D.A.R.C. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the D.A.R.C. Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	3 2 1 3 2 2 3 2 4 5% 73%	134728
	Night	4 3 1 3 2 2 3 2 4 5% 71%	132126
Transparency	Day	16% 3 7% 19% 29% 27%	129713
	Night	18% 3 9% 19% 26% 25%	128122
Seeing	Day	5% 7% 20% 32% 26% 10%	16615
	Night	5% 5% 16% 35% 31% 8%	26048

By Month

Percentage Cloud Cover SW (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	6% 5% 2 4 3 3 5% 3 4 8% 57%	3752
	11th to 20th	Day	5% 4 2 5% 5% 4 5% 4 4 6% 58%	4100
	21st to end	Day	10% 6% 2 6% 4 5% 5% 3 5% 7% 48%	4510
February	1st to 10th	Day	11% 6% 2 4 3 3 4 4 5% 7% 50%	1808
	11th to 20th	Day	5% 4 2 4 4 5% 5% 5% 6% 7% 53%	1950
	21st to end	Day	7% 6% 2 6% 4 4 3 4 6% 6% 51%	1650
March	1st to 10th	Day	7% 5% 2 5% 5% 5% 6% 5% 8% 10% 43%	4667
	11th to 20th	Day	5% 4 2 5% 4 4 5% 3 5% 8% 56%	5000
	21st to end	Day	5% 5% 2 5% 4 4 5% 5% 6% 9% 50%	5705
April	1st to 10th	Day	5% 5% 2 5% 4 4 4 3 6% 7% 56%	2400
	11th to 20th	Day	4 5% 1 3 3 3 3 3 5% 6% 62%	2369
	21st to end	Day	4 3 1 3 3 2 3 3 4 6% 69%	2400
May	1st to 10th	Day	2 2 1 2 2 1 2 2 3 4 81%	5440
	11th to 20th	Day	1 1 1 1 1 1 2 3 4 85%	5951
	21st to end	Day	1 1 1 1 1 1 1 2 4 87%	6902
June	1st to 10th	Day	2 2 2 2 2 2 2 4 6% 76%	2800
	11th to 20th	Day	1 1 1 1 1 2 4 89%	2800
	21st to end	Day	1 1 1 1 1 1 1 2 2 89%	2800
July	1st to 10th	Day	1 1 1 95%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 90%	6084
	21st to end	Day	1 1 1 2 3 90%	6402
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 89%	5675
	11th to 20th	Day	1 2 95%	5364
	21st to end	Day	1 1 1 1 1 2 2 90%	5984
September	1st to 10th	Day	1 1 1 1 1 1 2 92%	2400
	11th to 20th	Day	1 1 1 1 1 1 2 3 4 85%	2400
	21st to end	Day	1 1 2 1 2 1 1 3 4 83%	2179
October	1st to 10th	Day	1 1 1 2 1 1 2 2 3 5% 81%	3211
	11th to 20th	Day	1 1 2 1 2 2 2 3 4 81%	3161
	21st to end	Day	2 2 1 3 2 3 3 3 3 7% 70%	3448
November	1st to 10th	Day	4 3 2 3 3 3 4 3 6% 8% 61%	1850
	11th to 20th	Day	5% 3 1 4 3 3 3 4 4 7% 62%	1800
	21st to end	Day	6% 5% 2 4 3 3 3 3 5% 7% 59%	1795
December	1st to 10th	Day	4 4 1 6% 5% 4 4 3 5% 7% 57%	2620
	11th to 20th	Day	8% 7% 2 7% 5% 5% 6% 4 6% 8% 43%	3028
	21st to end	Day	8% 7% 2 6% 5% 5% 5% 5% 7% 10% 39%	4004
January	1st to 10th	Night	6% 5% 2 4 3 2 3 3 5% 7% 59%	6195
	11th to 20th	Night	7% 5% 2 5% 3 3 4 3 4 6% 58%	5880
	21st to end	Night	8% 5% 2 6% 4 4 4 3 5% 7% 53%	6468
February	1st to 10th	Night	9% 5% 2 4 4 3 4 2 5% 7% 55%	2590
	11th to 20th	Night	6% 3 1 3 4 3 4 4 6% 9% 57%	2338
	21st to end	Night	5% 5% 2 4 4 4 3 4 5% 6% 60%	1980
March	1st to 10th	Night	6% 5% 2 5% 5% 4 6% 5% 6% 7% 49%	5212
	11th to 20th	Night	3 4 2 5% 4 4 5% 4 5% 6% 60%	5000
