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 2025-05-11.

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	11% 6% 2 5% 4 3 4 3 5% 7% 50%	114550
	Night	11% 5% 2 4 3 3 3 3 4 6% 55%	114614
Transparency	Day	33% 5% 11% 25% 21% 6%	118256
	Night	29% 4 10% 25% 24% 8%	119037
Seeing	Day	13% 10% 26% 29% 18% 5%	16037
	Night	13% 9% 31% 33% 14% 1	26022

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	12% 11% 5% 6% 3 3 3 3 3 5% 46%	4077
	11th to 20th	Day	7% 5% 2 6% 3 3 3 3 3 5% 60%	4080
	21st to end	Day	18% 9% 2 4 3 2 2 2 2 4 50%	4488
February	1st to 10th	Day	14% 11% 4 7% 4 3 4 3 4 5% 43%	1800
	11th to 20th	Day	15% 8% 3 6% 3 4 4 3 5% 6% 45%	1919
	21st to end	Day	15% 8% 2 5% 4 3 3 3 4 6% 47%	1681
March	1st to 10th	Day	13% 6% 2 5% 2 3 3 2 3 5% 55%	4936
	11th to 20th	Day	8% 6% 2 5% 4 4 3 3 5% 7% 55%	4980
	21st to end	Day	14% 8% 3 7% 5% 4 4 3 5% 8% 39%	5462
April	1st to 10th	Day	12% 6% 2 5% 5% 3 4 3 5% 6% 48%	2200
	11th to 20th	Day	16% 5% 2 6% 4 4 3 3 6% 5% 46%	2366
	21st to end	Day	18% 7% 3 6% 4 3 4 3 4 5% 43%	2570
May	1st to 10th	Day	22% 7% 2 6% 4 3 3 3 4 5% 40%	4212
	11th to 20th	Day	12% 7% 2 7% 5% 4 4 3 6% 7% 44%	4083
	21st to end	Day	17% 6% 2 7% 5% 4 5% 4 6% 8% 36%	4477
June	1st to 10th	Day	7% 4 2 6% 5% 4 5% 4 7% 9% 48%	2470
	11th to 20th	Day	7% 4 2 5% 5% 5% 5% 4 6% 8% 49%	2470
	21st to end	Day	5% 3 1 4 4 4 4 4 6% 8% 57%	2470
July	1st to 10th	Day	8% 4 2 5% 4 5% 5% 5% 7% 9% 46%	4120
	11th to 20th	Day	5% 4 1 4 4 4 4 3 5% 9% 56%	4120
	21st to end	Day	6% 4 2 4 4 4 4 4 7% 10% 51%	4465
August	1st to 10th	Day	4 3 2 4 3 3 4 3 6% 9% 58%	4104
	11th to 20th	Day	8% 3 1 4 3 4 5% 4 7% 10% 51%	4041
	21st to end	Day	4 3 1 4 4 4 4 3 5% 9% 58%	4313
September	1st to 10th	Day	10% 5% 2 5% 4 3 4 3 5% 6% 54%	2112
	11th to 20th	Day	13% 4 2 5% 3 3 4 3 6% 6% 49%	2090
	21st to end	Day	8% 4 2 5% 3 3 3 3 4 8% 56%	2090
October	1st to 10th	Day	9% 3 2 4 3 3 3 2 5% 7% 58%	1977
	11th to 20th	Day	11% 4 1 4 3 2 2 3 2 5% 64%	1900
	21st to end	Day	15% 5% 2 4 3 2 3 2 2 5% 57%	2090
November	1st to 10th	Day	13% 4 2 4 2 2 3 3 5% 6% 58%	1958
	11th to 20th	Day	10% 5% 1 3 3 2 2 2 5% 6% 61%	1810
	21st to end	Day	14% 5% 2 3 3 2 2 3 5% 6% 55%	1800
December	1st to 10th	Day	10% 8% 2 5% 3 3 2 2 4 5% 56%	2940
	11th to 20th	Day	13% 7% 2 5% 3 3 3 2 4 6% 53%	3391
	21st to end	Day	22% 10% 3 6% 4 4 3 2 4 5% 37%	4488
January	1st to 10th	Night	14% 7% 2 7% 3 3 3 3 5% 5% 48%	5888
	11th to 20th	Night	6% 4 1 4 3 4 3 3 4 5% 61%	5900
	21st to end	Night	19% 7% 3 4 3 2 2 2 4 5% 49%	6490
February	1st to 10th	Night	15% 8% 2 6% 3 3 3 3 4 5% 48%	2600
	11th to 20th	Night	14% 7% 3 7% 4 3 3 2 3 5% 49%	2384
	21st to end	Night	14% 8% 2 4 3 4 4 2 3 5% 52%	1947
March	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 3 4 52%	4919
