Dr Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dr Sky Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	3 2 1 3 2 2 3 3 4 6% 71%	118365
	Night	3 2 1 3 2 2 3 3 4 6% 70%	119215
Transparency	Day	14% 3 5% 12% 25% 41%	118132
	Night	15% 3 4 11% 24% 42%	119038
Seeing	Day	4 3 13% 29% 34% 17%	15592
	Night	4 1 12% 45% 35% 3	25834

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	4 3 2 5% 3 2 2 2 4 5% 68%	4098
	11th to 20th	Day	3 3 1 3 2 2 3 2 3 4 74%	4100
	21st to end	Day	6% 4 2 5% 3 4 4 3 5% 7% 56%	4510
February	1st to 10th	Day	3 3 1 4 3 3 3 3 5% 7% 66%	1890
	11th to 20th	Day	6% 3 1 3 4 3 3 3 4 6% 62%	1950
	21st to end	Day	5% 4 2 3 3 3 3 3 4 5% 66%	1670
March	1st to 10th	Day	4 3 1 3 2 2 2 2 4 5% 70%	4954
	11th to 20th	Day	4 4 1 5% 3 4 3 3 4 6% 63%	5000
	21st to end	Day	4 4 2 4 3 3 3 2 4 5% 67%	5486
April	1st to 10th	Day	1 2 1 3 2 2 3 3 4 7% 73%	2200
	11th to 20th	Day	1 2 1 2 2 3 2 2 3 4 79%	2281
	21st to end	Day	1 1 1 1 1 2 2 3 5% 81%	2496
May	1st to 10th	Day	1 1 1 1 1 1 2 3 89%	4110
	11th to 20th	Day	1 1 1 2 1 1 2 2 3 4 81%	4071
	21st to end	Day	1 1 1 1 1 1 2 2 90%	4477
June	1st to 10th	Day	1 1 2 2 1 2 2 3 5% 81%	2470
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 2 89%	2470
	21st to end	Day	2 1 1 2 3 2 3 2 4 5% 77%	2470
July	1st to 10th	Day	1 2 1 3 2 2 2 3 4 5% 75%	4110
	11th to 20th	Day	2 2 1 3 3 3 5% 4 7% 12% 59%	4110
	21st to end	Day	2 3 1 3 4 4 5% 5% 8% 12% 53%	4454
August	1st to 10th	Day	2 2 1 4 3 3 4 4 7% 11% 59%	4092
	11th to 20th	Day	2 2 1 3 3 3 4 3 7% 8% 65%	4028
	21st to end	Day	2 2 1 3 2 3 3 3 5% 7% 68%	4099
September	1st to 10th	Day	3 4 1 4 3 3 3 3 5% 6% 67%	2090
	11th to 20th	Day	2 1 1 1 2 2 2 3 3 4 79%	2090
	21st to end	Day	2 2 1 3 2 3 3 2 3 5% 76%	2090
October	1st to 10th	Day	2 1 2 1 2 2 2 3 4 81%	3430
	11th to 20th	Day	2 1 1 1 1 1 1 2 3 87%	3150
	21st to end	Day	1 1 2 1 1 1 2 4 5% 82%	3438
November	1st to 10th	Day	2 1 1 2 1 2 2 2 4 7% 76%	1959
	11th to 20th	Day	1 2 1 3 2 2 2 2 3 5% 75%	1853
	21st to end	Day	5% 3 2 4 2 2 3 3 4 7% 65%	1800
December	1st to 10th	Day	3 2 1 3 2 2 2 2 3 3 77%	2950
	11th to 20th	Day	3 3 2 4 3 3 3 4 6% 9% 60%	3409
	21st to end	Day	6% 4 2 4 3 3 3 3 5% 6% 62%	4510
January	1st to 10th	Night	3 3 1 4 3 3 4 3 5% 6% 65%	5867
	11th to 20th	Night	3 1 1 2 2 2 2 2 3 4 80%	5880
	21st to end	Night	8% 4 2 5% 3 3 4 3 4 6% 57%	6468
February	1st to 10th	Night	4 3 2 3 3 2 3 3 4 6% 67%	2507
	11th to 20th	Night	5% 4 2 4 3 3 3 3 4 6% 62%	2338
	21st to end	Night	6% 3 1 3 3 4 3 3 4 5% 66%	1958
March	1st to 10th	Night	5% 3 1 3 2 2 3 3 4 6% 69%	4940
	11th to 20th	Night	5% 3 2 4 3 3 3 3 5% 6% 65%	5000
	21st to end	Night	4 2 1 2 2 2 3 2 5% 7% 68%	5489
