VSO
Clear Sky Chart
History

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

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	13% 10% 4 9% 6% 5% 5% 4 6% 7% 32%	132364
	Night	14% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	130679
Transparency	Day	49% 6% 12% 20% 11% 1	135959
	Night	48% 6% 12% 20% 12% 1	135170
Seeing	Day	19% 14% 28% 22% 12% 5%	16430
	Night	19% 11% 30% 27% 12% 1	25950

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% 13% 6% 15% 7% 6% 4 3 4 5% 21%	4703
	11th to 20th	Day	7% 20% 10% 21% 9% 7% 4 3 4 4 12%	5246
	21st to end	Day	14% 14% 6% 17% 9% 6% 5% 4 4 5% 17%	5780
February	1st to 10th	Day	14% 16% 6% 12% 7% 5% 5% 5% 5% 5% 20%	1890
	11th to 20th	Day	13% 14% 6% 13% 8% 5% 4 3 5% 5% 23%	1843
	21st to end	Day	17% 14% 5% 11% 6% 5% 4 4 4 6% 23%	1641
March	1st to 10th	Day	17% 11% 4 11% 7% 4 4 3 4 6% 30%	5042
	11th to 20th	Day	10% 12% 5% 10% 6% 5% 5% 4 6% 8% 30%	5090
	21st to end	Day	16% 11% 4 10% 6% 5% 4 3 5% 5% 30%	5586
April	1st to 10th	Day	16% 12% 3 8% 6% 5% 5% 4 5% 6% 29%	2451
	11th to 20th	Day	17% 10% 2 8% 5% 5% 5% 3 5% 6% 34%	2597
	21st to end	Day	18% 10% 3 9% 4 4 4 4 4 6% 33%	2730
May	1st to 10th	Day	18% 10% 3 8% 5% 5% 4 3 5% 6% 33%	6018
	11th to 20th	Day	14% 10% 4 9% 5% 5% 5% 4 5% 7% 31%	6267
	21st to end	Day	16% 7% 3 6% 4 5% 5% 4 6% 7% 39%	7103
June	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 6% 8% 39%	2800
	11th to 20th	Day	10% 6% 2 7% 6% 5% 5% 5% 7% 9% 38%	2800
	21st to end	Day	11% 6% 2 6% 4 6% 5% 4 7% 9% 39%	2836
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 5% 7% 11% 41%	6360
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	6230
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 47%	6424
August	1st to 10th	Day	16% 6% 2 6% 5% 5% 5% 4 6% 8% 37%	4234
	11th to 20th	Day	10% 7% 3 7% 5% 5% 5% 5% 9% 9% 35%	4174
	21st to end	Day	8% 6% 2 7% 4 5% 6% 4 7% 10% 42%	4520
September	1st to 10th	Day	11% 6% 2 7% 6% 4 5% 4 6% 8% 40%	2400
	11th to 20th	Day	9% 5% 2 6% 5% 4 4 4 5% 9% 48%	2240
	21st to end	Day	14% 7% 3 8% 6% 5% 4 5% 7% 8% 35%	2184
October	1st to 10th	Day	16% 9% 3 6% 5% 5% 5% 4 5% 7% 37%	2000
	11th to 20th	Day	17% 10% 3 9% 5% 5% 5% 3 6% 7% 32%	2000
	21st to end	Day	22% 11% 4 9% 5% 5% 4 4 5% 6% 25%	2200
November	1st to 10th	Day	13% 8% 3 9% 6% 5% 6% 4 6% 8% 33%	1821
	11th to 20th	Day	13% 13% 4 10% 6% 5% 6% 4 6% 8% 27%	1680
	21st to end	Day	18% 14% 5% 9% 6% 5% 5% 5% 6% 5% 22%	1680
December	1st to 10th	Day	15% 17% 5% 13% 7% 6% 6% 5% 5% 6% 15%	2680
	11th to 20th	Day	19% 18% 5% 9% 5% 4 5% 4 5% 5% 22%	3066
	21st to end	Day	24% 17% 7% 12% 7% 5% 5% 3 4 4 12%	4048
January	1st to 10th	Night	14% 13% 5% 12% 7% 6% 5% 4 5% 6% 22%	8174
	11th to 20th	Night	9% 15% 7% 16% 9% 6% 6% 4 5% 6% 17%	7639
	21st to end	Night	13% 13% 5% 13% 7% 6% 5% 4 5% 6% 21%	8401
February	1st to 10th	Night	14% 14% 5% 10% 7% 5% 5% 4 5% 6% 25%	2730
