Durham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Durham clear sky chart from 2005-11-06 to 2025-07-06.

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% 6% 2 6% 4 4 4 4 6% 8% 43%	119516
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	119275
Transparency	Day	38% 6% 12% 21% 18% 5%	123164
	Night	38% 5% 10% 20% 21% 6%	123764
Seeing	Day	17% 11% 26% 24% 16% 5%	16176
	Night	16% 10% 26% 31% 16% 2	26525

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	16% 6% 2 5% 2 2 2 2 3 4 56%	4056
	11th to 20th	Day	12% 6% 2 5% 3 2 3 2 3 4 57%	4060
	21st to end	Day	13% 8% 3 6% 4 4 4 3 4 4 47%	4466
February	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 6% 7% 44%	1800
	11th to 20th	Day	23% 6% 1 4 4 3 3 3 4 4 45%	1760
	21st to end	Day	18% 8% 2 7% 4 3 3 3 5% 7% 41%	1800
March	1st to 10th	Day	20% 7% 2 5% 4 4 4 3 5% 6% 40%	4935
	11th to 20th	Day	10% 5% 2 6% 3 3 3 3 4 7% 53%	4980
	21st to end	Day	15% 6% 2 6% 4 4 4 3 4 5% 47%	5465
April	1st to 10th	Day	11% 4 2 3 4 4 4 3 5% 8% 53%	2324
	11th to 20th	Day	10% 5% 2 6% 4 3 4 4 5% 9% 48%	2374
	21st to end	Day	13% 5% 2 5% 4 4 5% 3 5% 9% 44%	2400
May	1st to 10th	Day	9% 7% 3 7% 5% 6% 6% 5% 7% 10% 35%	5831
	11th to 20th	Day	19% 7% 2 7% 5% 5% 6% 4 7% 9% 29%	5861
	21st to end	Day	13% 5% 2 6% 4 5% 5% 4 6% 9% 43%	6433
June	1st to 10th	Day	9% 6% 1 6% 5% 5% 6% 5% 8% 10% 40%	2600
	11th to 20th	Day	8% 5% 2 6% 5% 5% 4 5% 7% 11% 43%	2600
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 9% 12% 40%	2600
July	1st to 10th	Day	6% 5% 2 8% 5% 6% 6% 6% 8% 10% 37%	4185
	11th to 20th	Day	10% 7% 2 7% 5% 5% 7% 5% 8% 11% 32%	4120
	21st to end	Day	10% 8% 3 9% 6% 6% 6% 6% 7% 10% 28%	4466
August	1st to 10th	Day	19% 8% 3 8% 5% 6% 6% 5% 7% 9% 24%	4104
	11th to 20th	Day	8% 6% 3 7% 6% 6% 7% 5% 8% 11% 36%	3924
	21st to end	Day	6% 4 2 5% 4 4 5% 4 6% 11% 49%	4180
September	1st to 10th	Day	9% 4 2 6% 5% 5% 5% 4 7% 8% 46%	2280
	11th to 20th	Day	15% 6% 3 7% 4 3 5% 4 5% 8% 41%	2150
	21st to end	Day	21% 7% 3 6% 4 3 3 3 6% 5% 39%	2048
October	1st to 10th	Day	13% 6% 2 6% 3 2 3 3 4 6% 50%	1900
	11th to 20th	Day	13% 4 2 4 3 3 3 3 4 6% 58%	1900
	21st to end	Day	11% 4 2 4 4 3 3 2 3 5% 57%	2090
November	1st to 10th	Day	12% 5% 2 5% 4 4 3 2 4 6% 54%	1957
	11th to 20th	Day	15% 4 2 5% 3 2 3 3 5% 6% 52%	1983
	21st to end	Day	14% 5% 2 4 4 3 3 3 5% 6% 51%	1668
December	1st to 10th	Day	13% 6% 2 6% 3 4 3 3 4 5% 53%	2600
	11th to 20th	Day	17% 5% 3 6% 4 3 4 3 4 6% 45%	3150
	21st to end	Day	16% 6% 2 5% 3 3 4 3 5% 7% 45%	4466
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 54%	5889
	11th to 20th	Night	13% 6% 2 5% 3 2 3 3 3 5% 56%	5900
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 6% 48%	6490
February	1st to 10th	Night	18% 5% 2 5% 4 3 3 3 6% 6% 45%	2600
	11th to 20th	Night	20% 7% 2 5% 4 3 3 3 4 5% 43%	2524
	21st to end	Night	15% 6% 1 6% 3 4 4 3 5% 6% 47%	1827
