Avon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Avon clear sky chart from 2005-11-06 to 2026-10-14.

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	11% 6% 2 7% 4 4 5% 4 6% 8% 42%	148298
	Night	11% 7% 2 7% 4 4 5% 4 6% 8% 41%	143433
Transparency	Day	28% 34% 11% 16% 9% 2	151892
	Night	28% 34% 10% 15% 11% 3	147897
Seeing	Day	15% 5% 19% 28% 25% 8%	18130
	Night	16% 7% 20% 28% 22% 6%	27702

By Month

Percentage Cloud Cover SE (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	9% 6% 3 6% 3 3 4 3 5% 7% 52%	5003
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 5% 5% 51%	5280
	21st to end	Day	14% 14% 4 8% 4 4 4 3 4 6% 36%	5780
February	1st to 10th	Day	15% 6% 2 7% 4 4 4 4 6% 8% 40%	1974
	11th to 20th	Day	19% 9% 3 6% 4 4 4 3 5% 5% 39%	2048
	21st to end	Day	13% 7% 2 6% 5% 4 4 4 6% 8% 40%	1730
March	1st to 10th	Day	17% 8% 3 7% 5% 5% 4 4 5% 6% 36%	4752
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 3 5% 6% 53%	5090
	21st to end	Day	11% 7% 2 4 3 4 3 3 5% 6% 51%	5856
April	1st to 10th	Day	11% 6% 2 4 3 3 4 4 6% 6% 50%	2520
	11th to 20th	Day	9% 4 1 5% 4 4 4 4 6% 7% 51%	2494
	21st to end	Day	9% 5% 1 4 4 3 4 3 5% 8% 53%	2520
May	1st to 10th	Day	11% 7% 3 7% 4 5% 5% 4 6% 9% 39%	7021
	11th to 20th	Day	12% 8% 2 7% 4 4 4 4 5% 8% 41%	7357
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	9045
June	1st to 10th	Day	8% 5% 3 6% 5% 5% 5% 5% 8% 9% 41%	2940
	11th to 20th	Day	8% 5% 2 5% 5% 5% 5% 4 6% 11% 44%	2940
	21st to end	Day	6% 4 1 5% 5% 5% 6% 5% 9% 12% 41%	2940
July	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 12% 41%	8280
	11th to 20th	Day	7% 6% 2 8% 5% 5% 6% 5% 8% 12% 36%	7767
	21st to end	Day	10% 6% 2 7% 6% 6% 7% 5% 8% 11% 32%	8340
August	1st to 10th	Day	10% 10% 4 10% 6% 6% 6% 5% 8% 12% 25%	5687
	11th to 20th	Day	6% 4 2 7% 4 4 5% 5% 7% 12% 45%	5488
	21st to end	Day	8% 6% 3 8% 5% 6% 6% 5% 7% 8% 38%	6080
September	1st to 10th	Day	11% 6% 2 8% 5% 4 5% 4 7% 9% 39%	2520
	11th to 20th	Day	15% 6% 2 7% 6% 5% 6% 4 6% 8% 37%	2490
	21st to end	Day	18% 7% 3 7% 4 5% 5% 3 5% 6% 37%	2310
October	1st to 10th	Day	13% 6% 3 6% 4 3 4 3 4 6% 46%	2116
	11th to 20th	Day	13% 6% 2 7% 5% 3 3 2 3 6% 50%	2030
	21st to end	Day	11% 6% 2 6% 3 4 3 3 5% 5% 50%	2200
November	1st to 10th	Day	16% 7% 2 6% 4 3 4 4 5% 6% 43%	1947
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 7% 45%	1890
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 7% 46%	1890
December	1st to 10th	Day	11% 6% 2 5% 3 3 4 4 5% 5% 51%	2855
	11th to 20th	Day	13% 7% 2 8% 5% 5% 5% 3 5% 7% 39%	3070
	21st to end	Day	11% 7% 2 5% 3 3 4 4 6% 7% 48%	4048
January	1st to 10th	Night	10% 7% 2 5% 3 3 3 3 5% 7% 50%	7873
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 52%	7680
	21st to end	Night	15% 8% 3 8% 4 5% 4 3 4 6% 39%	8401
February	1st to 10th	Night	18% 8% 3 5% 4 4 4 3 5% 7% 39%	2643
	11th to 20th	Night	18% 8% 3 8% 3 3 4 4 5% 6% 37%	2458
