Ash Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ash Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	15% 6% 2 6% 4 4 4 4 6% 8% 41%	149386
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	142329
Transparency	Day	40% 6% 12% 23% 17% 3	152916
	Night	40% 5% 11% 23% 17% 3	146887
Seeing	Day	18% 13% 26% 25% 14% 5%	17555
	Night	18% 9% 31% 31% 11% 1	26785

By Month

Percentage Cloud Cover NE (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	13% 7% 2 6% 4 4 4 3 5% 6% 46%	4661
	11th to 20th	Day	16% 7% 2 8% 4 3 3 2 3 5% 47%	4827
	21st to end	Day	16% 6% 2 5% 4 4 4 3 5% 6% 44%	5780
February	1st to 10th	Day	14% 8% 2 7% 4 4 5% 4 6% 6% 41%	1890
	11th to 20th	Day	17% 8% 2 6% 5% 4 4 4 5% 6% 39%	2012
	21st to end	Day	17% 10% 3 6% 4 4 3 3 5% 6% 39%	1730
March	1st to 10th	Day	20% 6% 2 5% 3 3 3 3 4 6% 47%	4576
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 6% 47%	5089
	21st to end	Day	17% 5% 2 5% 4 4 4 4 5% 6% 43%	5914
April	1st to 10th	Day	17% 7% 2 5% 4 3 4 4 5% 6% 42%	2520
	11th to 20th	Day	14% 6% 2 5% 5% 4 4 4 6% 7% 43%	2494
	21st to end	Day	16% 6% 2 5% 5% 4 5% 4 6% 7% 41%	2520
May	1st to 10th	Day	22% 7% 3 6% 4 4 4 4 5% 8% 35%	7455
	11th to 20th	Day	19% 6% 3 6% 4 4 4 3 5% 7% 39%	8221
	21st to end	Day	24% 6% 2 6% 4 4 4 3 5% 7% 36%	9060
June	1st to 10th	Day	11% 5% 2 7% 5% 4 5% 4 7% 9% 41%	2940
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 8% 40%	2940
	21st to end	Day	10% 4 2 6% 5% 4 5% 4 7% 9% 42%	2940
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 4 7% 11% 38%	8280
	11th to 20th	Day	8% 6% 3 7% 5% 5% 6% 5% 7% 11% 34%	8280
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 5% 7% 10% 38%	8801
August	1st to 10th	Day	14% 6% 2 8% 5% 5% 6% 5% 7% 10% 32%	5992
	11th to 20th	Day	11% 6% 2 7% 4 4 5% 4 7% 10% 40%	5488
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 7% 10% 43%	6080
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 5% 7% 45%	2520
	11th to 20th	Day	12% 5% 2 4 4 4 4 4 5% 7% 49%	2483
	21st to end	Day	18% 8% 3 8% 5% 4 5% 3 4 6% 36%	2310
October	1st to 10th	Day	17% 5% 2 5% 4 4 5% 3 5% 6% 46%	2109
	11th to 20th	Day	16% 5% 2 4 3 3 4 3 5% 6% 49%	2030
	21st to end	Day	16% 5% 2 5% 4 3 3 3 3 6% 49%	2135
November	1st to 10th	Day	13% 5% 2 4 3 3 3 2 4 7% 53%	1851
	11th to 20th	Day	14% 6% 2 4 4 3 4 5% 5% 7% 47%	1890
	21st to end	Day	15% 6% 1 5% 4 3 4 3 5% 6% 48%	1774
December	1st to 10th	Day	14% 6% 2 5% 3 4 4 4 6% 7% 45%	2680
	11th to 20th	Day	19% 10% 2 5% 3 3 3 3 3 6% 44%	3066
	21st to end	Day	15% 6% 2 6% 3 4 5% 4 7% 7% 41%	4048
January	1st to 10th	Night	12% 8% 2 6% 4 4 4 3 5% 6% 46%	8234
	11th to 20th	Night	16% 7% 2 6% 3 3 3 2 3 5% 50%	8055
	21st to end	Night	15% 7% 2 7% 4 3 3 3 4 5% 46%	8401
February	1st to 10th	Night	16% 8% 3 6% 3 4 4 4 5% 7% 41%	2730
	11th to 20th	Night	19% 8% 3 6% 4 3 3 3 4 6% 41%	2492
	21st to end	Night	17% 7% 3 5% 3 4 3 4 5% 6% 42%	2076
