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 2025-10-04.

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	17% 8% 2 6% 4 4 4 4 5% 7% 38%	126237
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	123120
Transparency	Day	44% 5% 12% 22% 15% 3	129768
	Night	44% 4 11% 22% 16% 3	127676
Seeing	Day	23% 17% 31% 19% 7% 2	16217
	Night	22% 15% 31% 24% 7% 1	25191

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% 8% 2 6% 5% 5% 5% 3 5% 5% 43%	3597
	11th to 20th	Day	14% 9% 3 6% 4 4 4 4 5% 5% 43%	3570
	21st to end	Day	19% 9% 3 7% 4 4 4 3 4 4 40%	4052
February	1st to 10th	Day	18% 9% 2 5% 4 5% 5% 4 5% 6% 37%	1882
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 3 5% 7% 40%	1948
	21st to end	Day	20% 9% 2 6% 4 4 4 4 4 5% 38%	1650
March	1st to 10th	Day	22% 10% 3 5% 4 3 3 3 3 5% 39%	4476
	11th to 20th	Day	14% 8% 2 6% 5% 4 5% 4 5% 7% 40%	4648
	21st to end	Day	20% 9% 3 6% 4 4 4 4 5% 7% 35%	5970
April	1st to 10th	Day	20% 9% 2 5% 4 3 4 3 5% 7% 37%	2400
	11th to 20th	Day	20% 7% 2 6% 4 3 4 3 5% 6% 40%	2374
	21st to end	Day	22% 5% 2 5% 4 4 4 3 5% 6% 39%	2520
May	1st to 10th	Day	30% 8% 3 6% 4 3 4 3 5% 8% 26%	5886
	11th to 20th	Day	20% 9% 3 7% 5% 4 4 4 5% 7% 33%	6088
	21st to end	Day	21% 7% 2 6% 4 4 4 4 6% 8% 34%	6949
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 4 7% 8% 35%	2800
	11th to 20th	Day	17% 7% 3 5% 4 4 5% 4 6% 8% 36%	2800
	21st to end	Day	14% 7% 2 7% 4 4 5% 3 6% 9% 38%	2800
July	1st to 10th	Day	15% 8% 2 7% 5% 5% 5% 4 7% 9% 32%	6360
	11th to 20th	Day	12% 7% 2 6% 5% 4 6% 5% 7% 11% 34%	6360
	21st to end	Day	15% 7% 2 6% 4 4 5% 4 6% 7% 41%	6926
August	1st to 10th	Day	20% 7% 2 7% 5% 5% 6% 4 6% 9% 30%	4372
	11th to 20th	Day	11% 6% 3 7% 5% 5% 4 4 6% 8% 41%	3890
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 4 8% 9% 42%	4312
September	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 5% 6% 46%	2400
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 4 7% 50%	2251
	21st to end	Day	19% 5% 3 6% 4 5% 4 3 5% 6% 40%	2200
October	1st to 10th	Day	21% 7% 2 5% 4 3 4 4 4 5% 41%	2108
	11th to 20th	Day	14% 6% 2 5% 4 4 4 4 5% 7% 47%	1875
	21st to end	Day	19% 7% 3 5% 4 3 4 3 5% 6% 41%	1881
November	1st to 10th	Day	13% 7% 2 5% 4 4 4 3 3 5% 49%	1761
	11th to 20th	Day	19% 7% 1 4 3 3 3 2 4 6% 48%	1800
	21st to end	Day	16% 7% 2 5% 4 4 4 3 5% 6% 44%	1785
December	1st to 10th	Day	16% 7% 2 6% 3 4 3 3 5% 6% 46%	2600
	11th to 20th	Day	20% 8% 2 5% 3 2 3 2 5% 5% 46%	2986
	21st to end	Day	19% 10% 3 6% 4 4 3 2 3 5% 39%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 3 4 4 5% 6% 46%	6349
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 4 5% 45%	6315
	21st to end	Night	17% 10% 3 7% 4 4 4 3 4 6% 37%	6889
February	1st to 10th	Night	17% 10% 3 7% 4 3 3 3 5% 6% 39%	2515
	11th to 20th	Night	17% 8% 3 6% 4 3 3 3 5% 7% 40%	2338
