ASKC Dark Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ASKC Dark Site clear sky chart from 2005-11-06 to 2025-07-06.

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	12% 7% 2 6% 4 4 4 3 5% 7% 45%	119715
	Night	12% 7% 2 6% 4 4 3 3 5% 6% 49%	119287
Transparency	Day	38% 3 11% 26% 18% 3	123368
	Night	36% 3 10% 26% 20% 4	123777
Seeing	Day	15% 9% 27% 27% 17% 4	16509
	Night	15% 8% 30% 33% 13% 1	26373

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	17% 9% 3 8% 5% 4 4 3 4 5% 38%	4077
	11th to 20th	Day	8% 9% 3 10% 6% 6% 5% 3 4 5% 41%	4080
	21st to end	Day	15% 12% 3 6% 3 3 4 3 4 6% 41%	4488
February	1st to 10th	Day	15% 12% 3 7% 5% 4 3 2 4 6% 39%	1800
	11th to 20th	Day	14% 10% 3 7% 4 4 4 3 4 5% 42%	1754
	21st to end	Day	13% 9% 3 6% 4 4 4 4 5% 7% 41%	1729
March	1st to 10th	Day	13% 8% 2 6% 4 3 3 2 3 5% 51%	4936
	11th to 20th	Day	10% 6% 2 4 3 3 3 3 4 7% 55%	4980
	21st to end	Day	13% 10% 3 8% 5% 5% 5% 4 5% 7% 35%	5462
April	1st to 10th	Day	14% 8% 3 6% 4 4 4 3 5% 6% 44%	2310
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 6% 7% 41%	2374
	21st to end	Day	20% 9% 3 6% 5% 4 3 3 5% 6% 35%	2505
May	1st to 10th	Day	18% 8% 2 6% 4 4 4 3 5% 7% 40%	5862
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 4 6% 8% 43%	5862
	21st to end	Day	20% 6% 2 5% 4 4 4 3 5% 7% 39%	6434
June	1st to 10th	Day	7% 4 2 5% 5% 5% 5% 5% 7% 9% 47%	2785
	11th to 20th	Day	8% 5% 1 5% 5% 4 5% 5% 6% 8% 48%	2800
	21st to end	Day	6% 4 1 4 4 3 4 4 7% 9% 55%	2702
July	1st to 10th	Day	10% 7% 3 6% 5% 5% 5% 4 6% 7% 43%	4186
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 6% 9% 52%	4120
	21st to end	Day	7% 4 2 5% 4 5% 6% 5% 9% 11% 42%	4465
August	1st to 10th	Day	5% 3 2 5% 4 4 4 4 6% 10% 52%	4104
	11th to 20th	Day	9% 6% 2 7% 4 5% 5% 3 6% 8% 46%	4041
	21st to end	Day	6% 4 1 4 4 4 4 4 6% 9% 56%	4226
September	1st to 10th	Day	12% 5% 1 5% 3 3 3 3 4 7% 54%	2280
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 6% 6% 52%	2109
	21st to end	Day	9% 4 2 4 4 4 4 3 5% 6% 55%	2074
October	1st to 10th	Day	11% 5% 2 4 3 3 4 3 5% 7% 54%	1900
	11th to 20th	Day	12% 5% 2 4 3 2 3 3 3 5% 57%	1900
	21st to end	Day	17% 7% 2 5% 3 3 3 3 3 5% 49%	2090
November	1st to 10th	Day	9% 7% 2 5% 3 3 2 3 4 6% 54%	1958
	11th to 20th	Day	16% 5% 3 4 3 3 3 3 4 7% 50%	1800
	21st to end	Day	16% 8% 2 5% 3 2 3 2 4 5% 49%	1600
December	1st to 10th	Day	7% 7% 2 6% 3 3 3 3 4 6% 55%	2610
	11th to 20th	Day	11% 7% 3 7% 4 3 3 2 3 5% 52%	3008
	21st to end	Day	20% 18% 6% 8% 4 4 3 3 4 5% 26%	4304
January	1st to 10th	Night	14% 9% 4 10% 5% 5% 3 3 4 5% 39%	5888
	11th to 20th	Night	10% 8% 3 10% 5% 7% 5% 3 4 4 43%	5900
	21st to end	Night	16% 9% 3 7% 3 3 3 3 3 5% 44%	6490
February	1st to 10th	Night	13% 11% 3 7% 4 4 4 3 5% 7% 39%	2600
	11th to 20th	Night	13% 9% 3 7% 4 3 4 4 4 6% 43%	2533
