Darklight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Darklight Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	13% 7% 3 6% 4 4 4 4 6% 8% 42%	148617
	Night	14% 7% 3 6% 4 4 4 3 5% 7% 44%	143009
Transparency	Day	40% 5% 11% 25% 16% 3	152247
	Night	40% 4 10% 24% 18% 3	147443
Seeing	Day	15% 10% 24% 25% 17% 7%	18000
	Night	18% 9% 25% 29% 17% 2	27679

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% 10% 6% 10% 4 4 5% 4 5% 6% 32%	4691
	11th to 20th	Day	12% 8% 3 7% 4 3 3 3 4 5% 49%	4766
	21st to end	Day	20% 9% 4 7% 4 4 4 3 4 5% 37%	5981
February	1st to 10th	Day	17% 12% 3 6% 4 3 3 3 5% 6% 37%	2100
	11th to 20th	Day	20% 9% 4 8% 4 3 4 3 4 5% 37%	2048
	21st to end	Day	18% 9% 4 7% 5% 4 4 4 4 5% 36%	1730
March	1st to 10th	Day	17% 13% 3 7% 4 3 3 3 4 5% 38%	4580
	11th to 20th	Day	13% 9% 3 6% 4 3 3 2 4 6% 49%	4772
	21st to end	Day	12% 8% 3 8% 5% 4 4 4 5% 6% 43%	6099
April	1st to 10th	Day	14% 5% 3 6% 5% 4 5% 3 5% 7% 44%	2520
	11th to 20th	Day	15% 7% 2 7% 4 5% 4 4 5% 6% 41%	2494
	21st to end	Day	17% 5% 2 7% 4 3 5% 3 5% 8% 40%	2520
May	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 8% 32%	7573
	11th to 20th	Day	16% 6% 3 7% 5% 4 5% 3 5% 8% 38%	7626
	21st to end	Day	19% 9% 3 7% 5% 4 5% 4 5% 7% 31%	8511
June	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 6% 9% 46%	2940
	11th to 20th	Day	5% 5% 2 5% 5% 5% 5% 5% 6% 10% 48%	2940
	21st to end	Day	6% 4 2 5% 5% 5% 5% 5% 7% 10% 46%	2940
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 10% 40%	8280
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 6% 9% 12% 38%	8280
	21st to end	Day	8% 7% 2 7% 5% 5% 5% 5% 7% 12% 37%	8754
August	1st to 10th	Day	8% 4 2 5% 3 4 5% 4 6% 9% 50%	5550
	11th to 20th	Day	8% 4 2 5% 4 5% 4 4 7% 9% 48%	5486
	21st to end	Day	4 3 1 3 3 3 3 3 5% 8% 63%	6080
September	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 5% 8% 48%	2520
	11th to 20th	Day	11% 4 2 5% 4 4 4 3 5% 7% 51%	2350
	21st to end	Day	13% 4 2 6% 4 4 4 3 4 7% 49%	2288
October	1st to 10th	Day	11% 4 1 4 3 3 3 3 4 7% 59%	2200
	11th to 20th	Day	13% 5% 2 4 3 4 4 2 4 6% 54%	2037
	21st to end	Day	19% 6% 2 5% 3 3 3 1 4 5% 47%	2052
November	1st to 10th	Day	12% 6% 2 5% 4 2 3 3 5% 6% 52%	1852
	11th to 20th	Day	15% 8% 2 5% 4 3 4 3 4 5% 48%	1890
	21st to end	Day	17% 9% 3 7% 3 3 3 2 5% 5% 42%	1890
December	1st to 10th	Day	15% 10% 2 6% 3 3 4 3 4 5% 44%	3020
	11th to 20th	Day	17% 11% 4 7% 4 4 4 3 4 6% 35%	3187
	21st to end	Day	19% 13% 4 9% 4 5% 5% 4 5% 6% 27%	4070
January	1st to 10th	Night	17% 11% 3 8% 3 3 4 3 5% 5% 37%	8232
	11th to 20th	Night	14% 10% 4 8% 4 4 3 3 3 4 43%	8220
	21st to end	Night	20% 10% 4 7% 4 3 3 3 4 5% 37%	8208
February	1st to 10th	Night	21% 11% 4 8% 5% 4 4 2 4 4 33%	2520
	11th to 20th	Night	20% 9% 3 8% 5% 3 3 2 4 6% 37%	2458
	21st to end	Night	21% 8% 3 7% 3 4 3 3 5% 5% 37%	2076