	21st to end	Night	20% 9% 2 5% 4 4 4 3 4 6% 39%	1980
March	1st to 10th	Night	21% 10% 2 5% 3 3 3 3 4 6% 40%	5364
	11th to 20th	Night	16% 9% 3 7% 4 3 4 3 5% 6% 40%	5294
	21st to end	Night	21% 9% 3 7% 4 3 4 3 5% 5% 37%	4962
April	1st to 10th	Night	20% 8% 3 6% 4 4 4 3 6% 6% 36%	2000
	11th to 20th	Night	20% 7% 2 6% 4 3 4 3 4 6% 40%	1975
	21st to end	Night	20% 7% 3 6% 4 3 3 3 5% 5% 41%	1879
May	1st to 10th	Night	27% 11% 4 7% 4 4 3 3 4 7% 26%	4054
	11th to 20th	Night	20% 11% 3 7% 4 5% 4 3 5% 7% 30%	3835
	21st to end	Night	22% 8% 3 7% 4 3 4 4 5% 7% 33%	3926
June	1st to 10th	Night	18% 6% 2 7% 5% 4 4 4 4 8% 39%	1600
	11th to 20th	Night	17% 6% 2 6% 5% 4 4 3 5% 9% 39%	1600
	21st to end	Night	13% 7% 2 6% 4 4 4 4 7% 9% 41%	1600
July	1st to 10th	Night	16% 8% 3 7% 4 4 4 4 6% 9% 34%	3600
	11th to 20th	Night	17% 9% 3 7% 4 4 4 4 7% 9% 32%	3600
	21st to end	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	3926
August	1st to 10th	Night	23% 7% 3 7% 5% 4 4 4 6% 7% 29%	2744
	11th to 20th	Night	13% 7% 2 7% 5% 5% 5% 4 5% 7% 40%	3153
	21st to end	Night	9% 4 1 4 3 4 4 3 5% 8% 54%	3586
September	1st to 10th	Night	14% 6% 3 6% 5% 4 4 4 5% 6% 44%	2000
	11th to 20th	Night	13% 5% 2 5% 3 4 3 2 5% 6% 53%	2148
	21st to end	Night	18% 8% 2 5% 3 4 4 3 5% 6% 41%	2200
October	1st to 10th	Night	19% 6% 2 6% 4 3 4 3 4 6% 41%	2136
	11th to 20th	Night	16% 6% 1 5% 3 3 3 3 4 6% 49%	2304
	21st to end	Night	21% 8% 2 5% 3 3 3 3 5% 5% 41%	2717
November	1st to 10th	Night	13% 5% 2 6% 4 3 4 3 5% 6% 49%	2537
	11th to 20th	Night	19% 7% 2 4 3 3 4 2 4 5% 46%	2600
	21st to end	Night	17% 6% 2 5% 3 4 4 3 4 7% 45%	2610
December	1st to 10th	Night	17% 8% 2 5% 3 3 3 3 4 4 47%	4580
	11th to 20th	Night	24% 8% 2 5% 3 3 3 3 3 4 41%	5208
	21st to end	Night	19% 10% 3 7% 3 3 3 2 4 5% 41%	6996

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	41% 4 7% 28% 17% 3	3598
	11th to 20th	Day	43% 4 10% 22% 15% 7%	3600
	21st to end	Day	48% 3 9% 23% 16% 2	4052
February	1st to 10th	Day	47% 4 12% 21% 14% 2	1882
	11th to 20th	Day	44% 4 10% 24% 16% 3	1957
	21st to end	Day	48% 4 10% 20% 14% 3	1650
March	1st to 10th	Day	49% 3 10% 16% 12% 10%	4479
	11th to 20th	Day	42% 3 13% 29% 11% 2	4648
	21st to end	Day	49% 4 7% 15% 19% 6%	5970
April	1st to 10th	Day	47% 4 9% 20% 16% 4	2400
	11th to 20th	Day	44% 3 10% 21% 19% 3	2374
	21st to end	Day	45% 4 9% 22% 16% 4	2520
May	1st to 10th	Day	57% 4 15% 14% 9% 1	5897
	11th to 20th	Day	50% 4 13% 19% 12% 1	6092
	21st to end	Day	47% 5% 11% 22% 13% 1	6949
June	1st to 10th	Day	45% 5% 12% 24% 12% 1	2800
	11th to 20th	Day	44% 5% 14% 22% 13% 2	2800
	21st to end	Day	42% 6% 13% 23% 14% 1	2800
July	1st to 10th	Day	47% 8% 11% 20% 14% 1	6290
	11th to 20th	Day	41% 7% 18% 22% 11% 1	6285
	21st to end	Day	41% 6% 11% 22% 19% 1	6728
August	1st to 10th	Day	49% 6% 15% 21% 8% 1	4358
	11th to 20th	Day	38% 6% 15% 30% 11% 1	3927
