Averted Vision Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Averted Vision Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	15% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	149251
	Night	16% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	141842
Transparency	Day	47% 8% 13% 20% 10% 1	152731
	Night	50% 7% 12% 18% 11% 1	146451
Seeing	Day	21% 16% 29% 21% 10% 3	17154
	Night	20% 12% 30% 28% 8% 1	26112

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% 10% 6% 14% 8% 6% 6% 5% 5% 5% 24%	4661
	11th to 20th	Day	12% 13% 6% 11% 7% 6% 6% 4 6% 6% 24%	4650
	21st to end	Day	11% 10% 3 9% 6% 6% 5% 4 6% 6% 33%	5241
February	1st to 10th	Day	13% 12% 4 10% 6% 5% 5% 4 6% 6% 29%	1890
	11th to 20th	Day	12% 12% 4 11% 6% 6% 5% 4 5% 5% 28%	2014
	21st to end	Day	18% 12% 4 7% 6% 4 5% 3 5% 5% 31%	1730
March	1st to 10th	Day	20% 11% 5% 9% 5% 4 4 3 3 5% 31%	4576
	11th to 20th	Day	15% 9% 3 8% 5% 5% 4 4 4 7% 37%	4801
	21st to end	Day	19% 10% 3 7% 5% 4 4 4 5% 5% 34%	6100
April	1st to 10th	Day	19% 9% 3 8% 5% 4 4 3 4 5% 35%	2520
	11th to 20th	Day	16% 7% 2 8% 6% 6% 5% 3 5% 6% 36%	2494
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 4 5% 5% 34%	2633
May	1st to 10th	Day	27% 8% 3 8% 5% 4 5% 4 5% 7% 25%	7815
	11th to 20th	Day	16% 8% 3 8% 5% 5% 5% 4 5% 7% 33%	8221
	21st to end	Day	20% 8% 3 6% 4 5% 5% 4 6% 8% 31%	9060
June	1st to 10th	Day	15% 6% 3 7% 6% 5% 6% 4 6% 9% 34%	2940
	11th to 20th	Day	17% 5% 2 6% 5% 5% 6% 4 6% 9% 34%	2940
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 5% 6% 8% 34%	2940
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 8% 11% 37%	8280
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 5% 8% 11% 42%	8280
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 6% 9% 39%	8993
August	1st to 10th	Day	13% 7% 3 8% 6% 5% 6% 4 7% 9% 32%	6483
	11th to 20th	Day	13% 5% 2 6% 5% 5% 6% 5% 8% 11% 34%	5537
	21st to end	Day	11% 5% 2 7% 5% 5% 6% 5% 8% 9% 36%	6080
September	1st to 10th	Day	13% 5% 2 5% 4 5% 4 4 6% 7% 45%	2520
	11th to 20th	Day	13% 5% 1 4 4 4 4 3 5% 8% 49%	2341
	21st to end	Day	16% 6% 2 4 4 4 5% 4 6% 8% 40%	2200
October	1st to 10th	Day	19% 7% 2 8% 6% 4 5% 4 5% 5% 35%	2090
	11th to 20th	Day	15% 6% 3 7% 5% 4 4 4 5% 8% 39%	1995
	21st to end	Day	23% 7% 3 7% 5% 4 5% 4 5% 6% 31%	1980
November	1st to 10th	Day	13% 8% 3 7% 5% 4 4 3 5% 7% 41%	1851
	11th to 20th	Day	15% 8% 3 7% 5% 4 5% 3 6% 6% 37%	1890
	21st to end	Day	18% 10% 3 7% 6% 5% 5% 3 5% 6% 32%	1711
December	1st to 10th	Day	17% 12% 5% 10% 6% 5% 5% 4 6% 6% 25%	2680
	11th to 20th	Day	22% 10% 3 6% 4 4 4 4 5% 6% 31%	3066
	21st to end	Day	22% 16% 4 9% 4 4 4 3 4 6% 26%	4048
January	1st to 10th	Night	13% 12% 4 11% 6% 5% 5% 4 5% 6% 28%	8234
	11th to 20th	Night	12% 13% 5% 13% 7% 7% 6% 4 6% 6% 21%	8235
	21st to end	Night	13% 9% 3 8% 6% 6% 6% 4 5% 6% 33%	8937
February	1st to 10th	Night	13% 11% 4 9% 7% 6% 5% 4 6% 6% 28%	2730
