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 2026-10-29.

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 5% 4 6% 8% 43%	148474
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	143590
Transparency	Day	38% 6% 13% 21% 17% 5%	152080
	Night	39% 5% 11% 20% 20% 6%	148056
Seeing	Day	17% 11% 26% 24% 17% 5%	17981
	Night	16% 10% 26% 31% 16% 2	28295

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	14% 7% 2 5% 3 2 3 2 4 5% 53%	5258
	11th to 20th	Day	12% 6% 2 4 3 2 3 2 3 4 59%	5280
	21st to end	Day	15% 8% 3 6% 4 4 4 3 4 5% 44%	5780
February	1st to 10th	Day	13% 7% 2 6% 5% 4 4 3 6% 6% 44%	1890
	11th to 20th	Day	22% 6% 2 4 4 3 3 3 4 4 44%	1851
	21st to end	Day	18% 8% 2 7% 4 3 3 3 5% 6% 41%	1887
March	1st to 10th	Day	20% 7% 2 5% 4 4 4 3 5% 6% 40%	5045
	11th to 20th	Day	10% 5% 2 6% 3 3 3 3 4 7% 53%	5090
	21st to end	Day	14% 6% 2 6% 4 4 4 3 4 5% 47%	5586
April	1st to 10th	Day	11% 4 1 3 4 4 4 3 6% 8% 53%	2440
	11th to 20th	Day	10% 4 2 5% 4 3 4 4 5% 9% 49%	2494
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 8% 45%	2520
May	1st to 10th	Day	11% 7% 3 7% 4 5% 5% 4 6% 9% 38%	7571
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 40%	7626
	21st to end	Day	15% 5% 2 7% 4 5% 5% 4 6% 8% 37%	8391
June	1st to 10th	Day	8% 6% 1 6% 5% 5% 5% 5% 8% 10% 41%	2730
	11th to 20th	Day	8% 5% 2 5% 5% 4 4 5% 7% 12% 43%	2730
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 9% 12% 40%	2730
July	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 6% 8% 12% 38%	7680
	11th to 20th	Day	9% 6% 3 7% 5% 6% 7% 6% 9% 13% 30%	7680
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 5% 7% 11% 33%	8340
August	1st to 10th	Day	17% 8% 3 8% 6% 6% 7% 5% 8% 11% 21%	6014
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 13% 38%	5772
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 6% 10% 47%	6080
September	1st to 10th	Day	9% 4 2 5% 5% 4 4 4 7% 8% 48%	2520
	11th to 20th	Day	14% 6% 3 7% 4 3 5% 4 5% 8% 42%	2376
	21st to end	Day	21% 7% 3 6% 4 3 3 3 6% 6% 38%	2264
October	1st to 10th	Day	14% 6% 2 6% 4 2 3 3 4 6% 50%	2100
	11th to 20th	Day	13% 4 2 3 2 3 3 3 3 5% 59%	2100
	21st to end	Day	11% 4 2 4 3 3 3 2 3 5% 58%	2280
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 2 4 6% 55%	2057
	11th to 20th	Day	14% 4 2 5% 3 2 3 3 5% 6% 53%	2082
	21st to end	Day	14% 5% 2 4 4 3 3 3 5% 6% 51%	1751
December	1st to 10th	Day	13% 6% 2 6% 3 4 3 3 4 5% 52%	2680
	11th to 20th	Day	17% 5% 3 6% 4 3 4 3 4 5% 46%	3234
	21st to end	Day	16% 6% 2 5% 3 4 4 3 5% 7% 45%	4565
January	1st to 10th	Night	12% 6% 2 6% 3 3 3 3 4 5% 53%	7635
	11th to 20th	Night	13% 6% 2 4 3 2 3 2 3 4 57%	7680
	21st to end	Night	17% 7% 3 6% 3 3 3 2 4 5% 46%	8401
February	1st to 10th	Night	17% 5% 2 5% 4 3 3 3 6% 6% 46%	2730
	11th to 20th	Night	20% 7% 2 5% 4 3 3 3 4 5% 43%	2653
	21st to end	Night	15% 6% 1 6% 3 4 4 3 5% 6% 46%	1915
