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 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	15% 6% 2 6% 4 4 4 4 5% 7% 41%	119493
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	118691
Transparency	Day	40% 6% 12% 23% 17% 3	123067
	Night	40% 5% 11% 23% 18% 4	123278
Seeing	Day	18% 13% 26% 25% 14% 5%	15751
	Night	18% 9% 31% 31% 11% 1	25111

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 5% 4 4 4 3 5% 5% 49%	3597
	11th to 20th	Day	14% 8% 3 9% 5% 4 3 2 3 5% 43%	3705
	21st to end	Day	18% 8% 3 5% 4 4 4 3 4 5% 44%	4466
February	1st to 10th	Day	15% 8% 2 7% 4 4 5% 4 6% 6% 40%	1800
	11th to 20th	Day	17% 8% 2 5% 5% 4 4 4 5% 6% 40%	1913
	21st to end	Day	17% 10% 3 6% 4 4 3 3 4 6% 39%	1650
March	1st to 10th	Day	20% 5% 2 5% 3 3 3 3 4 6% 47%	4476
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 4 5% 6% 47%	4979
	21st to end	Day	17% 5% 2 6% 4 4 4 4 5% 6% 43%	5786
April	1st to 10th	Day	17% 7% 2 5% 4 4 4 4 5% 6% 41%	2400
	11th to 20th	Day	14% 6% 2 5% 5% 4 4 4 6% 7% 43%	2374
	21st to end	Day	15% 6% 2 5% 5% 4 5% 4 6% 7% 41%	2400
May	1st to 10th	Day	25% 8% 3 6% 4 4 4 4 5% 8% 30%	5744
	11th to 20th	Day	23% 8% 3 7% 4 4 4 3 5% 7% 33%	6318
	21st to end	Day	19% 5% 2 6% 4 4 4 3 6% 8% 38%	6949
June	1st to 10th	Day	12% 5% 2 7% 5% 4 5% 4 7% 8% 40%	2786
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 8% 39%	2660
	21st to end	Day	10% 4 2 6% 5% 4 5% 4 7% 10% 43%	2660
July	1st to 10th	Day	7% 5% 2 6% 6% 6% 7% 5% 8% 12% 36%	4440
	11th to 20th	Day	10% 6% 2 6% 5% 4 6% 5% 7% 11% 39%	4440
	21st to end	Day	10% 8% 3 7% 6% 6% 6% 5% 7% 9% 32%	4706
August	1st to 10th	Day	17% 6% 2 7% 5% 5% 6% 5% 6% 8% 34%	4083
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 42%	3730
	21st to end	Day	9% 5% 2 5% 4 4 4 4 7% 9% 47%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 5% 6% 45%	2280
	11th to 20th	Day	12% 4 2 4 4 4 4 4 4 7% 50%	2247
	21st to end	Day	18% 7% 2 7% 5% 4 4 3 4 6% 39%	2090
October	1st to 10th	Day	18% 5% 2 5% 4 3 5% 3 5% 5% 45%	1909
	11th to 20th	Day	15% 5% 2 4 3 3 4 3 5% 6% 50%	1900
	21st to end	Day	17% 5% 2 5% 4 3 4 3 3 6% 48%	2028
November	1st to 10th	Day	13% 5% 2 4 4 3 3 2 5% 6% 53%	1761
	11th to 20th	Day	15% 6% 2 4 4 3 4 5% 5% 6% 47%	1800
	21st to end	Day	15% 5% 1 5% 4 3 4 3 5% 6% 48%	1690
December	1st to 10th	Day	14% 5% 2 5% 3 4 4 4 6% 7% 46%	2600
	11th to 20th	Day	19% 10% 2 5% 3 3 3 3 3 5% 43%	2986
	21st to end	Day	15% 6% 2 6% 3 4 5% 4 7% 7% 41%	3960
January	1st to 10th	Night	14% 7% 2 5% 3 3 4 3 5% 6% 49%	6349
	11th to 20th	Night	15% 6% 2 6% 4 3 3 2 3 5% 50%	6177
	21st to end	Night	17% 7% 2 7% 4 3 4 3 5% 5% 43%	6490
February	1st to 10th	Night	16% 8% 3 6% 3 4 4 4 5% 7% 40%	2600
	11th to 20th	Night	19% 8% 3 5% 4 3 3 3 4 6% 41%	2371
	21st to end	Night	17% 7% 3 5% 3 5% 3 4 5% 5% 42%	1980
March	1st to 10th	Night	18% 8% 2 6% 3 3 3 2 4 6% 45%	5349
