Darklight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Darklight Observatory clear sky chart from 2005-11-06 to 2024-03-18.

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% 7% 3 5% 5% 3 5% 5% 5% 7% 42%	78775
	Night	15% 7% 3 5% 5% 3 4 4 5% 7% 43%	81047
Transparency	Day	41% 6% 11% 23% 16% 3	78482
	Night	41% 5% 10% 23% 17% 4	80862
Seeing	Day	16% 10% 24% 26% 18% 8%	14439
	Night	18% 9% 25% 29% 17% 2	24423

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	15% 11% 5% 8% 6% 3 5% 5% 4 5% 32%	2527
	11th to 20th	Day	19% 8% 3 7% 6% 3 5% 4 4 5% 36%	2558
	21st to end	Day	24% 11% 5% 7% 5% 3 4 4 4 5% 30%	3226
February	1st to 10th	Day	18% 11% 3 5% 4 2 4 4 5% 6% 38%	1900
	11th to 20th	Day	20% 8% 4 7% 5% 2 3 4 4 5% 38%	1848
	21st to end	Day	19% 9% 5% 6% 5% 3 4 4 4 5% 35%	1570
March	1st to 10th	Day	17% 10% 3 6% 5% 2 4 4 5% 5% 39%	1900
	11th to 20th	Day	21% 9% 4 6% 6% 2 3 3 5% 5% 36%	1921
	21st to end	Day	18% 10% 4 7% 6% 2 4 4 6% 6% 33%	2376
April	1st to 10th	Day	13% 5% 3 5% 5% 3 5% 4 5% 7% 45%	2160
	11th to 20th	Day	16% 7% 3 6% 5% 4 5% 4 4 6% 41%	2134
	21st to end	Day	18% 5% 3 6% 5% 2 5% 4 5% 8% 40%	2160
May	1st to 10th	Day	16% 6% 3 6% 5% 4 5% 5% 5% 7% 39%	2325
	11th to 20th	Day	17% 6% 3 6% 5% 3 5% 4 4 7% 41%	2301
	21st to end	Day	8% 6% 2 6% 7% 4 6% 6% 8% 10% 39%	2567
June	1st to 10th	Day	10% 4 2 4 4 3 4 5% 6% 10% 47%	2520
	11th to 20th	Day	4 3 2 4 5% 4 5% 6% 7% 10% 49%	2520
	21st to end	Day	5% 3 2 4 5% 4 6% 6% 7% 11% 46%	2520
July	1st to 10th	Day	8% 5% 2 6% 6% 5% 6% 6% 8% 11% 38%	2520
	11th to 20th	Day	7% 4 2 5% 4 4 5% 6% 6% 9% 47%	2520
	21st to end	Day	8% 4 2 5% 6% 4 6% 6% 7% 11% 41%	2671
August	1st to 10th	Day	8% 5% 2 5% 5% 4 5% 6% 7% 10% 43%	2153
	11th to 20th	Day	10% 5% 2 5% 5% 4 4 6% 7% 8% 43%	2125
	21st to end	Day	8% 4 2 4 5% 4 4 5% 6% 10% 47%	2376
September	1st to 10th	Day	10% 5% 2 5% 5% 3 6% 5% 5% 8% 46%	2160
	11th to 20th	Day	12% 4 2 5% 5% 3 4 4 5% 7% 49%	2015
	21st to end	Day	12% 4 2 5% 4 3 4 4 4 7% 52%	1980
October	1st to 10th	Day	11% 3 2 3 4 2 3 4 4 7% 57%	1980
	11th to 20th	Day	14% 5% 2 4 4 3 5% 3 4 6% 51%	1834
	21st to end	Day	20% 6% 2 5% 4 2 4 2 4 5% 46%	1848
November	1st to 10th	Day	11% 5% 2 4 4 2 3 3 5% 6% 55%	1672
	11th to 20th	Day	16% 7% 3 4 4 2 3 4 4 5% 49%	1710
	21st to end	Day	18% 8% 3 5% 4 3 3 3 5% 6% 43%	1710
December	1st to 10th	Day	19% 10% 3 4 4 3 5% 5% 5% 6% 37%	1710
	11th to 20th	Day	16% 10% 4 6% 6% 3 4 4 5% 6% 35%	1975
	21st to end	Day	17% 12% 5% 7% 6% 3 5% 5% 5% 6% 30%	2783
January	1st to 10th	Night	19% 11% 3 5% 4 2 4 4 5% 6% 36%	4428
	11th to 20th	Night	17% 11% 4 9% 6% 3 4 4 4 5% 34%	4408
	21st to end	Night	25% 11% 4 5% 4 2 3 4 4 5% 32%	4436
February	1st to 10th	Night	22% 10% 4 7% 5% 3 4 3 4 5% 34%	2280
	11th to 20th	Night	18% 8% 4 7% 5% 2 4 3 4 5% 39%	2218
	21st to end	Night	22% 8% 3 6% 5% 3 4 4 5% 5% 35%	1884
