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 2024-01-12.

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% 7% 2 5% 5% 3 4 4 5% 7% 40%	74781
	Night	18% 7% 3 5% 4 3 4 4 5% 7% 41%	74923
Transparency	Day	43% 4 12% 23% 16% 2	74486
	Night	44% 4 11% 23% 16% 2	74752
Seeing	Day	23% 17% 31% 19% 8% 2	13908
	Night	22% 15% 31% 24% 7% 1	23051

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% 7% 3 5% 5% 2 5% 4 6% 5% 44%	1517
	11th to 20th	Day	17% 9% 3 4 5% 3 5% 6% 6% 6% 36%	1448
	21st to end	Day	15% 7% 3 5% 4 3 4 5% 5% 7% 43%	1620
February	1st to 10th	Day	19% 8% 2 5% 4 3 5% 5% 5% 6% 36%	1693
	11th to 20th	Day	15% 7% 3 5% 5% 3 4 4 5% 7% 41%	1748
	21st to end	Day	21% 9% 3 5% 5% 3 4 4 4 5% 37%	1480
March	1st to 10th	Day	16% 7% 3 4 5% 3 4 4 4 8% 42%	1797
	11th to 20th	Day	16% 7% 3 5% 6% 3 5% 5% 5% 8% 38%	1844
	21st to end	Day	20% 8% 4 6% 4 3 4 3 4 5% 39%	2376
April	1st to 10th	Day	19% 8% 3 4 4 2 4 4 5% 7% 39%	2160
	11th to 20th	Day	19% 6% 2 5% 4 2 4 4 5% 7% 42%	2134
	21st to end	Day	23% 4 2 4 4 3 5% 4 5% 6% 39%	2268
May	1st to 10th	Day	25% 7% 3 5% 4 3 3 4 5% 7% 34%	2340
	11th to 20th	Day	19% 6% 2 5% 5% 2 4 4 4 8% 40%	2388
	21st to end	Day	17% 6% 2 4 4 3 4 4 5% 7% 43%	2725
June	1st to 10th	Day	19% 5% 2 5% 5% 4 5% 5% 7% 8% 35%	2520
	11th to 20th	Day	17% 7% 3 5% 5% 3 5% 5% 6% 9% 36%	2520
	21st to end	Day	14% 6% 2 6% 5% 3 5% 4 6% 9% 38%	2520
July	1st to 10th	Day	14% 5% 2 5% 6% 4 5% 4 6% 8% 40%	2520
	11th to 20th	Day	13% 5% 3 4 5% 2 6% 6% 7% 10% 40%	2520
	21st to end	Day	13% 6% 2 5% 5% 3 6% 6% 7% 9% 39%	2744
August	1st to 10th	Day	12% 6% 2 5% 6% 4 6% 6% 8% 10% 36%	2404
	11th to 20th	Day	12% 6% 2 5% 6% 3 5% 4 6% 9% 41%	2152
	21st to end	Day	13% 6% 2 6% 5% 3 4 5% 7% 8% 42%	2376
September	1st to 10th	Day	15% 6% 2 4 5% 3 5% 5% 5% 6% 44%	2160
	11th to 20th	Day	13% 6% 2 4 4 3 3 4 4 6% 50%	2027
	21st to end	Day	19% 5% 3 5% 4 3 5% 4 5% 6% 43%	1980
October	1st to 10th	Day	22% 7% 2 5% 4 2 4 5% 4 5% 40%	1966
	11th to 20th	Day	15% 5% 2 4 5% 3 4 5% 4 8% 46%	1777
	21st to end	Day	20% 7% 4 4 5% 2 4 5% 5% 6% 40%	1782
November	1st to 10th	Day	14% 7% 2 5% 4 3 4 4 3 5% 48%	1671
	11th to 20th	Day	20% 6% 2 4 3 2 4 3 4 6% 47%	1710
	21st to end	Day	16% 6% 3 5% 4 2 4 4 5% 6% 45%	1696
December	1st to 10th	Day	20% 7% 2 5% 5% 4 5% 4 5% 5% 38%	1520
	11th to 20th	Day	22% 7% 2 4 4 2 3 2 5% 5% 44%	1906
	21st to end	Day	19% 12% 4 6% 5% 3 4 3 4 6% 33%	2772
January	1st to 10th	Night	16% 7% 2 4 5% 2 4 4 5% 6% 45%	2649
	11th to 20th	Night	21% 8% 3 5% 4 3 4 3 4 5% 39%	2541
	21st to end	Night	14% 8% 4 5% 3 2 4 4 4 7% 44%	2735