	11th to 20th	Night	10% 8% 2 5% 4 4 3 3 4 5% 51%	4979
	21st to end	Night	16% 7% 2 6% 4 4 4 3 4 5% 44%	5131
April	1st to 10th	Night	17% 7% 3 7% 4 4 3 4 5% 6% 40%	2000
	11th to 20th	Night	15% 6% 2 7% 4 3 4 4 5% 7% 42%	1975
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 6% 44%	2000
May	1st to 10th	Night	19% 9% 3 8% 5% 4 5% 4 6% 8% 29%	4226
	11th to 20th	Night	22% 8% 3 6% 4 4 4 4 6% 8% 32%	3575
	21st to end	Night	21% 6% 3 6% 4 3 4 3 5% 8% 37%	3926
June	1st to 10th	Night	15% 5% 2 5% 4 4 4 4 6% 8% 43%	1598
	11th to 20th	Night	8% 7% 3 7% 5% 5% 5% 3 7% 9% 41%	1520
	21st to end	Night	9% 5% 2 5% 4 5% 5% 4 8% 10% 42%	1520
July	1st to 10th	Night	7% 6% 3 7% 5% 6% 6% 5% 7% 10% 37%	2520
	11th to 20th	Night	10% 6% 2 6% 5% 5% 4 5% 7% 10% 38%	2520
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 4 7% 7% 36%	2830
August	1st to 10th	Night	20% 6% 3 6% 4 4 5% 4 6% 8% 33%	2825
	11th to 20th	Night	12% 7% 2 7% 4 5% 4 3 5% 7% 43%	3087
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 8% 49%	3476
September	1st to 10th	Night	13% 6% 2 7% 4 4 4 3 5% 7% 46%	1900
	11th to 20th	Night	10% 5% 2 5% 4 2 3 3 5% 7% 55%	1933
	21st to end	Night	19% 7% 2 6% 4 3 3 3 5% 7% 40%	2090
October	1st to 10th	Night	18% 7% 2 5% 4 3 3 3 4 6% 44%	2269
	11th to 20th	Night	15% 4 1 5% 3 4 3 2 5% 6% 51%	2280
	21st to end	Night	17% 6% 2 5% 3 2 4 3 5% 6% 48%	2569
November	1st to 10th	Night	11% 5% 1 5% 4 3 3 3 5% 5% 53%	2537
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 3 5% 49%	2600
	21st to end	Night	12% 6% 1 5% 5% 4 5% 4 6% 6% 46%	2705
December	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 6% 49%	4580
	11th to 20th	Night	22% 7% 2 5% 3 3 3 3 5% 7% 41%	5208
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 6% 40%	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	36% 4 9% 25% 23% 3	3598
	11th to 20th	Day	44% 3 8% 23% 17% 5%	3735
	21st to end	Day	43% 4 13% 24% 14% 3	4466
February	1st to 10th	Day	43% 6% 14% 24% 12% 2	1800
	11th to 20th	Day	43% 5% 9% 24% 15% 3	1922
	21st to end	Day	46% 5% 9% 22% 13% 5%	1650
March	1st to 10th	Day	39% 5% 8% 25% 17% 7%	4479
	11th to 20th	Day	35% 5% 9% 28% 16% 6%	4979
	21st to end	Day	41% 4 9% 17% 23% 5%	5786
April	1st to 10th	Day	42% 6% 11% 20% 16% 6%	2400
	11th to 20th	Day	40% 6% 12% 22% 16% 5%	2374
	21st to end	Day	40% 6% 11% 19% 20% 4	2400
May	1st to 10th	Day	51% 6% 14% 17% 10% 2	5753
	11th to 20th	Day	50% 6% 10% 20% 12% 1	6322
	21st to end	Day	42% 7% 11% 24% 15% 1	6949
June	1st to 10th	Day	38% 8% 13% 23% 16% 2	2786
	11th to 20th	Day	38% 10% 13% 24% 13% 3	2660
	21st to end	Day	34% 10% 15% 23% 17% 1	2660
July	1st to 10th	Day	37% 11% 18% 18% 14% 2	4370
	11th to 20th	Day	37% 8% 17% 23% 13% 1	4365
	21st to end	Day	45% 9% 12% 19% 14% 1	4556
August	1st to 10th	Day	45% 8% 15% 22% 9% 1	4070
	11th to 20th	Day	37% 7% 14% 28% 13% 1	3766
	21st to end	Day	32% 7% 13% 27% 20% 2	4180
