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 2024-11-30.

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% 8% 3 8% 5% 4 4 4 6% 7% 38%	101672
	Night	12% 8% 3 8% 5% 4 4 3 5% 7% 41%	98036
Transparency	Day	43% 7% 13% 23% 13% 1	105310
	Night	41% 6% 12% 24% 15% 2	102538
Seeing	Day	15% 10% 31% 27% 13% 3	15969
	Night	15% 10% 34% 31% 10% 1	25853

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	6% 13% 7% 23% 9% 6% 5% 4 4 5% 19%	2527
	11th to 20th	Day	8% 13% 5% 11% 7% 7% 8% 4 8% 8% 22%	2763
	21st to end	Day	13% 14% 9% 17% 7% 4 5% 4 4 6% 17%	3135
February	1st to 10th	Day	10% 12% 4 11% 7% 6% 4 5% 6% 7% 28%	1710
	11th to 20th	Day	10% 12% 5% 8% 5% 4 5% 5% 6% 7% 34%	1664
	21st to end	Day	14% 10% 4 9% 6% 5% 4 4 5% 5% 34%	1498
March	1st to 10th	Day	14% 9% 4 7% 4 3 4 3 4 5% 43%	3431
	11th to 20th	Day	11% 6% 3 7% 5% 4 4 4 5% 7% 43%	3480
	21st to end	Day	22% 14% 7% 9% 5% 5% 3 3 5% 5% 21%	3814
April	1st to 10th	Day	19% 10% 4 7% 4 4 4 4 5% 6% 32%	2280
	11th to 20th	Day	18% 9% 4 7% 6% 4 4 3 5% 5% 35%	2254
	21st to end	Day	21% 8% 4 7% 4 4 4 3 5% 6% 34%	2424
May	1st to 10th	Day	20% 8% 2 7% 4 5% 4 4 6% 8% 32%	4082
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 5% 6% 42%	4394
	21st to end	Day	16% 6% 3 6% 4 4 4 4 6% 7% 40%	4824
June	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 7% 9% 43%	2660
	11th to 20th	Day	11% 5% 2 7% 4 4 5% 4 6% 9% 43%	2660
	21st to end	Day	11% 6% 2 8% 5% 4 5% 4 7% 8% 40%	2660
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 8% 9% 38%	4440
	11th to 20th	Day	5% 4 2 4 4 3 4 3 5% 7% 58%	4440
	21st to end	Day	6% 4 2 6% 5% 6% 5% 5% 8% 10% 43%	4508
August	1st to 10th	Day	9% 6% 2 6% 4 4 5% 5% 7% 8% 45%	4104
	11th to 20th	Day	13% 5% 2 7% 5% 4 5% 5% 7% 9% 38%	4041
	21st to end	Day	7% 5% 2 7% 5% 4 5% 4 5% 8% 49%	4276
September	1st to 10th	Day	11% 6% 2 4 3 3 4 3 5% 7% 53%	2280
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 5% 7% 49%	2231
	21st to end	Day	11% 5% 2 5% 4 5% 3 3 5% 6% 49%	2009
October	1st to 10th	Day	15% 6% 2 4 5% 3 4 3 5% 6% 46%	1900
	11th to 20th	Day	15% 7% 2 6% 5% 4 3 3 4 6% 44%	1900
	21st to end	Day	16% 9% 3 6% 4 3 4 3 5% 7% 40%	2090
November	1st to 10th	Day	11% 7% 3 6% 4 3 4 3 7% 7% 45%	1775
	11th to 20th	Day	13% 10% 4 7% 5% 4 4 4 6% 7% 35%	1600
	21st to end	Day	12% 11% 4 6% 5% 4 4 4 5% 7% 37%	1597
December	1st to 10th	Day	12% 11% 3 10% 6% 5% 5% 3 6% 7% 32%	1520
	11th to 20th	Day	12% 11% 4 11% 6% 5% 5% 3 5% 8% 31%	1918
	21st to end	Day	20% 15% 7% 15% 5% 4 3 3 3 5% 19%	2783
January	1st to 10th	Night	9% 12% 5% 22% 7% 5% 4 3 5% 5% 23%	4428
	11th to 20th	Night	9% 14% 4 12% 6% 5% 7% 4 8% 8% 23%	4204
	21st to end	Night	14% 14% 6% 15% 6% 5% 4 3 4 6% 22%	4532
February	1st to 10th	Night	10% 12% 4 11% 7% 4 4 4 5% 6% 31%	2470
	11th to 20th	Night	10% 10% 3 8% 4 6% 4 4 5% 8% 39%	2403
	21st to end	Night	13% 9% 3 9% 6% 3 4 2 4 5% 40%	1956
