Dark Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Sky Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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	14% 5% 2 6% 4 4 4 4 6% 8% 43%	101084
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	97958
Transparency	Day	38% 4 11% 21% 19% 6%	104852
	Night	40% 4 10% 19% 20% 7%	102329
Seeing	Day	17% 8% 21% 26% 19% 10%	15052
	Night	17% 6% 21% 31% 21% 4	25086

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	18% 4 1 4 3 3 3 3 5% 8% 47%	2836
	11th to 20th	Day	15% 6% 2 5% 4 3 2 3 4 3 54%	2840
	21st to end	Day	27% 4 2 3 3 2 2 3 4 5% 46%	3124
February	1st to 10th	Day	17% 5% 2 5% 4 4 4 4 5% 7% 43%	1847
	11th to 20th	Day	21% 6% 2 5% 4 3 3 3 5% 5% 44%	1848
	21st to end	Day	21% 5% 2 6% 4 4 3 3 4 5% 42%	1570
March	1st to 10th	Day	25% 7% 2 6% 3 3 3 3 4 6% 39%	3338
	11th to 20th	Day	13% 4 2 4 3 3 4 3 5% 6% 53%	3480
	21st to end	Day	20% 5% 2 5% 4 4 4 3 5% 6% 42%	3815
April	1st to 10th	Day	13% 4 1 4 4 4 4 4 7% 8% 47%	2090
	11th to 20th	Day	12% 5% 2 5% 5% 4 4 3 5% 7% 47%	2249
	21st to end	Day	11% 4 1 4 3 3 4 3 6% 8% 52%	2470
May	1st to 10th	Day	12% 6% 3 7% 4 5% 5% 4 6% 8% 40%	4082
	11th to 20th	Day	17% 7% 3 6% 4 4 4 4 6% 7% 38%	4081
	21st to end	Day	7% 4 2 5% 4 4 6% 4 8% 10% 44%	4476
June	1st to 10th	Day	10% 6% 3 7% 5% 6% 6% 6% 8% 11% 33%	2470
	11th to 20th	Day	5% 4 2 5% 5% 5% 7% 7% 10% 11% 39%	2470
	21st to end	Day	7% 4 2 6% 6% 6% 5% 5% 8% 11% 39%	2470
July	1st to 10th	Day	7% 6% 2 8% 6% 6% 7% 6% 8% 11% 34%	4120
	11th to 20th	Day	8% 4 2 6% 5% 6% 6% 5% 8% 11% 38%	4120
	21st to end	Day	9% 7% 3 10% 8% 8% 8% 7% 9% 10% 20%	4466
August	1st to 10th	Day	13% 7% 2 6% 5% 5% 6% 5% 7% 11% 33%	4104
	11th to 20th	Day	9% 4 2 6% 4 6% 6% 4 8% 10% 41%	4044
	21st to end	Day	8% 4 2 5% 4 4 4 4 6% 8% 51%	4082
September	1st to 10th	Day	8% 4 2 7% 5% 5% 6% 5% 7% 9% 41%	2090
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 40%	2090
	21st to end	Day	16% 6% 2 5% 3 3 4 3 5% 8% 46%	2090
October	1st to 10th	Day	14% 4 2 4 2 3 3 2 3 6% 56%	2086
	11th to 20th	Day	13% 4 2 4 3 3 2 2 4 5% 58%	1900
	21st to end	Day	14% 5% 1 4 3 3 2 3 4 4 58%	2090
November	1st to 10th	Day	15% 5% 2 3 2 2 2 2 4 5% 56%	1839
	11th to 20th	Day	15% 5% 2 3 3 4 3 3 5% 6% 53%	1710
	21st to end	Day	17% 5% 2 4 4 3 3 3 5% 7% 49%	1710
December	1st to 10th	Day	23% 8% 2 5% 4 4 3 3 5% 6% 39%	1710
	11th to 20th	Day	18% 5% 1 5% 3 3 4 4 5% 5% 48%	2153
	21st to end	Day	23% 5% 1 4 3 3 4 3 5% 7% 42%	3124
January	1st to 10th	Night	18% 4 2 4 4 3 3 3 4 6% 48%	4109
	11th to 20th	Night	13% 6% 3 6% 4 3 3 4 5% 6% 48%	4120
	21st to end	Night	26% 6% 2 4 3 3 3 2 3 5% 43%	4532
February	1st to 10th	Night	16% 6% 2 5% 4 4 3 3 4 6% 47%	2328
	11th to 20th	Night	21% 6% 2 5% 5% 3 3 3 4 4 45%	2218
	21st to end	Night	19% 6% 2 5% 4 4 3 3 5% 5% 43%	1884
March	1st to 10th	Night	24% 6% 3 6% 4 3 3 3 4 6% 37%	3514
