The Deanery Project
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Deanery Project clear sky chart from 2005-11-06 to 2026-07-01.

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	22% 11% 4 8% 5% 4 4 3 5% 6% 28%	138420
	Night	23% 12% 4 8% 5% 4 4 3 5% 6% 28%	134143
Transparency	Day	36% 37% 8% 11% 6% 1	141967
	Night	37% 36% 7% 12% 7% 1	138677
Seeing	Day	27% 11% 27% 25% 8% 1	19721
	Night	27% 11% 28% 25% 8% 1	27397

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	13% 13% 5% 12% 6% 5% 5% 3 5% 6% 26%	4690
	11th to 20th	Day	19% 12% 8% 13% 6% 5% 5% 3 4 4 19%	4681
	21st to end	Day	14% 9% 4 8% 6% 5% 6% 5% 7% 7% 29%	5764
February	1st to 10th	Day	21% 12% 4 10% 7% 5% 4 4 4 5% 24%	1890
	11th to 20th	Day	17% 13% 4 9% 6% 5% 4 3 4 5% 28%	1843
	21st to end	Day	14% 14% 4 7% 5% 5% 5% 4 5% 6% 31%	1641
March	1st to 10th	Day	20% 14% 4 11% 5% 5% 3 2 4 4 26%	4874
	11th to 20th	Day	18% 17% 5% 9% 5% 3 3 2 4 5% 28%	5090
	21st to end	Day	28% 12% 4 10% 5% 5% 4 3 3 5% 22%	5680
April	1st to 10th	Day	26% 12% 4 7% 5% 4 3 3 4 5% 27%	2520
	11th to 20th	Day	25% 10% 3 7% 4 4 4 2 5% 6% 30%	2494
	21st to end	Day	23% 12% 4 7% 4 4 4 3 4 6% 30%	2728
May	1st to 10th	Day	31% 14% 4 7% 4 4 4 3 4 5% 22%	8014
	11th to 20th	Day	24% 11% 3 8% 4 4 4 3 5% 6% 29%	8220
	21st to end	Day	25% 13% 4 6% 5% 4 4 3 5% 6% 25%	9059
June	1st to 10th	Day	27% 11% 3 6% 6% 5% 4 4 4 7% 24%	2940
	11th to 20th	Day	23% 10% 3 7% 5% 4 4 3 5% 6% 31%	2940
	21st to end	Day	33% 11% 3 6% 4 4 4 3 5% 6% 21%	2940
July	1st to 10th	Day	26% 12% 3 7% 5% 5% 4 4 6% 8% 20%	6360
	11th to 20th	Day	25% 13% 3 7% 4 4 4 4 5% 6% 26%	6360
	21st to end	Day	18% 9% 3 6% 4 4 4 3 6% 9% 34%	6893
August	1st to 10th	Day	27% 12% 3 6% 3 4 4 3 5% 6% 26%	4235
	11th to 20th	Day	20% 8% 2 7% 4 5% 4 4 7% 8% 31%	4175
	21st to end	Day	12% 7% 2 5% 4 5% 5% 4 5% 8% 43%	4393
September	1st to 10th	Day	19% 7% 2 6% 5% 4 4 3 6% 7% 37%	2400
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 5% 7% 38%	2400
	21st to end	Day	20% 6% 2 6% 4 5% 4 3 5% 7% 37%	2087
October	1st to 10th	Day	19% 6% 2 7% 5% 4 5% 4 6% 7% 37%	2000
	11th to 20th	Day	21% 7% 3 7% 5% 4 5% 3 5% 7% 33%	2000
	21st to end	Day	23% 9% 3 7% 5% 4 3 3 5% 6% 31%	2158
November	1st to 10th	Day	24% 8% 3 7% 5% 5% 4 3 5% 7% 29%	1755
	11th to 20th	Day	24% 11% 3 9% 5% 4 4 2 5% 5% 28%	1680
	21st to end	Day	20% 10% 4 9% 6% 5% 5% 4 5% 7% 25%	1680
December	1st to 10th	Day	21% 11% 4 9% 5% 4 4 2 5% 6% 28%	2690
	11th to 20th	Day	30% 10% 3 6% 4 4 4 3 4 5% 28%	3076
	21st to end	Day	21% 11% 6% 10% 4 3 3 3 4 6% 29%	4070
January	1st to 10th	Night	15% 14% 4 9% 6% 5% 5% 3 5% 6% 27%	8234
	11th to 20th	Night	23% 13% 5% 13% 6% 5% 4 3 4 5% 19%	8234
	21st to end	Night	12% 12% 4 9% 6% 5% 6% 4 6% 8% 27%	8400
February	1st to 10th	Night	21% 12% 4 9% 5% 5% 5% 4 6% 5% 24%	2730
	11th to 20th	Night	18% 11% 4 10% 5% 5% 5% 4 5% 6% 27%	2663
	21st to end	Night	14% 14% 4 9% 6% 4 4 4 6% 6% 29%	2163
March	1st to 10th	Night	25% 12% 4 9% 6% 5% 5% 3 4 5% 23%	5170
