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 2024-11-02.

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% 7% 3 6% 4 4 4 4 5% 7% 42%	100970
	Night	14% 7% 3 6% 4 4 4 3 5% 7% 44%	98026
Transparency	Day	40% 5% 11% 24% 16% 3	100763
	Night	40% 5% 10% 24% 18% 3	97905
Seeing	Day	15% 10% 24% 26% 18% 8%	15392
	Night	18% 9% 25% 29% 17% 2	25161

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% 12% 5% 9% 5% 4 5% 3 4 5% 32%	2527
	11th to 20th	Day	18% 9% 3 7% 5% 4 5% 3 4 5% 36%	2558
	21st to end	Day	24% 12% 5% 7% 4 3 4 3 4 5% 29%	3226
February	1st to 10th	Day	18% 12% 3 6% 4 3 3 3 5% 6% 37%	1900
	11th to 20th	Day	19% 9% 4 8% 4 3 4 3 4 5% 38%	1848
	21st to end	Day	19% 10% 4 6% 5% 3 4 4 5% 5% 35%	1570
March	1st to 10th	Day	21% 14% 3 7% 4 3 3 3 4 5% 33%	3120
	11th to 20th	Day	14% 7% 3 6% 4 3 3 2 4 6% 49%	3267
	21st to end	Day	12% 8% 3 7% 5% 3 4 4 6% 6% 42%	4165
April	1st to 10th	Day	13% 6% 3 6% 5% 4 5% 3 4 7% 44%	2280
	11th to 20th	Day	16% 7% 2 7% 4 5% 3 4 5% 6% 41%	2254
	21st to end	Day	17% 6% 2 7% 5% 4 5% 3 5% 8% 40%	2280
May	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 4 7% 9% 32%	4067
	11th to 20th	Day	19% 7% 2 6% 4 3 4 3 5% 8% 38%	4081
	21st to end	Day	11% 7% 3 7% 5% 4 5% 4 6% 8% 41%	4540
June	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 6% 10% 46%	2660
	11th to 20th	Day	4 4 2 5% 4 5% 5% 5% 7% 10% 49%	2660
	21st to end	Day	5% 4 2 5% 5% 5% 5% 5% 7% 10% 46%	2660
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 8% 10% 40%	4440
	11th to 20th	Day	7% 3 2 5% 3 5% 5% 5% 7% 9% 49%	4440
	21st to end	Day	12% 10% 3 9% 6% 6% 5% 5% 6% 8% 29%	4688
August	1st to 10th	Day	6% 4 1 5% 3 4 4 4 6% 9% 54%	3790
	11th to 20th	Day	9% 5% 2 6% 5% 6% 5% 4 8% 8% 42%	3729
	21st to end	Day	5% 3 2 4 4 4 4 3 5% 8% 58%	4180
September	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 5% 8% 46%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 5% 7% 49%	2126
	21st to end	Day	13% 4 2 6% 3 3 4 4 4 7% 49%	2090
October	1st to 10th	Day	10% 3 1 4 3 3 3 3 4 7% 58%	2090
	11th to 20th	Day	13% 5% 2 4 3 4 4 2 4 6% 53%	1935
	21st to end	Day	19% 7% 2 5% 3 3 3 1 4 5% 48%	1950
November	1st to 10th	Day	11% 5% 2 5% 3 2 3 2 5% 6% 55%	1681
	11th to 20th	Day	15% 8% 2 5% 3 3 4 3 4 5% 49%	1710
	21st to end	Day	18% 8% 2 7% 3 3 3 2 5% 5% 43%	1710
December	1st to 10th	Day	19% 11% 3 5% 3 4 5% 3 5% 6% 37%	1710
	11th to 20th	Day	16% 10% 4 7% 5% 5% 4 4 5% 6% 34%	1975
	21st to end	Day	17% 12% 5% 8% 4 5% 5% 3 5% 6% 29%	2783
January	1st to 10th	Night	19% 11% 3 6% 3 3 4 3 5% 6% 36%	4428
	11th to 20th	Night	17% 12% 4 9% 5% 4 4 3 4 4 34%	4408
	21st to end	Night	25% 12% 4 6% 3 3 3 3 4 5% 32%	4436
February	1st to 10th	Night	21% 11% 4 7% 4 4 4 2 4 4 34%	2280
	11th to 20th	Night	18% 9% 3 8% 5% 3 4 3 4 6% 38%	2218
	21st to end	Night	22% 9% 3 8% 3 4 3 3 5% 5% 35%	1884
March	1st to 10th	Night	23% 10% 3 6% 4 3 3 3 4 4 36%	3733
	11th to 20th	Night	14% 7% 3 5% 4 3 3 3 4 6% 48%	3720
	21st to end	Night	15% 8% 3 7% 5% 3 3 3 5% 7% 41%	3466
April	1st to 10th	Night	12% 5% 1 5% 4 4 4 3 5% 8% 48%	1900
	11th to 20th	Night	16% 6% 2 5% 4 3 4 3 4 7% 46%	1875
	21st to end	Night	16% 6% 3 6% 5% 3 4 4 5% 8% 41%	1900