March	1st to 10th	Night	19% 8% 3 6% 4 4 3 3 4 5% 42%	4918
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 7% 54%	4980
	21st to end	Night	14% 5% 1 5% 4 4 4 3 5% 6% 49%	5467
April	1st to 10th	Night	14% 5% 2 4 4 4 4 3 5% 8% 48%	2075
	11th to 20th	Night	12% 5% 2 6% 4 4 3 3 5% 7% 51%	1975
	21st to end	Night	11% 5% 2 5% 5% 5% 5% 5% 5% 6% 47%	2000
May	1st to 10th	Night	12% 8% 2 7% 5% 5% 5% 4 6% 9% 37%	4093
	11th to 20th	Night	19% 8% 3 8% 4 4 4 3 5% 7% 34%	4032
	21st to end	Night	15% 5% 2 5% 3 4 4 3 5% 9% 43%	4421
June	1st to 10th	Night	10% 5% 3 5% 4 6% 5% 5% 7% 11% 39%	1800
	11th to 20th	Night	9% 6% 2 7% 5% 4 4 5% 7% 10% 41%	1800
	21st to end	Night	7% 5% 2 7% 4 5% 4 5% 9% 11% 41%	1800
July	1st to 10th	Night	7% 7% 3 7% 5% 5% 5% 6% 9% 10% 35%	2885
	11th to 20th	Night	14% 7% 2 6% 4 5% 5% 4 8% 11% 33%	2840
	21st to end	Night	15% 9% 3 9% 6% 6% 5% 4 6% 8% 29%	3087
August	1st to 10th	Night	22% 6% 3 7% 5% 5% 5% 4 8% 10% 23%	2825
	11th to 20th	Night	8% 5% 2 7% 6% 6% 6% 6% 9% 9% 37%	2871
	21st to end	Night	6% 4 2 6% 4 5% 5% 4 6% 9% 50%	3476
September	1st to 10th	Night	12% 6% 3 5% 3 4 4 4 6% 8% 46%	1900
	11th to 20th	Night	14% 6% 3 6% 4 3 5% 3 6% 7% 43%	2025
	21st to end	Night	20% 7% 2 7% 4 4 4 4 5% 6% 37%	2130
October	1st to 10th	Night	15% 6% 3 6% 4 3 3 3 4 5% 48%	2280
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 3 6% 54%	2280
	21st to end	Night	12% 5% 2 4 3 3 3 3 4 5% 56%	2508
November	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 4 5% 53%	2341
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 5% 50%	2415
	21st to end	Night	13% 4 2 4 3 3 3 3 5% 6% 52%	2725
December	1st to 10th	Night	12% 5% 2 6% 4 3 4 3 3 5% 52%	4580
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 4 4 40%	5026
	21st to end	Night	16% 6% 2 5% 3 3 3 3 4 7% 47%	6490

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	35% 2 4 21% 31% 7%	4057
	11th to 20th	Day	32% 3 4 17% 36% 8%	4060
	21st to end	Day	41% 4 5% 22% 22% 7%	4466
February	1st to 10th	Day	40% 4 12% 21% 20% 5%	1800
	11th to 20th	Day	43% 4 7% 18% 21% 6%	1769
	21st to end	Day	44% 4 10% 16% 21% 6%	1800
March	1st to 10th	Day	45% 5% 6% 17% 17% 11%	4935
	11th to 20th	Day	32% 4 7% 17% 24% 16%	4980
	21st to end	Day	40% 4 8% 21% 20% 6%	5465
April	1st to 10th	Day	30% 5% 11% 24% 22% 8%	2324
	11th to 20th	Day	33% 5% 12% 22% 20% 7%	2374
	21st to end	Day	38% 6% 12% 22% 18% 4	2400
May	1st to 10th	Day	41% 8% 19% 21% 10% 1	5866
	11th to 20th	Day	50% 8% 12% 20% 9% 1	5865
	21st to end	Day	37% 7% 14% 22% 18% 3	6447
June	1st to 10th	Day	36% 9% 15% 23% 15% 1	2600
	11th to 20th	Day	33% 9% 17% 23% 16% 2	2600
	21st to end	Day	33% 10% 18% 23% 15% 2	2600
July	1st to 10th	Day	38% 10% 18% 18% 13% 3	4120
	11th to 20th	Day	42% 9% 25% 17% 5% 1	4051
	21st to end	Day	48% 10% 15% 16% 10%	4324
August	1st to 10th	Day	55% 7% 18% 15% 5%	4091
	11th to 20th	Day	39% 9% 21% 22% 8% 1	3963
