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 2025-05-11.

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%	113672
	Night	17% 9% 3 8% 5% 5% 5% 4 5% 6% 34%	115259
Transparency	Day	47% 8% 13% 20% 11% 1	117198
	Night	50% 8% 13% 18% 11% 1	119897
Seeing	Day	21% 16% 29% 21% 9% 3	15373
	Night	20% 12% 30% 28% 8% 1	24538

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% 7% 13% 7% 6% 6% 4 5% 5% 26%	3597
	11th to 20th	Day	13% 12% 5% 10% 7% 6% 6% 5% 6% 6% 25%	3570
	21st to end	Day	13% 10% 4 10% 6% 6% 5% 4 6% 7% 31%	4049
February	1st to 10th	Day	13% 13% 4 10% 6% 5% 5% 4 6% 6% 28%	1800
	11th to 20th	Day	12% 13% 4 11% 6% 6% 5% 4 5% 5% 29%	1915
	21st to end	Day	19% 12% 4 7% 6% 4 5% 3 5% 4 31%	1650
March	1st to 10th	Day	20% 11% 5% 9% 5% 4 4 3 3 5% 30%	4476
	11th to 20th	Day	15% 9% 3 7% 5% 5% 4 3 4 7% 38%	4697
	21st to end	Day	19% 10% 3 7% 5% 4 4 4 5% 5% 35%	5968
April	1st to 10th	Day	19% 9% 3 8% 5% 4 4 3 4 5% 36%	2400
	11th to 20th	Day	16% 7% 2 8% 6% 5% 5% 3 5% 6% 37%	2374
	21st to end	Day	20% 8% 3 7% 5% 5% 5% 4 5% 5% 33%	2508
May	1st to 10th	Day	24% 9% 3 7% 4 4 4 3 5% 6% 30%	4333
	11th to 20th	Day	16% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	4398
	21st to end	Day	12% 6% 2 5% 5% 5% 5% 4 6% 8% 43%	4837
June	1st to 10th	Day	15% 7% 3 7% 6% 5% 6% 4 6% 9% 33%	2660
	11th to 20th	Day	17% 5% 2 6% 4 5% 6% 5% 6% 9% 35%	2660
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 5% 7% 8% 33%	2660
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 10% 38%	4440
	11th to 20th	Day	9% 6% 2 6% 5% 5% 6% 6% 10% 11% 35%	4440
	21st to end	Day	13% 6% 2 7% 5% 5% 6% 4 7% 8% 36%	4814
August	1st to 10th	Day	15% 5% 2 7% 5% 5% 6% 5% 7% 9% 35%	4423
	11th to 20th	Day	14% 6% 3 7% 5% 5% 6% 5% 7% 10% 33%	3764
	21st to end	Day	11% 6% 2 7% 5% 4 6% 5% 7% 8% 38%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 5% 4 4 6% 7% 45%	2280
	11th to 20th	Day	14% 5% 2 4 3 4 4 3 5% 7% 49%	2190
	21st to end	Day	16% 6% 2 4 4 4 5% 4 6% 7% 41%	2090
October	1st to 10th	Day	19% 8% 3 8% 6% 4 5% 4 5% 5% 34%	1986
	11th to 20th	Day	15% 6% 3 7% 5% 4 4 4 5% 7% 40%	1895
	21st to end	Day	24% 7% 3 7% 4 4 5% 4 5% 6% 30%	1881
November	1st to 10th	Day	13% 7% 3 7% 5% 4 4 3 5% 7% 42%	1761
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 3 6% 6% 38%	1800
	21st to end	Day	18% 9% 3 6% 6% 5% 5% 3 6% 6% 33%	1630
December	1st to 10th	Day	17% 12% 5% 10% 6% 5% 5% 4 6% 6% 24%	2600
	11th to 20th	Day	22% 10% 3 6% 4 4 4 4 6% 6% 31%	2986
	21st to end	Day	22% 16% 4 9% 3 3 4 3 4 6% 26%	3960
January	1st to 10th	Night	13% 11% 4 10% 6% 5% 5% 4 5% 6% 30%	6349
	11th to 20th	Night	11% 12% 5% 13% 7% 7% 6% 4 6% 6% 23%	6315
	21st to end	Night	16% 11% 4 8% 6% 5% 6% 4 5% 6% 30%	6904
February	1st to 10th	Night	14% 12% 4 10% 7% 6% 5% 4 6% 7% 27%	2600
	11th to 20th	Night	11% 12% 3 10% 6% 5% 5% 4 6% 5% 33%	2369