March	1st to 10th	Night	18% 8% 3 6% 4 4 3 3 4 5% 42%	5028
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 7% 54%	5090
	21st to end	Night	14% 5% 1 5% 4 4 4 3 5% 6% 49%	5588
April	1st to 10th	Night	13% 5% 2 4 4 4 3 3 5% 8% 49%	2179
	11th to 20th	Night	11% 5% 2 6% 4 4 3 3 5% 7% 51%	2075
	21st to end	Night	12% 5% 2 5% 5% 4 5% 4 5% 6% 47%	2100
May	1st to 10th	Night	12% 7% 2 7% 5% 5% 5% 4 7% 9% 38%	5325
	11th to 20th	Night	15% 7% 2 7% 3 4 4 3 5% 7% 44%	5234
	21st to end	Night	21% 6% 3 6% 4 4 4 3 5% 8% 36%	5761
June	1st to 10th	Night	10% 5% 3 5% 4 6% 5% 5% 7% 10% 40%	1890
	11th to 20th	Night	9% 6% 2 7% 5% 4 5% 5% 7% 10% 41%	1890
	21st to end	Night	7% 5% 2 7% 4 5% 4 5% 9% 11% 41%	1890
July	1st to 10th	Night	10% 7% 3 7% 5% 5% 5% 5% 8% 11% 35%	5278
	11th to 20th	Night	14% 7% 3 8% 5% 6% 6% 5% 8% 10% 28%	5277
	21st to end	Night	14% 7% 3 8% 5% 5% 5% 4 7% 10% 31%	5736
August	1st to 10th	Night	20% 8% 3 8% 5% 5% 6% 5% 8% 11% 21%	4135
	11th to 20th	Night	7% 5% 2 7% 6% 5% 6% 5% 9% 10% 38%	4212
	21st to end	Night	9% 5% 2 7% 4 5% 5% 4 6% 8% 46%	5045
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 4 6% 8% 48%	2100
	11th to 20th	Night	13% 6% 3 6% 4 4 5% 3 6% 7% 42%	2239
	21st to end	Night	21% 7% 2 7% 4 4 4 4 5% 6% 37%	2354
October	1st to 10th	Night	15% 7% 3 6% 3 3 3 3 4 5% 49%	2520
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 3 6% 54%	2520
	21st to end	Night	12% 5% 2 4 3 3 3 3 4 5% 56%	2737
November	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 4 5% 53%	2461
	11th to 20th	Night	12% 5% 2 5% 4 3 4 4 5% 5% 51%	2536
	21st to end	Night	13% 4 2 5% 4 3 3 4 5% 7% 51%	2861
December	1st to 10th	Night	12% 5% 2 6% 4 3 4 3 3 5% 52%	4720
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 4 4 41%	5162
	21st to end	Night	16% 6% 2 5% 3 3 3 3 4 7% 47%	6633

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 10% 21% 26% 6%	5259
	11th to 20th	Day	31% 2 4 14% 37% 11%	5280
	21st to end	Day	43% 3 4 22% 20% 8%	5780
February	1st to 10th	Day	39% 4 12% 20% 20% 5%	1890
	11th to 20th	Day	44% 4 7% 18% 21% 6%	1860
	21st to end	Day	45% 4 9% 16% 20% 5%	1887
March	1st to 10th	Day	45% 5% 7% 17% 17% 11%	5045
	11th to 20th	Day	32% 4 7% 17% 24% 16%	5090
	21st to end	Day	40% 4 8% 21% 20% 6%	5586
April	1st to 10th	Day	29% 5% 11% 24% 23% 8%	2440
	11th to 20th	Day	33% 5% 12% 23% 20% 7%	2494
	21st to end	Day	37% 6% 12% 23% 18% 4	2520
May	1st to 10th	Day	41% 6% 16% 21% 13% 2	7606
	11th to 20th	Day	41% 6% 12% 24% 15% 1	7630
	21st to end	Day	43% 7% 13% 19% 15% 3	8405
June	1st to 10th	Day	35% 9% 15% 23% 16% 2	2730
	11th to 20th	Day	33% 9% 17% 24% 16% 2	2730
	21st to end	Day	33% 10% 18% 24% 14% 2	2730
July	1st to 10th	Day	36% 8% 20% 25% 10% 2	7615
	11th to 20th	Day	43% 10% 25% 15% 6% 1	7611
	21st to end	Day	43% 8% 17% 19% 12%	8156
August	1st to 10th	Day	54% 10% 21% 11% 3	6001
	11th to 20th	Day	34% 8% 26% 24% 7% 1	5811
