Avon (east of)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Avon (east of) clear sky chart from 2005-11-06 to 2025-03-27.

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% 3 7% 4 4 4 3 5% 7% 46%	112496
	Night	9% 6% 3 7% 4 4 4 3 5% 7% 47%	115558
Transparency	Day	38% 6% 14% 25% 16% 2	112247
	Night	36% 6% 13% 26% 17% 2	115397
Seeing	Day	13% 10% 31% 30% 13% 2	16331
	Night	12% 9% 35% 34% 10% 1	26432

By Month

Percentage Cloud Cover SW (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	10% 12% 5% 15% 7% 7% 5% 3 4 4 27%	3638
	11th to 20th	Day	5% 11% 4 12% 7% 5% 5% 5% 7% 6% 33%	3640
	21st to end	Day	4 8% 4 10% 5% 4 5% 4 5% 5% 45%	4443
February	1st to 10th	Day	10% 12% 5% 8% 5% 5% 5% 4 5% 7% 34%	1800
	11th to 20th	Day	8% 11% 4 9% 5% 4 4 3 6% 6% 39%	1858
	21st to end	Day	10% 9% 3 9% 5% 4 3 3 5% 7% 41%	1650
March	1st to 10th	Day	9% 8% 3 7% 4 4 3 2 4 7% 48%	3326
	11th to 20th	Day	10% 7% 3 7% 3 3 4 3 5% 7% 50%	3590
	21st to end	Day	24% 11% 4 11% 4 4 4 3 4 5% 26%	3880
April	1st to 10th	Day	16% 7% 3 6% 5% 4 3 3 5% 6% 42%	2171
	11th to 20th	Day	18% 8% 2 5% 4 4 4 3 4 5% 42%	2430
	21st to end	Day	19% 7% 2 5% 4 4 5% 3 4 7% 40%	2470
May	1st to 10th	Day	16% 6% 3 6% 5% 5% 5% 4 6% 7% 38%	4120
	11th to 20th	Day	12% 6% 2 7% 4 3 3 3 4 6% 51%	4082
	21st to end	Day	17% 6% 2 6% 4 4 4 3 6% 7% 40%	4521
June	1st to 10th	Day	11% 4 2 4 4 3 4 3 6% 8% 51%	2836
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 8% 48%	2850
	21st to end	Day	7% 5% 1 5% 4 3 4 3 6% 9% 52%	2841
July	1st to 10th	Day	11% 4 2 5% 4 4 5% 4 7% 9% 46%	4430
	11th to 20th	Day	6% 4 1 5% 3 3 4 3 5% 7% 58%	4430
	21st to end	Day	4 3 1 5% 4 3 4 3 6% 9% 57%	4529
August	1st to 10th	Day	7% 5% 2 6% 4 4 4 4 7% 8% 49%	4103
	11th to 20th	Day	18% 6% 3 5% 4 3 4 4 6% 7% 41%	4041
	21st to end	Day	7% 4 2 5% 4 3 3 3 5% 6% 57%	4532
September	1st to 10th	Day	13% 5% 2 4 3 3 3 3 5% 6% 54%	2166
	11th to 20th	Day	8% 4 1 5% 3 3 3 3 5% 8% 58%	2090
	21st to end	Day	11% 4 1 4 3 3 3 3 3 7% 57%	2090
October	1st to 10th	Day	8% 4 1 3 2 2 2 2 4 5% 67%	3352
	11th to 20th	Day	7% 4 1 4 3 3 4 3 5% 6% 60%	3140
	21st to end	Day	11% 6% 2 6% 4 4 4 4 5% 7% 48%	3193
November	1st to 10th	Day	9% 6% 2 6% 4 4 4 4 6% 6% 48%	1763
	11th to 20th	Day	8% 6% 2 8% 6% 4 4 4 6% 6% 47%	1800
	21st to end	Day	9% 9% 3 7% 4 4 3 4 6% 7% 45%	1600