	21st to end	Day	34% 7% 13% 27% 17% 2	4312
September	1st to 10th	Day	38% 6% 12% 23% 19% 2	2400
	11th to 20th	Day	34% 4 11% 24% 24% 3	2251
	21st to end	Day	45% 5% 8% 21% 19% 2	2200
October	1st to 10th	Day	38% 5% 10% 19% 24% 5%	3485
	11th to 20th	Day	29% 3 8% 25% 31% 4	3091
	21st to end	Day	32% 4 13% 25% 22% 3	3081
November	1st to 10th	Day	38% 4 12% 22% 21% 2	1761
	11th to 20th	Day	39% 3 10% 24% 21% 4	1800
	21st to end	Day	41% 4 13% 27% 13% 3	1785
December	1st to 10th	Day	41% 3 15% 28% 12% 1	2600
	11th to 20th	Day	42% 3 14% 19% 19% 3	2986
	21st to end	Day	50% 2 8% 20% 15% 5%	3960
January	1st to 10th	Night	37% 4 9% 29% 18% 3	6349
	11th to 20th	Night	42% 3 13% 21% 16% 5%	6360
	21st to end	Night	49% 2 10% 19% 16% 4	6889
February	1st to 10th	Night	46% 3 12% 24% 12% 2	2515
	11th to 20th	Night	44% 4 9% 21% 18% 4	2340
	21st to end	Night	48% 3 7% 23% 15% 4	1980
March	1st to 10th	Night	47% 3 10% 19% 14% 6%	5374
	11th to 20th	Night	46% 3 15% 21% 12% 3	5295
	21st to end	Night	49% 3 5% 13% 24% 5%	4962
April	1st to 10th	Night	48% 4 9% 20% 15% 4	2000
	11th to 20th	Night	45% 3 10% 19% 20% 2	1975
	21st to end	Night	46% 3 9% 23% 15% 4	1879
May	1st to 10th	Night	60% 4 13% 13% 9% 1	4060
	11th to 20th	Night	53% 5% 13% 16% 10% 3	3836
	21st to end	Night	50% 5% 13% 22% 9% 1	3927
June	1st to 10th	Night	46% 5% 11% 21% 16% 2	1600
	11th to 20th	Night	43% 5% 14% 26% 11% 1	1600
	21st to end	Night	40% 5% 14% 26% 15% 1	1600
July	1st to 10th	Night	46% 6% 15% 21% 11% 1	3560
	11th to 20th	Night	47% 7% 17% 20% 9% 1	3552
	21st to end	Night	40% 5% 13% 24% 18% 1	3809
August	1st to 10th	Night	53% 6% 14% 16% 10% 1	2734
	11th to 20th	Night	43% 5% 14% 29% 8% 1	3182
	21st to end	Night	28% 6% 14% 33% 17% 1	3586
September	1st to 10th	Night	40% 5% 10% 26% 18% 1	2000
	11th to 20th	Night	34% 3 10% 24% 26% 2	2148
	21st to end	Night	45% 3 9% 21% 19% 3	2200
October	1st to 10th	Night	38% 5% 12% 24% 18% 2	3539
	11th to 20th	Night	26% 3 9% 27% 31% 3	3827
	21st to end	Night	33% 3 13% 29% 21% 2	4467
November	1st to 10th	Night	36% 4 12% 23% 21% 4	2537
	11th to 20th	Night	41% 3 9% 23% 21% 4	2600
	21st to end	Night	41% 3 12% 30% 13% 2	2610
December	1st to 10th	Night	42% 3 12% 29% 13% 1	4580
	11th to 20th	Night	49% 3 11% 17% 18% 2	5208
	21st to end	Night	48% 3 8% 20% 17% 4	6996

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	31% 24% 30% 15% 1	200
	11th to 20th	Day	32% 30% 26% 11% 2	200
	21st to end	Day	26% 29% 30% 9% 4 2	240
February	1st to 10th	Day	32% 25% 31% 12% 1	382
	11th to 20th	Day	30% 29% 30% 9% 1	496
	21st to end	Day	30% 24% 31% 12% 3	414
March	1st to 10th	Day	30% 24% 32% 12% 2	500
	11th to 20th	Day	27% 28% 30% 12% 2 1	500
	21st to end	Day	32% 22% 33% 9% 3 1	550
April	1st to 10th	Day	32% 25% 28% 9% 5%	500
	11th to 20th	Day	32% 21% 30% 12% 4 1	492
	21st to end	Day	30% 20% 32% 13% 3 1	500
