Ashton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashton Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	12% 8% 3 8% 5% 4 4 4 5% 7% 39%	126082
	Night	11% 8% 3 8% 4 4 4 3 5% 7% 42%	122266
Transparency	Day	42% 6% 13% 24% 14% 1	129682
	Night	41% 5% 12% 24% 15% 2	126743
Seeing	Day	16% 10% 30% 27% 14% 3	16902
	Night	16% 9% 32% 32% 11% 1	26724

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	7% 12% 5% 22% 7% 6% 5% 3 4 5% 23%	3617
	11th to 20th	Day	6% 10% 4 11% 6% 6% 7% 6% 6% 7% 31%	4004
	21st to end	Day	9% 13% 6% 14% 5% 5% 5% 4 4 6% 29%	4488
February	1st to 10th	Day	11% 14% 5% 9% 5% 6% 5% 3 6% 6% 29%	1800
	11th to 20th	Day	11% 11% 4 8% 5% 5% 6% 3 5% 6% 34%	1754
	21st to end	Day	13% 10% 5% 9% 5% 5% 4 3 5% 5% 36%	1599
March	1st to 10th	Day	17% 9% 3 6% 3 3 3 2 3 5% 46%	4922
	11th to 20th	Day	10% 7% 2 7% 4 3 4 3 5% 7% 47%	4980
	21st to end	Day	20% 14% 5% 8% 5% 4 4 3 5% 7% 25%	5497
April	1st to 10th	Day	17% 11% 4 6% 5% 4 4 4 6% 6% 34%	2400
	11th to 20th	Day	17% 9% 3 7% 4 3 4 4 6% 7% 35%	2374
	21st to end	Day	22% 8% 3 6% 5% 3 4 4 5% 6% 33%	2511
May	1st to 10th	Day	15% 7% 3 6% 5% 5% 5% 4 6% 8% 38%	5862
	11th to 20th	Day	16% 5% 2 5% 4 3 3 3 5% 7% 47%	6307
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	6935
June	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 4 6% 8% 45%	2800
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 6% 10% 45%	2800
	21st to end	Day	11% 7% 1 6% 5% 4 5% 5% 7% 8% 41%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 7% 10% 41%	6360
	11th to 20th	Day	8% 6% 2 6% 4 4 4 4 6% 8% 48%	6360
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	6452
August	1st to 10th	Day	6% 5% 2 6% 5% 4 5% 4 7% 10% 45%	4157
	11th to 20th	Day	10% 7% 3 7% 4 5% 5% 4 7% 9% 39%	4041
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 6% 8% 48%	4223
September	1st to 10th	Day	13% 5% 2 5% 3 3 3 2 5% 7% 54%	2280
	11th to 20th	Day	9% 4 2 5% 4 4 4 4 6% 7% 50%	2249
	21st to end	Day	11% 4 2 5% 4 4 4 3 5% 7% 50%	2000
October	1st to 10th	Day	13% 8% 2 6% 4 3 3 3 4 6% 48%	1900
	11th to 20th	Day	15% 8% 3 7% 4 4 3 3 4 4 45%	1900
	21st to end	Day	15% 9% 3 6% 4 4 4 3 4 6% 42%	2090
November	1st to 10th	Day	11% 7% 2 6% 4 4 5% 4 6% 8% 44%	1820
	11th to 20th	Day	13% 11% 4 7% 4 4 4 3 5% 7% 39%	1600
	21st to end	Day	15% 11% 4 6% 5% 3 4 3 5% 7% 38%	1600
December	1st to 10th	Day	8% 7% 3 7% 4 4 4 3 5% 8% 47%	2610
	11th to 20th	Day	13% 11% 4 16% 7% 5% 4 4 5% 5% 26%	3008
	21st to end	Day	23% 18% 9% 12% 4 4 3 3 4 5% 17%	3982
January	1st to 10th	Night	9% 12% 5% 19% 6% 5% 5% 3 4 6% 28%	6348
	11th to 20th	Night	9% 13% 4 10% 7% 5% 7% 4 7% 6% 29%	5974
	21st to end	Night	11% 13% 6% 12% 5% 4 4 3 5% 6% 31%	6490
February	1st to 10th	Night	11% 12% 4 10% 6% 4 5% 3 4 7% 34%	2600
	11th to 20th	Night	11% 11% 3 8% 5% 4 5% 4 5% 6% 39%	2533
