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 2026-07-31.

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 4 3 5% 7% 46%	146325
	Night	9% 6% 2 6% 4 4 4 4 5% 7% 47%	144042
Transparency	Day	38% 5% 13% 26% 16% 2	141004
	Night	36% 5% 13% 26% 17% 2	140375
Seeing	Day	14% 10% 32% 30% 13% 2	18324
	Night	12% 9% 35% 34% 9%	28168

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% 13% 5% 14% 7% 6% 4 3 3 4 30%	4720
	11th to 20th	Day	4 10% 4 11% 7% 7% 6% 5% 6% 6% 33%	4740
	21st to end	Day	4 8% 4 10% 6% 5% 6% 4 5% 5% 41%	5754
February	1st to 10th	Day	10% 12% 5% 8% 5% 5% 5% 5% 5% 7% 33%	1890
	11th to 20th	Day	8% 11% 4 9% 5% 4 4 3 6% 6% 41%	1953
	21st to end	Day	10% 9% 3 8% 5% 4 4 3 5% 7% 42%	1730
March	1st to 10th	Day	10% 7% 3 6% 4 4 3 2 3 6% 51%	4735
	11th to 20th	Day	8% 6% 2 6% 3 3 3 3 5% 7% 53%	5090
	21st to end	Day	22% 10% 3 10% 4 4 5% 3 5% 6% 27%	5585
April	1st to 10th	Day	16% 8% 2 6% 5% 4 4 3 5% 6% 41%	2399
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 4 6% 42%	2690
	21st to end	Day	20% 7% 2 5% 4 4 4 3 5% 7% 40%	2730
May	1st to 10th	Day	9% 4 2 5% 4 4 5% 4 6% 7% 50%	7655
	11th to 20th	Day	15% 5% 2 6% 4 3 4 3 4 6% 48%	7627
	21st to end	Day	20% 7% 3 6% 4 4 4 3 6% 7% 37%	8464
June	1st to 10th	Day	11% 4 2 4 4 3 4 4 6% 8% 50%	3135
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 5% 8% 49%	3150
	21st to end	Day	8% 5% 2 5% 4 3 4 3 6% 9% 51%	3139
July	1st to 10th	Day	9% 4 2 5% 4 4 5% 4 7% 9% 48%	6479
	11th to 20th	Day	10% 4 2 5% 4 3 4 3 5% 7% 53%	6480
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 9% 50%	6646
August	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 7% 9% 44%	5801
	11th to 20th	Day	14% 5% 2 5% 3 3 4 4 6% 9% 46%	5821
	21st to end	Day	10% 4 2 5% 4 3 3 3 5% 7% 55%	6490
September	1st to 10th	Day	13% 5% 2 4 3 3 3 3 5% 6% 54%	2279
	11th to 20th	Day	8% 4 1 5% 3 3 3 3 5% 8% 57%	2200
	21st to end	Day	10% 5% 1 4 3 3 3 3 3 7% 57%	2200
October	1st to 10th	Day	8% 4 1 3 2 2 2 2 4 5% 67%	3458
	11th to 20th	Day	8% 4 1 4 3 3 4 3 5% 6% 59%	3240
	21st to end	Day	11% 6% 2 6% 3 4 4 4 5% 7% 48%	3295
November	1st to 10th	Day	8% 6% 2 6% 3 4 4 4 6% 7% 48%	1853
	11th to 20th	Day	8% 6% 2 8% 6% 4 4 4 6% 6% 47%	1890
	21st to end	Day	9% 9% 3 7% 4 4 3 4 6% 7% 45%	1680
December	1st to 10th	Day	5% 6% 2 8% 7% 6% 5% 4 5% 8% 44%	2690
	11th to 20th	Day	7% 10% 4 7% 5% 4 5% 4 6% 7% 40%	2850
	21st to end	Day	12% 9% 4 13% 5% 4 4 3 5% 5% 35%	3787
January	1st to 10th	Night	8% 10% 4 12% 8% 6% 6% 4 5% 6% 30%	8201
	11th to 20th	Night	3 7% 4 9% 6% 6% 7% 5% 8% 7% 38%	8250
	21st to end	Night	3 8% 4 10% 5% 4 5% 4 6% 6% 45%	8445
February	1st to 10th	Night	10% 10% 4 8% 5% 5% 5% 3 5% 7% 37%	2730
	11th to 20th	Night	7% 10% 4 8% 5% 3 5% 4 5% 6% 42%	2571
	21st to end	Night	8% 8% 2 6% 4 4 5% 4 5% 6% 47%	2076
March	1st to 10th	Night	12% 6% 2 4 3 3 4 3 5% 6% 52%	5338