	21st to end	Night	16% 7% 3 7% 4 4 4 3 5% 7% 40%	2076
March	1st to 10th	Night	16% 8% 3 8% 5% 4 4 3 5% 5% 38%	5317
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 6% 49%	5090
	21st to end	Night	13% 8% 2 6% 4 3 4 3 4 5% 49%	5316
April	1st to 10th	Night	12% 6% 2 6% 3 3 4 3 5% 8% 47%	2100
	11th to 20th	Night	8% 6% 2 5% 5% 5% 5% 4 6% 8% 46%	2075
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 8% 49%	2100
May	1st to 10th	Night	10% 6% 3 8% 6% 6% 5% 5% 7% 9% 35%	5833
	11th to 20th	Night	13% 7% 2 7% 4 4 4 4 5% 9% 41%	5540
	21st to end	Night	9% 8% 3 8% 5% 5% 6% 4 8% 10% 34%	5109
June	1st to 10th	Night	11% 6% 2 6% 5% 5% 4 4 7% 10% 41%	1680
	11th to 20th	Night	7% 4 1 5% 4 4 5% 5% 8% 11% 44%	1680
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 5% 9% 13% 39%	1680
July	1st to 10th	Night	5% 4 2 6% 4 5% 6% 5% 8% 12% 43%	4678
	11th to 20th	Night	8% 5% 2 8% 5% 6% 6% 6% 9% 13% 32%	5172
	21st to end	Night	11% 6% 2 7% 5% 5% 6% 5% 9% 12% 30%	5736
August	1st to 10th	Night	8% 9% 4 10% 5% 6% 6% 5% 8% 12% 27%	4417
	11th to 20th	Night	5% 4 2 7% 6% 6% 7% 6% 8% 11% 38%	4557
	21st to end	Night	7% 6% 3 8% 6% 6% 6% 5% 8% 11% 34%	5045
September	1st to 10th	Night	12% 6% 2 7% 5% 5% 6% 5% 9% 9% 35%	2100
	11th to 20th	Night	14% 7% 3 8% 4 5% 5% 4 6% 8% 34%	2128
	21st to end	Night	18% 6% 3 7% 5% 6% 4 4 5% 7% 34%	2310
October	1st to 10th	Night	13% 7% 2 7% 4 4 4 3 5% 7% 44%	2499
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 4 6% 47%	2437
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 7% 50%	2640
November	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 42%	2568
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 4 5% 7% 40%	2730
	21st to end	Night	12% 6% 2 6% 4 3 4 3 5% 7% 47%	2730
December	1st to 10th	Night	11% 6% 2 6% 4 3 3 3 4 5% 54%	4540
	11th to 20th	Night	12% 8% 3 8% 5% 5% 5% 4 5% 7% 38%	5345
	21st to end	Night	12% 7% 2 5% 4 4 4 4 5% 8% 46%	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	27% 20% 10% 23% 16% 5%	5004
	11th to 20th	Day	29% 20% 3 15% 24% 8%	5280
	21st to end	Day	42% 23% 4 15% 12% 5%	5780
February	1st to 10th	Day	19% 59% 5% 9% 6% 1	1974
	11th to 20th	Day	22% 58% 4 8% 7% 1	2057
	21st to end	Day	17% 60% 3 9% 9% 2	1730
March	1st to 10th	Day	40% 28% 6% 10% 11% 5%	4752
	11th to 20th	Day	22% 27% 6% 15% 16% 13%	5090
	21st to end	Day	24% 27% 10% 21% 14% 4	5856
April	1st to 10th	Day	15% 59% 7% 9% 8% 1	2520
	11th to 20th	Day	13% 60% 6% 13% 6% 2	2494
	21st to end	Day	13% 59% 7% 14% 7%	2520
May	1st to 10th	Day	36% 23% 15% 18% 6% 1	7053
	11th to 20th	Day	34% 22% 14% 22% 8%	7361
	21st to end	Day	33% 25% 13% 19% 9% 1	9061
June	1st to 10th	Day	16% 60% 7% 10% 7% 1	2940
	11th to 20th	Day	13% 61% 10% 13% 4	2940
	21st to end	Day	12% 60% 10% 13% 5%	2940
July	1st to 10th	Day	29% 23% 25% 19% 4	8210
	11th to 20th	Day	34% 26% 24% 13% 3	7696
	21st to end	Day	38% 24% 14% 15% 8%	8156
August	1st to 10th	Day	42% 30% 20% 6% 1	5674
