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 2024-01-16.

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	3 4 2 3 4 2 4 5% 6% 8% 60%	74756
	Night	3 4 2 4 4 2 4 4 5% 7% 61%	75072
Transparency	Day	21% 7% 12% 23% 25% 11%	74473
	Night	22% 7% 12% 24% 25% 10%	74877
Seeing	Day	6% 13% 27% 31% 18% 4	14112
	Night	6% 5% 27% 43% 18% 2	24328

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	2 5% 2 4 2 1 2 3 4 8% 65%	1591
	11th to 20th	Day	2 6% 2 2 2 1 3 4 5% 6% 66%	1669
	21st to end	Day	3 4 2 4 4 2 3 3 5% 7% 64%	1782
February	1st to 10th	Day	1 5% 2 4 4 3 3 3 5% 8% 63%	1636
	11th to 20th	Day	3 5% 3 4 3 3 5% 3 4 8% 59%	1749
	21st to end	Day	2 6% 3 4 4 2 4 4 6% 9% 57%	1480
March	1st to 10th	Day	2 5% 2 4 2 3 4 5% 4 9% 60%	1855
	11th to 20th	Day	3 4 2 5% 5% 3 4 3 5% 7% 58%	1980
	21st to end	Day	3 4 2 5% 4 2 5% 5% 5% 8% 58%	2268
April	1st to 10th	Day	2 3 1 3 4 2 4 4 5% 8% 64%	2160
	11th to 20th	Day	2 4 1 4 3 3 5% 5% 5% 7% 60%	2131
	21st to end	Day	3 4 2 3 3 2 3 5% 5% 8% 61%	2160
May	1st to 10th	Day	3 3 1 2 3 2 4 5% 6% 7% 64%	2160
	11th to 20th	Day	4 3 1 4 4 2 5% 6% 6% 9% 55%	2311
	21st to end	Day	2 2 1 2 3 2 4 5% 5% 7% 65%	2724
June	1st to 10th	Day	1 1 1 2 3 3 5% 5% 6% 9% 63%	2520
	11th to 20th	Day	2 1 1 1 3 2 4 4 6% 7% 70%	2520
	21st to end	Day	4 3 1 2 4 2 4 6% 7% 9% 59%	2520
July	1st to 10th	Day	3 3 2 4 4 3 6% 6% 7% 12% 50%	2520
	11th to 20th	Day	2 2 1 3 4 4 7% 8% 12% 11% 47%	2416
	21st to end	Day	4 2 2 5% 7% 5% 8% 9% 11% 13% 34%	2545
August	1st to 10th	Day	3 3 2 4 5% 4 6% 9% 8% 10% 44%	2280
	11th to 20th	Day	3 3 2 4 5% 3 6% 7% 7% 10% 49%	2123
	21st to end	Day	3 3 1 4 5% 3 6% 5% 7% 11% 51%	2376
September	1st to 10th	Day	5% 3 2 3 5% 3 5% 7% 6% 8% 52%	2160
	11th to 20th	Day	4 3 1 4 4 2 4 4 6% 8% 60%	2160
	21st to end	Day	3 2 1 2 2 2 3 3 4 7% 69%	1968
October	1st to 10th	Day	5% 3 2 3 4 2 4 4 5% 8% 59%	1800
	11th to 20th	Day	3 2 1 1 2 2 2 2 2 4 78%	1800
	21st to end	Day	3 3 1 2 3 2 2 3 3 5% 72%	1980
November	1st to 10th	Day	2 2 1 2 2 2 3 3 3 6% 74%	1742
	11th to 20th	Day	1 2 1 2 3 1 2 3 4 7% 75%	1710
	21st to end	Day	3 7% 2 3 3 2 3 3 4 6% 64%	1710
December	1st to 10th	Day	3 5% 2 3 2 1 3 3 4 7% 67%	1530
	11th to 20th	Day	2 6% 2 4 3 3 4 5% 5% 9% 58%	1926
	21st to end	Day	2 7% 2 4 4 2 4 3 4 7% 62%	2794
January	1st to 10th	Night	3 4 2 3 3 2 3 2 4 7% 68%	2590
	11th to 20th	Night	3 5% 2 4 3 2 3 3 4 5% 65%	2418
	21st to end	Night	3 5% 2 4 3 2 3 4 4 7% 63%	2574
February	1st to 10th	Night	2 6% 3 4 3 2 2 3 4 7% 64%	2322
	11th to 20th	Night	3 8% 3 5% 4 2 4 4 5% 6% 58%	2098
	21st to end	Night	4 8% 1 4 3 3 2 3 4 6% 61%	1776
March	1st to 10th	Night	3 5% 2 3 4 2 3 4 5% 7% 63%	2102
	11th to 20th	Night	4 6% 2 4 3 2 3 3 4 8% 59%	1980
	21st to end	Night	4 5% 2 4 3 3 3 4 4 7% 60%	2088