	21st to end	Night	12% 9% 3 7% 5% 5% 3 3 4 5% 43%	2030
March	1st to 10th	Night	16% 8% 2 5% 2 3 3 3 4 6% 49%	4973
	11th to 20th	Night	9% 7% 2 6% 3 3 4 3 6% 7% 50%	4980
	21st to end	Night	16% 12% 5% 11% 5% 5% 4 4 5% 6% 27%	5432
April	1st to 10th	Night	15% 8% 3 6% 4 4 3 3 5% 8% 42%	2000
	11th to 20th	Night	13% 9% 4 7% 5% 4 4 3 5% 6% 41%	1975
	21st to end	Night	17% 7% 2 6% 5% 4 4 3 4 6% 42%	1885
May	1st to 10th	Night	15% 5% 2 6% 4 3 4 3 5% 6% 46%	4053
	11th to 20th	Night	12% 6% 3 5% 3 4 3 3 4 8% 49%	3606
	21st to end	Night	15% 8% 2 6% 4 4 4 3 5% 7% 41%	3942
June	1st to 10th	Night	7% 5% 2 6% 5% 5% 4 4 5% 8% 50%	1600
	11th to 20th	Night	6% 5% 2 5% 5% 3 3 4 7% 11% 49%	1600
	21st to end	Night	9% 5% 2 6% 4 3 4 4 6% 8% 49%	1600
July	1st to 10th	Night	8% 4 2 5% 4 4 5% 5% 7% 10% 47%	3618
	11th to 20th	Night	5% 4 2 5% 3 4 4 3 6% 8% 55%	3617
	21st to end	Night	8% 5% 2 7% 5% 5% 4 4 7% 8% 45%	4402
August	1st to 10th	Night	4 4 2 5% 3 4 5% 4 6% 9% 54%	2872
	11th to 20th	Night	8% 5% 2 5% 3 3 4 3 5% 7% 56%	2798
	21st to end	Night	8% 5% 2 5% 3 3 3 4 5% 7% 54%	3432
September	1st to 10th	Night	9% 4 2 5% 3 3 3 3 4 6% 58%	1900
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 6% 6% 54%	1928
	21st to end	Night	10% 4 1 4 3 3 3 3 4 7% 57%	2161
October	1st to 10th	Night	12% 7% 2 5% 3 3 3 2 4 6% 53%	2280
	11th to 20th	Night	14% 6% 3 5% 3 3 3 3 4 6% 49%	2280
	21st to end	Night	16% 9% 3 6% 3 3 4 3 5% 5% 44%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 52%	2474
	11th to 20th	Night	13% 8% 4 7% 5% 4 4 3 4 6% 42%	2800
	21st to end	Night	16% 9% 3 7% 4 4 4 3 5% 6% 41%	2800
December	1st to 10th	Night	6% 9% 3 7% 4 4 5% 4 6% 7% 45%	4580
	11th to 20th	Night	12% 10% 4 11% 5% 4 5% 3 5% 5% 35%	5199
	21st to end	Night	20% 17% 7% 13% 5% 4 3 3 4 4 19%	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	62% 5% 11% 15% 6%	3618
	11th to 20th	Day	52% 4 11% 18% 14% 1	4004
	21st to end	Day	57% 5% 12% 20% 6%	4488
February	1st to 10th	Day	54% 5% 15% 18% 8% 1	1800
	11th to 20th	Day	50% 5% 11% 21% 11% 2	1763
	21st to end	Day	49% 5% 12% 20% 11% 3	1599
March	1st to 10th	Day	44% 3 10% 25% 17% 1	4922
	11th to 20th	Day	35% 6% 8% 30% 19% 2	4980
	21st to end	Day	57% 6% 17% 15% 4 1	5499
April	1st to 10th	Day	48% 7% 11% 24% 9%	2400
	11th to 20th	Day	47% 7% 12% 21% 12% 1	2374
	21st to end	Day	50% 6% 12% 20% 11% 1	2511
May	1st to 10th	Day	43% 7% 13% 20% 17% 1	5900
	11th to 20th	Day	36% 6% 12% 28% 16% 3	6311
	21st to end	Day	43% 7% 12% 24% 14% 1	6949
June	1st to 10th	Day	35% 8% 16% 24% 15% 2	2800
	11th to 20th	Day	32% 8% 15% 23% 19% 2	2800
	21st to end	Day	38% 9% 15% 23% 14% 2	2800
July	1st to 10th	Day	37% 6% 16% 30% 11% 1	6291
	11th to 20th	Day	34% 5% 14% 29% 17% 1	6285
	21st to end	Day	35% 8% 20% 24% 12% 1	6269
August	1st to 10th	Day	32% 7% 17% 28% 15% 1	4143
	11th to 20th	Day	41% 6% 14% 25% 13% 1	4080