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 5% 8% 53%	5090
	21st to end	Night	16% 9% 3 7% 5% 4 5% 4 5% 7% 36%	5588
April	1st to 10th	Night	14% 8% 2 6% 3 4 4 4 5% 7% 44%	2218
	11th to 20th	Night	13% 7% 2 6% 4 3 4 3 5% 7% 46%	1876
	21st to end	Night	16% 6% 2 6% 4 3 5% 4 6% 6% 42%	1890
May	1st to 10th	Night	10% 4 1 4 4 4 4 3 6% 8% 52%	5292
	11th to 20th	Night	12% 5% 1 5% 3 3 4 3 6% 7% 51%	5265
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 7% 41%	5725
June	1st to 10th	Night	8% 4 1 4 3 4 3 3 7% 9% 54%	1480
	11th to 20th	Night	7% 5% 2 4 4 3 4 4 7% 8% 53%	1470
	21st to end	Night	9% 4 1 4 2 3 4 4 5% 7% 56%	1474
July	1st to 10th	Night	8% 4 2 5% 4 3 4 4 6% 9% 53%	3720
	11th to 20th	Night	9% 4 2 4 3 3 3 3 4 7% 57%	3720
	21st to end	Night	6% 3 1 4 3 3 4 4 6% 9% 55%	4445
August	1st to 10th	Night	7% 4 2 5% 4 4 5% 5% 7% 10% 47%	4011
	11th to 20th	Night	10% 4 2 4 3 3 4 4 7% 8% 50%	4028
	21st to end	Night	8% 4 1 5% 4 4 4 3 5% 8% 54%	4487
September	1st to 10th	Night	9% 5% 1 3 3 2 4 3 5% 6% 59%	2116
	11th to 20th	Night	8% 3 1 3 3 3 4 3 5% 7% 60%	2200
	21st to end	Night	9% 3 1 4 3 3 3 2 4 6% 62%	2200
October	1st to 10th	Night	9% 2 1 3 2 2 2 2 3 4 72%	3688
	11th to 20th	Night	8% 4 2 4 2 2 3 2 4 7% 63%	3899
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 7% 49%	4498
November	1st to 10th	Night	8% 4 2 6% 4 4 4 3 5% 7% 55%	2665
	11th to 20th	Night	8% 6% 2 7% 4 4 4 3 6% 7% 49%	2730
	21st to end	Night	8% 7% 2 6% 4 3 4 4 6% 7% 49%	2936
December	1st to 10th	Night	4 7% 3 7% 5% 4 5% 4 5% 6% 50%	4720
	11th to 20th	Night	8% 8% 3 9% 5% 5% 5% 4 6% 7% 41%	5572
	21st to end	Night	15% 10% 4 11% 5% 4 4 3 5% 5% 35%	7428

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% 4 12% 18% 7%	4723
	11th to 20th	Day	51% 3 14% 18% 14% 1	4740
	21st to end	Day	44% 5% 15% 22% 12% 1	5754
February	1st to 10th	Day	51% 6% 15% 17% 10% 1	1890
	11th to 20th	Day	44% 5% 11% 23% 14% 2	1962
	21st to end	Day	42% 6% 15% 22% 13% 2	1730
March	1st to 10th	Day	37% 4 9% 30% 18% 2	4735
	11th to 20th	Day	32% 4 11% 28% 24% 2	5090
	21st to end	Day	57% 6% 14% 17% 5% 2	5585
April	1st to 10th	Day	44% 4 12% 25% 13% 1	2399
	11th to 20th	Day	43% 5% 12% 22% 16% 2	2693
	21st to end	Day	44% 6% 12% 24% 14%	2730
May	1st to 10th	Day	31% 4 10% 29% 25% 1	7655
	11th to 20th	Day	38% 4 10% 23% 18% 7%	7631
	21st to end	Day	46% 6% 10% 25% 13% 1	8465
June	1st to 10th	Day	32% 8% 15% 31% 12% 2	3135
	11th to 20th	Day	34% 6% 15% 23% 18% 3	3150
	21st to end	Day	29% 7% 15% 30% 16% 3	3139
July	1st to 10th	Day	32% 6% 17% 30% 14% 1	6411
	11th to 20th	Day	31% 4 11% 32% 18% 4	6405
	21st to end	Day	30% 7% 18% 26% 17% 3	6459
August	1st to 10th	Day	31% 7% 14% 31% 13% 4	4220
	11th to 20th	Day	42% 6% 13% 25% 12% 1	4210
	21st to end	Day	29% 6% 16% 25% 20% 4	4674