April	1st to 10th	Night	3 4 1 4 4 2 4 4 5% 6% 63%	1800
	11th to 20th	Night	4 5% 2 3 3 3 4 5% 6% 7% 58%	1777
	21st to end	Night	4 4 2 3 4 2 4 5% 5% 7% 60%	1800
May	1st to 10th	Night	3 3 1 3 4 2 3 3 5% 7% 64%	1800
	11th to 20th	Night	6% 5% 2 4 5% 2 5% 4 6% 9% 54%	1579
	21st to end	Night	2 3 1 3 3 2 3 3 5% 8% 66%	1552
June	1st to 10th	Night	1 1 1 3 4 3 4 5% 6% 8% 64%	1440
	11th to 20th	Night	2 2 1 3 3 2 4 3 5% 6% 70%	1440
	21st to end	Night	3 3 1 3 5% 3 5% 5% 6% 11% 55%	1440
July	1st to 10th	Night	3 2 2 6% 7% 4 6% 7% 8% 9% 46%	1440
	11th to 20th	Night	1 2 1 3 5% 3 6% 7% 10% 12% 50%	1539
	21st to end	Night	4 3 2 4 6% 4 8% 9% 10% 14% 36%	1773
August	1st to 10th	Night	3 3 2 4 4 3 6% 7% 8% 12% 48%	1658
	11th to 20th	Night	4 4 2 7% 6% 4 6% 7% 9% 10% 42%	1778
	21st to end	Night	3 3 2 5% 4 4 5% 5% 8% 12% 47%	1980
September	1st to 10th	Night	6% 3 2 4 5% 3 5% 6% 7% 8% 52%	1800
	11th to 20th	Night	5% 2 1 2 3 2 4 5% 7% 7% 62%	1800
	21st to end	Night	5% 2 1 3 4 3 3 4 4 6% 66%	1988
October	1st to 10th	Night	6% 4 2 4 4 2 3 3 4 7% 62%	2160
	11th to 20th	Night	3 2 1 2 2 1 1 2 3 4 77%	2160
	21st to end	Night	4 3 1 3 3 2 2 3 4 6% 70%	2376
November	1st to 10th	Night	3 2 1 1 2 1 2 2 3 6% 77%	2315
	11th to 20th	Night	1 2 1 1 1 1 3 2 3 7% 78%	2470
	21st to end	Night	5% 5% 1 3 3 2 3 3 4 6% 65%	2470
December	1st to 10th	Night	4 6% 2 3 3 2 2 3 4 6% 64%	2645
	11th to 20th	Night	3 6% 2 4 4 2 4 4 5% 9% 57%	3271
	21st to end	Night	3 6% 3 5% 5% 2 4 4 4 6% 59%	4873

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	20% 6% 11% 36% 24% 3	1592
	11th to 20th	Day	20% 6% 13% 35% 24% 3	1669
	21st to end	Day	22% 5% 12% 31% 25% 6%	1782
February	1st to 10th	Day	22% 8% 16% 27% 22% 6%	1636
	11th to 20th	Day	24% 7% 12% 27% 25% 6%	1759
	21st to end	Day	25% 6% 10% 19% 28% 12%	1480
March	1st to 10th	Day	22% 7% 10% 23% 28% 9%	1855
	11th to 20th	Day	24% 7% 9% 25% 28% 7%	1980
	21st to end	Day	25% 6% 11% 24% 26% 7%	2268
April	1st to 10th	Day	19% 5% 10% 25% 26% 15%	2160
	11th to 20th	Day	22% 7% 9% 21% 29% 12%	2134
	21st to end	Day	20% 7% 10% 24% 28% 11%	2160
May	1st to 10th	Day	17% 6% 10% 21% 33% 12%	2160
	11th to 20th	Day	25% 8% 13% 21% 24% 8%	2315
	21st to end	Day	16% 7% 11% 20% 26% 20%	2724
June	1st to 10th	Day	16% 6% 13% 22% 23% 19%	2520
	11th to 20th	Day	12% 5% 8% 17% 25% 33%	2520
	21st to end	Day	20% 6% 11% 21% 23% 20%	2520
July	1st to 10th	Day	26% 12% 15% 16% 17% 15%	2451
	11th to 20th	Day	22% 14% 19% 24% 13% 8%	2342
	21st to end	Day	33% 17% 25% 19% 6% 1	2404
August	1st to 10th	Day	28% 15% 19% 16% 13% 9%	2265
	11th to 20th	Day	26% 11% 15% 22% 17% 9%	2123
	21st to end	Day	24% 11% 17% 24% 19% 6%	2376
September	1st to 10th	Day	26% 9% 13% 21% 20% 11%	2160
	11th to 20th	Day	22% 7% 11% 19% 26% 16%	2160
