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 2026-04-02.

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	15% 13% 5% 10% 6% 5% 4 3 5% 6% 28%	131577
	Night	15% 14% 6% 12% 6% 5% 4 3 4 5% 26%	130262
Transparency	Day	55% 6% 10% 17% 10% 1	135127
	Night	58% 6% 10% 16% 9% 1	134800
Seeing	Day	21% 14% 29% 22% 10% 3	16804
	Night	22% 12% 30% 26% 9% 1	26107

By Month

Percentage Cloud Cover NE (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	13% 18% 9% 20% 9% 6% 4 3 3 4 12%	4153
	11th to 20th	Day	8% 23% 11% 24% 13% 8% 3 2 2 2 4	5105
	21st to end	Day	10% 18% 8% 20% 10% 10% 6% 3 4 4 7%	5780
February	1st to 10th	Day	13% 21% 7% 19% 11% 6% 4 2 3 3 12%	1890
	11th to 20th	Day	13% 16% 9% 19% 10% 5% 4 2 4 4 12%	1843
	21st to end	Day	15% 21% 7% 13% 8% 6% 5% 4 4 4 15%	1675
March	1st to 10th	Day	16% 13% 5% 15% 8% 4 3 2 4 5% 24%	4941
	11th to 20th	Day	13% 15% 7% 11% 6% 4 4 4 5% 6% 24%	5090
	21st to end	Day	18% 17% 6% 10% 4 4 4 3 4 5% 24%	5586
April	1st to 10th	Day	17% 17% 5% 9% 4 3 4 3 5% 5% 26%	2316
	11th to 20th	Day	20% 13% 3 6% 3 3 3 3 5% 6% 35%	2550
	21st to end	Day	20% 12% 4 7% 4 4 3 4 4 7% 32%	2600
May	1st to 10th	Day	17% 10% 3 7% 4 4 4 4 5% 7% 35%	5888
	11th to 20th	Day	15% 10% 3 8% 5% 4 4 3 5% 7% 35%	6106
	21st to end	Day	21% 8% 2 6% 4 4 3 3 5% 7% 36%	6949
June	1st to 10th	Day	15% 8% 3 6% 5% 4 5% 4 6% 7% 37%	2860
	11th to 20th	Day	13% 6% 3 5% 4 4 5% 4 7% 9% 41%	3000
	21st to end	Day	18% 7% 2 5% 4 3 4 4 6% 8% 37%	3000
July	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 9% 37%	6819
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 6% 8% 44%	6486
	21st to end	Day	9% 6% 2 5% 4 4 5% 4 6% 9% 46%	6424
August	1st to 10th	Day	13% 7% 2 8% 5% 5% 6% 5% 6% 8% 35%	4234
	11th to 20th	Day	14% 7% 3 8% 5% 6% 5% 4 7% 9% 32%	4174
	21st to end	Day	10% 5% 2 7% 5% 5% 5% 4 7% 9% 41%	4611
September	1st to 10th	Day	14% 8% 2 8% 5% 4 4 4 6% 7% 38%	2390
	11th to 20th	Day	13% 5% 2 6% 4 4 3 3 6% 8% 45%	2200
	21st to end	Day	17% 8% 3 7% 4 4 5% 4 5% 8% 33%	2200
October	1st to 10th	Day	20% 9% 3 7% 4 4 4 3 5% 6% 34%	2015
	11th to 20th	Day	24% 13% 4 8% 5% 5% 4 4 5% 6% 22%	2000
	21st to end	Day	27% 17% 4 9% 4 4 5% 3 4 5% 19%	2155
November	1st to 10th	Day	18% 20% 4 9% 5% 5% 3 3 4 5% 25%	1805
	11th to 20th	Day	17% 19% 6% 11% 7% 4 5% 4 5% 6% 18%	1680
	21st to end	Day	18% 26% 8% 11% 6% 4 4 3 4 4 12%	1680