September	1st to 10th	Day	39% 7% 11% 19% 21% 3	2280
	11th to 20th	Day	33% 5% 11% 22% 25% 4	2247
	21st to end	Day	46% 5% 8% 18% 20% 2	2090
October	1st to 10th	Day	35% 5% 10% 17% 28% 5%	3169
	11th to 20th	Day	25% 4 10% 23% 29% 9%	3140
	21st to end	Day	25% 2 12% 27% 28% 5%	3318
November	1st to 10th	Day	33% 4 12% 26% 21% 4	1761
	11th to 20th	Day	36% 5% 10% 24% 22% 3	1800
	21st to end	Day	38% 4 12% 30% 14% 3	1690
December	1st to 10th	Day	37% 5% 17% 25% 14% 2	2600
	11th to 20th	Day	44% 3 11% 29% 10% 3	2986
	21st to end	Day	39% 6% 13% 22% 18% 2	3960
January	1st to 10th	Night	36% 4 8% 24% 22% 6%	6349
	11th to 20th	Night	38% 3 9% 24% 18% 7%	6222
	21st to end	Night	42% 4 9% 25% 18% 2	6490
February	1st to 10th	Night	43% 5% 13% 24% 13% 2	2600
	11th to 20th	Night	44% 4 9% 21% 17% 5%	2373
	21st to end	Night	43% 5% 9% 21% 16% 6%	1980
March	1st to 10th	Night	41% 4 9% 24% 13% 9%	5359
	11th to 20th	Night	34% 5% 13% 26% 16% 5%	4980
	21st to end	Night	42% 5% 8% 16% 24% 4	5131
April	1st to 10th	Night	44% 5% 9% 20% 18% 4	2000
	11th to 20th	Night	41% 5% 10% 19% 19% 5%	1975
	21st to end	Night	40% 6% 9% 20% 20% 5%	2000
May	1st to 10th	Night	52% 8% 11% 16% 11% 2	4233
	11th to 20th	Night	50% 8% 11% 19% 12% 1	3576
	21st to end	Night	45% 7% 12% 20% 14% 2	3927
June	1st to 10th	Night	38% 7% 11% 22% 21% 1	1598
	11th to 20th	Night	38% 9% 13% 25% 14% 2	1520
	21st to end	Night	34% 9% 15% 25% 15% 3	1520
July	1st to 10th	Night	40% 10% 19% 17% 12% 2	2480
	11th to 20th	Night	38% 9% 19% 23% 9% 1	2472
	21st to end	Night	44% 8% 14% 19% 14% 1	2740
August	1st to 10th	Night	47% 8% 18% 17% 9%	2814
	11th to 20th	Night	39% 6% 13% 27% 14% 1	3117
	21st to end	Night	33% 6% 11% 29% 20% 2	3476
September	1st to 10th	Night	38% 7% 12% 19% 21% 4	1900
	11th to 20th	Night	30% 6% 10% 24% 26% 4	1933
	21st to end	Night	44% 6% 8% 21% 19% 2	2090
October	1st to 10th	Night	40% 5% 10% 21% 21% 3	3789
	11th to 20th	Night	24% 4 10% 26% 30% 6%	3779
	21st to end	Night	25% 3 11% 31% 24% 5%	4229
November	1st to 10th	Night	33% 5% 11% 25% 20% 6%	2537
	11th to 20th	Night	39% 4 9% 22% 21% 5%	2600
	21st to end	Night	37% 5% 12% 26% 18% 2	2705
December	1st to 10th	Night	37% 4 12% 30% 15% 2	4580
	11th to 20th	Night	44% 3 11% 23% 15% 4	5208
	21st to end	Night	43% 5% 12% 20% 17% 2	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	22% 28% 26% 19% 5% 1	200
	11th to 20th	Day	19% 35% 28% 16% 2	229
	21st to end	Day	22% 23% 30% 18% 5% 2	330
February	1st to 10th	Day	22% 25% 31% 17% 5%	300
	11th to 20th	Day	27% 24% 34% 13% 2	461
	21st to end	Day	23% 19% 35% 19% 4 1	414
March	1st to 10th	Day	19% 24% 31% 20% 6%	500
	11th to 20th	Day	18% 23% 32% 19% 6% 1	500
	21st to end	Day	23% 18% 31% 22% 5% 1	550
April	1st to 10th	Day	22% 20% 33% 17% 7% 1	500
	11th to 20th	Day	18% 20% 27% 22% 10% 2	492
	21st to end	Day	20% 16% 31% 23% 8% 1	500