September	1st to 10th	Day	31% 6% 12% 28% 19% 4	2279
	11th to 20th	Day	27% 6% 13% 34% 19% 1	2200
	21st to end	Day	27% 4 10% 31% 25% 3	2200
October	1st to 10th	Day	22% 3 7% 21% 42% 5%	3458
	11th to 20th	Day	28% 3 14% 30% 20% 5%	3240
	21st to end	Day	36% 4 11% 28% 19% 3	3295
November	1st to 10th	Day	34% 6% 17% 25% 16% 2	1853
	11th to 20th	Day	37% 5% 15% 26% 14% 2	1890
	21st to end	Day	39% 4 17% 26% 14% 2	1680
December	1st to 10th	Day	39% 5% 18% 27% 10% 1	2690
	11th to 20th	Day	43% 4 19% 21% 11% 1	2847
	21st to end	Day	51% 4 12% 28% 5% 1	3787
January	1st to 10th	Night	54% 5% 13% 19% 8% 1	8202
	11th to 20th	Night	43% 5% 14% 23% 14% 1	8250
	21st to end	Night	38% 5% 14% 26% 16%	8445
February	1st to 10th	Night	46% 5% 16% 20% 12% 1	2730
	11th to 20th	Night	42% 5% 14% 22% 13% 4	2572
	21st to end	Night	37% 5% 14% 27% 14% 3	2076
March	1st to 10th	Night	32% 4 10% 30% 22% 2	5338
	11th to 20th	Night	30% 3 10% 28% 26% 3	5090
	21st to end	Night	48% 7% 14% 23% 8% 1	5588
April	1st to 10th	Night	40% 6% 13% 25% 15% 1	2218
	11th to 20th	Night	39% 5% 9% 26% 19% 2	1876
	21st to end	Night	41% 7% 12% 24% 14% 2	1890
May	1st to 10th	Night	30% 5% 10% 26% 27% 3	5293
	11th to 20th	Night	32% 5% 11% 21% 25% 6%	5266
	21st to end	Night	43% 6% 12% 24% 14% 1	5727
June	1st to 10th	Night	27% 8% 15% 28% 19% 3	1480
	11th to 20th	Night	28% 7% 14% 24% 22% 4	1470
	21st to end	Night	26% 6% 14% 32% 18% 4	1474
July	1st to 10th	Night	29% 5% 18% 32% 14% 3	3680
	11th to 20th	Night	29% 4 11% 34% 17% 4	3672
	21st to end	Night	25% 6% 20% 29% 17% 3	4319
August	1st to 10th	Night	29% 7% 17% 31% 14% 3	2916
	11th to 20th	Night	34% 7% 14% 29% 15% 2	2915
	21st to end	Night	26% 7% 15% 24% 22% 5%	3234
September	1st to 10th	Night	25% 6% 13% 28% 22% 6%	2116
	11th to 20th	Night	23% 7% 14% 34% 21% 2	2200
	21st to end	Night	25% 3 9% 30% 28% 4	2200
October	1st to 10th	Night	20% 3 6% 18% 47% 6%	3688
	11th to 20th	Night	24% 3 10% 32% 25% 7%	3899
	21st to end	Night	35% 4 13% 27% 19% 2	4498
November	1st to 10th	Night	29% 5% 15% 26% 21% 4	2665
	11th to 20th	Night	34% 5% 13% 25% 19% 4	2730
	21st to end	Night	34% 5% 15% 29% 15% 2	2936
December	1st to 10th	Night	35% 5% 15% 32% 13% 1	4720
	11th to 20th	Night	42% 6% 14% 29% 8%	5574
	21st to end	Night	52% 4 12% 23% 8%	7428

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	18% 10% 30% 29% 11% 2	330
	11th to 20th	Day	12% 16% 38% 28% 6%	330
	21st to end	Day	11% 10% 45% 26% 8% 1	363
February	1st to 10th	Day	10% 23% 38% 23% 5%	330
	11th to 20th	Day	14% 12% 39% 28% 6% 1	330
	21st to end	Day	12% 18% 40% 25% 4	273
March	1st to 10th	Day	14% 11% 46% 24% 5% 1	330
	11th to 20th	Day	18% 10% 40% 22% 11%	330
	21st to end	Day	24% 15% 32% 23% 5% 1	363
April	1st to 10th	Day	24% 17% 37% 19% 4	419
	11th to 20th	Day	22% 17% 30% 26% 5%	731
	21st to end	Day	23% 13% 31% 23% 8% 2	750