	21st to end	Night	13% 8% 3 6% 4 4 3 2 4 5% 48%	1916
March	1st to 10th	Night	15% 8% 3 5% 3 3 3 2 4 5% 49%	4919
	11th to 20th	Night	8% 6% 1 4 3 3 3 3 5% 7% 57%	4980
	21st to end	Night	13% 8% 3 6% 4 4 4 4 5% 8% 41%	5467
April	1st to 10th	Night	11% 7% 3 5% 3 4 3 3 6% 7% 49%	2090
	11th to 20th	Night	14% 8% 2 5% 4 3 3 3 4 6% 47%	1975
	21st to end	Night	16% 7% 3 7% 4 3 3 3 4 7% 41%	1890
May	1st to 10th	Night	17% 7% 2 6% 3 4 3 3 4 6% 45%	4053
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 6% 8% 47%	4053
	21st to end	Night	18% 5% 2 5% 3 3 3 3 4 6% 50%	4443
June	1st to 10th	Night	8% 3 1 4 3 3 3 4 7% 10% 53%	1609
	11th to 20th	Night	5% 4 2 5% 4 4 5% 5% 8% 8% 51%	1600
	21st to end	Night	5% 4 1 4 2 3 3 3 6% 8% 60%	1689
July	1st to 10th	Night	10% 6% 2 6% 3 4 4 4 6% 7% 49%	2895
	11th to 20th	Night	3 3 2 4 3 4 4 4 6% 9% 59%	2850
	21st to end	Night	6% 4 2 4 4 4 5% 5% 8% 11% 47%	3098
August	1st to 10th	Night	3 2 1 3 2 4 4 3 6% 9% 63%	2836
	11th to 20th	Night	9% 5% 2 6% 4 4 4 3 5% 7% 51%	2798
	21st to end	Night	4 3 1 4 3 4 4 4 6% 8% 61%	3428
September	1st to 10th	Night	10% 4 1 5% 3 3 3 3 4 7% 57%	1900
	11th to 20th	Night	9% 4 1 4 3 4 5% 3 5% 7% 53%	2066
	21st to end	Night	8% 4 2 4 4 3 3 3 5% 7% 57%	2090
October	1st to 10th	Night	9% 3 2 4 3 4 3 3 4 5% 60%	2275
	11th to 20th	Night	13% 6% 2 5% 3 2 3 2 4 5% 57%	2280
	21st to end	Night	15% 6% 2 5% 4 2 3 3 4 4 52%	2508
November	1st to 10th	Night	11% 5% 2 4 2 3 2 2 3 6% 60%	2340
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 4 6% 53%	2593
	21st to end	Night	16% 7% 2 4 3 3 3 3 3 5% 52%	2800
December	1st to 10th	Night	8% 7% 2 4 3 3 3 3 4 6% 57%	4580
	11th to 20th	Night	14% 8% 2 5% 3 3 3 3 5% 6% 47%	5199
	21st to end	Night	20% 17% 5% 8% 4 3 3 2 3 4 30%	6656

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	49% 2 10% 25% 12% 1	4078
	11th to 20th	Day	46% 4 9% 23% 17% 3	4080
	21st to end	Day	46% 2 9% 20% 17% 6%	4488
February	1st to 10th	Day	49% 4 10% 25% 9% 2	1800
	11th to 20th	Day	46% 2 12% 22% 15% 3	1763
	21st to end	Day	43% 2 8% 25% 15% 5%	1729
March	1st to 10th	Day	38% 3 7% 22% 23% 7%	4936
	11th to 20th	Day	31% 3 9% 23% 26% 9%	4980
	21st to end	Day	48% 3 16% 25% 7% 1	5464
April	1st to 10th	Day	41% 4 11% 25% 16% 3	2310
	11th to 20th	Day	43% 3 11% 26% 16% 2	2374
	21st to end	Day	49% 3 11% 25% 11% 1	2505
May	1st to 10th	Day	45% 3 11% 22% 17% 1	5900
	11th to 20th	Day	39% 4 13% 24% 17% 3	5866
	21st to end	Day	44% 4 12% 26% 13% 1	6447
June	1st to 10th	Day	32% 5% 17% 27% 17% 2	2785
	11th to 20th	Day	32% 5% 14% 31% 16% 2	2800
	21st to end	Day	25% 4 16% 33% 18% 3	2702
July	1st to 10th	Day	39% 5% 12% 23% 18% 3	4122
	11th to 20th	Day	28% 5% 12% 37% 16% 2	4051
	21st to end	Day	31% 7% 21% 29% 12%	4325
August	1st to 10th	Day	27% 5% 15% 29% 22% 2	4090
	11th to 20th	Day	36% 5% 17% 27% 14% 1	4080