March	1st to 10th	Night	13% 8% 3 6% 4 3 3 3 4 6% 47%	3449
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 5% 8% 49%	3480
	21st to end	Night	25% 13% 4 12% 4 4 3 3 4 4 23%	3817
April	1st to 10th	Night	14% 8% 3 7% 4 4 3 4 5% 6% 42%	1900
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 3 5% 7% 39%	1875
	21st to end	Night	17% 8% 2 6% 4 3 4 3 5% 7% 42%	1753
May	1st to 10th	Night	18% 7% 3 5% 4 4 3 4 5% 6% 41%	2823
	11th to 20th	Night	12% 7% 2 6% 4 4 4 4 4 5% 48%	2509
	21st to end	Night	12% 6% 2 6% 5% 3 4 3 5% 7% 46%	2743
June	1st to 10th	Night	8% 4 2 6% 4 3 4 4 7% 8% 48%	1520
	11th to 20th	Night	9% 5% 1 5% 4 4 5% 4 6% 11% 46%	1520
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 9% 49%	1520
July	1st to 10th	Night	8% 5% 2 5% 5% 5% 5% 4 6% 10% 46%	2530
	11th to 20th	Night	4 3 1 4 4 3 4 3 5% 9% 59%	2530
	21st to end	Night	4 3 2 5% 4 4 5% 4 8% 11% 51%	3053
August	1st to 10th	Night	5% 4 2 5% 5% 3 5% 4 6% 9% 52%	2836
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 4 7% 55%	2798
	21st to end	Night	9% 5% 2 5% 4 3 3 2 4 7% 55%	3383
September	1st to 10th	Night	9% 5% 2 5% 4 3 3 2 4 6% 57%	1900
	11th to 20th	Night	10% 5% 1 4 3 3 3 2 5% 8% 55%	1946
	21st to end	Night	9% 5% 1 4 3 3 4 4 5% 5% 57%	2151
October	1st to 10th	Night	12% 5% 3 5% 3 3 4 2 4 7% 53%	2280
	11th to 20th	Night	15% 6% 2 6% 3 4 4 3 4 7% 47%	2280
	21st to end	Night	15% 9% 3 5% 3 3 4 3 4 7% 44%	2508
November	1st to 10th	Night	12% 6% 2 5% 3 3 4 3 5% 7% 50%	2517
	11th to 20th	Night	12% 9% 4 8% 4 4 5% 3 5% 6% 41%	2800
	21st to end	Night	13% 9% 3 7% 4 3 3 3 4 7% 42%	2799
December	1st to 10th	Night	10% 11% 4 10% 5% 6% 5% 4 5% 7% 35%	2660
	11th to 20th	Night	11% 10% 4 10% 5% 5% 4 4 6% 6% 34%	3279
	21st to end	Night	19% 15% 6% 15% 5% 5% 4 3 4 4 20%	4884

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	66% 7% 10% 11% 6%	2528
	11th to 20th	Day	58% 6% 11% 19% 6%	2763
	21st to end	Day	68% 6% 11% 12% 4 1	3135
February	1st to 10th	Day	52% 6% 15% 18% 7% 1	1710
	11th to 20th	Day	47% 7% 13% 21% 11% 2	1673
	21st to end	Day	50% 7% 13% 18% 11% 2	1498
March	1st to 10th	Day	43% 6% 11% 24% 14% 2	3431
	11th to 20th	Day	39% 8% 11% 28% 12% 2	3480
	21st to end	Day	63% 8% 11% 13% 4 1	3814
April	1st to 10th	Day	50% 6% 12% 21% 9% 1	2280
	11th to 20th	Day	51% 5% 11% 19% 13% 1	2254
	21st to end	Day	50% 6% 10% 22% 12% 1	2424
May	1st to 10th	Day	47% 10% 14% 17% 12% 1	4120
	11th to 20th	Day	40% 8% 12% 27% 12% 1	4398
	21st to end	Day	40% 9% 13% 22% 15% 1	4837
June	1st to 10th	Day	35% 8% 16% 24% 15% 2	2660
	11th to 20th	Day	36% 8% 13% 22% 18% 2	2660
	21st to end	Day	40% 9% 15% 22% 13% 2	2660
July	1st to 10th	Day	40% 7% 17% 27% 8% 1	4371
	11th to 20th	Day	25% 7% 14% 33% 20% 2	4365
	21st to end	Day	31% 9% 18% 23% 17% 2	4366
August	1st to 10th	Day	35% 8% 16% 26% 14% 1	4090
	11th to 20th	Day	41% 8% 15% 24% 12% 1	4080