	21st to end	Night	16% 9% 3 9% 5% 5% 4 5% 6% 5% 32%	1980
March	1st to 10th	Night	21% 9% 3 8% 5% 5% 5% 3 5% 5% 30%	5364
	11th to 20th	Night	16% 10% 3 7% 4 4 5% 3 5% 7% 36%	5260
	21st to end	Night	20% 12% 3 9% 4 4 4 3 4 4 33%	4962
April	1st to 10th	Night	21% 8% 3 7% 5% 4 4 3 5% 5% 35%	2000
	11th to 20th	Night	19% 8% 4 7% 5% 5% 4 3 5% 6% 36%	1975
	21st to end	Night	20% 8% 2 7% 4 4 4 3 5% 5% 39%	1890
May	1st to 10th	Night	25% 9% 3 8% 4 3 3 4 5% 7% 30%	2844
	11th to 20th	Night	18% 9% 3 9% 5% 5% 5% 3 5% 6% 32%	2501
	21st to end	Night	13% 5% 3 6% 4 6% 5% 4 6% 7% 40%	2738
June	1st to 10th	Night	18% 5% 1 5% 5% 5% 6% 5% 7% 7% 37%	1520
	11th to 20th	Night	18% 6% 2 5% 4 3 4 4 5% 8% 40%	1520
	21st to end	Night	15% 5% 2 6% 5% 4 4 5% 6% 8% 40%	1520
July	1st to 10th	Night	11% 6% 2 7% 6% 6% 5% 4 7% 8% 38%	2520
	11th to 20th	Night	15% 6% 2 6% 5% 4 5% 4 6% 8% 40%	2520
	21st to end	Night	13% 7% 2 6% 5% 4 5% 4 7% 7% 39%	2739
August	1st to 10th	Night	19% 7% 2 7% 4 4 4 3 5% 8% 36%	2506
	11th to 20th	Night	16% 6% 3 7% 4 4 5% 4 7% 7% 37%	3027
	21st to end	Night	11% 5% 2 5% 4 4 5% 5% 8% 10% 41%	3476
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 7% 45%	1900
	11th to 20th	Night	13% 5% 2 5% 3 4 3 2 5% 6% 51%	1990
	21st to end	Night	15% 5% 2 5% 5% 4 5% 4 5% 6% 44%	2090
October	1st to 10th	Night	20% 6% 3 9% 6% 4 4 3 5% 5% 35%	2190
	11th to 20th	Night	16% 6% 2 7% 6% 4 5% 4 5% 6% 40%	2285
	21st to end	Night	23% 7% 3 8% 4 4 4 4 5% 6% 32%	2717
November	1st to 10th	Night	13% 9% 3 7% 5% 4 4 3 5% 5% 42%	2537
	11th to 20th	Night	19% 7% 3 8% 4 4 4 3 5% 5% 39%	2600
	21st to end	Night	16% 9% 3 7% 5% 5% 4 4 4 7% 36%	2767
December	1st to 10th	Night	16% 11% 4 9% 5% 5% 5% 4 6% 7% 27%	4580
	11th to 20th	Night	26% 9% 3 7% 4 4 4 3 5% 6% 30%	5208
	21st to end	Night	23% 14% 4 8% 4 3 4 3 4 6% 26%	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% 9% 11% 17% 5%	3598
	11th to 20th	Day	56% 8% 12% 12% 9% 2	3600
	21st to end	Day	52% 8% 11% 17% 11% 2	4049
February	1st to 10th	Day	57% 8% 12% 15% 7% 1	1800
	11th to 20th	Day	58% 6% 11% 15% 8% 1	1924
	21st to end	Day	55% 7% 11% 14% 11% 1	1650
March	1st to 10th	Day	58% 6% 10% 12% 8% 6%	4479
	11th to 20th	Day	46% 9% 18% 16% 8% 3	4697
	21st to end	Day	52% 6% 8% 14% 17% 4	5968
April	1st to 10th	Day	52% 7% 8% 19% 12% 3	2400
	11th to 20th	Day	48% 7% 11% 19% 14% 2	2374
	21st to end	Day	51% 8% 8% 17% 12% 3	2508
May	1st to 10th	Day	54% 9% 10% 18% 8% 1	4347
	11th to 20th	Day	45% 12% 15% 17% 9% 1	4402
	21st to end	Day	36% 11% 14% 25% 13% 1	4837
June	1st to 10th	Day	46% 11% 15% 17% 10% 1	2660
	11th to 20th	Day	44% 9% 13% 19% 13% 1	2660
	21st to end	Day	48% 9% 14% 19% 10%	2660
July	1st to 10th	Day	38% 9% 15% 24% 14% 1	4370
	11th to 20th	Day	37% 10% 18% 26% 9%	4365
	21st to end	Day	43% 8% 16% 21% 11%	4661
August	1st to 10th	Day	45% 9% 16% 20% 10%	4409
	11th to 20th	Day	43% 10% 17% 25% 5%	3801