February	1st to 10th	Night	18% 10% 3 6% 5% 3 3 3 4 7% 39%	2264
	11th to 20th	Night	17% 8% 4 5% 5% 2 4 4 5% 7% 39%	2098
	21st to end	Night	21% 8% 3 5% 4 3 4 5% 4 5% 39%	1776
March	1st to 10th	Night	17% 7% 2 4 4 3 3 4 5% 7% 43%	2150
	11th to 20th	Night	17% 7% 2 5% 5% 3 5% 4 5% 7% 39%	2112
	21st to end	Night	19% 7% 3 5% 4 2 4 3 5% 6% 42%	1980
April	1st to 10th	Night	19% 7% 4 6% 5% 3 5% 4 6% 6% 37%	1800
	11th to 20th	Night	19% 6% 2 5% 5% 3 4 4 4 7% 42%	1775
	21st to end	Night	21% 7% 3 6% 5% 2 3 4 4 5% 39%	1692
May	1st to 10th	Night	22% 7% 3 5% 6% 3 5% 5% 4 8% 31%	1620
	11th to 20th	Night	19% 7% 3 5% 4 3 4 5% 5% 8% 37%	1521
	21st to end	Night	18% 6% 3 5% 4 2 4 5% 5% 8% 39%	1550
June	1st to 10th	Night	18% 6% 3 6% 5% 3 5% 4 4 7% 39%	1440
	11th to 20th	Night	17% 6% 2 5% 6% 3 4 4 5% 9% 39%	1440
	21st to end	Night	12% 7% 3 5% 4 3 5% 4 6% 9% 42%	1440
July	1st to 10th	Night	13% 6% 2 5% 5% 4 5% 5% 7% 9% 39%	1440
	11th to 20th	Night	14% 5% 3 5% 5% 3 5% 6% 7% 9% 37%	1440
	21st to end	Night	12% 6% 2 5% 6% 3 5% 5% 7% 9% 40%	1574
August	1st to 10th	Night	9% 5% 3 6% 6% 4 5% 6% 7% 9% 41%	1515
	11th to 20th	Night	11% 6% 3 7% 7% 4 6% 7% 5% 8% 37%	1744
	21st to end	Night	14% 6% 2 4 4 3 5% 4 4 8% 46%	1980
September	1st to 10th	Night	14% 6% 3 5% 6% 3 4 5% 5% 6% 43%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 4 2 4 6% 53%	1932
	21st to end	Night	17% 8% 2 5% 4 2 4 5% 5% 5% 43%	1980
October	1st to 10th	Night	21% 6% 3 5% 5% 2 4 4 4 5% 41%	1991
	11th to 20th	Night	16% 6% 2 5% 4 2 3 4 4 6% 47%	2182
	21st to end	Night	22% 8% 2 5% 4 3 3 4 4 5% 40%	2574
November	1st to 10th	Night	14% 5% 2 5% 5% 2 4 4 4 6% 48%	2407
	11th to 20th	Night	20% 6% 2 4 3 2 4 4 4 6% 45%	2470
	21st to end	Night	17% 5% 2 5% 4 3 4 4 4 7% 45%	2479
December	1st to 10th	Night	21% 7% 3 5% 4 3 4 4 5% 5% 39%	2660
	11th to 20th	Night	21% 7% 2 4 4 3 4 4 4 5% 41%	3288
	21st to end	Night	22% 10% 3 6% 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	41% 3 10% 28% 14% 4	1518
	11th to 20th	Day	47% 6% 14% 18% 13% 2	1448
	21st to end	Day	41% 4 10% 25% 16% 3	1620
February	1st to 10th	Day	48% 4 12% 20% 14% 2	1693
	11th to 20th	Day	43% 4 10% 24% 16% 3	1757
	21st to end	Day	50% 4 9% 20% 14% 3	1480
March	1st to 10th	Day	44% 3 12% 23% 14% 4	1800
	11th to 20th	Day	45% 4 11% 24% 14% 3	1844
	21st to end	Day	50% 3 7% 18% 18% 5%	2376
April	1st to 10th	Day	45% 4 10% 21% 16% 4	2160
	11th to 20th	Day	43% 3 10% 22% 19% 3	2134
	21st to end	Day	46% 4 9% 22% 16% 4	2268
May	1st to 10th	Day	50% 3 9% 21% 14% 2	2340
	11th to 20th	Day	44% 4 11% 21% 17% 3	2392
