Craven E Williams Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Craven E Williams Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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	14% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	74939
	Night	15% 5% 2 5% 5% 3 4 5% 5% 7% 44%	74873
Transparency	Day	38% 7% 12% 20% 18% 5%	74662
	Night	39% 7% 11% 19% 19% 5%	74669
Seeing	Day	17% 8% 21% 25% 19% 10%	14056
	Night	17% 6% 23% 31% 20% 3	24193

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% 5% 2 4 4 2 4 4 5% 5% 47%	1706
	11th to 20th	Day	18% 7% 2 4 3 3 4 4 4 5% 45%	1665
	21st to end	Day	18% 5% 2 3 3 3 3 4 5% 6% 49%	1782
February	1st to 10th	Day	16% 6% 2 4 4 3 4 5% 6% 7% 43%	1644
	11th to 20th	Day	21% 5% 2 4 3 2 4 4 4 5% 45%	1748
	21st to end	Day	20% 6% 3 5% 6% 2 4 3 4 6% 40%	1484
March	1st to 10th	Day	16% 5% 2 4 4 2 3 3 4 6% 51%	1980
	11th to 20th	Day	18% 5% 3 5% 5% 3 4 5% 6% 7% 40%	1980
	21st to end	Day	18% 5% 2 5% 5% 3 4 4 5% 7% 43%	2178
April	1st to 10th	Day	12% 3 2 4 4 3 4 5% 6% 8% 50%	1980
	11th to 20th	Day	15% 5% 2 5% 4 3 4 5% 5% 8% 47%	2108
	21st to end	Day	13% 4 3 4 5% 2 4 4 5% 8% 48%	2340
May	1st to 10th	Day	9% 5% 3 5% 5% 3 5% 5% 6% 9% 46%	2340
	11th to 20th	Day	14% 5% 2 5% 5% 4 6% 7% 6% 8% 39%	2301
	21st to end	Day	12% 5% 2 5% 5% 3 4 5% 7% 10% 41%	2531
June	1st to 10th	Day	10% 6% 2 6% 5% 4 6% 6% 8% 12% 35%	2340
	11th to 20th	Day	8% 4 2 6% 5% 4 6% 7% 7% 11% 41%	2340
	21st to end	Day	8% 4 2 5% 6% 4 7% 7% 8% 10% 40%	2340
July	1st to 10th	Day	8% 5% 3 6% 8% 5% 7% 7% 8% 11% 33%	2340
	11th to 20th	Day	8% 5% 3 6% 6% 5% 7% 8% 9% 12% 32%	2340
	21st to end	Day	8% 4 2 6% 6% 4 7% 7% 8% 11% 35%	2547
August	1st to 10th	Day	11% 6% 2 7% 7% 5% 6% 7% 7% 11% 31%	2327
	11th to 20th	Day	12% 7% 3 6% 6% 4 6% 7% 7% 10% 30%	2303
	21st to end	Day	10% 5% 3 6% 6% 4 6% 6% 7% 11% 37%	2414
September	1st to 10th	Day	10% 5% 3 5% 6% 4 6% 5% 7% 8% 43%	1980
	11th to 20th	Day	13% 6% 2 5% 5% 4 4 4 6% 8% 43%	1980
	21st to end	Day	17% 6% 2 5% 5% 3 4 5% 4 6% 43%	1980
October	1st to 10th	Day	15% 5% 1 3 4 2 2 3 3 6% 56%	1905
	11th to 20th	Day	14% 5% 1 4 3 2 3 3 4 5% 58%	1800
	21st to end	Day	14% 4 1 4 4 2 3 3 4 4 56%	1980
November	1st to 10th	Day	13% 5% 2 4 3 1 3 3 4 5% 56%	1849
	11th to 20th	Day	14% 4 2 4 4 2 4 4 4 6% 53%	1710
	21st to end	Day	17% 4 1 3 4 3 4 4 5% 5% 51%	1710
December	1st to 10th	Day	20% 7% 2 5% 3 2 4 4 5% 6% 42%	1710
	11th to 20th	Day	17% 5% 2 3 4 1 3 3 5% 6% 51%	2153
	21st to end	Day	19% 6% 2 4 4 2 4 3 5% 6% 44%	3124
January	1st to 10th	Night	18% 4 2 5% 4 3 3 4 4 5% 49%	2459
	11th to 20th	Night	18% 5% 2 5% 5% 3 4 3 4 5% 45%	2417
	21st to end	Night	16% 6% 2 4 3 2 4 3 4 5% 51%	2574
February	1st to 10th	Night	17% 5% 2 4 4 3 4 4 4 6% 47%	2313
	11th to 20th	Night	20% 5% 2 5% 5% 3 4 3 4 5% 44%	2098
	21st to end	Night	17% 7% 2 6% 4 4 4 4 5% 5% 42%	1770