March	1st to 10th	Night	18% 8% 2 6% 3 3 3 2 4 6% 45%	5469
	11th to 20th	Night	10% 8% 2 5% 4 4 4 3 5% 5% 51%	5089
	21st to end	Night	16% 8% 2 6% 4 4 4 3 4 5% 44%	5245
April	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 5% 6% 41%	2100
	11th to 20th	Night	14% 6% 2 7% 4 3 4 4 5% 7% 42%	2075
	21st to end	Night	17% 6% 2 6% 4 3 3 3 5% 6% 44%	2100
May	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 4 6% 8% 33%	5487
	11th to 20th	Night	19% 7% 3 6% 4 4 4 4 6% 8% 35%	4639
	21st to end	Night	25% 7% 3 7% 4 3 4 3 4 6% 33%	5114
June	1st to 10th	Night	14% 5% 2 5% 4 4 4 4 6% 8% 44%	1680
	11th to 20th	Night	9% 8% 3 7% 5% 5% 5% 3 6% 9% 40%	1680
	21st to end	Night	10% 6% 2 5% 4 5% 5% 4 8% 10% 41%	1680
July	1st to 10th	Night	11% 6% 3 8% 5% 5% 6% 5% 6% 9% 37%	4680
	11th to 20th	Night	10% 7% 3 8% 5% 5% 5% 5% 8% 12% 34%	4680
	21st to end	Night	12% 7% 3 7% 4 5% 5% 4 6% 8% 40%	5259
August	1st to 10th	Night	17% 6% 3 8% 5% 5% 5% 5% 7% 9% 30%	4135
	11th to 20th	Night	13% 7% 2 7% 4 5% 4 4 6% 8% 41%	4533
	21st to end	Night	12% 5% 2 6% 4 5% 5% 3 5% 7% 46%	5045
September	1st to 10th	Night	12% 6% 2 7% 4 4 4 3 5% 7% 46%	2100
	11th to 20th	Night	11% 5% 2 5% 4 2 3 3 5% 7% 54%	2137
	21st to end	Night	20% 7% 2 6% 4 3 3 4 5% 7% 38%	2310
October	1st to 10th	Night	17% 7% 2 5% 4 3 3 3 5% 6% 45%	2508
	11th to 20th	Night	17% 4 2 5% 3 3 3 2 5% 6% 51%	2437
	21st to end	Night	16% 6% 2 5% 3 2 4 3 5% 6% 48%	2704
November	1st to 10th	Night	11% 5% 1 5% 4 3 3 3 5% 6% 54%	2667
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 3 5% 49%	2730
	21st to end	Night	12% 6% 1 5% 5% 4 5% 4 6% 6% 45%	2840
December	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 6% 49%	4720
	11th to 20th	Night	21% 7% 2 5% 3 3 3 3 5% 7% 41%	5348
	21st to end	Night	17% 7% 2 6% 4 3 4 3 5% 6% 40%	7150

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	39% 4 10% 26% 20% 3	4662
	11th to 20th	Day	42% 3 6% 23% 20% 6%	4857
	21st to end	Day	40% 4 13% 28% 13% 2	5780
February	1st to 10th	Day	42% 6% 14% 24% 13% 2	1890
	11th to 20th	Day	44% 5% 9% 23% 15% 3	2021
	21st to end	Day	46% 5% 9% 23% 13% 4	1730
March	1st to 10th	Day	39% 4 8% 25% 17% 7%	4579
	11th to 20th	Day	35% 5% 10% 28% 16% 5%	5089
	21st to end	Day	41% 4 10% 17% 22% 5%	5914
April	1st to 10th	Day	41% 6% 11% 20% 16% 6%	2520
	11th to 20th	Day	39% 6% 12% 22% 16% 4	2494
	21st to end	Day	41% 6% 11% 19% 19% 4	2520
May	1st to 10th	Day	48% 5% 13% 18% 15% 1	7464
	11th to 20th	Day	44% 6% 11% 24% 14% 2	8225
	21st to end	Day	47% 6% 9% 22% 14% 2	9061
June	1st to 10th	Day	37% 8% 13% 23% 17% 2	2940
	11th to 20th	Day	38% 9% 13% 23% 13% 3	2940
	21st to end	Day	35% 9% 14% 23% 17% 2	2940
July	1st to 10th	Day	39% 9% 15% 19% 16% 1	8210
	11th to 20th	Day	40% 9% 16% 24% 12% 1	8205
	21st to end	Day	38% 8% 11% 23% 20% 1	8606
August	1st to 10th	Day	45% 9% 18% 20% 8%	5979
	11th to 20th	Day	37% 7% 16% 28% 11% 1	5524