	21st to end	Day	33% 6% 16% 26% 17% 2	4275
September	1st to 10th	Day	32% 6% 13% 27% 20% 2	2280
	11th to 20th	Day	33% 8% 13% 30% 16%	2231
	21st to end	Day	35% 6% 11% 29% 19% 1	2009
October	1st to 10th	Day	23% 4 7% 29% 35% 2	3160
	11th to 20th	Day	31% 3 11% 28% 24% 2	3140
	21st to end	Day	41% 4 10% 29% 13% 3	3428
November	1st to 10th	Day	38% 6% 15% 23% 16% 2	1775
	11th to 20th	Day	47% 6% 14% 21% 11% 1	1600
	21st to end	Day	45% 7% 15% 22% 10% 1	1597
December	1st to 10th	Day	51% 5% 18% 17% 8% 2	1520
	11th to 20th	Day	51% 6% 18% 17% 6%	1915
	21st to end	Day	69% 5% 9% 12% 5% 1	2783
January	1st to 10th	Night	63% 6% 9% 13% 8% 1	4428
	11th to 20th	Night	56% 6% 13% 15% 9% 1	4204
	21st to end	Night	63% 7% 10% 13% 6% 1	4532
February	1st to 10th	Night	51% 6% 11% 20% 10% 1	2470
	11th to 20th	Night	44% 5% 13% 23% 13% 2	2404
	21st to end	Night	47% 5% 11% 20% 14% 3	1956
March	1st to 10th	Night	37% 7% 11% 28% 15% 2	3449
	11th to 20th	Night	34% 5% 12% 31% 15% 3	3480
	21st to end	Night	63% 8% 13% 10% 4 1	3817
April	1st to 10th	Night	42% 7% 11% 25% 12% 2	1900
	11th to 20th	Night	45% 5% 11% 18% 18% 2	1875
	21st to end	Night	41% 6% 12% 24% 15% 2	1753
May	1st to 10th	Night	42% 8% 11% 20% 19% 1	2850
	11th to 20th	Night	37% 6% 10% 26% 15% 5%	2510
	21st to end	Night	36% 8% 11% 25% 15% 3	2751
June	1st to 10th	Night	31% 9% 14% 27% 15% 3	1520
	11th to 20th	Night	30% 8% 14% 21% 20% 6%	1520
	21st to end	Night	31% 7% 16% 27% 16% 4	1520
July	1st to 10th	Night	32% 6% 17% 31% 12% 2	2490
	11th to 20th	Night	22% 6% 18% 35% 17% 2	2482
	21st to end	Night	24% 9% 18% 28% 19% 2	2956
August	1st to 10th	Night	29% 8% 15% 32% 14% 2	2826
	11th to 20th	Night	29% 6% 16% 33% 14% 2	2825
	21st to end	Night	30% 5% 16% 27% 19% 3	3383
September	1st to 10th	Night	29% 5% 11% 28% 25% 2	1900
	11th to 20th	Night	29% 5% 13% 31% 21% 1	1946
	21st to end	Night	28% 6% 11% 29% 24% 2	2151
October	1st to 10th	Night	22% 3 7% 27% 39% 3	3800
	11th to 20th	Night	27% 4 10% 28% 25% 6%	3779
	21st to end	Night	42% 5% 10% 26% 15% 2	4120
November	1st to 10th	Night	34% 6% 14% 26% 17% 3	2517
	11th to 20th	Night	43% 6% 13% 24% 13% 2	2800
	21st to end	Night	44% 5% 13% 26% 12% 2	2799
December	1st to 10th	Night	49% 6% 14% 22% 8% 1	2660
	11th to 20th	Night	47% 8% 15% 24% 6%	3281
	21st to end	Night	66% 6% 8% 13% 6%	4884

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	17% 11% 36% 23% 10% 2	270
	11th to 20th	Day	21% 13% 43% 20% 3	270
	21st to end	Day	18% 13% 37% 25% 5% 2	297
February	1st to 10th	Day	14% 19% 42% 22% 2 1	270
	11th to 20th	Day	14% 21% 35% 23% 5% 1	270
	21st to end	Day	22% 14% 44% 18% 3	225
March	1st to 10th	Day	16% 15% 44% 18% 6% 1	270
	11th to 20th	Day	21% 16% 33% 24% 5% 1	270
	21st to end	Day	30% 20% 28% 19% 2	297
April	1st to 10th	Day	26% 13% 37% 18% 5%	460
	11th to 20th	Day	22% 17% 31% 22% 7% 1	452
