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 2024-12-09.

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	16% 12% 4 9% 5% 5% 4 4 5% 7% 29%	101938
	Night	15% 14% 5% 11% 6% 5% 4 3 4 5% 28%	97857
Transparency	Day	54% 7% 11% 18% 10% 1	105498
	Night	56% 6% 10% 17% 9% 1	102386
Seeing	Day	21% 13% 29% 22% 11% 4	15152
	Night	22% 12% 30% 26% 10% 1	24355

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	14% 22% 11% 19% 7% 5% 3 3 3 3 10%	2239
	11th to 20th	Day	10% 22% 9% 22% 14% 8% 3 2 2 2 6%	2751
	21st to end	Day	16% 24% 9% 17% 7% 5% 4 3 3 4 9%	3124
February	1st to 10th	Day	13% 22% 7% 18% 11% 5% 4 2 4 3 11%	1710
	11th to 20th	Day	13% 16% 9% 19% 9% 5% 4 3 5% 5% 13%	1663
	21st to end	Day	15% 21% 7% 12% 8% 5% 5% 3 4 4 15%	1519
March	1st to 10th	Day	17% 15% 5% 11% 6% 4 4 2 4 5% 26%	3362
	11th to 20th	Day	12% 16% 9% 13% 6% 5% 4 3 4 5% 24%	3480
	21st to end	Day	16% 14% 5% 10% 5% 5% 5% 4 5% 6% 26%	3815
April	1st to 10th	Day	17% 17% 5% 10% 4 4 4 3 5% 5% 25%	2190
	11th to 20th	Day	20% 13% 3 6% 3 3 3 3 5% 6% 34%	2421
	21st to end	Day	21% 12% 4 7% 4 4 3 4 4 6% 32%	2470
May	1st to 10th	Day	18% 9% 3 7% 4 3 4 4 5% 8% 34%	4108
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 4 5% 8% 33%	4256
	21st to end	Day	16% 6% 2 5% 4 4 3 3 5% 7% 45%	4837
June	1st to 10th	Day	15% 9% 3 6% 5% 4 5% 3 6% 7% 36%	2717
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 4 7% 9% 41%	2850
	21st to end	Day	18% 7% 2 6% 4 3 4 4 7% 7% 38%	2850
July	1st to 10th	Day	13% 6% 3 7% 6% 5% 6% 4 7% 9% 34%	4760
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 6% 8% 41%	4524
	21st to end	Day	10% 6% 3 6% 5% 4 5% 4 7% 9% 41%	4466
August	1st to 10th	Day	13% 7% 2 8% 5% 5% 6% 5% 6% 8% 35%	4104
	11th to 20th	Day	14% 8% 3 8% 5% 5% 5% 4 7% 9% 32%	4044
	21st to end	Day	10% 5% 2 7% 5% 5% 5% 4 7% 9% 41%	4470
September	1st to 10th	Day	15% 8% 2 8% 5% 4 4 4 6% 7% 38%	2271
	11th to 20th	Day	14% 6% 2 6% 4 4 3 3 6% 8% 44%	2090
	21st to end	Day	17% 8% 3 7% 4 4 5% 4 6% 8% 33%	2090
October	1st to 10th	Day	21% 10% 3 8% 4 4 4 3 5% 6% 33%	1914
	11th to 20th	Day	24% 14% 4 8% 6% 5% 4 3 4 6% 21%	1900
	21st to end	Day	27% 17% 4 9% 4 4 5% 3 4 5% 17%	2047
November	1st to 10th	Day	17% 20% 4 9% 5% 5% 3 3 4 5% 26%	1717
	11th to 20th	Day	17% 19% 6% 11% 7% 4 5% 4 5% 6% 18%	1600
	21st to end	Day	19% 25% 8% 10% 6% 4 4 3 4 5% 12%	1600
December	1st to 10th	Day	15% 28% 10% 16% 6% 5% 4 3 3 3 7%	1584
	11th to 20th	Day	19% 22% 8% 15% 6% 5% 3 4 4 5% 10%	1906
	21st to end	Day	25% 23% 9% 15% 5% 3 3 2 4 4 8%	2489
January	1st to 10th	Night	14% 23% 10% 21% 8% 5% 3 2 3 3 7%	4716
	11th to 20th	Night	13% 16% 9% 23% 12% 8% 4 2 3 3 6%	4206
	21st to end	Night	16% 20% 10% 16% 8% 6% 5% 3 3 3 9%	4532
February	1st to 10th	Night	13% 21% 7% 14% 11% 6% 5% 3 4 3 13%	2470
	11th to 20th	Night	13% 16% 9% 15% 8% 5% 4 3 4 5% 17%	2403