	21st to end	Day	34% 7% 14% 26% 17% 2	6080
September	1st to 10th	Day	38% 6% 12% 21% 21% 3	2520
	11th to 20th	Day	33% 5% 11% 23% 23% 4	2483
	21st to end	Day	49% 5% 9% 17% 18% 2	2310
October	1st to 10th	Day	34% 5% 9% 18% 29% 5%	3369
	11th to 20th	Day	26% 4 10% 23% 29% 9%	3270
	21st to end	Day	25% 2 12% 27% 29% 5%	3425
November	1st to 10th	Day	32% 4 13% 26% 21% 4	1851
	11th to 20th	Day	36% 5% 10% 24% 22% 3	1890
	21st to end	Day	38% 4 11% 30% 14% 3	1774
December	1st to 10th	Day	38% 4 17% 25% 14% 2	2680
	11th to 20th	Day	44% 3 11% 30% 10% 3	3066
	21st to end	Day	39% 6% 13% 22% 18% 2	4048
January	1st to 10th	Night	39% 4 11% 24% 17% 5%	8234
	11th to 20th	Night	39% 3 8% 23% 19% 8%	8100
	21st to end	Night	40% 3 8% 27% 19% 3	8401
February	1st to 10th	Night	43% 5% 12% 24% 14% 3	2730
	11th to 20th	Night	44% 4 9% 20% 17% 5%	2494
	21st to end	Night	43% 5% 9% 22% 15% 6%	2076
March	1st to 10th	Night	41% 4 9% 24% 13% 9%	5479
	11th to 20th	Night	34% 5% 13% 26% 16% 5%	5090
	21st to end	Night	43% 5% 8% 16% 24% 4	5245
April	1st to 10th	Night	44% 5% 9% 20% 18% 4	2100
	11th to 20th	Night	40% 5% 11% 20% 19% 4	2075
	21st to end	Night	41% 5% 9% 21% 20% 5%	2100
May	1st to 10th	Night	48% 7% 11% 20% 13% 2	5494
	11th to 20th	Night	46% 7% 12% 23% 11% 1	4640
	21st to end	Night	51% 6% 9% 19% 12% 2	5115
June	1st to 10th	Night	37% 7% 11% 23% 21% 2	1680
	11th to 20th	Night	39% 9% 13% 24% 14% 2	1680
	21st to end	Night	35% 9% 14% 25% 15% 2	1680
July	1st to 10th	Night	43% 8% 14% 21% 12% 1	4640
	11th to 20th	Night	40% 9% 19% 22% 9% 1	4632
	21st to end	Night	39% 7% 14% 22% 17%	5140
August	1st to 10th	Night	48% 9% 17% 17% 8%	4124
	11th to 20th	Night	40% 7% 15% 26% 11% 1	4563
	21st to end	Night	36% 6% 11% 28% 18% 1	5045
September	1st to 10th	Night	38% 7% 12% 20% 20% 3	2100
	11th to 20th	Night	31% 5% 10% 24% 25% 4	2137
	21st to end	Night	46% 6% 8% 20% 18% 2	2310
October	1st to 10th	Night	39% 5% 10% 22% 21% 3	4028
	11th to 20th	Night	25% 4 11% 25% 29% 6%	3936
	21st to end	Night	25% 3 11% 31% 24% 5%	4364
November	1st to 10th	Night	33% 5% 11% 25% 21% 6%	2667
	11th to 20th	Night	39% 4 9% 21% 21% 6%	2730
	21st to end	Night	37% 5% 12% 26% 18% 2	2840
December	1st to 10th	Night	37% 4 12% 30% 15% 2	4720
	11th to 20th	Night	43% 3 11% 23% 15% 4	5348
	21st to end	Night	43% 6% 12% 20% 17% 2	7150

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	21% 26% 29% 19% 4 1	220
	11th to 20th	Day	20% 36% 28% 15% 2	252
	21st to end	Day	22% 25% 31% 17% 4 1	363
February	1st to 10th	Day	21% 25% 33% 17% 5%	330
	11th to 20th	Day	26% 23% 35% 13% 2	500
	21st to end	Day	24% 19% 35% 18% 4 1	446
March	1st to 10th	Day	21% 23% 31% 19% 6%	540
	11th to 20th	Day	20% 22% 32% 19% 6% 1	540
	21st to end	Day	22% 20% 31% 21% 5% 1	594
April	1st to 10th	Day	22% 19% 32% 17% 7% 1	540
	11th to 20th	Day	18% 20% 28% 22% 10% 2	532