	21st to end	Day	33% 6% 14% 32% 13% 1	6080
September	1st to 10th	Day	33% 8% 14% 21% 20% 5%	2520
	11th to 20th	Day	39% 7% 12% 18% 21% 3	2376
	21st to end	Day	45% 6% 10% 20% 15% 4	2264
October	1st to 10th	Day	34% 4 7% 20% 26% 8%	3360
	11th to 20th	Day	20% 3 5% 21% 28% 24%	3340
	21st to end	Day	25% 3 7% 24% 25% 16%	3618
November	1st to 10th	Day	33% 4 7% 22% 26% 7%	2057
	11th to 20th	Day	33% 3 8% 23% 26% 7%	2082
	21st to end	Day	34% 4 8% 23% 26% 5%	1751
December	1st to 10th	Day	36% 3 8% 22% 24% 7%	2680
	11th to 20th	Day	41% 4 10% 18% 21% 6%	3231
	21st to end	Day	38% 4 12% 26% 18% 2	4565
January	1st to 10th	Night	33% 3 10% 21% 24% 9%	7636
	11th to 20th	Night	32% 3 4 15% 36% 10%	7680
	21st to end	Night	42% 2 5% 19% 25% 7%	8401
February	1st to 10th	Night	39% 4 10% 21% 20% 5%	2730
	11th to 20th	Night	44% 4 9% 16% 21% 7%	2655
	21st to end	Night	38% 5% 9% 18% 22% 8%	1915
March	1st to 10th	Night	44% 5% 6% 14% 21% 10%	5028
	11th to 20th	Night	30% 4 6% 16% 25% 18%	5090
	21st to end	Night	36% 4 8% 24% 22% 6%	5588
April	1st to 10th	Night	35% 5% 11% 18% 24% 7%	2179
	11th to 20th	Night	32% 5% 10% 24% 21% 8%	2075
	21st to end	Night	36% 7% 11% 24% 17% 4	2100
May	1st to 10th	Night	41% 6% 16% 20% 14% 2	5352
	11th to 20th	Night	41% 6% 12% 22% 18% 1	5235
	21st to end	Night	46% 6% 12% 21% 13% 2	5771
June	1st to 10th	Night	37% 10% 15% 20% 16% 2	1890
	11th to 20th	Night	37% 10% 17% 20% 15% 2	1890
	21st to end	Night	33% 11% 17% 22% 14% 3	1890
July	1st to 10th	Night	42% 8% 17% 22% 10% 1	5235
	11th to 20th	Night	49% 12% 19% 12% 7% 1	5226
	21st to end	Night	47% 7% 16% 17% 11% 1	5606
August	1st to 10th	Night	55% 12% 16% 12% 4	4124
	11th to 20th	Night	37% 8% 22% 26% 6% 1	4239
	21st to end	Night	36% 6% 10% 27% 20% 1	5045
September	1st to 10th	Night	34% 8% 13% 23% 17% 5%	2100
	11th to 20th	Night	39% 8% 11% 20% 21% 2	2239
	21st to end	Night	47% 7% 12% 16% 14% 4	2354
October	1st to 10th	Night	39% 4 7% 19% 25% 6%	4040
	11th to 20th	Night	25% 3 5% 19% 28% 19%	4019
	21st to end	Night	24% 3 7% 26% 24% 16%	4349
November	1st to 10th	Night	35% 4 8% 20% 26% 8%	2461
	11th to 20th	Night	35% 4 8% 16% 25% 11%	2536
	21st to end	Night	33% 4 8% 22% 25% 7%	2861
December	1st to 10th	Night	36% 3 6% 19% 28% 7%	4720
	11th to 20th	Night	47% 3 9% 17% 16% 8%	5164
	21st to end	Night	38% 4 12% 24% 21% 2	6633

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% 25% 33% 13% 3	330
	11th to 20th	Day	28% 25% 33% 11% 3	330
	21st to end	Day	20% 23% 44% 10% 2	363
February	1st to 10th	Day	20% 20% 42% 15% 3	330
	11th to 20th	Day	38% 21% 30% 9% 1	338
	21st to end	Day	26% 20% 33% 17% 4	446
March	1st to 10th	Day	24% 18% 32% 19% 6%	540
	11th to 20th	Day	21% 20% 39% 14% 6% 1	540
	21st to end	Day	21% 18% 39% 17% 5%	594
April	1st to 10th	Day	20% 18% 35% 19% 8% 1	540
	11th to 20th	Day	14% 19% 33% 23% 10% 3	532
	21st to end	Day	17% 11% 30% 27% 14% 1	540
