ACT Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ACT Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	10% 6% 2 5% 4 4 4 3 5% 7% 50%	149044
	Night	10% 5% 2 4 3 3 3 3 5% 7% 55%	142438
Transparency	Day	32% 5% 11% 25% 21% 5%	152709
	Night	29% 5% 10% 26% 24% 7%	146840
Seeing	Day	13% 10% 26% 29% 18% 4	17998
	Night	13% 9% 31% 33% 14% 1	27730

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	10% 9% 4 6% 3 3 3 3 4 6% 49%	5289
	11th to 20th	Day	6% 5% 2 5% 3 3 3 2 3 4 64%	5310
	21st to end	Day	16% 9% 3 5% 3 2 2 2 3 5% 51%	5812
February	1st to 10th	Day	13% 11% 4 6% 4 3 3 3 4 5% 44%	1890
	11th to 20th	Day	14% 7% 2 5% 3 4 4 3 4 6% 46%	2018
	21st to end	Day	14% 8% 2 4 4 3 3 3 4 6% 48%	1763
March	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 3 5% 54%	5046
	11th to 20th	Day	8% 6% 2 5% 4 4 3 3 5% 7% 55%	5090
	21st to end	Day	13% 8% 3 7% 5% 4 4 3 6% 8% 39%	5583
April	1st to 10th	Day	12% 6% 2 6% 5% 4 4 3 6% 6% 47%	2310
	11th to 20th	Day	15% 5% 2 6% 4 4 3 3 6% 6% 46%	2486
	21st to end	Day	18% 7% 3 7% 4 3 4 3 4 5% 42%	2699
May	1st to 10th	Day	17% 7% 3 6% 4 4 4 3 4 6% 43%	7617
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 4 5% 7% 44%	7627
	21st to end	Day	19% 7% 3 8% 5% 4 5% 4 6% 7% 33%	8392
June	1st to 10th	Day	7% 4 2 6% 5% 4 5% 4 6% 9% 47%	2730
	11th to 20th	Day	8% 4 2 5% 5% 5% 5% 4 6% 8% 48%	2730
	21st to end	Day	5% 3 1 4 4 4 4 4 6% 8% 56%	2730
July	1st to 10th	Day	5% 4 2 5% 4 5% 5% 6% 8% 10% 46%	7680
	11th to 20th	Day	5% 4 1 6% 4 4 5% 4 6% 10% 49%	7680
	21st to end	Day	3 3 2 4 3 4 4 4 7% 11% 56%	8337
August	1st to 10th	Day	3 3 1 4 3 3 4 4 6% 8% 63%	6014
	11th to 20th	Day	5% 2 1 3 3 3 4 4 6% 9% 60%	5948
	21st to end	Day	4 3 2 4 3 4 4 3 6% 8% 59%	6269
September	1st to 10th	Day	9% 5% 2 5% 4 3 4 3 5% 6% 54%	2334
	11th to 20th	Day	12% 4 2 5% 3 4 5% 4 6% 7% 48%	2233
	21st to end	Day	8% 4 2 5% 3 3 3 3 4 8% 56%	2200
October	1st to 10th	Day	9% 3 2 4 3 3 3 2 5% 7% 59%	2081
	11th to 20th	Day	11% 4 1 4 2 2 2 3 3 5% 64%	2000
	21st to end	Day	16% 6% 2 4 3 2 3 2 2 5% 56%	2200
November	1st to 10th	Day	12% 4 2 3 2 2 3 3 4 6% 60%	2058
	11th to 20th	Day	10% 4 1 3 3 2 3 2 5% 5% 61%	1900
	21st to end	Day	14% 6% 2 3 3 2 2 3 5% 6% 54%	1890
December	1st to 10th	Day	9% 8% 2 5% 3 3 3 2 4 5% 56%	3030
	11th to 20th	Day	13% 6% 2 5% 3 3 3 2 4 6% 53%	3481
	21st to end	Day	22% 9% 3 6% 4 4 3 2 4 5% 38%	4587
January	1st to 10th	Night	13% 7% 2 6% 3 3 3 3 5% 6% 49%	7634
	11th to 20th	Night	5% 4 1 4 2 4 3 3 4 5% 66%	7680
	21st to end	Night	18% 7% 3 5% 3 3 3 2 4 4 47%	8402
February	1st to 10th	Night	14% 8% 2 6% 3 3 3 3 4 5% 49%	2730
	11th to 20th	Night	14% 7% 3 6% 3 2 3 2 4 5% 49%	2506
	21st to end	Night	13% 7% 2 4 3 4 4 2 3 5% 53%	2041
March	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 3 4 52%	5029