March	1st to 10th	Night	14% 5% 2 5% 4 2 3 4 4 6% 49%	1980
	11th to 20th	Night	16% 6% 3 4 5% 3 5% 5% 5% 6% 42%	1980
	21st to end	Night	16% 6% 3 5% 5% 3 4 4 6% 6% 42%	2178
April	1st to 10th	Night	13% 4 2 4 4 2 5% 4 5% 6% 50%	1980
	11th to 20th	Night	13% 5% 2 4 4 3 3 4 4 7% 51%	1812
	21st to end	Night	13% 4 2 5% 4 3 4 4 5% 7% 50%	1620
May	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 5% 4 7% 46%	1620
	11th to 20th	Night	14% 5% 2 6% 8% 4 6% 5% 6% 8% 36%	1592
	21st to end	Night	13% 6% 3 6% 5% 3 5% 5% 6% 8% 40%	1744
June	1st to 10th	Night	11% 5% 3 6% 5% 4 7% 7% 8% 11% 33%	1620
	11th to 20th	Night	11% 6% 3 5% 7% 3 6% 5% 7% 10% 38%	1620
	21st to end	Night	6% 5% 2 6% 8% 4 7% 7% 7% 12% 36%	1620
July	1st to 10th	Night	9% 5% 3 8% 7% 6% 6% 8% 8% 11% 29%	1620
	11th to 20th	Night	9% 5% 3 5% 6% 4 7% 8% 8% 12% 34%	1620
	21st to end	Night	7% 5% 3 6% 7% 4 8% 8% 10% 12% 31%	1771
August	1st to 10th	Night	11% 7% 3 8% 6% 3 6% 6% 9% 11% 30%	1611
	11th to 20th	Night	11% 7% 3 7% 6% 4 6% 7% 7% 11% 31%	1598
	21st to end	Night	10% 6% 3 6% 7% 5% 5% 7% 8% 11% 32%	1918
September	1st to 10th	Night	10% 6% 2 5% 6% 3 4 6% 6% 9% 42%	1980
	11th to 20th	Night	14% 7% 2 6% 5% 3 5% 5% 5% 9% 37%	1980
	21st to end	Night	15% 7% 2 6% 5% 2 4 4 5% 7% 42%	1980
October	1st to 10th	Night	17% 4 2 4 3 2 4 3 4 6% 51%	2053
	11th to 20th	Night	14% 5% 2 4 3 3 3 4 4 6% 52%	2160
	21st to end	Night	14% 4 2 4 3 2 3 3 4 5% 57%	2376
November	1st to 10th	Night	15% 4 2 4 4 3 2 3 3 5% 55%	2225
	11th to 20th	Night	14% 5% 2 3 4 2 3 4 4 6% 53%	2470
	21st to end	Night	15% 4 2 4 4 2 4 4 6% 6% 49%	2470
December	1st to 10th	Night	20% 6% 3 5% 5% 2 3 3 4 5% 43%	2470
	11th to 20th	Night	17% 5% 2 3 3 2 3 3 4 5% 52%	3042
	21st to end	Night	21% 5% 2 4 4 3 4 5% 5% 7% 42%	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	40% 4 8% 22% 21% 6%	1707
	11th to 20th	Day	43% 4 8% 18% 22% 6%	1665
	21st to end	Day	37% 3 6% 24% 23% 7%	1782
February	1st to 10th	Day	39% 6% 10% 21% 18% 6%	1644
	11th to 20th	Day	41% 5% 8% 20% 21% 6%	1758
	21st to end	Day	45% 6% 7% 15% 19% 8%	1484
March	1st to 10th	Day	35% 5% 8% 20% 25% 8%	1980
	11th to 20th	Day	43% 6% 11% 17% 17% 7%	1980
	21st to end	Day	41% 6% 12% 20% 18% 3	2178
April	1st to 10th	Day	32% 5% 11% 22% 25% 6%	1980
	11th to 20th	Day	36% 6% 9% 19% 21% 9%	2108
	21st to end	Day	36% 6% 11% 25% 18% 4	2340
May	1st to 10th	Day	34% 8% 16% 21% 18% 3	2340
	11th to 20th	Day	41% 10% 15% 19% 13% 2	2305
	21st to end	Day	36% 10% 14% 25% 14% 1	2531
June	1st to 10th	Day	39% 10% 16% 24% 11% 1	2340
	11th to 20th	Day	35% 12% 18% 21% 13% 1	2340
	21st to end	Day	35% 13% 17% 19% 13% 2	2340
July	1st to 10th	Day	42% 12% 16% 16% 11% 3	2275
	11th to 20th	Day	39% 14% 20% 16% 10% 1	2271
	21st to end	Day	38% 12% 19% 20% 9% 1	2405
August	1st to 10th	Day	44% 12% 17% 19% 8%	2314
	11th to 20th	Day	46% 13% 15% 16% 9% 1	2303