	11th to 20th	Night	11% 12% 4 10% 6% 5% 5% 4 6% 6% 33%	2490
	21st to end	Night	16% 10% 3 9% 5% 5% 5% 5% 6% 6% 32%	2076
March	1st to 10th	Night	21% 9% 3 8% 5% 5% 5% 3 5% 5% 30%	5484
	11th to 20th	Night	16% 10% 3 7% 4 4 5% 3 5% 7% 36%	5376
	21st to end	Night	20% 12% 3 9% 4 4 4 3 4 4 32%	5072
April	1st to 10th	Night	21% 8% 3 7% 5% 4 4 3 5% 5% 35%	2100
	11th to 20th	Night	19% 8% 4 7% 4 5% 3 3 5% 5% 36%	2075
	21st to end	Night	20% 8% 2 7% 3 4 3 3 5% 5% 40%	1985
May	1st to 10th	Night	26% 8% 3 8% 5% 4 5% 4 5% 6% 26%	5126
	11th to 20th	Night	16% 8% 3 9% 5% 5% 5% 3 6% 7% 32%	4639
	21st to end	Night	18% 7% 3 7% 4 5% 5% 4 6% 7% 33%	5114
June	1st to 10th	Night	18% 5% 1 5% 4 5% 6% 4 7% 6% 38%	1680
	11th to 20th	Night	18% 6% 2 6% 4 4 4 4 5% 8% 39%	1680
	21st to end	Night	14% 6% 2 6% 5% 4 4 5% 6% 8% 40%	1680
July	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 10% 38%	4680
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 4 6% 8% 43%	4680
	21st to end	Night	13% 5% 2 6% 4 4 5% 4 6% 7% 44%	5084
August	1st to 10th	Night	17% 8% 3 7% 4 4 5% 3 6% 8% 35%	3666
	11th to 20th	Night	16% 6% 2 6% 4 4 5% 4 7% 7% 37%	4457
	21st to end	Night	11% 6% 2 6% 4 5% 5% 5% 8% 10% 37%	5045
September	1st to 10th	Night	14% 6% 3 5% 4 4 4 4 6% 7% 45%	2100
	11th to 20th	Night	13% 5% 2 5% 3 4 3 2 5% 6% 52%	2126
	21st to end	Night	15% 5% 2 5% 5% 4 5% 3 5% 6% 44%	2200
October	1st to 10th	Night	20% 6% 3 8% 5% 4 4 3 5% 6% 36%	2305
	11th to 20th	Night	16% 5% 2 7% 6% 4 5% 4 5% 6% 40%	2405
	21st to end	Night	22% 7% 3 7% 4 4 4 4 5% 6% 33%	2860
November	1st to 10th	Night	14% 9% 3 7% 5% 4 4 3 5% 5% 41%	2667
	11th to 20th	Night	18% 7% 3 8% 4 4 4 3 5% 6% 39%	2730
	21st to end	Night	16% 9% 3 7% 5% 5% 4 4 5% 7% 35%	2906
December	1st to 10th	Night	16% 11% 4 9% 5% 5% 5% 4 6% 7% 27%	4720
	11th to 20th	Night	26% 9% 3 7% 4 4 4 3 5% 6% 30%	5348
	21st to end	Night	23% 14% 4 8% 4 3 4 3 4 6% 26%	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	59% 9% 11% 16% 4	4662
	11th to 20th	Day	59% 7% 11% 11% 10% 2	4680
	21st to end	Day	50% 7% 12% 21% 10% 1	5241
February	1st to 10th	Day	56% 8% 12% 15% 8% 1	1890
	11th to 20th	Day	58% 6% 11% 15% 8% 1	2023
	21st to end	Day	55% 7% 11% 15% 11% 1	1730
March	1st to 10th	Day	58% 6% 10% 13% 8% 6%	4579
	11th to 20th	Day	46% 9% 18% 16% 8% 3	4801
	21st to end	Day	52% 6% 8% 14% 17% 4	6100
April	1st to 10th	Day	52% 7% 8% 19% 11% 3	2520
	11th to 20th	Day	48% 7% 11% 18% 14% 2	2494
	21st to end	Day	51% 8% 9% 17% 13% 3	2633
May	1st to 10th	Day	58% 8% 10% 16% 8% 1	7827
	11th to 20th	Day	47% 8% 15% 20% 9% 1	8225
	21st to end	Day	48% 8% 13% 22% 8% 1	9061
June	1st to 10th	Day	46% 10% 15% 19% 10% 1	2940
	11th to 20th	Day	45% 8% 13% 19% 13% 1	2940
	21st to end	Day	47% 8% 14% 19% 11%	2940
July	1st to 10th	Day	40% 8% 16% 24% 11%	8210
	11th to 20th	Day	33% 8% 17% 29% 13% 1	8205
	21st to end	Day	41% 7% 14% 27% 11%	8795