	11th to 20th	Night	11% 4 1 4 3 3 4 4 6% 8% 52%	3480
	21st to end	Night	21% 5% 2 5% 4 4 4 3 4 5% 42%	3817
April	1st to 10th	Night	14% 4 2 5% 3 4 4 3 5% 6% 50%	2090
	11th to 20th	Night	13% 5% 2 5% 3 3 2 3 5% 7% 50%	1893
	21st to end	Night	11% 5% 2 5% 4 4 4 4 4 7% 51%	1710
May	1st to 10th	Night	13% 6% 3 7% 5% 4 4 3 6% 9% 38%	2813
	11th to 20th	Night	19% 6% 3 5% 4 4 4 4 5% 7% 38%	2812
	21st to end	Night	9% 5% 2 6% 4 5% 4 4 8% 10% 44%	3079
June	1st to 10th	Night	10% 7% 2 7% 5% 6% 7% 5% 8% 11% 34%	1710
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 5% 8% 11% 41%	1710
	21st to end	Night	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	1710
July	1st to 10th	Night	11% 8% 3 8% 6% 7% 7% 6% 8% 9% 28%	2840
	11th to 20th	Night	8% 5% 2 7% 5% 6% 7% 7% 10% 12% 31%	2840
	21st to end	Night	13% 9% 4 11% 7% 6% 6% 6% 7% 10% 21%	3087
August	1st to 10th	Night	13% 7% 3 8% 5% 6% 6% 5% 6% 10% 32%	2825
	11th to 20th	Night	13% 8% 3 9% 6% 6% 5% 5% 7% 10% 28%	2785
	21st to end	Night	9% 5% 2 7% 5% 5% 5% 4 6% 9% 43%	3522
September	1st to 10th	Night	10% 7% 2 7% 6% 6% 5% 4 7% 9% 38%	2090
	11th to 20th	Night	17% 7% 2 6% 5% 4 5% 4 6% 7% 35%	2090
	21st to end	Night	16% 5% 2 6% 4 4 4 3 6% 5% 45%	2090
October	1st to 10th	Night	14% 4 2 5% 3 3 4 3 5% 5% 51%	2091
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 5% 5% 55%	2280
	21st to end	Night	13% 4 1 4 3 3 2 3 4 5% 58%	2508
November	1st to 10th	Night	15% 4 2 5% 3 2 3 2 3 4 56%	2397
	11th to 20th	Night	16% 6% 2 4 3 2 3 2 3 6% 53%	2470
	21st to end	Night	17% 4 2 4 3 3 3 4 5% 5% 48%	2470
December	1st to 10th	Night	21% 6% 2 6% 4 4 3 3 3 5% 42%	2470
	11th to 20th	Night	18% 5% 2 4 3 2 3 3 4 5% 52%	3042
	21st to end	Night	22% 6% 2 5% 4 3 5% 4 5% 7% 37%	4532

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	38% 2 5% 24% 24% 8%	2837
	11th to 20th	Day	37% 2 6% 17% 31% 7%	2840
	21st to end	Day	42% 1 4 19% 27% 8%	3124
February	1st to 10th	Day	42% 2 7% 21% 20% 7%	1847
	11th to 20th	Day	43% 3 6% 19% 22% 7%	1858
	21st to end	Day	45% 3 6% 16% 20% 9%	1570
March	1st to 10th	Day	50% 2 6% 17% 15% 11%	3338
	11th to 20th	Day	34% 3 8% 15% 27% 13%	3480
	21st to end	Day	45% 2 5% 23% 21% 3	3815
April	1st to 10th	Day	34% 4 10% 21% 26% 6%	2090
	11th to 20th	Day	37% 3 7% 21% 22% 10%	2249
	21st to end	Day	31% 3 10% 30% 22% 4	2470
May	1st to 10th	Day	42% 3 14% 22% 15% 4	4120
	11th to 20th	Day	46% 3 12% 23% 15% 1	4085
	21st to end	Day	34% 4 16% 23% 21% 2	4489
June	1st to 10th	Day	41% 6% 18% 24% 9% 2	2470
	11th to 20th	Day	33% 7% 20% 25% 13% 2	2470
	21st to end	Day	35% 9% 18% 21% 14% 4	2470
July	1st to 10th	Day	40% 10% 20% 21% 7% 1	4055
	11th to 20th	Day	37% 9% 25% 21% 6% 2	4051
	21st to end	Day	55% 10% 17% 12% 5%	4324
August	1st to 10th	Day	43% 6% 16% 28% 7%	4091
	11th to 20th	Day	36% 7% 22% 19% 14% 2	4083
	21st to end	Day	30% 5% 15% 26% 22% 3	4082
