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 2024-08-30.

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	14% 10% 4 8% 5% 5% 5% 4 6% 7% 32%	95706
	Night	15% 10% 4 8% 5% 5% 5% 4 5% 7% 33%	93241
Transparency	Day	49% 7% 12% 20% 11% 1	95435
	Night	49% 7% 12% 20% 11% 1	93065
Seeing	Day	18% 13% 28% 23% 13% 5%	14232
	Night	19% 10% 30% 28% 12% 1	23653

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% 14% 6% 10% 6% 5% 4 3 4 7% 24%	2531
	11th to 20th	Day	11% 18% 8% 17% 8% 5% 4 3 4 5% 17%	2840
	21st to end	Day	21% 19% 7% 13% 6% 5% 4 3 4 4 14%	3124
February	1st to 10th	Day	14% 17% 6% 12% 7% 5% 5% 5% 5% 5% 19%	1710
	11th to 20th	Day	12% 14% 7% 13% 8% 5% 4 3 5% 5% 24%	1663
	21st to end	Day	18% 14% 5% 11% 5% 5% 4 4 4 5% 24%	1489
March	1st to 10th	Day	19% 12% 4 9% 7% 5% 5% 3 4 6% 27%	3432
	11th to 20th	Day	11% 14% 5% 10% 6% 5% 5% 4 5% 7% 29%	3480
	21st to end	Day	16% 10% 3 8% 5% 5% 5% 4 6% 6% 32%	3815
April	1st to 10th	Day	16% 12% 3 8% 6% 5% 5% 4 5% 6% 29%	2217
	11th to 20th	Day	18% 9% 2 7% 5% 5% 5% 3 5% 6% 35%	2347
	21st to end	Day	18% 10% 3 9% 4 4 4 4 4 6% 33%	2470
May	1st to 10th	Day	17% 8% 2 8% 5% 4 4 4 6% 7% 34%	4108
	11th to 20th	Day	14% 11% 3 9% 5% 5% 5% 4 6% 7% 30%	4267
	21st to end	Day	10% 6% 3 6% 4 4 4 3 6% 8% 46%	4837
June	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	2660
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 9% 39%	2660
	21st to end	Day	12% 6% 2 6% 4 6% 5% 4 7% 9% 39%	2693
July	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 7% 12% 41%	4440
	11th to 20th	Day	8% 5% 2 6% 4 5% 6% 5% 7% 10% 41%	4352
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 9% 42%	4466
August	1st to 10th	Day	9% 6% 2 6% 6% 6% 6% 4 7% 9% 39%	2457
	11th to 20th	Day	8% 6% 2 7% 6% 5% 6% 5% 9% 9% 37%	2433
	21st to end	Day	10% 7% 2 6% 4 5% 5% 4 6% 10% 41%	2610
September	1st to 10th	Day	12% 7% 3 7% 6% 4 5% 4 6% 8% 38%	2160
	11th to 20th	Day	10% 6% 2 6% 5% 4 4 4 5% 9% 46%	2016
	21st to end	Day	13% 6% 3 7% 6% 5% 4 5% 7% 8% 35%	1966
October	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 4 5% 6% 34%	1800
	11th to 20th	Day	18% 10% 3 9% 5% 5% 5% 3 6% 6% 30%	1800
	21st to end	Day	23% 11% 4 9% 5% 5% 4 4 4 6% 25%	1980
November	1st to 10th	Day	13% 8% 3 9% 6% 4 6% 4 6% 8% 33%	1645
	11th to 20th	Day	13% 13% 4 10% 5% 5% 6% 4 6% 8% 26%	1520
	21st to end	Day	18% 14% 5% 9% 6% 4 4 5% 7% 5% 23%	1520
December	1st to 10th	Day	13% 16% 5% 13% 7% 5% 5% 4 5% 7% 19%	1520
	11th to 20th	Day	17% 15% 6% 10% 6% 5% 6% 5% 6% 5% 20%	1906
	21st to end	Day	26% 19% 8% 12% 6% 5% 5% 3 3 4 10%	2772
January	1st to 10th	Night	16% 14% 6% 11% 7% 5% 5% 4 5% 6% 22%	4397
	11th to 20th	Night	14% 13% 6% 13% 8% 8% 7% 4 5% 5% 18%	4120
	21st to end	Night	22% 16% 6% 10% 6% 4 4 3 4 6% 19%	4532
February	1st to 10th	Night	15% 15% 5% 10% 7% 5% 5% 4 5% 6% 23%	2470
	11th to 20th	Night	13% 13% 5% 10% 6% 5% 5% 4 6% 6% 26%	2403