	21st to end	Day	20% 16% 31% 23% 8% 1	540
May	1st to 10th	Day	27% 17% 28% 17% 9% 2	543
	11th to 20th	Day	20% 13% 26% 25% 12% 2	567
	21st to end	Day	22% 8% 20% 26% 17% 7%	617
June	1st to 10th	Day	12% 9% 23% 31% 20% 5%	570
	11th to 20th	Day	15% 10% 23% 31% 17% 4	570
	21st to end	Day	12% 7% 25% 31% 19% 6%	570
July	1st to 10th	Day	12% 4 18% 32% 24% 10%	565
	11th to 20th	Day	6% 2 20% 34% 27% 10%	564
	21st to end	Day	11% 3 20% 34% 24% 8%	620
August	1st to 10th	Day	12% 5% 19% 31% 24% 10%	566
	11th to 20th	Day	13% 4 15% 33% 26% 9%	559
	21st to end	Day	10% 4 24% 30% 19% 12%	627
September	1st to 10th	Day	15% 8% 22% 24% 22% 9%	570
	11th to 20th	Day	13% 5% 22% 30% 19% 11%	570
	21st to end	Day	24% 10% 23% 26% 12% 5%	570
October	1st to 10th	Day	21% 6% 24% 26% 15% 9%	570
	11th to 20th	Day	18% 15% 23% 24% 16% 4	542
	21st to end	Day	18% 11% 32% 24% 11% 3	547
November	1st to 10th	Day	14% 12% 33% 25% 14% 2	425
	11th to 20th	Day	17% 16% 35% 24% 9%	430
	21st to end	Day	17% 18% 30% 25% 9% 1	314
December	1st to 10th	Day	22% 21% 34% 16% 7%	220
	11th to 20th	Day	27% 20% 32% 15% 6%	220
	21st to end	Day	25% 19% 27% 18% 9% 1	242
January	1st to 10th	Night	19% 14% 40% 23% 4	840
	11th to 20th	Night	22% 15% 36% 24% 3	840
	21st to end	Night	17% 14% 40% 25% 2	924
February	1st to 10th	Night	20% 12% 40% 26% 3	840
	11th to 20th	Night	25% 15% 40% 19% 1	823
	21st to end	Night	22% 14% 41% 21% 3	692
March	1st to 10th	Night	19% 13% 37% 25% 5%	840
	11th to 20th	Night	18% 14% 40% 22% 5% 1	840
	21st to end	Night	21% 9% 42% 23% 5%	924
April	1st to 10th	Night	21% 15% 38% 20% 5%	840
	11th to 20th	Night	16% 12% 37% 30% 5%	831
	21st to end	Night	20% 11% 32% 32% 6%	840
May	1st to 10th	Night	19% 11% 34% 29% 7%	716
	11th to 20th	Night	16% 6% 35% 36% 7%	414
	21st to end	Night	18% 5% 21% 37% 16% 2	455
June	1st to 10th	Night	15% 4 26% 40% 15%	420
	11th to 20th	Night	12% 5% 26% 45% 12%	420
	21st to end	Night	10% 4 21% 51% 14%	420
July	1st to 10th	Night	11% 2 18% 46% 22% 1	410
	11th to 20th	Night	8% 3 15% 51% 22% 1	408
	21st to end	Night	9% 3 18% 45% 24% 2	521
August	1st to 10th	Night	13% 2 13% 42% 28% 2	627
	11th to 20th	Night	14% 4 14% 40% 26% 2	819
	21st to end	Night	13% 3 17% 40% 24% 3	924
September	1st to 10th	Night	15% 3 21% 38% 20% 2	840
	11th to 20th	Night	13% 4 15% 38% 26% 4	840
	21st to end	Night	24% 5% 21% 32% 18% 1	840
October	1st to 10th	Night	20% 5% 26% 30% 16% 4	840
	11th to 20th	Night	19% 8% 26% 33% 13% 1	813
	21st to end	Night	20% 8% 32% 30% 10% 1	880
November	1st to 10th	Night	14% 7% 35% 32% 12%	820
	11th to 20th	Night	19% 10% 35% 28% 8%	840
	21st to end	Night	14% 14% 36% 28% 8%	840
December	1st to 10th	Night	22% 12% 34% 24% 7% 1	840
	11th to 20th	Night	18% 12% 35% 29% 6%	840
	21st to end	Night	21% 13% 34% 30% 2	924

Page generated at 2026-10-11 05:06:30. Version ac07

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