	11th to 20th	Night	8% 4 1 3 2 3 3 3 4 6% 63%	4980
	21st to end	Night	13% 5% 2 5% 3 4 3 3 6% 8% 48%	5467
April	1st to 10th	Night	12% 6% 1 3 4 3 3 3 5% 7% 53%	2200
	11th to 20th	Night	14% 4 2 5% 3 3 3 3 4 5% 53%	1980
	21st to end	Night	15% 6% 2 5% 4 3 3 3 4 6% 49%	1825
May	1st to 10th	Night	18% 6% 2 5% 3 3 3 2 4 7% 46%	2913
	11th to 20th	Night	15% 6% 3 6% 4 3 4 3 6% 6% 46%	2828
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 9% 48%	3090
June	1st to 10th	Night	5% 3 2 4 3 4 4 4 6% 8% 58%	1710
	11th to 20th	Night	5% 4 1 3 4 3 3 5% 7% 9% 58%	1710
	21st to end	Night	3 3 1 3 2 3 4 4 5% 8% 65%	1710
July	1st to 10th	Night	6% 6% 2 4 4 4 4 4 6% 9% 53%	2850
	11th to 20th	Night	2 2 1 4 3 4 3 3 4 9% 66%	2850
	21st to end	Night	6% 3 2 4 2 3 3 3 5% 8% 62%	3098
August	1st to 10th	Night	2 2 1 3 2 3 4 4 6% 9% 65%	2836
	11th to 20th	Night	5% 3 1 5% 4 4 4 4 6% 8% 57%	2798
	21st to end	Night	3 2 1 3 2 3 3 3 5% 9% 65%	3346
September	1st to 10th	Night	9% 4 2 4 3 3 3 3 4 7% 60%	2064
	11th to 20th	Night	11% 4 2 6% 4 4 3 3 6% 7% 52%	2090
	21st to end	Night	8% 4 1 3 4 4 3 3 4 6% 60%	2090
October	1st to 10th	Night	7% 3 1 5% 3 3 4 3 4 6% 62%	2182
	11th to 20th	Night	11% 5% 1 3 3 2 3 2 4 4 62%	2280
	21st to end	Night	14% 5% 2 4 2 2 2 3 4 4 58%	2508
November	1st to 10th	Night	12% 5% 1 3 2 2 2 3 3 4 63%	2340
	11th to 20th	Night	10% 5% 2 4 3 3 3 2 4 5% 60%	2585
	21st to end	Night	12% 6% 2 4 3 3 3 2 4 5% 56%	2600
December	1st to 10th	Night	8% 7% 2 5% 3 3 3 2 3 6% 58%	4250
	11th to 20th	Night	15% 5% 3 5% 3 3 2 2 3 4 55%	4816
	21st to end	Night	19% 10% 2 5% 3 2 3 2 4 5% 46%	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	42% 3 10% 25% 19% 2	4078
	11th to 20th	Day	27% 3 11% 25% 29% 5%	4080
	21st to end	Day	41% 2 6% 20% 27% 5%	4488
February	1st to 10th	Day	45% 4 9% 21% 17% 4	1800
	11th to 20th	Day	40% 4 8% 25% 18% 4	1929
	21st to end	Day	39% 4 6% 21% 20% 9%	1681
March	1st to 10th	Day	33% 4 5% 20% 19% 19%	4936
	11th to 20th	Day	30% 4 10% 20% 20% 17%	4980
	21st to end	Day	42% 6% 11% 28% 12% 1	5464
April	1st to 10th	Day	36% 6% 10% 23% 19% 6%	2200
	11th to 20th	Day	38% 5% 11% 22% 18% 6%	2366
	21st to end	Day	44% 5% 10% 21% 19% 2	2570
May	1st to 10th	Day	46% 6% 9% 20% 16% 3	4250
	11th to 20th	Day	38% 7% 13% 23% 19% 1	4087
	21st to end	Day	43% 10% 13% 19% 11% 4	4489
June	1st to 10th	Day	31% 8% 16% 27% 15% 3	2470
	11th to 20th	Day	30% 7% 14% 28% 17% 3	2470
	21st to end	Day	24% 7% 14% 29% 22% 4	2470
July	1st to 10th	Day	31% 8% 16% 26% 16% 3	4056
	11th to 20th	Day	24% 5% 11% 32% 25% 2	4051
	21st to end	Day	24% 7% 18% 34% 16% 1	4325
August	1st to 10th	Day	22% 5% 13% 28% 25% 6%	4090
	11th to 20th	Day	27% 7% 20% 27% 16% 2	4080
	21st to end	Day	23% 7% 14% 40% 13% 3	4312
September	1st to 10th	Day	30% 4 10% 23% 23% 9%	2112
