Darlington Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Darlington Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	2 3 1 4 3 3 4 4 6% 8% 61%	140042
	Night	3 4 2 5% 3 3 4 3 5% 7% 60%	139668
Transparency	Day	18% 7% 12% 24% 26% 11%	135040
	Night	21% 7% 12% 24% 25% 10%	135677
Seeing	Day	5% 12% 27% 32% 19% 5%	17135
	Night	5% 5% 27% 43% 17% 1	27345

By Month

Percentage Cloud Cover SW (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	1 7% 3 7% 3 2 3 2 4 7% 62%	4957
	11th to 20th	Day	1 2 2 3 2 2 2 4 4 5% 72%	5340
	21st to end	Day	2 5% 2 5% 3 3 4 3 5% 6% 60%	5844
February	1st to 10th	Day	1 5% 2 4 3 3 4 3 5% 8% 62%	1909
	11th to 20th	Day	2 5% 2 5% 3 3 4 3 5% 8% 59%	2049
	21st to end	Day	2 6% 2 4 3 3 3 3 5% 8% 61%	1730
March	1st to 10th	Day	1 5% 2 5% 3 4 3 3 4 6% 64%	4756
	11th to 20th	Day	2 6% 2 6% 4 4 4 3 6% 8% 52%	5110
	21st to end	Day	1 4 2 4 3 4 5% 4 6% 9% 58%	5831
April	1st to 10th	Day	2 4 1 4 3 3 4 4 5% 7% 63%	2520
	11th to 20th	Day	2 4 1 4 3 4 4 4 5% 7% 61%	2444
	21st to end	Day	3 4 2 3 3 3 4 4 6% 8% 62%	2400
May	1st to 10th	Day	4 4 1 4 3 3 5% 4 6% 8% 58%	5440
	11th to 20th	Day	2 3 1 4 3 3 4 4 6% 7% 63%	5903
	21st to end	Day	1 2 1 2 2 3 4 3 6% 9% 67%	6925
June	1st to 10th	Day	2 2 1 3 3 4 4 4 6% 9% 62%	2800
	11th to 20th	Day	2 1 1 2 2 3 3 4 6% 6% 71%	2800
	21st to end	Day	4 3 1 3 4 3 4 5% 7% 8% 57%	2800
July	1st to 10th	Day	2 2 1 4 3 4 4 4 6% 8% 63%	6339
	11th to 20th	Day	1 1 1 2 3 4 7% 7% 11% 14% 48%	6068
	21st to end	Day	2 2 1 5% 5% 5% 7% 6% 9% 11% 46%	6401
August	1st to 10th	Day	2 2 1 5% 4 5% 6% 7% 10% 11% 46%	5656
	11th to 20th	Day	1 2 1 3 2 3 5% 5% 8% 11% 57%	5366
	21st to end	Day	2 3 1 5% 4 5% 6% 6% 9% 13% 45%	5984
September	1st to 10th	Day	5% 3 1 3 4 4 5% 5% 6% 9% 53%	2400
	11th to 20th	Day	4 3 1 4 3 3 4 3 7% 8% 60%	2400
	21st to end	Day	3 2 1 3 2 3 3 2 4 7% 68%	2185
October	1st to 10th	Day	3 2 1 2 2 2 2 2 4 6% 74%	3270
	11th to 20th	Day	4 4 2 4 3 3 4 3 5% 7% 61%	3250
	21st to end	Day	2 2 1 3 2 2 2 2 3 5% 75%	3548
November	1st to 10th	Day	2 3 1 2 1 3 2 3 4 6% 73%	1929
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 7% 74%	1890
	21st to end	Day	3 7% 1 4 2 2 3 2 4 6% 64%	1890
December	1st to 10th	Day	2 3 1 2 1 1 2 2 3 6% 77%	2710
	11th to 20th	Day	1 4 1 3 2 3 3 3 5% 10% 66%	3106
	21st to end	Day	2 5% 2 4 3 3 3 2 5% 6% 66%	4092
January	1st to 10th	Night	1 6% 3 7% 3 3 4 3 5% 7% 58%	8021
	11th to 20th	Night	1 4 2 7% 3 3 3 3 4 6% 65%	7650
	21st to end	Night	3 7% 2 5% 3 3 4 3 4 7% 60%	8370
February	1st to 10th	Night	2 6% 3 5% 3 2 3 2 4 7% 64%	2709
	11th to 20th	Night	3 8% 2 5% 3 3 4 3 5% 6% 57%	2458
	21st to end	Night	4 7% 1 4 3 3 2 2 4 7% 63%	2076
March	1st to 10th	Night	2 7% 2 4 3 3 3 3 5% 6% 63%	5342
	11th to 20th	Night	4 7% 3 7% 3 4 4 3 4 6% 56%	5110