December	1st to 10th	Day	16% 27% 11% 16% 6% 5% 4 3 3 4 7%	2680
	11th to 20th	Day	19% 24% 7% 16% 5% 4 3 3 3 4 11%	3066
	21st to end	Day	24% 21% 7% 14% 6% 5% 4 2 4 4 11%	3626
January	1st to 10th	Night	12% 18% 10% 23% 9% 5% 4 3 4 4 8%	8757
	11th to 20th	Night	10% 21% 11% 24% 12% 7% 4 2 3 2 4	7781
	21st to end	Night	10% 15% 9% 19% 12% 8% 6% 3 4 4 10%	8401
February	1st to 10th	Night	13% 20% 7% 15% 10% 6% 5% 3 4 4 13%	2730
	11th to 20th	Night	13% 16% 8% 16% 8% 5% 4 3 4 5% 17%	2663
	21st to end	Night	15% 16% 6% 13% 7% 5% 4 4 5% 5% 19%	2128
March	1st to 10th	Night	14% 15% 5% 10% 7% 5% 4 4 4 5% 26%	5117
	11th to 20th	Night	9% 14% 7% 9% 5% 5% 5% 4 6% 7% 29%	5089
	21st to end	Night	20% 12% 5% 12% 5% 5% 4 2 4 4 27%	5588
April	1st to 10th	Night	19% 14% 5% 10% 4 4 3 3 4 6% 27%	2107
	11th to 20th	Night	18% 12% 4 6% 4 5% 4 3 5% 6% 35%	1817
	21st to end	Night	17% 13% 4 8% 5% 5% 4 4 5% 5% 31%	1800
May	1st to 10th	Night	16% 9% 3 7% 5% 4 4 3 4 7% 37%	4053
	11th to 20th	Night	14% 7% 2 7% 5% 4 4 4 5% 6% 42%	3771
	21st to end	Night	20% 7% 2 6% 4 3 3 3 5% 6% 42%	3926
June	1st to 10th	Night	15% 7% 3 6% 5% 4 4 3 6% 8% 39%	1535
	11th to 20th	Night	13% 5% 2 7% 5% 5% 4 5% 7% 9% 39%	1400
	21st to end	Night	14% 8% 2 5% 3 5% 4 4 6% 7% 42%	1400
July	1st to 10th	Night	10% 8% 3 6% 4 4 4 3 5% 8% 45%	3160
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 7% 48%	3467
	21st to end	Night	10% 6% 2 6% 4 4 4 3 5% 8% 48%	4428
August	1st to 10th	Night	12% 7% 2 7% 4 4 4 3 5% 6% 46%	2915
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 8% 39%	2875
	21st to end	Night	14% 6% 2 6% 4 4 4 4 4 6% 46%	3252
September	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 6% 7% 45%	2006
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 5% 7% 50%	2200
	21st to end	Night	15% 8% 3 8% 5% 5% 4 3 4 6% 38%	2200
October	1st to 10th	Night	18% 8% 3 7% 4 4 4 3 5% 7% 38%	2384
	11th to 20th	Night	23% 12% 3 8% 5% 4 4 3 5% 6% 27%	2400
	21st to end	Night	24% 15% 3 8% 5% 5% 4 4 5% 5% 23%	2682
November	1st to 10th	Night	18% 17% 4 9% 5% 5% 4 3 4 5% 26%	2712
	11th to 20th	Night	17% 19% 5% 10% 5% 5% 5% 4 5% 5% 19%	2940
	21st to end	Night	21% 21% 7% 10% 5% 5% 4 3 5% 5% 14%	2940
December	1st to 10th	Night	16% 26% 8% 14% 6% 5% 5% 3 4 4 9%	4720
	11th to 20th	Night	13% 23% 10% 16% 7% 5% 4 3 4 3 12%	5348
	21st to end	Night	20% 22% 9% 15% 6% 5% 4 3 4 4 9%	7570