	11th to 20th	Night	13% 13% 5% 10% 6% 5% 5% 4 6% 6% 26%	2663
	21st to end	Night	14% 13% 5% 12% 5% 5% 4 4 6% 6% 26%	2160
March	1st to 10th	Night	15% 9% 4 9% 6% 5% 5% 4 5% 6% 32%	5018
	11th to 20th	Night	10% 8% 4 8% 5% 5% 5% 3 6% 8% 37%	5089
	21st to end	Night	17% 10% 3 8% 5% 4 4 3 4 5% 38%	5588
April	1st to 10th	Night	16% 10% 4 8% 6% 5% 5% 3 5% 6% 31%	2168
	11th to 20th	Night	16% 9% 3 8% 5% 4 5% 4 5% 6% 35%	1989
	21st to end	Night	14% 8% 3 7% 5% 5% 4 4 6% 6% 38%	1890
May	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 4 6% 34%	4143
	11th to 20th	Night	13% 8% 3 9% 5% 5% 5% 4 5% 7% 34%	3843
	21st to end	Night	17% 5% 2 5% 4 3 3 4 5% 8% 43%	4014
June	1st to 10th	Night	10% 6% 1 6% 5% 5% 6% 4 6% 9% 42%	1600
	11th to 20th	Night	10% 5% 2 6% 4 5% 5% 5% 8% 8% 43%	1600
	21st to end	Night	11% 4 2 6% 5% 5% 4 4 6% 8% 45%	1479
July	1st to 10th	Night	6% 5% 2 7% 5% 5% 4 4 5% 9% 48%	3600
	11th to 20th	Night	8% 5% 2 6% 3 4 4 4 6% 8% 49%	3727
	21st to end	Night	7% 5% 2 6% 4 4 4 3 6% 8% 51%	4428
August	1st to 10th	Night	10% 6% 3 6% 4 5% 4 4 6% 9% 44%	2915
	11th to 20th	Night	11% 6% 2 5% 3 4 4 3 6% 9% 46%	2875
	21st to end	Night	8% 5% 2 5% 4 4 5% 4 6% 8% 49%	3345
September	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 4 5% 8% 46%	2000
	11th to 20th	Night	9% 4 2 5% 3 4 4 3 5% 7% 55%	2156
	21st to end	Night	15% 7% 2 7% 4 4 4 4 6% 7% 39%	2215
October	1st to 10th	Night	15% 7% 2 6% 4 3 4 3 5% 7% 43%	2400
	11th to 20th	Night	17% 9% 3 7% 5% 5% 5% 3 5% 7% 36%	2400
	21st to end	Night	20% 9% 3 8% 4 5% 5% 4 5% 6% 32%	2640
November	1st to 10th	Night	13% 8% 3 7% 4 4 4 4 6% 7% 40%	2692
	11th to 20th	Night	14% 11% 3 8% 5% 6% 5% 4 5% 8% 30%	2940
	21st to end	Night	17% 12% 4 8% 6% 5% 5% 4 6% 8% 25%	2940
December	1st to 10th	Night	12% 13% 5% 11% 7% 6% 6% 6% 7% 7% 19%	4720
	11th to 20th	Night	16% 17% 4 10% 5% 5% 4 4 5% 6% 24%	5348
	21st to end	Night	24% 17% 5% 12% 6% 5% 4 3 4 5% 15%	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	61% 7% 11% 11% 9% 1	4704
	11th to 20th	Day	75% 5% 8% 8% 4	5280
	21st to end	Day	68% 5% 12% 9% 6%	5780
February	1st to 10th	Day	65% 5% 9% 15% 5% 1	1890
	11th to 20th	Day	63% 5% 9% 15% 7% 1	1852
	21st to end	Day	62% 5% 10% 15% 7% 1	1641
March	1st to 10th	Day	55% 5% 15% 15% 9%	5045
	11th to 20th	Day	50% 6% 17% 18% 7% 2	5090
	21st to end	Day	53% 6% 8% 21% 11% 1	5586
April	1st to 10th	Day	55% 5% 10% 18% 12% 1	2451
	11th to 20th	Day	50% 6% 10% 21% 12% 1	2597
	21st to end	Day	51% 6% 9% 20% 11% 2	2730
May	1st to 10th	Day	51% 7% 10% 17% 10% 6%	6027
	11th to 20th	Day	50% 8% 10% 18% 12% 2	6271
	21st to end	Day	43% 7% 11% 21% 16% 3	7103
June	1st to 10th	Day	41% 7% 11% 23% 16% 2	2800
	11th to 20th	Day	40% 7% 13% 24% 15% 1	2800
	21st to end	Day	39% 7% 13% 25% 17%	2836
July	1st to 10th	Day	35% 7% 13% 27% 17% 2	6290
	11th to 20th	Day	34% 7% 14% 29% 16% 1	6155
	21st to end	Day	32% 8% 13% 27% 20% 1	6240