May	1st to 10th	Day	33% 19% 30% 13% 5% 1	503
	11th to 20th	Day	25% 19% 35% 14% 6% 1	527
	21st to end	Day	22% 14% 33% 22% 8% 1	573
June	1st to 10th	Day	21% 14% 32% 22% 10% 2	530
	11th to 20th	Day	19% 16% 35% 22% 7% 1	530
	21st to end	Day	17% 15% 33% 25% 9% 2	530
July	1st to 10th	Day	18% 8% 34% 27% 12% 2	525
	11th to 20th	Day	15% 8% 32% 30% 12% 2	524
	21st to end	Day	16% 10% 38% 27% 7% 2	576
August	1st to 10th	Day	13% 9% 31% 32% 10% 4	526
	11th to 20th	Day	14% 8% 32% 29% 14% 3	519
	21st to end	Day	12% 11% 35% 24% 12% 6%	583
September	1st to 10th	Day	20% 11% 28% 22% 15% 4	530
	11th to 20th	Day	13% 11% 30% 27% 12% 7%	530
	21st to end	Day	22% 11% 35% 19% 8% 3	530
October	1st to 10th	Day	27% 12% 25% 20% 12% 4	502
	11th to 20th	Day	17% 19% 32% 22% 8% 2	476
	21st to end	Day	26% 16% 32% 18% 7% 1	429
November	1st to 10th	Day	21% 17% 30% 22% 9% 2	395
	11th to 20th	Day	27% 23% 27% 18% 5%	400
	21st to end	Day	25% 22% 28% 19% 6%	385
December	1st to 10th	Day	25% 25% 32% 14% 6%	200
	11th to 20th	Day	35% 25% 30% 9% 2	200
	21st to end	Day	27% 22% 29% 17% 5%	220
January	1st to 10th	Night	23% 23% 35% 16% 3	800
	11th to 20th	Night	28% 20% 31% 18% 3	800
	21st to end	Night	23% 22% 33% 19% 3	880
February	1st to 10th	Night	23% 21% 34% 21% 2	800
	11th to 20th	Night	25% 23% 35% 15% 1	783
	21st to end	Night	26% 21% 34% 16% 3	660
March	1st to 10th	Night	26% 21% 32% 17% 3	800
	11th to 20th	Night	23% 20% 34% 19% 3 1	800
	21st to end	Night	24% 23% 34% 17% 3	880
April	1st to 10th	Night	25% 20% 34% 18% 2	800
	11th to 20th	Night	24% 19% 33% 20% 3	791
	21st to end	Night	23% 18% 36% 18% 5%	679
May	1st to 10th	Night	26% 17% 32% 20% 6% 1	600
	11th to 20th	Night	22% 17% 36% 21% 4	511
	21st to end	Night	21% 7% 29% 34% 7% 1	433
June	1st to 10th	Night	20% 8% 30% 33% 8% 1	400
	11th to 20th	Night	20% 7% 31% 34% 8%	400
	21st to end	Night	16% 6% 33% 37% 8%	400
July	1st to 10th	Night	15% 4 30% 37% 15%	390
	11th to 20th	Night	15% 4 25% 41% 15% 1	388
	21st to end	Night	14% 6% 29% 38% 11% 1	438
August	1st to 10th	Night	12% 5% 31% 38% 13% 1	491
	11th to 20th	Night	13% 6% 23% 40% 16% 1	758
	21st to end	Night	14% 6% 26% 35% 16% 2	880
September	1st to 10th	Night	17% 5% 30% 33% 12% 2	800
	11th to 20th	Night	17% 7% 25% 31% 18% 2	800
	21st to end	Night	22% 7% 28% 28% 14% 1	800
October	1st to 10th	Night	23% 9% 27% 26% 12% 3	773
	11th to 20th	Night	19% 14% 31% 27% 9% 1	760
	21st to end	Night	28% 11% 32% 21% 7%	836
November	1st to 10th	Night	17% 14% 34% 24% 10% 1	780
	11th to 20th	Night	24% 18% 33% 19% 6% 1	800
	21st to end	Night	23% 18% 30% 23% 5% 1	800
December	1st to 10th	Night	27% 17% 29% 20% 6% 1	800
	11th to 20th	Night	26% 21% 30% 21% 3	800
	21st to end	Night	26% 18% 35% 18% 2	880

Page generated at 2025-10-01 01:39:06. Version ac07

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