May	1st to 10th	Day	14% 14% 32% 28% 11% 1	753
	11th to 20th	Day	19% 11% 29% 25% 14% 1	775
	21st to end	Day	19% 8% 31% 28% 12% 3	844
June	1st to 10th	Day	10% 7% 29% 31% 18% 4	780
	11th to 20th	Day	13% 8% 35% 29% 14% 2	780
	21st to end	Day	11% 7% 29% 35% 17% 2	780
July	1st to 10th	Day	8% 2 21% 39% 25% 4	771
	11th to 20th	Day	6% 3 25% 43% 20% 3	768
	21st to end	Day	6% 2 24% 48% 17% 3	845
August	1st to 10th	Day	9% 3 27% 41% 19% 3	726
	11th to 20th	Day	10% 3 23% 39% 20% 3	715
	21st to end	Day	7% 4 27% 38% 21% 3	803
September	1st to 10th	Day	12% 6% 30% 32% 17% 3	409
	11th to 20th	Day	8% 6% 36% 26% 19% 5%	330
	21st to end	Day	14% 11% 28% 28% 14% 4	330
October	1st to 10th	Day	17% 6% 29% 28% 16% 4	330
	11th to 20th	Day	18% 12% 38% 22% 8% 1	330
	21st to end	Day	18% 17% 37% 19% 9% 1	363
November	1st to 10th	Day	15% 15% 35% 22% 11% 3	330
	11th to 20th	Day	13% 15% 42% 23% 8%	330
	21st to end	Day	18% 18% 30% 24% 10% 1	330
December	1st to 10th	Day	12% 10% 38% 30% 9% 1	330
	11th to 20th	Day	15% 16% 38% 23% 7%	330
	21st to end	Day	18% 13% 37% 26% 5%	363
January	1st to 10th	Night	11% 10% 38% 32% 8% 1	1050
	11th to 20th	Night	9% 16% 37% 31% 6% 1	1050
	21st to end	Night	11% 8% 44% 29% 7%	1155
February	1st to 10th	Night	14% 11% 41% 28% 6%	1050
	11th to 20th	Night	11% 12% 41% 31% 5% 1	937
	21st to end	Night	13% 10% 37% 32% 7% 1	692
March	1st to 10th	Night	14% 10% 41% 30% 5%	775
	11th to 20th	Night	16% 10% 41% 28% 5%	630
	21st to end	Night	18% 9% 41% 26% 6%	693
April	1st to 10th	Night	17% 13% 39% 28% 4	630
	11th to 20th	Night	18% 13% 35% 28% 6%	623
	21st to end	Night	17% 11% 39% 26% 6% 1	630
May	1st to 10th	Night	14% 9% 35% 33% 9%	630
	11th to 20th	Night	15% 9% 33% 33% 11%	621
	21st to end	Night	17% 7% 33% 32% 11%	683
June	1st to 10th	Night	9% 7% 26% 42% 16% 1	630
	11th to 20th	Night	9% 4 36% 41% 10%	630
	21st to end	Night	9% 4 28% 46% 14%	630
July	1st to 10th	Night	6% 3 22% 47% 22%	615
	11th to 20th	Night	6% 1 24% 55% 14%	612
	21st to end	Night	5% 1 24% 53% 16%	687
August	1st to 10th	Night	7% 4 24% 46% 18%	597
	11th to 20th	Night	8% 2 24% 44% 20% 1	594
	21st to end	Night	7% 2 28% 46% 16% 1	660
September	1st to 10th	Night	10% 5% 27% 41% 15% 2	600
	11th to 20th	Night	7% 5% 31% 40% 15% 2	600
	21st to end	Night	10% 7% 33% 36% 14% 1	600
October	1st to 10th	Night	16% 8% 33% 32% 11% 1	659
	11th to 20th	Night	14% 11% 36% 30% 8%	800
	21st to end	Night	15% 12% 35% 30% 7%	1025
November	1st to 10th	Night	11% 9% 38% 33% 8% 1	1025
	11th to 20th	Night	12% 12% 38% 30% 8%	1050
	21st to end	Night	11% 11% 36% 32% 10%	1050
December	1st to 10th	Night	11% 12% 39% 31% 5%	1050
	11th to 20th	Night	14% 9% 38% 31% 8%	1050
	21st to end	Night	18% 11% 36% 29% 6% 1	1155

Page generated at 2026-07-28 02:25:19. Version ac07

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