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 2024-11-03.

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%	100681
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	98250
Transparency	Day	39% 7% 12% 21% 17% 4	100493
	Night	39% 6% 11% 20% 19% 5%	98108
Seeing	Day	17% 11% 26% 24% 17% 5%	15311
	Night	16% 10% 25% 31% 16% 2	25611

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	18% 6% 1 5% 2 2 3 2 4 5% 52%	2836
	11th to 20th	Day	14% 7% 3 5% 4 2 3 3 4 5% 52%	2840
	21st to end	Day	18% 10% 3 6% 3 4 4 3 4 5% 41%	3124
February	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 6% 6% 44%	1710
	11th to 20th	Day	22% 6% 1 4 4 3 3 3 4 4 46%	1669
	21st to end	Day	18% 8% 2 7% 4 3 3 3 5% 7% 40%	1712
March	1st to 10th	Day	23% 7% 2 5% 3 4 4 4 5% 7% 37%	3435
	11th to 20th	Day	10% 5% 2 5% 3 3 4 2 5% 7% 54%	3480
	21st to end	Day	19% 7% 2 5% 4 3 4 3 4 5% 43%	3815
April	1st to 10th	Day	11% 4 1 3 4 3 4 3 5% 7% 54%	2208
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 8% 48%	2254
	21st to end	Day	14% 5% 2 5% 4 4 4 3 5% 9% 45%	2280
May	1st to 10th	Day	10% 7% 3 7% 5% 6% 6% 5% 7% 9% 34%	4065
	11th to 20th	Day	17% 6% 2 7% 5% 5% 5% 4 6% 9% 33%	4081
	21st to end	Day	7% 4 2 6% 4 5% 5% 4 6% 9% 49%	4476
June	1st to 10th	Day	9% 5% 1 6% 5% 5% 6% 5% 8% 10% 40%	2470
	11th to 20th	Day	8% 5% 2 6% 5% 4 4 5% 6% 11% 44%	2470
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 9% 11% 39%	2470
July	1st to 10th	Day	6% 5% 2 8% 5% 6% 6% 6% 8% 10% 37%	4120
	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% 3 3 3 2 4 6% 55%	1877
	11th to 20th	Day	14% 4 2 5% 4 2 3 3 5% 6% 52%	1884
	21st to end	Day	14% 5% 2 4 4 4 3 3 5% 6% 50%	1585
December	1st to 10th	Day	18% 7% 2 6% 3 4 4 3 5% 6% 42%	1520
	11th to 20th	Day	15% 4 2 4 3 2 2 3 4 5% 56%	2014
	21st to end	Day	17% 6% 2 6% 3 4 4 3 5% 8% 42%	3124
January	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 47%	4109
	11th to 20th	Night	14% 8% 2 6% 3 3 4 3 4 6% 47%	4120
	21st to end	Night	19% 7% 2 5% 3 3 4 3 4 5% 44%	4532
February	1st to 10th	Night	17% 5% 2 5% 4 3 3 3 6% 6% 45%	2470
	11th to 20th	Night	20% 7% 3 5% 4 3 3 3 4 5% 43%	2395
	21st to end	Night	15% 6% 1 6% 3 4 4 3 5% 6% 45%	1739
March	1st to 10th	Night	22% 9% 3 7% 4 4 4 3 4 4 34%	3418
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 4 6% 52%	3480
	21st to end	Night	18% 5% 1 4 3 3 3 3 5% 6% 49%	3817
April	1st to 10th	Night	13% 6% 2 4 4 4 4 3 5% 8% 48%	1971
	11th to 20th	Night	12% 5% 2 6% 4 4 2 3 5% 7% 50%	1875
	21st to end	Night	12% 5% 2 5% 4 5% 5% 4 5% 6% 48%	1900
May	1st to 10th	Night	12% 8% 2 7% 5% 4 5% 4 6% 8% 39%	2859
	11th to 20th	Night	20% 8% 3 8% 4 4 4 3 4 6% 37%	2812
	21st to end	Night	8% 5% 2 5% 3 4 5% 4 6% 11% 47%	3079
June	1st to 10th	Night	10% 5% 3 5% 4 6% 5% 5% 7% 11% 39%	1710
	11th to 20th	Night	9% 6% 2 7% 5% 4 5% 5% 7% 10% 41%	1710
	21st to end	Night	8% 5% 2 7% 4 5% 4 5% 9% 11% 40%	1710
July	1st to 10th	Night	7% 7% 3 7% 5% 5% 5% 6% 9% 10% 35%	2840
	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 3 3 3 5% 53%	2246
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 5% 50%	2294
	21st to end	Night	14% 4 2 5% 3 3 4 3 5% 6% 51%	2588
December	1st to 10th	Night	17% 6% 3 7% 5% 4 4 4 3 5% 42%	2660
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 4 5% 48%	3162
	21st to end	Night	17% 7% 2 5% 3 3 3 3 4 7% 46%	4532

