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 2024-11-05.

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	18% 7% 2 6% 4 4 4 4 5% 7% 38%	101242
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 40%	97801
Transparency	Day	44% 4 12% 23% 15% 2	104791
	Night	44% 4 11% 23% 15% 2	102341
Seeing	Day	23% 17% 31% 19% 7% 2	15040
	Night	22% 15% 32% 24% 7% 1	24052

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	16% 7% 2 6% 5% 4 4 3 5% 5% 45%	2517
	11th to 20th	Day	17% 10% 3 8% 4 4 4 4 5% 4 37%	2520
	21st to end	Day	23% 12% 4 6% 3 4 3 3 4 4 33%	2836
February	1st to 10th	Day	18% 9% 2 5% 4 5% 5% 4 5% 6% 37%	1787
	11th to 20th	Day	15% 8% 2 6% 5% 4 4 3 5% 7% 41%	1848
	21st to end	Day	21% 10% 2 6% 4 4 4 4 4 5% 37%	1570
March	1st to 10th	Day	27% 11% 3 6% 4 3 3 2 3 5% 33%	3116
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 8% 42%	3260
	21st to end	Day	21% 7% 3 7% 3 4 4 4 5% 6% 36%	4166
April	1st to 10th	Day	19% 9% 2 5% 4 3 4 3 5% 7% 37%	2280
	11th to 20th	Day	20% 7% 2 5% 4 3 4 3 5% 7% 40%	2254
	21st to end	Day	22% 5% 2 5% 4 4 4 3 5% 6% 39%	2394
May	1st to 10th	Day	28% 9% 3 6% 4 3 3 3 5% 7% 30%	4106
	11th to 20th	Day	23% 8% 3 6% 5% 4 4 4 5% 7% 32%	4238
	21st to end	Day	15% 6% 2 5% 3 4 4 3 6% 8% 45%	4837
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 4 7% 8% 35%	2660
	11th to 20th	Day	16% 7% 2 5% 4 4 5% 4 7% 9% 37%	2660
	21st to end	Day	14% 7% 2 7% 4 4 5% 3 6% 9% 38%	2660
July	1st to 10th	Day	14% 6% 2 8% 6% 5% 5% 4 7% 9% 34%	4440
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 4 7% 10% 38%	4440
	21st to end	Day	17% 8% 3 7% 4 4 5% 4 6% 7% 34%	4814
August	1st to 10th	Day	20% 7% 3 7% 5% 5% 6% 4 6% 9% 30%	4238
	11th to 20th	Day	11% 6% 3 7% 5% 5% 4 4 6% 8% 41%	3769
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 4 8% 9% 42%	4180
September	1st to 10th	Day	14% 7% 2 5% 4 4 5% 4 5% 6% 46%	2280
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 7% 50%	2139
	21st to end	Day	19% 5% 3 6% 4 4 4 3 5% 5% 41%	2090
October	1st to 10th	Day	21% 7% 2 5% 4 3 4 4 4 5% 40%	2075
	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	14% 8% 2 5% 4 4 4 3 3 5% 49%	1708
	11th to 20th	Day	19% 7% 1 5% 3 3 3 2 4 6% 46%	1710
	21st to end	Day	16% 7% 2 5% 4 3 4 3 5% 6% 45%	1696
December	1st to 10th	Day	20% 7% 2 6% 4 5% 4 4 5% 5% 38%	1520
	11th to 20th	Day	22% 7% 2 5% 4 2 3 2 5% 5% 44%	1906
	21st to end	Day	19% 12% 3 7% 5% 5% 4 3 4 6% 33%	2772
January	1st to 10th	Night	17% 7% 2 5% 4 3 3 3 5% 6% 45%	4429
	11th to 20th	Night	17% 9% 4 7% 3 4 3 2 3 4 43%	4440
	21st to end	Night	24% 12% 3 7% 4 4 3 2 3 5% 32%	4819
February	1st to 10th	Night	17% 10% 3 7% 4 3 3 3 5% 6% 40%	2390
	11th to 20th	Night	17% 8% 3 6% 4 3 4 3 5% 6% 40%	2218