	21st to end	Day	25% 5% 12% 40% 14% 4	4225
September	1st to 10th	Day	32% 3 11% 28% 18% 7%	2280
	11th to 20th	Day	32% 3 9% 27% 25% 2	2109
	21st to end	Day	31% 3 9% 27% 26% 4	2074
October	1st to 10th	Day	19% 2 6% 21% 41% 10%	3160
	11th to 20th	Day	20% 2 4 27% 39% 9%	3140
	21st to end	Day	32% 2 8% 28% 26% 4	3428
November	1st to 10th	Day	32% 3 10% 30% 21% 5%	1958
	11th to 20th	Day	36% 2 8% 26% 23% 5%	1800
	21st to end	Day	40% 2 8% 30% 18% 1	1600
December	1st to 10th	Day	32% 2 10% 39% 17%	2610
	11th to 20th	Day	37% 2 9% 33% 17% 2	3005
	21st to end	Day	63% 2 10% 18% 6% 1	4304
January	1st to 10th	Night	49% 3 12% 23% 12% 1	5888
	11th to 20th	Night	46% 3 8% 24% 16% 3	5900
	21st to end	Night	44% 3 9% 21% 17% 6%	6490
February	1st to 10th	Night	45% 4 11% 23% 15% 2	2600
	11th to 20th	Night	43% 2 11% 25% 16% 3	2534
	21st to end	Night	40% 3 7% 24% 19% 7%	1916
March	1st to 10th	Night	39% 2 8% 21% 24% 5%	4919
	11th to 20th	Night	28% 3 7% 22% 33% 7%	4980
	21st to end	Night	42% 4 15% 26% 12% 2	5467
April	1st to 10th	Night	36% 3 10% 23% 23% 5%	2090
	11th to 20th	Night	39% 2 8% 27% 20% 5%	1975
	21st to end	Night	44% 3 10% 26% 15% 1	1890
May	1st to 10th	Night	42% 3 14% 24% 14% 2	4080
	11th to 20th	Night	37% 3 13% 26% 15% 7%	4054
	21st to end	Night	37% 4 10% 28% 20% 2	4452
June	1st to 10th	Night	26% 4 18% 30% 18% 3	1609
	11th to 20th	Night	26% 6% 16% 29% 21% 2	1600
	21st to end	Night	22% 4 14% 33% 23% 4	1689
July	1st to 10th	Night	35% 4 12% 25% 20% 3	2850
	11th to 20th	Night	21% 4 12% 37% 22% 2	2796
	21st to end	Night	27% 5% 20% 31% 15% 1	2999
August	1st to 10th	Night	18% 4 18% 33% 20% 7%	2826
	11th to 20th	Night	33% 5% 12% 29% 20% 1	2825
	21st to end	Night	21% 4 13% 40% 19% 4	3428
September	1st to 10th	Night	28% 4 10% 28% 21% 9%	1900
	11th to 20th	Night	30% 4 9% 26% 26% 5%	2066
	21st to end	Night	27% 3 8% 27% 30% 5%	2090
October	1st to 10th	Night	16% 2 5% 20% 42% 14%	3795
	11th to 20th	Night	23% 3 4 23% 32% 15%	3779
	21st to end	Night	31% 3 6% 31% 25% 5%	4120
November	1st to 10th	Night	28% 2 11% 29% 24% 6%	2340
	11th to 20th	Night	34% 2 6% 27% 26% 5%	2593
	21st to end	Night	37% 3 8% 28% 21% 3	2800
December	1st to 10th	Night	30% 3 13% 37% 17% 1	4580
	11th to 20th	Night	39% 2 10% 28% 19% 2	5201
	21st to end	Night	60% 2 8% 21% 7% 1	6656

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% 12% 38% 24% 4	300
	11th to 20th	Day	27% 14% 38% 19% 2	300
	21st to end	Day	17% 13% 44% 22% 4	330
February	1st to 10th	Day	23% 19% 37% 17% 3	300
	11th to 20th	Day	20% 20% 42% 13% 4 1	300
	21st to end	Day	21% 22% 41% 12% 3	249
March	1st to 10th	Day	20% 17% 41% 19% 3	300
	11th to 20th	Day	26% 15% 33% 21% 5% 1	300
	21st to end	Day	28% 18% 39% 12% 2 1	330
April	1st to 10th	Day	20% 19% 35% 19% 6% 1	410
	11th to 20th	Day	21% 16% 34% 22% 6%	492
	21st to end	Day	24% 12% 34% 23% 7%	605
