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 2026-04-02.

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% 44%	131643
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	130196
Transparency	Day	25% 37% 8% 13% 12% 5%	135315
	Night	26% 36% 7% 13% 14% 5%	134594
Seeing	Day	17% 8% 22% 25% 18% 9%	17018
	Night	17% 6% 23% 31% 20% 3	27162

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	16% 7% 2 4 2 2 3 2 4 5% 52%	5258
	11th to 20th	Day	11% 5% 1 4 3 2 2 2 2 3 63%	5280
	21st to end	Day	20% 7% 2 6% 4 3 3 2 4 5% 43%	5780
February	1st to 10th	Day	15% 7% 2 5% 4 4 5% 4 6% 6% 45%	1919
	11th to 20th	Day	22% 6% 2 5% 3 3 3 3 4 5% 44%	2048
	21st to end	Day	18% 7% 3 6% 5% 4 4 2 4 6% 41%	1735
March	1st to 10th	Day	20% 7% 3 6% 4 3 3 3 5% 6% 42%	5045
	11th to 20th	Day	10% 5% 3 5% 3 3 3 3 4 6% 54%	5090
	21st to end	Day	18% 6% 2 5% 3 3 3 3 4 5% 48%	5586
April	1st to 10th	Day	12% 4 2 4 4 4 4 4 6% 8% 49%	2211
	11th to 20th	Day	13% 5% 1 5% 3 4 3 4 5% 7% 48%	2347
	21st to end	Day	13% 5% 2 5% 5% 4 4 4 5% 8% 46%	2600
May	1st to 10th	Day	10% 7% 2 7% 5% 5% 6% 5% 7% 10% 36%	5862
	11th to 20th	Day	18% 8% 3 6% 5% 5% 5% 4 7% 8% 31%	5861
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	6433
June	1st to 10th	Day	10% 6% 2 6% 4 6% 5% 5% 8% 12% 35%	2600
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 6% 8% 11% 41%	2600
	21st to end	Day	7% 4 2 6% 5% 5% 7% 5% 8% 9% 42%	2600
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 5% 8% 10% 37%	5900
	11th to 20th	Day	6% 4 2 7% 5% 6% 7% 6% 9% 12% 35%	5900
	21st to end	Day	9% 7% 3 9% 6% 7% 7% 6% 9% 10% 28%	6424
August	1st to 10th	Day	18% 6% 3 7% 6% 5% 6% 5% 7% 9% 27%	4234
	11th to 20th	Day	9% 6% 3 6% 5% 5% 5% 6% 7% 12% 36%	4174
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 9% 53%	4365
September	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 8% 44%	2200
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 3 6% 8% 42%	2200
	21st to end	Day	18% 6% 2 7% 4 4 4 4 5% 6% 41%	2200
October	1st to 10th	Day	14% 5% 1 4 3 3 3 2 4 6% 56%	2116
	11th to 20th	Day	13% 5% 1 4 2 2 3 2 4 4 60%	2000
	21st to end	Day	15% 4 1 4 3 3 3 2 3 4 57%	2200
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 4 5% 55%	2047
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 6% 53%	1890
	21st to end	Day	16% 5% 1 4 3 4 3 3 5% 5% 51%	1890
December	1st to 10th	Day	16% 5% 2 4 2 3 3 3 4 5% 53%	3020
	11th to 20th	Day	19% 7% 2 6% 4 3 3 3 5% 6% 42%	3463
	21st to end	Day	22% 6% 2 4 2 3 3 2 5% 6% 47%	4565
January	1st to 10th	Night	14% 5% 2 4 3 3 2 2 3 4 57%	7635
	11th to 20th	Night	12% 5% 2 5% 3 3 2 2 3 4 58%	7680
	21st to end	Night	19% 7% 2 5% 3 3 3 2 4 5% 46%	8401
February	1st to 10th	Night	15% 5% 2 5% 4 3 4 3 5% 6% 47%	2698
	11th to 20th	Night	20% 6% 2 5% 4 5% 3 3 4 4 43%	2458
	21st to end	Night	16% 7% 2 6% 4 4 4 3 5% 5% 44%	2068
