ACME Planetary Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ACME Planetary 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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	119861
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	118386
Transparency	Day	39% 4 11% 21% 19% 6%	123509
	Night	39% 4 10% 19% 22% 7%	122875
Seeing	Day	16% 8% 23% 25% 19% 9%	16305
	Night	16% 7% 24% 33% 18% 2	26446

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	14% 5% 2 5% 3 3 3 2 2 5% 57%	4056
	11th to 20th	Day	12% 6% 2 4 3 3 2 2 3 4 59%	4060
	21st to end	Day	14% 7% 3 7% 4 4 4 3 5% 4 45%	4466
February	1st to 10th	Day	15% 7% 3 6% 4 4 3 3 6% 7% 43%	1800
	11th to 20th	Day	21% 7% 2 5% 3 3 2 3 4 5% 44%	1793
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 5% 42%	1813
March	1st to 10th	Day	20% 7% 2 6% 4 4 4 3 5% 5% 40%	4935
	11th to 20th	Day	10% 5% 2 5% 4 3 4 3 4 7% 53%	4980
	21st to end	Day	14% 6% 2 6% 4 4 4 3 4 5% 48%	5465
April	1st to 10th	Day	10% 4 1 4 4 4 3 4 5% 9% 52%	2290
	11th to 20th	Day	11% 4 2 5% 4 4 4 3 5% 6% 51%	2374
	21st to end	Day	11% 5% 2 5% 4 5% 4 4 5% 6% 49%	2400
May	1st to 10th	Day	8% 7% 3 8% 5% 6% 5% 4 7% 11% 35%	5830
	11th to 20th	Day	18% 6% 3 7% 5% 6% 5% 5% 7% 9% 30%	5861
	21st to end	Day	12% 5% 2 5% 5% 5% 5% 4 6% 8% 44%	6433
June	1st to 10th	Day	10% 5% 3 6% 5% 5% 6% 5% 7% 10% 39%	2587
	11th to 20th	Day	8% 5% 2 4 6% 5% 5% 5% 8% 11% 41%	2470
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 11% 38%	2470
July	1st to 10th	Day	7% 6% 3 8% 6% 6% 7% 6% 9% 11% 32%	4120
	11th to 20th	Day	9% 6% 2 9% 6% 6% 6% 6% 9% 12% 29%	4120
	21st to end	Day	9% 9% 3 9% 6% 5% 6% 5% 9% 10% 28%	4466
August	1st to 10th	Day	22% 9% 3 7% 6% 5% 5% 5% 7% 9% 21%	4104
	11th to 20th	Day	10% 6% 2 8% 6% 6% 6% 5% 7% 10% 34%	3926
	21st to end	Day	6% 4 2 5% 3 4 5% 5% 7% 11% 49%	4180
September	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 5% 7% 8% 44%	2280
	11th to 20th	Day	12% 6% 2 7% 5% 5% 4 4 6% 7% 42%	2099
	21st to end	Day	18% 6% 2 7% 4 4 4 3 5% 7% 39%	2074
October	1st to 10th	Day	14% 6% 2 4 3 3 3 3 5% 6% 49%	1900
	11th to 20th	Day	11% 4 2 5% 3 3 3 2 4 5% 58%	1900
	21st to end	Day	11% 4 2 5% 3 2 3 2 4 5% 59%	2090
November	1st to 10th	Day	13% 5% 2 6% 3 2 3 3 5% 5% 54%	1957
	11th to 20th	Day	14% 4 2 4 3 3 4 3 4 6% 53%	1993
	21st to end	Day	13% 5% 2 4 4 3 3 3 5% 7% 50%	1800
December	1st to 10th	Day	12% 6% 2 6% 4 3 3 3 4 6% 52%	2930
	11th to 20th	Day	17% 4 2 6% 4 3 4 3 5% 7% 46%	3373
	21st to end	Day	17% 6% 2 6% 4 3 3 3 4 5% 46%	4466
January	1st to 10th	Night	12% 5% 2 4 3 3 3 3 4 5% 57%	5889
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 4 5% 52%	5900
	21st to end	Night	14% 8% 3 6% 3 3 4 3 4 6% 46%	6490
February	1st to 10th	Night	16% 6% 2 5% 4 4 3 4 4 6% 45%	2600
	11th to 20th	Night	19% 6% 2 5% 4 4 3 3 5% 5% 42%	2488
	21st to end	Night	14% 6% 2 5% 4 3 3 4 4 7% 47%	1815