	21st to end	Night	16% 16% 6% 13% 7% 5% 4 4 5% 5% 20%	1932
March	1st to 10th	Night	13% 15% 5% 9% 5% 4 4 4 5% 5% 31%	3477
	11th to 20th	Night	10% 16% 7% 10% 5% 4 4 3 4 5% 30%	3479
	21st to end	Night	15% 11% 5% 12% 6% 6% 5% 3 4 5% 30%	3817
April	1st to 10th	Night	19% 14% 5% 10% 4 4 3 3 4 6% 28%	1990
	11th to 20th	Night	19% 12% 3 6% 4 5% 4 3 5% 6% 35%	1725
	21st to end	Night	17% 13% 3 8% 5% 5% 4 4 5% 5% 32%	1710
May	1st to 10th	Night	15% 8% 3 6% 4 4 4 4 4 7% 41%	2833
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 4 5% 6% 40%	2635
	21st to end	Night	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2738
June	1st to 10th	Night	15% 7% 3 6% 5% 4 4 3 6% 8% 39%	1458
	11th to 20th	Night	13% 5% 2 7% 5% 5% 4 5% 7% 9% 41%	1330
	21st to end	Night	14% 7% 2 6% 3 5% 3 4 6% 7% 43%	1330
July	1st to 10th	Night	8% 7% 3 7% 5% 3 4 4 6% 7% 48%	2210
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 7% 50%	2423
	21st to end	Night	8% 7% 2 7% 4 4 5% 4 6% 8% 46%	3087
August	1st to 10th	Night	12% 7% 2 7% 4 4 4 3 5% 6% 46%	2825
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 4 8% 39%	2785
	21st to end	Night	14% 6% 2 6% 4 4 4 3 5% 6% 46%	3152
September	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 6% 7% 45%	1905
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 5% 7% 49%	2090
	21st to end	Night	15% 8% 3 8% 5% 5% 4 3 4 7% 38%	2090
October	1st to 10th	Night	18% 8% 3 8% 4 4 4 3 5% 7% 36%	2265
	11th to 20th	Night	23% 13% 3 8% 5% 5% 4 3 5% 5% 26%	2280
	21st to end	Night	24% 15% 3 8% 5% 5% 4 4 5% 6% 21%	2548
November	1st to 10th	Night	18% 16% 4 8% 6% 5% 4 3 5% 5% 26%	2580
	11th to 20th	Night	17% 19% 5% 10% 5% 5% 5% 4 5% 5% 20%	2800
	21st to end	Night	21% 21% 6% 10% 5% 5% 4 3 5% 5% 15%	2800
December	1st to 10th	Night	19% 26% 9% 14% 6% 4 4 2 3 4 9%	2779
	11th to 20th	Night	15% 22% 9% 14% 6% 5% 4 4 5% 4 13%	3288
	21st to end	Night	23% 23% 8% 16% 6% 4 3 2 3 3 7%	5169

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	77% 5% 8% 7% 2	2240
	11th to 20th	Day	87% 4 3 4 2	2751
	21st to end	Day	80% 5% 7% 6% 3	3124
February	1st to 10th	Day	79% 4 4 9% 2 1	1710
	11th to 20th	Day	74% 4 6% 10% 5% 1	1672
	21st to end	Day	72% 5% 7% 11% 5%	1519
March	1st to 10th	Day	62% 5% 10% 13% 8% 1	3365
	11th to 20th	Day	63% 4 9% 16% 6% 1	3480
	21st to end	Day	56% 5% 12% 15% 9% 2	3815
April	1st to 10th	Day	61% 5% 8% 17% 9%	2190
	11th to 20th	Day	51% 5% 9% 20% 14% 1	2421
	21st to end	Day	54% 5% 10% 17% 12% 2	2470
May	1st to 10th	Day	47% 6% 13% 19% 14% 1	4120
	11th to 20th	Day	48% 8% 13% 23% 8% 1	4260
	21st to end	Day	38% 6% 12% 21% 18% 5%	4837
June	1st to 10th	Day	45% 7% 10% 22% 14% 1	2717
	11th to 20th	Day	39% 7% 11% 25% 17% 1	2850
	21st to end	Day	43% 8% 13% 23% 13% 1	2850
July	1st to 10th	Day	44% 8% 12% 18% 15% 2	4686
	11th to 20th	Day	37% 8% 12% 30% 12% 1	4443
	21st to end	Day	37% 8% 12% 23% 18% 2	4324