	21st to end	Day	15% 5% 8% 24% 35% 13%	1968
October	1st to 10th	Day	23% 8% 9% 22% 24% 15%	1800
	11th to 20th	Day	12% 4 5% 18% 42% 20%	1800
	21st to end	Day	16% 4 8% 26% 35% 11%	1980
November	1st to 10th	Day	12% 4 11% 25% 36% 12%	1742
	11th to 20th	Day	12% 4 10% 34% 34% 7%	1710
	21st to end	Day	23% 5% 8% 25% 30% 9%	1710
December	1st to 10th	Day	19% 6% 17% 29% 21% 7%	1530
	11th to 20th	Day	23% 9% 12% 25% 26% 5%	1924
	21st to end	Day	21% 5% 12% 25% 30% 7%	2794
January	1st to 10th	Night	19% 5% 13% 32% 27% 4	2590
	11th to 20th	Night	22% 7% 11% 31% 25% 4	2418
	21st to end	Night	23% 6% 10% 33% 23% 5%	2574
February	1st to 10th	Night	23% 7% 14% 27% 22% 7%	2322
	11th to 20th	Night	28% 7% 11% 25% 24% 5%	2098
	21st to end	Night	25% 6% 7% 22% 31% 10%	1776
March	1st to 10th	Night	21% 7% 10% 26% 28% 8%	2102
	11th to 20th	Night	25% 5% 10% 25% 27% 8%	1980
	21st to end	Night	23% 6% 10% 24% 30% 6%	2088
April	1st to 10th	Night	21% 5% 11% 25% 27% 12%	1800
	11th to 20th	Night	23% 8% 9% 20% 26% 13%	1777
	21st to end	Night	23% 7% 11% 23% 26% 10%	1800
May	1st to 10th	Night	20% 5% 10% 20% 32% 12%	1800
	11th to 20th	Night	26% 9% 14% 19% 22% 10%	1580
	21st to end	Night	16% 6% 13% 20% 24% 20%	1552
June	1st to 10th	Night	16% 7% 17% 20% 20% 20%	1440
	11th to 20th	Night	15% 5% 7% 14% 25% 35%	1440
	21st to end	Night	21% 8% 12% 19% 22% 19%	1440
July	1st to 10th	Night	29% 11% 16% 17% 11% 16%	1400
	11th to 20th	Night	21% 14% 21% 22% 13% 9%	1490
	21st to end	Night	31% 16% 29% 18% 5% 1	1674
August	1st to 10th	Night	24% 16% 23% 16% 11% 11%	1649
	11th to 20th	Night	31% 13% 17% 21% 11% 8%	1778
	21st to end	Night	24% 11% 20% 22% 17% 6%	1980
September	1st to 10th	Night	27% 8% 15% 23% 17% 10%	1800
	11th to 20th	Night	19% 7% 15% 18% 24% 17%	1800
	21st to end	Night	19% 6% 9% 20% 33% 14%	1988
October	1st to 10th	Night	23% 6% 10% 20% 26% 16%	2160
	11th to 20th	Night	12% 3 7% 17% 41% 19%	2160
	21st to end	Night	17% 5% 7% 25% 35% 12%	2376
November	1st to 10th	Night	10% 3 9% 27% 40% 12%	2315
	11th to 20th	Night	10% 4 8% 34% 35% 8%	2470
	21st to end	Night	22% 5% 8% 24% 31% 10%	2470
December	1st to 10th	Night	23% 5% 11% 32% 24% 4	2645
	11th to 20th	Night	24% 7% 12% 26% 26% 4	3272
	21st to end	Night	27% 6% 12% 25% 27% 4	4873

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	5% 19% 34% 33% 8% 2	270
	11th to 20th	Day	8% 13% 36% 35% 6% 2	257
	21st to end	Day	4 10% 36% 38% 11% 1	264
February	1st to 10th	Day	2 14% 39% 34% 10% 2	240
	11th to 20th	Day	4 20% 42% 28% 7%	240
	21st to end	Day	9% 26% 34% 28% 3	198
March	1st to 10th	Day	5% 24% 35% 28% 8% 1	295
	11th to 20th	Day	11% 25% 29% 25% 9% 2	420
	21st to end	Day	11% 33% 34% 16% 6%	462
April	1st to 10th	Day	7% 28% 36% 20% 8% 1	420
	11th to 20th	Day	10% 30% 38% 15% 6% 1	412
