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 2026-10-29.

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% 7% 2 6% 4 4 4 3 5% 7% 46%	148805
	Night	11% 7% 2 6% 4 4 4 3 5% 7% 49%	143628
Transparency	Day	37% 4 12% 27% 17% 3	152416
	Night	35% 3 11% 27% 19% 4	148097
Seeing	Day	15% 9% 28% 27% 16% 4	18342
	Night	15% 8% 31% 33% 13% 1	28089

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	15% 9% 3 8% 5% 4 4 3 4 5% 41%	5289
	11th to 20th	Day	6% 8% 3 10% 6% 6% 4 3 4 6% 44%	5310
	21st to end	Day	14% 11% 4 6% 3 3 3 3 4 5% 43%	5812
February	1st to 10th	Day	14% 11% 3 7% 5% 4 3 2 4 6% 40%	1890
	11th to 20th	Day	14% 9% 3 7% 4 4 4 3 4 5% 43%	1844
	21st to end	Day	13% 8% 3 6% 4 4 4 3 5% 7% 42%	1814
March	1st to 10th	Day	13% 8% 2 6% 3 3 3 2 3 5% 50%	5046
	11th to 20th	Day	10% 6% 2 4 3 3 3 3 4 7% 55%	5090
	21st to end	Day	13% 10% 3 8% 5% 5% 5% 4 5% 7% 36%	5583
April	1st to 10th	Day	14% 8% 2 6% 4 4 4 3 5% 6% 44%	2425
	11th to 20th	Day	15% 7% 2 6% 5% 4 4 3 6% 6% 41%	2494
	21st to end	Day	20% 9% 3 6% 5% 4 4 3 5% 6% 35%	2630
May	1st to 10th	Day	17% 7% 2 6% 4 4 4 3 5% 7% 42%	7617
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 6% 8% 44%	7627
	21st to end	Day	21% 6% 3 6% 4 4 4 3 5% 7% 36%	8392
June	1st to 10th	Day	7% 4 2 5% 5% 5% 5% 5% 7% 9% 46%	2924
	11th to 20th	Day	8% 5% 1 5% 5% 4 5% 5% 6% 8% 48%	2940
	21st to end	Day	6% 4 1 4 4 4 4 4 7% 9% 54%	2837
July	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 6% 9% 46%	7680
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 6% 9% 50%	7680
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 10% 40%	8337
August	1st to 10th	Day	6% 4 2 5% 4 4 4 4 6% 9% 52%	6014
	11th to 20th	Day	6% 5% 2 6% 4 4 4 4 7% 10% 48%	5948
	21st to end	Day	6% 4 1 4 4 4 4 4 6% 10% 55%	6154
September	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 4 6% 55%	2520
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 6% 7% 51%	2331
	21st to end	Day	9% 5% 2 5% 4 4 4 3 5% 6% 52%	2292
October	1st to 10th	Day	11% 5% 2 4 3 3 3 3 5% 6% 55%	2100
	11th to 20th	Day	13% 5% 2 4 4 2 3 3 3 5% 56%	2100
	21st to end	Day	18% 7% 3 5% 3 3 3 3 3 5% 48%	2280
November	1st to 10th	Day	9% 7% 2 5% 3 3 2 3 4 6% 55%	2058
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 4 6% 51%	1890
	21st to end	Day	16% 8% 2 6% 3 2 3 2 4 5% 47%	1680
December	1st to 10th	Day	8% 7% 2 6% 3 3 3 3 4 6% 55%	2690
	11th to 20th	Day	11% 7% 3 7% 4 3 3 2 3 5% 52%	3088
	21st to end	Day	20% 18% 6% 8% 4 4 3 3 4 5% 27%	4399
January	1st to 10th	Night	13% 9% 4 9% 5% 5% 3 2 4 4 41%	7634
	11th to 20th	Night	8% 7% 2 9% 5% 6% 4 3 3 4 48%	7680
	21st to end	Night	15% 9% 3 7% 4 5% 4 3 4 5% 42%	8402
February	1st to 10th	Night	12% 11% 3 7% 4 3 4 3 5% 7% 40%	2730
	11th to 20th	Night	13% 9% 2 7% 4 3 4 3 4 6% 43%	2663
	21st to end	Night	13% 8% 2 6% 3 4 3 2 4 5% 48%	2008