August	1st to 10th	Day	44% 9% 13% 21% 13% 1	4091
	11th to 20th	Day	46% 8% 15% 22% 9%	4083
	21st to end	Day	37% 9% 14% 29% 11% 1	4470
September	1st to 10th	Day	45% 8% 11% 23% 11% 2	2271
	11th to 20th	Day	38% 6% 12% 29% 13% 1	2090
	21st to end	Day	48% 8% 12% 23% 8% 1	2090
October	1st to 10th	Day	47% 8% 11% 21% 12% 1	3174
	11th to 20th	Day	50% 7% 11% 19% 11% 1	3140
	21st to end	Day	61% 6% 11% 17% 5%	3349
November	1st to 10th	Day	61% 6% 10% 16% 7%	1717
	11th to 20th	Day	66% 7% 10% 10% 7% 1	1600
	21st to end	Day	74% 5% 9% 10% 1	1600
December	1st to 10th	Day	83% 5% 5% 5% 1	1584
	11th to 20th	Day	76% 5% 9% 7% 3	1906
	21st to end	Day	82% 6% 6% 5% 1	2489
January	1st to 10th	Night	83% 4 6% 5% 2	4716
	11th to 20th	Night	84% 6% 5% 3 3	4206
	21st to end	Night	80% 5% 7% 5% 3 1	4532
February	1st to 10th	Night	76% 5% 7% 10% 2	2470
	11th to 20th	Night	69% 5% 10% 12% 5%	2405
	21st to end	Night	65% 6% 8% 13% 7%	1932
March	1st to 10th	Night	54% 6% 13% 19% 8% 1	3487
	11th to 20th	Night	54% 7% 11% 16% 9% 2	3480
	21st to end	Night	55% 4 10% 19% 11% 1	3817
April	1st to 10th	Night	58% 5% 7% 17% 11% 1	1990
	11th to 20th	Night	50% 5% 8% 19% 16% 1	1725
	21st to end	Night	54% 6% 9% 15% 15% 2	1710
May	1st to 10th	Night	42% 7% 10% 21% 15% 5%	2840
	11th to 20th	Night	43% 9% 11% 24% 12% 2	2636
	21st to end	Night	35% 6% 12% 24% 21% 3	2739
June	1st to 10th	Night	42% 6% 11% 24% 16% 2	1458
	11th to 20th	Night	38% 7% 13% 25% 16% 1	1330
	21st to end	Night	40% 7% 10% 27% 15% 2	1330
July	1st to 10th	Night	33% 6% 14% 23% 18% 5%	2175
	11th to 20th	Night	34% 5% 11% 33% 15% 1	2381
	21st to end	Night	35% 8% 13% 26% 16% 1	2988
August	1st to 10th	Night	39% 6% 11% 28% 16% 1	2814
	11th to 20th	Night	43% 8% 14% 23% 12% 1	2812
	21st to end	Night	37% 6% 11% 31% 13% 1	3152
September	1st to 10th	Night	37% 7% 11% 28% 14% 2	1905
	11th to 20th	Night	35% 5% 13% 29% 17% 1	2090
	21st to end	Night	46% 7% 12% 24% 11% 1	2090
October	1st to 10th	Night	39% 7% 11% 25% 18% 1	3785
	11th to 20th	Night	51% 5% 13% 18% 13%	3779
	21st to end	Night	52% 7% 16% 19% 6%	4196
November	1st to 10th	Night	59% 7% 10% 18% 7% 1	2580
	11th to 20th	Night	64% 6% 11% 12% 5% 2	2800
	21st to end	Night	70% 6% 11% 10% 2	2800
December	1st to 10th	Night	79% 6% 8% 5% 2	2779
	11th to 20th	Night	74% 5% 11% 9% 2	3288
	21st to end	Night	83% 5% 6% 5% 1	5169

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% 19% 36% 17% 4 1	189
	11th to 20th	Day	19% 24% 37% 14% 5%	270
	21st to end	Day	26% 16% 35% 19% 3	297
February	1st to 10th	Day	26% 24% 36% 10% 4	270
	11th to 20th	Day	24% 24% 40% 11% 1	270
	21st to end	Day	32% 29% 30% 8% 1	225
March	1st to 10th	Day	25% 18% 37% 14% 6% 1	412
	11th to 20th	Day	25% 15% 37% 17% 5% 1	460
	21st to end	Day	28% 15% 32% 18% 6% 1	506
April	1st to 10th	Day	25% 17% 34% 18% 5% 1	460
	11th to 20th	Day	31% 18% 23% 15% 9% 4	452
	21st to end	Day	28% 12% 30% 21% 7% 2	460