August	1st to 10th	Day	48% 9% 16% 17% 10% 1	6469
	11th to 20th	Day	40% 10% 20% 25% 5%	5574
	21st to end	Day	39% 9% 17% 26% 9% 1	6080
September	1st to 10th	Day	36% 7% 13% 29% 13% 1	2520
	11th to 20th	Day	34% 7% 12% 27% 19% 1	2341
	21st to end	Day	40% 8% 14% 25% 13% 1	2200
October	1st to 10th	Day	46% 7% 10% 23% 14% 1	3402
	11th to 20th	Day	38% 6% 16% 25% 14% 1	3223
	21st to end	Day	38% 6% 14% 28% 12% 1	3180
November	1st to 10th	Day	42% 8% 16% 23% 11% 1	1851
	11th to 20th	Day	47% 8% 12% 17% 14% 1	1890
	21st to end	Day	53% 7% 16% 17% 6% 1	1711
December	1st to 10th	Day	59% 7% 13% 16% 4	2680
	11th to 20th	Day	54% 7% 12% 18% 8% 1	3066
	21st to end	Day	60% 5% 10% 14% 10% 1	4048
January	1st to 10th	Night	57% 8% 13% 13% 8% 1	8234
	11th to 20th	Night	62% 7% 11% 10% 8% 2	8280
	21st to end	Night	51% 6% 11% 14% 14% 3	8937
February	1st to 10th	Night	56% 8% 12% 17% 6% 1	2730
	11th to 20th	Night	54% 8% 11% 17% 9% 1	2492
	21st to end	Night	52% 8% 13% 18% 8% 2	2076
March	1st to 10th	Night	54% 8% 9% 14% 12% 2	5494
	11th to 20th	Night	47% 8% 18% 17% 8% 2	5377
	21st to end	Night	56% 5% 6% 13% 17% 2	5072
April	1st to 10th	Night	52% 5% 11% 18% 12% 1	2100
	11th to 20th	Night	50% 7% 9% 20% 13% 1	2075
	21st to end	Night	47% 7% 9% 19% 15% 3	1985
May	1st to 10th	Night	58% 7% 11% 12% 11% 1	5132
	11th to 20th	Night	50% 9% 12% 19% 9% 1	4640
	21st to end	Night	50% 8% 11% 20% 10% 1	5115
June	1st to 10th	Night	43% 8% 14% 22% 12% 1	1680
	11th to 20th	Night	42% 9% 15% 22% 12% 1	1680
	21st to end	Night	41% 9% 14% 23% 12% 1	1680
July	1st to 10th	Night	42% 8% 17% 22% 11%	4640
	11th to 20th	Night	40% 7% 11% 23% 18% 1	4632
	21st to end	Night	38% 7% 14% 30% 12% 1	4967
August	1st to 10th	Night	48% 9% 16% 18% 9% 2	3656
	11th to 20th	Night	44% 9% 18% 21% 7%	4486
	21st to end	Night	40% 9% 17% 25% 9%	5045
September	1st to 10th	Night	38% 8% 13% 27% 14% 1	2100
	11th to 20th	Night	33% 7% 13% 28% 18% 1	2126
	21st to end	Night	39% 9% 12% 23% 16% 2	2200
October	1st to 10th	Night	49% 7% 11% 19% 13% 1	3773
	11th to 20th	Night	36% 6% 15% 24% 18% 1	3916
	21st to end	Night	44% 7% 12% 28% 9% 1	4610
November	1st to 10th	Night	45% 7% 11% 21% 13% 2	2667
	11th to 20th	Night	47% 7% 12% 20% 12% 2	2730
	21st to end	Night	50% 8% 17% 18% 7%	2906
December	1st to 10th	Night	58% 10% 13% 16% 4	4720
	11th to 20th	Night	57% 6% 13% 13% 11% 1	5348
	21st to end	Night	60% 6% 9% 15% 10% 1	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	25% 21% 28% 18% 7%	220
	11th to 20th	Day	28% 28% 29% 13% 1 1	220
	21st to end	Day	22% 22% 33% 17% 5% 1	263
February	1st to 10th	Day	25% 28% 32% 12% 3	330
	11th to 20th	Day	27% 25% 33% 13% 3	502
	21st to end	Day	27% 20% 35% 15% 2	446
March	1st to 10th	Day	27% 18% 32% 19% 3	540
	11th to 20th	Day	27% 22% 32% 14% 5% 1	540
	21st to end	Day	30% 23% 26% 13% 7% 1	594
April	1st to 10th	Day	32% 22% 25% 15% 6% 1	540
	11th to 20th	Day	29% 25% 25% 13% 7% 2	532