	21st to end	Night	4 7% 2 5% 3 3 3 3 4 7% 59%	5376
April	1st to 10th	Night	3 4 1 5% 3 3 4 3 5% 6% 61%	2100
	11th to 20th	Night	4 5% 2 4 3 4 4 4 6% 7% 58%	2037
	21st to end	Night	3 5% 1 4 3 2 4 4 6% 6% 61%	2000
May	1st to 10th	Night	5% 5% 2 5% 4 3 4 3 6% 8% 54%	4540
	11th to 20th	Night	4 4 2 4 4 4 4 3 6% 8% 59%	4007
	21st to end	Night	2 2 1 3 3 3 3 3 5% 8% 66%	3972
June	1st to 10th	Night	2 2 1 3 4 4 4 4 6% 8% 62%	1600
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 6% 70%	1600
	21st to end	Night	3 4 1 4 5% 4 4 5% 7% 10% 53%	1600
July	1st to 10th	Night	2 2 2 6% 5% 5% 5% 4 7% 7% 55%	3640
	11th to 20th	Night	1 2 1 5% 4 5% 6% 6% 11% 15% 45%	3907
	21st to end	Night	4 3 2 5% 6% 7% 8% 7% 10% 11% 37%	4472
August	1st to 10th	Night	3 3 2 6% 5% 6% 7% 6% 8% 10% 44%	4155
	11th to 20th	Night	2 3 1 6% 4 5% 6% 5% 10% 14% 43%	4483
	21st to end	Night	4 5% 3 8% 6% 7% 7% 6% 9% 11% 33%	4993
September	1st to 10th	Night	6% 3 2 5% 4 4 5% 5% 7% 8% 52%	2000
	11th to 20th	Night	5% 3 1 2 3 3 4 3 7% 7% 61%	2000
	21st to end	Night	5% 2 1 4 3 3 3 3 4 6% 66%	2211
October	1st to 10th	Night	3 3 1 3 2 2 2 2 4 6% 72%	3920
	11th to 20th	Night	6% 4 1 4 3 3 3 3 5% 5% 64%	3898
	21st to end	Night	3 2 1 3 2 2 2 2 3 5% 76%	4252
November	1st to 10th	Night	2 2 1 2 1 2 2 2 3 6% 77%	2566
	11th to 20th	Night	1 2 1 1 1 2 2 2 4 6% 78%	2730
	21st to end	Night	4 5% 1 3 2 2 3 3 5% 6% 65%	2730
December	1st to 10th	Night	2 4 1 3 2 1 2 2 3 5% 75%	4694
	11th to 20th	Night	2 3 1 3 2 2 2 2 4 8% 70%	5321
	21st to end	Night	2 5% 2 5% 3 3 4 3 4 7% 62%	7128

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	24% 5% 10% 35% 24% 3	4960
	11th to 20th	Day	12% 9% 13% 28% 29% 9%	5340
	21st to end	Day	22% 6% 9% 23% 32% 9%	5844
February	1st to 10th	Day	22% 7% 14% 26% 23% 8%	1909
	11th to 20th	Day	23% 7% 12% 26% 26% 6%	2059
	21st to end	Day	23% 6% 8% 19% 29% 14%	1730
March	1st to 10th	Day	18% 6% 7% 17% 37% 14%	4756
	11th to 20th	Day	24% 8% 10% 26% 27% 5%	5110
	21st to end	Day	20% 7% 9% 38% 20% 5%	5831
April	1st to 10th	Day	20% 6% 9% 25% 27% 14%	2520
	11th to 20th	Day	21% 6% 9% 22% 28% 13%	2447
	21st to end	Day	19% 7% 10% 24% 30% 11%	2400
May	1st to 10th	Day	18% 9% 14% 20% 25% 14%	5440
	11th to 20th	Day	17% 7% 10% 18% 34% 14%	5907
	21st to end	Day	11% 6% 11% 17% 32% 24%	6926
June	1st to 10th	Day	16% 7% 13% 22% 24% 18%	2800
	11th to 20th	Day	12% 5% 8% 17% 25% 33%	2800
	21st to end	Day	21% 6% 11% 20% 23% 20%	2800
July	1st to 10th	Day	18% 8% 11% 17% 21% 25%	6271
	11th to 20th	Day	16% 13% 27% 31% 8% 6%	5994
	21st to end	Day	25% 12% 21% 27% 12% 2	6217
August	1st to 10th	Day	28% 14% 22% 21% 9% 5%	4118
	11th to 20th	Day	16% 9% 18% 25% 20% 11%	3882
	21st to end	Day	24% 13% 23% 23% 15% 3	4311
September	1st to 10th	Day	24% 9% 13% 22% 21% 11%	2400
	11th to 20th	Day	21% 7% 11% 19% 27% 15%	2400