March	1st to 10th	Night	19% 6% 2 5% 4 3 3 3 4 7% 44%	5028
	11th to 20th	Night	9% 4 1 4 3 3 4 3 6% 7% 56%	5090
	21st to end	Night	16% 5% 2 5% 4 4 4 3 5% 7% 47%	5588
April	1st to 10th	Night	13% 5% 2 5% 4 3 4 3 5% 6% 49%	2212
	11th to 20th	Night	13% 5% 2 5% 3 4 3 3 4 7% 51%	2014
	21st to end	Night	12% 4 2 5% 5% 4 4 3 5% 7% 49%	1800
May	1st to 10th	Night	13% 7% 3 7% 5% 5% 5% 4 5% 9% 38%	4033
	11th to 20th	Night	21% 7% 2 6% 5% 4 5% 4 6% 8% 31%	4032
	21st to end	Night	14% 5% 2 7% 4 5% 4 4 6% 9% 40%	4421
June	1st to 10th	Night	10% 6% 3 7% 5% 6% 7% 5% 8% 10% 34%	1800
	11th to 20th	Night	10% 6% 2 6% 7% 5% 5% 4 7% 10% 37%	1800
	21st to end	Night	6% 6% 2 7% 6% 6% 6% 6% 8% 12% 36%	1800
July	1st to 10th	Night	10% 7% 3 9% 6% 7% 6% 5% 8% 10% 29%	4058
	11th to 20th	Night	9% 6% 3 7% 6% 6% 7% 6% 10% 12% 28%	4057
	21st to end	Night	11% 9% 3 10% 6% 6% 7% 6% 9% 10% 23%	4427
August	1st to 10th	Night	18% 10% 3 9% 5% 4 5% 5% 8% 9% 24%	2915
	11th to 20th	Night	12% 8% 3 10% 6% 6% 7% 5% 7% 9% 28%	2875
	21st to end	Night	6% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	3493
September	1st to 10th	Night	10% 6% 2 6% 5% 4 4 5% 6% 9% 43%	2200
	11th to 20th	Night	14% 7% 2 7% 4 5% 6% 4 5% 9% 36%	2200
	21st to end	Night	16% 7% 2 7% 4 4 4 3 5% 7% 40%	2200
October	1st to 10th	Night	15% 4 2 5% 3 4 4 3 4 7% 49%	2282
	11th to 20th	Night	13% 6% 1 5% 3 3 3 3 4 5% 53%	2400
	21st to end	Night	14% 4 2 4 2 3 3 2 4 5% 57%	2640
November	1st to 10th	Night	14% 5% 2 5% 4 4 2 3 3 5% 54%	2466
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 4 5% 54%	2730
	21st to end	Night	15% 5% 2 4 4 3 4 3 5% 6% 50%	2730
December	1st to 10th	Night	16% 5% 2 5% 3 2 3 3 4 5% 52%	4380
	11th to 20th	Night	21% 7% 3 5% 3 4 3 3 4 5% 44%	4952
	21st to end	Night	21% 4 2 4 3 3 4 3 4 6% 47%	6633

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	29% 20% 7% 16% 22% 6%	5259
	11th to 20th	Day	21% 20% 3 8% 33% 15%	5280
	21st to end	Day	37% 21% 4 14% 16% 9%	5780
February	1st to 10th	Day	16% 59% 3 11% 8% 2	1919
	11th to 20th	Day	20% 58% 2 8% 9% 2	2058
	21st to end	Day	19% 59% 3 4 11% 3	1735
March	1st to 10th	Day	36% 29% 4 9% 11% 11%	5045
	11th to 20th	Day	21% 28% 4 10% 20% 18%	5090
	21st to end	Day	28% 27% 4 19% 17% 5%	5586
April	1st to 10th	Day	15% 62% 5% 7% 9% 2	2211
	11th to 20th	Day	16% 63% 3 9% 7% 2	2347
	21st to end	Day	15% 63% 5% 10% 7% 1	2600
May	1st to 10th	Day	32% 32% 13% 15% 7% 1	5897
	11th to 20th	Day	38% 30% 10% 14% 7%	5865
	21st to end	Day	28% 31% 10% 14% 14% 3	6447
June	1st to 10th	Day	16% 64% 6% 9% 4 1	2600
	11th to 20th	Day	11% 64% 9% 10% 5%	2600
	21st to end	Day	14% 62% 8% 10% 5%	2600
July	1st to 10th	Day	29% 30% 14% 19% 7%	5835
	11th to 20th	Day	30% 29% 27% 13% 1	5831
	21st to end	Day	37% 30% 13% 16% 4 1	6240
August	1st to 10th	Day	37% 37% 11% 13% 2	4221
	11th to 20th	Day	22% 38% 17% 13% 9% 1	4213
