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 2025-06-10.

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% 7% 2 7% 4 4 5% 4 6% 8% 42%	119060
	Night	12% 7% 2 7% 4 4 4 4 6% 8% 42%	119174
Transparency	Day	25% 40% 9% 14% 9% 2	122695
	Night	26% 39% 8% 13% 11% 3	123663
Seeing	Day	15% 6% 19% 28% 25% 8%	16289
	Night	16% 7% 21% 29% 22% 6%	25962

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	10% 7% 3 7% 3 3 3 2 4 7% 50%	3855
	11th to 20th	Day	12% 6% 2 6% 4 3 4 3 5% 5% 51%	4060
	21st to end	Day	12% 13% 4 8% 4 4 4 3 5% 6% 37%	4466
February	1st to 10th	Day	15% 6% 2 7% 4 4 4 4 6% 8% 39%	1880
	11th to 20th	Day	19% 9% 3 6% 4 4 3 3 5% 5% 39%	1948
	21st to end	Day	12% 7% 2 6% 5% 4 4 4 6% 8% 41%	1650
March	1st to 10th	Day	17% 8% 3 7% 5% 4 4 4 5% 6% 36%	4649
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 3 5% 6% 53%	4980
	21st to end	Day	12% 7% 2 4 3 4 3 3 4 6% 51%	5730
April	1st to 10th	Day	12% 6% 3 4 4 3 4 4 6% 6% 50%	2400
	11th to 20th	Day	9% 4 1 6% 4 4 4 4 6% 7% 50%	2374
	21st to end	Day	9% 5% 1 5% 4 4 4 3 5% 8% 53%	2400
May	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 5% 7% 10% 39%	5405
	11th to 20th	Day	15% 9% 3 9% 5% 5% 5% 4 6% 9% 30%	5661
	21st to end	Day	11% 6% 2 7% 4 4 4 4 6% 9% 42%	6934
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 5% 8% 9% 41%	2786
	11th to 20th	Day	8% 5% 2 5% 5% 4 5% 4 6% 11% 45%	2660
	21st to end	Day	6% 5% 2 5% 5% 5% 6% 5% 9% 12% 39%	2660
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 12% 38%	4440
	11th to 20th	Day	7% 6% 3 8% 6% 6% 5% 5% 8% 11% 35%	4165
	21st to end	Day	9% 6% 3 8% 6% 7% 6% 5% 8% 10% 32%	4466
August	1st to 10th	Day	12% 11% 4 10% 5% 5% 5% 5% 7% 10% 26%	3882
	11th to 20th	Day	7% 4 2 7% 4 5% 5% 4 6% 9% 45%	3730
	21st to end	Day	7% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	4180
September	1st to 10th	Day	11% 7% 2 8% 5% 4 5% 4 8% 10% 36%	2280
	11th to 20th	Day	15% 6% 2 7% 6% 5% 6% 4 6% 7% 37%	2253
	21st to end	Day	18% 7% 3 7% 4 5% 5% 3 5% 6% 37%	2090
October	1st to 10th	Day	12% 6% 2 6% 4 3 4 4 4 7% 48%	1914
	11th to 20th	Day	13% 6% 2 6% 4 3 3 2 4 6% 51%	1900
	21st to end	Day	11% 6% 2 6% 3 4 4 3 5% 5% 50%	2090
November	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 5% 6% 42%	1852
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 7% 44%	1800
	21st to end	Day	12% 6% 2 5% 4 4 4 4 5% 7% 46%	1800
December	1st to 10th	Day	10% 6% 2 5% 3 4 4 4 5% 5% 51%	2770
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 3 6% 7% 39%	2990
	21st to end	Day	11% 7% 2 5% 3 3 4 4 6% 7% 48%	3960
January	1st to 10th	Night	10% 8% 3 5% 3 3 3 3 4 6% 52%	6073
	11th to 20th	Night	13% 6% 2 5% 4 3 4 4 5% 6% 48%	5900
	21st to end	Night	13% 8% 3 7% 5% 5% 4 3 5% 6% 41%	6490
February	1st to 10th	Night	18% 8% 3 5% 4 4 4 4 5% 7% 38%	2517
	11th to 20th	Night	18% 9% 3 8% 3 3 4 4 5% 6% 37%	2338
	21st to end	Night	15% 7% 3 7% 4 4 4 3 5% 7% 41%	1980