	21st to end	Day	25% 15% 32% 18% 8% 2	604
May	1st to 10th	Day	22% 13% 34% 20% 9% 2	653
	11th to 20th	Day	23% 12% 29% 24% 10% 3	675
	21st to end	Day	19% 8% 28% 27% 16% 3	734
June	1st to 10th	Day	9% 7% 28% 33% 20% 4	680
	11th to 20th	Day	12% 6% 33% 33% 14% 3	680
	21st to end	Day	12% 8% 34% 31% 11% 4	680
July	1st to 10th	Day	8% 4 19% 35% 24% 10%	671
	11th to 20th	Day	6% 4 28% 35% 21% 6%	668
	21st to end	Day	6% 2 25% 38% 23% 5%	739
August	1st to 10th	Day	8% 4 21% 37% 25% 5%	676
	11th to 20th	Day	12% 5% 21% 33% 23% 6%	665
	21st to end	Day	9% 4 23% 37% 21% 6%	596
September	1st to 10th	Day	13% 5% 23% 34% 19% 6%	490
	11th to 20th	Day	9% 4 25% 37% 18% 6%	441
	21st to end	Day	11% 12% 28% 32% 14% 3	300
October	1st to 10th	Day	19% 10% 26% 25% 16% 4	300
	11th to 20th	Day	18% 18% 38% 18% 7% 1	300
	21st to end	Day	21% 15% 37% 18% 7% 2	330
November	1st to 10th	Day	16% 12% 42% 21% 7% 2	300
	11th to 20th	Day	19% 21% 34% 19% 7%	300
	21st to end	Day	20% 15% 40% 21% 3 1	299
December	1st to 10th	Day	11% 15% 44% 21% 7% 1	270
	11th to 20th	Day	16% 13% 37% 25% 7% 1	270
	21st to end	Day	25% 15% 42% 16% 2	297
January	1st to 10th	Night	12% 15% 40% 26% 7%	950
	11th to 20th	Night	16% 12% 39% 26% 6%	950
	21st to end	Night	18% 11% 37% 28% 6%	1045
February	1st to 10th	Night	17% 11% 38% 28% 6%	950
	11th to 20th	Night	16% 13% 40% 26% 5% 1	929
	21st to end	Night	19% 14% 38% 24% 5% 1	700
March	1st to 10th	Night	17% 16% 37% 21% 7% 1	589
	11th to 20th	Night	20% 9% 38% 26% 7% 1	570
	21st to end	Night	22% 14% 37% 20% 7% 1	627
April	1st to 10th	Night	19% 14% 42% 22% 4	570
	11th to 20th	Night	20% 14% 36% 24% 6%	563
	21st to end	Night	21% 11% 35% 26% 7% 1	570
May	1st to 10th	Night	17% 9% 34% 32% 7% 1	570
	11th to 20th	Night	17% 10% 29% 33% 10%	561
	21st to end	Night	15% 8% 28% 36% 12%	617
June	1st to 10th	Night	9% 6% 29% 42% 14% 1	570
	11th to 20th	Night	11% 5% 27% 46% 11%	570
	21st to end	Night	10% 4 30% 41% 15%	570
July	1st to 10th	Night	6% 2 18% 44% 27% 3	555
	11th to 20th	Night	5% 2 22% 44% 24% 2	552
	21st to end	Night	4 2 17% 53% 22% 2	624
August	1st to 10th	Night	7% 2 19% 47% 24% 1	567
	11th to 20th	Night	10% 3 21% 40% 24% 3	564
	21st to end	Night	8% 4 23% 40% 23% 2	627
September	1st to 10th	Night	11% 5% 21% 39% 22% 2	570
	11th to 20th	Night	12% 3 26% 40% 18% 2	570
	21st to end	Night	12% 6% 32% 34% 16% 1	631
October	1st to 10th	Night	14% 8% 33% 31% 13% 1	760
	11th to 20th	Night	18% 11% 37% 27% 7%	760
	21st to end	Night	21% 11% 34% 27% 6%	836
November	1st to 10th	Night	16% 10% 37% 29% 7% 1	821
	11th to 20th	Night	16% 13% 44% 21% 5%	1000
	21st to end	Night	17% 13% 37% 26% 7%	1000
December	1st to 10th	Night	16% 13% 40% 25% 6% 1	950
	11th to 20th	Night	20% 9% 36% 28% 6%	950
	21st to end	Night	23% 13% 38% 21% 4	1045

Page generated at 2024-11-27 16:30:52. Version ac07

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