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 2025-05-26.

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 3 5% 7% 42%	113893
	Night	15% 8% 3 6% 4 4 4 3 5% 6% 44%	115505
Transparency	Day	40% 5% 10% 24% 18% 3	117569
	Night	40% 4 9% 24% 19% 4	119959
Seeing	Day	16% 11% 24% 25% 17% 7%	16075
	Night	18% 9% 25% 29% 17% 2	25958

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	16% 10% 6% 9% 4 4 5% 4 5% 6% 31%	3617
	11th to 20th	Day	14% 8% 3 8% 4 3 4 3 4 5% 43%	3662
	21st to end	Day	20% 9% 4 6% 4 4 4 3 4 5% 38%	4618
February	1st to 10th	Day	18% 12% 3 6% 4 3 3 3 5% 6% 37%	2000
	11th to 20th	Day	20% 9% 4 8% 4 3 4 3 4 5% 37%	1948
	21st to end	Day	19% 9% 4 6% 5% 4 4 4 4 5% 36%	1650
March	1st to 10th	Day	17% 13% 3 7% 4 3 3 3 4 5% 38%	4480
	11th to 20th	Day	13% 9% 3 6% 4 3 3 2 4 6% 49%	4669
	21st to end	Day	12% 8% 3 8% 5% 4 4 4 5% 6% 42%	5967
April	1st to 10th	Day	14% 6% 3 6% 5% 4 5% 3 5% 7% 44%	2400
	11th to 20th	Day	15% 7% 2 7% 4 5% 3 4 5% 6% 42%	2374
	21st to end	Day	17% 6% 2 7% 5% 4 5% 3 5% 8% 40%	2400
May	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 4 7% 9% 31%	4196
	11th to 20th	Day	18% 7% 2 6% 4 3 4 3 5% 8% 38%	4211
	21st to end	Day	11% 7% 3 7% 5% 4 5% 4 6% 8% 41%	4605
June	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 6% 10% 46%	2660
	11th to 20th	Day	4 4 2 5% 4 5% 5% 5% 7% 10% 49%	2660
	21st to end	Day	5% 4 2 5% 5% 5% 5% 5% 7% 10% 46%	2660
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 8% 10% 40%	4440
	11th to 20th	Day	7% 3 2 5% 3 5% 5% 5% 7% 9% 49%	4440
	21st to end	Day	12% 10% 3 9% 6% 6% 5% 5% 6% 8% 29%	4688
August	1st to 10th	Day	6% 4 1 5% 3 4 4 4 6% 9% 54%	3790
	11th to 20th	Day	9% 5% 2 6% 5% 6% 5% 4 8% 8% 42%	3729
	21st to end	Day	5% 3 2 4 4 4 4 3 5% 8% 58%	4180
September	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 5% 8% 46%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 5% 7% 49%	2126
	21st to end	Day	13% 4 2 6% 3 3 4 4 4 7% 49%	2090
October	1st to 10th	Day	10% 3 1 4 3 3 3 3 4 7% 58%	2090
	11th to 20th	Day	13% 5% 2 4 3 4 4 2 4 6% 53%	1935
	21st to end	Day	19% 7% 2 5% 3 3 3 1 4 5% 48%	1950
November	1st to 10th	Day	12% 6% 2 5% 3 2 3 2 5% 6% 53%	1762
	11th to 20th	Day	15% 8% 2 5% 3 3 4 3 4 5% 49%	1800
	21st to end	Day	17% 9% 2 7% 3 3 3 2 5% 5% 42%	1800
December	1st to 10th	Day	16% 10% 2 6% 3 3 4 3 4 5% 44%	2930
	11th to 20th	Day	17% 11% 4 7% 4 4 4 3 4 6% 35%	3104
	21st to end	Day	19% 13% 4 9% 4 4 5% 4 5% 6% 27%	3982
January	1st to 10th	Night	17% 12% 4 9% 3 3 4 3 5% 6% 34%	6348
	11th to 20th	Night	17% 11% 4 8% 4 4 3 3 3 4 40%	6314
	21st to end	Night	21% 10% 4 6% 4 3 3 3 4 5% 39%	6338
February	1st to 10th	Night	22% 11% 4 7% 5% 4 4 2 4 4 33%	2400
	11th to 20th	Night	20% 9% 3 8% 5% 3 3 2 4 5% 37%	2338
	21st to end	Night	21% 9% 3 7% 3 4 3 3 5% 5% 36%	1980
March	1st to 10th	Night	19% 11% 4 7% 4 3 3 3 4 5% 37%	5373