March	1st to 10th	Night	16% 8% 3 8% 5% 4 4 3 5% 5% 38%	5200
	11th to 20th	Night	14% 5% 2 5% 4 3 3 3 5% 6% 49%	4980
	21st to end	Night	13% 8% 2 6% 4 3 4 3 4 5% 49%	5201
April	1st to 10th	Night	12% 6% 2 6% 3 3 4 4 5% 8% 47%	2000
	11th to 20th	Night	8% 6% 2 5% 5% 5% 5% 4 7% 8% 45%	1975
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 8% 49%	2000
May	1st to 10th	Night	8% 6% 3 8% 5% 6% 6% 6% 8% 9% 36%	4490
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 4 6% 9% 30%	4270
	21st to end	Night	10% 9% 3 8% 5% 4 5% 4 7% 9% 35%	3921
June	1st to 10th	Night	11% 6% 2 6% 4 5% 4 4 7% 10% 40%	1598
	11th to 20th	Night	7% 3 1 5% 4 4 5% 6% 8% 12% 45%	1520
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 5% 9% 13% 36%	1520
July	1st to 10th	Night	7% 5% 2 6% 4 4 6% 5% 8% 12% 40%	2520
	11th to 20th	Night	9% 6% 2 7% 6% 6% 5% 7% 9% 12% 31%	2791
	21st to end	Night	10% 6% 2 7% 6% 5% 6% 6% 9% 12% 31%	3087
August	1st to 10th	Night	12% 11% 4 10% 5% 5% 6% 4 8% 11% 25%	3017
	11th to 20th	Night	6% 4 2 7% 6% 6% 6% 6% 8% 9% 40%	3097
	21st to end	Night	8% 6% 2 8% 5% 5% 6% 5% 8% 10% 39%	3476
September	1st to 10th	Night	12% 6% 2 7% 5% 5% 6% 5% 9% 9% 33%	1900
	11th to 20th	Night	14% 7% 3 9% 4 5% 6% 4 6% 8% 34%	1925
	21st to end	Night	18% 6% 3 7% 6% 6% 4 4 6% 7% 34%	2090
October	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 7% 45%	2261
	11th to 20th	Night	15% 6% 2 4 3 3 4 3 4 6% 48%	2280
	21st to end	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	2508
November	1st to 10th	Night	15% 7% 2 6% 5% 5% 4 3 5% 7% 41%	2443
	11th to 20th	Night	15% 7% 3 7% 5% 4 4 4 5% 7% 39%	2600
	21st to end	Night	13% 6% 2 6% 4 3 4 3 5% 7% 47%	2600
December	1st to 10th	Night	10% 6% 2 6% 4 3 3 3 4 5% 55%	4405
	11th to 20th	Night	13% 8% 3 8% 5% 5% 5% 4 5% 7% 37%	5205
	21st to end	Night	12% 8% 2 5% 4 4 4 3 5% 8% 46%	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	27% 26% 2 20% 19% 6%	3856
	11th to 20th	Day	26% 26% 3 16% 24% 6%	4060
	21st to end	Day	37% 29% 5% 11% 14% 4	4466
February	1st to 10th	Day	18% 62% 5% 9% 5%	1880
	11th to 20th	Day	20% 61% 4 7% 6% 2	1957
	21st to end	Day	15% 62% 3 9% 9% 2	1650
March	1st to 10th	Day	40% 28% 6% 10% 11% 5%	4649
	11th to 20th	Day	22% 28% 6% 15% 17% 13%	4980
	21st to end	Day	24% 27% 9% 21% 14% 4	5730
April	1st to 10th	Day	15% 62% 6% 8% 8% 1	2400
	11th to 20th	Day	13% 63% 5% 11% 5% 2	2374
	21st to end	Day	11% 62% 7% 13% 7%	2400
May	1st to 10th	Day	31% 30% 16% 17% 5% 1	5437
	11th to 20th	Day	40% 28% 13% 16% 3	5665
	21st to end	Day	29% 29% 10% 23% 8%	6949
June	1st to 10th	Day	15% 63% 6% 9% 6%	2786
	11th to 20th	Day	9% 66% 9% 11% 4	2660
	21st to end	Day	12% 66% 9% 10% 3	2660
July	1st to 10th	Day	26% 37% 19% 13% 4	4370
	11th to 20th	Day	30% 40% 22% 7% 1	4094
	21st to end	Day	32% 41% 11% 10% 7%	4324
August	1st to 10th	Day	40% 39% 14% 5% 1	3869
