Dark Sky Walker Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Sky Walker 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% 6% 2 5% 4 4 4 3 5% 7% 47%	114572
	Night	15% 6% 2 4 3 3 3 3 4 6% 51%	114932
Transparency	Day	36% 5% 9% 20% 19% 11%	118418
	Night	35% 4 10% 19% 19% 13%	119215
Seeing	Day	15% 9% 24% 30% 18% 4	15471
	Night	17% 8% 26% 35% 13% 1	25496

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	14% 9% 3 6% 3 3 2 2 3 4 50%	4077
	11th to 20th	Day	15% 6% 2 4 2 2 3 3 5% 5% 54%	4080
	21st to end	Day	23% 8% 2 5% 2 2 3 2 4 7% 43%	4930
February	1st to 10th	Day	18% 7% 2 6% 4 4 3 2 4 4 46%	2000
	11th to 20th	Day	16% 8% 3 5% 4 4 4 3 5% 5% 46%	1949
	21st to end	Day	18% 6% 2 4 4 2 2 3 4 5% 50%	1650
March	1st to 10th	Day	13% 6% 2 4 3 3 3 3 4 6% 54%	4808
	11th to 20th	Day	15% 8% 2 6% 3 3 3 2 3 5% 51%	4980
	21st to end	Day	16% 8% 2 6% 3 3 3 3 4 7% 45%	5462
April	1st to 10th	Day	14% 7% 3 7% 4 3 3 3 5% 5% 45%	2200
	11th to 20th	Day	20% 6% 2 5% 3 4 4 3 4 5% 43%	2224
	21st to end	Day	14% 8% 3 6% 5% 4 4 3 4 6% 43%	2510
May	1st to 10th	Day	22% 9% 2 7% 5% 4 4 3 5% 6% 32%	4212
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 5% 7% 43%	4212
	21st to end	Day	11% 7% 2 6% 5% 5% 5% 5% 7% 9% 37%	4543
June	1st to 10th	Day	6% 6% 2 5% 5% 5% 6% 5% 6% 9% 47%	2470
	11th to 20th	Day	8% 6% 2 6% 5% 4 5% 5% 6% 8% 45%	2470
	21st to end	Day	8% 6% 2 7% 6% 6% 6% 5% 7% 9% 38%	2470
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 4 6% 8% 46%	4120
	11th to 20th	Day	5% 4 2 5% 4 5% 6% 4 7% 11% 48%	4120
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 5% 6% 11% 38%	4465
August	1st to 10th	Day	1 1 1 2 3 3 3 3 5% 8% 69%	3978
	11th to 20th	Day	3 3 1 3 3 4 3 3 6% 11% 58%	3726
	21st to end	Day	4 4 2 7% 4 5% 5% 4 7% 11% 46%	4054
September	1st to 10th	Day	11% 7% 3 7% 5% 4 5% 3 5% 6% 43%	2090
	11th to 20th	Day	12% 6% 2 7% 4 5% 6% 4 7% 9% 39%	2090
	21st to end	Day	8% 5% 3 6% 4 4 4 3 7% 7% 49%	2090
October	1st to 10th	Day	7% 4 2 5% 3 4 5% 3 5% 6% 55%	2090
	11th to 20th	Day	12% 6% 2 4 4 3 4 3 5% 6% 51%	2000
	21st to end	Day	12% 4 2 4 4 3 3 2 4 5% 57%	2090
November	1st to 10th	Day	14% 7% 3 6% 4 4 4 3 5% 6% 44%	1958
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 6% 49%	1835
	21st to end	Day	16% 7% 3 5% 4 3 3 3 5% 6% 46%	1800
December	1st to 10th	Day	27% 8% 3 6% 3 2 2 2 3 6% 39%	2940
	11th to 20th	Day	17% 8% 2 6% 4 4 3 3 5% 5% 44%	3391
	21st to end	Day	13% 6% 2 4 3 2 2 2 3 4 59%	4488
January	1st to 10th	Night	18% 6% 3 5% 2 3 2 2 3 5% 51%	5888
	11th to 20th	Night	12% 3 1 3 2 2 2 3 4 5% 64%	5900
	21st to end	Night	27% 7% 2 5% 3 3 3 2 3 5% 41%	6076
February	1st to 10th	Night	17% 8% 2 5% 3 3 3 2 4 5% 49%	2400
	11th to 20th	Night	18% 8% 2 5% 3 3 2 2 3 5% 50%	2338
	21st to end	Night	20% 7% 1 4 3 3 3 3 4 5% 47%	1980