March	1st to 10th	Night	18% 8% 3 5% 4 2 5% 4 4 5% 42%	2280
	11th to 20th	Night	19% 7% 4 5% 5% 3 4 3 4 6% 39%	2218
	21st to end	Night	19% 9% 3 7% 5% 3 4 4 6% 7% 35%	1980
April	1st to 10th	Night	12% 5% 2 4 4 3 5% 4 5% 8% 49%	1800
	11th to 20th	Night	16% 6% 2 4 4 2 4 4 4 7% 46%	1775
	21st to end	Night	17% 5% 3 5% 5% 3 4 5% 5% 8% 41%	1800
May	1st to 10th	Night	17% 6% 2 5% 5% 2 4 3 5% 8% 43%	1634
	11th to 20th	Night	12% 6% 3 5% 5% 3 4 5% 7% 8% 42%	1592
	21st to end	Night	6% 4 2 4 6% 3 7% 5% 8% 10% 45%	1704
June	1st to 10th	Night	13% 6% 2 4 4 3 3 4 6% 8% 47%	1440
	11th to 20th	Night	5% 3 2 3 3 2 4 6% 6% 9% 56%	1440
	21st to end	Night	3 2 1 3 5% 4 6% 5% 7% 14% 50%	1440
July	1st to 10th	Night	8% 3 2 4 6% 4 6% 5% 7% 10% 46%	1440
	11th to 20th	Night	8% 3 2 4 5% 3 5% 5% 5% 9% 52%	1440
	21st to end	Night	7% 4 2 5% 5% 3 5% 6% 6% 10% 49%	1642
August	1st to 10th	Night	6% 3 1 5% 4 3 5% 5% 7% 9% 52%	1780
	11th to 20th	Night	8% 4 2 4 4 2 5% 4 6% 10% 50%	1776
	21st to end	Night	7% 4 2 5% 5% 3 4 4 6% 10% 51%	1980
September	1st to 10th	Night	11% 5% 2 4 4 2 4 4 5% 7% 53%	1800
	11th to 20th	Night	10% 5% 3 4 4 3 4 4 5% 6% 52%	1926
	21st to end	Night	11% 4 2 4 3 2 4 3 3 7% 57%	1980
October	1st to 10th	Night	10% 4 2 3 4 2 3 3 3 6% 60%	1980
	11th to 20th	Night	16% 5% 1 4 3 2 3 3 3 5% 53%	2123
	21st to end	Night	19% 6% 2 4 5% 2 3 3 4 5% 46%	2506
November	1st to 10th	Night	12% 5% 2 4 3 2 2 3 4 6% 56%	2406
	11th to 20th	Night	14% 5% 3 5% 4 2 3 4 4 6% 50%	2470
	21st to end	Night	17% 7% 3 4 4 2 3 3 4 5% 47%	2470
December	1st to 10th	Night	20% 11% 3 5% 5% 3 3 5% 4 6% 36%	2470
	11th to 20th	Night	17% 9% 3 5% 5% 2 4 4 4 5% 43%	3217
	21st to end	Night	17% 12% 5% 7% 7% 4 5% 4 4 6% 29%	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	53% 5% 7% 24% 9% 2	2528
	11th to 20th	Day	51% 4 8% 23% 13% 1	2558
	21st to end	Day	57% 4 9% 20% 9% 1	3226
February	1st to 10th	Day	49% 4 9% 24% 13% 2	1900
	11th to 20th	Day	50% 4 8% 21% 13% 4	1858
	21st to end	Day	51% 4 9% 17% 14% 5%	1570
March	1st to 10th	Day	48% 4 6% 21% 17% 4	1900
	11th to 20th	Day	51% 5% 9% 21% 12% 2	1921
	21st to end	Day	52% 5% 11% 20% 11% 1	2376
April	1st to 10th	Day	38% 5% 10% 24% 20% 2	2160
	11th to 20th	Day	44% 4 9% 22% 18% 4	2134
	21st to end	Day	44% 4 9% 27% 13% 2	2160
May	1st to 10th	Day	44% 6% 13% 24% 12% 1	2325
	11th to 20th	Day	44% 5% 12% 22% 15% 1	2305
	21st to end	Day	39% 9% 17% 21% 12% 2	2567
June	1st to 10th	Day	32% 7% 14% 24% 20% 3	2520
	11th to 20th	Day	27% 9% 14% 28% 19% 3	2520
	21st to end	Day	30% 8% 16% 24% 17% 5%	2520
July	1st to 10th	Day	38% 9% 14% 23% 12% 4	2451
	11th to 20th	Day	33% 7% 17% 25% 15% 3	2445
	21st to end	Day	33% 9% 16% 27% 12% 2	2523
August	1st to 10th	Day	34% 9% 15% 28% 15% 1	2140
	11th to 20th	Day	37% 9% 12% 25% 16% 2	2125
	21st to end	Day	32% 7% 13% 26% 17% 4	2376