	21st to end	Day	26% 20% 30% 16% 6% 1	540
May	1st to 10th	Day	31% 18% 28% 13% 8% 2	543
	11th to 20th	Day	18% 17% 34% 24% 6% 2	567
	21st to end	Day	17% 14% 32% 25% 10% 2	617
June	1st to 10th	Day	16% 16% 31% 24% 11% 2	570
	11th to 20th	Day	18% 14% 33% 24% 9% 2	570
	21st to end	Day	17% 14% 31% 26% 10% 2	570
July	1st to 10th	Day	12% 9% 31% 31% 15% 3	565
	11th to 20th	Day	10% 10% 31% 30% 16% 3	564
	21st to end	Day	15% 7% 34% 28% 13% 3	620
August	1st to 10th	Day	11% 8% 26% 31% 19% 5%	566
	11th to 20th	Day	15% 6% 29% 26% 19% 5%	559
	21st to end	Day	11% 10% 26% 32% 14% 7%	627
September	1st to 10th	Day	15% 11% 23% 28% 16% 8%	570
	11th to 20th	Day	13% 9% 25% 21% 20% 12%	542
	21st to end	Day	18% 9% 27% 26% 14% 7%	530
October	1st to 10th	Day	23% 13% 25% 21% 13% 6%	530
	11th to 20th	Day	18% 18% 29% 23% 10% 2	527
	21st to end	Day	29% 15% 27% 23% 6% 1	462
November	1st to 10th	Day	21% 16% 28% 23% 11%	425
	11th to 20th	Day	25% 16% 30% 21% 7% 1	430
	21st to end	Day	28% 22% 29% 18% 3	251
December	1st to 10th	Day	24% 23% 33% 16% 4	220
	11th to 20th	Day	33% 20% 32% 13% 2	220
	21st to end	Day	26% 21% 31% 19% 3	242
January	1st to 10th	Night	21% 17% 35% 25% 3	840
	11th to 20th	Night	25% 18% 31% 23% 3	840
	21st to end	Night	17% 15% 39% 25% 3	924
February	1st to 10th	Night	20% 15% 39% 24% 2	840
	11th to 20th	Night	19% 19% 39% 21% 2	823
	21st to end	Night	23% 19% 35% 21% 3	692
March	1st to 10th	Night	23% 16% 37% 22% 3	840
	11th to 20th	Night	24% 18% 31% 22% 4	840
	21st to end	Night	23% 17% 33% 21% 6%	924
April	1st to 10th	Night	26% 17% 33% 22% 3	840
	11th to 20th	Night	24% 18% 31% 21% 6%	831
	21st to end	Night	24% 15% 29% 26% 6%	725
May	1st to 10th	Night	26% 11% 29% 25% 9%	587
	11th to 20th	Night	18% 10% 37% 29% 6%	414
	21st to end	Night	15% 11% 27% 36% 11% 1	455
June	1st to 10th	Night	20% 5% 25% 36% 13%	420
	11th to 20th	Night	19% 6% 28% 37% 10%	420
	21st to end	Night	14% 4 30% 41% 10%	420
July	1st to 10th	Night	11% 5% 22% 44% 16% 1	410
	11th to 20th	Night	13% 4 22% 43% 17% 1	408
	21st to end	Night	12% 3 23% 44% 15% 1	460
August	1st to 10th	Night	12% 2 21% 47% 17% 1	418
	11th to 20th	Night	14% 5% 23% 36% 20% 3	780
	21st to end	Night	13% 7% 20% 39% 20% 1	924
September	1st to 10th	Night	15% 5% 23% 37% 18% 2	840
	11th to 20th	Night	14% 7% 23% 30% 21% 5%	813
	21st to end	Night	16% 7% 25% 33% 17% 2	800
October	1st to 10th	Night	23% 11% 23% 27% 15% 2	800
	11th to 20th	Night	18% 11% 27% 34% 9% 1	800
	21st to end	Night	27% 10% 30% 27% 6%	880
November	1st to 10th	Night	18% 13% 33% 28% 7%	820
	11th to 20th	Night	20% 14% 33% 25% 7% 1	840
	21st to end	Night	20% 15% 35% 25% 5%	840
December	1st to 10th	Night	25% 16% 33% 21% 4	840
	11th to 20th	Night	26% 15% 32% 23% 4	840
	21st to end	Night	29% 14% 34% 20% 3	924

Page generated at 2026-09-11 04:11:56. Version ac07

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