	21st to end	Day	9% 32% 37% 18% 4	420
May	1st to 10th	Day	6% 25% 44% 19% 5% 1	423
	11th to 20th	Day	7% 16% 28% 31% 14% 3	556
	21st to end	Day	6% 14% 32% 33% 14% 1	679
June	1st to 10th	Day	1 8% 26% 40% 23% 2	630
	11th to 20th	Day	2 9% 29% 35% 21% 3	630
	21st to end	Day	6% 7% 21% 36% 24% 6%	630
July	1st to 10th	Day	5% 2 15% 34% 36% 8%	621
	11th to 20th	Day	3 1 12% 33% 39% 12%	515
	21st to end	Day	5% 1 9% 31% 38% 16%	487
August	1st to 10th	Day	4 4 13% 38% 30% 11%	447
	11th to 20th	Day	3 4 20% 38% 26% 9%	437
	21st to end	Day	3 2 17% 41% 26% 10%	495
September	1st to 10th	Day	6% 3 20% 33% 30% 9%	450
	11th to 20th	Day	4 4 25% 39% 22% 6%	450
	21st to end	Day	3 6% 28% 30% 25% 8%	280
October	1st to 10th	Day	4 17% 21% 27% 22% 9%	270
	11th to 20th	Day	6% 11% 25% 31% 21% 6%	270
	21st to end	Day	9% 12% 24% 31% 21% 3	297
November	1st to 10th	Day	5% 17% 27% 34% 13% 3	270
	11th to 20th	Day	3 8% 29% 38% 20% 1	270
	21st to end	Day	9% 12% 37% 25% 13% 5%	270
December	1st to 10th	Day	9% 14% 34% 29% 12% 3	270
	11th to 20th	Day	4 14% 32% 39% 10% 1	270
	21st to end	Day	12% 17% 34% 30% 6% 1	297
January	1st to 10th	Night	5% 4 33% 42% 14% 1	893
	11th to 20th	Night	7% 4 33% 42% 12% 2	744
	21st to end	Night	6% 4 32% 44% 11% 2	792
February	1st to 10th	Night	5% 6% 38% 42% 9%	720
	11th to 20th	Night	8% 6% 39% 39% 7% 1	704
	21st to end	Night	10% 6% 42% 38% 3	592
March	1st to 10th	Night	6% 10% 38% 38% 9%	720
	11th to 20th	Night	9% 7% 29% 40% 14% 1	720
	21st to end	Night	8% 9% 40% 35% 8%	702
April	1st to 10th	Night	6% 13% 41% 35% 4	540
	11th to 20th	Night	6% 13% 35% 38% 8%	533
	21st to end	Night	5% 10% 42% 36% 7%	540
May	1st to 10th	Night	5% 7% 34% 43% 11%	540
	11th to 20th	Night	7% 5% 27% 44% 17% 1	531
	21st to end	Night	4 7% 29% 45% 14%	584
June	1st to 10th	Night	1 2 22% 53% 21%	540
	11th to 20th	Night	2 4 22% 54% 18% 1	540
	21st to end	Night	3 2 17% 49% 28% 1	540
July	1st to 10th	Night	3 2 8% 47% 38% 2	525
	11th to 20th	Night	2 1 6% 40% 49% 4	522
	21st to end	Night	4 1 4 30% 54% 8%	591
August	1st to 10th	Night	3 1 6% 33% 51% 7%	537
	11th to 20th	Night	4 2 7% 45% 40% 2	534
	21st to end	Night	3 6% 47% 40% 4	594
September	1st to 10th	Night	6% 1 11% 48% 32% 2	540
	11th to 20th	Night	4 1 10% 54% 29% 2	540
	21st to end	Night	5% 3 20% 50% 22% 1	558
October	1st to 10th	Night	7% 3 23% 46% 19% 2	720
	11th to 20th	Night	4 3 29% 44% 17% 3	720
	21st to end	Night	6% 4 26% 41% 19% 4	792
November	1st to 10th	Night	4 4 25% 49% 17% 1	835
	11th to 20th	Night	2 5% 30% 46% 15% 2	950
	21st to end	Night	8% 7% 27% 40% 17% 1	950
December	1st to 10th	Night	7% 8% 32% 41% 11% 1	950
	11th to 20th	Night	8% 7% 32% 42% 11%	950
	21st to end	Night	8% 7% 35% 43% 7% 1	1045

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