August	1st to 10th	Day	43% 7% 13% 22% 14% 1	4221
	11th to 20th	Day	41% 8% 15% 25% 10% 1	4213
	21st to end	Day	36% 7% 11% 33% 13% 1	4520
September	1st to 10th	Day	39% 8% 13% 27% 12% 1	2400
	11th to 20th	Day	33% 5% 14% 32% 16%	2240
	21st to end	Day	44% 8% 13% 24% 10% 1	2184
October	1st to 10th	Day	37% 5% 15% 27% 15% 1	3260
	11th to 20th	Day	41% 5% 13% 24% 16% 1	3240
	21st to end	Day	50% 6% 14% 22% 8%	3538
November	1st to 10th	Day	48% 6% 13% 23% 8% 1	1821
	11th to 20th	Day	53% 7% 16% 15% 10% 1	1680
	21st to end	Day	58% 7% 16% 14% 5% 1	1680
December	1st to 10th	Day	67% 7% 13% 9% 3	2680
	11th to 20th	Day	62% 6% 12% 10% 9%	3066
	21st to end	Day	73% 7% 7% 8% 5%	4048
January	1st to 10th	Night	58% 8% 12% 16% 7%	8174
	11th to 20th	Night	63% 7% 10% 12% 8% 1	7680
	21st to end	Night	60% 5% 10% 14% 10%	8401
February	1st to 10th	Night	59% 6% 10% 18% 6% 1	2730
	11th to 20th	Night	56% 7% 11% 17% 9% 1	2665
	21st to end	Night	58% 6% 10% 18% 8% 1	2160
March	1st to 10th	Night	50% 4 16% 18% 11% 2	5028
	11th to 20th	Night	42% 6% 17% 21% 12% 1	5090
	21st to end	Night	48% 5% 8% 20% 17% 2	5588
April	1st to 10th	Night	53% 6% 10% 18% 12% 1	2168
	11th to 20th	Night	48% 6% 8% 20% 15% 2	1989
	21st to end	Night	46% 6% 9% 21% 15% 3	1890
May	1st to 10th	Night	51% 5% 9% 15% 13% 7%	4150
	11th to 20th	Night	47% 8% 12% 19% 11% 2	3844
	21st to end	Night	38% 7% 12% 24% 17% 2	4015
June	1st to 10th	Night	38% 6% 11% 24% 18% 3	1600
	11th to 20th	Night	36% 7% 12% 25% 18% 2	1600
	21st to end	Night	36% 7% 13% 24% 19% 2	1479
July	1st to 10th	Night	33% 6% 12% 24% 22% 3	3560
	11th to 20th	Night	31% 6% 13% 28% 19% 3	3679
	21st to end	Night	30% 7% 13% 25% 23% 2	4297
August	1st to 10th	Night	36% 7% 13% 26% 17% 2	2904
	11th to 20th	Night	34% 7% 16% 31% 12% 1	2902
	21st to end	Night	31% 6% 13% 32% 16% 1	3345
September	1st to 10th	Night	37% 6% 12% 29% 15% 2	2000
	11th to 20th	Night	29% 5% 12% 33% 19% 1	2156
	21st to end	Night	43% 6% 12% 26% 13% 1	2215
October	1st to 10th	Night	32% 5% 9% 28% 24% 1	3920
	11th to 20th	Night	43% 5% 12% 25% 15%	3899
	21st to end	Night	40% 5% 14% 28% 13% 1	4252
November	1st to 10th	Night	42% 7% 13% 24% 14% 1	2692
	11th to 20th	Night	50% 7% 13% 19% 10% 1	2940
	21st to end	Night	55% 7% 16% 16% 5%	2940
December	1st to 10th	Night	58% 9% 16% 13% 4	4720
	11th to 20th	Night	59% 6% 11% 14% 10% 1	5348
	21st to end	Night	69% 8% 9% 9% 5%	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	23% 19% 36% 14% 7% 1	228
	11th to 20th	Day	27% 24% 32% 12% 3 2	330
	21st to end	Day	25% 21% 34% 17% 4 1	363
February	1st to 10th	Day	26% 21% 36% 13% 4	330
	11th to 20th	Day	26% 26% 35% 12% 1	330
	21st to end	Day	33% 25% 30% 10% 2	273
March	1st to 10th	Day	23% 17% 33% 20% 7%	540
	11th to 20th	Day	27% 18% 33% 15% 6% 1	540
	21st to end	Day	29% 15% 31% 17% 7% 1	594
April	1st to 10th	Day	24% 19% 31% 20% 6% 1	540
	11th to 20th	Day	26% 19% 27% 18% 8% 3	532