	21st to end	Night	21% 9% 2 5% 3 4 4 4 4 6% 39%	1884
March	1st to 10th	Night	26% 12% 2 5% 3 3 3 3 4 5% 34%	3724
	11th to 20th	Night	11% 8% 2 7% 4 4 4 4 5% 7% 42%	3696
	21st to end	Night	22% 8% 3 6% 3 3 3 3 5% 6% 39%	3466
April	1st to 10th	Night	19% 8% 3 6% 4 4 4 3 6% 6% 36%	1900
	11th to 20th	Night	19% 6% 2 6% 4 3 4 3 4 6% 40%	1875
	21st to end	Night	20% 7% 3 7% 4 3 3 3 5% 5% 40%	1785
May	1st to 10th	Night	23% 11% 4 8% 5% 4 4 3 5% 7% 28%	2834
	11th to 20th	Night	21% 10% 3 8% 4 5% 4 4 5% 7% 29%	2671
	21st to end	Night	18% 7% 3 5% 3 3 4 4 5% 7% 41%	2738
June	1st to 10th	Night	18% 6% 2 7% 5% 3 4 4 4 8% 39%	1520
	11th to 20th	Night	16% 6% 2 6% 5% 4 4 3 5% 9% 39%	1520
	21st to end	Night	13% 7% 2 7% 4 4 4 4 7% 9% 40%	1520
July	1st to 10th	Night	15% 7% 2 7% 5% 5% 5% 5% 7% 9% 33%	2520
	11th to 20th	Night	17% 7% 2 6% 5% 4 4 4 7% 8% 36%	2520
	21st to end	Night	19% 10% 3 7% 5% 4 4 4 5% 7% 33%	2739
August	1st to 10th	Night	23% 7% 3 8% 5% 4 5% 4 6% 7% 28%	2659
	11th to 20th	Night	13% 7% 2 7% 5% 5% 5% 4 5% 7% 39%	3054
	21st to end	Night	9% 4 1 4 3 4 4 3 5% 8% 54%	3476
September	1st to 10th	Night	14% 6% 3 6% 5% 4 4 4 5% 6% 44%	1900
	11th to 20th	Night	14% 5% 2 5% 3 4 3 2 5% 6% 53%	2040
	21st to end	Night	17% 8% 2 6% 3 4 4 3 5% 5% 41%	2090
October	1st to 10th	Night	20% 6% 2 6% 4 4 4 3 4 6% 41%	2102
	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 4 6% 49%	2470
	11th to 20th	Night	19% 7% 2 4 3 3 4 3 4 6% 45%	2470
	21st to end	Night	17% 6% 2 5% 3 4 4 3 4 7% 45%	2479
December	1st to 10th	Night	21% 8% 3 5% 4 4 4 3 5% 5% 39%	2660
	11th to 20th	Night	21% 7% 2 5% 3 4 4 3 4 5% 41%	3288
	21st to end	Night	22% 11% 3 7% 4 3 4 3 4 5% 35%	4884

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	43% 3 8% 32% 12% 3	2518
	11th to 20th	Day	50% 4 10% 24% 12% 1	2520
	21st to end	Day	56% 3 8% 22% 10% 2	2836
February	1st to 10th	Day	47% 4 12% 21% 14% 2	1787
	11th to 20th	Day	43% 4 10% 25% 16% 3	1857
	21st to end	Day	49% 4 10% 20% 13% 3	1570
March	1st to 10th	Day	56% 2 10% 18% 11% 2	3119
	11th to 20th	Day	39% 3 12% 32% 13% 2	3260
	21st to end	Day	48% 3 6% 18% 20% 5%	4166
April	1st to 10th	Day	46% 4 9% 21% 16% 4	2280
	11th to 20th	Day	44% 3 10% 21% 19% 3	2254
	21st to end	Day	46% 4 9% 22% 16% 4	2394
May	1st to 10th	Day	55% 3 14% 15% 11% 1	4120
	11th to 20th	Day	52% 3 15% 20% 10% 2	4242
	21st to end	Day	38% 4 12% 26% 18% 2	4837
June	1st to 10th	Day	45% 5% 12% 25% 11% 1	2660
	11th to 20th	Day	43% 5% 14% 23% 13% 2	2660
	21st to end	Day	42% 6% 13% 23% 14% 1	2660
July	1st to 10th	Day	45% 7% 11% 20% 15% 1	4370
	11th to 20th	Day	40% 6% 17% 23% 13% 1	4365
	21st to end	Day	48% 6% 12% 17% 16% 2	4661
August	1st to 10th	Day	50% 6% 15% 21% 8%	4224
	11th to 20th	Day	39% 6% 14% 30% 11% 1	3806