	21st to end	Day	32% 7% 16% 26% 16% 2	4222
September	1st to 10th	Day	32% 5% 13% 24% 22% 4	2280
	11th to 20th	Day	30% 7% 14% 31% 17% 1	2249
	21st to end	Day	33% 5% 11% 31% 19% 1	2000
October	1st to 10th	Day	23% 3 6% 28% 37% 2	3160
	11th to 20th	Day	30% 2 9% 30% 24% 5%	3140
	21st to end	Day	39% 4 10% 29% 15% 3	3428
November	1st to 10th	Day	37% 7% 16% 23% 17% 2	1820
	11th to 20th	Day	45% 4 13% 22% 15% 1	1600
	21st to end	Day	46% 5% 15% 24% 9% 1	1600
December	1st to 10th	Day	36% 4 15% 37% 8% 1	2610
	11th to 20th	Day	58% 4 14% 15% 9%	3005
	21st to end	Day	71% 5% 8% 12% 4	3982
January	1st to 10th	Night	57% 5% 15% 14% 8% 1	6348
	11th to 20th	Night	53% 4 13% 13% 15% 2	5974
	21st to end	Night	54% 4 11% 18% 12% 1	6490
February	1st to 10th	Night	51% 5% 12% 21% 10% 1	2600
	11th to 20th	Night	45% 5% 11% 25% 12% 2	2534
	21st to end	Night	44% 5% 11% 21% 16% 3	2030
March	1st to 10th	Night	38% 3 8% 30% 19% 2	4973
	11th to 20th	Night	32% 5% 9% 32% 18% 4	4980
	21st to end	Night	56% 8% 15% 13% 6% 2	5432
April	1st to 10th	Night	41% 5% 11% 25% 15% 2	2000
	11th to 20th	Night	44% 5% 10% 22% 16% 3	1975
	21st to end	Night	43% 5% 14% 23% 13% 2	1885
May	1st to 10th	Night	37% 6% 11% 26% 19% 1	4080
	11th to 20th	Night	34% 5% 13% 26% 18% 4	3607
	21st to end	Night	42% 6% 12% 22% 16% 2	3950
June	1st to 10th	Night	32% 7% 14% 28% 16% 4	1600
	11th to 20th	Night	28% 7% 16% 22% 22% 6%	1600
	21st to end	Night	31% 7% 13% 26% 19% 4	1600
July	1st to 10th	Night	31% 6% 15% 32% 14% 2	3580
	11th to 20th	Night	26% 6% 16% 30% 20% 2	3572
	21st to end	Night	33% 7% 19% 26% 14% 1	4275
August	1st to 10th	Night	25% 7% 16% 35% 15% 2	2862
	11th to 20th	Night	29% 6% 16% 31% 16% 1	2825
	21st to end	Night	28% 6% 15% 28% 17% 5%	3432
September	1st to 10th	Night	28% 6% 9% 27% 26% 4	1900
	11th to 20th	Night	30% 5% 10% 32% 20% 2	1928
	21st to end	Night	28% 5% 9% 31% 24% 3	2161
October	1st to 10th	Night	22% 2 7% 24% 42% 3	3800
	11th to 20th	Night	27% 2 9% 26% 28% 8%	3779
	21st to end	Night	42% 4 10% 24% 17% 3	4120
November	1st to 10th	Night	33% 5% 15% 25% 18% 3	2474
	11th to 20th	Night	43% 5% 13% 24% 14% 2	2800
	21st to end	Night	45% 4 12% 26% 12% 1	2800
December	1st to 10th	Night	37% 5% 13% 36% 8% 1	4580
	11th to 20th	Night	51% 4 14% 24% 6%	5201
	21st to end	Night	68% 5% 10% 12% 5%	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	19% 12% 36% 22% 9% 2	300
	11th to 20th	Day	19% 14% 44% 19% 3	300
	21st to end	Day	21% 11% 40% 23% 4 1	330
February	1st to 10th	Day	21% 14% 42% 19% 3 1	300
	11th to 20th	Day	17% 18% 36% 22% 6% 1	300
	21st to end	Day	22% 14% 41% 20% 2	249
March	1st to 10th	Day	18% 17% 44% 14% 6%	300
	11th to 20th	Day	23% 16% 33% 20% 7% 1	300
	21st to end	Day	34% 16% 33% 14% 3	340
April	1st to 10th	Day	26% 15% 34% 20% 5% 1	500
	11th to 20th	Day	24% 16% 30% 23% 6%	492
	21st to end	Day	26% 13% 34% 20% 5% 2	611