	21st to end	Day	40% 6% 17% 22% 14% 2	2725
June	1st to 10th	Day	46% 5% 13% 24% 10% 1	2520
	11th to 20th	Day	45% 5% 13% 22% 13% 1	2520
	21st to end	Day	42% 6% 13% 24% 13% 1	2520
July	1st to 10th	Day	42% 5% 11% 22% 17% 2	2450
	11th to 20th	Day	39% 5% 15% 27% 13% 2	2445
	21st to end	Day	40% 6% 14% 21% 18% 3	2591
August	1st to 10th	Day	40% 7% 15% 25% 13%	2390
	11th to 20th	Day	40% 5% 16% 24% 15% 1	2152
	21st to end	Day	40% 5% 11% 23% 19% 3	2376
September	1st to 10th	Day	39% 6% 12% 22% 18% 2	2160
	11th to 20th	Day	35% 4 11% 24% 24% 3	2027
	21st to end	Day	43% 4 8% 22% 21% 3	1980
October	1st to 10th	Day	46% 4 8% 23% 16% 3	1966
	11th to 20th	Day	38% 3 12% 29% 17% 1	1777
	21st to end	Day	46% 4 9% 21% 17% 3	1782
November	1st to 10th	Day	39% 4 12% 22% 20% 2	1671
	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% 3 12% 26% 16% 3	2649
	11th to 20th	Night	49% 3 13% 22% 10% 2	2541
	21st to end	Night	41% 3 10% 24% 20% 3	2735
February	1st to 10th	Night	48% 3 12% 23% 12% 2	2264
	11th to 20th	Night	45% 4 9% 21% 16% 4	2100
	21st to end	Night	48% 3 7% 23% 15% 4	1776
March	1st to 10th	Night	42% 3 12% 24% 16% 4	2160
	11th to 20th	Night	45% 4 11% 22% 16% 4	2113
	21st to end	Night	44% 3 8% 20% 19% 5%	1980
April	1st to 10th	Night	47% 4 10% 20% 15% 3	1800
	11th to 20th	Night	44% 3 10% 20% 20% 2	1775
	21st to end	Night	48% 3 9% 22% 13% 5%	1692
May	1st to 10th	Night	52% 5% 10% 18% 13% 2	1620
	11th to 20th	Night	46% 5% 11% 20% 15% 3	1522
	21st to end	Night	43% 5% 18% 20% 13% 1	1551
June	1st to 10th	Night	46% 5% 11% 20% 16% 2	1440
	11th to 20th	Night	44% 5% 13% 26% 11% 1	1440
	21st to end	Night	39% 6% 14% 27% 14%	1440
July	1st to 10th	Night	41% 6% 16% 21% 15% 2	1400
	11th to 20th	Night	41% 5% 17% 27% 9% 1	1392
	21st to end	Night	39% 4 16% 24% 14% 2	1486
August	1st to 10th	Night	37% 6% 18% 22% 15% 2	1505
	11th to 20th	Night	45% 4 14% 25% 11% 1	1744
	21st to end	Night	39% 4 11% 26% 19% 1	1980
September	1st to 10th	Night	42% 5% 10% 26% 17% 1	1800
	11th to 20th	Night	35% 3 10% 23% 26% 3	1932
	21st to end	Night	43% 3 8% 22% 21% 3	1980
October	1st to 10th	Night	47% 4 9% 23% 14% 3	1991
	11th to 20th	Night	39% 3 12% 29% 16% 1	2182
	21st to end	Night	47% 3 11% 22% 15% 1	2574
November	1st to 10th	Night	37% 4 12% 22% 21% 4	2407
	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	31% 29% 27% 11% 2	162
	21st to end	Day	23% 32% 31% 9% 3 2	192
February	1st to 10th	Day	34% 23% 29% 13% 1	313
	11th to 20th	Day	28% 30% 32% 9% 1	416
	21st to end	Day	31% 24% 30% 12% 3	346
March	1st to 10th	Day	26% 24% 34% 14% 2	420
	11th to 20th	Day	26% 29% 31% 11% 3 1	420
