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 2024-12-09.

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	14% 6% 2 6% 4 4 4 4 6% 8% 43%	101488
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	98250
Transparency	Day	21% 47% 7% 12% 10% 3	105204
	Night	22% 47% 6% 10% 11% 4	102673
Seeing	Day	17% 8% 22% 25% 18% 10%	15295
	Night	17% 6% 23% 31% 20% 3	25376

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	17% 4 1 4 3 3 3 2 5% 6% 52%	2836
	11th to 20th	Day	15% 7% 2 5% 3 3 3 3 3 4 53%	2840
	21st to end	Day	26% 5% 2 3 3 3 3 2 4 5% 45%	3124
February	1st to 10th	Day	15% 7% 2 5% 3 4 4 4 6% 7% 43%	1735
	11th to 20th	Day	20% 6% 2 5% 3 3 3 3 4 5% 45%	1848
	21st to end	Day	19% 7% 3 6% 5% 4 4 2 4 7% 40%	1575
March	1st to 10th	Day	24% 7% 2 5% 3 3 3 2 4 6% 41%	3435
	11th to 20th	Day	12% 4 2 5% 3 4 3 3 5% 6% 53%	3480
	21st to end	Day	21% 5% 2 5% 3 3 3 4 4 6% 44%	3815
April	1st to 10th	Day	12% 3 2 4 4 4 4 4 6% 8% 49%	2090
	11th to 20th	Day	14% 5% 1 6% 3 4 3 4 5% 8% 46%	2228
	21st to end	Day	13% 5% 2 5% 5% 4 4 4 5% 7% 48%	2470
May	1st to 10th	Day	11% 7% 2 7% 5% 5% 6% 4 6% 9% 37%	4082
	11th to 20th	Day	16% 8% 3 7% 4 4 5% 4 5% 7% 37%	4081
	21st to end	Day	8% 4 2 5% 4 4 5% 4 7% 10% 47%	4476
June	1st to 10th	Day	10% 6% 2 7% 4 6% 5% 5% 8% 12% 35%	2470
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 6% 7% 11% 41%	2470
	21st to end	Day	8% 4 2 6% 5% 6% 7% 6% 9% 9% 40%	2470
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 5% 9% 10% 35%	4120
	11th to 20th	Day	8% 5% 3 7% 5% 6% 6% 6% 8% 11% 36%	4120
	21st to end	Day	11% 8% 3 10% 6% 7% 7% 6% 9% 10% 23%	4466
August	1st to 10th	Day	17% 6% 3 7% 6% 5% 6% 5% 7% 10% 28%	4104
	11th to 20th	Day	8% 6% 3 6% 5% 5% 5% 6% 7% 12% 36%	4044
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 9% 54%	4232
September	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 8% 43%	2090
	11th to 20th	Day	14% 7% 2 6% 4 5% 4 3 6% 8% 41%	2090
	21st to end	Day	18% 6% 2 7% 4 4 4 4 5% 6% 42%	2090
October	1st to 10th	Day	14% 5% 1 4 3 3 3 2 4 6% 56%	2010
	11th to 20th	Day	13% 5% 1 4 2 2 3 2 4 5% 60%	1900
	21st to end	Day	14% 4 1 5% 3 3 3 2 3 4 57%	2090
November	1st to 10th	Day	14% 5% 3 5% 3 3 3 2 4 5% 54%	1948
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 4 6% 52%	1800
	21st to end	Day	17% 5% 1 4 3 4 3 3 5% 5% 51%	1800
December	1st to 10th	Day	19% 7% 2 5% 3 3 4 3 5% 6% 43%	1782
	11th to 20th	Day	17% 5% 2 4 3 2 3 3 6% 6% 51%	2153
	21st to end	Day	19% 6% 2 4 3 3 3 3 6% 6% 44%	3124
January	1st to 10th	Night	17% 4 2 5% 3 3 3 3 4 5% 50%	4109
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 6% 50%	4120
	21st to end	Night	25% 7% 1 4 3 3 3 2 4 5% 45%	4532
February	1st to 10th	Night	16% 5% 2 5% 3 4 4 3 5% 6% 47%	2442
	11th to 20th	Night	20% 6% 2 5% 4 4 3 2 4 4 44%	2218