	21st to end	Day	29% 7% 14% 34% 14% 2	4180
September	1st to 10th	Day	34% 9% 14% 19% 19% 5%	2280
	11th to 20th	Day	40% 7% 12% 17% 21% 3	2150
	21st to end	Day	45% 6% 9% 20% 16% 4	2048
October	1st to 10th	Day	33% 4 8% 20% 26% 9%	3160
	11th to 20th	Day	20% 3 5% 21% 27% 24%	3140
	21st to end	Day	25% 3 7% 25% 24% 15%	3428
November	1st to 10th	Day	34% 4 7% 22% 26% 7%	1957
	11th to 20th	Day	34% 3 8% 22% 26% 7%	1983
	21st to end	Day	34% 4 8% 22% 26% 5%	1668
December	1st to 10th	Day	36% 3 7% 22% 25% 7%	2600
	11th to 20th	Day	41% 4 10% 18% 21% 6%	3147
	21st to end	Day	39% 4 12% 26% 18% 2	4466
January	1st to 10th	Night	32% 3 5% 18% 30% 11%	5889
	11th to 20th	Night	33% 3 5% 16% 35% 7%	5900
	21st to end	Night	39% 3 6% 19% 28% 6%	6490
February	1st to 10th	Night	40% 4 10% 21% 20% 5%	2600
	11th to 20th	Night	44% 4 9% 15% 21% 7%	2526
	21st to end	Night	37% 5% 9% 19% 21% 9%	1827
March	1st to 10th	Night	44% 5% 6% 13% 21% 10%	4918
	11th to 20th	Night	31% 4 6% 16% 25% 18%	4980
	21st to end	Night	36% 4 7% 24% 22% 6%	5467
April	1st to 10th	Night	36% 5% 11% 18% 24% 7%	2075
	11th to 20th	Night	33% 5% 10% 23% 21% 8%	1975
	21st to end	Night	36% 7% 12% 24% 17% 4	2000
May	1st to 10th	Night	42% 7% 17% 19% 13% 2	4120
	11th to 20th	Night	48% 7% 12% 18% 14% 1	4033
	21st to end	Night	36% 7% 14% 24% 16% 2	4429
June	1st to 10th	Night	37% 10% 16% 20% 16% 1	1800
	11th to 20th	Night	37% 10% 17% 20% 15% 2	1800
	21st to end	Night	34% 12% 17% 21% 14% 3	1800
July	1st to 10th	Night	41% 12% 17% 17% 12% 2	2840
	11th to 20th	Night	43% 11% 23% 17% 5% 1	2786
	21st to end	Night	52% 9% 13% 16% 10%	2988
August	1st to 10th	Night	53% 8% 18% 15% 5% 1	2814
	11th to 20th	Night	38% 10% 21% 24% 7% 1	2898
	21st to end	Night	30% 8% 11% 28% 21% 2	3476
September	1st to 10th	Night	35% 8% 13% 21% 17% 5%	1900
	11th to 20th	Night	39% 8% 11% 18% 21% 3	2025
	21st to end	Night	47% 8% 11% 16% 15% 4	2130
October	1st to 10th	Night	39% 4 7% 19% 24% 6%	3800
	11th to 20th	Night	25% 3 6% 19% 28% 20%	3779
	21st to end	Night	23% 3 7% 27% 24% 16%	4120
November	1st to 10th	Night	35% 4 8% 20% 26% 8%	2341
	11th to 20th	Night	36% 4 8% 16% 25% 11%	2415
	21st to end	Night	32% 4 8% 23% 26% 7%	2725
December	1st to 10th	Night	36% 4 6% 19% 29% 7%	4580
	11th to 20th	Night	47% 3 8% 18% 16% 8%	5028
	21st to end	Night	38% 4 12% 24% 21% 2	6490

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	27% 25% 33% 13% 3	300
	11th to 20th	Day	26% 26% 33% 12% 3	300
	21st to end	Day	19% 23% 44% 12% 2	330
February	1st to 10th	Day	20% 20% 41% 15% 3	300
	11th to 20th	Day	39% 20% 31% 8% 2	307
	21st to end	Day	24% 20% 34% 19% 4	414
March	1st to 10th	Day	25% 19% 33% 18% 6%	500
	11th to 20th	Day	21% 20% 40% 14% 5% 1	500
	21st to end	Day	23% 18% 38% 16% 5%	550
April	1st to 10th	Day	21% 18% 36% 17% 7% 1	500
	11th to 20th	Day	14% 19% 32% 22% 9% 3	492
	21st to end	Day	18% 10% 31% 27% 13% 1	500