March	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 5% 6% 46%	5039
	11th to 20th	Night	16% 9% 2 5% 2 3 2 2 3 5% 51%	4980
	21st to end	Night	20% 9% 2 4 3 3 3 3 4 5% 44%	5467
April	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 5% 6% 47%	2200
	11th to 20th	Night	19% 7% 2 5% 3 3 3 3 4 6% 46%	2121
	21st to end	Night	20% 7% 2 5% 4 3 3 2 4 4 46%	1890
May	1st to 10th	Night	26% 9% 2 5% 3 3 3 2 4 5% 37%	2913
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 4 6% 50%	2913
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 9% 44%	3140
June	1st to 10th	Night	6% 4 2 4 2 3 3 3 5% 8% 59%	1710
	11th to 20th	Night	5% 4 1 4 4 4 4 3 5% 8% 58%	1710
	21st to end	Night	8% 5% 2 4 5% 3 5% 3 6% 10% 50%	1710
July	1st to 10th	Night	6% 5% 1 4 3 3 4 4 6% 9% 54%	2850
	11th to 20th	Night	4 3 2 4 4 4 5% 5% 7% 10% 53%	2850
	21st to end	Night	11% 8% 3 6% 4 4 4 4 6% 9% 41%	3098
August	1st to 10th	Night	1 1 1 2 2 2 3 2 4 8% 73%	2956
	11th to 20th	Night	3 3 1 3 3 4 4 3 6% 12% 60%	3113
	21st to end	Night	5% 5% 1 4 4 3 4 4 6% 10% 53%	3606
September	1st to 10th	Night	9% 6% 2 6% 4 4 4 3 6% 7% 49%	2090
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 9% 45%	2090
	21st to end	Night	9% 5% 2 5% 4 3 4 4 6% 8% 52%	2090
October	1st to 10th	Night	6% 4 2 4 3 3 3 3 5% 7% 60%	2090
	11th to 20th	Night	13% 5% 1 5% 4 3 4 3 4 5% 54%	2160
	21st to end	Night	15% 4 1 4 2 2 2 2 3 4 61%	2508
November	1st to 10th	Night	15% 6% 2 5% 4 3 3 2 3 5% 52%	2340
	11th to 20th	Night	11% 6% 1 5% 3 3 3 3 4 6% 55%	2560
	21st to end	Night	18% 6% 2 4 2 3 3 3 4 7% 48%	2600
December	1st to 10th	Night	24% 8% 2 6% 3 3 3 3 4 5% 39%	4250
	11th to 20th	Night	19% 9% 3 5% 3 3 2 2 3 3 48%	4816
	21st to end	Night	17% 6% 2 3 2 2 2 2 3 4 57%	6490

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	40% 2 4 18% 23% 13%	4078
	11th to 20th	Day	34% 2 5% 15% 25% 20%	4080
	21st to end	Day	44% 2 5% 10% 25% 13%	4930
February	1st to 10th	Day	43% 3 5% 12% 20% 17%	2000
	11th to 20th	Day	42% 3 6% 18% 18% 14%	1959
	21st to end	Day	38% 4 6% 16% 20% 16%	1650
March	1st to 10th	Day	34% 3 5% 17% 17% 24%	4808
	11th to 20th	Day	39% 3 4 8% 15% 31%	4980
	21st to end	Day	40% 3 9% 17% 20% 11%	5464
April	1st to 10th	Day	42% 4 8% 17% 17% 13%	2200
	11th to 20th	Day	43% 4 7% 16% 18% 12%	2224
	21st to end	Day	43% 5% 6% 19% 17% 10%	2510
May	1st to 10th	Day	52% 5% 9% 14% 14% 6%	4250
	11th to 20th	Day	40% 6% 9% 18% 20% 7%	4216
	21st to end	Day	39% 9% 11% 29% 11% 2	4555
June	1st to 10th	Day	32% 8% 11% 25% 20% 3	2470
	11th to 20th	Day	35% 7% 10% 25% 19% 4	2470
	21st to end	Day	40% 9% 10% 23% 14% 4	2470
July	1st to 10th	Day	34% 6% 16% 37% 6% 1	4056
	11th to 20th	Day	29% 6% 18% 30% 15% 2	4051
	21st to end	Day	37% 6% 18% 29% 9%	4325
August	1st to 10th	Day	15% 4 11% 44% 26%	3964
	11th to 20th	Day	21% 4 24% 38% 11% 2	3762
	21st to end	Day	31% 7% 18% 24% 18% 2	4053
