11th to 20th	Day	32% 4 7% 29% 25% 4	3159
	21st to end	Day	33% 5% 10% 29% 20% 3	3180
November	1st to 10th	Day	39% 4 12% 26% 18% 2	1851
	11th to 20th	Day	38% 4 8% 24% 22% 3	1890
	21st to end	Day	42% 5% 13% 26% 13% 2	1890
December	1st to 10th	Day	39% 4 14% 28% 15% 1	2753
	11th to 20th	Day	39% 3 13% 18% 20% 5%	3066
	21st to end	Day	49% 4 8% 21% 14% 4	4048
January	1st to 10th	Night	38% 3 11% 30% 16% 2	8234
	11th to 20th	Night	47% 4 10% 19% 15% 5%	8280
	21st to end	Night	46% 4 9% 18% 20% 4	9057
February	1st to 10th	Night	45% 4 12% 21% 15% 2	2653
	11th to 20th	Night	45% 4 9% 22% 16% 3	2460
	21st to end	Night	45% 4 8% 24% 16% 4	2076
March	1st to 10th	Night	49% 3 8% 16% 17% 7%	5494
	11th to 20th	Night	46% 6% 14% 18% 13% 2	5287
	21st to end	Night	54% 3 5% 12% 20% 6%	5274
April	1st to 10th	Night	48% 4 9% 19% 16% 3	2100
	11th to 20th	Night	46% 5% 9% 20% 18% 3	2054
	21st to end	Night	47% 5% 9% 18% 17% 3	1890
May	1st to 10th	Night	55% 6% 13% 14% 11% 2	5265
	11th to 20th	Night	48% 8% 14% 19% 10% 2	5181
	21st to end	Night	52% 6% 12% 20% 9% 1	5115
June	1st to 10th	Night	45% 7% 12% 23% 12% 1	1680
	11th to 20th	Night	44% 7% 13% 22% 14% 1	1680
	21st to end	Night	40% 8% 13% 25% 13% 1	1680
July	1st to 10th	Night	44% 6% 14% 21% 15%	4640
	11th to 20th	Night	33% 7% 16% 25% 17% 1	4632
	21st to end	Night	38% 6% 11% 27% 17% 1	4967
August	1st to 10th	Night	44% 8% 18% 18% 11% 1	4027
	11th to 20th	Night	39% 9% 16% 26% 10%	4166
	21st to end	Night	33% 7% 15% 27% 17%	5045
September	1st to 10th	Night	38% 6% 11% 25% 19% 1	2110
	11th to 20th	Night	33% 5% 10% 23% 26% 3	2310
	21st to end	Night	42% 6% 8% 22% 19% 2	2289
October	1st to 10th	Night	38% 5% 13% 24% 16% 3	3590
	11th to 20th	Night	27% 4 10% 27% 29% 3	3976
	21st to end	Night	34% 4 11% 30% 19% 2	4610
November	1st to 10th	Night	37% 5% 10% 24% 20% 4	2667
	11th to 20th	Night	40% 3 9% 23% 21% 4	2730
	21st to end	Night	40% 5% 13% 26% 14% 2	2730
December	1st to 10th	Night	43% 4 12% 28% 13% 1	4645
	11th to 20th	Night	46% 4 14% 15% 20% 2	5348
	21st to end	Night	46% 4 7% 23% 17% 2	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	26% 25% 30% 14% 4	220
	11th to 20th	Day	24% 28% 36% 9% 2	220
	21st to end	Day	21% 26% 38% 11% 4	242
February	1st to 10th	Day	26% 20% 35% 16% 2	444
	11th to 20th	Day	24% 22% 38% 14% 2	536
	21st to end	Day	24% 19% 36% 17% 3	446
March	1st to 10th	Day	25% 23% 33% 16% 3	540
	11th to 20th	Day	26% 24% 33% 13% 4 1	540
	21st to end	Day	31% 21% 33% 8% 6% 1	594
April	1st to 10th	Day	32% 24% 25% 13% 5% 1	540
	11th to 20th	Day	29% 26% 30% 11% 3 1	532
	21st to end	Day	26% 20% 34% 16% 3 1	540
May	1st to 10th	Day	30% 24% 30% 12% 2 1	543
	11th to 20th	Day	23% 19% 37% 14% 5% 1	567