May	1st to 10th	Day	16% 17% 31% 24% 9% 3	503
	11th to 20th	Day	17% 10% 24% 28% 17% 4	527
	21st to end	Day	18% 7% 22% 26% 20% 7%	573
June	1st to 10th	Day	11% 6% 27% 31% 20% 6%	530
	11th to 20th	Day	8% 6% 22% 30% 25% 8%	530
	21st to end	Day	9% 3 19% 34% 27% 8%	530
July	1st to 10th	Day	10% 1 12% 30% 37% 10%	505
	11th to 20th	Day	7% 2 11% 30% 37% 12%	484
	21st to end	Day	8% 3 16% 36% 27% 10%	532
August	1st to 10th	Day	15% 3 19% 28% 25% 10%	486
	11th to 20th	Day	10% 5% 14% 30% 28% 14%	479
	21st to end	Day	11% 3 12% 29% 29% 16%	539
September	1st to 10th	Day	12% 4 19% 24% 26% 16%	490
	11th to 20th	Day	16% 3 13% 31% 24% 11%	490
	21st to end	Day	24% 6% 24% 24% 17% 4	490
October	1st to 10th	Day	16% 8% 22% 24% 22% 9%	490
	11th to 20th	Day	14% 10% 25% 27% 18% 5%	490
	21st to end	Day	14% 11% 25% 29% 16% 5%	539
November	1st to 10th	Day	16% 11% 30% 27% 13% 3	495
	11th to 20th	Day	18% 14% 29% 25% 12% 1	483
	21st to end	Day	15% 19% 30% 21% 12% 2	233
December	1st to 10th	Day	22% 21% 39% 12% 7% 1	200
	11th to 20th	Day	20% 16% 33% 24% 7%	235
	21st to end	Day	24% 17% 31% 17% 9% 1	330
January	1st to 10th	Night	20% 21% 31% 24% 4	800
	11th to 20th	Night	19% 18% 33% 25% 6%	800
	21st to end	Night	19% 15% 38% 25% 3	880
February	1st to 10th	Night	20% 15% 35% 26% 3	800
	11th to 20th	Night	25% 20% 35% 20% 1	783
	21st to end	Night	18% 16% 38% 25% 3	660
March	1st to 10th	Night	16% 18% 33% 28% 6%	800
	11th to 20th	Night	20% 16% 35% 21% 8%	800
	21st to end	Night	18% 13% 41% 24% 4	880
April	1st to 10th	Night	18% 18% 33% 25% 6%	800
	11th to 20th	Night	15% 14% 35% 25% 10%	791
	21st to end	Night	14% 9% 33% 35% 9%	800
May	1st to 10th	Night	14% 11% 33% 32% 9%	634
	11th to 20th	Night	17% 7% 25% 32% 17% 1	591
	21st to end	Night	13% 4 17% 40% 25% 1	650
June	1st to 10th	Night	12% 3 19% 42% 23% 2	600
	11th to 20th	Night	11% 3 19% 39% 26% 4	600
	21st to end	Night	8% 3 16% 42% 30% 3	600
July	1st to 10th	Night	8% 1 13% 37% 37% 4	570
	11th to 20th	Night	8% 2 9% 41% 36% 4	552
	21st to end	Night	7% 1 15% 39% 34% 4	624
August	1st to 10th	Night	14% 2 10% 37% 34% 4	567
	11th to 20th	Night	12% 2 11% 37% 32% 5%	611
	21st to end	Night	11% 2 12% 31% 38% 6%	836
September	1st to 10th	Night	14% 3 11% 34% 33% 6%	760
	11th to 20th	Night	17% 3 15% 31% 31% 4	760
	21st to end	Night	23% 5% 17% 33% 20% 1	760
October	1st to 10th	Night	17% 5% 17% 31% 24% 6%	760
	11th to 20th	Night	16% 9% 24% 29% 20% 2	760
	21st to end	Night	14% 10% 25% 29% 20% 2	836
November	1st to 10th	Night	16% 8% 27% 33% 14% 2	780
	11th to 20th	Night	16% 12% 28% 33% 11%	800
	21st to end	Night	16% 13% 28% 29% 14% 1	800
December	1st to 10th	Night	21% 13% 30% 28% 7%	800
	11th to 20th	Night	19% 11% 31% 33% 7%	800
	21st to end	Night	21% 15% 25% 31% 7%	880

Page generated at 2025-07-03 01:30:16. Version ac07

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