	11th to 20th	Day	22% 40% 10% 21% 8%	3766
	21st to end	Day	21% 38% 15% 19% 7%	4180
September	1st to 10th	Day	18% 62% 10% 6% 3 1	2280
	11th to 20th	Day	21% 61% 6% 8% 3 1	2253
	21st to end	Day	22% 60% 7% 6% 4	2090
October	1st to 10th	Day	21% 38% 4 17% 15% 6%	3174
	11th to 20th	Day	17% 37% 3 14% 16% 13%	3140
	21st to end	Day	12% 37% 3 28% 19% 2	3428
November	1st to 10th	Day	20% 56% 4 10% 8% 1	1852
	11th to 20th	Day	19% 57% 3 9% 9% 4	1800
	21st to end	Day	15% 57% 4 12% 10% 1	1800
December	1st to 10th	Day	21% 36% 4 16% 20% 2	2770
	11th to 20th	Day	31% 31% 9% 14% 11% 4	2986
	21st to end	Day	28% 27% 12% 24% 8% 1	3960
January	1st to 10th	Night	25% 25% 3 15% 26% 5%	6073
	11th to 20th	Night	27% 26% 4 14% 24% 5%	5900
	21st to end	Night	33% 28% 6% 12% 18% 3	6490
February	1st to 10th	Night	17% 62% 5% 11% 5% 1	2517
	11th to 20th	Night	21% 61% 4 7% 7% 1	2340
	21st to end	Night	18% 62% 3 7% 7% 3	1980
March	1st to 10th	Night	40% 28% 5% 11% 10% 5%	5200
	11th to 20th	Night	25% 28% 6% 9% 20% 12%	4980
	21st to end	Night	28% 29% 7% 20% 13% 4	5201
April	1st to 10th	Night	15% 62% 6% 8% 8% 1	2000
	11th to 20th	Night	14% 63% 5% 12% 4 1	1975
	21st to end	Night	14% 63% 5% 12% 6%	2000
May	1st to 10th	Night	33% 30% 17% 16% 4 1	4520
	11th to 20th	Night	40% 30% 10% 14% 5%	4271
	21st to end	Night	36% 29% 10% 17% 8%	3928
June	1st to 10th	Night	16% 63% 7% 8% 6% 1	1598
	11th to 20th	Night	8% 68% 8% 10% 6%	1520
	21st to end	Night	12% 66% 8% 11% 3	1520
July	1st to 10th	Night	23% 39% 21% 14% 3	2480
	11th to 20th	Night	31% 41% 19% 8% 1	2739
	21st to end	Night	30% 43% 11% 7% 9%	2988
August	1st to 10th	Night	38% 38% 15% 7% 1	3006
	11th to 20th	Night	23% 39% 13% 19% 6%	3127
	21st to end	Night	22% 38% 12% 21% 6%	3476
September	1st to 10th	Night	19% 61% 8% 7% 4	1900
	11th to 20th	Night	20% 61% 6% 7% 5% 1	1925
	21st to end	Night	22% 61% 6% 9% 3	2090
October	1st to 10th	Night	27% 37% 5% 11% 17% 4	3768
	11th to 20th	Night	17% 38% 2 10% 17% 16%	3779
	21st to end	Night	12% 37% 4 26% 18% 4	4120
November	1st to 10th	Night	21% 56% 5% 11% 7% 1	2443
	11th to 20th	Night	21% 57% 4 7% 8% 4	2600
	21st to end	Night	16% 57% 5% 12% 10% 1	2600
December	1st to 10th	Night	19% 35% 3 13% 25% 4	4405
	11th to 20th	Night	32% 32% 11% 10% 11% 4	5208
	21st to end	Night	29% 26% 13% 21% 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	16% 13% 39% 25% 7% 1	257
	11th to 20th	Day	18% 10% 32% 32% 7% 1	300
	21st to end	Day	19% 10% 35% 25% 10% 1	330
February	1st to 10th	Day	17% 9% 34% 31% 8% 1	380
	11th to 20th	Day	26% 12% 32% 25% 5%	496
	21st to end	Day	17% 9% 29% 32% 12% 1	414
March	1st to 10th	Day	17% 13% 31% 25% 11% 2	500
	11th to 20th	Day	17% 10% 31% 28% 12% 1	500
	21st to end	Day	14% 9% 31% 31% 12% 2	550
April	1st to 10th	Day	16% 10% 30% 27% 15% 2	500
	11th to 20th	Day	10% 5% 24% 37% 21% 2	492
	21st to end	Day	9% 5% 22% 38% 21% 6%	500
May	1st to 10th	Day	13% 4 21% 35% 21% 6%	503