	21st to end	Day	15% 5% 8% 24% 35% 13%	2185
October	1st to 10th	Day	13% 5% 5% 18% 37% 23%	3270
	11th to 20th	Day	19% 7% 10% 20% 32% 12%	3250
	21st to end	Day	11% 4 6% 26% 42% 11%	3548
November	1st to 10th	Day	13% 3 10% 26% 36% 11%	1929
	11th to 20th	Day	12% 3 9% 34% 34% 7%	1890
	21st to end	Day	22% 5% 8% 26% 30% 9%	1890
December	1st to 10th	Day	11% 4 11% 36% 32% 6%	2710
	11th to 20th	Day	16% 6% 8% 36% 29% 4	3104
	21st to end	Day	17% 5% 12% 31% 30% 5%	4092
January	1st to 10th	Night	25% 6% 12% 33% 19% 4	8022
	11th to 20th	Night	18% 10% 12% 27% 26% 7%	7650
	21st to end	Night	24% 5% 8% 27% 29% 6%	8370
February	1st to 10th	Night	22% 7% 13% 26% 24% 8%	2709
	11th to 20th	Night	27% 7% 12% 25% 24% 5%	2458
	21st to end	Night	23% 6% 6% 23% 31% 11%	2076
March	1st to 10th	Night	20% 6% 7% 16% 38% 12%	5342
	11th to 20th	Night	26% 6% 8% 24% 29% 6%	5110
	21st to end	Night	22% 7% 9% 37% 19% 5%	5376
April	1st to 10th	Night	21% 5% 11% 25% 26% 12%	2100
	11th to 20th	Night	23% 8% 10% 20% 26% 14%	2037
	21st to end	Night	22% 7% 11% 23% 27% 10%	2000
May	1st to 10th	Night	23% 8% 15% 19% 21% 14%	4537
	11th to 20th	Night	20% 8% 11% 22% 22% 17%	4008
	21st to end	Night	14% 7% 13% 14% 26% 26%	3972
June	1st to 10th	Night	17% 8% 17% 19% 21% 19%	1600
	11th to 20th	Night	15% 5% 7% 14% 24% 35%	1600
	21st to end	Night	23% 8% 12% 19% 21% 18%	1600
July	1st to 10th	Night	24% 10% 13% 13% 15% 25%	3600
	11th to 20th	Night	22% 13% 31% 20% 7% 7%	3858
	21st to end	Night	32% 14% 22% 22% 9% 1	4344
August	1st to 10th	Night	35% 15% 22% 13% 8% 6%	3017
	11th to 20th	Night	25% 12% 21% 18% 15% 10%	3243
	21st to end	Night	35% 13% 20% 17% 11% 3	3597
September	1st to 10th	Night	26% 8% 14% 24% 18% 10%	2000
	11th to 20th	Night	19% 7% 14% 19% 25% 17%	2000
	21st to end	Night	19% 6% 9% 20% 33% 13%	2211
October	1st to 10th	Night	14% 4 7% 18% 39% 18%	3920
	11th to 20th	Night	19% 6% 9% 18% 34% 13%	3898
	21st to end	Night	12% 4 7% 27% 39% 11%	4252
November	1st to 10th	Night	11% 3 8% 27% 40% 11%	2566
	11th to 20th	Night	10% 4 8% 35% 35% 8%	2730
	21st to end	Night	21% 5% 9% 26% 30% 9%	2730
December	1st to 10th	Night	15% 3 9% 40% 28% 4	4694
	11th to 20th	Night	15% 5% 8% 34% 34% 4	5322
	21st to end	Night	22% 6% 13% 29% 27% 3	7128

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	4 18% 32% 36% 8% 2	330
	11th to 20th	Day	7% 11% 33% 38% 8% 2	330
	21st to end	Day	4 9% 32% 41% 12% 3	363
February	1st to 10th	Day	4 15% 35% 35% 9% 3	330
	11th to 20th	Day	6% 19% 41% 27% 7%	330
	21st to end	Day	9% 22% 30% 33% 6%	273
March	1st to 10th	Day	5% 23% 31% 30% 10% 1	395
	11th to 20th	Day	10% 24% 29% 26% 9% 2	540
	21st to end	Day	9% 30% 33% 19% 7% 1	594
April	1st to 10th	Day	8% 25% 35% 21% 9% 1	540
	11th to 20th	Day	10% 29% 39% 16% 6% 1	517