March	1st to 10th	Night	19% 11% 4 7% 4 4 3 3 4 5% 36%	5493
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 49%	5450
	21st to end	Night	15% 7% 3 7% 5% 4 4 3 5% 7% 40%	5072
April	1st to 10th	Night	13% 5% 1 5% 3 4 4 3 5% 8% 47%	2100
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 8% 46%	2075
	21st to end	Night	16% 6% 3 6% 5% 4 4 4 5% 8% 40%	2100
May	1st to 10th	Night	17% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	5279
	11th to 20th	Night	13% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	5234
	21st to end	Night	18% 8% 3 8% 5% 5% 5% 4 6% 7% 32%	5630
June	1st to 10th	Night	12% 6% 2 5% 3 4 4 3 6% 8% 47%	1680
	11th to 20th	Night	7% 4 1 4 3 3 4 5% 7% 9% 53%	1680
	21st to end	Night	4 2 1 4 4 5% 6% 4 7% 13% 50%	1680
July	1st to 10th	Night	7% 4 2 7% 4 5% 5% 4 7% 9% 46%	4708
	11th to 20th	Night	6% 4 2 6% 4 5% 5% 4 8% 11% 44%	4707
	21st to end	Night	6% 6% 2 7% 5% 5% 5% 5% 8% 12% 39%	5338
August	1st to 10th	Night	6% 4 1 5% 3 4 4 3 5% 7% 56%	4594
	11th to 20th	Night	4 3 1 4 3 3 4 4 6% 9% 58%	4560
	21st to end	Night	4 3 1 4 3 3 3 3 6% 8% 63%	5046
September	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 5% 7% 55%	2100
	11th to 20th	Night	9% 5% 2 5% 3 4 3 3 5% 6% 54%	2247
	21st to end	Night	12% 5% 2 5% 3 3 4 3 4 7% 54%	2289
October	1st to 10th	Night	10% 4 2 4 3 2 3 2 3 6% 61%	2200
	11th to 20th	Night	15% 5% 1 4 3 3 3 2 3 5% 56%	2359
	21st to end	Night	19% 6% 2 5% 4 3 3 3 4 5% 47%	2786
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 5% 6% 54%	2666
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 50%	2730
	21st to end	Night	18% 8% 3 5% 4 3 3 2 4 5% 46%	2730
December	1st to 10th	Night	18% 9% 2 7% 4 3 3 3 4 5% 43%	4380
	11th to 20th	Night	19% 9% 3 5% 3 3 4 3 4 5% 42%	5232
	21st to end	Night	22% 12% 5% 8% 5% 4 4 3 4 5% 28%	7150

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	52% 4 12% 23% 7% 1	4692
	11th to 20th	Day	39% 2 5% 29% 22% 2	4766
	21st to end	Day	50% 2 9% 20% 13% 5%	5981
February	1st to 10th	Day	49% 4 10% 24% 12% 2	2100
	11th to 20th	Day	51% 4 8% 20% 13% 4	2058
	21st to end	Day	51% 3 9% 18% 14% 5%	1730
March	1st to 10th	Day	51% 3 4 19% 17% 6%	4580
	11th to 20th	Day	39% 4 8% 25% 21% 4	4772
	21st to end	Day	42% 4 10% 27% 17% 1	6101
April	1st to 10th	Day	39% 5% 11% 23% 19% 2	2520
	11th to 20th	Day	43% 4 9% 22% 17% 4	2494
	21st to end	Day	43% 4 10% 28% 14% 2	2520
May	1st to 10th	Day	49% 6% 13% 21% 11% 1	7609
	11th to 20th	Day	45% 6% 13% 21% 14% 1	7630
	21st to end	Day	52% 6% 10% 17% 13% 2	8525
June	1st to 10th	Day	34% 7% 13% 24% 20% 3	2940
	11th to 20th	Day	29% 8% 14% 27% 19% 3	2940
	21st to end	Day	30% 8% 16% 25% 17% 5%	2940
July	1st to 10th	Day	38% 8% 13% 29% 12% 1	8211
	11th to 20th	Day	35% 10% 20% 28% 7% 1	8205
	21st to end	Day	38% 8% 22% 25% 7% 1	8561
August	1st to 10th	Day	29% 6% 15% 28% 21% 1	5537
	11th to 20th	Day	32% 6% 16% 28% 17% 1	5522