	11th to 20th	Day	34% 6% 10% 22% 22% 5%	2090
	21st to end	Day	27% 5% 8% 23% 29% 7%	2090
October	1st to 10th	Day	17% 3 7% 14% 42% 16%	3289
	11th to 20th	Day	16% 2 4 21% 40% 17%	3140
	21st to end	Day	27% 3 7% 28% 28% 7%	3428
November	1st to 10th	Day	28% 3 8% 26% 25% 10%	1958
	11th to 20th	Day	26% 2 9% 28% 24% 10%	1810
	21st to end	Day	31% 3 8% 26% 25% 7%	1800
December	1st to 10th	Day	32% 3 7% 35% 21% 2	2940
	11th to 20th	Day	34% 3 5% 24% 28% 6%	3389
	21st to end	Day	49% 4 8% 23% 12% 3	4488
January	1st to 10th	Night	38% 4 9% 29% 17% 3	5888
	11th to 20th	Night	23% 4 13% 20% 33% 7%	5900
	21st to end	Night	39% 3 7% 18% 29% 4	6490
February	1st to 10th	Night	39% 3 10% 24% 19% 5%	2600
	11th to 20th	Night	40% 3 7% 25% 20% 6%	2384
	21st to end	Night	37% 4 7% 18% 25% 10%	1947
March	1st to 10th	Night	36% 4 6% 18% 21% 14%	4919
	11th to 20th	Night	23% 4 8% 24% 23% 19%	4980
	21st to end	Night	33% 6% 12% 30% 17% 2	5467
April	1st to 10th	Night	30% 6% 10% 21% 23% 10%	2200
	11th to 20th	Night	33% 5% 11% 24% 19% 9%	1980
	21st to end	Night	38% 4 9% 24% 22% 3	1825
May	1st to 10th	Night	38% 7% 11% 20% 20% 5%	2940
	11th to 20th	Night	38% 6% 13% 24% 16% 2	2829
	21st to end	Night	31% 9% 15% 24% 17% 4	3099
June	1st to 10th	Night	23% 7% 17% 31% 19% 3	1710
	11th to 20th	Night	21% 7% 15% 33% 20% 4	1710
	21st to end	Night	17% 5% 13% 31% 26% 7%	1710
July	1st to 10th	Night	27% 7% 15% 29% 17% 4	2805
	11th to 20th	Night	16% 5% 10% 39% 27% 3	2796
	21st to end	Night	20% 6% 15% 37% 21% 2	2999
August	1st to 10th	Night	15% 5% 14% 30% 27% 8%	2826
	11th to 20th	Night	25% 6% 15% 29% 21% 4	2825
	21st to end	Night	17% 5% 11% 43% 19% 5%	3346
September	1st to 10th	Night	26% 3 10% 25% 24% 11%	2064
	11th to 20th	Night	31% 5% 11% 21% 24% 8%	2090
	21st to end	Night	25% 5% 8% 24% 31% 8%	2090
October	1st to 10th	Night	14% 2 7% 14% 37% 26%	3637
	11th to 20th	Night	19% 3 5% 17% 38% 19%	3779
	21st to end	Night	25% 4 7% 27% 28% 9%	4120
November	1st to 10th	Night	26% 4 8% 22% 30% 10%	2340
	11th to 20th	Night	28% 3 7% 23% 27% 11%	2585
	21st to end	Night	32% 3 8% 22% 28% 8%	2600
December	1st to 10th	Night	29% 4 9% 35% 20% 3	4250
	11th to 20th	Night	35% 3 5% 23% 26% 8%	4817
	21st to end	Night	42% 4 8% 24% 18% 5%	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	21% 15% 32% 25% 7%	300
	11th to 20th	Day	21% 18% 38% 19% 4	300
	21st to end	Day	16% 16% 42% 20% 7%	330
February	1st to 10th	Day	26% 15% 40% 16% 3	300
	11th to 20th	Day	21% 21% 41% 16% 2	300
	21st to end	Day	24% 22% 36% 14% 3	249
March	1st to 10th	Day	23% 18% 33% 21% 4	300
	11th to 20th	Day	22% 22% 36% 16% 4	300
	21st to end	Day	20% 25% 36% 14% 5%	330
April	1st to 10th	Day	18% 24% 32% 18% 7%	300
	11th to 20th	Day	21% 18% 39% 19% 3	484
	21st to end	Day	20% 17% 34% 19% 8% 1	670
May	1st to 10th	Day	15% 13% 36% 29% 6% 1	703