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	37% 3 5% 27% 22% 6%	2837
	11th to 20th	Day	36% 3 6% 18% 31% 5%	2840
	21st to end	Day	46% 5% 6% 17% 20% 6%	3124
February	1st to 10th	Day	40% 4 12% 20% 20% 5%	1710
	11th to 20th	Day	42% 4 6% 18% 22% 7%	1678
	21st to end	Day	45% 5% 10% 16% 19% 6%	1712
March	1st to 10th	Day	46% 6% 8% 18% 14% 8%	3435
	11th to 20th	Day	30% 5% 8% 19% 25% 13%	3480
	21st to end	Day	44% 5% 7% 21% 19% 4	3815
April	1st to 10th	Day	29% 6% 11% 24% 23% 9%	2208
	11th to 20th	Day	34% 5% 12% 21% 21% 8%	2254
	21st to end	Day	37% 6% 12% 23% 18% 4	2280
May	1st to 10th	Day	43% 10% 16% 18% 12% 2	4103
	11th to 20th	Day	45% 8% 11% 23% 11% 1	4085
	21st to end	Day	30% 8% 16% 23% 20% 4	4489
June	1st to 10th	Day	36% 10% 16% 23% 15% 1	2470
	11th to 20th	Day	33% 9% 17% 24% 16% 2	2470
	21st to end	Day	34% 10% 17% 22% 14% 2	2470
July	1st to 10th	Day	39% 10% 18% 17% 13% 3	4055
	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	36% 4 10% 22% 23% 4	1900
	11th to 20th	Day	29% 4 8% 27% 22% 9%	1900
	21st to end	Day	31% 4 7% 24% 25% 9%	2090
November	1st to 10th	Day	33% 4 8% 22% 27% 8%	1877
	11th to 20th	Day	33% 3 8% 22% 26% 7%	1884
	21st to end	Day	34% 4 9% 22% 25% 6%	1585
December	1st to 10th	Day	43% 4 11% 21% 17% 4	1520
	11th to 20th	Day	32% 3 8% 26% 23% 9%	2011
	21st to end	Day	39% 5% 11% 23% 19% 2	3124
January	1st to 10th	Night	37% 4 7% 21% 25% 5%	4109
	11th to 20th	Night	39% 4 7% 18% 26% 6%	4120
	21st to end	Night	43% 4 5% 19% 22% 7%	4532
February	1st to 10th	Night	39% 4 10% 21% 21% 5%	2470
	11th to 20th	Night	44% 4 8% 15% 21% 8%	2397
	21st to end	Night	38% 5% 9% 19% 20% 8%	1739
March	1st to 10th	Night	52% 6% 8% 14% 16% 5%	3418
	11th to 20th	Night	34% 6% 7% 14% 23% 17%	3480
	21st to end	Night	37% 5% 7% 23% 22% 6%	3817
April	1st to 10th	Night	35% 5% 10% 17% 25% 8%	1971
	11th to 20th	Night	34% 5% 9% 23% 21% 8%	1875
	21st to end	Night	35% 7% 11% 24% 18% 5%	1900
May	1st to 10th	Night	41% 8% 16% 20% 13% 2	2886
	11th to 20th	Night	47% 7% 10% 20% 16% 1	2813
	21st to end	Night	29% 9% 17% 26% 17% 2	3087
June	1st to 10th	Night	37% 10% 16% 21% 15% 1	1710
	11th to 20th	Night	36% 9% 16% 20% 16% 2	1710
	21st to end	Night	34% 12% 17% 21% 14% 3	1710
July	1st to 10th	Night	41% 12% 17% 17% 12% 2	2795
	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	38% 5% 9% 21% 23% 5%	2280
	11th to 20th	Night	34% 4 8% 22% 24% 9%	2280
	21st to end	Night	31% 4 8% 22% 25% 11%	2508
November	1st to 10th	Night	35% 4 8% 18% 26% 8%	2246
	11th to 20th	Night	35% 4 9% 16% 24% 12%	2294
	21st to end	Night	33% 4 8% 22% 26% 7%	2588
December	1st to 10th	Night	45% 5% 8% 19% 19% 4	2660
	11th to 20th	Night	38% 4 8% 21% 22% 8%	3164
	21st to end	Night	39% 4 7% 24% 23% 2	4532