	11th to 20th	Night	23% 13% 5% 7% 4 3 4 3 5% 6% 28%	5089
	21st to end	Night	31% 11% 4 9% 4 3 3 2 3 4 25%	5494
April	1st to 10th	Night	27% 11% 3 6% 5% 4 4 4 4 5% 27%	2100
	11th to 20th	Night	24% 10% 3 6% 5% 4 4 3 5% 5% 32%	2075
	21st to end	Night	24% 10% 3 6% 5% 3 3 3 5% 6% 31%	1890
May	1st to 10th	Night	33% 15% 5% 8% 4 3 4 3 4 5% 18%	4903
	11th to 20th	Night	27% 13% 3 6% 4 4 4 3 4 6% 26%	4670
	21st to end	Night	23% 17% 4 7% 4 3 3 3 4 6% 27%	5147
June	1st to 10th	Night	30% 11% 3 6% 4 3 3 3 4 5% 27%	1680
	11th to 20th	Night	25% 10% 3 5% 5% 3 3 3 5% 6% 32%	1680
	21st to end	Night	34% 13% 3 7% 4 4 3 3 4 5% 21%	1680
July	1st to 10th	Night	28% 13% 4 8% 5% 4 4 3 5% 6% 20%	3620
	11th to 20th	Night	27% 19% 3 7% 4 3 3 3 3 4 25%	3620
	21st to end	Night	23% 10% 4 6% 4 3 4 3 5% 7% 32%	3964
August	1st to 10th	Night	31% 10% 2 6% 3 3 2 3 4 6% 30%	2915
	11th to 20th	Night	21% 13% 3 6% 4 3 4 3 6% 8% 29%	2872
	21st to end	Night	16% 5% 1 5% 2 4 4 3 4 6% 50%	3503
September	1st to 10th	Night	20% 8% 2 4 3 3 3 3 5% 8% 40%	2000
	11th to 20th	Night	20% 8% 2 5% 3 3 3 3 6% 6% 43%	2000
	21st to end	Night	21% 10% 3 6% 4 4 3 3 3 6% 38%	2307
October	1st to 10th	Night	18% 6% 3 6% 4 4 4 3 4 7% 41%	2400
	11th to 20th	Night	20% 8% 2 5% 4 4 3 3 6% 6% 38%	2400
	21st to end	Night	26% 8% 2 5% 4 4 3 3 4 6% 33%	2680
November	1st to 10th	Night	22% 9% 3 7% 4 4 4 3 6% 6% 33%	2757
	11th to 20th	Night	22% 10% 3 8% 5% 4 4 3 5% 6% 29%	2940
	21st to end	Night	22% 10% 3 8% 5% 4 4 3 5% 6% 30%	2940
December	1st to 10th	Night	18% 9% 4 9% 6% 4 4 4 5% 6% 31%	4720
	11th to 20th	Night	28% 11% 3 6% 3 3 3 3 4 5% 30%	5353
	21st to end	Night	25% 11% 3 7% 4 4 4 3 5% 5% 29%	7150

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	43% 25% 14% 12% 6%	4691
	11th to 20th	Day	48% 23% 11% 12% 4	4710
	21st to end	Day	39% 25% 9% 11% 12% 3	5764
February	1st to 10th	Day	27% 60% 3 5% 5% 1	1890
	11th to 20th	Day	23% 59% 5% 8% 6% 1	1851
	21st to end	Day	21% 59% 4 8% 7% 1	1641
March	1st to 10th	Day	44% 29% 7% 9% 7% 4	4878
	11th to 20th	Day	39% 33% 11% 13% 3	5090
	21st to end	Day	48% 29% 5% 11% 6% 1	5680
April	1st to 10th	Day	24% 59% 4 5% 6% 2	2520
	11th to 20th	Day	24% 61% 4 7% 4	2494
	21st to end	Day	24% 59% 5% 5% 5% 1	2728
May	1st to 10th	Day	50% 27% 7% 11% 5%	8027
	11th to 20th	Day	43% 25% 9% 16% 5%	8224
	21st to end	Day	44% 28% 8% 12% 8%	9060
June	1st to 10th	Day	22% 61% 5% 8% 4	2940
	11th to 20th	Day	21% 60% 6% 9% 5% 1	2940
	21st to end	Day	26% 59% 3 7% 5%	2940
July	1st to 10th	Day	49% 31% 7% 7% 6%	6290
	11th to 20th	Day	47% 30% 7% 12% 5%	6284
	21st to end	Day	34% 30% 11% 16% 9%	6696
August	1st to 10th	Day	40% 39% 5% 10% 5%	4222
	11th to 20th	Day	32% 41% 12% 14% 2	4214
	21st to end	Day	24% 37% 8% 19% 12% 1	4393
September	1st to 10th	Day	19% 58% 6% 9% 7% 1	2400
	11th to 20th	Day	21% 58% 4 12% 5%	2400
	21st to end	Day	17% 57% 5% 13% 7% 1	2087
October	1st to 10th	Day	30% 37% 8% 15% 9%	3260
	11th to 20th	Day	23% 37% 8% 18% 14%	3240