April	1st to 10th	Night	2 2 1 3 3 2 3 3 5% 6% 71%	2200
	11th to 20th	Night	2 3 1 2 2 2 2 2 3 4 77%	2066
	21st to end	Night	2 2 1 1 1 2 1 3 5% 83%	1795
May	1st to 10th	Night	1 1 1 1 1 1 1 1 2 4 87%	2860
	11th to 20th	Night	1 1 2 1 2 2 2 3 4 83%	2834
	21st to end	Night	1 1 1 1 1 1 1 2 91%	3110
June	1st to 10th	Night	1 2 2 2 2 2 2 3 6% 78%	1710
	11th to 20th	Night	1 1 1 1 2 2 3 88%	1710
	21st to end	Night	2 1 1 2 2 3 2 2 4 5% 75%	1710
July	1st to 10th	Night	2 2 1 3 3 2 3 3 4 6% 72%	2860
	11th to 20th	Night	1 2 1 3 3 4 5% 5% 7% 12% 57%	2860
	21st to end	Night	2 2 1 4 4 5% 7% 6% 10% 16% 43%	3109
August	1st to 10th	Night	2 2 1 5% 5% 5% 7% 6% 12% 14% 40%	2847
	11th to 20th	Night	2 2 1 3 3 4 6% 4 7% 9% 59%	2811
	21st to end	Night	2 3 1 4 3 3 4 4 5% 7% 64%	3567
September	1st to 10th	Night	3 3 1 4 3 2 5% 4 6% 8% 61%	2090
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 5% 76%	2090
	21st to end	Night	1 1 1 3 3 2 3 2 3 5% 75%	2090
October	1st to 10th	Night	1 1 1 2 2 2 2 2 3 6% 79%	3547
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 87%	3778
	21st to end	Night	1 1 1 1 1 1 2 2 3 5% 82%	4120
November	1st to 10th	Night	2 2 1 2 2 2 3 2 4 5% 76%	2340
	11th to 20th	Night	1 2 1 3 2 2 3 2 3 4 78%	2545
	21st to end	Night	4 3 1 3 3 3 3 3 4 6% 67%	2600
December	1st to 10th	Night	3 2 1 3 1 1 2 2 4 6% 74%	4240
	11th to 20th	Night	5% 3 1 5% 3 3 3 3 4 5% 65%	4811
	21st to end	Night	5% 4 2 4 3 4 3 3 4 5% 64%	6468

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	19% 2 4 12% 39% 23%	4099
	11th to 20th	Day	15% 3 3 14% 30% 36%	4100
	21st to end	Day	25% 5% 5% 12% 26% 27%	4510
February	1st to 10th	Day	19% 2 5% 15% 25% 34%	1890
	11th to 20th	Day	24% 3 5% 13% 27% 28%	1960
	21st to end	Day	19% 3 4 13% 29% 32%	1670
March	1st to 10th	Day	16% 3 4 12% 34% 31%	4954
	11th to 20th	Day	20% 5% 6% 17% 26% 26%	5000
	21st to end	Day	19% 4 3 11% 34% 29%	5486
April	1st to 10th	Day	12% 2 3 8% 34% 41%	2200
	11th to 20th	Day	11% 2 3 6% 24% 54%	2284
	21st to end	Day	8% 1 3 7% 23% 57%	2496
May	1st to 10th	Day	5% 1 2 7% 16% 68%	4110
	11th to 20th	Day	8% 2 3 6% 18% 64%	4074
	21st to end	Day	4 1 2 3 13% 78%	4477
June	1st to 10th	Day	8% 1 2 4 20% 66%	2470
	11th to 20th	Day	6% 1 1 2 9% 81%	2470
	21st to end	Day	12% 2 4 6% 13% 64%	2470
July	1st to 10th	Day	12% 3 5% 6% 11% 63%	4046
	11th to 20th	Day	15% 6% 9% 22% 27% 19%	4041
	21st to end	Day	19% 7% 10% 26% 25% 13%	4314
August	1st to 10th	Day	18% 5% 11% 26% 21% 18%	4078
	11th to 20th	Day	15% 5% 8% 18% 28% 25%	4067
	21st to end	Day	14% 5% 7% 14% 25% 35%	4098
September	1st to 10th	Day	18% 4 7% 12% 24% 36%	2090
	11th to 20th	Day	10% 2 4 7% 28% 49%	2090
	21st to end	Day	11% 4 4 6% 21% 54%	2090