March	1st to 10th	Night	19% 8% 3 7% 4 4 3 3 5% 5% 41%	4918
	11th to 20th	Night	11% 5% 1 4 3 3 4 3 5% 7% 53%	4980
	21st to end	Night	14% 4 2 6% 4 4 3 3 5% 5% 49%	5467
April	1st to 10th	Night	12% 6% 2 5% 3 3 4 4 5% 6% 50%	2108
	11th to 20th	Night	11% 5% 2 4 4 3 4 3 5% 6% 52%	1975
	21st to end	Night	10% 4 2 5% 4 5% 6% 4 5% 7% 50%	2000
May	1st to 10th	Night	9% 8% 3 7% 5% 5% 5% 4 6% 9% 39%	4093
	11th to 20th	Night	17% 8% 3 7% 4 4 4 4 6% 8% 35%	4032
	21st to end	Night	13% 5% 2 6% 4 4 5% 4 6% 9% 41%	4421
June	1st to 10th	Night	10% 6% 2 7% 6% 6% 4 5% 8% 10% 36%	1797
	11th to 20th	Night	11% 5% 2 7% 6% 5% 6% 4 7% 10% 39%	1710
	21st to end	Night	7% 5% 2 6% 5% 4 5% 6% 8% 11% 38%	1710
July	1st to 10th	Night	9% 7% 3 8% 6% 6% 7% 5% 8% 10% 31%	2840
	11th to 20th	Night	12% 8% 3 8% 5% 5% 7% 5% 8% 12% 28%	2840
	21st to end	Night	15% 11% 4 8% 7% 5% 5% 5% 7% 8% 25%	3087
August	1st to 10th	Night	22% 8% 3 7% 5% 5% 5% 5% 7% 9% 24%	2825
	11th to 20th	Night	9% 6% 2 7% 6% 6% 7% 5% 8% 11% 35%	2868
	21st to end	Night	6% 4 2 5% 5% 4 4 4 8% 11% 47%	3476
September	1st to 10th	Night	9% 5% 2 6% 5% 5% 4 4 5% 10% 44%	1900
	11th to 20th	Night	13% 7% 3 7% 5% 5% 4 3 6% 7% 41%	2077
	21st to end	Night	18% 7% 3 6% 5% 4 4 4 4 7% 38%	2104
October	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 4 5% 51%	2280
	11th to 20th	Night	12% 5% 2 4 3 3 2 3 4 5% 56%	2280
	21st to end	Night	11% 5% 2 4 2 3 3 3 4 5% 57%	2508
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 54%	2341
	11th to 20th	Night	13% 7% 3 4 3 3 3 3 4 7% 49%	2405
	21st to end	Night	13% 5% 1 4 3 4 3 3 5% 6% 52%	2600
December	1st to 10th	Night	13% 7% 2 4 3 3 3 3 4 5% 54%	4250
	11th to 20th	Night	16% 8% 3 8% 4 4 4 3 4 5% 41%	4822
	21st to end	Night	16% 6% 2 6% 3 4 3 2 4 6% 47%	6490

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	34% 1 3 20% 33% 9%	4057
	11th to 20th	Day	32% 2 3 17% 35% 12%	4060
	21st to end	Day	43% 3 3 18% 23% 11%	4466
February	1st to 10th	Day	41% 2 10% 21% 20% 6%	1800
	11th to 20th	Day	44% 2 6% 18% 23% 7%	1802
	21st to end	Day	44% 3 6% 19% 21% 7%	1813
March	1st to 10th	Day	48% 2 5% 16% 19% 11%	4935
	11th to 20th	Day	33% 2 7% 15% 27% 16%	4980
	21st to end	Day	39% 3 7% 22% 21% 7%	5465
April	1st to 10th	Day	30% 3 11% 24% 26% 7%	2290
	11th to 20th	Day	34% 3 10% 24% 22% 8%	2374
	21st to end	Day	36% 4 11% 25% 19% 4	2400
May	1st to 10th	Day	43% 5% 18% 23% 10% 1	5865
	11th to 20th	Day	51% 5% 12% 21% 11% 1	5865
	21st to end	Day	38% 4 13% 22% 20% 3	6447
June	1st to 10th	Day	38% 5% 16% 25% 15% 1	2587
	11th to 20th	Day	34% 6% 18% 24% 15% 2	2470
	21st to end	Day	36% 8% 16% 23% 15% 2	2470
July	1st to 10th	Day	41% 8% 19% 20% 11% 2	4055
	11th to 20th	Day	44% 9% 26% 16% 4 1	4051
	21st to end	Day	47% 10% 16% 17% 10%	4324
August	1st to 10th	Day	58% 6% 17% 12% 6% 1	4091