	21st to end	Night	15% 13% 5% 11% 6% 5% 4 4 6% 6% 26%	1960
March	1st to 10th	Night	18% 10% 3 8% 6% 5% 5% 4 5% 5% 31%	3408
	11th to 20th	Night	13% 10% 4 8% 5% 4 3 3 6% 8% 37%	3479
	21st to end	Night	16% 10% 3 7% 4 4 4 3 4 5% 40%	3817
April	1st to 10th	Night	17% 10% 4 7% 6% 5% 5% 3 4 7% 33%	1962
	11th to 20th	Night	16% 9% 3 7% 5% 4 5% 3 5% 6% 37%	1797
	21st to end	Night	15% 8% 4 7% 5% 5% 4 4 6% 6% 37%	1710
May	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 4 7% 37%	2833
	11th to 20th	Night	14% 8% 3 9% 6% 5% 5% 4 6% 7% 32%	2623
	21st to end	Night	10% 5% 2 5% 4 4 3 4 6% 8% 50%	2738
June	1st to 10th	Night	10% 6% 1 6% 6% 5% 6% 4 6% 9% 41%	1520
	11th to 20th	Night	10% 5% 1 6% 4 5% 5% 4 8% 8% 43%	1520
	21st to end	Night	11% 4 2 6% 5% 4 4 4 6% 8% 45%	1405
July	1st to 10th	Night	5% 5% 2 6% 5% 5% 4 3 5% 9% 51%	2520
	11th to 20th	Night	9% 4 2 5% 3 4 4 3 6% 9% 50%	2605
	21st to end	Night	6% 4 2 6% 4 4 4 3 7% 9% 49%	3087
August	1st to 10th	Night	7% 5% 2 6% 4 5% 5% 3 8% 9% 46%	1701
	11th to 20th	Night	7% 5% 1 5% 4 5% 4 3 7% 10% 49%	1688
	21st to end	Night	8% 5% 2 5% 5% 4 5% 5% 5% 8% 48%	1937
September	1st to 10th	Night	11% 5% 3 6% 5% 4 5% 4 5% 8% 44%	1800
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 5% 7% 53%	1940
	21st to end	Night	14% 6% 2 7% 4 4 4 4 7% 7% 40%	1993
October	1st to 10th	Night	16% 8% 2 7% 5% 3 4 3 5% 7% 41%	2160
	11th to 20th	Night	17% 9% 3 7% 5% 5% 5% 4 5% 7% 34%	2160
	21st to end	Night	21% 10% 3 8% 4 5% 5% 4 5% 6% 29%	2376
November	1st to 10th	Night	14% 8% 2 6% 4 4 5% 4 7% 7% 40%	2428
	11th to 20th	Night	15% 11% 3 8% 5% 6% 5% 4 5% 8% 30%	2660
	21st to end	Night	17% 11% 3 8% 6% 5% 5% 4 6% 8% 26%	2660
December	1st to 10th	Night	18% 14% 5% 11% 6% 5% 5% 4 6% 6% 20%	2660
	11th to 20th	Night	15% 17% 4 10% 5% 5% 5% 4 5% 6% 23%	3288
	21st to end	Night	28% 19% 6% 12% 6% 5% 4 3 4 4 10%	4884

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	57% 9% 13% 11% 9%	2532
	11th to 20th	Day	69% 7% 10% 10% 4	2840
	21st to end	Day	73% 7% 8% 8% 4	3124
February	1st to 10th	Day	66% 5% 10% 14% 4 1	1710
	11th to 20th	Day	62% 5% 9% 15% 8% 1	1672
	21st to end	Day	62% 5% 10% 14% 7% 1	1489
March	1st to 10th	Day	57% 6% 12% 18% 6%	3435
	11th to 20th	Day	52% 7% 14% 18% 7% 2	3480
	21st to end	Day	48% 8% 9% 22% 11% 1	3815
April	1st to 10th	Day	55% 5% 11% 17% 12%	2217
	11th to 20th	Day	49% 6% 10% 21% 13% 1	2347
	21st to end	Day	51% 6% 10% 19% 12% 3	2470
May	1st to 10th	Day	48% 7% 10% 20% 12% 3	4120
	11th to 20th	Day	50% 9% 12% 21% 7% 1	4271
	21st to end	Day	35% 8% 12% 23% 19% 3	4837
June	1st to 10th	Day	41% 7% 11% 23% 16% 2	2660
	11th to 20th	Day	40% 7% 13% 24% 16% 1	2660
	21st to end	Day	39% 7% 13% 25% 17%	2693
July	1st to 10th	Day	34% 7% 15% 27% 15% 2	4370
	11th to 20th	Day	34% 7% 14% 31% 13%	4277
	21st to end	Day	35% 9% 14% 22% 19% 1	4324
August	1st to 10th	Day	38% 8% 14% 24% 13% 1	2444