	11th to 20th	Day	23% 29% 19% 23% 5%	5524
	21st to end	Day	31% 28% 15% 19% 6%	6080
September	1st to 10th	Day	19% 57% 10% 9% 5% 1	2520
	11th to 20th	Day	23% 55% 7% 11% 3 1	2490
	21st to end	Day	25% 55% 9% 7% 4	2310
October	1st to 10th	Day	23% 36% 4 17% 15% 5%	3376
	11th to 20th	Day	19% 36% 3 14% 16% 13%	3270
	21st to end	Day	12% 36% 3 27% 19% 2	3538
November	1st to 10th	Day	21% 53% 5% 11% 9% 1	1947
	11th to 20th	Day	19% 54% 3 11% 9% 4	1890
	21st to end	Day	16% 54% 5% 13% 11% 1	1890
December	1st to 10th	Day	22% 35% 5% 17% 20% 2	2855
	11th to 20th	Day	31% 31% 10% 14% 11% 4	3066
	21st to end	Day	27% 26% 13% 25% 8% 1	4048
January	1st to 10th	Night	28% 20% 10% 17% 21% 4	7874
	11th to 20th	Night	26% 20% 5% 15% 26% 7%	7680
	21st to end	Night	38% 22% 5% 13% 17% 4	8401
February	1st to 10th	Night	18% 59% 5% 11% 6% 1	2643
	11th to 20th	Night	22% 58% 4 8% 7% 1	2460
	21st to end	Night	21% 59% 3 7% 7% 3	2076
March	1st to 10th	Night	40% 28% 6% 11% 10% 5%	5317
	11th to 20th	Night	25% 28% 7% 9% 20% 11%	5090
	21st to end	Night	28% 28% 7% 20% 13% 3	5316
April	1st to 10th	Night	16% 59% 6% 9% 9% 1	2100
	11th to 20th	Night	15% 60% 6% 13% 4 1	2075
	21st to end	Night	15% 60% 5% 14% 6%	2100
May	1st to 10th	Night	37% 24% 16% 17% 6%	5863
	11th to 20th	Night	34% 24% 11% 21% 11%	5541
	21st to end	Night	37% 26% 11% 16% 9%	5116
June	1st to 10th	Night	17% 60% 7% 9% 7% 1	1680
	11th to 20th	Night	12% 62% 9% 11% 6%	1680
	21st to end	Night	13% 59% 9% 14% 4	1680
July	1st to 10th	Night	26% 23% 25% 18% 7%	4640
	11th to 20th	Night	35% 27% 19% 15% 3	5123
	21st to end	Night	38% 24% 15% 14% 8%	5606
August	1st to 10th	Night	40% 30% 20% 8% 2	4406
	11th to 20th	Night	28% 28% 19% 21% 4	4587
	21st to end	Night	31% 29% 15% 19% 6%	5045
September	1st to 10th	Night	20% 56% 8% 11% 5%	2100
	11th to 20th	Night	22% 56% 7% 9% 6% 1	2128
	21st to end	Night	25% 55% 8% 9% 3	2310
October	1st to 10th	Night	29% 35% 5% 11% 17% 4	4006
	11th to 20th	Night	19% 36% 2 10% 17% 15%	3936
	21st to end	Night	13% 35% 4 26% 18% 4	4252
November	1st to 10th	Night	21% 53% 5% 12% 8% 1	2568
	11th to 20th	Night	21% 54% 4 8% 9% 4	2730
	21st to end	Night	17% 54% 5% 13% 10% 1	2730
December	1st to 10th	Night	21% 34% 3 13% 25% 4	4540
	11th to 20th	Night	32% 31% 11% 11% 11% 5%	5348
	21st to end	Night	29% 26% 13% 22% 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	16% 12% 38% 27% 7% 1	283
	11th to 20th	Day	18% 10% 33% 31% 7% 1	330
	21st to end	Day	21% 10% 35% 25% 9% 1	363
February	1st to 10th	Day	17% 10% 33% 32% 8%	414
	11th to 20th	Day	26% 12% 32% 26% 5%	536
	21st to end	Day	18% 9% 29% 31% 12% 1	446
March	1st to 10th	Day	17% 12% 29% 26% 13% 2	540
	11th to 20th	Day	17% 10% 31% 29% 12% 1	540
	21st to end	Day	13% 9% 31% 31% 14% 2	594
April	1st to 10th	Day	15% 10% 29% 27% 16% 3	540
	11th to 20th	Day	10% 5% 23% 37% 23% 2	532
	21st to end	Day	9% 4 21% 37% 22% 6%	540