May	1st to 10th	Day	27% 17% 26% 17% 10% 2	503
	11th to 20th	Day	21% 13% 26% 25% 13% 2	527
	21st to end	Day	19% 8% 20% 27% 18% 8%	573
June	1st to 10th	Day	13% 9% 22% 30% 20% 5%	526
	11th to 20th	Day	14% 8% 24% 32% 18% 4	490
	21st to end	Day	11% 7% 25% 32% 18% 7%	490
July	1st to 10th	Day	12% 4 19% 31% 25% 10%	485
	11th to 20th	Day	7% 2 18% 36% 27% 10%	484
	21st to end	Day	12% 3 19% 33% 25% 8%	532
August	1st to 10th	Day	11% 5% 19% 31% 25% 10%	486
	11th to 20th	Day	13% 3 14% 32% 27% 10%	479
	21st to end	Day	11% 4 23% 29% 20% 13%	539
September	1st to 10th	Day	16% 8% 18% 24% 23% 11%	490
	11th to 20th	Day	13% 5% 22% 28% 20% 12%	490
	21st to end	Day	23% 9% 22% 27% 12% 6%	490
October	1st to 10th	Day	22% 6% 22% 26% 15% 9%	490
	11th to 20th	Day	16% 14% 24% 25% 17% 4	490
	21st to end	Day	18% 10% 32% 25% 12% 3	506
November	1st to 10th	Day	14% 10% 33% 26% 15% 2	395
	11th to 20th	Day	15% 15% 35% 25% 9%	400
	21st to end	Day	17% 17% 29% 27% 9% 1	290
December	1st to 10th	Day	22% 20% 34% 17% 8%	200
	11th to 20th	Day	28% 19% 32% 15% 7%	200
	21st to end	Day	24% 20% 26% 19% 10% 1	220
January	1st to 10th	Night	19% 15% 40% 23% 4	800
	11th to 20th	Night	22% 14% 36% 25% 3	800
	21st to end	Night	18% 13% 41% 26% 3	880
February	1st to 10th	Night	21% 11% 40% 26% 2	800
	11th to 20th	Night	25% 15% 39% 19% 1	783
	21st to end	Night	22% 14% 41% 21% 3	660
March	1st to 10th	Night	19% 14% 37% 25% 5%	800
	11th to 20th	Night	18% 14% 40% 22% 6% 1	800
	21st to end	Night	20% 8% 42% 24% 5%	880
April	1st to 10th	Night	21% 15% 39% 21% 5%	800
	11th to 20th	Night	17% 12% 36% 30% 5%	791
	21st to end	Night	20% 11% 32% 31% 7%	800
May	1st to 10th	Night	19% 10% 33% 30% 7%	682
	11th to 20th	Night	17% 6% 35% 35% 7%	394
	21st to end	Night	17% 4 21% 39% 17% 2	433
June	1st to 10th	Night	16% 3 26% 40% 15%	400
	11th to 20th	Night	11% 4 24% 47% 13% 1	380
	21st to end	Night	9% 3 21% 53% 13%	380
July	1st to 10th	Night	10% 2 20% 45% 22% 1	370
	11th to 20th	Night	8% 2 14% 52% 23% 1	368
	21st to end	Night	9% 3 18% 45% 23% 2	471
August	1st to 10th	Night	12% 1 14% 43% 28% 2	567
	11th to 20th	Night	14% 3 14% 40% 26% 2	740
	21st to end	Night	14% 3 16% 39% 26% 3	836
September	1st to 10th	Night	15% 3 21% 38% 21% 2	760
	11th to 20th	Night	13% 4 15% 37% 27% 4	760
	21st to end	Night	23% 4 21% 32% 19% 1	760
October	1st to 10th	Night	21% 4 26% 29% 16% 5%	760
	11th to 20th	Night	18% 7% 27% 34% 13% 1	760
	21st to end	Night	21% 8% 31% 30% 10% 1	836
November	1st to 10th	Night	13% 7% 35% 32% 12% 1	780
	11th to 20th	Night	19% 9% 35% 29% 9% 1	800
	21st to end	Night	14% 13% 37% 28% 8% 1	800
December	1st to 10th	Night	23% 12% 34% 24% 7% 1	800
	11th to 20th	Night	18% 11% 36% 30% 6%	800
	21st to end	Night	21% 12% 34% 31% 2	880

Page generated at 2025-06-07 10:28:20. Version ac07

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