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% 23% 35% 13% 3	270
	11th to 20th	Day	27% 24% 33% 12% 3	270
	21st to end	Day	20% 23% 44% 11% 2	297
February	1st to 10th	Day	21% 20% 40% 16% 4	270
	11th to 20th	Day	38% 21% 32% 8% 2	276
	21st to end	Day	25% 20% 34% 18% 3	382
March	1st to 10th	Day	24% 18% 33% 18% 7%	460
	11th to 20th	Day	21% 21% 40% 12% 5% 1	460
	21st to end	Day	23% 18% 37% 17% 5%	506
April	1st to 10th	Day	21% 18% 35% 17% 7% 1	460
	11th to 20th	Day	14% 19% 31% 22% 9% 4	452
	21st to end	Day	19% 10% 31% 26% 12% 1	460
May	1st to 10th	Day	17% 18% 29% 23% 9% 3	463
	11th to 20th	Day	16% 10% 23% 29% 18% 4	487
	21st to end	Day	16% 6% 21% 27% 22% 7%	529
June	1st to 10th	Day	11% 5% 26% 31% 21% 6%	490
	11th to 20th	Day	8% 7% 23% 29% 26% 8%	490
	21st to end	Day	10% 3 21% 35% 25% 6%	490
July	1st to 10th	Day	10% 1 12% 30% 37% 9%	485
	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	17% 11% 29% 27% 14% 3	463
	11th to 20th	Day	18% 15% 28% 25% 12% 1	444
	21st to end	Day	15% 18% 30% 21% 13% 2	210
December	1st to 10th	Day	22% 20% 39% 11% 7% 1	180
	11th to 20th	Day	19% 17% 32% 23% 8%	211
	21st to end	Day	24% 17% 31% 16% 10% 1	297
January	1st to 10th	Night	20% 20% 31% 25% 4	760
	11th to 20th	Night	19% 18% 32% 25% 6%	760
	21st to end	Night	19% 16% 37% 25% 3	836
February	1st to 10th	Night	19% 15% 35% 27% 3	760
	11th to 20th	Night	24% 20% 36% 19% 1	743
	21st to end	Night	19% 16% 38% 24% 4	628
March	1st to 10th	Night	16% 17% 33% 28% 6%	760
	11th to 20th	Night	20% 16% 34% 21% 8%	760
	21st to end	Night	19% 13% 40% 24% 4	836
April	1st to 10th	Night	17% 18% 33% 25% 6%	760
	11th to 20th	Night	15% 14% 34% 26% 11%	751
	21st to end	Night	14% 10% 33% 35% 8%	760
May	1st to 10th	Night	14% 11% 33% 33% 9%	603
	11th to 20th	Night	17% 7% 24% 33% 18% 1	561
	21st to end	Night	12% 3 16% 41% 26% 1	617
June	1st to 10th	Night	12% 2 19% 42% 23% 2	570
	11th to 20th	Night	10% 2 18% 39% 26% 4	570
	21st to end	Night	8% 3 17% 42% 28% 2	570
July	1st to 10th	Night	8% 1 13% 37% 37% 4	555
	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% 32% 14% 2	749
	11th to 20th	Night	15% 12% 28% 33% 11%	760
	21st to end	Night	15% 13% 27% 30% 14% 1	760
December	1st to 10th	Night	21% 13% 29% 29% 7%	760
	11th to 20th	Night	18% 11% 32% 32% 7%	760
	21st to end	Night	21% 15% 26% 30% 7%	836

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