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 2024-11-01.

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 4 4 3 5% 7% 50%	101235
	Night	11% 5% 2 4 3 3 3 3 5% 6% 55%	97813
Transparency	Day	34% 6% 11% 25% 19% 4	101049
	Night	30% 5% 11% 26% 22% 6%	97670
Seeing	Day	13% 9% 26% 29% 19% 5%	15414
	Night	13% 8% 30% 34% 14% 1	25238

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	11% 11% 5% 5% 3 3 4 3 4 5% 46%	2847
	11th to 20th	Day	8% 6% 2 5% 2 3 3 3 4 6% 58%	2850
	21st to end	Day	21% 10% 3 5% 3 2 2 2 2 5% 46%	3135
February	1st to 10th	Day	13% 11% 4 6% 4 3 4 3 4 5% 43%	1710
	11th to 20th	Day	14% 7% 3 6% 3 3 4 3 5% 6% 46%	1820
	21st to end	Day	15% 8% 2 5% 4 4 4 3 4 5% 46%	1599
March	1st to 10th	Day	15% 7% 2 5% 2 3 3 2 3 5% 54%	3436
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 5% 7% 52%	3480
	21st to end	Day	16% 9% 3 7% 5% 4 4 3 5% 7% 36%	3814
April	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 6% 6% 48%	2090
	11th to 20th	Day	15% 5% 2 6% 4 4 3 3 6% 6% 46%	2246
	21st to end	Day	18% 7% 3 6% 4 3 4 3 4 5% 43%	2441
May	1st to 10th	Day	22% 7% 2 6% 4 3 3 3 4 5% 40%	4082
	11th to 20th	Day	12% 7% 2 7% 5% 4 4 3 6% 7% 44%	4082
	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	11% 4 2 4 2 2 3 3 5% 6% 59%	1858
	11th to 20th	Day	10% 5% 1 3 3 2 3 2 5% 6% 61%	1720
	21st to end	Day	15% 5% 2 3 3 2 2 3 5% 6% 54%	1710
December	1st to 10th	Day	13% 6% 2 5% 4 3 3 2 5% 5% 52%	1710
	11th to 20th	Day	15% 7% 3 6% 4 3 3 2 4 6% 47%	2161
	21st to end	Day	17% 9% 4 7% 4 5% 4 3 5% 6% 35%	3135
January	1st to 10th	Night	12% 8% 3 6% 3 3 4 3 4 4 49%	4108
	11th to 20th	Night	8% 5% 2 5% 3 5% 3 3 5% 6% 56%	4120
	21st to end	Night	22% 7% 2 4 2 2 2 2 4 4 48%	4532
February	1st to 10th	Night	15% 8% 2 6% 3 3 3 3 4 5% 48%	2470
	11th to 20th	Night	14% 7% 3 6% 3 2 3 2 3 5% 51%	2262
	21st to end	Night	14% 8% 2 4 3 4 4 2 3 5% 51%	1853
March	1st to 10th	Night	14% 7% 2 4 3 3 3 3 4 5% 51%	3419
	11th to 20th	Night	11% 4 2 3 3 3 3 3 4 6% 59%	3480
	21st to end	Night	15% 5% 2 5% 4 4 3 3 6% 7% 47%	3817
April	1st to 10th	Night	11% 5% 1 3 4 3 3 3 6% 6% 54%	2090
	11th to 20th	Night	14% 4 2 5% 3 3 4 3 5% 5% 52%	1880
	21st to end	Night	15% 6% 2 5% 4 3 3 2 4 6% 49%	1734
May	1st to 10th	Night	18% 6% 2 5% 3 3 3 2 4 7% 45%	2823
	11th to 20th	Night	15% 6% 3 6% 4 3 4 3 6% 6% 46%	2823
	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	10% 5% 1 3 2 2 2 3 3 4 65%	2227
	11th to 20th	Night	10% 5% 2 4 3 3 3 2 4 5% 60%	2455
	21st to end	Night	13% 6% 2 4 3 3 3 2 4 5% 55%	2470
December	1st to 10th	Night	12% 5% 3 4 3 3 4 3 4 7% 53%	2470
	11th to 20th	Night	12% 6% 3 6% 4 3 3 3 4 4 51%	3036
	21st to end	Night	17% 8% 2 5% 3 3 3 2 4 6% 46%	4532