September	1st to 10th	Day	37% 6% 14% 19% 20% 4	2090
	11th to 20th	Day	42% 5% 11% 20% 20% 3	2090
	21st to end	Day	38% 3 11% 22% 20% 6%	2090
October	1st to 10th	Day	28% 3 6% 18% 35% 11%	3463
	11th to 20th	Day	21% 1 5% 16% 30% 27%	3140
	21st to end	Day	29% 2 4 22% 29% 15%	3428
November	1st to 10th	Day	32% 2 7% 19% 29% 10%	1839
	11th to 20th	Day	35% 2 5% 22% 27% 9%	1710
	21st to end	Day	37% 2 7% 22% 25% 7%	1710
December	1st to 10th	Day	47% 3 6% 19% 18% 7%	1710
	11th to 20th	Day	40% 2 5% 20% 22% 12%	2150
	21st to end	Day	44% 1 5% 23% 24% 3	3124
January	1st to 10th	Night	39% 2 5% 18% 28% 8%	4109
	11th to 20th	Night	39% 2 4 18% 27% 9%	4120
	21st to end	Night	45% 2 3 17% 27% 6%	4532
February	1st to 10th	Night	40% 2 7% 21% 23% 7%	2328
	11th to 20th	Night	45% 2 5% 18% 23% 7%	2219
	21st to end	Night	43% 3 7% 18% 20% 9%	1884
March	1st to 10th	Night	50% 2 7% 16% 17% 8%	3514
	11th to 20th	Night	32% 3 8% 16% 27% 15%	3480
	21st to end	Night	45% 2 5% 19% 19% 9%	3817
April	1st to 10th	Night	36% 3 8% 21% 26% 7%	2090
	11th to 20th	Night	34% 3 6% 23% 24% 10%	1893
	21st to end	Night	34% 3 9% 28% 22% 4	1710
May	1st to 10th	Night	43% 3 13% 23% 16% 2	2840
	11th to 20th	Night	46% 3 13% 21% 16% 1	2813
	21st to end	Night	35% 4 13% 27% 19% 2	3087
June	1st to 10th	Night	41% 8% 16% 20% 11% 2	1710
	11th to 20th	Night	35% 7% 18% 25% 13% 2	1710
	21st to end	Night	36% 8% 18% 18% 15% 4	1710
July	1st to 10th	Night	50% 9% 19% 16% 6% 1	2795
	11th to 20th	Night	39% 12% 25% 18% 4 1	2786
	21st to end	Night	56% 9% 17% 13% 4 1	2988
August	1st to 10th	Night	45% 8% 21% 21% 5%	2814
	11th to 20th	Night	50% 9% 16% 17% 9% 1	2812
	21st to end	Night	37% 7% 13% 22% 19% 2	3522
September	1st to 10th	Night	40% 7% 14% 19% 16% 4	2090
	11th to 20th	Night	46% 6% 11% 18% 16% 3	2090
	21st to end	Night	41% 4 11% 19% 19% 7%	2090
October	1st to 10th	Night	29% 4 10% 14% 33% 9%	3494
	11th to 20th	Night	21% 3 5% 17% 30% 25%	3779
	21st to end	Night	28% 2 4 21% 27% 18%	4120
November	1st to 10th	Night	34% 2 6% 19% 27% 11%	2397
	11th to 20th	Night	35% 2 4 18% 29% 12%	2470
	21st to end	Night	37% 4 5% 21% 26% 7%	2470
December	1st to 10th	Night	47% 2 6% 19% 20% 7%	2470
	11th to 20th	Night	37% 2 7% 17% 27% 10%	3044
	21st to end	Night	47% 2 5% 22% 19% 5%	4532

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	23% 14% 36% 22% 5%	270
	11th to 20th	Day	26% 17% 29% 23% 5% 1	270
	21st to end	Day	22% 18% 38% 17% 4 1	297
February	1st to 10th	Day	23% 16% 32% 25% 4	270
	11th to 20th	Day	34% 15% 35% 16% 1	270
	21st to end	Day	28% 23% 36% 11% 2	225
March	1st to 10th	Day	21% 13% 29% 27% 9% 1	397
	11th to 20th	Day	20% 12% 39% 21% 7% 1	460
	21st to end	Day	24% 12% 31% 22% 10% 1	506
April	1st to 10th	Day	18% 15% 28% 23% 13% 2	460
	11th to 20th	Day	15% 12% 32% 24% 14% 3	452
	21st to end	Day	18% 8% 23% 26% 18% 7%	460