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	75% 5% 8% 6% 5% 1	4154
	11th to 20th	Day	89% 4 2 3 2	5139
	21st to end	Day	80% 5% 6% 6% 2	5780
February	1st to 10th	Day	79% 4 4 10% 2 1	1890
	11th to 20th	Day	75% 4 6% 9% 4 1	1852
	21st to end	Day	72% 5% 7% 11% 5%	1675
March	1st to 10th	Day	63% 5% 13% 11% 7% 1	4944
	11th to 20th	Day	60% 4 11% 17% 6% 2	5090
	21st to end	Day	61% 4 10% 16% 8% 1	5586
April	1st to 10th	Day	60% 5% 8% 17% 9%	2316
	11th to 20th	Day	51% 5% 9% 21% 13% 1	2550
	21st to end	Day	53% 6% 10% 17% 12% 2	2600
May	1st to 10th	Day	47% 6% 12% 17% 11% 5%	5897
	11th to 20th	Day	48% 7% 9% 17% 16% 1	6110
	21st to end	Day	47% 6% 10% 19% 14% 4	6949
June	1st to 10th	Day	45% 7% 10% 22% 15% 1	2860
	11th to 20th	Day	40% 7% 11% 25% 16% 1	3000
	21st to end	Day	43% 8% 13% 23% 13% 1	3000
July	1st to 10th	Day	42% 8% 12% 21% 17% 1	6746
	11th to 20th	Day	36% 7% 11% 29% 16% 1	6405
	21st to end	Day	33% 7% 12% 27% 19% 2	6240
August	1st to 10th	Day	43% 9% 13% 22% 13% 1	4221
	11th to 20th	Day	46% 8% 14% 22% 9%	4213
	21st to end	Day	36% 9% 14% 29% 11% 1	4611
September	1st to 10th	Day	44% 8% 12% 23% 11% 2	2390
	11th to 20th	Day	37% 6% 13% 31% 13% 1	2200
	21st to end	Day	48% 7% 12% 23% 9% 1	2200
October	1st to 10th	Day	46% 7% 11% 22% 12% 1	3275
	11th to 20th	Day	50% 7% 11% 20% 11% 1	3240
	21st to end	Day	61% 6% 11% 16% 6%	3457
November	1st to 10th	Day	61% 6% 10% 16% 7%	1805
	11th to 20th	Day	66% 7% 10% 10% 6% 1	1680
	21st to end	Day	75% 5% 9% 10% 2	1680
December	1st to 10th	Day	81% 5% 8% 6%	2680
	11th to 20th	Day	77% 4 8% 8% 3	3066
	21st to end	Day	78% 6% 6% 6% 4	3626
January	1st to 10th	Night	78% 6% 7% 6% 3	8757
	11th to 20th	Night	86% 5% 4 3 2	7822
	21st to end	Night	75% 6% 7% 5% 6%	8401
February	1st to 10th	Night	74% 5% 7% 11% 2	2730
	11th to 20th	Night	69% 4 10% 12% 5%	2665
	21st to end	Night	66% 6% 8% 13% 7%	2128
March	1st to 10th	Night	57% 6% 12% 15% 9% 1	5127
	11th to 20th	Night	52% 7% 15% 17% 9% 1	5090
	21st to end	Night	60% 4 8% 15% 12% 2	5588
April	1st to 10th	Night	59% 5% 7% 16% 11% 1	2107
	11th to 20th	Night	51% 5% 8% 19% 16% 1	1817
	21st to end	Night	54% 6% 9% 15% 15% 2	1800
May	1st to 10th	Night	46% 6% 9% 17% 13% 8%	4060
	11th to 20th	Night	41% 10% 10% 22% 16% 1	3772
	21st to end	Night	42% 6% 11% 24% 15% 2	3927
June	1st to 10th	Night	42% 6% 11% 24% 16% 2	1535
	11th to 20th	Night	39% 7% 13% 25% 16% 1	1400
	21st to end	Night	40% 7% 10% 26% 15% 1	1400
July	1st to 10th	Night	36% 7% 13% 23% 18% 3	3125
	11th to 20th	Night	36% 5% 10% 29% 18% 1	3425
	21st to end	Night	34% 6% 10% 30% 17% 2	4297
August	1st to 10th	Night	38% 6% 11% 28% 16% 1	2904
	11th to 20th	Night	43% 8% 14% 23% 12% 1	2902
	21st to end	Night	38% 6% 11% 31% 13% 1	3252
September	1st to 10th	Night	37% 7% 11% 29% 13% 2	2006
	11th to 20th	Night	33% 5% 13% 30% 17% 1	2200
	21st to end	Night	47% 7% 12% 23% 11% 1	2200
October	1st to 10th	Night	38% 7% 11% 25% 18% 1	3904
	11th to 20th	Night	51% 5% 13% 18% 14%	3899
	21st to end	Night	52% 7% 16% 19% 7%	4330
November	1st to 10th	Night	59% 6% 10% 18% 6% 1	2712
	11th to 20th	Night	64% 6% 11% 12% 5% 1	2940
	21st to end	Night	71% 6% 11% 10% 2	2940
December	1st to 10th	Night	77% 7% 10% 4 1	4720
	11th to 20th	Night	75% 5% 9% 9% 2	5348
	21st to end	Night	79% 5% 7% 7% 2	7570

