DarkSkies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the DarkSkies Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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	16% 8% 3 7% 4 4 4 4 5% 7% 37%	113112
	Night	16% 8% 3 6% 4 4 4 3 5% 7% 39%	115166
Transparency	Day	44% 6% 12% 22% 15% 2	116667
	Night	43% 5% 12% 22% 16% 3	119749
Seeing	Day	24% 19% 32% 17% 7% 2	15441
	Night	22% 18% 33% 21% 5%	24527

By Month

Percentage Cloud Cover NE (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	10% 8% 3 11% 5% 6% 6% 5% 5% 6% 35%	3597
	11th to 20th	Day	12% 8% 3 7% 4 4 4 3 5% 5% 43%	3570
	21st to end	Day	15% 10% 3 6% 4 3 4 4 4 5% 42%	3960
February	1st to 10th	Day	16% 8% 3 7% 5% 6% 5% 4 5% 7% 33%	1846
	11th to 20th	Day	12% 7% 3 8% 6% 5% 5% 5% 6% 7% 36%	1948
	21st to end	Day	17% 11% 3 7% 5% 4 5% 3 4 4 36%	1650
March	1st to 10th	Day	18% 13% 3 7% 4 4 4 3 4 5% 34%	4476
	11th to 20th	Day	15% 8% 2 6% 5% 5% 5% 4 5% 6% 38%	4699
	21st to end	Day	23% 10% 3 5% 3 3 4 3 5% 7% 32%	5873
April	1st to 10th	Day	17% 11% 3 8% 5% 4 4 4 4 6% 35%	2400
	11th to 20th	Day	16% 8% 2 6% 5% 4 4 4 7% 6% 37%	2254
	21st to end	Day	20% 8% 3 6% 6% 5% 4 3 5% 5% 34%	2456
May	1st to 10th	Day	23% 10% 3 8% 5% 4 3 4 5% 7% 27%	4106
	11th to 20th	Day	22% 9% 3 8% 5% 4 4 3 5% 6% 33%	4254
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 6% 9% 42%	4837
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 5% 6% 8% 33%	2660
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 5% 6% 9% 35%	2660
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 6% 9% 32%	2660
July	1st to 10th	Day	12% 6% 3 6% 5% 5% 5% 5% 7% 9% 35%	4440
	11th to 20th	Day	12% 5% 2 6% 4 5% 6% 4 7% 11% 37%	4440
	21st to end	Day	18% 8% 2 6% 5% 5% 5% 4 6% 7% 33%	4814
August	1st to 10th	Day	17% 7% 2 6% 5% 6% 5% 4 6% 9% 33%	4234
	11th to 20th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 8% 35%	3924
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	4180
September	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 5% 8% 44%	2280
	11th to 20th	Day	14% 7% 2 4 3 3 3 3 5% 6% 50%	2114
	21st to end	Day	16% 7% 2 6% 4 5% 3 4 5% 7% 42%	2090
October	1st to 10th	Day	22% 8% 3 5% 5% 4 3 3 4 5% 38%	2090
	11th to 20th	Day	13% 7% 3 5% 3 4 4 4 5% 7% 45%	1837
	21st to end	Day	20% 8% 2 7% 5% 4 3 3 4 6% 38%	1881
November	1st to 10th	Day	13% 7% 2 7% 3 3 3 3 4 5% 49%	1761
	11th to 20th	Day	18% 8% 1 5% 3 3 3 3 4 5% 46%	1800
	21st to end	Day	17% 9% 2 5% 4 4 4 3 5% 5% 40%	1775
December	1st to 10th	Day	13% 9% 3 7% 4 4 4 4 5% 6% 42%	2600
	11th to 20th	Day	18% 9% 3 5% 3 4 4 3 4 5% 41%	2986
	21st to end	Day	18% 11% 3 7% 5% 4 4 3 4 6% 36%	3960
January	1st to 10th	Night	12% 7% 3 7% 4 4 5% 4 6% 7% 41%	6349
	11th to 20th	Night	11% 9% 3 9% 6% 5% 5% 3 5% 6% 38%	6315
	21st to end	Night	15% 9% 3 7% 5% 4 5% 3 5% 6% 38%	6996
February	1st to 10th	Night	15% 9% 3 7% 4 4 4 5% 6% 6% 37%	2548