December	1st to 10th	Day	5% 6% 2 8% 7% 6% 5% 4 5% 8% 44%	2610
	11th to 20th	Day	7% 10% 4 7% 5% 4 5% 4 6% 7% 40%	2776
	21st to end	Day	12% 10% 4 13% 5% 4 4 3 5% 5% 35%	3705
January	1st to 10th	Night	9% 11% 5% 13% 9% 7% 7% 4 5% 6% 24%	6327
	11th to 20th	Night	4 9% 4 9% 6% 6% 7% 5% 7% 6% 38%	6340
	21st to end	Night	3 7% 4 10% 5% 4 5% 4 6% 7% 45%	6520
February	1st to 10th	Night	10% 10% 4 8% 6% 5% 4 3 5% 7% 38%	2600
	11th to 20th	Night	7% 11% 4 8% 5% 3 5% 4 5% 6% 42%	2446
	21st to end	Night	9% 8% 2 6% 4 4 4 4 5% 6% 47%	1980
March	1st to 10th	Night	10% 6% 2 5% 3 3 4 3 5% 6% 52%	3740
	11th to 20th	Night	10% 6% 2 5% 3 3 3 3 5% 8% 51%	3590
	21st to end	Night	19% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	3889
April	1st to 10th	Night	14% 8% 2 6% 3 4 4 4 5% 7% 44%	2007
	11th to 20th	Night	15% 8% 2 7% 4 3 4 3 5% 7% 44%	1696
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 6% 6% 42%	1710
May	1st to 10th	Night	16% 5% 2 5% 4 4 3 3 5% 7% 47%	2850
	11th to 20th	Night	11% 4 2 5% 3 3 4 3 7% 8% 51%	2823
	21st to end	Night	15% 5% 2 6% 4 4 4 3 5% 8% 45%	3048
June	1st to 10th	Night	8% 4 1 4 3 4 3 4 7% 9% 54%	1340
	11th to 20th	Night	8% 5% 2 5% 4 3 3 4 7% 8% 52%	1330
	21st to end	Night	7% 4 1 4 2 3 4 4 5% 7% 59%	1334
July	1st to 10th	Night	8% 4 2 5% 4 3 4 4 5% 9% 52%	2540
	11th to 20th	Night	5% 3 2 3 2 3 3 3 5% 8% 63%	2540
	21st to end	Night	3 2 3 3 3 4 4 6% 9% 62%	3031
August	1st to 10th	Night	6% 4 2 5% 4 4 4 4 6% 9% 51%	2836
	11th to 20th	Night	13% 5% 2 5% 3 3 4 4 6% 8% 48%	2798
	21st to end	Night	7% 3 1 4 4 3 4 3 5% 9% 57%	3135
September	1st to 10th	Night	8% 5% 1 4 3 2 4 4 5% 6% 59%	2010
	11th to 20th	Night	8% 3 1 3 3 3 3 3 5% 6% 62%	2090
	21st to end	Night	9% 3 1 4 3 3 3 2 4 6% 62%	2090
October	1st to 10th	Night	9% 2 1 3 2 2 2 2 3 4 71%	3575
	11th to 20th	Night	7% 3 2 4 2 2 3 2 4 7% 64%	3779
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 8% 49%	4359
November	1st to 10th	Night	8% 4 2 5% 4 4 4 3 5% 7% 55%	2535
	11th to 20th	Night	8% 6% 2 7% 4 3 4 3 6% 6% 50%	2600
	21st to end	Night	8% 7% 2 5% 4 3 4 4 6% 7% 50%	2796
December	1st to 10th	Night	5% 7% 3 7% 5% 4 5% 4 5% 6% 50%	4580
	11th to 20th	Night	7% 8% 3 9% 5% 5% 5% 4 6% 7% 41%	5426
	21st to end	Night	15% 10% 4 11% 5% 4 4 3 5% 5% 35%	7268