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 5% 6% 49%	5332
	21st to end	Night	15% 7% 3 7% 5% 4 4 3 5% 7% 40%	4962
April	1st to 10th	Night	14% 5% 1 5% 4 4 5% 3 5% 7% 47%	2000
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 7% 46%	1975
	21st to end	Night	16% 6% 3 6% 5% 4 4 4 5% 8% 40%	2000
May	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 8% 37%	2918
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 6% 7% 41%	2902
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 7% 8% 44%	3051
June	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 6% 8% 47%	1520
	11th to 20th	Night	5% 3 1 4 3 3 4 5% 6% 9% 55%	1520
	21st to end	Night	3 3 1 4 4 5% 5% 4 7% 14% 50%	1520
July	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 4 7% 9% 46%	2530
	11th to 20th	Night	6% 4 2 5% 4 4 4 3 5% 8% 56%	2530
	21st to end	Night	8% 8% 3 9% 6% 6% 6% 5% 7% 8% 33%	2867
August	1st to 10th	Night	5% 3 1 4 3 3 4 3 5% 7% 61%	3134
	11th to 20th	Night	5% 3 2 5% 4 4 5% 4 6% 9% 52%	3100
	21st to end	Night	4 3 1 4 3 3 3 3 6% 9% 62%	3476
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 2 5% 7% 54%	1900
	11th to 20th	Night	10% 5% 3 5% 4 4 4 3 5% 6% 52%	2033
	21st to end	Night	12% 5% 2 5% 3 3 3 3 4 7% 54%	2090
October	1st to 10th	Night	10% 4 2 4 3 2 3 2 3 6% 61%	2090
	11th to 20th	Night	15% 5% 1 4 2 3 3 2 3 5% 55%	2241
	21st to end	Night	18% 6% 2 4 4 3 3 3 4 5% 47%	2646
November	1st to 10th	Night	13% 5% 2 5% 3 3 3 2 5% 6% 54%	2536
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 50%	2600
	21st to end	Night	17% 8% 3 5% 4 3 3 2 4 5% 47%	2600
December	1st to 10th	Night	18% 9% 2 7% 4 3 3 3 4 4 43%	4250
	11th to 20th	Night	20% 9% 3 5% 3 3 3 3 4 5% 42%	5095
	21st to end	Night	22% 13% 5% 8% 5% 4 4 3 4 5% 28%	6996

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	53% 4 11% 21% 9% 2	3618
	11th to 20th	Day	44% 3 7% 25% 19% 2	3662
	21st to end	Day	49% 3 9% 19% 16% 5%	4618
February	1st to 10th	Day	49% 4 10% 23% 13% 2	2000
	11th to 20th	Day	51% 4 8% 20% 13% 4	1958
	21st to end	Day	50% 3 9% 18% 15% 5%	1650
March	1st to 10th	Day	50% 3 4 20% 18% 6%	4480
	11th to 20th	Day	39% 4 7% 25% 21% 4	4669
	21st to end	Day	43% 4 10% 26% 17% 1	5969
April	1st to 10th	Day	40% 5% 10% 23% 19% 2	2400
	11th to 20th	Day	43% 4 9% 23% 18% 4	2374
	21st to end	Day	44% 4 9% 27% 14% 2	2400
May	1st to 10th	Day	48% 7% 13% 20% 11% 1	4234
	11th to 20th	Day	45% 6% 11% 25% 13% 1	4215
	21st to end	Day	40% 7% 11% 19% 21% 1	4618
June	1st to 10th	Day	33% 7% 14% 23% 20% 3	2660
	11th to 20th	Day	27% 8% 14% 27% 20% 3	2660
	21st to end	Day	30% 8% 16% 23% 17% 5%	2660
July	1st to 10th	Day	37% 7% 15% 28% 11% 2	4371
	11th to 20th	Day	29% 6% 14% 37% 12% 2	4365
	21st to end	Day	51% 7% 12% 21% 8% 1	4540
August	1st to 10th	Day	24% 6% 15% 31% 23% 1	3777
	11th to 20th	Day	37% 7% 12% 25% 18% 2	3765