	21st to end	Night	16% 7% 2 6% 4 4 4 3 5% 5% 42%	1877
March	1st to 10th	Night	23% 8% 2 6% 4 3 3 4 4 6% 37%	3418
	11th to 20th	Night	10% 5% 2 4 3 3 4 3 5% 7% 53%	3480
	21st to end	Night	19% 5% 2 5% 4 4 3 3 5% 6% 43%	3817
April	1st to 10th	Night	13% 5% 2 5% 4 3 4 4 5% 6% 49%	2090
	11th to 20th	Night	13% 5% 2 5% 3 4 3 3 4 7% 50%	1913
	21st to end	Night	13% 4 2 5% 5% 4 4 3 5% 7% 49%	1710
May	1st to 10th	Night	14% 7% 2 6% 5% 5% 4 4 5% 8% 40%	2813
	11th to 20th	Night	17% 7% 2 6% 5% 4 4 4 6% 7% 38%	2812
	21st to end	Night	9% 4 2 6% 5% 5% 5% 4 7% 11% 42%	3079
June	1st to 10th	Night	11% 6% 2 7% 4 6% 6% 5% 9% 10% 33%	1710
	11th to 20th	Night	10% 6% 2 6% 7% 5% 5% 4 7% 10% 37%	1710
	21st to end	Night	6% 6% 2 7% 7% 6% 6% 6% 8% 12% 36%	1710
July	1st to 10th	Night	10% 7% 4 9% 6% 7% 6% 5% 8% 10% 29%	2840
	11th to 20th	Night	9% 5% 2 6% 5% 5% 6% 6% 10% 12% 33%	2840
	21st to end	Night	12% 10% 3 10% 6% 7% 7% 6% 9% 10% 20%	3087
August	1st to 10th	Night	17% 10% 3 9% 5% 4 5% 5% 8% 9% 24%	2825
	11th to 20th	Night	11% 8% 3 9% 6% 6% 7% 5% 7% 10% 29%	2785
	21st to end	Night	6% 4 2 6% 5% 5% 6% 5% 7% 9% 44%	3386
September	1st to 10th	Night	10% 6% 2 7% 5% 4 5% 5% 6% 9% 42%	2090
	11th to 20th	Night	15% 7% 2 7% 4 5% 5% 4 5% 9% 35%	2090
	21st to end	Night	16% 7% 2 7% 4 3 4 3 5% 7% 41%	2090
October	1st to 10th	Night	16% 5% 2 5% 3 3 4 3 4 7% 50%	2168
	11th to 20th	Night	13% 5% 1 5% 3 3 3 3 4 5% 53%	2280
	21st to end	Night	14% 4 2 4 2 3 3 2 4 5% 58%	2508
November	1st to 10th	Night	15% 5% 2 5% 4 4 2 3 3 5% 53%	2346
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 5% 53%	2600
	21st to end	Night	15% 4 2 4 4 3 4 4 6% 6% 50%	2600
December	1st to 10th	Night	19% 7% 2 5% 4 3 3 3 4 5% 44%	2581
	11th to 20th	Night	17% 5% 2 4 3 3 2 3 4 5% 52%	3042
	21st to end	Night	21% 5% 2 5% 3 4 4 3 5% 7% 41%	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	21% 36% 3 17% 17% 7%	2837
	11th to 20th	Day	22% 37% 4 9% 25% 4	2840
	21st to end	Day	27% 38% 4 9% 18% 3	3124
February	1st to 10th	Day	14% 65% 3 9% 7% 2	1735
	11th to 20th	Day	17% 65% 2 6% 8% 2	1858
	21st to end	Day	18% 65% 3 3 8% 3	1575
March	1st to 10th	Day	33% 41% 5% 8% 6% 6%	3435
	11th to 20th	Day	15% 40% 5% 10% 20% 10%	3480
	21st to end	Day	25% 39% 3 14% 16% 3	3815
April	1st to 10th	Day	13% 66% 4 6% 9% 2	2090
	11th to 20th	Day	16% 66% 3 7% 6% 2	2228
	21st to end	Day	13% 66% 4 10% 7% 1	2470
May	1st to 10th	Day	27% 45% 9% 11% 7% 1	4120
	11th to 20th	Day	29% 42% 5% 15% 9%	4085
	21st to end	Day	16% 44% 12% 13% 13% 2	4489
June	1st to 10th	Day	15% 67% 6% 8% 4 1	2470
	11th to 20th	Day	10% 67% 8% 10% 5%	2470
	21st to end	Day	14% 66% 7% 9% 4	2470
July	1st to 10th	Day	25% 40% 15% 14% 5%	4055
	11th to 20th	Day	25% 39% 20% 15% 1	4051
	21st to end	Day	39% 39% 9% 9% 3 1	4324
August	1st to 10th	Day	36% 38% 11% 13% 2	4091