May	1st to 10th	Day	21% 12% 35% 22% 9% 1	703
	11th to 20th	Day	19% 9% 29% 27% 12% 3	725
	21st to end	Day	17% 7% 29% 27% 16% 5%	789
June	1st to 10th	Day	8% 4 18% 35% 26% 9%	730
	11th to 20th	Day	8% 5% 25% 35% 23% 4	730
	21st to end	Day	6% 4 32% 35% 20% 3	730
July	1st to 10th	Day	10% 3 13% 31% 32% 11%	697
	11th to 20th	Day	4 1 20% 35% 32% 7%	668
	21st to end	Day	8% 2 12% 32% 35% 11%	739
August	1st to 10th	Day	8% 4 12% 38% 31% 7%	676
	11th to 20th	Day	8% 2 17% 36% 31% 6%	665
	21st to end	Day	9% 2 11% 30% 36% 12%	572
September	1st to 10th	Day	12% 2 13% 33% 31% 7%	490
	11th to 20th	Day	7% 1 20% 37% 28% 7%	319
	21st to end	Day	13% 5% 26% 34% 16% 7%	300
October	1st to 10th	Day	12% 8% 25% 29% 18% 8%	300
	11th to 20th	Day	12% 12% 39% 28% 8% 1	300
	21st to end	Day	24% 15% 33% 22% 6%	330
November	1st to 10th	Day	17% 9% 37% 23% 10% 3	300
	11th to 20th	Day	19% 16% 29% 24% 10% 1	300
	21st to end	Day	24% 15% 33% 21% 7% 1	300
December	1st to 10th	Day	13% 13% 43% 22% 6% 4	300
	11th to 20th	Day	19% 16% 36% 23% 6%	300
	21st to end	Day	26% 20% 37% 16% 2	330
January	1st to 10th	Night	15% 12% 38% 29% 6% 1	1000
	11th to 20th	Night	17% 13% 39% 27% 4 1	1000
	21st to end	Night	15% 13% 39% 30% 4	1100
February	1st to 10th	Night	19% 13% 40% 25% 3	1000
	11th to 20th	Night	19% 14% 37% 27% 2	979
	21st to end	Night	20% 12% 39% 25% 3	596
March	1st to 10th	Night	18% 12% 40% 27% 4	600
	11th to 20th	Night	21% 11% 35% 27% 6% 1	600
	21st to end	Night	22% 12% 37% 25% 3	660
April	1st to 10th	Night	15% 12% 38% 29% 6%	600
	11th to 20th	Night	18% 14% 33% 27% 7%	593
	21st to end	Night	19% 8% 39% 26% 9%	600
May	1st to 10th	Night	18% 6% 32% 36% 8%	600
	11th to 20th	Night	16% 5% 29% 33% 16% 1	591
	21st to end	Night	15% 6% 27% 32% 18% 2	650
June	1st to 10th	Night	7% 4 17% 43% 28% 1	600
	11th to 20th	Night	5% 2 24% 49% 19% 2	600
	21st to end	Night	5% 3 26% 47% 18% 1	600
July	1st to 10th	Night	9% 3 11% 40% 35% 3	570
	11th to 20th	Night	4 1 14% 45% 33% 3	552
	21st to end	Night	8% 2 12% 39% 38% 2	624
August	1st to 10th	Night	6% 2 13% 45% 33% 2	567
	11th to 20th	Night	8% 1 12% 48% 28% 3	564
	21st to end	Night	7% 1 10% 41% 38% 3	627
September	1st to 10th	Night	13% 1 14% 43% 25% 4	570
	11th to 20th	Night	10% 2 18% 38% 27% 5%	570
	21st to end	Night	11% 3 21% 40% 21% 4	570
October	1st to 10th	Night	10% 4 25% 38% 22% 2	755
	11th to 20th	Night	15% 6% 38% 31% 9%	760
	21st to end	Night	20% 11% 32% 29% 8%	836
November	1st to 10th	Night	13% 9% 34% 32% 12% 1	780
	11th to 20th	Night	16% 11% 35% 29% 9%	959
	21st to end	Night	20% 11% 35% 28% 7%	1000
December	1st to 10th	Night	16% 10% 38% 29% 6% 1	1000
	11th to 20th	Night	21% 10% 34% 30% 5%	1000
	21st to end	Night	20% 14% 35% 27% 4	1100

Page generated at 2025-07-03 00:30:15. Version ac07

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