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 2024-01-15.

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% 4 7% 6% 3 5% 5% 5% 7% 38%	74713
	Night	12% 8% 3 7% 5% 3 4 4 5% 7% 42%	75083
Transparency	Day	44% 7% 14% 22% 12% 1	74423
	Night	42% 6% 13% 23% 14% 2	74893
Seeing	Day	16% 10% 31% 27% 13% 3	14714
	Night	15% 10% 33% 31% 10% 1	24715

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	7% 12% 7% 8% 9% 5% 6% 6% 5% 8% 27%	1517
	11th to 20th	Day	10% 12% 6% 10% 9% 4 6% 4 6% 9% 24%	1608
	21st to end	Day	13% 10% 7% 12% 8% 3 7% 6% 6% 7% 22%	1782
February	1st to 10th	Day	10% 12% 5% 11% 8% 4 5% 5% 6% 7% 28%	1620
	11th to 20th	Day	11% 11% 5% 8% 6% 2 5% 6% 6% 7% 33%	1574
	21st to end	Day	15% 10% 5% 8% 7% 4 4 5% 5% 5% 32%	1412
March	1st to 10th	Day	13% 11% 6% 8% 6% 3 5% 4 5% 7% 33%	1975
	11th to 20th	Day	14% 8% 4 8% 7% 3 4 5% 5% 9% 31%	1980
	21st to end	Day	20% 14% 5% 6% 5% 4 4 4 6% 6% 27%	2178
April	1st to 10th	Day	20% 9% 4 7% 5% 3 5% 4 5% 7% 32%	2160
	11th to 20th	Day	18% 9% 4 7% 7% 3 4 4 4 5% 35%	2134
	21st to end	Day	21% 7% 4 7% 5% 3 5% 4 5% 6% 34%	2296
May	1st to 10th	Day	19% 9% 3 6% 5% 3 5% 6% 6% 8% 30%	2340
	11th to 20th	Day	18% 9% 3 6% 5% 3 4 4 5% 6% 37%	2474
	21st to end	Day	18% 7% 3 6% 6% 3 5% 5% 6% 7% 35%	2725
June	1st to 10th	Day	12% 5% 2 5% 5% 3 4 5% 6% 9% 43%	2520
	11th to 20th	Day	11% 5% 2 6% 5% 2 5% 5% 5% 9% 44%	2520
	21st to end	Day	11% 6% 3 7% 6% 4 5% 5% 7% 8% 40%	2520
July	1st to 10th	Day	7% 5% 2 4 4 3 5% 6% 6% 9% 49%	2520
	11th to 20th	Day	6% 4 3 4 5% 3 4 5% 6% 8% 51%	2520
	21st to end	Day	7% 3 2 4 6% 4 6% 6% 8% 9% 47%	2575
August	1st to 10th	Day	8% 5% 3 5% 6% 3 6% 6% 6% 8% 44%	2328
	11th to 20th	Day	11% 4 3 5% 6% 3 5% 5% 6% 8% 44%	2301
	21st to end	Day	9% 4 3 5% 5% 2 4 4 5% 8% 50%	2430
September	1st to 10th	Day	12% 6% 2 4 3 3 3 4 5% 7% 52%	2160
	11th to 20th	Day	10% 4 2 4 5% 3 5% 5% 5% 7% 49%	2114
	21st to end	Day	12% 5% 2 5% 4 4 4 4 5% 7% 48%	1904
October	1st to 10th	Day	16% 6% 2 4 6% 3 4 4 5% 7% 44%	1800
	11th to 20th	Day	16% 7% 3 6% 6% 3 4 4 4 6% 43%	1800
	21st to end	Day	17% 9% 3 6% 5% 2 3 4 5% 6% 40%	1980
November	1st to 10th	Day	10% 6% 3 5% 5% 2 4 4 6% 7% 46%	1685
	11th to 20th	Day	14% 9% 5% 7% 5% 3 5% 5% 6% 7% 36%	1520
	21st to end	Day	13% 10% 4 5% 6% 3 4 5% 4 8% 38%	1520
December	1st to 10th	Day	12% 11% 3 8% 7% 3 6% 4 5% 8% 32%	1520
	11th to 20th	Day	12% 11% 4 10% 7% 4 5% 4 5% 8% 31%	1918
	21st to end	Day	20% 14% 8% 14% 7% 3 3 3 3 5% 19%	2783
January	1st to 10th	Night	9% 9% 5% 9% 6% 4 5% 5% 6% 7% 36%	2648
	11th to 20th	Night	11% 11% 5% 11% 8% 4 6% 5% 6% 7% 26%	2438
	21st to end	Night	13% 12% 5% 9% 7% 3 5% 4 5% 6% 30%	2574
February	1st to 10th	Night	11% 12% 5% 10% 8% 3 5% 5% 5% 6% 31%	2340