	21st to end	Day	8% 29% 39% 19% 5%	500
May	1st to 10th	Day	8% 24% 41% 19% 6% 2	503
	11th to 20th	Day	7% 16% 29% 31% 14% 3	649
	21st to end	Day	6% 14% 32% 33% 15% 1	789
June	1st to 10th	Day	2 8% 25% 40% 23% 2	730
	11th to 20th	Day	2 8% 29% 35% 22% 4	730
	21st to end	Day	6% 7% 22% 34% 25% 7%	730
July	1st to 10th	Day	5% 2 16% 35% 36% 7%	721
	11th to 20th	Day	2 1 12% 31% 40% 14%	603
	21st to end	Day	5% 2 9% 31% 37% 16%	575
August	1st to 10th	Day	4 4 13% 36% 30% 13%	527
	11th to 20th	Day	3 3 18% 36% 28% 12%	517
	21st to end	Day	3 2 16% 40% 28% 11%	583
September	1st to 10th	Day	5% 3 18% 32% 32% 10%	530
	11th to 20th	Day	4 4 26% 38% 22% 6%	530
	21st to end	Day	3 6% 26% 30% 27% 9%	340
October	1st to 10th	Day	4 15% 19% 29% 24% 9%	330
	11th to 20th	Day	7% 12% 27% 30% 19% 5%	330
	21st to end	Day	7% 11% 23% 32% 23% 3	363
November	1st to 10th	Day	6% 15% 25% 35% 16% 3	330
	11th to 20th	Day	4 8% 26% 40% 21% 2	330
	21st to end	Day	8% 10% 35% 30% 13% 4	330
December	1st to 10th	Day	8% 12% 30% 33% 15% 2	330
	11th to 20th	Day	4 12% 29% 40% 14% 2	330
	21st to end	Day	10% 15% 33% 33% 8% 1	363
January	1st to 10th	Night	5% 4 34% 43% 13% 1	987
	11th to 20th	Night	6% 4 32% 44% 12% 2	840
	21st to end	Night	6% 5% 33% 44% 12% 1	924
February	1st to 10th	Night	5% 6% 38% 43% 9%	840
	11th to 20th	Night	7% 8% 41% 38% 6% 1	824
	21st to end	Night	10% 6% 43% 37% 3	692
March	1st to 10th	Night	5% 10% 38% 38% 8%	840
	11th to 20th	Night	8% 7% 31% 41% 12% 1	840
	21st to end	Night	8% 9% 39% 36% 8%	819
April	1st to 10th	Night	7% 13% 40% 36% 4	630
	11th to 20th	Night	6% 13% 37% 38% 7%	611
	21st to end	Night	5% 10% 41% 37% 7%	600
May	1st to 10th	Night	5% 7% 34% 43% 11%	600
	11th to 20th	Night	6% 6% 28% 44% 15% 1	591
	21st to end	Night	4 7% 30% 46% 13%	650
June	1st to 10th	Night	2 2 22% 54% 20%	600
	11th to 20th	Night	2 4 22% 55% 18% 1	600
	21st to end	Night	4 3 16% 49% 28% 1	600
July	1st to 10th	Night	3 2 9% 48% 36% 2	585
	11th to 20th	Night	2 1 6% 39% 49% 4	582
	21st to end	Night	4 1 5% 32% 51% 7%	657
August	1st to 10th	Night	3 1 6% 34% 50% 6%	597
	11th to 20th	Night	4 2 8% 45% 41% 2	594
	21st to end	Night	3 1 6% 47% 39% 3	660
September	1st to 10th	Night	5% 1 11% 49% 31% 2	600
	11th to 20th	Night	4 2 10% 55% 27% 2	600
	21st to end	Night	5% 3 19% 52% 20% 1	621
October	1st to 10th	Night	7% 3 22% 47% 20% 2	800
	11th to 20th	Night	6% 3 28% 44% 16% 3	800
	21st to end	Night	5% 4 25% 43% 19% 4	880
November	1st to 10th	Night	4 4 25% 50% 15% 1	926
	11th to 20th	Night	2 5% 30% 47% 14% 2	1050
	21st to end	Night	7% 7% 28% 42% 15% 1	1050
December	1st to 10th	Night	7% 7% 31% 43% 11% 1	1050
	11th to 20th	Night	7% 6% 33% 42% 11%	1050
	21st to end	Night	7% 6% 35% 44% 7% 1	1155

Page generated at 2026-04-14 00:44:24. Version ac07

Copyright © 2026 Attilla Danko.