September	1st to 10th	Day	36% 7% 10% 26% 18% 4	2160
	11th to 20th	Day	35% 6% 9% 24% 23% 3	2015
	21st to end	Day	32% 5% 8% 24% 24% 6%	1980
October	1st to 10th	Day	29% 4 10% 24% 27% 5%	1980
	11th to 20th	Day	35% 4 8% 24% 23% 7%	1834
	21st to end	Day	42% 3 6% 25% 20% 4	1848
November	1st to 10th	Day	32% 3 10% 24% 25% 6%	1672
	11th to 20th	Day	38% 4 8% 24% 22% 5%	1710
	21st to end	Day	44% 3 7% 26% 15% 5%	1710
December	1st to 10th	Day	48% 4 11% 23% 12% 2	1710
	11th to 20th	Day	49% 5% 9% 23% 13% 2	1972
	21st to end	Day	54% 5% 13% 18% 11%	2783
January	1st to 10th	Night	49% 3 8% 27% 12% 1	4428
	11th to 20th	Night	53% 3 8% 21% 12% 2	4408
	21st to end	Night	55% 3 9% 22% 10% 1	4436
February	1st to 10th	Night	54% 4 8% 19% 13% 2	2280
	11th to 20th	Night	49% 4 6% 22% 15% 4	2219
	21st to end	Night	52% 4 9% 16% 16% 4	1884
March	1st to 10th	Night	46% 4 8% 16% 20% 6%	2280
	11th to 20th	Night	46% 4 11% 22% 13% 4	2218
	21st to end	Night	49% 5% 11% 21% 12% 2	1980
April	1st to 10th	Night	34% 4 10% 27% 21% 3	1800
	11th to 20th	Night	39% 5% 8% 21% 21% 7%	1775
	21st to end	Night	41% 5% 9% 25% 18% 2	1800
May	1st to 10th	Night	40% 5% 10% 23% 18% 2	1634
	11th to 20th	Night	38% 8% 14% 23% 15% 1	1593
	21st to end	Night	32% 9% 17% 22% 17% 2	1705
June	1st to 10th	Night	35% 5% 12% 24% 22% 3	1440
	11th to 20th	Night	22% 6% 18% 27% 22% 4	1440
	21st to end	Night	24% 9% 17% 28% 17% 6%	1440
July	1st to 10th	Night	30% 9% 15% 26% 16% 4	1400
	11th to 20th	Night	28% 7% 19% 25% 17% 6%	1392
	21st to end	Night	29% 8% 14% 32% 15% 1	1551
August	1st to 10th	Night	28% 8% 16% 27% 20% 1	1769
	11th to 20th	Night	29% 8% 15% 27% 20% 2	1776
	21st to end	Night	29% 8% 13% 27% 18% 6%	1980
September	1st to 10th	Night	32% 5% 9% 27% 21% 5%	1800
	11th to 20th	Night	32% 5% 8% 26% 24% 5%	1926
	21st to end	Night	30% 4 7% 26% 25% 7%	1980
October	1st to 10th	Night	28% 4 8% 24% 29% 7%	1980
	11th to 20th	Night	36% 4 7% 21% 25% 8%	2123
	21st to end	Night	42% 3 6% 23% 21% 5%	2506
November	1st to 10th	Night	31% 3 7% 25% 27% 6%	2406
	11th to 20th	Night	35% 3 7% 24% 22% 9%	2470
	21st to end	Night	41% 3 8% 24% 21% 3	2470
December	1st to 10th	Night	50% 4 9% 21% 13% 2	2470
	11th to 20th	Night	45% 3 8% 23% 19% 2	3219
	21st to end	Night	56% 5% 8% 18% 10% 2	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	26% 20% 34% 17% 3	270
	11th to 20th	Day	28% 21% 35% 13% 2	270
	21st to end	Day	23% 23% 39% 11% 3	297
February	1st to 10th	Day	28% 21% 29% 18% 3 1	270
	11th to 20th	Day	30% 24% 34% 9% 2	270
	21st to end	Day	29% 22% 34% 12% 3	225
March	1st to 10th	Day	23% 21% 40% 12% 3 1	270
	11th to 20th	Day	27% 22% 33% 14% 4 1	293
	21st to end	Day	26% 19% 31% 18% 6%	462
April	1st to 10th	Day	20% 13% 33% 25% 8% 1	420
	11th to 20th	Day	21% 17% 26% 26% 9% 2	412
	21st to end	Day	21% 14% 31% 23% 8% 3	420
May	1st to 10th	Day	20% 17% 24% 29% 7% 3	423