	21st to end	Day	38% 8% 17% 26% 10% 1	4180
September	1st to 10th	Day	37% 8% 13% 29% 13% 1	2280
	11th to 20th	Day	35% 7% 12% 26% 18% 1	2190
	21st to end	Day	40% 8% 14% 24% 13% 1	2090
October	1st to 10th	Day	47% 7% 10% 23% 13% 1	3298
	11th to 20th	Day	38% 6% 16% 25% 14% 1	3123
	21st to end	Day	38% 6% 14% 28% 12% 1	3081
November	1st to 10th	Day	41% 8% 16% 23% 11% 1	1761
	11th to 20th	Day	46% 8% 13% 17% 15% 1	1800
	21st to end	Day	52% 8% 16% 17% 6% 1	1630
December	1st to 10th	Day	59% 7% 14% 16% 4	2600
	11th to 20th	Day	55% 7% 12% 17% 9% 1	2986
	21st to end	Day	60% 5% 10% 15% 10% 1	3960
January	1st to 10th	Night	56% 9% 14% 13% 8%	6349
	11th to 20th	Night	60% 9% 13% 11% 7% 1	6360
	21st to end	Night	55% 7% 11% 12% 12% 3	6904
February	1st to 10th	Night	58% 8% 12% 15% 6% 1	2600
	11th to 20th	Night	54% 9% 11% 17% 9% 1	2371
	21st to end	Night	52% 8% 13% 18% 7% 2	1980
March	1st to 10th	Night	54% 8% 9% 14% 12% 2	5374
	11th to 20th	Night	47% 9% 18% 17% 8% 2	5261
	21st to end	Night	55% 5% 6% 14% 17% 2	4962
April	1st to 10th	Night	52% 5% 11% 19% 11% 1	2000
	11th to 20th	Night	50% 7% 9% 20% 13% 1	1975
	21st to end	Night	47% 7% 9% 19% 14% 3	1890
May	1st to 10th	Night	54% 9% 12% 14% 10% 1	2850
	11th to 20th	Night	51% 11% 12% 17% 8% 1	2502
	21st to end	Night	40% 11% 13% 19% 15% 2	2739
June	1st to 10th	Night	44% 8% 15% 21% 11% 1	1520
	11th to 20th	Night	40% 9% 16% 22% 12% 1	1520
	21st to end	Night	41% 10% 14% 22% 12% 1	1520
July	1st to 10th	Night	42% 9% 14% 19% 14% 1	2480
	11th to 20th	Night	43% 9% 14% 22% 13%	2472
	21st to end	Night	42% 8% 12% 27% 10% 1	2651
August	1st to 10th	Night	47% 9% 15% 20% 8%	2496
	11th to 20th	Night	44% 10% 18% 19% 9%	3056
	21st to end	Night	37% 7% 17% 28% 11%	3476
September	1st to 10th	Night	37% 8% 13% 27% 14% 1	1900
	11th to 20th	Night	33% 7% 13% 28% 18% 1	1990
	21st to end	Night	39% 9% 12% 23% 16% 1	2090
October	1st to 10th	Night	50% 7% 11% 19% 12% 1	3658
	11th to 20th	Night	36% 6% 15% 24% 18% 1	3796
	21st to end	Night	44% 7% 12% 28% 9% 1	4467
November	1st to 10th	Night	45% 7% 11% 21% 13% 2	2537
	11th to 20th	Night	47% 7% 12% 20% 12% 2	2600
	21st to end	Night	50% 8% 17% 19% 7%	2767
December	1st to 10th	Night	58% 10% 13% 16% 3	4580
	11th to 20th	Night	57% 6% 13% 13% 11%	5208
	21st to end	Night	60% 6% 9% 15% 10% 1	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	26% 21% 30% 16% 8% 1	200
	11th to 20th	Day	28% 27% 30% 14% 2 1	200
	21st to end	Day	21% 23% 32% 18% 5% 1	239
February	1st to 10th	Day	27% 31% 33% 9%	300
	11th to 20th	Day	28% 26% 33% 11% 2	463
	21st to end	Day	27% 19% 35% 16% 3	414
March	1st to 10th	Day	28% 17% 33% 19% 3	500
	11th to 20th	Day	25% 22% 32% 14% 5% 1	500
	21st to end	Day	31% 21% 26% 13% 7% 1	550
April	1st to 10th	Day	31% 21% 26% 16% 6% 1	500
	11th to 20th	Day	29% 25% 24% 13% 7% 2	492
	21st to end	Day	27% 19% 31% 16% 6% 1	500