May	1st to 10th	Day	17% 18% 31% 23% 9% 3	543
	11th to 20th	Day	16% 10% 25% 28% 17% 4	567
	21st to end	Day	18% 7% 21% 27% 20% 6%	617
June	1st to 10th	Day	11% 6% 26% 31% 20% 6%	570
	11th to 20th	Day	8% 7% 23% 31% 24% 7%	570
	21st to end	Day	9% 3 20% 34% 27% 8%	570
July	1st to 10th	Day	10% 1 12% 30% 37% 10%	565
	11th to 20th	Day	7% 2 11% 30% 35% 13%	564
	21st to end	Day	9% 3 16% 34% 27% 11%	620
August	1st to 10th	Day	16% 4 17% 29% 25% 9%	566
	11th to 20th	Day	10% 4 14% 31% 28% 13%	559
	21st to end	Day	11% 3 14% 30% 29% 14%	627
September	1st to 10th	Day	11% 3 18% 26% 26% 15%	570
	11th to 20th	Day	15% 3 16% 30% 25% 11%	570
	21st to end	Day	24% 6% 24% 24% 17% 4	570
October	1st to 10th	Day	16% 7% 21% 25% 22% 9%	570
	11th to 20th	Day	15% 10% 26% 27% 17% 4	570
	21st to end	Day	15% 12% 25% 28% 16% 4	615
November	1st to 10th	Day	16% 11% 32% 26% 13% 3	535
	11th to 20th	Day	18% 15% 30% 25% 11% 1	522
	21st to end	Day	16% 20% 29% 21% 12% 2	256
December	1st to 10th	Day	23% 20% 36% 14% 6%	220
	11th to 20th	Day	21% 17% 31% 24% 7%	259
	21st to end	Day	23% 18% 33% 17% 9% 1	363
January	1st to 10th	Night	19% 20% 32% 24% 4	840
	11th to 20th	Night	19% 19% 33% 24% 6%	840
	21st to end	Night	19% 15% 38% 24% 3	924
February	1st to 10th	Night	20% 15% 35% 26% 3	840
	11th to 20th	Night	25% 20% 35% 20% 1	823
	21st to end	Night	19% 16% 37% 24% 3	692
March	1st to 10th	Night	16% 17% 33% 28% 5%	840
	11th to 20th	Night	20% 16% 34% 22% 8%	840
	21st to end	Night	18% 13% 41% 24% 4	924
April	1st to 10th	Night	17% 17% 34% 25% 6%	840
	11th to 20th	Night	14% 14% 36% 26% 10%	831
	21st to end	Night	14% 9% 33% 35% 8%	840
May	1st to 10th	Night	14% 11% 34% 32% 9%	666
	11th to 20th	Night	16% 7% 24% 34% 17% 1	621
	21st to end	Night	15% 4 18% 39% 24% 1	683
June	1st to 10th	Night	12% 3 20% 41% 23% 2	630
	11th to 20th	Night	10% 3 19% 40% 24% 3	630
	21st to end	Night	8% 3 16% 42% 29% 2	630
July	1st to 10th	Night	8% 1 12% 38% 37% 4	615
	11th to 20th	Night	8% 2 8% 41% 36% 4	612
	21st to end	Night	8% 1 14% 38% 33% 5%	690
August	1st to 10th	Night	16% 2 10% 37% 32% 4	627
	11th to 20th	Night	11% 2 11% 37% 33% 5%	676
	21st to end	Night	11% 2 13% 32% 37% 5%	924
September	1st to 10th	Night	13% 3 11% 35% 32% 5%	840
	11th to 20th	Night	15% 3 15% 31% 31% 4	840
	21st to end	Night	23% 5% 18% 33% 20% 1	840
October	1st to 10th	Night	16% 5% 17% 31% 25% 5%	840
	11th to 20th	Night	17% 9% 24% 29% 19% 1	840
	21st to end	Night	15% 10% 25% 29% 20% 2	913
November	1st to 10th	Night	16% 8% 27% 33% 14% 2	820
	11th to 20th	Night	15% 13% 29% 32% 10%	840
	21st to end	Night	15% 13% 29% 29% 13%	840
December	1st to 10th	Night	22% 13% 30% 28% 7%	840
	11th to 20th	Night	18% 11% 31% 33% 7%	840
	21st to end	Night	20% 16% 27% 30% 7%	924

Page generated at 2026-10-26 04:37:49. Version ac07

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