	21st to end	Day	31% 21% 33% 10% 4 1	462
April	1st to 10th	Day	29% 25% 30% 10% 5%	420
	11th to 20th	Day	29% 21% 31% 14% 5% 1	412
	21st to end	Day	30% 21% 32% 12% 4 1	420
May	1st to 10th	Day	33% 19% 30% 12% 6% 1	423
	11th to 20th	Day	25% 19% 36% 14% 5% 1	447
	21st to end	Day	21% 14% 31% 24% 9% 2	485
June	1st to 10th	Day	21% 16% 29% 23% 10% 2	450
	11th to 20th	Day	19% 16% 35% 22% 7% 1	450
	21st to end	Day	16% 12% 34% 26% 10% 2	450
July	1st to 10th	Day	17% 7% 33% 28% 13% 2	445
	11th to 20th	Day	16% 6% 30% 31% 14% 3	444
	21st to end	Day	15% 9% 39% 28% 8% 2	488
August	1st to 10th	Day	13% 8% 30% 35% 10% 4	446
	11th to 20th	Day	15% 8% 32% 27% 14% 4	439
	21st to end	Day	14% 9% 34% 25% 12% 7%	495
September	1st to 10th	Day	21% 10% 27% 21% 17% 4	450
	11th to 20th	Day	15% 12% 31% 24% 11% 7%	450
	21st to end	Day	22% 12% 34% 18% 9% 4	450
October	1st to 10th	Day	29% 11% 25% 20% 11% 4	450
	11th to 20th	Day	16% 20% 31% 24% 8% 1	438
	21st to end	Day	27% 17% 31% 17% 7% 1	396
November	1st to 10th	Day	21% 14% 30% 23% 9% 2	365
	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% 19% 3	727
	21st to end	Night	22% 23% 33% 20% 3	792
February	1st to 10th	Night	23% 20% 34% 21% 2	720
	11th to 20th	Night	24% 23% 35% 16% 1	703
	21st to end	Night	27% 21% 34% 16% 2	592
March	1st to 10th	Night	25% 21% 33% 18% 3	720
	11th to 20th	Night	23% 19% 35% 19% 3 1	720
	21st to end	Night	23% 23% 34% 18% 3	792
April	1st to 10th	Night	25% 20% 34% 19% 3	720
	11th to 20th	Night	23% 18% 34% 22% 4	711
	21st to end	Night	24% 17% 35% 19% 5%	612
May	1st to 10th	Night	25% 17% 32% 20% 6% 1	540
	11th to 20th	Night	23% 15% 37% 21% 4	460
	21st to end	Night	21% 7% 29% 35% 8% 1	389
June	1st to 10th	Night	21% 7% 30% 32% 9% 1	360
	11th to 20th	Night	21% 6% 30% 34% 9%	360
	21st to end	Night	16% 4 33% 38% 8%	360
July	1st to 10th	Night	15% 3 29% 37% 16%	350
	11th to 20th	Night	14% 2 24% 43% 16% 1	348
	21st to end	Night	14% 6% 29% 39% 11% 1	394
August	1st to 10th	Night	10% 5% 34% 39% 12% 1	441
	11th to 20th	Night	14% 5% 23% 40% 16% 1	680
	21st to end	Night	15% 7% 24% 36% 16% 2	792
September	1st to 10th	Night	18% 5% 30% 34% 12% 2	720
	11th to 20th	Night	18% 7% 26% 29% 18% 2	720
	21st to end	Night	21% 7% 28% 28% 15% 1	720
October	1st to 10th	Night	25% 10% 26% 25% 12% 3	720
	11th to 20th	Night	20% 14% 30% 28% 8%	720
	21st to end	Night	30% 11% 33% 20% 6%	792
November	1st to 10th	Night	17% 14% 34% 24% 10% 1	740
	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 2024-01-09 07:14:40. Version ac07

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