	11th to 20th	Day	21% 39% 18% 13% 9% 1	4083
	21st to end	Day	12% 38% 9% 26% 13% 2	4232
September	1st to 10th	Day	15% 61% 4 9% 9% 1	2090
	11th to 20th	Day	17% 61% 6% 7% 7% 2	2090
	21st to end	Day	21% 60% 5% 8% 6% 1	2090
October	1st to 10th	Day	18% 37% 4 14% 19% 8%	3335
	11th to 20th	Day	9% 36% 3 10% 20% 22%	3140
	21st to end	Day	16% 36% 2 17% 14% 14%	3428
November	1st to 10th	Day	20% 56% 2 9% 10% 3	1948
	11th to 20th	Day	16% 57% 2 9% 13% 3	1800
	21st to end	Day	15% 57% 3 10% 12% 3	1800
December	1st to 10th	Day	20% 57% 3 10% 6% 3	1782
	11th to 20th	Day	16% 47% 2 10% 16% 9%	2150
	21st to end	Day	27% 37% 4 16% 14% 2	3124
January	1st to 10th	Night	24% 36% 5% 10% 20% 6%	4109
	11th to 20th	Night	21% 38% 4 10% 21% 5%	4120
	21st to end	Night	29% 40% 3 9% 16% 4	4532
February	1st to 10th	Night	14% 65% 3 8% 8% 1	2442
	11th to 20th	Night	18% 63% 3 6% 7% 2	2219
	21st to end	Night	16% 65% 3 6% 7% 3	1877
March	1st to 10th	Night	34% 43% 7% 7% 6% 3	3418
	11th to 20th	Night	14% 40% 3 12% 16% 14%	3480
	21st to end	Night	26% 40% 3 11% 13% 6%	3817
April	1st to 10th	Night	15% 65% 4 6% 8% 2	2090
	11th to 20th	Night	15% 66% 3 8% 5% 3	1913
	21st to end	Night	13% 65% 3 10% 7% 1	1710
May	1st to 10th	Night	27% 43% 7% 14% 8% 1	2840
	11th to 20th	Night	25% 44% 8% 14% 10%	2813
	21st to end	Night	17% 43% 11% 18% 10% 1	3087
June	1st to 10th	Night	15% 67% 5% 8% 3 1	1710
	11th to 20th	Night	12% 66% 8% 8% 5%	1710
	21st to end	Night	13% 67% 8% 5% 6% 1	1710
July	1st to 10th	Night	32% 39% 12% 11% 5%	2795
	11th to 20th	Night	25% 41% 18% 15% 1	2786
	21st to end	Night	41% 40% 8% 8% 2 1	2988
August	1st to 10th	Night	39% 39% 12% 9% 1	2814
	11th to 20th	Night	33% 39% 14% 9% 5%	2812
	21st to end	Night	17% 39% 7% 22% 14% 2	3386
September	1st to 10th	Night	13% 62% 7% 9% 7% 1	2090
	11th to 20th	Night	20% 62% 6% 6% 6% 1	2090
	21st to end	Night	20% 60% 7% 8% 4	2090
October	1st to 10th	Night	24% 38% 5% 11% 16% 6%	3623
	11th to 20th	Night	10% 36% 3 12% 20% 19%	3779
	21st to end	Night	17% 36% 2 15% 18% 12%	4120
November	1st to 10th	Night	20% 56% 2 9% 9% 4	2346
	11th to 20th	Night	16% 57% 3 7% 12% 6%	2600
	21st to end	Night	15% 57% 2 10% 13% 2	2600
December	1st to 10th	Night	21% 57% 3 8% 8% 2	2581
	11th to 20th	Night	16% 48% 2 9% 17% 7%	3044
	21st to end	Night	28% 38% 5% 14% 14% 1	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	26% 21% 26% 21% 5% 1	270
	21st to end	Day	23% 19% 36% 17% 5%	297
February	1st to 10th	Day	20% 20% 34% 17% 7% 1	270
	11th to 20th	Day	34% 17% 34% 12% 2	270
	21st to end	Day	29% 21% 33% 14% 2	230
March	1st to 10th	Day	23% 12% 30% 25% 8% 1	460
	11th to 20th	Day	20% 12% 38% 22% 6% 1	460
	21st to end	Day	23% 17% 31% 23% 6%	506
April	1st to 10th	Day	18% 16% 31% 23% 11% 2	460
	11th to 20th	Day	16% 15% 27% 25% 15% 3	452