October	1st to 10th	Day	9% 2 3 7% 17% 63%	3430
	11th to 20th	Day	6% 1 2 5% 27% 59%	3150
	21st to end	Day	7% 1 3 6% 26% 58%	3438
November	1st to 10th	Day	10% 2 3 12% 32% 41%	1959
	11th to 20th	Day	14% 2 3 10% 26% 45%	1853
	21st to end	Day	20% 2 5% 12% 25% 36%	1800
December	1st to 10th	Day	14% 2 2 14% 39% 29%	2950
	11th to 20th	Day	20% 3 5% 13% 31% 27%	3408
	21st to end	Day	21% 4 7% 20% 34% 14%	4510
January	1st to 10th	Night	18% 4 4 12% 38% 24%	5867
	11th to 20th	Night	11% 2 2 14% 34% 37%	5880
	21st to end	Night	26% 6% 5% 9% 26% 27%	6468
February	1st to 10th	Night	19% 3 5% 12% 23% 39%	2507
	11th to 20th	Night	23% 3 4 15% 25% 30%	2339
	21st to end	Night	22% 3 3 11% 25% 36%	1958
March	1st to 10th	Night	16% 3 3 12% 31% 36%	4940
	11th to 20th	Night	19% 4 5% 16% 29% 28%	5000
	21st to end	Night	14% 3 4 10% 36% 33%	5489
April	1st to 10th	Night	14% 3 4 8% 25% 47%	2200
	11th to 20th	Night	12% 2 3 5% 23% 56%	2066
	21st to end	Night	8% 1 2 6% 21% 63%	1795
May	1st to 10th	Night	5% 1 2 5% 20% 67%	2860
	11th to 20th	Night	7% 2 3 5% 17% 67%	2835
	21st to end	Night	4 1 1 2 9% 83%	3110
June	1st to 10th	Night	9% 1 3 4 14% 70%	1710
	11th to 20th	Night	3 1 2 3 10% 80%	1710
	21st to end	Night	13% 2 4 5% 10% 66%	1710
July	1st to 10th	Night	14% 3 5% 7% 10% 61%	2815
	11th to 20th	Night	16% 6% 9% 18% 28% 22%	2806
	21st to end	Night	23% 8% 10% 25% 20% 14%	3011
August	1st to 10th	Night	26% 7% 13% 22% 13% 18%	2838
	11th to 20th	Night	19% 6% 8% 16% 22% 27%	2838
	21st to end	Night	19% 5% 7% 12% 18% 40%	3567
September	1st to 10th	Night	20% 4 9% 15% 18% 34%	2090
	11th to 20th	Night	11% 3 3 9% 21% 52%	2090
	21st to end	Night	12% 4 4 6% 18% 56%	2090
October	1st to 10th	Night	9% 2 3 7% 14% 65%	3547
	11th to 20th	Night	5% 1 2 6% 21% 65%	3778
	21st to end	Night	8% 1 2 6% 17% 65%	4120
November	1st to 10th	Night	13% 2 3 9% 26% 48%	2340
	11th to 20th	Night	13% 2 3 8% 25% 48%	2545
	21st to end	Night	19% 2 5% 10% 26% 38%	2600
December	1st to 10th	Night	12% 2 3 15% 34% 33%	4240
	11th to 20th	Night	22% 2 4 10% 27% 35%	4811
	21st to end	Night	21% 4 6% 17% 31% 20%	6468

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	8% 3 13% 40% 27% 10%	300
	11th to 20th	Day	9% 2 12% 34% 34% 10%	300
	21st to end	Day	4 5% 18% 38% 24% 11%	330
February	1st to 10th	Day	6% 3 19% 31% 32% 9%	390
	11th to 20th	Day	9% 6% 23% 33% 24% 4	495
	21st to end	Day	7% 7% 23% 40% 19% 4	414
March	1st to 10th	Day	6% 9% 23% 35% 20% 7%	500
	11th to 20th	Day	6% 7% 22% 30% 22% 13%	500
	21st to end	Day	5% 8% 22% 33% 25% 6%	550
April	1st to 10th	Day	4 7% 16% 35% 25% 13%	500
	11th to 20th	Day	2 9% 25% 29% 26% 9%	492
	21st to end	Day	3 5% 22% 31% 28% 12%	481
