Ashby
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashby 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	7% 6% 2 5% 4 4 4 3 5% 7% 53%	146646
	Night	7% 6% 2 6% 4 4 4 4 5% 7% 50%	143814
Transparency	Day	31% 4 12% 28% 20% 5%	141475
	Night	33% 4 13% 27% 19% 4	139997
Seeing	Day	11% 8% 32% 31% 15% 3	18232
	Night	10% 9% 38% 35% 8% 1	28123

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	3 8% 4 9% 6% 4 4 2 4 5% 52%	4720
	11th to 20th	Day	3 8% 3 9% 4 4 4 4 6% 7% 48%	4740
	21st to end	Day	3 5% 3 8% 5% 5% 5% 3 5% 6% 52%	5188
February	1st to 10th	Day	5% 10% 4 8% 5% 4 5% 4 5% 7% 43%	1986
	11th to 20th	Day	6% 9% 3 8% 4 4 4 3 4 7% 47%	2049
	21st to end	Day	6% 8% 2 4 3 3 4 3 6% 6% 55%	1730
March	1st to 10th	Day	9% 9% 2 5% 3 2 3 3 4 6% 54%	4600
	11th to 20th	Day	7% 6% 2 4 3 3 3 3 4 6% 59%	4820
	21st to end	Day	10% 9% 3 7% 4 4 4 3 6% 7% 44%	5953
April	1st to 10th	Day	10% 8% 2 5% 5% 4 3 3 6% 7% 48%	2520
	11th to 20th	Day	13% 8% 3 6% 4 4 4 4 4 6% 45%	2491
	21st to end	Day	14% 8% 2 6% 4 3 3 3 4 6% 45%	2705
May	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 6% 7% 50%	7655
	11th to 20th	Day	11% 5% 2 5% 3 4 4 3 5% 8% 51%	7774
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 6% 45%	9026
June	1st to 10th	Day	6% 4 2 5% 3 4 4 3 6% 9% 54%	2940
	11th to 20th	Day	8% 4 1 4 4 3 4 4 6% 8% 53%	2940
	21st to end	Day	5% 3 1 4 3 4 4 3 5% 8% 60%	2940
July	1st to 10th	Day	6% 4 2 4 4 4 5% 4 6% 8% 54%	6479
	11th to 20th	Day	4 4 1 4 3 4 5% 4 7% 9% 54%	6480
	21st to end	Day	6% 4 2 4 3 4 5% 4 6% 9% 53%	7044
August	1st to 10th	Day	9% 5% 2 4 3 4 4 3 6% 9% 52%	5958
	11th to 20th	Day	5% 4 2 4 3 3 4 4 6% 8% 56%	5601
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 48%	5984
September	1st to 10th	Day	6% 3 1 4 2 3 3 3 4 6% 63%	2400
	11th to 20th	Day	7% 3 1 4 3 3 4 3 5% 7% 61%	2239
	21st to end	Day	8% 3 2 4 2 3 3 2 4 6% 63%	2200
October	1st to 10th	Day	6% 4 1 2 1 2 2 1 3 4 74%	3590
	11th to 20th	Day	6% 5% 2 5% 3 3 3 3 4 7% 59%	3163
	21st to end	Day	6% 7% 2 5% 3 3 4 3 5% 8% 54%	3190
November	1st to 10th	Day	6% 5% 2 5% 3 3 3 2 6% 8% 58%	1853
	11th to 20th	Day	4 5% 2 4 3 3 4 3 6% 6% 59%	1890
	21st to end	Day	5% 6% 2 5% 3 3 3 3 6% 7% 56%	1890
December	1st to 10th	Day	3 5% 2 5% 4 3 4 3 4 6% 61%	2710
	11th to 20th	Day	3 3 2 4 3 3 4 3 6% 8% 61%	3106
	21st to end	Day	7% 11% 3 6% 4 4 4 3 4 7% 47%	4092
January	1st to 10th	Night	5% 10% 4 9% 6% 4 4 3 4 6% 45%	8201
	11th to 20th	Night	2 5% 3 8% 5% 5% 5% 4 6% 7% 49%	8250
	21st to end	Night	2 5% 3 8% 6% 5% 5% 4 5% 6% 51%	9026
February	1st to 10th	Night	6% 10% 3 9% 4 4 4 4 6% 7% 44%	2652
	11th to 20th	Night	5% 10% 3 9% 4 4 4 4 5% 8% 44%	2458
	21st to end	Night	4 7% 3 7% 3 3 4 3 4 7% 53%	2076
March	1st to 10th	Night	10% 11% 2 5% 3 3 3 3 3 5% 52%	5494
	11th to 20th	Night	7% 7% 2 6% 4 3 3 2 4 7% 54%	5357