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	61% 5% 12% 16% 6%	3639
	11th to 20th	Day	50% 4 15% 16% 14% 1	3640
	21st to end	Day	41% 5% 15% 24% 13% 2	4443
February	1st to 10th	Day	51% 7% 14% 17% 10% 1	1800
	11th to 20th	Day	46% 5% 12% 22% 14% 2	1867
	21st to end	Day	43% 6% 15% 21% 13% 3	1650
March	1st to 10th	Day	36% 6% 11% 29% 17% 2	3326
	11th to 20th	Day	35% 4 12% 28% 17% 3	3590
	21st to end	Day	60% 7% 15% 14% 4 1	3880
April	1st to 10th	Day	42% 4 13% 26% 13% 1	2171
	11th to 20th	Day	44% 5% 12% 21% 16% 2	2433
	21st to end	Day	44% 6% 13% 24% 13%	2470
May	1st to 10th	Day	42% 6% 13% 22% 16% 1	4120
	11th to 20th	Day	34% 6% 11% 29% 17% 3	4086
	21st to end	Day	42% 7% 11% 25% 14% 1	4521
June	1st to 10th	Day	31% 9% 15% 31% 12% 2	2836
	11th to 20th	Day	35% 7% 14% 24% 18% 3	2850
	21st to end	Day	27% 8% 16% 31% 15% 3	2841
July	1st to 10th	Day	34% 7% 17% 26% 14% 1	4361
	11th to 20th	Day	26% 5% 11% 34% 19% 4	4355
	21st to end	Day	23% 7% 16% 28% 22% 4	4385
August	1st to 10th	Day	31% 7% 14% 31% 14% 4	4090
	11th to 20th	Day	42% 7% 13% 25% 12% 1	4080
	21st to end	Day	28% 6% 16% 25% 20% 4	4531
September	1st to 10th	Day	31% 6% 11% 28% 20% 4	2166
	11th to 20th	Day	26% 6% 13% 35% 20% 1	2090
	21st to end	Day	28% 5% 11% 31% 24% 3	2090
October	1st to 10th	Day	22% 3 7% 20% 42% 5%	3352
	11th to 20th	Day	27% 3 15% 30% 20% 5%	3140
	21st to end	Day	36% 4 11% 28% 19% 3	3193
November	1st to 10th	Day	34% 6% 18% 24% 16% 2	1763
	11th to 20th	Day	37% 5% 14% 27% 14% 2	1800
	21st to end	Day	38% 4 17% 25% 14% 2	1600
December	1st to 10th	Day	39% 5% 18% 27% 10% 1	2610
	11th to 20th	Day	43% 4 20% 21% 11% 1	2773
	21st to end	Day	52% 4 12% 28% 5% 1	3705
January	1st to 10th	Night	60% 5% 11% 16% 8%	6327
	11th to 20th	Night	44% 5% 14% 21% 14% 2	6340
	21st to end	Night	38% 6% 13% 27% 15% 1	6520
February	1st to 10th	Night	46% 5% 16% 20% 12% 1	2600
	11th to 20th	Night	42% 5% 15% 21% 12% 4	2447
	21st to end	Night	37% 6% 14% 26% 14% 4	1980
March	1st to 10th	Night	32% 5% 12% 25% 23% 2	3740
	11th to 20th	Night	32% 4 11% 30% 20% 3	3590
	21st to end	Night	51% 9% 16% 18% 5% 1	3889
April	1st to 10th	Night	39% 6% 13% 24% 16% 1	2007
	11th to 20th	Night	40% 5% 10% 25% 19% 2	1696
	21st to end	Night	41% 7% 12% 24% 14% 2	1710
May	1st to 10th	Night	37% 6% 14% 20% 18% 4	2850
	11th to 20th	Night	29% 8% 13% 26% 22% 2	2824
	21st to end	Night	37% 8% 13% 24% 16% 1	3049
June	1st to 10th	Night	26% 8% 15% 29% 19% 3	1340
	11th to 20th	Night	28% 8% 14% 26% 21% 4	1330
	21st to end	Night	23% 7% 14% 34% 19% 4	1334
July	1st to 10th	Night	30% 6% 21% 27% 13% 4	2500
	11th to 20th	Night	22% 4 12% 37% 20% 5%	2492
	21st to end	Night	17% 7% 18% 31% 22% 4	2936
August	1st to 10th	Night	28% 6% 16% 31% 14% 3	2826
	11th to 20th	Night	35% 7% 14% 29% 14% 2	2825
	21st to end	Night	26% 7% 15% 24% 22% 5%	3135
September	1st to 10th	Night	26% 6% 11% 28% 23% 6%	2010
	11th to 20th	Night	23% 6% 12% 35% 22% 2	2090
	21st to end	Night	25% 3 9% 30% 28% 4	2090
October	1st to 10th	Night	20% 3 6% 17% 48% 6%	3575
	11th to 20th	Night	23% 3 10% 32% 25% 7%	3779
	21st to end	Night	35% 4 14% 27% 19% 2	4359
November	1st to 10th	Night	30% 5% 15% 25% 21% 5%	2535
	11th to 20th	Night	35% 5% 13% 25% 19% 4	2600
	21st to end	Night	33% 5% 15% 29% 16% 2	2796
December	1st to 10th	Night	35% 5% 15% 32% 13% 1	4580
	11th to 20th	Night	42% 6% 15% 29% 8%	5428
	21st to end	Night	53% 4 12% 23% 8%	7268