	21st to end	Night	6% 4 1 4 3 3 4 4 6% 8% 56%	5270
April	1st to 10th	Night	4 4 1 4 3 4 4 4 5% 7% 61%	2000
	11th to 20th	Night	3 3 1 3 3 3 3 3 4 6% 66%	1979
	21st to end	Night	2 1 1 2 2 2 3 3 4 7% 74%	2000
May	1st to 10th	Night	1 1 1 1 1 2 1 3 4 85%	4560
	11th to 20th	Night	1 1 1 1 1 1 2 4 88%	3950
	21st to end	Night	1 1 1 1 1 2 2 91%	3972
June	1st to 10th	Night	1 1 2 2 2 3 3 4 5% 78%	1600
	11th to 20th	Night	1 1 1 1 1 2 3 89%	1600
	21st to end	Night	1 1 1 1 2 3 92%	1600
July	1st to 10th	Night	1 1 96%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 92%	3869
	21st to end	Night	1 1 1 1 1 2 3 90%	4472
August	1st to 10th	Night	1 1 1 1 1 1 2 91%	4141
	11th to 20th	Night	1 1 1 95%	4506
	21st to end	Night	1 1 1 1 1 1 2 2 90%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 91%	2000
	11th to 20th	Night	1 1 1 1 1 1 2 2 2 87%	2000
	21st to end	Night	2 1 2 1 1 1 2 2 4 86%	2218
October	1st to 10th	Night	2 1 1 1 1 2 2 3 4 83%	3836
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 82%	3777
	21st to end	Night	2 2 1 2 2 1 1 2 3 5% 78%	4121
November	1st to 10th	Night	4 2 1 2 2 2 3 3 4 7% 70%	2446
	11th to 20th	Night	5% 3 1 4 3 2 3 3 5% 8% 63%	2600
	21st to end	Night	5% 4 1 3 3 3 4 3 5% 7% 61%	2603
December	1st to 10th	Night	4 3 1 3 3 4 4 3 4 7% 62%	4560
	11th to 20th	Night	9% 6% 2 6% 5% 5% 5% 4 8% 11% 42%	5176
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 5% 7% 9% 39%	6952

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	26% 5% 19% 27% 17% 5%	3752
	11th to 20th	Day	29% 4 10% 14% 26% 18%	4100
	21st to end	Day	35% 4 11% 13% 25% 11%	4510
February	1st to 10th	Day	33% 4 8% 24% 20% 11%	1808
	11th to 20th	Day	29% 5% 12% 23% 24% 8%	1960
	21st to end	Day	32% 6% 9% 24% 22% 7%	1650
March	1st to 10th	Day	32% 6% 16% 18% 25% 2	4667
	11th to 20th	Day	27% 4 10% 34% 16% 9%	5000
	21st to end	Day	29% 7% 13% 32% 16% 3	5705
April	1st to 10th	Day	27% 5% 11% 27% 21% 8%	2400
	11th to 20th	Day	22% 5% 9% 29% 27% 8%	2372
	21st to end	Day	18% 3 8% 32% 31% 7%	2400
May	1st to 10th	Day	11% 3 5% 27% 38% 17%	5440
	11th to 20th	Day	7% 2 3 25% 46% 17%	5954
	21st to end	Day	6% 1 4 23% 52% 14%	6904
June	1st to 10th	Day	12% 3 7% 21% 35% 21%	2800
	11th to 20th	Day	4 1 4 16% 39% 37%	2800
	21st to end	Day	6% 1 1 7% 29% 56%	2800
July	1st to 10th	Day	2 1 2 19% 77%	6251
	11th to 20th	Day	5% 1 2 3 17% 72%	6010
	21st to end	Day	3 1 1 16% 31% 48%	6217
August	1st to 10th	Day	6% 1 1 9% 30% 52%	4126
	11th to 20th	Day	2 1 1 5% 35% 57%	3880
	21st to end	Day	3 1 2 8% 30% 56%	4311
September	1st to 10th	Day	4 1 2 9% 32% 52%	2400
	11th to 20th	Day	7% 2 2 18% 44% 27%	2400
	21st to end	Day	8% 2 3 16% 41% 30%	2179
October	1st to 10th	Day	9% 2 4 11% 25% 49%	3211
