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 2025-02-06.

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 9% 5% 5% 5% 4 6% 7% 31%	108649
	Night	14% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	109492
Transparency	Day	49% 7% 12% 20% 11% 1	112289
	Night	48% 7% 12% 20% 12% 1	114015
Seeing	Day	18% 13% 28% 23% 12% 5%	14711
	Night	19% 10% 30% 28% 12% 1	24290

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% 14% 7% 15% 7% 5% 4 3 4 5% 25%	3625
	11th to 20th	Day	9% 19% 9% 20% 9% 7% 4 3 4 4 13%	4026
	21st to end	Day	17% 15% 6% 17% 7% 5% 4 3 4 4 17%	4466
February	1st to 10th	Day	14% 17% 6% 12% 7% 5% 5% 5% 5% 5% 19%	1755
	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	17% 6% 2 6% 5% 5% 5% 4 6% 8% 36%	4104
	11th to 20th	Day	10% 7% 3 7% 5% 5% 6% 5% 9% 9% 35%	4044
	21st to end	Day	8% 6% 2 7% 4 5% 6% 4 7% 10% 42%	4382
September	1st to 10th	Day	11% 6% 3 7% 6% 4 5% 4 6% 8% 40%	2280
	11th to 20th	Day	9% 6% 2 6% 5% 4 4 4 5% 9% 47%	2128
	21st to end	Day	14% 7% 3 8% 6% 5% 4 5% 7% 8% 34%	2075
October	1st to 10th	Day	16% 9% 3 7% 5% 5% 5% 4 5% 7% 35%	1900
	11th to 20th	Day	18% 10% 3 9% 5% 5% 4 3 6% 6% 31%	1900
	21st to end	Day	22% 11% 4 9% 5% 5% 4 4 5% 6% 25%	2090
November	1st to 10th	Day	13% 8% 3 9% 6% 5% 6% 4 6% 8% 33%	1732
	11th to 20th	Day	13% 13% 4 10% 5% 5% 6% 4 6% 8% 27%	1600
	21st to end	Day	18% 14% 5% 9% 6% 5% 4 5% 7% 5% 22%	1600
December	1st to 10th	Day	15% 17% 5% 13% 7% 6% 6% 5% 5% 6% 14%	2600
	11th to 20th	Day	19% 18% 5% 8% 5% 4 5% 4 5% 4 23%	2986
	21st to end	Day	24% 17% 7% 12% 7% 5% 5% 3 4 4 12%	3960
January	1st to 10th	Night	13% 11% 5% 12% 7% 6% 5% 4 5% 6% 25%	6303
	11th to 20th	Night	11% 14% 6% 15% 9% 7% 7% 4 5% 5% 16%	5859
	21st to end	Night	17% 15% 6% 13% 6% 5% 5% 3 4 7% 21%	6490
February	1st to 10th	Night	15% 15% 5% 10% 6% 5% 5% 4 5% 6% 23%	2536
	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	10% 6% 3 6% 4 5% 4 4 6% 9% 44%	2825
	11th to 20th	Night	11% 6% 2 5% 3 4 4 3 6% 9% 46%	2785
	21st to end	Night	8% 5% 2 5% 4 5% 5% 4 6% 8% 49%	3242
September	1st to 10th	Night	11% 5% 2 6% 5% 4 5% 4 5% 7% 46%	1900
	11th to 20th	Night	9% 4 2 5% 3 4 4 3 5% 7% 54%	2048
	21st to end	Night	15% 7% 2 7% 4 5% 4 4 7% 7% 38%	2104
October	1st to 10th	Night	15% 8% 2 7% 4 3 4 3 5% 7% 42%	2280
	11th to 20th	Night	17% 9% 3 7% 5% 5% 5% 3 5% 7% 35%	2280
	21st to end	Night	20% 10% 3 7% 4 5% 5% 4 6% 6% 31%	2508
November	1st to 10th	Night	14% 8% 2 6% 4 4 4 4 7% 7% 40%	2561
	11th to 20th	Night	15% 11% 3 8% 5% 6% 5% 4 5% 8% 30%	2800
	21st to end	Night	17% 11% 3 8% 6% 5% 5% 4 6% 8% 26%	2800
December	1st to 10th	Night	12% 13% 5% 11% 7% 6% 6% 6% 7% 7% 19%	4580
	11th to 20th	Night	16% 17% 4 10% 5% 5% 4 4 5% 5% 24%	5208
	21st to end	Night	24% 17% 5% 11% 6% 5% 4 3 4 5% 14%	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	58% 8% 11% 11% 11% 1	3626
	11th to 20th	Day	73% 6% 9% 8% 3 1	4060
	21st to end	Day	70% 5% 11% 7% 6%	4466
February	1st to 10th	Day	66% 5% 10% 14% 4 1	1755
	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	44% 7% 13% 22% 13% 1	4091
	11th to 20th	Day	40% 8% 16% 25% 10% 1	4083
	21st to end	Day	36% 7% 11% 32% 13% 1	4382
September	1st to 10th	Day	40% 8% 13% 26% 12% 1	2280