	11th to 20th	Day	15% 3 18% 30% 26% 7%	527
	21st to end	Day	15% 2 10% 30% 26% 17%	573
June	1st to 10th	Day	12% 2 12% 31% 33% 10%	526
	11th to 20th	Day	10% 2 10% 28% 35% 14%	490
	21st to end	Day	8% 1 9% 22% 48% 11%	490
July	1st to 10th	Day	12% 2 6% 20% 44% 17%	485
	11th to 20th	Day	9% 2 4 22% 46% 18%	484
	21st to end	Day	9% 2 7% 27% 45% 10%	532
August	1st to 10th	Day	12% 2 8% 26% 37% 15%	486
	11th to 20th	Day	10% 1 6% 21% 37% 24%	479
	21st to end	Day	14% 1 6% 24% 40% 16%	539
September	1st to 10th	Day	13% 1 10% 24% 30% 21%	490
	11th to 20th	Day	18% 3 9% 28% 32% 10%	490
	21st to end	Day	20% 4 12% 25% 30% 9%	490
October	1st to 10th	Day	17% 3 14% 23% 32% 11%	490
	11th to 20th	Day	15% 4 19% 31% 23% 7%	490
	21st to end	Day	13% 6% 19% 29% 26% 8%	539
November	1st to 10th	Day	19% 8% 16% 30% 21% 6%	442
	11th to 20th	Day	19% 7% 27% 30% 15% 2	400
	21st to end	Day	14% 9% 27% 30% 20% 1	400
December	1st to 10th	Day	16% 10% 36% 27% 10% 1	305
	11th to 20th	Day	18% 12% 32% 26% 11% 3	200
	21st to end	Day	17% 7% 29% 30% 15% 1	220
January	1st to 10th	Night	16% 19% 33% 22% 10% 1	800
	11th to 20th	Night	18% 15% 28% 29% 10% 1	800
	21st to end	Night	21% 13% 33% 26% 7%	880
February	1st to 10th	Night	21% 10% 32% 28% 9% 1	800
	11th to 20th	Night	25% 13% 31% 25% 6%	783
	21st to end	Night	18% 13% 28% 30% 10% 1	660
March	1st to 10th	Night	15% 12% 33% 28% 11% 1	800
	11th to 20th	Night	17% 10% 28% 31% 10% 4	800
	21st to end	Night	17% 10% 27% 31% 12% 2	880
April	1st to 10th	Night	14% 8% 33% 30% 14% 2	800
	11th to 20th	Night	10% 7% 25% 36% 18% 4	791
	21st to end	Night	12% 6% 23% 35% 22% 2	800
May	1st to 10th	Night	15% 6% 18% 39% 20% 4	800
	11th to 20th	Night	15% 4 19% 37% 20% 5%	699
	21st to end	Night	15% 2 9% 29% 31% 15%	434
June	1st to 10th	Night	12% 4 13% 32% 33% 8%	400
	11th to 20th	Night	9% 1 8% 33% 39% 11%	380
	21st to end	Night	10% 4 7% 34% 36% 10%	380
July	1st to 10th	Night	9% 1 6% 29% 43% 12%	370
	11th to 20th	Night	11% 2 3 24% 44% 15%	518
	21st to end	Night	11% 2 6% 30% 41% 9%	624
August	1st to 10th	Night	14% 1 8% 26% 41% 9%	681
	11th to 20th	Night	10% 2 5% 25% 38% 19%	750
	21st to end	Night	15% 2 5% 22% 41% 15%	836
September	1st to 10th	Night	14% 3 9% 23% 34% 18%	760
	11th to 20th	Night	18% 3 10% 22% 38% 9%	760
	21st to end	Night	22% 3 11% 22% 34% 8%	760
October	1st to 10th	Night	14% 4 14% 26% 31% 11%	760
	11th to 20th	Night	18% 6% 18% 29% 24% 4	760
	21st to end	Night	15% 6% 22% 25% 25% 8%	836
November	1st to 10th	Night	18% 7% 18% 32% 20% 5%	780
	11th to 20th	Night	19% 10% 20% 33% 16% 3	800
	21st to end	Night	16% 12% 23% 28% 19% 2	800
December	1st to 10th	Night	18% 11% 32% 25% 13% 1	800
	11th to 20th	Night	17% 11% 30% 29% 13% 2	800
	21st to end	Night	16% 12% 28% 31% 12% 1	880

Page generated at 2025-06-07 10:28:16. Version ac07

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