	11th to 20th	Day	17% 9% 24% 31% 14% 5%	447
	21st to end	Day	10% 6% 19% 33% 21% 12%	521
June	1st to 10th	Day	10% 5% 19% 29% 24% 12%	630
	11th to 20th	Day	4 3 21% 36% 25% 11%	630
	21st to end	Day	5% 4 22% 35% 23% 11%	630
July	1st to 10th	Day	9% 2 12% 28% 29% 19%	621
	11th to 20th	Day	8% 4 15% 26% 34% 14%	618
	21st to end	Day	7% 1 14% 32% 30% 15%	612
August	1st to 10th	Day	11% 3 11% 30% 30% 15%	446
	11th to 20th	Day	13% 3 13% 32% 31% 8%	439
	21st to end	Day	9% 2 10% 26% 36% 18%	495
September	1st to 10th	Day	10% 3 13% 28% 32% 14%	450
	11th to 20th	Day	10% 2 11% 28% 32% 17%	450
	21st to end	Day	15% 6% 18% 26% 23% 11%	450
October	1st to 10th	Day	10% 3 23% 35% 22% 7%	450
	11th to 20th	Day	14% 12% 33% 26% 11% 4	304
	21st to end	Day	28% 14% 27% 23% 7% 2	297
November	1st to 10th	Day	12% 8% 38% 27% 14% 1	270
	11th to 20th	Day	17% 19% 31% 27% 6% 1	270
	21st to end	Day	24% 12% 33% 22% 8% 1	270
December	1st to 10th	Day	21% 21% 33% 18% 8%	270
	11th to 20th	Day	23% 19% 32% 22% 4	270
	21st to end	Day	26% 20% 38% 13% 2	297
January	1st to 10th	Night	25% 18% 31% 22% 3	950
	11th to 20th	Night	27% 18% 32% 20% 4	950
	21st to end	Night	22% 16% 38% 22% 2	988
February	1st to 10th	Night	27% 14% 35% 21% 2	760
	11th to 20th	Night	25% 17% 36% 20% 2	743
	21st to end	Night	28% 14% 36% 20% 1	628
March	1st to 10th	Night	23% 14% 32% 26% 5%	760
	11th to 20th	Night	22% 13% 35% 23% 6% 1	748
	21st to end	Night	23% 12% 28% 30% 7% 1	594
April	1st to 10th	Night	14% 10% 37% 31% 8% 1	540
	11th to 20th	Night	19% 11% 29% 29% 12% 1	533
	21st to end	Night	19% 9% 29% 31% 10% 1	540
May	1st to 10th	Night	20% 8% 31% 26% 14% 1	540
	11th to 20th	Night	13% 4 24% 37% 20% 2	531
	21st to end	Night	8% 3 16% 41% 29% 4	584
June	1st to 10th	Night	14% 3 16% 33% 30% 4	540
	11th to 20th	Night	6% 1 12% 44% 34% 3	540
	21st to end	Night	3 2 14% 41% 35% 5%	540
July	1st to 10th	Night	8% 1 9% 30% 42% 10%	525
	11th to 20th	Night	9% 1 9% 30% 44% 7%	522
	21st to end	Night	7% 2 11% 35% 36% 9%	591
August	1st to 10th	Night	7% 1 7% 35% 40% 9%	537
	11th to 20th	Night	9% 2 9% 37% 38% 5%	534
	21st to end	Night	10% 7% 30% 44% 8%	594
September	1st to 10th	Night	12% 1 14% 30% 38% 5%	540
	11th to 20th	Night	11% 2 11% 31% 38% 6%	666
	21st to end	Night	11% 4 19% 31% 29% 5%	720
October	1st to 10th	Night	13% 5% 17% 32% 28% 5%	720
	11th to 20th	Night	18% 7% 25% 33% 16% 2	720
	21st to end	Night	22% 8% 29% 28% 13% 1	792
November	1st to 10th	Night	13% 8% 29% 33% 14% 1	740
	11th to 20th	Night	16% 13% 33% 29% 10% 1	760
	21st to end	Night	22% 13% 26% 28% 11% 1	760
December	1st to 10th	Night	26% 12% 30% 26% 6% 1	760
	11th to 20th	Night	24% 10% 32% 27% 6%	888
	21st to end	Night	28% 13% 31% 24% 4	1045

Page generated at 2024-03-15 13:01:45. Version ac07

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