May	1st to 10th	Day	14% 4 23% 34% 20% 5%	543
	11th to 20th	Day	14% 3 18% 30% 27% 7%	567
	21st to end	Day	14% 2 11% 30% 27% 16%	617
June	1st to 10th	Day	11% 3 13% 31% 32% 10%	570
	11th to 20th	Day	11% 2 9% 29% 36% 13%	570
	21st to end	Day	7% 1 8% 22% 48% 13%	570
July	1st to 10th	Day	10% 2 5% 21% 43% 18%	565
	11th to 20th	Day	9% 2 4 21% 45% 20%	564
	21st to end	Day	10% 1 8% 25% 43% 13%	620
August	1st to 10th	Day	11% 2 8% 24% 38% 16%	566
	11th to 20th	Day	10% 1 5% 21% 40% 23%	559
	21st to end	Day	15% 2 6% 25% 37% 15%	627
September	1st to 10th	Day	13% 1 9% 24% 33% 21%	570
	11th to 20th	Day	18% 2 9% 27% 33% 11%	570
	21st to end	Day	21% 4 11% 25% 30% 9%	570
October	1st to 10th	Day	18% 3 14% 24% 31% 11%	570
	11th to 20th	Day	17% 4 19% 30% 23% 7%	542
	21st to end	Day	14% 5% 20% 29% 24% 8%	583
November	1st to 10th	Day	19% 8% 16% 31% 21% 5%	477
	11th to 20th	Day	18% 7% 27% 31% 14% 2	430
	21st to end	Day	13% 8% 28% 30% 19% 1	430
December	1st to 10th	Day	18% 9% 35% 28% 9% 1	330
	11th to 20th	Day	18% 11% 32% 27% 10% 3	220
	21st to end	Day	16% 7% 31% 31% 13% 1	242
January	1st to 10th	Night	16% 18% 33% 23% 10% 1	840
	11th to 20th	Night	17% 15% 29% 28% 9% 1	840
	21st to end	Night	21% 14% 33% 25% 7%	924
February	1st to 10th	Night	21% 10% 32% 28% 9%	840
	11th to 20th	Night	24% 13% 31% 26% 6%	823
	21st to end	Night	20% 13% 28% 29% 10% 1	692
March	1st to 10th	Night	15% 12% 32% 28% 12% 2	840
	11th to 20th	Night	17% 10% 29% 31% 10% 3	840
	21st to end	Night	17% 10% 27% 31% 13% 2	924
April	1st to 10th	Night	14% 8% 31% 29% 15% 2	840
	11th to 20th	Night	10% 7% 25% 36% 19% 4	831
	21st to end	Night	12% 5% 23% 36% 22% 2	840
May	1st to 10th	Night	15% 6% 18% 38% 19% 3	840
	11th to 20th	Night	14% 4 18% 36% 22% 6%	734
	21st to end	Night	14% 2 10% 29% 31% 14%	456
June	1st to 10th	Night	12% 3 13% 32% 33% 7%	420
	11th to 20th	Night	9% 9% 34% 37% 11%	420
	21st to end	Night	9% 3 7% 32% 38% 11%	420
July	1st to 10th	Night	8% 1 6% 29% 44% 12%	410
	11th to 20th	Night	11% 2 3 24% 45% 15%	574
	21st to end	Night	11% 2 7% 29% 40% 11%	690
August	1st to 10th	Night	14% 2 8% 26% 42% 8%	753
	11th to 20th	Night	10% 2 5% 24% 40% 18%	830
	21st to end	Night	16% 2 5% 22% 40% 15%	924
September	1st to 10th	Night	14% 2 9% 23% 35% 17%	840
	11th to 20th	Night	18% 3 10% 22% 38% 9%	840
	21st to end	Night	22% 3 11% 22% 34% 8%	840
October	1st to 10th	Night	15% 4 14% 25% 31% 11%	840
	11th to 20th	Night	19% 6% 19% 29% 23% 4	813
	21st to end	Night	15% 6% 23% 25% 24% 7%	880
November	1st to 10th	Night	18% 7% 19% 32% 20% 5%	820
	11th to 20th	Night	18% 10% 21% 33% 16% 2	840
	21st to end	Night	15% 11% 24% 29% 19% 2	840
December	1st to 10th	Night	19% 10% 31% 26% 12% 1	840
	11th to 20th	Night	16% 11% 30% 29% 13% 2	840
	21st to end	Night	16% 12% 29% 31% 12% 1	924

Page generated at 2026-10-11 05:06:31. Version ac07

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