September	1st to 10th	Day	41% 7% 10% 18% 16% 8%	2090
	11th to 20th	Day	39% 9% 11% 18% 18% 5%	2090
	21st to end	Day	32% 7% 9% 22% 17% 13%	2090
October	1st to 10th	Day	19% 4 6% 12% 25% 35%	3480
	11th to 20th	Day	26% 3 8% 18% 22% 22%	3305
	21st to end	Day	27% 4 5% 17% 34% 13%	3428
November	1st to 10th	Day	41% 4 7% 18% 17% 12%	1958
	11th to 20th	Day	36% 4 7% 16% 21% 16%	1835
	21st to end	Day	41% 4 6% 15% 20% 14%	1800
December	1st to 10th	Day	51% 2 5% 19% 14% 10%	2940
	11th to 20th	Day	42% 3 8% 12% 20% 14%	3389
	21st to end	Day	32% 2 4 15% 29% 18%	4488
January	1st to 10th	Night	38% 2 4 17% 23% 15%	5888
	11th to 20th	Night	25% 2 4 18% 27% 23%	5900
	21st to end	Night	48% 2 3 9% 21% 16%	6076
February	1st to 10th	Night	39% 3 6% 12% 21% 19%	2400
	11th to 20th	Night	40% 2 6% 16% 19% 16%	2338
	21st to end	Night	41% 4 6% 14% 19% 17%	1980
March	1st to 10th	Night	40% 3 6% 17% 17% 17%	5039
	11th to 20th	Night	38% 3 6% 10% 15% 27%	4980
	21st to end	Night	42% 3 8% 18% 17% 12%	5467
April	1st to 10th	Night	38% 4 9% 17% 15% 16%	2200
	11th to 20th	Night	41% 4 8% 14% 17% 15%	2121
	21st to end	Night	43% 4 9% 18% 17% 10%	1890
May	1st to 10th	Night	50% 4 8% 16% 13% 9%	2940
	11th to 20th	Night	35% 7% 10% 21% 19% 8%	2914
	21st to end	Night	38% 6% 14% 27% 11% 4	3149
June	1st to 10th	Night	23% 6% 17% 31% 21% 3	1710
	11th to 20th	Night	25% 5% 14% 30% 21% 5%	1710
	21st to end	Night	30% 7% 17% 27% 16% 3	1710
July	1st to 10th	Night	25% 7% 25% 36% 5% 1	2805
	11th to 20th	Night	25% 7% 21% 30% 15% 1	2796
	21st to end	Night	38% 6% 22% 26% 7% 1	2999
August	1st to 10th	Night	12% 4 16% 46% 22%	2946
	11th to 20th	Night	19% 6% 31% 34% 9% 2	3143
	21st to end	Night	26% 11% 20% 25% 16% 2	3606
September	1st to 10th	Night	34% 7% 15% 20% 17% 8%	2090
	11th to 20th	Night	36% 8% 15% 21% 15% 5%	2090
	21st to end	Night	30% 7% 12% 21% 18% 12%	2090
October	1st to 10th	Night	17% 4 8% 14% 21% 37%	3480
	11th to 20th	Night	24% 3 9% 15% 30% 19%	3581
	21st to end	Night	26% 2 6% 18% 32% 15%	4120
November	1st to 10th	Night	37% 4 8% 18% 20% 14%	2340
	11th to 20th	Night	32% 4 8% 18% 20% 17%	2560
	21st to end	Night	38% 4 8% 16% 20% 15%	2600
December	1st to 10th	Night	49% 3 6% 15% 15% 12%	4250
	11th to 20th	Night	44% 2 5% 10% 21% 18%	4817
	21st to end	Night	34% 2 4 14% 28% 18%	6490

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% 18% 35% 20% 4	300
	11th to 20th	Day	27% 16% 30% 21% 6% 1	300
	21st to end	Day	22% 18% 36% 19% 5%	330
February	1st to 10th	Day	28% 17% 28% 24% 3	300
	11th to 20th	Day	27% 21% 37% 12% 3	300
	21st to end	Day	25% 17% 44% 12% 2	249
March	1st to 10th	Day	23% 19% 40% 16% 1	300
	11th to 20th	Day	19% 20% 41% 14% 5%	300
	21st to end	Day	22% 17% 41% 16% 4 1	330
April	1st to 10th	Day	20% 18% 42% 17% 3	300
	11th to 20th	Day	23% 18% 34% 21% 4	342