	11th to 20th	Night	12% 9% 3 6% 5% 4 5% 5% 6% 7% 38%	2338
	21st to end	Night	15% 9% 3 7% 5% 4 4 3 5% 6% 38%	1980
March	1st to 10th	Night	20% 9% 2 5% 4 4 4 3 5% 7% 36%	5364
	11th to 20th	Night	15% 8% 3 7% 5% 5% 5% 4 4 7% 38%	5259
	21st to end	Night	23% 10% 3 5% 4 3 5% 3 5% 6% 34%	5054
April	1st to 10th	Night	18% 9% 2 7% 5% 4 4 5% 6% 6% 34%	2000
	11th to 20th	Night	18% 8% 2 5% 5% 4 5% 3 5% 6% 39%	1882
	21st to end	Night	20% 6% 3 7% 4 4 4 3 5% 5% 39%	1724
May	1st to 10th	Night	22% 11% 3 8% 4 4 5% 4 5% 7% 26%	2834
	11th to 20th	Night	17% 10% 4 9% 6% 5% 5% 4 5% 7% 27%	2668
	21st to end	Night	16% 7% 2 6% 4 4 4 3 5% 8% 41%	2738
June	1st to 10th	Night	18% 9% 3 6% 5% 4 4 4 5% 5% 38%	1520
	11th to 20th	Night	18% 7% 3 6% 5% 3 4 4 6% 7% 37%	1520
	21st to end	Night	15% 6% 2 6% 3 4 4 4 7% 8% 40%	1520
July	1st to 10th	Night	12% 7% 2 7% 5% 5% 6% 4 6% 8% 37%	2520
	11th to 20th	Night	15% 6% 3 6% 5% 4 4 4 5% 9% 40%	2520
	21st to end	Night	16% 8% 3 7% 5% 4 4 3 5% 7% 39%	2739
August	1st to 10th	Night	22% 7% 2 7% 4 4 5% 3 5% 6% 34%	2662
	11th to 20th	Night	13% 8% 3 9% 5% 4 5% 4 6% 7% 37%	2900
	21st to end	Night	9% 4 1 4 3 3 3 3 5% 8% 58%	3476
September	1st to 10th	Night	12% 7% 3 5% 4 4 3 3 6% 7% 47%	1900
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 6% 51%	2065
	21st to end	Night	15% 6% 3 6% 4 4 4 3 4 6% 44%	2090
October	1st to 10th	Night	19% 7% 2 6% 4 3 4 3 4 6% 40%	2090
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 6% 7% 45%	2337
	21st to end	Night	20% 9% 3 6% 3 3 4 3 5% 6% 38%	2717
November	1st to 10th	Night	14% 6% 2 6% 3 3 4 3 4 6% 47%	2537
	11th to 20th	Night	16% 7% 2 5% 5% 4 4 2 4 6% 45%	2600
	21st to end	Night	14% 8% 3 5% 4 4 4 3 6% 7% 42%	2620
December	1st to 10th	Night	16% 11% 3 5% 4 4 3 3 5% 6% 39%	4580
	11th to 20th	Night	22% 9% 3 6% 4 3 3 3 4 5% 39%	5208
	21st to end	Night	18% 10% 3 7% 3 3 3 3 5% 6% 39%	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	44% 4 9% 27% 15% 2	3598
	11th to 20th	Day	41% 4 11% 21% 18% 4	3600
	21st to end	Day	43% 5% 8% 23% 19% 3	3960
February	1st to 10th	Day	48% 6% 13% 18% 12% 2	1846
	11th to 20th	Day	44% 8% 10% 20% 15% 3	1957
	21st to end	Day	50% 5% 9% 19% 14% 3	1650
March	1st to 10th	Day	51% 4 10% 14% 12% 9%	4479
	11th to 20th	Day	43% 7% 15% 24% 10% 2	4699
	21st to end	Day	50% 3 7% 16% 18% 5%	5873
April	1st to 10th	Day	50% 5% 9% 18% 15% 3	2400
	11th to 20th	Day	43% 7% 12% 19% 16% 2	2254
	21st to end	Day	49% 6% 8% 18% 16% 3	2456
May	1st to 10th	Day	55% 8% 12% 14% 10% 1	4120
	11th to 20th	Day	51% 7% 14% 17% 9% 1	4258
	21st to end	Day	35% 9% 13% 25% 17% 1	4837
June	1st to 10th	Day	45% 8% 15% 22% 10% 1	2660
	11th to 20th	Day	43% 8% 14% 20% 14% 2	2660
	21st to end	Day	47% 8% 13% 20% 12% 1	2660
July	1st to 10th	Day	43% 6% 14% 21% 15% 1	4370
	11th to 20th	Day	39% 6% 17% 26% 12% 1	4365
	21st to end	Day	47% 5% 13% 20% 13% 2	4661