	11th to 20th	Day	9% 2 5% 25% 40% 19%	3161
	21st to end	Day	16% 2 4 17% 43% 18%	3448
November	1st to 10th	Day	20% 5% 10% 22% 27% 16%	1850
	11th to 20th	Day	21% 5% 12% 26% 27% 9%	1800
	21st to end	Day	25% 4 9% 23% 25% 13%	1795
December	1st to 10th	Day	27% 4 7% 23% 30% 9%	2620
	11th to 20th	Day	37% 6% 12% 19% 20% 5%	3028
	21st to end	Day	36% 9% 19% 23% 11% 3	4004
January	1st to 10th	Night	24% 4 18% 31% 19% 4	6195
	11th to 20th	Night	29% 4 11% 15% 30% 12%	5880
	21st to end	Night	32% 3 11% 17% 24% 13%	6468
February	1st to 10th	Night	30% 4 8% 22% 24% 12%	2590
	11th to 20th	Night	22% 5% 13% 25% 26% 9%	2339
	21st to end	Night	25% 5% 11% 31% 21% 7%	1980
March	1st to 10th	Night	31% 6% 16% 19% 24% 4	5212
	11th to 20th	Night	24% 5% 16% 30% 16% 9%	5000
	21st to end	Night	26% 5% 19% 30% 17% 3	5270
April	1st to 10th	Night	23% 4 15% 28% 23% 8%	2000
	11th to 20th	Night	18% 4 13% 29% 26% 10%	1979
	21st to end	Night	12% 3 12% 33% 31% 9%	2000
May	1st to 10th	Night	6% 2 6% 30% 38% 18%	4557
	11th to 20th	Night	5% 1 3 19% 46% 26%	3950
	21st to end	Night	4 1 3 21% 50% 21%	3972
June	1st to 10th	Night	11% 3 8% 19% 34% 26%	1600
	11th to 20th	Night	4 2 4 15% 36% 39%	1600
	21st to end	Night	3 1 1 9% 27% 60%	1600
July	1st to 10th	Night	2 2 13% 83%	3600
	11th to 20th	Night	4 1 2 12% 80%	3821
	21st to end	Night	4 1 1 10% 29% 56%	4345
August	1st to 10th	Night	6% 1 1 6% 23% 64%	3011
	11th to 20th	Night	3 1 4 24% 68%	3256
	21st to end	Night	4 1 2 7% 27% 59%	3608
September	1st to 10th	Night	4 2 2 7% 25% 60%	2000
	11th to 20th	Night	7% 1 2 16% 40% 34%	2000
	21st to end	Night	7% 2 2 15% 41% 34%	2218
October	1st to 10th	Night	8% 1 4 11% 19% 57%	3836
	11th to 20th	Night	9% 3 4 24% 36% 24%	3777
	21st to end	Night	12% 2 3 18% 44% 22%	4121
November	1st to 10th	Night	15% 4 9% 24% 28% 19%	2446
	11th to 20th	Night	20% 4 11% 23% 31% 11%	2600
	21st to end	Night	23% 5% 10% 21% 29% 13%	2603
December	1st to 10th	Night	23% 4 7% 27% 29% 11%	4560
	11th to 20th	Night	36% 5% 15% 19% 21% 4	5176
	21st to end	Night	40% 6% 18% 23% 11% 3	6952

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	19% 9% 27% 25% 17% 3	248
	11th to 20th	Day	8% 8% 24% 36% 20% 5%	400
	21st to end	Day	10% 7% 24% 29% 25% 5%	440
February	1st to 10th	Day	15% 13% 24% 22% 20% 6%	405
	11th to 20th	Day	9% 14% 27% 31% 17% 2	495
	21st to end	Day	10% 13% 28% 34% 13% 2	414
March	1st to 10th	Day	13% 17% 25% 31% 12% 1	500
	11th to 20th	Day	14% 12% 29% 27% 14% 3	500
	21st to end	Day	11% 13% 31% 32% 12% 1	550
April	1st to 10th	Day	10% 12% 28% 30% 18% 3	500
	11th to 20th	Day	7% 11% 29% 30% 18% 4	492
	21st to end	Day	4 13% 25% 33% 18% 8%	500
May	1st to 10th	Day	6% 10% 29% 31% 19% 5%	503