	21st to end	Day	39% 11% 16% 21% 9% 3	2414
September	1st to 10th	Day	38% 8% 12% 21% 17% 4	1980
	11th to 20th	Day	40% 7% 13% 17% 20% 3	1980
	21st to end	Day	43% 6% 9% 19% 17% 6%	1980
October	1st to 10th	Day	32% 5% 9% 21% 28% 5%	1905
	11th to 20th	Day	30% 4 9% 24% 25% 8%	1800
	21st to end	Day	32% 3 6% 22% 26% 10%	1980
November	1st to 10th	Day	32% 3 8% 19% 30% 9%	1849
	11th to 20th	Day	33% 5% 6% 22% 25% 9%	1710
	21st to end	Day	36% 4 7% 21% 25% 7%	1710
December	1st to 10th	Day	43% 6% 9% 20% 16% 5%	1710
	11th to 20th	Day	35% 3 8% 21% 23% 11%	2150
	21st to end	Day	41% 4 8% 23% 21% 3	3124
January	1st to 10th	Night	39% 5% 9% 17% 24% 7%	2459
	11th to 20th	Night	42% 4 7% 16% 25% 6%	2417
	21st to end	Night	37% 3 7% 21% 27% 6%	2574
February	1st to 10th	Night	38% 5% 11% 19% 20% 6%	2313
	11th to 20th	Night	43% 4 7% 17% 22% 6%	2099
	21st to end	Night	43% 6% 9% 15% 20% 7%	1770
March	1st to 10th	Night	36% 5% 8% 17% 24% 11%	1980
	11th to 20th	Night	41% 6% 9% 18% 19% 7%	1980
	21st to end	Night	42% 6% 9% 20% 19% 4	2178
April	1st to 10th	Night	35% 6% 10% 19% 24% 6%	1980
	11th to 20th	Night	34% 5% 8% 22% 23% 8%	1812
	21st to end	Night	35% 6% 10% 23% 21% 4	1620
May	1st to 10th	Night	37% 7% 13% 21% 18% 3	1620
	11th to 20th	Night	44% 11% 10% 17% 16% 1	1593
	21st to end	Night	40% 8% 12% 26% 12% 2	1745
June	1st to 10th	Night	40% 11% 15% 21% 11% 1	1620
	11th to 20th	Night	40% 10% 18% 21% 10% 2	1620
	21st to end	Night	37% 13% 18% 15% 13% 4	1620
July	1st to 10th	Night	44% 13% 19% 14% 9% 2	1575
	11th to 20th	Night	37% 14% 23% 18% 6% 2	1566
	21st to end	Night	37% 17% 20% 19% 7% 1	1672
August	1st to 10th	Night	42% 15% 19% 15% 8%	1600
	11th to 20th	Night	43% 14% 19% 15% 9% 1	1598
	21st to end	Night	42% 12% 16% 19% 8% 2	1918
September	1st to 10th	Night	37% 10% 14% 19% 15% 5%	1980
	11th to 20th	Night	42% 9% 14% 15% 18% 2	1980
	21st to end	Night	42% 6% 13% 17% 18% 5%	1980
October	1st to 10th	Night	35% 5% 11% 20% 24% 5%	2053
	11th to 20th	Night	34% 6% 8% 24% 20% 8%	2160
	21st to end	Night	31% 4 6% 20% 29% 11%	2376
November	1st to 10th	Night	34% 4 6% 18% 26% 11%	2225
	11th to 20th	Night	34% 3 8% 17% 26% 12%	2470
	21st to end	Night	35% 4 6% 22% 26% 7%	2470
December	1st to 10th	Night	44% 5% 8% 17% 20% 6%	2470
	11th to 20th	Night	36% 4 8% 17% 25% 10%	3044
	21st to end	Night	42% 4 9% 21% 20% 3	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	23% 15% 34% 23% 4	270
	11th to 20th	Day	25% 20% 26% 22% 5% 1	255
	21st to end	Day	20% 20% 37% 18% 5%	264
February	1st to 10th	Day	21% 21% 34% 17% 6% 1	240
	11th to 20th	Day	36% 15% 35% 12% 2	240
	21st to end	Day	27% 23% 33% 14% 2	202
March	1st to 10th	Day	22% 12% 31% 26% 8%	420
	11th to 20th	Day	21% 12% 37% 22% 6% 1	420
	21st to end	Day	23% 15% 31% 24% 6%	462
April	1st to 10th	Day	16% 15% 30% 24% 12% 2	420
	11th to 20th	Day	17% 14% 27% 24% 16% 3	412