	11th to 20th	Night	8% 4 1 3 2 3 3 3 4 6% 63%	5090
	21st to end	Night	13% 5% 2 5% 3 4 3 3 6% 8% 48%	5588
April	1st to 10th	Night	12% 5% 1 3 4 3 3 3 5% 7% 53%	2310
	11th to 20th	Night	14% 4 1 5% 3 3 3 3 5% 6% 52%	2080
	21st to end	Night	15% 6% 2 5% 4 3 3 3 4 6% 48%	1916
May	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 5% 7% 45%	5266
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 6% 7% 49%	5265
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 8% 42%	5793
June	1st to 10th	Night	7% 4 2 4 3 3 4 4 5% 8% 56%	1890
	11th to 20th	Night	6% 4 1 4 4 3 3 5% 7% 9% 56%	1890
	21st to end	Night	3 3 1 3 3 3 4 4 5% 8% 64%	1890
July	1st to 10th	Night	4 4 1 4 3 4 5% 4 8% 11% 51%	5308
	11th to 20th	Night	3 3 1 5% 4 4 4 3 6% 10% 57%	5307
	21st to end	Night	3 2 1 3 2 2 3 3 5% 10% 65%	5770
August	1st to 10th	Night	2 2 1 3 2 2 3 4 6% 8% 68%	4156
	11th to 20th	Night	3 2 1 4 3 4 4 3 5% 7% 63%	4118
	21st to end	Night	3 2 1 3 2 3 4 3 6% 8% 65%	4864
September	1st to 10th	Night	8% 4 2 4 3 3 3 3 4 7% 59%	2282
	11th to 20th	Night	11% 4 2 6% 4 4 3 3 6% 7% 51%	2233
	21st to end	Night	8% 4 1 3 4 4 4 3 4 6% 59%	2200
October	1st to 10th	Night	7% 2 1 4 3 3 4 3 4 6% 63%	2297
	11th to 20th	Night	11% 5% 1 3 3 3 3 2 4 4 61%	2400
	21st to end	Night	15% 5% 2 4 2 2 2 2 4 4 58%	2640
November	1st to 10th	Night	11% 5% 1 3 2 2 2 3 3 4 64%	2460
	11th to 20th	Night	10% 4 2 4 3 3 3 2 3 5% 61%	2714
	21st to end	Night	13% 6% 2 4 3 2 3 2 5% 5% 55%	2730
December	1st to 10th	Night	8% 7% 2 5% 3 3 3 2 3 6% 58%	4380
	11th to 20th	Night	14% 5% 3 5% 3 3 2 2 3 4 55%	4946
	21st to end	Night	19% 9% 2 5% 3 2 3 2 4 5% 46%	6633

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	38% 3 9% 27% 20% 3	5290
	11th to 20th	Day	26% 2 9% 26% 31% 6%	5310
	21st to end	Day	39% 2 5% 22% 26% 5%	5812
February	1st to 10th	Day	44% 4 9% 23% 17% 4	1890
	11th to 20th	Day	40% 4 9% 25% 18% 5%	2028
	21st to end	Day	38% 4 7% 21% 22% 9%	1763
March	1st to 10th	Day	33% 4 5% 20% 18% 19%	5046
	11th to 20th	Day	29% 4 10% 20% 20% 16%	5090
	21st to end	Day	41% 6% 11% 28% 12% 1	5585
April	1st to 10th	Day	36% 6% 10% 23% 19% 6%	2310
	11th to 20th	Day	38% 5% 11% 21% 18% 6%	2486
	21st to end	Day	45% 5% 10% 20% 18% 2	2699
May	1st to 10th	Day	42% 6% 10% 21% 18% 4	7655
	11th to 20th	Day	37% 6% 11% 21% 20% 4	7631
	21st to end	Day	48% 9% 12% 15% 13% 3	8405
June	1st to 10th	Day	32% 8% 15% 26% 16% 3	2730
	11th to 20th	Day	31% 7% 14% 27% 17% 3	2730
	21st to end	Day	25% 7% 15% 29% 21% 4	2730
July	1st to 10th	Day	28% 9% 17% 30% 15% 2	7616
	11th to 20th	Day	27% 8% 16% 25% 20% 3	7611
	21st to end	Day	20% 7% 17% 39% 16% 1	8155
August	1st to 10th	Day	18% 5% 12% 25% 31% 8%	6000
	11th to 20th	Day	20% 6% 16% 32% 23% 3	5987
	21st to end	Day	23% 6% 16% 36% 16% 3	6268
September	1st to 10th	Day	30% 5% 10% 23% 23% 9%	2334