May	1st to 10th	Day	21% 13% 32% 21% 11% 1	703
	11th to 20th	Day	21% 13% 28% 22% 13% 3	725
	21st to end	Day	17% 8% 29% 27% 15% 3	789
June	1st to 10th	Day	8% 7% 26% 32% 21% 6%	730
	11th to 20th	Day	10% 6% 29% 35% 16% 4	730
	21st to end	Day	11% 7% 33% 32% 13% 4	730
July	1st to 10th	Day	9% 3 17% 33% 27% 10%	721
	11th to 20th	Day	7% 4 26% 33% 24% 6%	718
	21st to end	Day	7% 2 21% 39% 26% 5%	794
August	1st to 10th	Day	7% 3 22% 36% 27% 5%	697
	11th to 20th	Day	12% 4 18% 35% 24% 6%	665
	21st to end	Day	9% 4 19% 38% 24% 6%	569
September	1st to 10th	Day	14% 6% 20% 30% 22% 7%	490
	11th to 20th	Day	8% 4 24% 35% 23% 6%	459
	21st to end	Day	12% 9% 30% 29% 18% 2	300
October	1st to 10th	Day	17% 6% 32% 25% 15% 6%	300
	11th to 20th	Day	15% 23% 35% 19% 7% 1	300
	21st to end	Day	22% 15% 32% 21% 7% 3	330
November	1st to 10th	Day	16% 11% 40% 24% 7% 2	300
	11th to 20th	Day	20% 17% 34% 19% 9% 1	300
	21st to end	Day	23% 14% 38% 21% 5%	300
December	1st to 10th	Day	11% 13% 44% 20% 10% 1	300
	11th to 20th	Day	17% 14% 37% 23% 7% 2	300
	21st to end	Day	27% 17% 38% 16% 3	330
January	1st to 10th	Night	14% 12% 39% 28% 7%	1000
	11th to 20th	Night	16% 13% 36% 28% 7%	1000
	21st to end	Night	19% 10% 37% 28% 5%	1100
February	1st to 10th	Night	18% 12% 37% 28% 5%	1000
	11th to 20th	Night	16% 13% 40% 27% 4	979
	21st to end	Night	18% 11% 42% 24% 3 1	710
March	1st to 10th	Night	20% 13% 37% 23% 6% 1	630
	11th to 20th	Night	16% 11% 41% 26% 6% 1	600
	21st to end	Night	23% 12% 41% 18% 5% 1	660
April	1st to 10th	Night	19% 14% 41% 24% 3	600
	11th to 20th	Night	18% 15% 35% 26% 6% 1	593
	21st to end	Night	20% 10% 36% 26% 8% 1	600
May	1st to 10th	Night	18% 9% 33% 32% 8%	600
	11th to 20th	Night	18% 8% 32% 30% 12%	591
	21st to end	Night	16% 6% 28% 36% 14%	650
June	1st to 10th	Night	8% 5% 26% 43% 17% 1	600
	11th to 20th	Night	8% 3 29% 48% 12% 1	600
	21st to end	Night	10% 4 31% 40% 15%	600
July	1st to 10th	Night	5% 2 14% 46% 29% 3	585
	11th to 20th	Night	5% 2 20% 46% 25% 2	582
	21st to end	Night	7% 2 18% 48% 24% 1	657
August	1st to 10th	Night	6% 3 16% 47% 26% 2	579
	11th to 20th	Night	9% 2 18% 44% 26% 2	564
	21st to end	Night	9% 2 19% 42% 25% 2	627
September	1st to 10th	Night	12% 3 20% 42% 20% 3	570
	11th to 20th	Night	14% 2 22% 38% 22% 2	570
	21st to end	Night	11% 6% 29% 37% 15% 1	641
October	1st to 10th	Night	14% 8% 31% 33% 14% 1	760
	11th to 20th	Night	17% 11% 34% 30% 8%	760
	21st to end	Night	20% 11% 33% 28% 7%	836
November	1st to 10th	Night	15% 9% 37% 30% 9%	780
	11th to 20th	Night	17% 13% 39% 23% 6% 1	1000
	21st to end	Night	20% 10% 36% 26% 8%	1000
December	1st to 10th	Night	16% 13% 39% 27% 5% 1	1000
	11th to 20th	Night	20% 10% 35% 29% 6%	1000
	21st to end	Night	24% 13% 33% 24% 5%	1100

Page generated at 2025-08-02 00:32:23. Version ac07

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