	11th to 20th	Night	11% 9% 3 7% 5% 4 5% 5% 5% 7% 38%	2273
	21st to end	Night	13% 9% 4 8% 8% 2 5% 3 4 5% 39%	1844
March	1st to 10th	Night	15% 8% 4 6% 5% 3 4 5% 4 7% 40%	1998
	11th to 20th	Night	14% 8% 3 7% 5% 3 4 4 4 8% 39%	1980
	21st to end	Night	18% 12% 5% 7% 6% 3 4 4 5% 6% 30%	2178
April	1st to 10th	Night	14% 8% 3 6% 5% 3 4 5% 5% 6% 42%	1800
	11th to 20th	Night	17% 9% 3 5% 5% 3 4 4 5% 7% 38%	1775
	21st to end	Night	17% 8% 2 5% 4 2 4 3 4 8% 43%	1661
May	1st to 10th	Night	16% 6% 3 5% 5% 3 4 5% 5% 9% 40%	1620
	11th to 20th	Night	16% 7% 2 4 5% 3 5% 6% 5% 6% 41%	1419
	21st to end	Night	13% 6% 2 5% 6% 3 4 5% 6% 9% 43%	1551
June	1st to 10th	Night	7% 3 3 5% 5% 3 4 5% 7% 9% 49%	1440
	11th to 20th	Night	9% 4 2 5% 5% 3 4 5% 6% 10% 48%	1440
	21st to end	Night	10% 3 2 4 4 3 4 3 6% 10% 50%	1440
July	1st to 10th	Night	5% 3 2 3 5% 4 4 5% 5% 9% 57%	1440
	11th to 20th	Night	5% 2 1 3 4 3 4 4 4 10% 59%	1440
	21st to end	Night	4 2 2 3 4 2 4 5% 7% 9% 58%	1741
August	1st to 10th	Night	7% 4 3 5% 5% 2 6% 5% 6% 9% 50%	1611
	11th to 20th	Night	9% 4 3 4 3 2 3 4 4 7% 58%	1600
	21st to end	Night	8% 4 3 4 4 2 3 3 4 6% 58%	1926
September	1st to 10th	Night	10% 5% 2 5% 5% 3 3 3 4 6% 55%	1800
	11th to 20th	Night	10% 5% 1 4 4 3 3 3 5% 8% 55%	1843
	21st to end	Night	10% 5% 1 3 3 2 4 5% 5% 5% 57%	2037
October	1st to 10th	Night	13% 5% 3 5% 4 2 4 3 4 7% 51%	2160
	11th to 20th	Night	16% 6% 2 6% 4 3 4 4 4 7% 45%	2160
	21st to end	Night	15% 9% 3 5% 4 2 4 4 4 7% 44%	2376
November	1st to 10th	Night	11% 6% 2 4 4 3 3 4 5% 7% 51%	2387
	11th to 20th	Night	13% 8% 4 7% 5% 3 5% 4 5% 7% 41%	2660
	21st to end	Night	14% 8% 4 6% 5% 2 4 4 4 7% 43%	2660
December	1st to 10th	Night	10% 10% 4 8% 7% 4 5% 5% 5% 7% 35%	2660
	11th to 20th	Night	12% 9% 5% 9% 6% 4 5% 5% 6% 6% 34%	3279
	21st to end	Night	19% 14% 6% 14% 7% 3 4 3 4 4 21%	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	54% 7% 14% 14% 10%	1518
	11th to 20th	Day	57% 6% 14% 16% 7% 1	1608
	21st to end	Day	59% 7% 14% 13% 6% 1	1782
February	1st to 10th	Day	54% 6% 16% 17% 6% 1	1620
	11th to 20th	Day	49% 7% 13% 20% 10% 2	1583
	21st to end	Day	52% 7% 13% 16% 10% 2	1412
March	1st to 10th	Day	50% 7% 15% 19% 8% 2	1975
	11th to 20th	Day	50% 7% 13% 20% 9% 1	1980
	21st to end	Day	57% 7% 12% 17% 6% 1	2178
April	1st to 10th	Day	51% 6% 12% 21% 9% 1	2160
	11th to 20th	Day	52% 5% 11% 19% 12% 1	2134
	21st to end	Day	50% 6% 10% 22% 11% 1	2296
May	1st to 10th	Day	50% 9% 13% 20% 8% 1	2340
	11th to 20th	Day	48% 7% 12% 23% 10% 1	2478
	21st to end	Day	47% 8% 13% 18% 12% 2	2725
June	1st to 10th	Day	36% 8% 16% 24% 15% 1	2520
	11th to 20th	Day	35% 8% 14% 23% 18% 2	2520
	21st to end	Day	39% 9% 15% 22% 13% 1	2520