	11th to 20th	Day	33% 7% 11% 23% 21% 5%	2233
	21st to end	Day	27% 5% 8% 23% 29% 7%	2200
October	1st to 10th	Day	17% 3 7% 14% 42% 16%	3393
	11th to 20th	Day	16% 2 4 21% 40% 17%	3240
	21st to end	Day	27% 3 7% 28% 28% 7%	3538
November	1st to 10th	Day	27% 3 8% 26% 27% 10%	2058
	11th to 20th	Day	26% 2 9% 28% 25% 9%	1900
	21st to end	Day	32% 4 8% 26% 24% 6%	1890
December	1st to 10th	Day	32% 3 7% 35% 21% 2	3030
	11th to 20th	Day	34% 3 6% 24% 28% 5%	3479
	21st to end	Day	48% 4 8% 24% 13% 3	4587
January	1st to 10th	Night	36% 3 9% 29% 20% 2	7635
	11th to 20th	Night	20% 4 11% 21% 36% 8%	7680
	21st to end	Night	41% 3 7% 19% 26% 5%	8402
February	1st to 10th	Night	38% 3 10% 25% 19% 5%	2730
	11th to 20th	Night	39% 3 7% 25% 20% 6%	2506
	21st to end	Night	35% 4 7% 19% 25% 11%	2041
March	1st to 10th	Night	37% 4 6% 18% 21% 14%	5029
	11th to 20th	Night	23% 4 8% 24% 23% 19%	5090
	21st to end	Night	33% 6% 12% 30% 17% 2	5588
April	1st to 10th	Night	31% 6% 10% 22% 23% 9%	2310
	11th to 20th	Night	33% 5% 11% 23% 18% 9%	2080
	21st to end	Night	38% 4 9% 24% 22% 3	1916
May	1st to 10th	Night	38% 6% 11% 20% 21% 4	5293
	11th to 20th	Night	33% 6% 12% 22% 20% 6%	5266
	21st to end	Night	38% 9% 15% 22% 13% 4	5805
June	1st to 10th	Night	25% 7% 17% 30% 19% 3	1890
	11th to 20th	Night	23% 7% 15% 32% 20% 3	1890
	21st to end	Night	18% 5% 14% 30% 25% 7%	1890
July	1st to 10th	Night	23% 8% 17% 33% 17% 3	5265
	11th to 20th	Night	21% 8% 15% 31% 21% 4	5256
	21st to end	Night	15% 5% 14% 44% 21% 1	5641
August	1st to 10th	Night	14% 4 12% 28% 33% 9%	4146
	11th to 20th	Night	21% 5% 12% 33% 25% 4	4145
	21st to end	Night	17% 5% 12% 40% 21% 5%	4864
September	1st to 10th	Night	25% 4 10% 26% 25% 11%	2282
	11th to 20th	Night	31% 5% 11% 22% 23% 7%	2233
	21st to end	Night	26% 5% 8% 23% 30% 8%	2200
October	1st to 10th	Night	14% 2 7% 14% 37% 26%	3752
	11th to 20th	Night	19% 3 5% 17% 38% 18%	3899
	21st to end	Night	25% 4 7% 26% 29% 9%	4252
November	1st to 10th	Night	25% 3 8% 22% 31% 10%	2460
	11th to 20th	Night	28% 3 8% 25% 26% 10%	2714
	21st to end	Night	32% 3 8% 22% 27% 8%	2730
December	1st to 10th	Night	29% 4 9% 35% 20% 3	4380
	11th to 20th	Night	34% 3 5% 23% 26% 8%	4947
	21st to end	Night	41% 4 8% 24% 18% 5%	6633

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	20% 15% 35% 24% 6%	330
	11th to 20th	Day	21% 18% 38% 20% 3	330
	21st to end	Day	17% 15% 42% 20% 7%	363
February	1st to 10th	Day	24% 15% 42% 16% 3	330
	11th to 20th	Day	21% 20% 42% 15% 2	330
	21st to end	Day	22% 23% 37% 14% 3	273
March	1st to 10th	Day	25% 19% 33% 19% 4	330
	11th to 20th	Day	22% 22% 37% 16% 4	330
	21st to end	Day	19% 24% 38% 14% 5%	363
April	1st to 10th	Day	18% 23% 32% 18% 8%	330
	11th to 20th	Day	21% 19% 39% 18% 3	524
	21st to end	Day	21% 18% 34% 17% 8% 1	719