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	41% 4 8% 23% 22% 2	2848
	11th to 20th	Day	29% 3 15% 28% 21% 5%	2850
	21st to end	Day	44% 2 7% 20% 24% 3	3135
February	1st to 10th	Day	44% 4 10% 22% 17% 4	1710
	11th to 20th	Day	39% 4 8% 25% 19% 4	1830
	21st to end	Day	41% 4 7% 21% 20% 8%	1599
March	1st to 10th	Day	33% 4 6% 22% 23% 11%	3436
	11th to 20th	Day	33% 5% 13% 25% 20% 4	3480
	21st to end	Day	45% 7% 11% 25% 11% 2	3814
April	1st to 10th	Day	36% 6% 10% 23% 18% 7%	2090
	11th to 20th	Day	39% 5% 11% 22% 16% 6%	2246
	21st to end	Day	43% 5% 10% 21% 19% 2	2441
May	1st to 10th	Day	46% 6% 9% 20% 16% 4	4120
	11th to 20th	Day	38% 7% 13% 23% 19% 1	4086
	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	27% 5% 11% 21% 27% 8%	1977
	11th to 20th	Day	25% 3 6% 21% 33% 11%	1900
	21st to end	Day	34% 3 8% 26% 20% 9%	2090
November	1st to 10th	Day	27% 3 8% 27% 26% 9%	1858
	11th to 20th	Day	26% 3 10% 28% 25% 8%	1720
	21st to end	Day	32% 4 8% 24% 25% 7%	1710
December	1st to 10th	Day	35% 4 10% 27% 22% 2	1710
	11th to 20th	Day	39% 4 7% 21% 25% 4	2159
	21st to end	Day	46% 6% 10% 22% 13% 3	3135
January	1st to 10th	Night	37% 5% 7% 25% 22% 4	4108
	11th to 20th	Night	27% 5% 17% 21% 25% 3	4120
	21st to end	Night	41% 3 8% 21% 23% 4	4532
February	1st to 10th	Night	39% 4 10% 23% 19% 5%	2470
	11th to 20th	Night	38% 3 6% 26% 21% 6%	2262
	21st to end	Night	38% 4 7% 19% 23% 10%	1853
March	1st to 10th	Night	36% 5% 7% 19% 20% 12%	3419
	11th to 20th	Night	27% 5% 10% 32% 20% 6%	3480
	21st to end	Night	35% 6% 11% 31% 15% 2	3817
April	1st to 10th	Night	30% 6% 10% 21% 23% 10%	2090
	11th to 20th	Night	33% 5% 11% 24% 18% 9%	1880
	21st to end	Night	38% 4 9% 24% 22% 3	1734
May	1st to 10th	Night	38% 7% 11% 20% 20% 5%	2850
	11th to 20th	Night	38% 6% 13% 24% 16% 2	2824
	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	23% 4 11% 22% 28% 12%	2182
	11th to 20th	Night	26% 4 6% 18% 32% 13%	2280
	21st to end	Night	30% 3 7% 23% 26% 10%	2508
November	1st to 10th	Night	24% 4 8% 23% 31% 10%	2227
	11th to 20th	Night	29% 3 7% 24% 27% 10%	2455
	21st to end	Night	33% 3 7% 20% 29% 8%	2470
December	1st to 10th	Night	32% 4 11% 24% 22% 6%	2470
	11th to 20th	Night	36% 4 7% 21% 26% 6%	3037
	21st to end	Night	39% 5% 9% 22% 20% 5%	4532

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% 16% 31% 26% 7%	270
	11th to 20th	Day	21% 20% 38% 17% 4	270
	21st to end	Day	16% 15% 40% 20% 8%	297
February	1st to 10th	Day	25% 15% 41% 16% 3	270
	11th to 20th	Day	20% 20% 40% 17% 2	270
	21st to end	Day	27% 24% 36% 12% 2	225
March	1st to 10th	Day	22% 19% 34% 21% 4	270
	11th to 20th	Day	21% 21% 35% 17% 5%	270
	21st to end	Day	20% 25% 36% 13% 5%	297
April	1st to 10th	Day	16% 24% 34% 19% 7%	270
	11th to 20th	Day	20% 18% 38% 20% 4	444
	21st to end	Day	20% 17% 36% 18% 8% 1	621
May	1st to 10th	Day	15% 13% 37% 28% 6% 1	653
	11th to 20th	Day	15% 11% 29% 29% 13% 2	675
	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% 14% 30% 24% 10% 2	330
November	1st to 10th	Day	14% 10% 38% 23% 12% 2	270
	11th to 20th	Day	12% 14% 37% 24% 11% 2	270
	21st to end	Day	23% 14% 36% 20% 7%	270
December	1st to 10th	Day	16% 14% 39% 21% 9% 1	270
	11th to 20th	Day	21% 14% 39% 20% 6%	270
	21st to end	Day	17% 19% 38% 23% 4	297
January	1st to 10th	Night	14% 13% 37% 29% 6%	950
	11th to 20th	Night	16% 14% 37% 28% 4	950
	21st to end	Night	15% 12% 37% 31% 5%	1045
February	1st to 10th	Night	20% 11% 38% 27% 4	950
	11th to 20th	Night	18% 16% 36% 27% 3	788
	21st to end	Night	18% 16% 38% 24% 3	597
March	1st to 10th	Night	16% 12% 36% 32% 4	570
	11th to 20th	Night	19% 12% 36% 27% 5%	570
	21st to end	Night	17% 14% 40% 24% 5%	627
April	1st to 10th	Night	13% 13% 43% 27% 4	570
	11th to 20th	Night	17% 11% 38% 28% 6%	563
	21st to end	Night	17% 11% 39% 24% 8% 1	570
May	1st to 10th	Night	12% 11% 39% 33% 6%	570
	11th to 20th	Night	13% 7% 32% 32% 15% 1	561
	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	11% 6% 38% 31% 13% 1	743
	11th to 20th	Night	12% 12% 34% 32% 9%	935
	21st to end	Night	16% 12% 35% 27% 9%	950
December	1st to 10th	Night	16% 12% 34% 32% 6%	950
	11th to 20th	Night	19% 9% 38% 28% 5%	950
	21st to end	Night	18% 12% 36% 28% 6%	1045

Page generated at 2024-10-29 10:20:48. Version ac07

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