	21st to end	Day	18% 10% 25% 28% 13% 6%	420
May	1st to 10th	Day	12% 12% 31% 22% 18% 4	423
	11th to 20th	Day	15% 6% 17% 30% 22% 11%	447
	21st to end	Day	15% 5% 15% 24% 25% 16%	485
June	1st to 10th	Day	13% 4 13% 32% 23% 16%	450
	11th to 20th	Day	10% 4 15% 26% 24% 22%	450
	21st to end	Day	10% 4 16% 28% 27% 16%	450
July	1st to 10th	Day	9% 2 8% 24% 29% 27%	445
	11th to 20th	Day	8% 2 7% 31% 28% 25%	444
	21st to end	Day	7% 2 9% 28% 33% 22%	488
August	1st to 10th	Day	16% 2 13% 25% 24% 20%	446
	11th to 20th	Day	12% 3 8% 28% 31% 17%	439
	21st to end	Day	12% 3 10% 25% 27% 24%	495
September	1st to 10th	Day	13% 2 10% 24% 32% 20%	450
	11th to 20th	Day	17% 3 12% 21% 30% 18%	450
	21st to end	Day	22% 2 18% 26% 24% 6%	450
October	1st to 10th	Day	19% 5% 18% 25% 23% 10%	450
	11th to 20th	Day	14% 5% 21% 32% 21% 7%	450
	21st to end	Day	18% 7% 21% 33% 16% 4	495
November	1st to 10th	Day	20% 8% 24% 26% 17% 4	447
	11th to 20th	Day	19% 10% 35% 26% 8% 2	270
	21st to end	Day	13% 11% 34% 26% 13% 3	270
December	1st to 10th	Day	25% 13% 32% 23% 6%	270
	11th to 20th	Day	22% 13% 32% 23% 10% 1	270
	21st to end	Day	26% 14% 31% 20% 8% 1	297
January	1st to 10th	Night	20% 11% 33% 29% 7%	760
	11th to 20th	Night	22% 10% 32% 32% 4	744
	21st to end	Night	19% 9% 37% 32% 3	792
February	1st to 10th	Night	19% 10% 33% 35% 3	720
	11th to 20th	Night	22% 12% 39% 25% 1	703
	21st to end	Night	21% 11% 38% 27% 3	592
March	1st to 10th	Night	17% 11% 31% 37% 4	720
	11th to 20th	Night	19% 10% 36% 26% 10% 1	720
	21st to end	Night	20% 8% 33% 32% 7%	792
April	1st to 10th	Night	16% 12% 31% 32% 10%	720
	11th to 20th	Night	16% 10% 27% 30% 16% 1	706
	21st to end	Night	16% 6% 25% 35% 18%	540
May	1st to 10th	Night	13% 7% 27% 34% 19%	540
	11th to 20th	Night	16% 4 20% 33% 24% 2	531
	21st to end	Night	17% 2 10% 30% 37% 5%	584
June	1st to 10th	Night	12% 3 11% 36% 33% 5%	540
	11th to 20th	Night	13% 2 14% 29% 32% 11%	540
	21st to end	Night	8% 1 13% 33% 41% 5%	540
July	1st to 10th	Night	11% 1 8% 24% 46% 10%	525
	11th to 20th	Night	11% 2 6% 25% 43% 13%	522
	21st to end	Night	8% 1 7% 28% 48% 8%	591
August	1st to 10th	Night	15% 2 7% 27% 41% 9%	537
	11th to 20th	Night	13% 2 9% 26% 41% 9%	534
	21st to end	Night	12% 2 9% 26% 45% 7%	692
September	1st to 10th	Night	11% 2 8% 28% 42% 8%	720
	11th to 20th	Night	17% 2 13% 25% 36% 7%	720
	21st to end	Night	19% 3 17% 30% 29% 2	720
October	1st to 10th	Night	18% 3 18% 28% 27% 6%	720
	11th to 20th	Night	17% 5% 23% 30% 22% 2	720
	21st to end	Night	16% 5% 22% 34% 19% 3	792
November	1st to 10th	Night	16% 4 24% 36% 18% 3	740
	11th to 20th	Night	16% 7% 27% 37% 12% 1	760
	21st to end	Night	17% 5% 28% 36% 14% 1	760
December	1st to 10th	Night	23% 9% 26% 34% 7%	760
	11th to 20th	Night	19% 7% 27% 36% 10% 1	760
	21st to end	Night	26% 8% 27% 32% 8%	836

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