August	1st to 10th	Day	46% 6% 16% 22% 9%	4220
	11th to 20th	Day	45% 5% 15% 26% 8% 1	3963
	21st to end	Day	34% 5% 12% 31% 16% 2	4180
September	1st to 10th	Day	38% 6% 12% 25% 18% 2	2280
	11th to 20th	Day	35% 5% 9% 26% 23% 2	2114
	21st to end	Day	41% 5% 9% 22% 19% 3	2090
October	1st to 10th	Day	44% 4 8% 24% 19% 2	3480
	11th to 20th	Day	32% 4 8% 28% 23% 4	3014
	21st to end	Day	35% 4 10% 30% 17% 3	3081
November	1st to 10th	Day	38% 5% 13% 25% 16% 2	1761
	11th to 20th	Day	39% 5% 10% 21% 22% 3	1800
	21st to end	Day	43% 6% 14% 23% 11% 2	1775
December	1st to 10th	Day	41% 5% 15% 29% 11% 1	2600
	11th to 20th	Day	43% 4 14% 18% 20% 2	2986
	21st to end	Day	49% 6% 7% 18% 14% 6%	3960
January	1st to 10th	Night	38% 4 11% 32% 14% 2	6349
	11th to 20th	Night	45% 5% 13% 20% 14% 4	6360
	21st to end	Night	47% 4 10% 16% 18% 5%	6996
February	1st to 10th	Night	45% 7% 15% 20% 12% 2	2548
	11th to 20th	Night	42% 7% 12% 20% 16% 3	2340
	21st to end	Night	45% 5% 10% 22% 13% 4	1980
March	1st to 10th	Night	46% 4 10% 17% 17% 6%	5374
	11th to 20th	Night	45% 7% 17% 18% 11% 2	5260
	21st to end	Night	50% 4 6% 14% 22% 5%	5054
April	1st to 10th	Night	48% 6% 11% 18% 15% 3	2000
	11th to 20th	Night	44% 7% 10% 20% 18% 1	1882
	21st to end	Night	46% 5% 10% 19% 15% 4	1724
May	1st to 10th	Night	54% 9% 12% 16% 8% 1	2840
	11th to 20th	Night	53% 10% 14% 13% 8% 2	2669
	21st to end	Night	40% 8% 14% 22% 15% 1	2739
June	1st to 10th	Night	46% 8% 10% 24% 12% 1	1520
	11th to 20th	Night	44% 8% 13% 20% 13% 1	1520
	21st to end	Night	39% 10% 14% 23% 13%	1520
July	1st to 10th	Night	42% 7% 13% 22% 15%	2480
	11th to 20th	Night	40% 6% 16% 23% 14%	2472
	21st to end	Night	45% 5% 13% 21% 15% 1	2651
August	1st to 10th	Night	50% 7% 13% 20% 10% 1	2652
	11th to 20th	Night	46% 6% 15% 24% 9%	2927
	21st to end	Night	26% 5% 16% 32% 20% 1	3476
September	1st to 10th	Night	36% 7% 12% 25% 19% 1	1900
	11th to 20th	Night	34% 5% 11% 24% 25% 2	2065
	21st to end	Night	41% 5% 9% 26% 16% 3	2090
October	1st to 10th	Night	40% 4 14% 24% 16% 2	3480
	11th to 20th	Night	27% 4 11% 27% 27% 4	3873
	21st to end	Night	35% 4 12% 32% 14% 2	4467
November	1st to 10th	Night	37% 7% 11% 22% 20% 3	2537
	11th to 20th	Night	40% 5% 11% 21% 20% 3	2600
	21st to end	Night	40% 6% 15% 25% 13% 1	2620
December	1st to 10th	Night	44% 4 12% 30% 11% 1	4580
	11th to 20th	Night	47% 4 15% 13% 20% 2	5208
	21st to end	Night	46% 5% 8% 21% 16% 4	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	32% 30% 23% 15% 1	200
	11th to 20th	Day	31% 28% 32% 7% 3	200
	21st to end	Day	28% 29% 32% 9% 2	220
February	1st to 10th	Day	33% 32% 27% 8% 1	368
	11th to 20th	Day	32% 30% 29% 8%	496
	21st to end	Day	32% 23% 33% 10% 2	414
March	1st to 10th	Day	31% 25% 35% 8% 2	500
	11th to 20th	Day	27% 31% 29% 8% 3 1	500
	21st to end	Day	32% 24% 31% 8% 4 1	550
April	1st to 10th	Day	34% 25% 27% 9% 5%	500
	11th to 20th	Day	28% 24% 33% 11% 4 1	452