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	17% 10% 30% 29% 12% 3	300
	11th to 20th	Day	12% 15% 37% 29% 7%	300
	21st to end	Day	11% 10% 43% 26% 8% 1	330
February	1st to 10th	Day	10% 24% 37% 24% 5%	300
	11th to 20th	Day	14% 11% 40% 27% 6% 1	300
	21st to end	Day	14% 17% 38% 27% 4	249
March	1st to 10th	Day	13% 12% 45% 24% 5% 1	300
	11th to 20th	Day	17% 10% 40% 22% 11%	300
	21st to end	Day	26% 14% 32% 22% 5% 1	315
April	1st to 10th	Day	22% 18% 37% 19% 4	351
	11th to 20th	Day	23% 16% 29% 26% 5%	631
	21st to end	Day	22% 15% 30% 23% 8% 2	650
May	1st to 10th	Day	16% 14% 33% 28% 9%	653
	11th to 20th	Day	18% 11% 30% 25% 15% 1	675
	21st to end	Day	18% 9% 31% 28% 11% 3	734
June	1st to 10th	Day	10% 7% 28% 31% 19% 5%	680
	11th to 20th	Day	13% 7% 35% 29% 14% 2	680
	21st to end	Day	9% 7% 28% 36% 19% 2	680
July	1st to 10th	Day	8% 2 21% 38% 26% 4	671
	11th to 20th	Day	6% 3 26% 44% 18% 2	668
	21st to end	Day	5% 2 24% 48% 18% 3	739
August	1st to 10th	Day	8% 3 26% 41% 19% 3	676
	11th to 20th	Day	11% 3 24% 40% 20% 3	665
	21st to end	Day	6% 4 27% 38% 21% 4	748
September	1st to 10th	Day	12% 6% 31% 31% 17% 4	376
	11th to 20th	Day	8% 6% 36% 26% 20% 4	300
	21st to end	Day	15% 12% 30% 26% 13% 3	300
October	1st to 10th	Day	18% 7% 28% 28% 16% 4	300
	11th to 20th	Day	16% 12% 37% 24% 9% 1	300
	21st to end	Day	17% 17% 37% 20% 8% 1	330
November	1st to 10th	Day	15% 15% 33% 23% 11% 3	300
	11th to 20th	Day	14% 16% 40% 23% 8%	300
	21st to end	Day	17% 18% 30% 25% 9%	300
December	1st to 10th	Day	12% 10% 37% 30% 10% 1	300
	11th to 20th	Day	15% 15% 38% 24% 8%	300
	21st to end	Day	19% 15% 35% 26% 5%	330
January	1st to 10th	Night	11% 10% 38% 32% 8% 1	1000
	11th to 20th	Night	9% 16% 36% 31% 6% 1	1000
	21st to end	Night	12% 8% 44% 29% 7%	1100
February	1st to 10th	Night	15% 11% 41% 27% 6%	1000
	11th to 20th	Night	11% 12% 41% 30% 5% 1	892
	21st to end	Night	14% 10% 37% 32% 7% 1	660
March	1st to 10th	Night	14% 10% 41% 31% 5%	738
	11th to 20th	Night	15% 10% 41% 28% 5%	600
	21st to end	Night	18% 8% 41% 25% 7%	647
April	1st to 10th	Night	17% 13% 38% 28% 4	570
	11th to 20th	Night	19% 13% 33% 29% 6%	563
	21st to end	Night	16% 11% 38% 27% 6% 1	570
May	1st to 10th	Night	15% 10% 36% 31% 8%	570
	11th to 20th	Night	15% 7% 33% 32% 12%	561
	21st to end	Night	16% 7% 34% 31% 11%	617
June	1st to 10th	Night	9% 6% 27% 41% 17% 1	570
	11th to 20th	Night	9% 3 36% 41% 11%	570
	21st to end	Night	6% 4 27% 47% 16%	570
July	1st to 10th	Night	6% 3 22% 46% 23% 1	555
	11th to 20th	Night	5% 1 24% 55% 15%	552
	21st to end	Night	3 1 24% 54% 17%	624
August	1st to 10th	Night	7% 4 23% 47% 19%	567
	11th to 20th	Night	9% 2 24% 44% 19% 1	564
	21st to end	Night	7% 2 28% 46% 16% 1	627
September	1st to 10th	Night	10% 5% 27% 41% 15% 2	570
	11th to 20th	Night	7% 5% 31% 39% 15% 2	570
	21st to end	Night	10% 7% 33% 35% 14% 1	570
October	1st to 10th	Night	17% 8% 33% 32% 10% 1	626
	11th to 20th	Night	14% 11% 36% 31% 9%	760
	21st to end	Night	15% 12% 35% 31% 8%	974
November	1st to 10th	Night	11% 9% 37% 34% 8% 1	975
	11th to 20th	Night	12% 13% 38% 30% 8%	1000
	21st to end	Night	11% 11% 37% 32% 9%	1000
December	1st to 10th	Night	12% 13% 39% 32% 5%	1000
	11th to 20th	Night	13% 9% 39% 31% 8%	1000
	21st to end	Night	18% 11% 35% 29% 6% 1	1100

Page generated at 2025-03-24 08:04:26. Version ac07

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