	21st to end	Night	8% 8% 3 9% 5% 5% 4 5% 6% 8% 38%	5234
April	1st to 10th	Night	12% 7% 2 6% 4 3 4 4 6% 8% 44%	2100
	11th to 20th	Night	11% 8% 2 5% 4 4 4 3 5% 6% 48%	2077
	21st to end	Night	15% 8% 1 6% 4 4 3 4 6% 7% 42%	1913
May	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 6% 47%	5292
	11th to 20th	Night	10% 6% 2 4 4 4 4 3 5% 8% 51%	5102
	21st to end	Night	17% 6% 2 6% 4 4 4 4 6% 7% 40%	5181
June	1st to 10th	Night	8% 5% 2 4 4 3 5% 4 6% 10% 50%	1680
	11th to 20th	Night	10% 4 1 4 4 4 4 5% 7% 8% 48%	1680
	21st to end	Night	7% 4 2 5% 4 4 5% 4 7% 10% 48%	1680
July	1st to 10th	Night	6% 4 2 4 3 4 5% 4 6% 8% 53%	3720
	11th to 20th	Night	7% 5% 2 4 4 4 5% 4 8% 10% 49%	3720
	21st to end	Night	6% 6% 2 6% 5% 5% 5% 4 6% 8% 47%	4050
August	1st to 10th	Night	9% 5% 2 8% 6% 5% 7% 5% 7% 9% 36%	3854
	11th to 20th	Night	7% 4 2 5% 4 4 5% 5% 7% 8% 48%	4246
	21st to end	Night	7% 7% 3 6% 5% 5% 5% 4 6% 8% 44%	4993
September	1st to 10th	Night	7% 4 2 5% 3 3 3 2 5% 6% 60%	2000
	11th to 20th	Night	8% 4 2 4 2 3 3 2 4 7% 60%	2160
	21st to end	Night	9% 3 1 4 3 3 2 3 4 6% 62%	2200
October	1st to 10th	Night	7% 3 1 3 2 1 2 1 2 3 74%	3590
	11th to 20th	Night	8% 4 1 3 3 2 2 2 4 6% 63%	3949
	21st to end	Night	7% 7% 2 5% 4 3 3 2 5% 6% 55%	4610
November	1st to 10th	Night	5% 5% 2 5% 3 3 3 4 6% 7% 56%	2665
	11th to 20th	Night	4 5% 2 6% 5% 3 4 3 6% 7% 56%	2730
	21st to end	Night	5% 6% 2 5% 3 3 4 3 5% 8% 56%	2730
December	1st to 10th	Night	2 4 2 5% 4 4 4 3 5% 6% 62%	4695
	11th to 20th	Night	3 5% 3 6% 4 4 4 4 6% 7% 54%	5321
	21st to end	Night	8% 10% 3 6% 4 4 4 3 5% 7% 45%	7128

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	37% 2 11% 30% 19% 1	4723
	11th to 20th	Day	35% 4 13% 27% 18% 3	4740
	21st to end	Day	34% 3 15% 26% 19% 4	5188
February	1st to 10th	Day	41% 5% 13% 22% 14% 4	1986
	11th to 20th	Day	37% 4 13% 25% 17% 4	2059
	21st to end	Day	28% 5% 12% 32% 18% 5%	1730
March	1st to 10th	Day	32% 3 6% 26% 29% 4	4600
	11th to 20th	Day	28% 3 11% 28% 29% 1	4820
	21st to end	Day	40% 4 18% 24% 12% 1	5953
April	1st to 10th	Day	36% 4 15% 28% 14% 3	2520
	11th to 20th	Day	39% 5% 12% 22% 18% 3	2494
	21st to end	Day	40% 3 13% 26% 16% 1	2705
May	1st to 10th	Day	31% 6% 12% 30% 19% 3	7655
	11th to 20th	Day	33% 4 12% 31% 17% 3	7778
	21st to end	Day	39% 4 13% 26% 16% 2	9027
June	1st to 10th	Day	28% 5% 20% 31% 14% 2	2940
	11th to 20th	Day	28% 5% 14% 30% 18% 5%	2940
	21st to end	Day	22% 5% 11% 31% 26% 6%	2940
July	1st to 10th	Day	29% 4 8% 27% 28% 4	6411
	11th to 20th	Day	26% 5% 14% 30% 19% 6%	6405
	21st to end	Day	27% 5% 13% 23% 23% 10%	6846
August	1st to 10th	Day	30% 5% 13% 28% 20% 5%	4328
	11th to 20th	Day	30% 6% 13% 24% 22% 5%	4055
	21st to end	Day	23% 5% 13% 24% 23% 13%	4311
September	1st to 10th	Day	22% 4 9% 29% 24% 12%	2400