	11th to 20th	Day	15% 11% 29% 29% 13% 2	676
	21st to end	Day	18% 7% 28% 26% 14% 6%	734
June	1st to 10th	Day	6% 5% 22% 36% 25% 6%	680
	11th to 20th	Day	7% 3 23% 41% 22% 4	680
	21st to end	Day	4 2 20% 43% 25% 5%	680
July	1st to 10th	Day	7% 2 13% 32% 31% 14%	671
	11th to 20th	Day	3 1 14% 37% 35% 9%	668
	21st to end	Day	6% 2 8% 33% 38% 13%	739
August	1st to 10th	Day	5% 2 11% 43% 34% 6%	676
	11th to 20th	Day	9% 3 12% 35% 34% 6%	665
	21st to end	Day	5% 2 7% 34% 41% 11%	620
September	1st to 10th	Day	7% 1 12% 31% 34% 16%	322
	11th to 20th	Day	7% 1 14% 41% 29% 9%	300
	21st to end	Day	11% 5% 16% 37% 23% 7%	300
October	1st to 10th	Day	9% 4 29% 31% 19% 8%	300
	11th to 20th	Day	9% 12% 35% 33% 10% 1	300
	21st to end	Day	21% 15% 29% 24% 10% 2	330
November	1st to 10th	Day	17% 10% 38% 22% 11% 2	300
	11th to 20th	Day	13% 14% 36% 23% 12% 2	300
	21st to end	Day	21% 15% 36% 20% 7%	300
December	1st to 10th	Day	16% 16% 38% 20% 8% 1	300
	11th to 20th	Day	20% 16% 39% 20% 5%	300
	21st to end	Day	18% 20% 36% 22% 4	330
January	1st to 10th	Night	15% 13% 38% 28% 6%	1000
	11th to 20th	Night	16% 14% 38% 28% 4	1000
	21st to end	Night	15% 12% 38% 31% 5%	1100
February	1st to 10th	Night	20% 12% 38% 27% 4	1000
	11th to 20th	Night	19% 17% 34% 26% 3	830
	21st to end	Night	17% 16% 39% 25% 3	627
March	1st to 10th	Night	17% 13% 35% 32% 4	600
	11th to 20th	Night	19% 13% 37% 26% 5%	600
	21st to end	Night	17% 14% 40% 24% 5%	660
April	1st to 10th	Night	14% 12% 43% 27% 4	600
	11th to 20th	Night	17% 12% 39% 28% 5%	593
	21st to end	Night	17% 11% 38% 25% 8% 1	600
May	1st to 10th	Night	12% 10% 38% 34% 6%	600
	11th to 20th	Night	13% 7% 32% 33% 15% 1	563
	21st to end	Night	8% 6% 31% 35% 18% 1	617
June	1st to 10th	Night	6% 4 19% 42% 28% 2	570
	11th to 20th	Night	5% 3 25% 49% 18%	570
	21st to end	Night	3 2 21% 52% 20% 2	570
July	1st to 10th	Night	5% 1 11% 45% 34% 4	555
	11th to 20th	Night	3 1 14% 45% 34% 3	552
	21st to end	Night	7% 7% 41% 41% 3	624
August	1st to 10th	Night	3 1 12% 48% 34% 2	567
	11th to 20th	Night	6% 1 11% 46% 32% 5%	564
	21st to end	Night	2 2 10% 42% 41% 4	627
September	1st to 10th	Night	10% 1 12% 39% 36% 1	570
	11th to 20th	Night	14% 1 17% 40% 25% 4	570
	21st to end	Night	9% 2 20% 40% 26% 3	570
October	1st to 10th	Night	8% 2 28% 37% 22% 3	662
	11th to 20th	Night	13% 7% 36% 34% 11%	760
	21st to end	Night	17% 10% 29% 32% 11% 1	836
November	1st to 10th	Night	12% 6% 38% 32% 12% 1	780
	11th to 20th	Night	12% 12% 34% 32% 9%	985
	21st to end	Night	15% 12% 37% 27% 9%	1000
December	1st to 10th	Night	15% 12% 35% 32% 6%	1000
	11th to 20th	Night	19% 9% 38% 28% 5%	1000
	21st to end	Night	19% 13% 35% 28% 6%	1100

Page generated at 2025-05-08 08:33:59. Version ac07

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