	21st to end	Day	30% 21% 30% 14% 3 1	460
May	1st to 10th	Day	32% 23% 26% 15% 3 1	463
	11th to 20th	Day	21% 19% 39% 16% 4 2	487
	21st to end	Day	19% 15% 34% 22% 8% 1	529
June	1st to 10th	Day	19% 15% 37% 19% 8% 1	490
	11th to 20th	Day	17% 14% 39% 21% 8% 1	490
	21st to end	Day	18% 16% 39% 18% 8% 2	490
July	1st to 10th	Day	17% 9% 32% 29% 12% 2	485
	11th to 20th	Day	14% 9% 34% 32% 10% 2	484
	21st to end	Day	15% 10% 37% 28% 8% 2	532
August	1st to 10th	Day	14% 10% 34% 29% 12% 1	486
	11th to 20th	Day	18% 9% 33% 25% 14% 2	479
	21st to end	Day	13% 11% 32% 28% 12% 4	539
September	1st to 10th	Day	18% 12% 29% 22% 13% 5%	490
	11th to 20th	Day	19% 13% 26% 21% 14% 8%	490
	21st to end	Day	19% 14% 33% 20% 9% 4	490
October	1st to 10th	Day	29% 13% 26% 19% 11% 3	490
	11th to 20th	Day	19% 21% 32% 20% 6% 2	448
	21st to end	Day	29% 17% 32% 17% 4 1	429
November	1st to 10th	Day	22% 22% 31% 17% 8% 1	395
	11th to 20th	Day	31% 24% 27% 15% 3 1	400
	21st to end	Day	32% 21% 29% 13% 4	375
December	1st to 10th	Day	25% 31% 29% 13% 3	200
	11th to 20th	Day	40% 24% 31% 5% 2	200
	21st to end	Day	28% 30% 29% 10% 4	220
January	1st to 10th	Night	24% 26% 37% 12% 2	800
	11th to 20th	Night	27% 25% 32% 14% 2	800
	21st to end	Night	21% 25% 37% 15% 2	880
February	1st to 10th	Night	24% 24% 38% 14% 1	800
	11th to 20th	Night	25% 27% 36% 11% 1	783
	21st to end	Night	27% 26% 33% 12% 2	660
March	1st to 10th	Night	26% 24% 34% 14% 2	800
	11th to 20th	Night	23% 26% 34% 15% 3	800
	21st to end	Night	26% 23% 32% 16% 4	880
April	1st to 10th	Night	26% 22% 33% 16% 2	800
	11th to 20th	Night	24% 22% 32% 17% 4	754
	21st to end	Night	24% 21% 35% 18% 2	584
May	1st to 10th	Night	26% 17% 35% 19% 2	570
	11th to 20th	Night	20% 17% 38% 21% 4	482
	21st to end	Night	19% 10% 36% 30% 4	411
June	1st to 10th	Night	19% 14% 35% 26% 7%	380
	11th to 20th	Night	19% 9% 36% 29% 5% 1	380
	21st to end	Night	17% 5% 43% 31% 5%	380
July	1st to 10th	Night	15% 6% 33% 37% 9%	370
	11th to 20th	Night	16% 4 31% 39% 9% 1	368
	21st to end	Night	12% 5% 35% 38% 10%	416
August	1st to 10th	Night	13% 5% 34% 37% 12%	469
	11th to 20th	Night	13% 6% 32% 36% 11% 1	628
	21st to end	Night	16% 5% 27% 36% 14% 1	836
September	1st to 10th	Night	16% 8% 33% 31% 11% 1	760
	11th to 20th	Night	15% 9% 30% 28% 17% 2	760
	21st to end	Night	19% 11% 31% 26% 11% 2	760
October	1st to 10th	Night	25% 13% 28% 23% 10% 1	760
	11th to 20th	Night	18% 17% 32% 23% 8% 1	760
	21st to end	Night	26% 18% 32% 19% 5%	836
November	1st to 10th	Night	19% 20% 35% 19% 7% 1	780
	11th to 20th	Night	23% 24% 30% 16% 6% 1	800
	21st to end	Night	24% 24% 34% 14% 4	800
December	1st to 10th	Night	27% 22% 34% 13% 4	800
	11th to 20th	Night	27% 24% 32% 14% 3	800
	21st to end	Night	27% 26% 31% 14% 2	880

Page generated at 2025-04-08 09:17:25. Version ac07

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