	11th to 20th	Day	38% 10% 15% 25% 11% 1	2433
	21st to end	Day	38% 7% 12% 29% 14% 1	2610
September	1st to 10th	Day	41% 8% 13% 26% 11% 1	2160
	11th to 20th	Day	35% 5% 14% 30% 15%	2016
	21st to end	Day	43% 8% 14% 25% 10% 1	1966
October	1st to 10th	Day	50% 5% 13% 20% 11% 1	1800
	11th to 20th	Day	53% 6% 12% 20% 8% 1	1800
	21st to end	Day	60% 6% 12% 15% 7% 1	1980
November	1st to 10th	Day	47% 7% 14% 23% 8% 1	1645
	11th to 20th	Day	53% 7% 17% 14% 9% 1	1520
	21st to end	Day	58% 7% 16% 14% 5% 1	1520
December	1st to 10th	Day	63% 7% 13% 12% 4	1520
	11th to 20th	Day	62% 8% 14% 9% 6%	1906
	21st to end	Day	77% 8% 7% 7% 2	2772
January	1st to 10th	Night	60% 9% 12% 13% 5% 1	4397
	11th to 20th	Night	62% 11% 12% 11% 3	4120
	21st to end	Night	66% 7% 9% 13% 4	4532
February	1st to 10th	Night	60% 6% 10% 17% 5% 1	2470
	11th to 20th	Night	56% 7% 11% 16% 9% 1	2405
	21st to end	Night	57% 6% 10% 18% 8% 1	1960
March	1st to 10th	Night	52% 5% 14% 21% 7% 1	3418
	11th to 20th	Night	44% 8% 15% 20% 13% 2	3480
	21st to end	Night	45% 6% 7% 23% 17% 2	3817
April	1st to 10th	Night	51% 7% 9% 19% 13% 1	1962
	11th to 20th	Night	48% 6% 8% 20% 16% 3	1797
	21st to end	Night	47% 6% 9% 19% 16% 3	1710
May	1st to 10th	Night	47% 5% 10% 19% 15% 4	2840
	11th to 20th	Night	48% 9% 14% 19% 8% 2	2624
	21st to end	Night	30% 9% 12% 24% 23% 2	2739
June	1st to 10th	Night	39% 6% 11% 24% 18% 2	1520
	11th to 20th	Night	36% 7% 12% 25% 19% 2	1520
	21st to end	Night	36% 7% 13% 24% 18% 1	1405
July	1st to 10th	Night	31% 5% 12% 25% 23% 4	2480
	11th to 20th	Night	30% 6% 13% 31% 18% 2	2557
	21st to end	Night	30% 8% 16% 25% 19% 2	2988
August	1st to 10th	Night	33% 7% 14% 29% 14% 2	1690
	11th to 20th	Night	28% 9% 16% 32% 14% 1	1688
	21st to end	Night	32% 7% 14% 30% 15% 2	1937
September	1st to 10th	Night	39% 6% 12% 26% 14% 2	1800
	11th to 20th	Night	31% 5% 12% 32% 19% 1	1940
	21st to end	Night	42% 6% 12% 26% 13% 1	1993
October	1st to 10th	Night	43% 6% 10% 24% 16% 1	2160
	11th to 20th	Night	49% 6% 12% 22% 10%	2160
	21st to end	Night	55% 6% 10% 18% 11% 1	2376
November	1st to 10th	Night	42% 7% 12% 24% 14% 1	2428
	11th to 20th	Night	50% 7% 13% 19% 10% 1	2660
	21st to end	Night	54% 7% 17% 17% 5%	2660
December	1st to 10th	Night	61% 8% 12% 14% 5%	2660
	11th to 20th	Night	59% 7% 14% 15% 5%	3288
	21st to end	Night	74% 9% 8% 6% 2	4884

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	20% 18% 35% 16% 9% 2	186
	11th to 20th	Day	23% 23% 35% 13% 4 2	270
	21st to end	Day	22% 20% 34% 20% 4 1	297
February	1st to 10th	Day	25% 20% 40% 11% 4	270
	11th to 20th	Day	22% 27% 37% 13% 1	270
	21st to end	Day	35% 23% 31% 10% 1	225
March	1st to 10th	Day	22% 15% 34% 22% 8%	460
	11th to 20th	Day	22% 18% 36% 17% 6% 1	460
	21st to end	Day	28% 14% 31% 18% 8% 1	506
April	1st to 10th	Day	20% 18% 32% 22% 7% 1	460
	11th to 20th	Day	25% 18% 26% 18% 9% 4	452
	21st to end	Day	22% 16% 32% 20% 7% 3	460