July	1st to 10th	Day	31% 7% 16% 30% 13% 3	2451
	11th to 20th	Day	29% 9% 16% 26% 17% 3	2445
	21st to end	Day	29% 9% 19% 26% 15% 3	2432
August	1st to 10th	Day	36% 10% 15% 25% 12% 2	2314
	11th to 20th	Day	35% 9% 14% 27% 13% 1	2301
	21st to end	Day	31% 7% 13% 26% 20% 3	2430
September	1st to 10th	Day	33% 6% 13% 26% 19% 2	2160
	11th to 20th	Day	32% 8% 13% 31% 16%	2114
	21st to end	Day	35% 6% 11% 29% 18% 1	1904
October	1st to 10th	Day	40% 7% 11% 25% 16% 1	1800
	11th to 20th	Day	43% 5% 10% 23% 18% 2	1800
	21st to end	Day	45% 5% 13% 22% 14% 1	1980
November	1st to 10th	Day	36% 7% 14% 23% 17% 2	1685
	11th to 20th	Day	46% 6% 14% 22% 11% 1	1520
	21st to end	Day	45% 6% 15% 22% 10% 1	1520
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	46% 6% 14% 19% 13% 1	2648
	11th to 20th	Night	56% 7% 15% 15% 7%	2438
	21st to end	Night	54% 6% 13% 17% 10% 1	2574
February	1st to 10th	Night	53% 6% 11% 19% 10% 1	2340
	11th to 20th	Night	44% 5% 14% 22% 13% 2	2274
	21st to end	Night	48% 5% 11% 19% 14% 3	1844
March	1st to 10th	Night	43% 7% 14% 26% 9% 1	1998
	11th to 20th	Night	45% 5% 13% 22% 14% 1	1980
	21st to end	Night	55% 7% 13% 16% 8% 1	2178
April	1st to 10th	Night	42% 7% 12% 24% 13% 2	1800
	11th to 20th	Night	46% 5% 11% 18% 17% 2	1775
	21st to end	Night	41% 6% 12% 24% 14% 2	1661
May	1st to 10th	Night	41% 8% 14% 22% 14% 1	1620
	11th to 20th	Night	41% 8% 12% 25% 12% 1	1420
	21st to end	Night	39% 9% 13% 23% 13% 3	1552
June	1st to 10th	Night	30% 9% 15% 27% 16% 2	1440
	11th to 20th	Night	30% 8% 14% 21% 20% 6%	1440
	21st to end	Night	30% 7% 16% 28% 16% 4	1440
July	1st to 10th	Night	24% 8% 15% 33% 17% 3	1400
	11th to 20th	Night	22% 7% 18% 32% 17% 4	1392
	21st to end	Night	20% 7% 17% 32% 21% 3	1644
August	1st to 10th	Night	29% 10% 14% 29% 14% 3	1601
	11th to 20th	Night	28% 6% 12% 34% 17% 3	1600
	21st to end	Night	28% 6% 14% 24% 23% 5%	1926
September	1st to 10th	Night	31% 5% 11% 26% 25% 2	1800
	11th to 20th	Night	29% 5% 13% 30% 22% 1	1843
	21st to end	Night	28% 6% 11% 30% 23% 2	2037
October	1st to 10th	Night	34% 5% 12% 26% 21% 3	2160
	11th to 20th	Night	39% 6% 11% 22% 18% 3	2160
	21st to end	Night	40% 6% 12% 23% 17% 1	2376
November	1st to 10th	Night	33% 6% 14% 26% 18% 4	2387
	11th to 20th	Night	43% 6% 13% 24% 12% 1	2660
	21st to end	Night	43% 5% 13% 26% 12% 2	2660
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% 42% 21% 3	252
	21st to end	Day	19% 13% 39% 24% 4 2	264
February	1st to 10th	Day	15% 19% 43% 20% 2 1	240
	11th to 20th	Day	16% 22% 35% 20% 5% 1	240
	21st to end	Day	24% 13% 41% 18% 3 1	198
March	1st to 10th	Day	15% 16% 45% 17% 6% 2	240
	11th to 20th	Day	22% 14% 34% 24% 5% 1	240
	21st to end	Day	31% 21% 26% 20% 3	264