March	1st to 10th	Night	15% 9% 2 5% 3 3 3 2 4 5% 48%	5029
	11th to 20th	Night	8% 6% 1 4 3 3 3 3 5% 7% 57%	5090
	21st to end	Night	12% 8% 3 6% 4 4 4 4 5% 8% 41%	5588
April	1st to 10th	Night	12% 6% 3 6% 3 4 3 3 6% 6% 48%	2195
	11th to 20th	Night	14% 8% 2 5% 4 3 3 3 4 7% 47%	2075
	21st to end	Night	16% 7% 3 7% 4 3 3 3 4 7% 41%	1985
May	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 4 6% 46%	5266
	11th to 20th	Night	13% 7% 3 6% 4 4 4 3 5% 7% 46%	5265
	21st to end	Night	19% 6% 2 5% 3 3 3 3 5% 7% 43%	5793
June	1st to 10th	Night	7% 3 2 4 4 3 3 4 7% 10% 52%	1689
	11th to 20th	Night	5% 4 2 5% 4 4 5% 5% 8% 9% 50%	1680
	21st to end	Night	5% 4 1 4 3 3 3 3 6% 8% 59%	1774
July	1st to 10th	Night	7% 5% 2 5% 3 4 4 4 6% 9% 50%	5308
	11th to 20th	Night	5% 4 2 4 4 4 4 4 6% 9% 54%	5307
	21st to end	Night	5% 4 2 6% 4 4 5% 5% 8% 11% 45%	5770
August	1st to 10th	Night	4 3 1 4 3 4 4 4 5% 8% 61%	4156
	11th to 20th	Night	7% 4 1 5% 4 4 4 4 5% 8% 53%	4118
	21st to end	Night	4 3 1 4 3 3 4 3 5% 8% 62%	4972
September	1st to 10th	Night	10% 4 1 5% 3 3 3 3 4 7% 56%	2100
	11th to 20th	Night	8% 4 1 4 4 5% 5% 3 5% 7% 53%	2284
	21st to end	Night	8% 5% 2 4 4 3 3 3 5% 7% 56%	2310
October	1st to 10th	Night	9% 3 2 4 3 4 3 3 4 5% 61%	2515
	11th to 20th	Night	13% 6% 2 5% 2 3 3 2 4 5% 57%	2520
	21st to end	Night	16% 6% 3 5% 4 3 3 3 4 4 51%	2737
November	1st to 10th	Night	11% 5% 2 4 2 3 2 2 3 6% 60%	2460
	11th to 20th	Night	11% 7% 2 5% 3 3 3 3 4 6% 54%	2723
	21st to end	Night	16% 8% 2 4 3 3 3 3 3 5% 51%	2940
December	1st to 10th	Night	8% 7% 2 4 3 3 3 3 4 6% 56%	4720
	11th to 20th	Night	14% 7% 3 5% 3 3 3 3 5% 6% 47%	5339
	21st to end	Night	20% 17% 5% 8% 4 3 3 2 3 4 31%	6803

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	47% 2 9% 28% 12% 2	5290
	11th to 20th	Day	42% 4 9% 26% 16% 4	5310
	21st to end	Day	45% 2 9% 21% 17% 5%	5812
February	1st to 10th	Day	48% 4 10% 26% 10% 2	1890
	11th to 20th	Day	45% 2 12% 22% 15% 3	1853
	21st to end	Day	43% 2 8% 25% 17% 5%	1814
March	1st to 10th	Day	39% 2 7% 22% 23% 7%	5046
	11th to 20th	Day	31% 3 9% 23% 26% 9%	5090
	21st to end	Day	48% 3 16% 25% 7% 1	5585
April	1st to 10th	Day	41% 4 11% 25% 16% 3	2425
	11th to 20th	Day	43% 3 11% 25% 17% 1	2494
	21st to end	Day	50% 3 11% 25% 10% 1	2630
May	1st to 10th	Day	43% 3 11% 24% 17% 1	7655
	11th to 20th	Day	38% 4 13% 24% 19% 3	7631
	21st to end	Day	48% 4 11% 24% 13% 1	8405
June	1st to 10th	Day	32% 6% 17% 26% 17% 2	2924
	11th to 20th	Day	33% 5% 14% 31% 16% 2	2940
	21st to end	Day	26% 5% 16% 32% 17% 3	2837
July	1st to 10th	Day	34% 5% 14% 27% 19% 2	7616
	11th to 20th	Day	30% 6% 13% 30% 16% 5%	7611
	21st to end	Day	36% 6% 17% 33% 8%	8155
August	1st to 10th	Day	29% 5% 13% 29% 23% 1	6000
	11th to 20th	Day	31% 5% 16% 33% 14% 1	5987
	21st to end	Day	26% 5% 13% 39% 14% 3	6153