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	24% 20% 37% 15% 3	231
	11th to 20th	Day	20% 28% 36% 12% 4	330
	21st to end	Day	24% 19% 36% 18% 3	363
February	1st to 10th	Day	25% 25% 35% 10% 5%	330
	11th to 20th	Day	26% 23% 38% 12% 1	330
	21st to end	Day	31% 27% 31% 10% 1	273
March	1st to 10th	Day	26% 18% 38% 13% 5% 1	487
	11th to 20th	Day	28% 17% 34% 16% 5% 1	540
	21st to end	Day	29% 17% 32% 16% 5% 1	594
April	1st to 10th	Day	26% 18% 33% 18% 5% 1	504
	11th to 20th	Day	31% 18% 23% 15% 9% 4	492
	21st to end	Day	28% 13% 30% 21% 6% 2	500
May	1st to 10th	Day	24% 11% 31% 23% 9% 2	503
	11th to 20th	Day	20% 11% 29% 20% 16% 3	527
	21st to end	Day	20% 10% 26% 26% 14% 4	573
June	1st to 10th	Day	16% 14% 27% 29% 12% 2	590
	11th to 20th	Day	15% 9% 33% 30% 10% 3	730
	21st to end	Day	20% 8% 34% 26% 10% 2	730
July	1st to 10th	Day	11% 8% 29% 28% 19% 5%	721
	11th to 20th	Day	14% 8% 29% 29% 14% 7%	637
	21st to end	Day	10% 6% 30% 32% 15% 6%	576
August	1st to 10th	Day	11% 8% 22% 31% 18% 10%	526
	11th to 20th	Day	16% 5% 20% 26% 22% 10%	519
	21st to end	Day	15% 6% 25% 28% 19% 8%	583
September	1st to 10th	Day	17% 10% 24% 26% 15% 9%	530
	11th to 20th	Day	10% 7% 19% 26% 25% 13%	530
	21st to end	Day	16% 14% 24% 27% 14% 4	530
October	1st to 10th	Day	22% 11% 28% 21% 15% 3	530
	11th to 20th	Day	27% 19% 30% 19% 5% 1	530
	21st to end	Day	34% 17% 28% 18% 4	538
November	1st to 10th	Day	34% 16% 30% 15% 6%	305
	11th to 20th	Day	27% 23% 35% 10% 2 2	220
	21st to end	Day	34% 20% 32% 12% 1	220
December	1st to 10th	Day	30% 27% 27% 14% 2	220
	11th to 20th	Day	30% 25% 32% 10% 2 1	220
	21st to end	Day	36% 19% 32% 10% 3	242
January	1st to 10th	Night	23% 18% 36% 18% 4	1050
	11th to 20th	Night	27% 19% 31% 18% 3 1	891
	21st to end	Night	19% 13% 37% 25% 6%	924
February	1st to 10th	Night	22% 18% 37% 19% 3	840
	11th to 20th	Night	22% 17% 36% 22% 4	823
	21st to end	Night	23% 20% 36% 19% 2	692
March	1st to 10th	Night	20% 17% 38% 19% 5%	840
	11th to 20th	Night	21% 18% 36% 19% 5%	840
	21st to end	Night	24% 13% 30% 24% 7% 1	924
April	1st to 10th	Night	27% 13% 32% 24% 4	805
	11th to 20th	Night	24% 13% 29% 22% 10% 1	633
	21st to end	Night	25% 10% 31% 26% 8%	600
May	1st to 10th	Night	20% 6% 30% 32% 11% 1	600
	11th to 20th	Night	18% 6% 29% 36% 10% 1	486
	21st to end	Night	15% 5% 28% 36% 15%	433
June	1st to 10th	Night	18% 6% 32% 35% 10% 1	400
	11th to 20th	Night	13% 8% 28% 40% 11%	400
	21st to end	Night	18% 6% 26% 36% 13% 1	400
July	1st to 10th	Night	8% 4 19% 50% 17% 1	390
	11th to 20th	Night	15% 3 23% 39% 19% 1	442
	21st to end	Night	12% 3 21% 44% 20% 1	657
August	1st to 10th	Night	12% 3 17% 40% 28% 1	597
	11th to 20th	Night	13% 4 16% 37% 27% 3	594
	21st to end	Night	17% 3 22% 30% 26% 2	676
September	1st to 10th	Night	15% 6% 24% 32% 18% 5%	800
	11th to 20th	Night	14% 3 22% 35% 21% 5%	800
	21st to end	Night	20% 8% 23% 32% 15% 3	800
October	1st to 10th	Night	21% 8% 29% 28% 14% 1	800
	11th to 20th	Night	29% 12% 26% 24% 8% 1	800
	21st to end	Night	31% 13% 32% 21% 4	880
November	1st to 10th	Night	28% 13% 29% 23% 7% 1	820
	11th to 20th	Night	27% 19% 30% 18% 5%	840
	21st to end	Night	29% 16% 34% 19% 2	840
December	1st to 10th	Night	30% 18% 33% 15% 4	840
	11th to 20th	Night	22% 21% 39% 15% 3	840
	21st to end	Night	32% 16% 34% 16% 2	1110

Page generated at 2026-03-30 02:18:39. Version ac07

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