	11th to 20th	Day	43% 7% 16% 23% 10% 1	3965
	21st to end	Day	29% 6% 14% 34% 16% 2	4180
September	1st to 10th	Day	36% 5% 17% 18% 19% 5%	2280
	11th to 20th	Day	41% 4 12% 17% 21% 4	2099
	21st to end	Day	45% 4 8% 22% 16% 5%	2074
October	1st to 10th	Day	34% 3 7% 21% 27% 8%	3160
	11th to 20th	Day	20% 2 5% 19% 26% 28%	3140
	21st to end	Day	24% 2 4 25% 26% 18%	3428
November	1st to 10th	Day	33% 3 6% 20% 29% 9%	1957
	11th to 20th	Day	34% 2 7% 21% 28% 8%	1993
	21st to end	Day	35% 3 7% 23% 24% 9%	1800
December	1st to 10th	Day	35% 3 6% 19% 26% 11%	2930
	11th to 20th	Day	40% 3 11% 18% 21% 6%	3370
	21st to end	Day	42% 2 8% 24% 23% 2	4466
January	1st to 10th	Night	31% 2 4 17% 38% 8%	5889
	11th to 20th	Night	37% 2 4 15% 34% 9%	5900
	21st to end	Night	41% 2 4 16% 28% 8%	6490
February	1st to 10th	Night	42% 2 8% 21% 20% 7%	2600
	11th to 20th	Night	45% 2 8% 17% 22% 7%	2490
	21st to end	Night	38% 2 8% 19% 23% 9%	1815
March	1st to 10th	Night	47% 2 5% 13% 26% 8%	4918
	11th to 20th	Night	33% 2 6% 13% 27% 19%	4980
	21st to end	Night	38% 2 7% 21% 23% 7%	5467
April	1st to 10th	Night	35% 3 9% 21% 23% 9%	2108
	11th to 20th	Night	33% 3 8% 24% 22% 9%	1975
	21st to end	Night	34% 4 12% 26% 20% 5%	2000
May	1st to 10th	Night	41% 4 18% 21% 14% 2	4120
	11th to 20th	Night	48% 5% 14% 19% 13%	4033
	21st to end	Night	40% 4 11% 25% 17% 2	4429
June	1st to 10th	Night	40% 7% 17% 22% 12% 2	1797
	11th to 20th	Night	39% 7% 16% 22% 14% 2	1710
	21st to end	Night	37% 9% 18% 21% 12% 3	1710
July	1st to 10th	Night	45% 10% 18% 16% 9% 1	2795
	11th to 20th	Night	47% 10% 25% 13% 5% 1	2786
	21st to end	Night	55% 7% 14% 16% 9%	2988
August	1st to 10th	Night	57% 5% 18% 16% 4 1	2814
	11th to 20th	Night	41% 8% 22% 21% 7% 1	2895
	21st to end	Night	31% 6% 13% 30% 18% 3	3476
September	1st to 10th	Night	37% 6% 13% 21% 16% 6%	1900
	11th to 20th	Night	43% 4 13% 17% 20% 3	2077
	21st to end	Night	47% 3 11% 18% 15% 5%	2104
October	1st to 10th	Night	37% 3 6% 21% 25% 7%	3800
	11th to 20th	Night	23% 2 5% 17% 29% 25%	3779
	21st to end	Night	26% 2 5% 24% 25% 18%	4120
November	1st to 10th	Night	34% 3 6% 20% 25% 13%	2341
	11th to 20th	Night	37% 2 6% 17% 27% 12%	2405
	21st to end	Night	34% 3 6% 22% 26% 9%	2600
December	1st to 10th	Night	34% 2 6% 18% 29% 10%	4250
	11th to 20th	Night	46% 4 8% 16% 17% 9%	4824
	21st to end	Night	41% 2 8% 24% 23% 2	6490

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% 21% 36% 22% 3	300
	11th to 20th	Day	19% 22% 36% 16% 7%	300
	21st to end	Day	19% 18% 41% 17% 4	330
February	1st to 10th	Day	18% 16% 40% 20% 6%	300
	11th to 20th	Day	35% 15% 36% 13% 1	300
	21st to end	Day	23% 14% 36% 22% 6%	412
March	1st to 10th	Day	21% 15% 31% 25% 8%	500
	11th to 20th	Day	18% 15% 39% 21% 6% 1	500
	21st to end	Day	21% 12% 35% 24% 7% 2	550
April	1st to 10th	Day	17% 15% 33% 22% 11% 1	500
	11th to 20th	Day	12% 12% 34% 23% 14% 5%	492