	21st to end	Day	23% 16% 32% 19% 7% 3	540
May	1st to 10th	Day	24% 15% 29% 19% 9% 5%	543
	11th to 20th	Day	17% 11% 30% 25% 14% 4	567
	21st to end	Day	14% 12% 26% 27% 15% 6%	617
June	1st to 10th	Day	13% 14% 31% 27% 13% 2	530
	11th to 20th	Day	13% 11% 28% 27% 18% 4	530
	21st to end	Day	15% 9% 28% 30% 13% 5%	566
July	1st to 10th	Day	10% 4 22% 31% 23% 10%	525
	11th to 20th	Day	13% 7% 21% 30% 19% 11%	524
	21st to end	Day	10% 5% 25% 33% 19% 8%	576
August	1st to 10th	Day	10% 5% 22% 31% 21% 12%	526
	11th to 20th	Day	9% 6% 23% 29% 19% 13%	519
	21st to end	Day	11% 5% 22% 31% 20% 10%	583
September	1st to 10th	Day	13% 9% 21% 26% 19% 13%	530
	11th to 20th	Day	8% 5% 18% 28% 25% 15%	530
	21st to end	Day	14% 12% 28% 24% 15% 8%	530
October	1st to 10th	Day	20% 11% 25% 23% 15% 7%	530
	11th to 20th	Day	18% 17% 34% 20% 9% 2	530
	21st to end	Day	27% 16% 30% 20% 7%	583
November	1st to 10th	Day	23% 22% 31% 16% 6% 2	329
	11th to 20th	Day	27% 23% 25% 17% 7% 2	220
	21st to end	Day	28% 20% 35% 12% 4	220
December	1st to 10th	Day	27% 21% 28% 19% 3 1	220
	11th to 20th	Day	30% 25% 32% 11% 2	220
	21st to end	Day	28% 20% 36% 13% 3	242
January	1st to 10th	Night	21% 17% 39% 18% 5% 1	840
	11th to 20th	Night	27% 18% 33% 18% 4	840
	21st to end	Night	19% 16% 37% 22% 6%	924
February	1st to 10th	Night	22% 17% 37% 22% 3	840
	11th to 20th	Night	22% 17% 35% 23% 4	823
	21st to end	Night	23% 17% 35% 22% 3	692
March	1st to 10th	Night	20% 13% 35% 26% 6%	840
	11th to 20th	Night	22% 14% 36% 22% 6%	840
	21st to end	Night	21% 14% 30% 26% 8% 1	924
April	1st to 10th	Night	23% 14% 31% 27% 5%	840
	11th to 20th	Night	21% 12% 32% 26% 8% 1	745
	21st to end	Night	18% 8% 31% 33% 10%	630
May	1st to 10th	Night	19% 6% 30% 29% 14% 2	630
	11th to 20th	Night	18% 6% 26% 37% 12% 2	498
	21st to end	Night	15% 4 27% 34% 19% 1	455
June	1st to 10th	Night	13% 3 33% 37% 13% 1	400
	11th to 20th	Night	12% 5% 31% 36% 16% 1	400
	21st to end	Night	12% 6% 26% 40% 16%	400
July	1st to 10th	Night	5% 2 24% 42% 24% 2	390
	11th to 20th	Night	10% 3 23% 41% 21% 2	437
	21st to end	Night	9% 2 19% 44% 24% 2	657
August	1st to 10th	Night	8% 3 16% 40% 29% 3	597
	11th to 20th	Night	10% 3 14% 37% 34% 3	594
	21st to end	Night	10% 2 21% 36% 28% 3	730
September	1st to 10th	Night	13% 5% 21% 33% 24% 5%	800
	11th to 20th	Night	10% 4 17% 31% 32% 6%	800
	21st to end	Night	17% 8% 22% 29% 21% 5%	800
October	1st to 10th	Night	18% 8% 26% 25% 20% 3	800
	11th to 20th	Night	21% 10% 32% 27% 8% 1	800
	21st to end	Night	26% 11% 31% 28% 4	880
November	1st to 10th	Night	21% 12% 30% 25% 11% 1	820
	11th to 20th	Night	21% 16% 31% 22% 9% 1	840
	21st to end	Night	24% 15% 33% 22% 7%	840
December	1st to 10th	Night	25% 18% 32% 20% 5%	840
	11th to 20th	Night	23% 20% 35% 18% 4	840
	21st to end	Night	30% 14% 33% 20% 3	924

Page generated at 2026-05-29 01:53:25. Version ac07

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