	21st to end	Day	24% 5% 11% 36% 22% 3	4179
September	1st to 10th	Day	36% 7% 9% 25% 19% 5%	2280
	11th to 20th	Day	35% 6% 9% 24% 23% 3	2126
	21st to end	Day	34% 5% 9% 23% 23% 6%	2090
October	1st to 10th	Day	18% 3 6% 18% 47% 8%	3480
	11th to 20th	Day	23% 3 7% 27% 30% 11%	3188
	21st to end	Day	30% 2 5% 28% 29% 6%	3188
November	1st to 10th	Day	33% 4 9% 24% 25% 5%	1762
	11th to 20th	Day	38% 4 7% 24% 22% 5%	1800
	21st to end	Day	45% 3 7% 26% 15% 5%	1800
December	1st to 10th	Day	43% 3 7% 28% 17% 1	2930
	11th to 20th	Day	50% 4 9% 21% 13% 3	3101
	21st to end	Day	57% 4 12% 19% 8%	3982
January	1st to 10th	Night	52% 3 9% 24% 11% 1	6348
	11th to 20th	Night	49% 3 6% 22% 17% 2	6314
	21st to end	Night	49% 2 7% 22% 15% 4	6338
February	1st to 10th	Night	55% 4 9% 18% 13% 2	2400
	11th to 20th	Night	51% 4 6% 21% 15% 4	2339
	21st to end	Night	50% 4 8% 18% 16% 3	1980
March	1st to 10th	Night	51% 3 6% 15% 19% 6%	5373
	11th to 20th	Night	36% 3 9% 22% 19% 10%	5332
	21st to end	Night	44% 4 10% 25% 15% 2	4962
April	1st to 10th	Night	37% 4 10% 26% 20% 3	2000
	11th to 20th	Night	38% 5% 8% 22% 21% 6%	1975
	21st to end	Night	42% 5% 9% 24% 18% 2	2000
May	1st to 10th	Night	45% 6% 14% 22% 12% 1	2945
	11th to 20th	Night	40% 7% 13% 24% 15% 1	2903
	21st to end	Night	35% 8% 14% 24% 18% 1	3059
June	1st to 10th	Night	35% 5% 12% 23% 21% 3	1520
	11th to 20th	Night	23% 6% 17% 26% 24% 4	1520
	21st to end	Night	23% 8% 17% 27% 18% 6%	1520
July	1st to 10th	Night	32% 7% 14% 29% 14% 3	2490
	11th to 20th	Night	26% 5% 13% 36% 17% 3	2482
	21st to end	Night	47% 8% 13% 22% 9% 1	2776
August	1st to 10th	Night	22% 6% 14% 37% 21% 1	3123
	11th to 20th	Night	27% 6% 17% 30% 19% 2	3130
	21st to end	Night	21% 6% 10% 37% 23% 3	3476
September	1st to 10th	Night	32% 5% 9% 26% 23% 6%	1900
	11th to 20th	Night	33% 5% 8% 26% 24% 4	2033
	21st to end	Night	31% 4 8% 25% 24% 7%	2090
October	1st to 10th	Night	19% 3 5% 16% 46% 12%	3480
	11th to 20th	Night	21% 2 5% 21% 40% 10%	3714
	21st to end	Night	29% 2 7% 24% 28% 10%	4358
November	1st to 10th	Night	33% 3 7% 25% 26% 5%	2536
	11th to 20th	Night	36% 3 7% 24% 22% 9%	2600
	21st to end	Night	41% 3 8% 24% 20% 4	2600
December	1st to 10th	Night	45% 3 8% 24% 19% 1	4250
	11th to 20th	Night	45% 3 9% 23% 19% 2	5097
	21st to end	Night	58% 4 8% 19% 8% 2	6996

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	26% 21% 33% 17% 3	300
	11th to 20th	Day	26% 23% 36% 13% 2	300
	21st to end	Day	22% 22% 42% 12% 2	330
February	1st to 10th	Day	28% 21% 30% 17% 3 1	300
	11th to 20th	Day	30% 26% 33% 9% 2	300
	21st to end	Day	27% 21% 37% 13% 3	249
March	1st to 10th	Day	23% 21% 40% 12% 3 1	300
	11th to 20th	Day	26% 22% 35% 14% 3 1	336
	21st to end	Day	25% 19% 35% 16% 5%	550
April	1st to 10th	Day	23% 14% 32% 22% 7% 1	500
	11th to 20th	Day	21% 20% 26% 25% 8% 1	492
	21st to end	Day	20% 14% 33% 21% 9% 3	500