May	1st to 10th	Day	1 4 21% 30% 30% 14%	463
	11th to 20th	Day	1 2 17% 30% 36% 14%	488
	21st to end	Day	1 4 17% 31% 35% 11%	529
June	1st to 10th	Day	2 2 8% 28% 40% 19%	490
	11th to 20th	Day	1 3 11% 32% 33% 20%	490
	21st to end	Day	2 1 7% 27% 43% 21%	490
July	1st to 10th	Day	2 1 4 20% 47% 27%	486
	11th to 20th	Day	4 4 15% 43% 35%	484
	21st to end	Day	4 3 15% 41% 37%	531
August	1st to 10th	Day	2 2 18% 40% 38%	487
	11th to 20th	Day	3 1 2 16% 44% 34%	477
	21st to end	Day	1 1 2 16% 43% 38%	539
September	1st to 10th	Day	4 1 6% 25% 41% 22%	490
	11th to 20th	Day	4 1 9% 25% 43% 19%	490
	21st to end	Day	3 2 12% 28% 32% 22%	490
October	1st to 10th	Day	3 3 12% 32% 35% 16%	456
	11th to 20th	Day	2 1 12% 26% 37% 21%	300
	21st to end	Day	1 2 13% 26% 37% 21%	330
November	1st to 10th	Day	3 2 13% 33% 37% 12%	300
	11th to 20th	Day	3 1 9% 34% 38% 14%	300
	21st to end	Day	6% 15% 33% 36% 10%	300
December	1st to 10th	Day	8% 1 8% 38% 34% 12%	300
	11th to 20th	Day	5% 2 12% 39% 31% 11%	300
	21st to end	Day	10% 3 17% 36% 24% 9%	330
January	1st to 10th	Night	6% 2 12% 52% 27% 2	800
	11th to 20th	Night	6% 2 12% 50% 27% 2	800
	21st to end	Night	10% 2 16% 49% 21% 2	880
February	1st to 10th	Night	6% 2 16% 47% 26% 3	800
	11th to 20th	Night	8% 2 19% 52% 19% 1	784
	21st to end	Night	8% 3 18% 51% 19% 1	660
March	1st to 10th	Night	5% 3 20% 54% 16% 2	800
	11th to 20th	Night	6% 2 17% 43% 27% 5%	800
	21st to end	Night	5% 3 16% 48% 26% 2	880
April	1st to 10th	Night	3 3 17% 51% 25% 2	800
	11th to 20th	Night	4 2 22% 48% 22% 1	682
	21st to end	Night	2 2 20% 50% 23% 3	587
May	1st to 10th	Night	2 2 18% 51% 25% 2	570
	11th to 20th	Night	2 1 16% 50% 30% 2	561
	21st to end	Night	3 13% 53% 29% 2	617
June	1st to 10th	Night	2 1 9% 52% 35% 2	570
	11th to 20th	Night	1 9% 52% 37% 2	570
	21st to end	Night	2 1 5% 40% 49% 2	570
July	1st to 10th	Night	3 4 38% 50% 4	555
	11th to 20th	Night	3 3 25% 62% 7%	552
	21st to end	Night	4 1 3 18% 64% 9%	624
August	1st to 10th	Night	3 1 1 20% 65% 10%	567
	11th to 20th	Night	1 4 25% 64% 6%	564
	21st to end	Night	4 3 21% 61% 11%	745
September	1st to 10th	Night	4 1 5% 30% 53% 7%	760
	11th to 20th	Night	2 1 8% 37% 49% 4	760
	21st to end	Night	2 1 9% 42% 42% 5%	760
October	1st to 10th	Night	3 10% 48% 38% 1	760
	11th to 20th	Night	2 1 10% 48% 38% 2	760
	21st to end	Night	2 1 11% 44% 37% 5%	836
November	1st to 10th	Night	4 1 8% 49% 35% 3	780
	11th to 20th	Night	2 11% 48% 36% 3	800
	21st to end	Night	5% 2 13% 49% 28% 2	800
December	1st to 10th	Night	7% 2 10% 48% 32% 1	800
	11th to 20th	Night	10% 2 9% 51% 27% 2	800
	21st to end	Night	8% 1 14% 51% 24% 3	880

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