	11th to 20th	Day	4 9% 27% 34% 19% 8%	528
	21st to end	Day	4 9% 20% 32% 25% 9%	573
June	1st to 10th	Day	3 6% 22% 33% 25% 12%	530
	11th to 20th	Day	2 5% 22% 35% 28% 8%	530
	21st to end	Day	2 3 16% 36% 28% 15%	530
July	1st to 10th	Day	1 1 11% 36% 34% 17%	526
	11th to 20th	Day	5% 35% 39% 20%	524
	21st to end	Day	1 7% 35% 38% 18%	575
August	1st to 10th	Day	2 1 12% 37% 33% 15%	527
	11th to 20th	Day	1 13% 31% 38% 17%	517
	21st to end	Day	1 7% 32% 40% 20%	583
September	1st to 10th	Day	1 3 9% 34% 36% 17%	530
	11th to 20th	Day	3 3 17% 34% 29% 14%	530
	21st to end	Day	2 5% 13% 30% 39% 12%	530
October	1st to 10th	Day	1 4 19% 34% 32% 10%	503
	11th to 20th	Day	2 6% 15% 31% 36% 10%	490
	21st to end	Day	4 5% 18% 30% 33% 10%	539
November	1st to 10th	Day	6% 7% 20% 25% 33% 9%	386
	11th to 20th	Day	4 7% 19% 31% 28% 12%	300
	21st to end	Day	6% 5% 23% 34% 25% 7%	297
December	1st to 10th	Day	10% 7% 19% 31% 24% 11%	200
	11th to 20th	Day	8% 12% 19% 30% 23% 9%	200
	21st to end	Day	10% 15% 25% 28% 15% 7%	220
January	1st to 10th	Night	12% 5% 17% 33% 24% 10%	800
	11th to 20th	Night	7% 4 16% 36% 28% 9%	800
	21st to end	Night	10% 5% 15% 39% 24% 7%	880
February	1st to 10th	Night	12% 7% 16% 34% 24% 8%	800
	11th to 20th	Night	8% 9% 19% 32% 24% 8%	784
	21st to end	Night	8% 7% 19% 43% 20% 3	660
March	1st to 10th	Night	8% 10% 21% 36% 21% 5%	800
	11th to 20th	Night	7% 8% 21% 32% 26% 6%	800
	21st to end	Night	8% 9% 22% 35% 22% 4	880
April	1st to 10th	Night	5% 8% 24% 36% 23% 3	800
	11th to 20th	Night	5% 11% 24% 33% 21% 7%	791
	21st to end	Night	3 8% 26% 31% 26% 6%	800
May	1st to 10th	Night	3 5% 27% 35% 23% 7%	800
	11th to 20th	Night	2 6% 24% 37% 28% 4	546
	21st to end	Night	2 6% 24% 36% 29% 3	433
June	1st to 10th	Night	2 4 19% 45% 29% 2	400
	11th to 20th	Night	1 4 21% 45% 26% 5%	400
	21st to end	Night	2 10% 47% 36% 6%	400
July	1st to 10th	Night	1 11% 38% 44% 5%	390
	11th to 20th	Night	1 3 36% 49% 10%	487
	21st to end	Night	1 1 4 31% 51% 13%	657
August	1st to 10th	Night	1 8% 38% 44% 9%	640
	11th to 20th	Night	1 12% 37% 43% 8%	792
	21st to end	Night	1 1 6% 29% 55% 9%	880
September	1st to 10th	Night	1 9% 34% 45% 10%	800
	11th to 20th	Night	2 2 14% 34% 39% 9%	800
	21st to end	Night	2 3 12% 31% 41% 12%	800
October	1st to 10th	Night	4 3 10% 38% 35% 11%	772
	11th to 20th	Night	3 2 13% 32% 37% 12%	760
	21st to end	Night	4 2 11% 33% 34% 16%	836
November	1st to 10th	Night	4 4 13% 34% 33% 12%	780
	11th to 20th	Night	6% 3 14% 37% 30% 11%	800
	21st to end	Night	6% 4 16% 35% 30% 9%	800
December	1st to 10th	Night	9% 3 14% 39% 27% 8%	800
	11th to 20th	Night	12% 5% 15% 37% 26% 5%	800
	21st to end	Night	10% 9% 21% 35% 20% 5%	880

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