May	1st to 10th	Day	22% 13% 28% 21% 10% 6%	463
	11th to 20th	Day	17% 8% 27% 27% 16% 4	487
	21st to end	Day	12% 9% 26% 29% 17% 7%	529
June	1st to 10th	Day	12% 13% 30% 29% 14% 2	490
	11th to 20th	Day	13% 9% 28% 28% 19% 4	490
	21st to end	Day	15% 8% 27% 31% 13% 5%	523
July	1st to 10th	Day	10% 3 21% 31% 24% 11%	485
	11th to 20th	Day	14% 6% 20% 30% 19% 12%	484
	21st to end	Day	9% 5% 24% 34% 19% 8%	532
August	1st to 10th	Day	11% 5% 22% 29% 21% 12%	486
	11th to 20th	Day	9% 6% 22% 29% 20% 14%	479
	21st to end	Day	11% 5% 22% 31% 21% 11%	532
September	1st to 10th	Day	13% 7% 22% 29% 17% 12%	450
	11th to 20th	Day	9% 6% 19% 28% 24% 14%	450
	21st to end	Day	13% 12% 29% 23% 15% 9%	450
October	1st to 10th	Day	22% 11% 24% 21% 16% 7%	450
	11th to 20th	Day	18% 19% 34% 19% 8% 2	450
	21st to end	Day	28% 15% 30% 19% 8%	495
November	1st to 10th	Day	19% 23% 31% 18% 7% 2	273
	11th to 20th	Day	27% 22% 24% 17% 7% 2	180
	21st to end	Day	24% 19% 38% 14% 4 1	180
December	1st to 10th	Day	26% 21% 29% 18% 4 2	180
	11th to 20th	Day	27% 24% 34% 13% 2	180
	21st to end	Day	27% 19% 40% 11% 3	198
January	1st to 10th	Night	21% 17% 38% 18% 5% 1	760
	11th to 20th	Night	27% 16% 33% 20% 4	760
	21st to end	Night	18% 14% 38% 24% 6%	836
February	1st to 10th	Night	23% 16% 37% 21% 3	760
	11th to 20th	Night	22% 15% 36% 24% 4	743
	21st to end	Night	23% 16% 35% 23% 3	628
March	1st to 10th	Night	18% 12% 36% 27% 7%	760
	11th to 20th	Night	21% 14% 36% 22% 6%	760
	21st to end	Night	20% 13% 30% 27% 9% 1	836
April	1st to 10th	Night	22% 13% 31% 28% 6%	760
	11th to 20th	Night	20% 11% 33% 26% 8% 1	673
	21st to end	Night	18% 8% 31% 33% 10%	570
May	1st to 10th	Night	19% 5% 29% 29% 15% 2	570
	11th to 20th	Night	18% 5% 24% 38% 13% 2	450
	21st to end	Night	13% 4 26% 35% 21% 1	411
June	1st to 10th	Night	13% 2 32% 37% 14% 1	380
	11th to 20th	Night	11% 5% 31% 37% 16% 1	380
	21st to end	Night	12% 5% 26% 41% 16%	380
July	1st to 10th	Night	5% 2 23% 42% 25% 2	370
	11th to 20th	Night	10% 2 23% 41% 21% 2	414
	21st to end	Night	9% 2 19% 44% 24% 2	624
August	1st to 10th	Night	9% 3 17% 40% 28% 3	567
	11th to 20th	Night	9% 2 14% 37% 34% 3	564
	21st to end	Night	9% 2 22% 35% 29% 3	689
September	1st to 10th	Night	13% 5% 20% 33% 23% 6%	720
	11th to 20th	Night	11% 5% 19% 31% 30% 5%	720
	21st to end	Night	16% 7% 21% 29% 22% 5%	720
October	1st to 10th	Night	20% 8% 26% 24% 19% 3	720
	11th to 20th	Night	21% 11% 33% 26% 8% 1	720
	21st to end	Night	28% 10% 30% 27% 4	792
November	1st to 10th	Night	21% 10% 30% 26% 11% 1	740
	11th to 20th	Night	20% 15% 33% 21% 9% 1	760
	21st to end	Night	23% 14% 33% 23% 7%	760
December	1st to 10th	Night	26% 17% 32% 19% 5% 1	760
	11th to 20th	Night	23% 19% 36% 19% 5%	760
	21st to end	Night	29% 14% 33% 20% 3	836

Page generated at 2024-08-27 09:49:37. Version ac07

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