	21st to end	Day	35% 7% 13% 27% 16% 2	4180
September	1st to 10th	Day	38% 6% 12% 23% 19% 2	2280
	11th to 20th	Day	35% 4 11% 24% 24% 3	2139
	21st to end	Day	45% 5% 8% 21% 20% 3	2090
October	1st to 10th	Day	38% 5% 10% 19% 24% 5%	3452
	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	39% 4 12% 22% 21% 2	1708
	11th to 20th	Day	40% 3 11% 23% 20% 3	1710
	21st to end	Day	41% 4 13% 26% 13% 3	1696
December	1st to 10th	Day	48% 3 14% 18% 15% 1	1520
	11th to 20th	Day	44% 2 14% 19% 18% 2	1906
	21st to end	Day	54% 3 10% 23% 10% 1	2772
January	1st to 10th	Night	40% 2 10% 32% 12% 4	4429
	11th to 20th	Night	47% 3 11% 23% 13% 3	4440
	21st to end	Night	57% 2 8% 20% 11% 2	4819
February	1st to 10th	Night	46% 3 12% 24% 12% 2	2390
	11th to 20th	Night	45% 4 9% 21% 17% 4	2220
	21st to end	Night	47% 3 8% 23% 15% 4	1884
March	1st to 10th	Night	54% 2 9% 21% 11% 2	3734
	11th to 20th	Night	41% 3 15% 24% 15% 3	3697
	21st to end	Night	48% 2 6% 14% 25% 5%	3466
April	1st to 10th	Night	48% 4 10% 20% 15% 3	1900
	11th to 20th	Night	45% 3 10% 20% 20% 2	1875
	21st to end	Night	47% 3 9% 23% 14% 5%	1785
May	1st to 10th	Night	58% 3 13% 16% 9% 1	2840
	11th to 20th	Night	55% 4 14% 16% 9% 2	2672
	21st to end	Night	42% 4 15% 25% 13% 1	2739
June	1st to 10th	Night	45% 5% 11% 21% 16% 2	1520
	11th to 20th	Night	42% 5% 14% 27% 11% 1	1520
	21st to end	Night	40% 6% 13% 26% 14%	1520
July	1st to 10th	Night	45% 7% 15% 19% 12% 1	2480
	11th to 20th	Night	43% 6% 18% 23% 10% 1	2472
	21st to end	Night	50% 4 14% 15% 16% 2	2651
August	1st to 10th	Night	54% 7% 14% 15% 9% 1	2649
	11th to 20th	Night	43% 5% 14% 29% 8% 1	3083
	21st to end	Night	28% 6% 14% 33% 17% 1	3476
September	1st to 10th	Night	41% 5% 10% 26% 18% 1	1900
	11th to 20th	Night	34% 3 10% 24% 26% 3	2040
	21st to end	Night	44% 3 9% 21% 20% 3	2090
October	1st to 10th	Night	39% 5% 12% 24% 18% 2	3505
	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	37% 4 12% 23% 21% 4	2470
	11th to 20th	Night	42% 3 9% 23% 19% 3	2470
	21st to end	Night	41% 3 12% 30% 13% 2	2479
December	1st to 10th	Night	47% 4 12% 21% 14% 1	2660
	11th to 20th	Night	45% 4 13% 18% 18% 2	3288
	21st to end	Night	53% 3 9% 23% 12% 1	4884

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% 23% 32% 17% 1	180
	11th to 20th	Day	32% 28% 27% 11% 2	180
	21st to end	Day	24% 29% 32% 9% 4 2	216
February	1st to 10th	Day	31% 25% 31% 13% 1	347
	11th to 20th	Day	28% 29% 32% 9% 1	456
	21st to end	Day	31% 23% 30% 13% 3	382
March	1st to 10th	Day	28% 23% 33% 13% 2	460
	11th to 20th	Day	26% 29% 30% 12% 2 1	460
	21st to end	Day	32% 21% 33% 9% 3 1	506
April	1st to 10th	Day	30% 25% 29% 10% 5%	460
	11th to 20th	Day	31% 21% 30% 13% 5% 1	452
	21st to end	Day	29% 21% 32% 13% 3 2	460