September	1st to 10th	Day	32% 3 11% 29% 18% 7%	2520
	11th to 20th	Day	32% 4 10% 29% 23% 2	2331
	21st to end	Day	33% 4 9% 26% 25% 4	2292
October	1st to 10th	Day	19% 2 6% 22% 41% 10%	3360
	11th to 20th	Day	21% 2 4 27% 38% 9%	3340
	21st to end	Day	33% 2 8% 28% 25% 4	3618
November	1st to 10th	Day	31% 3 10% 31% 21% 4	2058
	11th to 20th	Day	36% 2 8% 27% 22% 5%	1890
	21st to end	Day	41% 2 8% 29% 18% 1	1680
December	1st to 10th	Day	32% 2 10% 38% 17%	2690
	11th to 20th	Day	37% 2 9% 33% 17% 2	3085
	21st to end	Day	62% 2 10% 19% 6% 1	4399
January	1st to 10th	Night	47% 3 12% 26% 12% 1	7635
	11th to 20th	Night	40% 2 7% 27% 19% 4	7680
	21st to end	Night	45% 3 10% 21% 16% 5%	8402
February	1st to 10th	Night	44% 3 11% 24% 16% 2	2730
	11th to 20th	Night	42% 2 11% 25% 16% 3	2664
	21st to end	Night	40% 3 7% 24% 20% 7%	2008
March	1st to 10th	Night	40% 2 7% 21% 24% 5%	5029
	11th to 20th	Night	28% 3 8% 22% 32% 7%	5090
	21st to end	Night	41% 4 15% 26% 12% 2	5588
April	1st to 10th	Night	37% 3 10% 22% 23% 4	2195
	11th to 20th	Night	39% 3 8% 26% 20% 5%	2075
	21st to end	Night	44% 3 10% 26% 15% 1	1985
May	1st to 10th	Night	40% 4 12% 26% 15% 2	5293
	11th to 20th	Night	38% 3 11% 24% 18% 6%	5266
	21st to end	Night	42% 5% 11% 25% 17% 2	5805
June	1st to 10th	Night	27% 5% 19% 29% 17% 3	1689
	11th to 20th	Night	27% 6% 16% 29% 20% 2	1680
	21st to end	Night	23% 4 14% 33% 22% 4	1774
July	1st to 10th	Night	30% 5% 14% 28% 22% 2	5265
	11th to 20th	Night	25% 7% 14% 32% 18% 5%	5256
	21st to end	Night	30% 5% 17% 38% 10%	5641
August	1st to 10th	Night	20% 4 15% 31% 24% 5%	4146
	11th to 20th	Night	29% 5% 13% 34% 18% 1	4145
	21st to end	Night	21% 4 13% 40% 19% 3	4972
September	1st to 10th	Night	29% 4 9% 28% 21% 8%	2100
	11th to 20th	Night	30% 4 9% 27% 24% 5%	2284
	21st to end	Night	29% 3 9% 26% 29% 5%	2310
October	1st to 10th	Night	16% 2 5% 20% 42% 13%	4035
	11th to 20th	Night	23% 3 4 23% 32% 15%	4019
	21st to end	Night	32% 3 6% 31% 24% 4	4349
November	1st to 10th	Night	28% 3 10% 30% 24% 6%	2460
	11th to 20th	Night	33% 2 7% 28% 25% 4	2723
	21st to end	Night	38% 3 9% 27% 20% 3	2940
December	1st to 10th	Night	30% 3 13% 36% 17% 1	4720
	11th to 20th	Night	39% 3 10% 28% 19% 2	5341
	21st to end	Night	60% 2 8% 21% 8% 1	6803

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% 39% 24% 4	330
	11th to 20th	Day	26% 15% 39% 18% 2	330
	21st to end	Day	18% 13% 45% 21% 3	363
February	1st to 10th	Day	22% 20% 37% 18% 4	330
	11th to 20th	Day	21% 19% 42% 14% 4 1	330
	21st to end	Day	22% 22% 40% 13% 3	273
March	1st to 10th	Day	23% 18% 40% 17% 3	330
	11th to 20th	Day	27% 14% 34% 20% 5% 1	330
	21st to end	Day	26% 20% 40% 11% 2 1	363
April	1st to 10th	Day	20% 19% 35% 19% 6% 1	445
	11th to 20th	Day	21% 17% 35% 21% 6%	532
	21st to end	Day	24% 13% 34% 22% 7%	650