May	1st to 10th	Day	25% 11% 29% 24% 9% 2	463
	11th to 20th	Day	20% 11% 28% 22% 16% 3	487
	21st to end	Day	19% 9% 26% 28% 15% 4	529
June	1st to 10th	Day	17% 13% 26% 30% 12% 2	547
	11th to 20th	Day	15% 9% 32% 30% 11% 3	680
	21st to end	Day	20% 8% 34% 26% 11% 2	680
July	1st to 10th	Day	12% 8% 28% 28% 19% 5%	671
	11th to 20th	Day	14% 8% 29% 28% 14% 7%	591
	21st to end	Day	10% 5% 30% 33% 15% 7%	532
August	1st to 10th	Day	12% 8% 23% 28% 18% 10%	486
	11th to 20th	Day	16% 5% 20% 25% 22% 11%	479
	21st to end	Day	15% 6% 25% 28% 19% 8%	539
September	1st to 10th	Day	17% 10% 24% 27% 14% 9%	490
	11th to 20th	Day	11% 8% 20% 26% 24% 11%	490
	21st to end	Day	17% 13% 26% 26% 14% 4	490
October	1st to 10th	Day	23% 11% 29% 20% 14% 3	490
	11th to 20th	Day	27% 20% 29% 18% 5% 1	490
	21st to end	Day	35% 18% 26% 17% 4	496
November	1st to 10th	Day	32% 15% 30% 16% 6%	277
	11th to 20th	Day	27% 22% 37% 10% 2 3	200
	21st to end	Day	32% 22% 34% 11% 2	200
December	1st to 10th	Day	30% 26% 28% 14% 2	196
	11th to 20th	Day	29% 24% 33% 10% 2 1	180
	21st to end	Day	35% 19% 35% 10% 1	198
January	1st to 10th	Night	24% 17% 36% 18% 5% 1	950
	11th to 20th	Night	28% 18% 31% 19% 4 1	807
	21st to end	Night	19% 12% 37% 26% 6%	836
February	1st to 10th	Night	23% 19% 36% 19% 3	760
	11th to 20th	Night	21% 16% 36% 22% 4	743
	21st to end	Night	24% 19% 36% 19% 3	628
March	1st to 10th	Night	20% 16% 39% 19% 6% 1	760
	11th to 20th	Night	20% 17% 37% 20% 6% 1	760
	21st to end	Night	23% 12% 31% 25% 8% 1	836
April	1st to 10th	Night	27% 13% 32% 25% 4	760
	11th to 20th	Night	25% 13% 28% 22% 10% 1	601
	21st to end	Night	25% 10% 31% 26% 9%	570
May	1st to 10th	Night	20% 6% 29% 32% 11% 1	570
	11th to 20th	Night	19% 5% 28% 36% 11% 1	462
	21st to end	Night	14% 4 29% 37% 16%	411
June	1st to 10th	Night	18% 5% 32% 34% 10% 1	380
	11th to 20th	Night	12% 8% 28% 40% 12%	380
	21st to end	Night	18% 6% 26% 36% 14% 1	380
July	1st to 10th	Night	8% 5% 19% 51% 18% 1	370
	11th to 20th	Night	15% 3 23% 38% 20% 1	419
	21st to end	Night	12% 3 21% 43% 20% 1	624
August	1st to 10th	Night	12% 3 17% 38% 28% 1	567
	11th to 20th	Night	13% 3 16% 37% 28% 3	564
	21st to end	Night	17% 3 23% 30% 26% 2	642
September	1st to 10th	Night	15% 6% 24% 32% 18% 5%	760
	11th to 20th	Night	15% 3 23% 34% 20% 5%	760
	21st to end	Night	20% 7% 23% 31% 15% 4	760
October	1st to 10th	Night	22% 8% 29% 26% 14% 1	760
	11th to 20th	Night	29% 12% 26% 24% 8% 1	760
	21st to end	Night	31% 13% 32% 20% 3	836
November	1st to 10th	Night	28% 13% 30% 23% 7% 1	780
	11th to 20th	Night	27% 18% 30% 19% 5% 1	800
	21st to end	Night	29% 16% 34% 19% 3	800
December	1st to 10th	Night	31% 17% 32% 15% 5%	795
	11th to 20th	Night	23% 20% 39% 15% 3	760
	21st to end	Night	32% 16% 34% 16% 2	1004

Page generated at 2024-12-06 03:54:50. Version ac07

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