	11th to 20th	Day	34% 5% 13% 30% 16%	2128
	21st to end	Day	44% 8% 13% 24% 9% 1	2075
October	1st to 10th	Day	38% 6% 15% 25% 15% 1	3160
	11th to 20th	Day	41% 5% 13% 24% 16% 1	3140
	21st to end	Day	50% 6% 14% 22% 8%	3428
November	1st to 10th	Day	47% 7% 14% 23% 8% 1	1732
	11th to 20th	Day	53% 7% 16% 14% 9% 1	1600
	21st to end	Day	58% 7% 16% 14% 5% 1	1600
December	1st to 10th	Day	67% 8% 13% 9% 3	2600
	11th to 20th	Day	62% 6% 12% 10% 9%	2986
	21st to end	Day	73% 8% 7% 8% 5%	3960
January	1st to 10th	Night	55% 8% 11% 17% 8% 1	6303
	11th to 20th	Night	63% 9% 11% 13% 4	5900
	21st to end	Night	62% 6% 10% 12% 10% 1	6490
February	1st to 10th	Night	60% 6% 10% 17% 5% 1	2536
	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	37% 7% 12% 26% 16% 2	2814
	11th to 20th	Night	34% 7% 16% 30% 11% 1	2812
	21st to end	Night	31% 6% 13% 32% 16% 1	3242
September	1st to 10th	Night	38% 6% 11% 27% 16% 2	1900
	11th to 20th	Night	30% 5% 12% 32% 20% 1	2048
	21st to end	Night	44% 6% 12% 26% 12% 1	2104
October	1st to 10th	Night	33% 5% 10% 27% 24% 1	3800
	11th to 20th	Night	43% 5% 12% 25% 15%	3779
	21st to end	Night	41% 5% 14% 27% 12% 1	4120
November	1st to 10th	Night	41% 7% 13% 24% 14% 1	2561
	11th to 20th	Night	50% 7% 13% 19% 10% 1	2800
	21st to end	Night	54% 7% 17% 16% 5%	2800
December	1st to 10th	Night	58% 9% 16% 13% 4	4580
	11th to 20th	Night	59% 6% 11% 14% 10% 1	5208
	21st to end	Night	69% 8% 9% 9% 5%	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	22% 18% 36% 15% 8% 1	207
	11th to 20th	Day	24% 23% 34% 13% 4 2	300
	21st to end	Day	24% 21% 34% 18% 3 1	330
February	1st to 10th	Day	25% 20% 40% 11% 4	285
	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% 10%	539
September	1st to 10th	Day	12% 8% 22% 27% 18% 12%	490
	11th to 20th	Day	8% 6% 19% 27% 24% 16%	490
	21st to end	Day	14% 12% 28% 23% 14% 8%	490
October	1st to 10th	Day	21% 11% 25% 22% 14% 7%	490
	11th to 20th	Day	18% 18% 34% 19% 9% 2	490
	21st to end	Day	27% 16% 30% 19% 8%	539
November	1st to 10th	Day	20% 23% 31% 17% 6% 2	300
	11th to 20th	Day	27% 23% 26% 17% 7% 2	200
	21st to end	Day	26% 21% 37% 13% 4 1	200
December	1st to 10th	Day	27% 22% 29% 18% 4 2	200
	11th to 20th	Day	29% 25% 33% 12% 2	200
	21st to end	Day	26% 19% 38% 14% 3	220
January	1st to 10th	Night	21% 17% 38% 18% 5% 1	800
	11th to 20th	Night	26% 17% 34% 19% 4	800
	21st to end	Night	19% 15% 37% 23% 6%	880
February	1st to 10th	Night	23% 16% 37% 21% 3	781
	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	10% 2 21% 35% 29% 3	693
September	1st to 10th	Night	13% 5% 21% 33% 23% 6%	760
	11th to 20th	Night	11% 4 18% 31% 31% 6%	760
	21st to end	Night	17% 8% 22% 29% 21% 5%	760
October	1st to 10th	Night	19% 8% 27% 24% 19% 3	760
	11th to 20th	Night	21% 11% 31% 27% 9% 1	760
	21st to end	Night	26% 11% 30% 28% 4	836
November	1st to 10th	Night	21% 11% 30% 26% 11% 1	780
	11th to 20th	Night	21% 14% 32% 22% 9% 1	800
	21st to end	Night	24% 15% 33% 22% 7%	800
December	1st to 10th	Night	25% 18% 32% 19% 5% 1	800
	11th to 20th	Night	24% 19% 35% 18% 5%	800
	21st to end	Night	29% 14% 33% 21% 3	880

Page generated at 2025-02-03 03:05:55. Version ac07

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