May	1st to 10th	Day	32% 16% 28% 14% 8% 2	503
	11th to 20th	Day	17% 16% 34% 24% 7% 2	487
	21st to end	Day	15% 14% 31% 27% 11% 3	529
June	1st to 10th	Day	16% 17% 31% 23% 10% 2	490
	11th to 20th	Day	18% 12% 33% 26% 10% 2	490
	21st to end	Day	18% 15% 32% 24% 9% 2	490
July	1st to 10th	Day	13% 9% 29% 30% 16% 3	485
	11th to 20th	Day	11% 8% 31% 31% 16% 3	484
	21st to end	Day	14% 7% 36% 30% 12% 2	532
August	1st to 10th	Day	11% 9% 27% 30% 17% 5%	486
	11th to 20th	Day	15% 6% 28% 26% 19% 5%	479
	21st to end	Day	11% 11% 23% 33% 15% 7%	539
September	1st to 10th	Day	14% 11% 23% 28% 16% 9%	490
	11th to 20th	Day	14% 10% 25% 21% 18% 12%	490
	21st to end	Day	18% 9% 27% 26% 14% 7%	490
October	1st to 10th	Day	24% 13% 25% 21% 12% 6%	490
	11th to 20th	Day	17% 18% 29% 23% 10% 2	487
	21st to end	Day	29% 14% 26% 23% 6% 1	429
November	1st to 10th	Day	20% 16% 28% 24% 12%	395
	11th to 20th	Day	25% 16% 31% 21% 6% 1	400
	21st to end	Day	28% 21% 30% 18% 3	230
December	1st to 10th	Day	25% 24% 31% 16% 5% 1	200
	11th to 20th	Day	33% 21% 32% 13% 3	200
	21st to end	Day	25% 23% 29% 20% 3	220
January	1st to 10th	Night	21% 17% 35% 25% 3	800
	11th to 20th	Night	25% 17% 31% 24% 3	800
	21st to end	Night	17% 15% 39% 26% 3	880
February	1st to 10th	Night	21% 16% 41% 22% 2	800
	11th to 20th	Night	19% 20% 39% 20% 2	783
	21st to end	Night	23% 19% 34% 21% 3	660
March	1st to 10th	Night	23% 17% 36% 22% 3	800
	11th to 20th	Night	24% 17% 32% 23% 4	800
	21st to end	Night	23% 16% 33% 22% 6%	880
April	1st to 10th	Night	26% 16% 33% 22% 3	800
	11th to 20th	Night	23% 18% 31% 21% 6%	791
	21st to end	Night	24% 16% 29% 25% 6%	690
May	1st to 10th	Night	27% 11% 28% 25% 9%	559
	11th to 20th	Night	19% 10% 37% 28% 6% 1	376
	21st to end	Night	13% 11% 26% 37% 12% 1	411
June	1st to 10th	Night	20% 5% 26% 37% 12% 1	380
	11th to 20th	Night	19% 6% 27% 38% 11%	380
	21st to end	Night	13% 4 30% 42% 11%	380
July	1st to 10th	Night	12% 5% 22% 44% 16% 1	370
	11th to 20th	Night	14% 4 20% 43% 17% 1	368
	21st to end	Night	13% 4 24% 44% 16% 1	416
August	1st to 10th	Night	12% 2 22% 46% 17% 1	378
	11th to 20th	Night	14% 4 23% 36% 20% 3	704
	21st to end	Night	14% 6% 19% 39% 21% 1	836
September	1st to 10th	Night	15% 4 23% 37% 18% 3	760
	11th to 20th	Night	15% 7% 23% 29% 20% 5%	760
	21st to end	Night	16% 7% 26% 32% 17% 2	760
October	1st to 10th	Night	24% 11% 23% 27% 15% 1	760
	11th to 20th	Night	18% 11% 27% 34% 10% 1	760
	21st to end	Night	27% 10% 30% 26% 6%	836
November	1st to 10th	Night	17% 12% 34% 29% 8%	780
	11th to 20th	Night	20% 14% 33% 25% 7% 1	800
	21st to end	Night	20% 14% 35% 25% 6% 1	800
December	1st to 10th	Night	26% 15% 33% 21% 4	800
	11th to 20th	Night	27% 14% 31% 24% 4	800
	21st to end	Night	28% 13% 34% 21% 4	880

Page generated at 2025-05-08 09:33:56. Version ac07

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