	21st to end	Day	14% 37% 9% 26% 14% 1	4365
September	1st to 10th	Day	16% 58% 5% 10% 9% 1	2200
	11th to 20th	Day	18% 58% 7% 8% 8% 1	2200
	21st to end	Day	22% 58% 7% 8% 5% 1	2200
October	1st to 10th	Day	18% 35% 4 15% 19% 8%	3441
	11th to 20th	Day	9% 35% 3 10% 20% 22%	3240
	21st to end	Day	17% 35% 2 17% 14% 14%	3538
November	1st to 10th	Day	20% 53% 3 9% 11% 4	2047
	11th to 20th	Day	16% 54% 3 10% 13% 3	1890
	21st to end	Day	16% 54% 4 11% 13% 3	1890
December	1st to 10th	Day	22% 34% 4 12% 19% 9%	3020
	11th to 20th	Day	32% 31% 5% 9% 16% 6%	3460
	21st to end	Day	30% 26% 6% 23% 13% 2	4565
January	1st to 10th	Night	26% 20% 8% 18% 23% 6%	7636
	11th to 20th	Night	23% 21% 3 11% 32% 10%	7680
	21st to end	Night	34% 22% 4 14% 17% 9%	8401
February	1st to 10th	Night	16% 59% 4 10% 9% 2	2698
	11th to 20th	Night	21% 58% 4 7% 8% 2	2459
	21st to end	Night	18% 59% 3 6% 10% 4	2068
March	1st to 10th	Night	32% 30% 5% 8% 19% 6%	5028
	11th to 20th	Night	17% 28% 3 11% 23% 17%	5090
	21st to end	Night	27% 28% 4 18% 16% 7%	5588
April	1st to 10th	Night	16% 62% 4 7% 9% 2	2212
	11th to 20th	Night	15% 63% 4 9% 6% 3	2014
	21st to end	Night	14% 62% 4 11% 7% 1	1800
May	1st to 10th	Night	33% 31% 11% 16% 9% 1	4060
	11th to 20th	Night	36% 32% 11% 13% 8%	4033
	21st to end	Night	29% 30% 10% 18% 11% 2	4429
June	1st to 10th	Night	17% 64% 6% 9% 4 1	1800
	11th to 20th	Night	14% 64% 9% 9% 5%	1800
	21st to end	Night	14% 64% 9% 6% 7% 1	1800
July	1st to 10th	Night	37% 29% 14% 15% 5%	4015
	11th to 20th	Night	36% 31% 21% 11% 1	4006
	21st to end	Night	43% 29% 12% 12% 3	4297
August	1st to 10th	Night	40% 38% 12% 9% 1	2904
	11th to 20th	Night	34% 38% 14% 9% 5%	2902
	21st to end	Night	19% 38% 7% 21% 13% 2	3493
September	1st to 10th	Night	14% 59% 8% 9% 8% 1	2200
	11th to 20th	Night	21% 59% 7% 6% 7% 1	2200
	21st to end	Night	22% 58% 8% 8% 4	2200
October	1st to 10th	Night	24% 37% 5% 12% 16% 6%	3737
	11th to 20th	Night	10% 35% 3 12% 20% 19%	3899
	21st to end	Night	18% 35% 2 14% 19% 12%	4252
November	1st to 10th	Night	21% 53% 2 10% 10% 4	2466
	11th to 20th	Night	15% 54% 3 8% 14% 6%	2730
	21st to end	Night	16% 54% 3 11% 13% 3	2730
December	1st to 10th	Night	23% 34% 3 14% 20% 6%	4380
	11th to 20th	Night	33% 31% 5% 11% 14% 7%	4954
	21st to end	Night	30% 26% 9% 18% 17% 1	6633

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% 14% 36% 22% 4	330
	11th to 20th	Day	23% 22% 29% 20% 5% 1	330
	21st to end	Day	22% 19% 36% 18% 4	363
February	1st to 10th	Day	19% 20% 35% 19% 6% 1	330
	11th to 20th	Day	36% 16% 34% 12% 2	330
	21st to end	Day	29% 19% 32% 16% 4	278
March	1st to 10th	Day	23% 12% 30% 26% 8% 1	540
	11th to 20th	Day	21% 12% 38% 23% 6% 1	540
	21st to end	Day	21% 17% 31% 25% 6% 1	594
April	1st to 10th	Day	19% 15% 30% 23% 11% 2	504
	11th to 20th	Day	16% 14% 28% 26% 14% 3	492
	21st to end	Day	17% 10% 26% 29% 13% 5%	500