	11th to 20th	Day	24% 4 9% 30% 26% 9%	2239
	21st to end	Day	24% 3 8% 30% 30% 6%	2200
October	1st to 10th	Day	17% 2 7% 21% 34% 19%	3590
	11th to 20th	Day	27% 3 11% 35% 21% 4	3163
	21st to end	Day	30% 2 12% 31% 20% 6%	3190
November	1st to 10th	Day	25% 4 14% 30% 21% 6%	1853
	11th to 20th	Day	25% 3 12% 29% 26% 5%	1890
	21st to end	Day	28% 3 13% 31% 21% 4	1890
December	1st to 10th	Day	25% 3 15% 33% 22% 3	2710
	11th to 20th	Day	22% 3 17% 39% 16% 3	3104
	21st to end	Day	39% 3 7% 35% 14% 2	4092
January	1st to 10th	Night	41% 3 10% 27% 16% 3	8202
	11th to 20th	Night	33% 4 16% 27% 18% 2	8250
	21st to end	Night	35% 3 12% 28% 19% 4	9026
February	1st to 10th	Night	39% 4 13% 28% 13% 3	2652
	11th to 20th	Night	38% 4 14% 26% 14% 4	2458
	21st to end	Night	31% 3 12% 29% 20% 5%	2076
March	1st to 10th	Night	35% 3 5% 25% 27% 4	5494
	11th to 20th	Night	32% 3 9% 24% 29% 3	5357
	21st to end	Night	43% 5% 18% 22% 11% 1	5234
April	1st to 10th	Night	36% 5% 15% 23% 16% 4	2100
	11th to 20th	Night	37% 5% 12% 25% 18% 3	2077
	21st to end	Night	41% 5% 14% 27% 12% 2	1913
May	1st to 10th	Night	37% 5% 12% 27% 18% 1	5293
	11th to 20th	Night	32% 5% 13% 31% 17% 3	5103
	21st to end	Night	43% 6% 13% 26% 11% 1	5182
June	1st to 10th	Night	29% 7% 23% 26% 13% 3	1680
	11th to 20th	Night	33% 6% 18% 25% 14% 5%	1680
	21st to end	Night	30% 7% 16% 27% 17% 4	1680
July	1st to 10th	Night	28% 4 15% 29% 21% 2	3680
	11th to 20th	Night	30% 5% 17% 27% 17% 4	3672
	21st to end	Night	35% 6% 13% 22% 19% 5%	3936
August	1st to 10th	Night	37% 6% 17% 24% 13% 2	2808
	11th to 20th	Night	36% 5% 14% 25% 17% 4	3068
	21st to end	Night	28% 6% 13% 23% 20% 10%	3597
September	1st to 10th	Night	25% 4 11% 26% 27% 8%	2000
	11th to 20th	Night	27% 3 11% 28% 24% 7%	2160
	21st to end	Night	25% 3 8% 30% 27% 8%	2200
October	1st to 10th	Night	19% 2 5% 20% 42% 12%	3590
	11th to 20th	Night	24% 3 10% 36% 22% 5%	3949
	21st to end	Night	31% 3 12% 28% 20% 7%	4610
November	1st to 10th	Night	25% 4 13% 29% 23% 6%	2665
	11th to 20th	Night	28% 3 12% 28% 23% 5%	2730
	21st to end	Night	27% 3 12% 31% 23% 4	2730
December	1st to 10th	Night	24% 3 16% 35% 19% 4	4695
	11th to 20th	Night	30% 3 13% 36% 15% 2	5322
	21st to end	Night	38% 4 10% 32% 15% 2	7128

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	9% 9% 41% 26% 11% 3	330
	11th to 20th	Day	12% 13% 38% 29% 6% 1	330
	21st to end	Day	7% 9% 52% 22% 8% 2	363
February	1st to 10th	Day	10% 13% 51% 22% 4 1	330
	11th to 20th	Day	9% 11% 50% 22% 6% 2	330
	21st to end	Day	8% 14% 50% 23% 4	273
March	1st to 10th	Day	12% 11% 37% 27% 12% 1	330
	11th to 20th	Day	12% 11% 39% 25% 10% 3	330
	21st to end	Day	15% 13% 41% 24% 6% 2	524
April	1st to 10th	Day	16% 17% 35% 24% 8% 1	540
	11th to 20th	Day	17% 18% 36% 21% 7% 1	532
	21st to end	Day	20% 12% 28% 27% 11% 2	725