May	1st to 10th	Day	21% 13% 34% 22% 9% 1	753
	11th to 20th	Day	19% 10% 30% 27% 11% 3	775
	21st to end	Day	17% 7% 28% 27% 16% 4	844
June	1st to 10th	Day	9% 4 19% 35% 24% 8%	780
	11th to 20th	Day	9% 6% 25% 34% 22% 4	780
	21st to end	Day	8% 4 33% 34% 19% 3	780
July	1st to 10th	Day	9% 3 15% 31% 31% 11%	771
	11th to 20th	Day	4 2 19% 36% 31% 8%	768
	21st to end	Day	9% 2 12% 34% 32% 10%	849
August	1st to 10th	Day	8% 4 15% 37% 29% 6%	776
	11th to 20th	Day	7% 2 17% 37% 31% 6%	765
	21st to end	Day	9% 2 11% 33% 33% 11%	664
September	1st to 10th	Day	12% 3 12% 36% 30% 7%	570
	11th to 20th	Day	6% 2 19% 38% 28% 7%	381
	21st to end	Day	13% 4 24% 34% 17% 8%	360
October	1st to 10th	Day	12% 7% 24% 31% 19% 8%	360
	11th to 20th	Day	14% 11% 38% 27% 7% 2	360
	21st to end	Day	25% 13% 32% 22% 8%	387
November	1st to 10th	Day	16% 10% 39% 23% 9% 3	330
	11th to 20th	Day	18% 16% 31% 24% 10% 1	330
	21st to end	Day	24% 15% 33% 21% 7% 1	330
December	1st to 10th	Day	14% 14% 42% 21% 5% 3	330
	11th to 20th	Day	18% 16% 37% 22% 6%	330
	21st to end	Day	24% 20% 38% 17% 1	363
January	1st to 10th	Night	15% 12% 38% 29% 6%	1050
	11th to 20th	Night	16% 13% 40% 26% 4 1	1050
	21st to end	Night	15% 13% 40% 29% 4	1155
February	1st to 10th	Night	18% 12% 41% 25% 3	1050
	11th to 20th	Night	20% 13% 38% 27% 2	1029
	21st to end	Night	20% 13% 40% 25% 3	624
March	1st to 10th	Night	19% 12% 39% 26% 4	630
	11th to 20th	Night	21% 11% 36% 26% 5% 1	630
	21st to end	Night	22% 12% 38% 24% 3	693
April	1st to 10th	Night	16% 12% 38% 29% 5%	630
	11th to 20th	Night	18% 15% 34% 26% 6%	623
	21st to end	Night	19% 8% 40% 25% 8%	630
May	1st to 10th	Night	18% 6% 33% 36% 8%	630
	11th to 20th	Night	17% 5% 29% 33% 15% 1	621
	21st to end	Night	15% 6% 27% 32% 18% 2	683
June	1st to 10th	Night	7% 4 18% 43% 27% 1	630
	11th to 20th	Night	5% 3 24% 48% 18% 2	630
	21st to end	Night	6% 4 27% 45% 18% 1	630
July	1st to 10th	Night	8% 3 11% 41% 35% 3	615
	11th to 20th	Night	4 2 15% 44% 33% 3	612
	21st to end	Night	8% 2 12% 40% 36% 2	690
August	1st to 10th	Night	6% 2 15% 45% 30% 2	627
	11th to 20th	Night	8% 1 12% 48% 28% 3	624
	21st to end	Night	6% 1 10% 43% 37% 3	693
September	1st to 10th	Night	13% 2 14% 44% 24% 3	630
	11th to 20th	Night	9% 2 18% 39% 27% 5%	630
	21st to end	Night	10% 4 21% 40% 20% 4	630
October	1st to 10th	Night	9% 3 25% 38% 22% 2	835
	11th to 20th	Night	15% 7% 37% 31% 9%	840
	21st to end	Night	20% 11% 32% 29% 8%	913
November	1st to 10th	Night	13% 9% 35% 32% 11% 1	820
	11th to 20th	Night	15% 11% 36% 29% 9%	1007
	21st to end	Night	20% 11% 34% 28% 6%	1050
December	1st to 10th	Night	16% 11% 38% 28% 6% 1	1050
	11th to 20th	Night	21% 10% 35% 30% 5%	1050
	21st to end	Night	20% 14% 35% 27% 4	1155

Page generated at 2026-10-26 03:37:48. Version ac07

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