May	1st to 10th	Day	13% 11% 28% 27% 13% 8%	463
	11th to 20th	Day	18% 4 18% 28% 22% 11%	487
	21st to end	Day	15% 7% 18% 24% 22% 14%	529
June	1st to 10th	Day	14% 5% 13% 32% 24% 12%	490
	11th to 20th	Day	7% 4 14% 29% 25% 21%	490
	21st to end	Day	10% 2 12% 32% 24% 20%	490
July	1st to 10th	Day	10% 3 8% 22% 32% 25%	485
	11th to 20th	Day	8% 2 7% 25% 34% 23%	484
	21st to end	Day	7% 2 6% 31% 35% 20%	532
August	1st to 10th	Day	15% 4 13% 25% 24% 20%	486
	11th to 20th	Day	11% 3 8% 26% 35% 18%	479
	21st to end	Day	14% 3 7% 26% 30% 21%	539
September	1st to 10th	Day	10% 2 11% 30% 30% 17%	490
	11th to 20th	Day	20% 4 10% 21% 29% 16%	490
	21st to end	Day	19% 4 15% 27% 26% 10%	490
October	1st to 10th	Day	19% 4 14% 30% 23% 10%	490
	11th to 20th	Day	13% 6% 21% 31% 23% 6%	490
	21st to end	Day	18% 6% 20% 30% 21% 5%	539
November	1st to 10th	Day	22% 7% 24% 26% 16% 5%	395
	11th to 20th	Day	17% 12% 31% 30% 8% 2	270
	21st to end	Day	12% 13% 31% 29% 13% 3	270
December	1st to 10th	Day	31% 10% 29% 21% 8%	270
	11th to 20th	Day	23% 12% 30% 25% 9% 1	270
	21st to end	Day	31% 14% 30% 15% 7% 3	297
January	1st to 10th	Night	22% 11% 31% 30% 5%	760
	11th to 20th	Night	22% 11% 31% 31% 5%	760
	21st to end	Night	21% 12% 36% 28% 3	836
February	1st to 10th	Night	19% 8% 30% 37% 6%	760
	11th to 20th	Night	25% 10% 34% 27% 4	743
	21st to end	Night	21% 11% 34% 30% 4	628
March	1st to 10th	Night	18% 9% 31% 35% 7%	760
	11th to 20th	Night	16% 10% 33% 31% 10%	760
	21st to end	Night	20% 9% 29% 31% 11%	836
April	1st to 10th	Night	17% 12% 27% 34% 11%	760
	11th to 20th	Night	16% 8% 26% 33% 16% 1	641
	21st to end	Night	13% 4 24% 36% 21% 2	570
May	1st to 10th	Night	13% 5% 23% 36% 22% 1	570
	11th to 20th	Night	16% 5% 17% 33% 26% 3	561
	21st to end	Night	15% 2 10% 32% 35% 6%	617
June	1st to 10th	Night	13% 2 10% 31% 36% 7%	570
	11th to 20th	Night	10% 3 11% 26% 35% 14%	570
	21st to end	Night	9% 2 6% 35% 38% 9%	570
July	1st to 10th	Night	12% 3 7% 24% 40% 14%	555
	11th to 20th	Night	13% 2 4 22% 44% 16%	552
	21st to end	Night	9% 3 5% 25% 46% 12%	624
August	1st to 10th	Night	14% 3 7% 25% 38% 14%	567
	11th to 20th	Night	13% 2 8% 27% 40% 10%	564
	21st to end	Night	14% 2 5% 25% 42% 12%	669
September	1st to 10th	Night	11% 2 10% 29% 38% 11%	760
	11th to 20th	Night	19% 4 11% 26% 35% 6%	760
	21st to end	Night	17% 4 12% 34% 29% 4	760
October	1st to 10th	Night	15% 5% 17% 29% 28% 7%	760
	11th to 20th	Night	14% 6% 22% 34% 22% 2	760
	21st to end	Night	15% 6% 20% 33% 23% 3	836
November	1st to 10th	Night	17% 5% 18% 36% 20% 4	771
	11th to 20th	Night	19% 8% 27% 32% 13% 2	760
	21st to end	Night	19% 6% 23% 37% 15% 1	760
December	1st to 10th	Night	25% 7% 25% 33% 10% 1	760
	11th to 20th	Night	21% 7% 27% 33% 12% 1	760
	21st to end	Night	27% 8% 25% 31% 9%	836

Page generated at 2024-11-05 08:15:47. Version ac07

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