	21st to end	Day	20% 15% 38% 21% 6% 1	617
June	1st to 10th	Day	19% 15% 36% 20% 7% 2	570
	11th to 20th	Day	17% 17% 38% 20% 6% 1	570
	21st to end	Day	15% 15% 38% 22% 8% 1	570
July	1st to 10th	Day	19% 8% 33% 29% 10% 1	565
	11th to 20th	Day	15% 8% 33% 30% 11% 2	564
	21st to end	Day	14% 9% 37% 30% 9% 2	620
August	1st to 10th	Day	10% 9% 33% 33% 12% 2	566
	11th to 20th	Day	15% 8% 34% 28% 14% 3	559
	21st to end	Day	12% 11% 35% 25% 13% 4	627
September	1st to 10th	Day	17% 11% 31% 25% 13% 3	570
	11th to 20th	Day	19% 11% 29% 21% 14% 7%	570
	21st to end	Day	23% 14% 31% 20% 8% 4	562
October	1st to 10th	Day	25% 13% 26% 21% 11% 4	530
	11th to 20th	Day	19% 19% 30% 21% 9% 2	467
	21st to end	Day	26% 16% 32% 17% 8% 1	462
November	1st to 10th	Day	20% 18% 34% 17% 9% 1	425
	11th to 20th	Day	24% 22% 34% 15% 4 1	430
	21st to end	Day	26% 23% 28% 17% 5% 1	430
December	1st to 10th	Day	23% 22% 36% 17% 3	267
	11th to 20th	Day	31% 25% 32% 9% 3	220
	21st to end	Day	26% 22% 34% 14% 2 2	242
January	1st to 10th	Night	22% 19% 39% 17% 3	840
	11th to 20th	Night	27% 19% 35% 17% 2	840
	21st to end	Night	19% 20% 37% 20% 4	924
February	1st to 10th	Night	21% 17% 39% 21% 1	840
	11th to 20th	Night	23% 17% 40% 18% 1	823
	21st to end	Night	24% 19% 35% 20% 2	692
March	1st to 10th	Night	24% 17% 37% 19% 3	840
	11th to 20th	Night	23% 18% 37% 17% 4	840
	21st to end	Night	25% 19% 35% 18% 4	924
April	1st to 10th	Night	26% 19% 31% 20% 4	840
	11th to 20th	Night	24% 19% 34% 19% 5%	810
	21st to end	Night	24% 16% 37% 21% 2	630
May	1st to 10th	Night	29% 16% 31% 20% 4	630
	11th to 20th	Night	19% 16% 40% 21% 4	606
	21st to end	Night	20% 10% 36% 30% 4 1	455
June	1st to 10th	Night	20% 8% 35% 30% 6%	420
	11th to 20th	Night	15% 11% 37% 31% 5% 1	420
	21st to end	Night	17% 6% 36% 35% 6%	420
July	1st to 10th	Night	13% 5% 34% 39% 9%	410
	11th to 20th	Night	13% 4 34% 38% 11%	408
	21st to end	Night	13% 4 33% 40% 10% 1	460
August	1st to 10th	Night	12% 5% 34% 39% 10% 1	581
	11th to 20th	Night	11% 6% 31% 40% 11%	643
	21st to end	Night	14% 6% 26% 38% 14% 2	924
September	1st to 10th	Night	15% 5% 32% 34% 13%	840
	11th to 20th	Night	15% 8% 27% 30% 18% 2	840
	21st to end	Night	23% 8% 26% 29% 12% 2	833
October	1st to 10th	Night	22% 10% 27% 27% 12% 2	800
	11th to 20th	Night	19% 13% 31% 26% 11% 1	800
	21st to end	Night	25% 14% 33% 22% 6%	880
November	1st to 10th	Night	18% 15% 33% 25% 7% 1	820
	11th to 20th	Night	22% 20% 33% 19% 7%	840
	21st to end	Night	21% 21% 33% 20% 5%	840
December	1st to 10th	Night	25% 17% 35% 18% 4	840
	11th to 20th	Night	24% 20% 32% 21% 4	840
	21st to end	Night	25% 17% 35% 20% 3	924

Page generated at 2026-09-26 04:18:38. Version ac07

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