	21st to end	Day	14% 7% 28% 30% 18% 3	500
May	1st to 10th	Day	12% 10% 35% 24% 15% 5%	503
	11th to 20th	Day	17% 4 18% 29% 24% 8%	527
	21st to end	Day	17% 3 16% 25% 23% 15%	573
June	1st to 10th	Day	13% 3 16% 31% 26% 11%	526
	11th to 20th	Day	10% 2 15% 30% 28% 15%	490
	21st to end	Day	8% 3 11% 29% 34% 16%	490
July	1st to 10th	Day	9% 2 4 25% 35% 24%	485
	11th to 20th	Day	10% 2 5% 27% 34% 22%	484
	21st to end	Day	8% 2 8% 30% 38% 16%	532
August	1st to 10th	Day	14% 3 8% 27% 27% 21%	486
	11th to 20th	Day	13% 2 6% 24% 33% 22%	479
	21st to end	Day	11% 3 8% 26% 27% 25%	539
September	1st to 10th	Day	9% 2 13% 25% 25% 26%	490
	11th to 20th	Day	17% 2 11% 26% 29% 15%	490
	21st to end	Day	21% 3 16% 30% 23% 7%	490
October	1st to 10th	Day	16% 3 21% 25% 24% 10%	490
	11th to 20th	Day	13% 8% 23% 27% 21% 6%	490
	21st to end	Day	11% 8% 24% 30% 22% 5%	539
November	1st to 10th	Day	19% 5% 25% 31% 15% 4	495
	11th to 20th	Day	17% 10% 32% 28% 11% 2	493
	21st to end	Day	15% 11% 32% 27% 11% 4	300
December	1st to 10th	Day	20% 13% 42% 19% 6%	300
	11th to 20th	Day	22% 10% 34% 22% 9% 2	300
	21st to end	Day	24% 13% 34% 20% 7% 2	330
January	1st to 10th	Night	20% 12% 35% 26% 6% 1	800
	11th to 20th	Night	17% 12% 33% 33% 6%	800
	21st to end	Night	20% 10% 37% 31% 3	880
February	1st to 10th	Night	20% 8% 38% 30% 5%	800
	11th to 20th	Night	24% 13% 38% 23% 1	783
	21st to end	Night	19% 11% 37% 28% 5%	660
March	1st to 10th	Night	18% 11% 34% 32% 5% 1	800
	11th to 20th	Night	18% 12% 36% 27% 8% 1	800
	21st to end	Night	18% 10% 36% 29% 7%	880
April	1st to 10th	Night	16% 13% 33% 31% 7%	800
	11th to 20th	Night	13% 10% 32% 33% 11% 1	791
	21st to end	Night	12% 5% 31% 40% 11%	800
May	1st to 10th	Night	12% 8% 29% 40% 11%	634
	11th to 20th	Night	15% 4 22% 41% 17% 1	591
	21st to end	Night	14% 2 12% 39% 30% 3	650
June	1st to 10th	Night	13% 2 15% 42% 26% 3	599
	11th to 20th	Night	12% 3 13% 34% 32% 7%	570
	21st to end	Night	10% 1 11% 40% 34% 4	570
July	1st to 10th	Night	8% 2 7% 39% 39% 5%	555
	11th to 20th	Night	8% 2 4 36% 43% 7%	552
	21st to end	Night	8% 4 7% 39% 38% 4	624
August	1st to 10th	Night	12% 2 5% 37% 39% 6%	567
	11th to 20th	Night	12% 2 6% 39% 34% 7%	608
	21st to end	Night	12% 1 7% 32% 41% 7%	836
September	1st to 10th	Night	11% 1 8% 32% 42% 6%	760
	11th to 20th	Night	17% 2 10% 31% 36% 4	760
	21st to end	Night	23% 3 15% 33% 25% 3	760
October	1st to 10th	Night	17% 3 16% 31% 28% 5%	760
	11th to 20th	Night	14% 5% 25% 32% 21% 3	760
	21st to end	Night	15% 5% 23% 33% 20% 3	836
November	1st to 10th	Night	16% 6% 23% 37% 15% 3	780
	11th to 20th	Night	17% 9% 28% 34% 12%	800
	21st to end	Night	16% 6% 30% 32% 15% 1	800
December	1st to 10th	Night	20% 9% 28% 36% 7%	800
	11th to 20th	Night	19% 8% 29% 34% 10%	800
	21st to end	Night	21% 9% 26% 35% 8% 1	880

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