May	1st to 10th	Day	19% 18% 25% 28% 7% 3	503
	11th to 20th	Day	16% 10% 24% 30% 14% 5%	527
	21st to end	Day	10% 7% 22% 32% 19% 10%	587
June	1st to 10th	Day	11% 5% 21% 29% 23% 11%	680
	11th to 20th	Day	4 3 20% 35% 27% 11%	680
	21st to end	Day	6% 4 22% 35% 22% 11%	680
July	1st to 10th	Day	9% 3 13% 28% 30% 18%	671
	11th to 20th	Day	8% 4 14% 27% 34% 13%	668
	21st to end	Day	9% 2 14% 32% 29% 14%	663
August	1st to 10th	Day	10% 3 12% 30% 30% 16%	486
	11th to 20th	Day	12% 3 15% 32% 31% 8%	479
	21st to end	Day	8% 2 10% 26% 35% 20%	539
September	1st to 10th	Day	10% 3 13% 29% 31% 13%	490
	11th to 20th	Day	11% 2 11% 28% 32% 16%	490
	21st to end	Day	17% 6% 19% 26% 22% 10%	490
October	1st to 10th	Day	9% 3 21% 35% 24% 8%	490
	11th to 20th	Day	13% 11% 33% 24% 13% 5%	335
	21st to end	Day	25% 13% 29% 23% 8% 2	330
November	1st to 10th	Day	13% 10% 37% 26% 13% 1	300
	11th to 20th	Day	17% 20% 30% 27% 6% 1	300
	21st to end	Day	24% 12% 33% 22% 7% 2	300
December	1st to 10th	Day	21% 20% 33% 18% 7%	300
	11th to 20th	Day	23% 19% 31% 22% 4	300
	21st to end	Day	27% 19% 38% 14% 2	330
January	1st to 10th	Night	25% 18% 32% 23% 3	1000
	11th to 20th	Night	27% 17% 33% 20% 4	1000
	21st to end	Night	22% 16% 38% 22% 2	1039
February	1st to 10th	Night	27% 14% 36% 21% 2	800
	11th to 20th	Night	26% 17% 36% 19% 2	783
	21st to end	Night	27% 14% 37% 21% 1	660
March	1st to 10th	Night	23% 15% 32% 26% 5%	800
	11th to 20th	Night	22% 13% 35% 24% 6% 1	795
	21st to end	Night	22% 13% 30% 28% 6% 1	660
April	1st to 10th	Night	17% 11% 36% 28% 8% 1	600
	11th to 20th	Night	18% 11% 31% 28% 11% 1	593
	21st to end	Night	18% 10% 30% 31% 10% 1	600
May	1st to 10th	Night	21% 7% 30% 27% 14% 1	600
	11th to 20th	Night	14% 5% 23% 38% 19% 2	591
	21st to end	Night	8% 4 17% 41% 27% 4	634
June	1st to 10th	Night	14% 3 16% 35% 29% 3	570
	11th to 20th	Night	6% 2 13% 43% 34% 3	570
	21st to end	Night	3 2 13% 42% 34% 5%	570
July	1st to 10th	Night	8% 1 10% 30% 41% 10%	555
	11th to 20th	Night	9% 1 8% 31% 43% 7%	552
	21st to end	Night	8% 1 12% 36% 35% 8%	624
August	1st to 10th	Night	7% 1 8% 35% 40% 9%	567
	11th to 20th	Night	9% 2 9% 38% 37% 5%	564
	21st to end	Night	9% 7% 31% 45% 8%	627
September	1st to 10th	Night	12% 2 14% 31% 37% 5%	570
	11th to 20th	Night	12% 2 11% 32% 38% 6%	703
	21st to end	Night	12% 4 19% 31% 28% 4	760
October	1st to 10th	Night	12% 4 17% 33% 29% 5%	760
	11th to 20th	Night	18% 6% 24% 33% 17% 2	760
	21st to end	Night	21% 7% 29% 28% 13% 1	836
November	1st to 10th	Night	15% 8% 29% 33% 14% 1	780
	11th to 20th	Night	16% 13% 32% 30% 10% 1	800
	21st to end	Night	22% 13% 27% 28% 10% 1	800
December	1st to 10th	Night	26% 12% 30% 27% 6% 1	800
	11th to 20th	Night	24% 10% 33% 27% 6%	935
	21st to end	Night	29% 13% 31% 24% 3	1100

Page generated at 2025-05-23 08:21:00. Version ac07

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