April	1st to 10th	Day	26% 13% 36% 19% 5%	420
	11th to 20th	Day	21% 16% 32% 22% 8% 1	412
	21st to end	Day	24% 14% 33% 18% 9% 2	556
May	1st to 10th	Day	23% 13% 34% 21% 9% 1	603
	11th to 20th	Day	25% 12% 28% 23% 9% 3	625
	21st to end	Day	19% 7% 27% 28% 16% 3	679
June	1st to 10th	Day	9% 6% 27% 33% 21% 4	630
	11th to 20th	Day	12% 5% 32% 34% 15% 3	630
	21st to end	Day	11% 8% 33% 32% 12% 4	630
July	1st to 10th	Day	8% 3 18% 36% 25% 10%	621
	11th to 20th	Day	6% 4 29% 34% 21% 6%	618
	21st to end	Day	7% 2 26% 38% 23% 5%	684
August	1st to 10th	Day	8% 4 20% 37% 26% 6%	626
	11th to 20th	Day	12% 5% 19% 33% 25% 6%	615
	21st to end	Day	10% 4 23% 36% 22% 6%	549
September	1st to 10th	Day	14% 5% 23% 34% 18% 6%	450
	11th to 20th	Day	10% 4 26% 36% 18% 6%	404
	21st to end	Day	11% 13% 29% 31% 14% 3	270
October	1st to 10th	Day	21% 9% 25% 26% 15% 3	270
	11th to 20th	Day	19% 19% 38% 16% 7%	270
	21st to end	Day	22% 15% 35% 18% 8% 2	297
November	1st to 10th	Day	13% 13% 44% 20% 7% 2	270
	11th to 20th	Day	18% 21% 34% 21% 6%	270
	21st to end	Day	21% 14% 40% 21% 3 1	270
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	17% 13% 37% 27% 6%	921
	21st to end	Night	19% 11% 38% 27% 5%	990
February	1st to 10th	Night	17% 12% 38% 28% 5%	900
	11th to 20th	Night	16% 13% 41% 25% 5% 1	879
	21st to end	Night	19% 14% 36% 24% 5% 1	660
March	1st to 10th	Night	18% 16% 35% 21% 8% 1	558
	11th to 20th	Night	20% 9% 37% 26% 7% 1	540
	21st to end	Night	21% 14% 36% 21% 7% 1	594
April	1st to 10th	Night	18% 14% 41% 23% 4	540
	11th to 20th	Night	20% 13% 36% 24% 6%	533
	21st to end	Night	20% 10% 36% 26% 8% 1	540
May	1st to 10th	Night	17% 8% 34% 32% 7% 1	540
	11th to 20th	Night	18% 10% 30% 32% 10%	531
	21st to end	Night	16% 7% 28% 37% 13%	584
June	1st to 10th	Night	9% 5% 28% 44% 15% 1	540
	11th to 20th	Night	11% 4 27% 46% 11%	540
	21st to end	Night	10% 3 30% 41% 16%	540
July	1st to 10th	Night	5% 2 18% 44% 28% 3	525
	11th to 20th	Night	5% 2 23% 44% 24% 2	522
	21st to end	Night	4 2 17% 53% 22% 2	591
August	1st to 10th	Night	7% 2 18% 47% 24% 1	537
	11th to 20th	Night	9% 3 20% 39% 25% 4	534
	21st to end	Night	7% 4 24% 39% 23% 2	594
September	1st to 10th	Night	11% 5% 21% 39% 21% 2	540
	11th to 20th	Night	12% 3 26% 38% 19% 2	540
	21st to end	Night	12% 6% 34% 33% 15% 1	597
October	1st to 10th	Night	15% 8% 34% 31% 12% 1	720
	11th to 20th	Night	19% 11% 36% 27% 7%	720
	21st to end	Night	21% 11% 34% 27% 7%	792
November	1st to 10th	Night	15% 10% 38% 29% 7% 1	778
	11th to 20th	Night	17% 12% 44% 21% 5%	950
	21st to end	Night	16% 13% 37% 26% 7%	950
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 2024-01-11 21:33:49. Version ac07

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