May	1st to 10th	Day	15% 9% 30% 29% 13% 3	753
	11th to 20th	Day	19% 9% 27% 28% 15% 2	775
	21st to end	Day	17% 8% 30% 28% 13% 3	844
June	1st to 10th	Day	8% 6% 28% 31% 22% 6%	780
	11th to 20th	Day	9% 8% 35% 33% 13% 2	780
	21st to end	Day	8% 4 33% 33% 18% 3	780
July	1st to 10th	Day	7% 4 23% 39% 22% 5%	771
	11th to 20th	Day	4 2 21% 43% 25% 5%	768
	21st to end	Day	7% 2 19% 44% 23% 4	845
August	1st to 10th	Day	8% 2 21% 40% 22% 6%	726
	11th to 20th	Day	6% 2 25% 40% 23% 4	625
	21st to end	Day	5% 4 25% 36% 24% 6%	583
September	1st to 10th	Day	6% 4 26% 38% 21% 5%	530
	11th to 20th	Day	5% 4 24% 38% 22% 8%	369
	21st to end	Day	9% 7% 26% 32% 20% 5%	330
October	1st to 10th	Day	14% 7% 25% 26% 21% 6%	330
	11th to 20th	Day	11% 15% 35% 26% 12% 1	330
	21st to end	Day	11% 13% 39% 23% 12% 2	363
November	1st to 10th	Day	10% 9% 41% 23% 14% 3	330
	11th to 20th	Day	7% 12% 39% 32% 8% 2	330
	21st to end	Day	10% 7% 38% 32% 9% 3	330
December	1st to 10th	Day	10% 9% 47% 26% 8%	330
	11th to 20th	Day	11% 13% 42% 27% 7%	330
	21st to end	Day	14% 13% 44% 21% 7% 1	363
January	1st to 10th	Night	8% 12% 40% 29% 9% 1	1050
	11th to 20th	Night	7% 15% 44% 28% 6%	1050
	21st to end	Night	9% 12% 44% 30% 6%	1155
February	1st to 10th	Night	12% 12% 47% 26% 4	887
	11th to 20th	Night	9% 9% 49% 28% 5%	824
	21st to end	Night	8% 9% 54% 26% 3	692
March	1st to 10th	Night	14% 10% 39% 32% 5%	840
	11th to 20th	Night	12% 9% 41% 31% 6% 1	754
	21st to end	Night	12% 10% 42% 30% 6%	693
April	1st to 10th	Night	15% 13% 40% 27% 4	630
	11th to 20th	Night	15% 11% 46% 26% 3	623
	21st to end	Night	17% 11% 32% 32% 7%	630
May	1st to 10th	Night	13% 8% 35% 34% 10%	630
	11th to 20th	Night	18% 6% 35% 29% 11% 1	621
	21st to end	Night	18% 6% 35% 31% 9%	683
June	1st to 10th	Night	9% 5% 35% 36% 14% 2	630
	11th to 20th	Night	12% 8% 38% 36% 6%	630
	21st to end	Night	7% 4 32% 46% 11%	630
July	1st to 10th	Night	7% 2 29% 46% 15% 1	615
	11th to 20th	Night	5% 2 29% 51% 11%	612
	21st to end	Night	6% 2 29% 47% 15% 1	687
August	1st to 10th	Night	8% 3 23% 50% 14% 1	597
	11th to 20th	Night	6% 1 28% 52% 13%	594
	21st to end	Night	7% 3 27% 49% 13%	660
September	1st to 10th	Night	9% 3 29% 48% 10% 1	600
	11th to 20th	Night	10% 5% 29% 42% 14% 1	600
	21st to end	Night	12% 3 34% 41% 9% 1	600
October	1st to 10th	Night	15% 6% 32% 36% 12%	600
	11th to 20th	Night	11% 8% 34% 36% 10% 1	826
	21st to end	Night	11% 9% 36% 34% 8% 1	1100
November	1st to 10th	Night	8% 8% 40% 36% 7% 1	1025
	11th to 20th	Night	6% 12% 42% 32% 7% 1	1050
	21st to end	Night	8% 8% 40% 36% 8%	1050
December	1st to 10th	Night	6% 13% 47% 27% 6%	1050
	11th to 20th	Night	8% 12% 44% 30% 6%	1050
	21st to end	Night	12% 13% 40% 29% 5% 1	1155

Page generated at 2026-07-28 01:25:21. Version ac07

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