May	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 8% 38%	2827
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 6% 7% 42%	2812
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 7% 8% 44%	3000
June	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 6% 8% 47%	1520
	11th to 20th	Night	5% 3 1 4 3 3 4 5% 6% 9% 55%	1520
	21st to end	Night	3 3 1 4 4 5% 5% 4 7% 14% 50%	1520
July	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 4 7% 9% 46%	2530
	11th to 20th	Night	6% 4 2 5% 4 4 4 3 5% 8% 56%	2530
	21st to end	Night	8% 8% 3 9% 6% 6% 6% 5% 7% 8% 33%	2867
August	1st to 10th	Night	5% 3 1 4 3 3 4 3 5% 7% 61%	3134
	11th to 20th	Night	5% 3 2 5% 4 4 5% 4 6% 9% 52%	3100
	21st to end	Night	4 3 1 4 3 3 3 3 6% 9% 62%	3476
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 2 5% 7% 54%	1900
	11th to 20th	Night	10% 5% 3 5% 4 4 4 3 5% 6% 52%	2033
	21st to end	Night	12% 5% 2 5% 3 3 3 3 4 7% 54%	2090
October	1st to 10th	Night	10% 4 2 4 3 2 3 2 3 6% 61%	2090
	11th to 20th	Night	15% 5% 1 4 2 3 3 2 3 5% 55%	2241
	21st to end	Night	18% 6% 2 4 4 3 3 3 4 5% 48%	2646
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 2 5% 6% 55%	2431
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 6% 50%	2470
	21st to end	Night	17% 7% 2 5% 4 3 3 2 4 5% 47%	2470
December	1st to 10th	Night	20% 11% 3 6% 4 4 3 4 4 6% 36%	2470
	11th to 20th	Night	16% 9% 3 6% 4 3 4 3 4 5% 43%	3217
	21st to end	Night	17% 13% 5% 8% 6% 5% 5% 3 4 5% 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	55% 4 5% 17% 13% 7%	3120
	11th to 20th	Day	37% 4 8% 28% 18% 4	3267
	21st to end	Day	42% 4 11% 23% 18% 1	4165
April	1st to 10th	Day	39% 5% 10% 24% 19% 2	2280
	11th to 20th	Day	43% 4 9% 22% 18% 4	2254
	21st to end	Day	44% 4 9% 27% 14% 2	2280
May	1st to 10th	Day	47% 7% 13% 20% 11% 1	4105
	11th to 20th	Day	45% 6% 11% 25% 13% 1	4085
	21st to end	Day	40% 8% 11% 19% 21% 1	4553
June	1st to 10th	Day	33% 7% 14% 23% 20% 3	2660
	11th to 20th	Day	27% 8% 14% 27% 20% 3	2660
	21st to end	Day	30% 8% 16% 23% 17% 5%	2660
July	1st to 10th	Day	37% 7% 15% 28% 11% 2	4371
	11th to 20th	Day	29% 6% 14% 37% 12% 2	4365
	21st to end	Day	51% 7% 12% 21% 8% 1	4540
August	1st to 10th	Day	24% 6% 15% 31% 23% 1	3777
	11th to 20th	Day	37% 7% 12% 25% 18% 2	3765
	21st to end	Day	24% 5% 11% 36% 22% 3	4179
September	1st to 10th	Day	36% 7% 9% 25% 19% 5%	2280
	11th to 20th	Day	35% 6% 9% 24% 23% 3	2126
	21st to end	Day	34% 5% 9% 23% 23% 6%	2090
October	1st to 10th	Day	28% 4 10% 23% 30% 6%	2090
	11th to 20th	Day	34% 3 8% 24% 24% 7%	1935
	21st to end	Day	41% 3 6% 25% 20% 5%	1950
November	1st to 10th	Day	32% 3 10% 24% 26% 5%	1681
	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	53% 4 7% 13% 18% 5%	3733
	11th to 20th	Night	38% 4 11% 23% 14% 9%	3720
	21st to end	Night	44% 4 10% 24% 17% 1	3466
April	1st to 10th	Night	35% 4 10% 27% 20% 3	1900
	11th to 20th	Night	39% 5% 8% 21% 21% 7%	1875
	21st to end	Night	41% 5% 8% 25% 18% 2	1900
May	1st to 10th	Night	44% 6% 14% 22% 12% 1	2854
	11th to 20th	Night	40% 7% 13% 24% 15% 1	2813
	21st to end	Night	34% 8% 14% 24% 18% 1	3008
June	1st to 10th	Night	35% 5% 12% 23% 21% 3	1520