	21st to end	Day	20% 4 11% 39% 24% 2	6079
September	1st to 10th	Day	34% 7% 9% 26% 20% 4	2520
	11th to 20th	Day	34% 6% 9% 26% 23% 3	2350
	21st to end	Day	36% 5% 8% 22% 23% 6%	2288
October	1st to 10th	Day	18% 3 6% 18% 47% 9%	3590
	11th to 20th	Day	23% 3 7% 27% 31% 10%	3290
	21st to end	Day	30% 2 5% 28% 28% 6%	3290
November	1st to 10th	Day	34% 4 9% 24% 24% 5%	1852
	11th to 20th	Day	38% 3 8% 23% 22% 5%	1890
	21st to end	Day	45% 3 7% 26% 15% 4	1890
December	1st to 10th	Day	44% 3 8% 28% 17% 1	3020
	11th to 20th	Day	50% 4 9% 21% 14% 2	3184
	21st to end	Day	57% 4 12% 19% 8%	4070
January	1st to 10th	Night	50% 2 12% 25% 10% 1	8233
	11th to 20th	Night	46% 2 6% 23% 20% 3	8220
	21st to end	Night	52% 2 8% 21% 14% 3	8208
February	1st to 10th	Night	55% 4 10% 18% 13% 1	2520
	11th to 20th	Night	51% 4 6% 22% 15% 3	2459
	21st to end	Night	50% 4 8% 18% 17% 3	2076
March	1st to 10th	Night	51% 3 6% 15% 19% 6%	5493
	11th to 20th	Night	37% 3 9% 22% 19% 10%	5450
	21st to end	Night	44% 4 10% 26% 15% 2	5072
April	1st to 10th	Night	36% 4 11% 26% 19% 3	2100
	11th to 20th	Night	37% 5% 9% 21% 21% 6%	2075
	21st to end	Night	41% 5% 8% 25% 18% 2	2100
May	1st to 10th	Night	47% 5% 13% 23% 11% 1	5306
	11th to 20th	Night	45% 7% 13% 20% 15% 1	5235
	21st to end	Night	50% 7% 13% 19% 11% 1	5640
June	1st to 10th	Night	35% 5% 11% 24% 21% 3	1680
	11th to 20th	Night	25% 6% 16% 25% 23% 4	1680
	21st to end	Night	24% 8% 16% 28% 18% 6%	1680
July	1st to 10th	Night	34% 9% 14% 26% 16% 2	4670
	11th to 20th	Night	32% 11% 21% 26% 9% 2	4662
	21st to end	Night	36% 8% 21% 26% 8%	5219
August	1st to 10th	Night	27% 6% 14% 32% 20%	4583
	11th to 20th	Night	23% 5% 18% 36% 18% 1	4590
	21st to end	Night	20% 5% 9% 40% 25% 2	5046
September	1st to 10th	Night	30% 5% 9% 27% 23% 5%	2100
	11th to 20th	Night	31% 5% 8% 27% 25% 4	2247
	21st to end	Night	32% 4 8% 24% 24% 7%	2289
October	1st to 10th	Night	19% 3 5% 16% 46% 12%	3590
	11th to 20th	Night	21% 2 5% 21% 40% 10%	3832
	21st to end	Night	30% 2 7% 24% 28% 10%	4498
November	1st to 10th	Night	33% 3 8% 25% 26% 5%	2666
	11th to 20th	Night	35% 3 7% 24% 22% 9%	2730
	21st to end	Night	42% 3 8% 24% 19% 3	2730
December	1st to 10th	Night	45% 3 8% 24% 18% 1	4380
	11th to 20th	Night	45% 3 9% 23% 19% 2	5234
	21st to end	Night	58% 4 8% 19% 8% 2	7150

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	25% 21% 33% 18% 4	330
	11th to 20th	Day	25% 24% 36% 13% 2	330
	21st to end	Day	23% 22% 42% 11% 2	363
February	1st to 10th	Day	27% 20% 32% 17% 3 1	330
	11th to 20th	Day	29% 26% 34% 9% 2	330
	21st to end	Day	26% 21% 37% 13% 3	273
March	1st to 10th	Day	23% 21% 41% 11% 3 1	330
	11th to 20th	Day	24% 23% 36% 14% 3 1	368
	21st to end	Day	23% 19% 36% 17% 5%	594
April	1st to 10th	Day	22% 14% 32% 24% 7% 1	540
	11th to 20th	Day	20% 19% 27% 24% 8% 1	532
	21st to end	Day	19% 13% 35% 22% 9% 3	540