May	1st to 10th	Day	13% 12% 31% 24% 16% 4	503
	11th to 20th	Day	17% 7% 18% 29% 20% 9%	527
	21st to end	Day	15% 6% 18% 24% 23% 14%	573
June	1st to 10th	Day	12% 5% 17% 31% 22% 14%	530
	11th to 20th	Day	8% 3 15% 29% 24% 20%	530
	21st to end	Day	9% 3 14% 28% 30% 16%	530
July	1st to 10th	Day	9% 2 8% 24% 30% 26%	525
	11th to 20th	Day	7% 2 6% 29% 30% 26%	524
	21st to end	Day	8% 2 8% 27% 32% 22%	576
August	1st to 10th	Day	19% 2 13% 25% 24% 17%	526
	11th to 20th	Day	13% 3 9% 28% 30% 18%	519
	21st to end	Day	11% 2 10% 25% 28% 24%	583
September	1st to 10th	Day	12% 2 11% 28% 30% 17%	530
	11th to 20th	Day	17% 3 12% 21% 30% 18%	530
	21st to end	Day	23% 3 20% 26% 23% 6%	530
October	1st to 10th	Day	17% 4 18% 26% 25% 10%	530
	11th to 20th	Day	13% 5% 22% 33% 22% 6%	530
	21st to end	Day	19% 7% 21% 32% 16% 5%	583
November	1st to 10th	Day	20% 7% 26% 27% 16% 4	525
	11th to 20th	Day	18% 11% 35% 25% 8% 2	330
	21st to end	Day	12% 14% 34% 25% 13% 3	330
December	1st to 10th	Day	26% 12% 34% 22% 5%	330
	11th to 20th	Day	22% 12% 33% 21% 11% 1	330
	21st to end	Day	25% 14% 30% 20% 9% 1	363
January	1st to 10th	Night	19% 11% 33% 30% 6%	840
	11th to 20th	Night	21% 11% 34% 31% 4	840
	21st to end	Night	19% 9% 37% 32% 2	924
February	1st to 10th	Night	18% 10% 34% 35% 3	840
	11th to 20th	Night	23% 13% 39% 24% 1	823
	21st to end	Night	20% 12% 38% 27% 3	692
March	1st to 10th	Night	18% 11% 30% 37% 5%	840
	11th to 20th	Night	17% 11% 36% 27% 9% 1	840
	21st to end	Night	19% 9% 34% 31% 7%	924
April	1st to 10th	Night	16% 12% 32% 31% 9%	805
	11th to 20th	Night	15% 11% 28% 30% 15% 1	784
	21st to end	Night	15% 6% 25% 36% 19%	600
May	1st to 10th	Night	13% 7% 27% 34% 20%	600
	11th to 20th	Night	17% 5% 19% 34% 23% 2	591
	21st to end	Night	17% 3 12% 30% 34% 4	650
June	1st to 10th	Night	11% 3 13% 37% 32% 4	600
	11th to 20th	Night	12% 2 14% 29% 33% 11%	600
	21st to end	Night	8% 1 12% 32% 43% 5%	600
July	1st to 10th	Night	11% 1 8% 24% 45% 10%	585
	11th to 20th	Night	11% 2 5% 24% 43% 14%	582
	21st to end	Night	9% 1 7% 28% 47% 8%	657
August	1st to 10th	Night	16% 3 8% 27% 39% 8%	597
	11th to 20th	Night	14% 2 9% 27% 40% 9%	594
	21st to end	Night	12% 2 9% 26% 45% 7%	770
September	1st to 10th	Night	11% 2 10% 30% 40% 8%	800
	11th to 20th	Night	17% 3 12% 26% 35% 7%	800
	21st to end	Night	20% 3 16% 31% 28% 2	800
October	1st to 10th	Night	17% 3 18% 30% 27% 5%	800
	11th to 20th	Night	17% 5% 23% 32% 21% 2	800
	21st to end	Night	16% 5% 22% 34% 20% 3	880
November	1st to 10th	Night	16% 4 23% 37% 17% 2	820
	11th to 20th	Night	16% 8% 28% 37% 11% 1	840
	21st to end	Night	17% 5% 29% 36% 13% 1	840
December	1st to 10th	Night	23% 9% 27% 34% 7%	840
	11th to 20th	Night	20% 7% 27% 36% 9% 1	840
	21st to end	Night	25% 7% 26% 33% 8%	924

Page generated at 2026-03-30 02:18:45. Version ac07

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