	11th to 20th	Night	23% 6% 17% 26% 24% 4	1520
	21st to end	Night	23% 8% 17% 27% 18% 6%	1520
July	1st to 10th	Night	32% 7% 14% 29% 14% 3	2490
	11th to 20th	Night	26% 5% 13% 36% 17% 3	2482
	21st to end	Night	47% 8% 13% 22% 9% 1	2776
August	1st to 10th	Night	22% 6% 14% 37% 21% 1	3123
	11th to 20th	Night	27% 6% 17% 30% 19% 2	3130
	21st to end	Night	21% 6% 10% 37% 23% 3	3476
September	1st to 10th	Night	32% 5% 9% 26% 23% 6%	1900
	11th to 20th	Night	33% 5% 8% 26% 24% 4	2033
	21st to end	Night	31% 4 8% 25% 24% 7%	2090
October	1st to 10th	Night	27% 4 8% 23% 31% 8%	2090
	11th to 20th	Night	34% 4 7% 21% 27% 8%	2241
	21st to end	Night	40% 3 7% 23% 22% 5%	2646
November	1st to 10th	Night	31% 3 7% 26% 27% 6%	2431
	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	25% 23% 34% 14% 3 1	304
	21st to end	Day	26% 18% 33% 17% 6%	506
April	1st to 10th	Day	22% 15% 32% 23% 7% 1	460
	11th to 20th	Day	22% 18% 26% 25% 8% 2	452
	21st to end	Day	20% 15% 33% 22% 8% 3	460
May	1st to 10th	Day	19% 17% 25% 28% 8% 3	463
	11th to 20th	Day	16% 9% 24% 31% 14% 6%	487
	21st to end	Day	10% 7% 22% 31% 19% 11%	567
June	1st to 10th	Day	11% 5% 21% 29% 23% 11%	680
	11th to 20th	Day	4 3 20% 35% 27% 11%	680
	21st to end	Day	6% 4 22% 35% 22% 11%	680
July	1st to 10th	Day	9% 3 13% 28% 30% 18%	671
	11th to 20th	Day	8% 4 14% 27% 34% 13%	668
	21st to end	Day	9% 2 14% 32% 29% 14%	663
August	1st to 10th	Day	10% 3 12% 30% 30% 16%	486
	11th to 20th	Day	12% 3 15% 32% 31% 8%	479
	21st to end	Day	8% 2 10% 26% 35% 20%	539
September	1st to 10th	Day	10% 3 13% 29% 31% 13%	490
	11th to 20th	Day	11% 2 11% 28% 32% 16%	490
	21st to end	Day	17% 6% 19% 26% 22% 10%	490
October	1st to 10th	Day	9% 3 21% 35% 24% 8%	490
	11th to 20th	Day	13% 11% 33% 24% 13% 5%	335
	21st to end	Day	26% 12% 29% 23% 8% 2	330
November	1st to 10th	Day	12% 8% 37% 27% 14% 1	273
	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% 24% 6% 1	755
	21st to end	Night	22% 13% 29% 29% 6% 1	627
April	1st to 10th	Night	15% 11% 36% 30% 8% 1	570
	11th to 20th	Night	19% 11% 30% 28% 12% 1	563
	21st to end	Night	19% 10% 31% 30% 10% 1	570
May	1st to 10th	Night	20% 8% 30% 26% 15% 1	570
	11th to 20th	Night	13% 4 22% 38% 20% 2	561
	21st to end	Night	8% 4 16% 41% 27% 4	617
June	1st to 10th	Night	14% 3 16% 35% 29% 3	570
	11th to 20th	Night	6% 2 13% 43% 34% 3	570
	21st to end	Night	3 2 13% 42% 34% 5%	570
July	1st to 10th	Night	8% 1 10% 30% 41% 10%	555
	11th to 20th	Night	9% 1 8% 31% 43% 7%	552
	21st to end	Night	8% 1 12% 36% 35% 8%	624
August	1st to 10th	Night	7% 1 8% 35% 40% 9%	567
	11th to 20th	Night	9% 2 9% 38% 37% 5%	564
	21st to end	Night	9% 7% 31% 45% 8%	627
September	1st to 10th	Night	12% 2 14% 31% 37% 5%	570
	11th to 20th	Night	12% 2 11% 32% 38% 6%	703
	21st to end	Night	12% 4 19% 31% 28% 4	760
October	1st to 10th	Night	12% 4 17% 33% 29% 5%	760
	11th to 20th	Night	18% 6% 24% 33% 17% 2	760
	21st to end	Night	21% 7% 30% 28% 13% 1	836
November	1st to 10th	Night	13% 8% 29% 33% 14% 1	748
	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 2024-10-30 12:19:25. Version ac07

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