	21st to end	Day	17% 10% 26% 28% 13% 6%	460
May	1st to 10th	Day	13% 12% 31% 24% 17% 5%	463
	11th to 20th	Day	16% 6% 17% 30% 22% 10%	487
	21st to end	Day	14% 5% 16% 24% 25% 15%	529
June	1st to 10th	Day	12% 4 15% 33% 22% 14%	490
	11th to 20th	Day	9% 3 15% 27% 24% 21%	490
	21st to end	Day	10% 3 15% 28% 28% 16%	490
July	1st to 10th	Day	9% 2 8% 24% 30% 27%	485
	11th to 20th	Day	8% 2 7% 31% 28% 25%	484
	21st to end	Day	8% 3 9% 29% 31% 20%	532
August	1st to 10th	Day	17% 2 13% 25% 25% 19%	486
	11th to 20th	Day	12% 3 9% 29% 31% 16%	479
	21st to end	Day	11% 2 9% 24% 28% 25%	539
September	1st to 10th	Day	12% 2 10% 26% 31% 19%	490
	11th to 20th	Day	18% 3 13% 20% 29% 17%	490
	21st to end	Day	23% 2 19% 26% 23% 6%	490
October	1st to 10th	Day	18% 5% 18% 26% 24% 10%	490
	11th to 20th	Day	13% 5% 21% 32% 22% 6%	490
	21st to end	Day	18% 7% 19% 32% 18% 6%	539
November	1st to 10th	Day	21% 7% 24% 27% 16% 4	486
	11th to 20th	Day	19% 10% 35% 25% 8% 3	300
	21st to end	Day	12% 13% 33% 26% 13% 3	300
December	1st to 10th	Day	23% 14% 33% 23% 6%	294
	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	21% 10% 33% 32% 4	760
	21st to end	Night	19% 10% 36% 32% 3	836
February	1st to 10th	Night	18% 10% 34% 35% 3	760
	11th to 20th	Night	22% 13% 38% 25% 1	743
	21st to end	Night	20% 12% 38% 27% 3	628
March	1st to 10th	Night	18% 11% 30% 37% 4	760
	11th to 20th	Night	18% 10% 36% 26% 9% 1	760
	21st to end	Night	20% 8% 34% 31% 7%	836
April	1st to 10th	Night	16% 12% 32% 31% 9%	760
	11th to 20th	Night	16% 11% 27% 30% 15% 1	745
	21st to end	Night	15% 6% 25% 35% 18%	570
May	1st to 10th	Night	13% 7% 26% 33% 20%	570
	11th to 20th	Night	16% 5% 20% 33% 24% 2	561
	21st to end	Night	16% 2 10% 31% 36% 5%	617
June	1st to 10th	Night	11% 3 12% 37% 32% 4	570
	11th to 20th	Night	12% 2 13% 29% 33% 11%	570
	21st to end	Night	8% 1 13% 32% 42% 5%	570
July	1st to 10th	Night	11% 1 8% 24% 45% 10%	555
	11th to 20th	Night	11% 2 5% 25% 44% 13%	552
	21st to end	Night	9% 1 7% 29% 46% 8%	624
August	1st to 10th	Night	15% 3 7% 27% 40% 8%	567
	11th to 20th	Night	13% 2 9% 28% 40% 9%	564
	21st to end	Night	11% 2 9% 25% 46% 7%	731
September	1st to 10th	Night	11% 2 10% 28% 41% 8%	760
	11th to 20th	Night	18% 3 13% 25% 34% 7%	760
	21st to end	Night	19% 3 17% 31% 28% 2	760
October	1st to 10th	Night	18% 3 18% 29% 27% 5%	760
	11th to 20th	Night	17% 5% 23% 32% 22% 2	760
	21st to end	Night	15% 5% 21% 34% 20% 3	836
November	1st to 10th	Night	17% 4 23% 37% 17% 3	780
	11th to 20th	Night	17% 7% 27% 37% 12% 1	800
	21st to end	Night	17% 5% 28% 35% 14% 1	800
December	1st to 10th	Night	22% 9% 27% 34% 7%	795
	11th to 20th	Night	19% 7% 27% 36% 10% 1	760
	21st to end	Night	26% 8% 27% 32% 8%	836

Page generated at 2024-12-06 03:54:52. Version ac07

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