May	1st to 10th	Day	32% 19% 29% 13% 6% 1	463
	11th to 20th	Day	25% 17% 36% 15% 6% 1	487
	21st to end	Day	20% 14% 33% 23% 9% 2	529
June	1st to 10th	Day	21% 15% 30% 22% 10% 2	490
	11th to 20th	Day	17% 17% 35% 22% 8% 1	490
	21st to end	Day	17% 14% 34% 24% 9% 2	490
July	1st to 10th	Day	18% 6% 34% 27% 13% 2	485
	11th to 20th	Day	15% 7% 33% 30% 13% 2	484
	21st to end	Day	16% 10% 38% 27% 8% 2	532
August	1st to 10th	Day	14% 9% 30% 33% 9% 4	486
	11th to 20th	Day	14% 8% 32% 27% 14% 3	479
	21st to end	Day	13% 9% 34% 25% 13% 6%	539
September	1st to 10th	Day	20% 10% 27% 22% 16% 4	490
	11th to 20th	Day	14% 11% 30% 24% 13% 8%	490
	21st to end	Day	23% 12% 34% 19% 8% 3	490
October	1st to 10th	Day	28% 12% 25% 20% 12% 4	490
	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% 15% 30% 23% 9% 2	378
	11th to 20th	Day	28% 22% 26% 19% 5%	370
	21st to end	Day	24% 21% 28% 20% 7%	356
December	1st to 10th	Day	24% 23% 32% 14% 6%	180
	11th to 20th	Day	33% 26% 31% 9% 2	180
	21st to end	Day	28% 24% 27% 17% 4 1	198
January	1st to 10th	Night	22% 22% 36% 17% 3	760
	11th to 20th	Night	28% 20% 31% 18% 3	760
	21st to end	Night	23% 22% 33% 19% 3	836
February	1st to 10th	Night	23% 20% 34% 21% 2	760
	11th to 20th	Night	24% 24% 36% 15% 1	743
	21st to end	Night	27% 21% 33% 16% 3	628
March	1st to 10th	Night	26% 21% 33% 17% 3	760
	11th to 20th	Night	23% 20% 35% 19% 3 1	760
	21st to end	Night	24% 23% 33% 17% 3	836
April	1st to 10th	Night	24% 20% 35% 19% 2	760
	11th to 20th	Night	23% 19% 34% 21% 3	751
	21st to end	Night	23% 17% 36% 18% 5%	645
May	1st to 10th	Night	25% 17% 32% 20% 6% 1	570
	11th to 20th	Night	22% 16% 36% 21% 4	485
	21st to end	Night	21% 7% 29% 35% 8% 1	411
June	1st to 10th	Night	21% 8% 29% 33% 9% 1	380
	11th to 20th	Night	20% 6% 31% 34% 8%	380
	21st to end	Night	17% 7% 32% 37% 8%	380
July	1st to 10th	Night	15% 3 30% 36% 15%	370
	11th to 20th	Night	15% 3 25% 42% 15% 1	368
	21st to end	Night	15% 7% 29% 38% 11% 1	416
August	1st to 10th	Night	12% 5% 32% 38% 12% 1	466
	11th to 20th	Night	14% 5% 24% 39% 16% 2	719
	21st to end	Night	14% 6% 25% 36% 17% 2	836
September	1st to 10th	Night	17% 5% 30% 33% 12% 2	760
	11th to 20th	Night	17% 7% 26% 30% 18% 3	760
	21st to end	Night	22% 7% 28% 28% 15% 1	760
October	1st to 10th	Night	24% 10% 27% 26% 12% 3	760
	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% 23% 10% 1	760
	11th to 20th	Night	24% 16% 33% 19% 6% 1	760
	21st to end	Night	22% 18% 31% 23% 5% 1	760
December	1st to 10th	Night	28% 17% 29% 20% 5% 1	760
	11th to 20th	Night	24% 22% 30% 21% 3	760
	21st to end	Night	27% 18% 36% 17% 2	836

Page generated at 2024-11-03 02:11:23. Version ac07

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