May	1st to 10th	Day	16% 12% 37% 28% 6% 1	753
	11th to 20th	Day	16% 13% 30% 27% 12% 2	775
	21st to end	Day	18% 9% 28% 26% 14% 5%	844
June	1st to 10th	Day	7% 6% 23% 35% 23% 6%	780
	11th to 20th	Day	9% 5% 24% 38% 21% 4	780
	21st to end	Day	4 3 22% 43% 23% 5%	780
July	1st to 10th	Day	6% 2 14% 34% 30% 13%	771
	11th to 20th	Day	3 2 14% 38% 35% 8%	768
	21st to end	Day	6% 2 9% 35% 37% 11%	849
August	1st to 10th	Day	5% 2 12% 44% 32% 6%	776
	11th to 20th	Day	8% 3 11% 37% 35% 6%	765
	21st to end	Day	6% 3 9% 34% 38% 9%	716
September	1st to 10th	Day	6% 2 13% 31% 34% 14%	384
	11th to 20th	Day	6% 1 14% 41% 29% 9%	339
	21st to end	Day	11% 5% 17% 36% 23% 8%	330
October	1st to 10th	Day	8% 4 27% 34% 19% 7%	330
	11th to 20th	Day	9% 12% 35% 33% 11% 1	330
	21st to end	Day	22% 14% 29% 24% 9% 1	363
November	1st to 10th	Day	16% 10% 39% 23% 10% 2	330
	11th to 20th	Day	12% 13% 38% 23% 11% 2	330
	21st to end	Day	22% 15% 36% 20% 7% 1	330
December	1st to 10th	Day	16% 16% 39% 19% 8% 1	330
	11th to 20th	Day	19% 16% 40% 20% 5%	330
	21st to end	Day	17% 20% 37% 21% 5%	363
January	1st to 10th	Night	15% 13% 38% 28% 6%	1050
	11th to 20th	Night	15% 15% 39% 27% 4	1050
	21st to end	Night	15% 12% 38% 30% 5%	1155
February	1st to 10th	Night	19% 11% 39% 27% 3	1050
	11th to 20th	Night	19% 18% 34% 26% 3	872
	21st to end	Night	17% 15% 40% 25% 3	657
March	1st to 10th	Night	17% 13% 35% 31% 4	630
	11th to 20th	Night	19% 13% 37% 26% 5%	630
	21st to end	Night	17% 14% 41% 24% 4	693
April	1st to 10th	Night	15% 12% 42% 27% 4	630
	11th to 20th	Night	17% 13% 38% 27% 5%	623
	21st to end	Night	18% 11% 38% 24% 7% 1	630
May	1st to 10th	Night	11% 11% 38% 33% 6%	630
	11th to 20th	Night	13% 8% 32% 32% 15%	621
	21st to end	Night	10% 6% 30% 36% 18% 1	683
June	1st to 10th	Night	7% 4 20% 40% 26% 2	630
	11th to 20th	Night	5% 3 25% 48% 18%	630
	21st to end	Night	3 2 23% 51% 19% 2	630
July	1st to 10th	Night	5% 1 12% 45% 34% 3	615
	11th to 20th	Night	3 1 14% 45% 34% 3	612
	21st to end	Night	6% 8% 43% 40% 3	690
August	1st to 10th	Night	3 1 13% 49% 33% 2	627
	11th to 20th	Night	5% 12% 46% 32% 5%	624
	21st to end	Night	3 2 11% 43% 39% 3	693
September	1st to 10th	Night	10% 2 12% 40% 36% 1	630
	11th to 20th	Night	13% 1 16% 41% 25% 4	609
	21st to end	Night	9% 3 20% 39% 26% 3	600
October	1st to 10th	Night	7% 2 28% 38% 22% 2	697
	11th to 20th	Night	12% 7% 36% 34% 11%	800
	21st to end	Night	18% 10% 29% 31% 11% 1	880
November	1st to 10th	Night	12% 7% 38% 31% 12%	820
	11th to 20th	Night	12% 11% 35% 32% 9%	1034
	21st to end	Night	16% 12% 36% 26% 9%	1050
December	1st to 10th	Night	15% 12% 36% 31% 6%	1050
	11th to 20th	Night	19% 9% 38% 28% 5%	1050
	21st to end	Night	18% 13% 35% 28% 6%	1155

Page generated at 2026-09-11 03:11:51. Version ac07

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