May	1st to 10th	Day	19% 17% 26% 28% 7% 3	543
	11th to 20th	Day	16% 10% 26% 30% 13% 5%	567
	21st to end	Day	13% 7% 22% 31% 17% 9%	659
June	1st to 10th	Day	11% 5% 21% 30% 22% 11%	780
	11th to 20th	Day	6% 4 21% 34% 26% 10%	780
	21st to end	Day	6% 3 21% 33% 23% 13%	780
July	1st to 10th	Day	10% 3 12% 26% 30% 19%	771
	11th to 20th	Day	8% 3 13% 28% 35% 13%	768
	21st to end	Day	8% 2 14% 32% 29% 15%	765
August	1st to 10th	Day	10% 3 12% 29% 31% 16%	566
	11th to 20th	Day	11% 3 13% 31% 32% 9%	559
	21st to end	Day	7% 2 11% 27% 36% 19%	627
September	1st to 10th	Day	9% 3 13% 29% 31% 15%	570
	11th to 20th	Day	10% 2 10% 27% 33% 18%	570
	21st to end	Day	17% 6% 18% 27% 22% 9%	562
October	1st to 10th	Day	10% 3 21% 35% 24% 8%	530
	11th to 20th	Day	12% 11% 32% 25% 15% 5%	367
	21st to end	Day	25% 13% 31% 22% 7% 1	363
November	1st to 10th	Day	14% 11% 37% 25% 12% 1	330
	11th to 20th	Day	16% 19% 32% 27% 6% 1	330
	21st to end	Day	24% 13% 32% 22% 7% 2	330
December	1st to 10th	Day	21% 19% 35% 18% 7%	330
	11th to 20th	Day	23% 19% 32% 22% 4	330
	21st to end	Day	27% 19% 39% 13% 2	363
January	1st to 10th	Night	25% 18% 31% 23% 3	1050
	11th to 20th	Night	26% 17% 33% 20% 3	1050
	21st to end	Night	21% 16% 38% 22% 2	1091
February	1st to 10th	Night	26% 13% 37% 21% 2	840
	11th to 20th	Night	27% 17% 35% 20% 1	823
	21st to end	Night	27% 14% 37% 22% 1	692
March	1st to 10th	Night	23% 15% 32% 26% 5%	840
	11th to 20th	Night	22% 13% 35% 24% 6%	835
	21st to end	Night	21% 14% 31% 28% 6% 1	693
April	1st to 10th	Night	17% 10% 36% 30% 8%	630
	11th to 20th	Night	18% 11% 32% 28% 11% 1	623
	21st to end	Night	18% 10% 30% 31% 10% 1	630
May	1st to 10th	Night	20% 7% 30% 28% 13% 1	630
	11th to 20th	Night	14% 5% 23% 38% 19% 2	621
	21st to end	Night	10% 4 17% 40% 25% 4	683
June	1st to 10th	Night	13% 4 16% 36% 28% 3	630
	11th to 20th	Night	7% 2 14% 41% 33% 3	630
	21st to end	Night	3 2 13% 41% 36% 5%	630
July	1st to 10th	Night	8% 1 10% 28% 43% 9%	615
	11th to 20th	Night	9% 1 8% 31% 44% 7%	612
	21st to end	Night	7% 1 12% 34% 37% 8%	690
August	1st to 10th	Night	7% 1 8% 35% 41% 8%	627
	11th to 20th	Night	8% 2 8% 38% 38% 5%	624
	21st to end	Night	9% 7% 32% 44% 7%	693
September	1st to 10th	Night	11% 2 13% 31% 38% 5%	630
	11th to 20th	Night	11% 2 10% 32% 38% 7%	777
	21st to end	Night	12% 5% 18% 33% 27% 4	833
October	1st to 10th	Night	12% 4 17% 34% 29% 5%	800
	11th to 20th	Night	17% 6% 24% 34% 18% 2	800
	21st to end	Night	22% 8% 29% 28% 12% 1	880
November	1st to 10th	Night	14% 9% 29% 34% 13% 1	820
	11th to 20th	Night	15% 12% 33% 30% 9%	840
	21st to end	Night	22% 13% 27% 28% 10%	840
December	1st to 10th	Night	25% 12% 30% 27% 5%	840
	11th to 20th	Night	24% 10% 33% 27% 6%	982
	21st to end	Night	29% 13% 31% 24% 3	1155

Page generated at 2026-09-26 03:18:39. Version ac07

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