	21st to end	Day	19% 17% 40% 21% 3	610
May	1st to 10th	Day	21% 13% 37% 25% 5%	703
	11th to 20th	Day	19% 8% 33% 32% 6% 2	725
	21st to end	Day	17% 7% 31% 33% 12% 1	760
June	1st to 10th	Day	8% 4 19% 43% 23% 3	680
	11th to 20th	Day	8% 2 22% 48% 18% 3	680
	21st to end	Day	9% 5% 14% 39% 29% 5%	680
July	1st to 10th	Day	10% 2 14% 35% 30% 8%	671
	11th to 20th	Day	7% 7% 40% 42% 5%	668
	21st to end	Day	5% 2 8% 37% 40% 7%	739
August	1st to 10th	Day	3 1 4 43% 39% 10%	602
	11th to 20th	Day	6% 2 5% 41% 38% 8%	477
	21st to end	Day	4 2 8% 32% 40% 14%	465
September	1st to 10th	Day	12% 3 9% 29% 37% 9%	300
	11th to 20th	Day	7% 1 6% 33% 42% 11%	300
	21st to end	Day	6% 3 10% 36% 35% 9%	300
October	1st to 10th	Day	6% 2 17% 40% 30% 6%	300
	11th to 20th	Day	9% 4 24% 42% 17% 3	300
	21st to end	Day	19% 13% 26% 23% 15% 3	330
November	1st to 10th	Day	18% 7% 33% 28% 12% 2	300
	11th to 20th	Day	21% 10% 31% 24% 11% 3	300
	21st to end	Day	26% 16% 34% 15% 8% 1	300
December	1st to 10th	Day	21% 8% 36% 26% 8% 2	300
	11th to 20th	Day	21% 16% 33% 22% 7% 1	300
	21st to end	Day	20% 14% 37% 22% 6% 1	330
January	1st to 10th	Night	20% 10% 30% 32% 8%	1000
	11th to 20th	Night	21% 13% 32% 29% 6%	1000
	21st to end	Night	24% 11% 30% 29% 6%	920
February	1st to 10th	Night	24% 13% 31% 26% 6%	800
	11th to 20th	Night	28% 13% 31% 25% 3	784
	21st to end	Night	26% 13% 39% 20% 2	660
March	1st to 10th	Night	23% 12% 37% 25% 3	659
	11th to 20th	Night	24% 10% 38% 24% 4	600
	21st to end	Night	20% 12% 41% 23% 3	660
April	1st to 10th	Night	17% 14% 38% 27% 3	600
	11th to 20th	Night	21% 14% 39% 24% 2	593
	21st to end	Night	23% 9% 42% 21% 6% 1	600
May	1st to 10th	Night	20% 11% 39% 26% 4	600
	11th to 20th	Night	15% 9% 36% 32% 7%	591
	21st to end	Night	15% 8% 35% 33% 8%	634
June	1st to 10th	Night	8% 2 26% 44% 18% 1	570
	11th to 20th	Night	6% 2 34% 47% 10% 1	570
	21st to end	Night	8% 2 20% 46% 22% 3	570
July	1st to 10th	Night	11% 2 14% 44% 26% 2	555
	11th to 20th	Night	4 1 12% 56% 26% 2	552
	21st to end	Night	7% 1 10% 53% 26% 3	624
August	1st to 10th	Night	1 1 10% 54% 31% 2	567
	11th to 20th	Night	5% 1 8% 54% 28% 3	564
	21st to end	Night	7% 2 6% 46% 33% 6%	627
September	1st to 10th	Night	11% 1 10% 43% 32% 5%	570
	11th to 20th	Night	13% 1 8% 43% 31% 4	570
	21st to end	Night	11% 1 11% 40% 33% 4	570
October	1st to 10th	Night	8% 2 12% 48% 25% 4	570
	11th to 20th	Night	14% 4 21% 39% 19% 2	640
	21st to end	Night	16% 9% 24% 29% 21% 2	836
November	1st to 10th	Night	16% 7% 21% 39% 16% 2	780
	11th to 20th	Night	15% 8% 27% 35% 14% 1	960
	21st to end	Night	21% 12% 26% 30% 11% 1	1000
December	1st to 10th	Night	20% 8% 28% 33% 9% 1	1000
	11th to 20th	Night	24% 10% 28% 31% 7%	1000
	21st to end	Night	20% 10% 28% 35% 7%	1100

Page generated at 2025-05-23 08:21:01. Version ac07

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