	21st to end	Day	27% 36% 12% 14% 9% 2	3463
November	1st to 10th	Day	25% 55% 6% 9% 4 1	1755
	11th to 20th	Day	29% 56% 6% 6% 2 1	1680
	21st to end	Day	29% 55% 6% 6% 4	1680
December	1st to 10th	Day	36% 36% 8% 16% 4	2690
	11th to 20th	Day	40% 32% 9% 9% 9%	3075
	21st to end	Day	42% 28% 11% 12% 7%	4070
January	1st to 10th	Night	44% 25% 12% 15% 5%	8234
	11th to 20th	Night	52% 25% 9% 10% 4	8280
	21st to end	Night	41% 25% 10% 11% 12% 1	8400
February	1st to 10th	Night	28% 59% 3 4 4 1	2730
	11th to 20th	Night	23% 58% 3 9% 6%	2666
	21st to end	Night	22% 60% 4 9% 6% 1	2163
March	1st to 10th	Night	46% 29% 5% 9% 8% 3	5179
	11th to 20th	Night	42% 32% 8% 13% 6%	5090
	21st to end	Night	50% 29% 5% 12% 4	5494
April	1st to 10th	Night	25% 59% 4 5% 6% 1	2100
	11th to 20th	Night	22% 60% 4 8% 4 1	2075
	21st to end	Night	23% 59% 3 8% 6% 1	1890
May	1st to 10th	Night	55% 26% 5% 9% 4 1	4908
	11th to 20th	Night	47% 26% 7% 14% 5%	4671
	21st to end	Night	48% 24% 8% 12% 7% 1	5148
June	1st to 10th	Night	23% 60% 5% 8% 5%	1680
	11th to 20th	Night	21% 59% 3 12% 5%	1680
	21st to end	Night	26% 58% 5% 7% 4	1680
July	1st to 10th	Night	53% 26% 5% 10% 4 1	3582
	11th to 20th	Night	52% 26% 4 11% 7% 1	3572
	21st to end	Night	39% 27% 8% 15% 10%	3847
August	1st to 10th	Night	38% 37% 6% 15% 4	2903
	11th to 20th	Night	33% 40% 11% 14% 2	2899
	21st to end	Night	20% 36% 10% 21% 13%	3502
September	1st to 10th	Night	18% 57% 5% 12% 7% 1	2000
	11th to 20th	Night	19% 57% 4 14% 6%	2000
	21st to end	Night	17% 58% 5% 14% 6%	2307
October	1st to 10th	Night	32% 37% 9% 14% 7%	3920
	11th to 20th	Night	20% 36% 9% 19% 15%	3899
	21st to end	Night	27% 36% 10% 16% 10%	4316
November	1st to 10th	Night	26% 54% 7% 9% 4	2757
	11th to 20th	Night	28% 56% 5% 7% 4	2940
	21st to end	Night	27% 55% 7% 6% 4	2940
December	1st to 10th	Night	32% 37% 9% 16% 5%	4720
	11th to 20th	Night	39% 32% 7% 12% 9% 1	5355
	21st to end	Night	41% 28% 9% 14% 8%	7150

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	26% 23% 30% 18% 2	430
	11th to 20th	Day	28% 17% 33% 16% 6%	430
	21st to end	Day	21% 16% 38% 22% 3	473
February	1st to 10th	Day	27% 17% 31% 19% 5%	430
	11th to 20th	Day	26% 21% 32% 17% 3	426
	21st to end	Day	20% 16% 39% 21% 4	355
March	1st to 10th	Day	21% 16% 40% 20% 3	499
	11th to 20th	Day	22% 16% 33% 24% 5%	540
	21st to end	Day	28% 15% 34% 18% 5% 1	594
April	1st to 10th	Day	32% 17% 27% 20% 3	540
	11th to 20th	Day	30% 12% 34% 19% 5%	532
	21st to end	Day	23% 11% 29% 28% 7% 1	540
May	1st to 10th	Day	32% 9% 29% 22% 5% 2	674
	11th to 20th	Day	28% 6% 27% 29% 8% 2	774
	21st to end	Day	31% 7% 28% 28% 6% 1	845
June	1st to 10th	Day	31% 8% 21% 28% 11%	780
	11th to 20th	Day	26% 6% 25% 32% 10% 1	780
	21st to end	Day	39% 6% 17% 27% 10% 1	780
July	1st to 10th	Day	30% 6% 19% 31% 13% 1	720
	11th to 20th	Day	31% 7% 14% 32% 16% 1	718
	21st to end	Day	30% 5% 18% 34% 12% 1	786
August	1st to 10th	Day	24% 5% 17% 37% 14% 3	526
	11th to 20th	Day	25% 6% 19% 32% 14% 4	519
	21st to end	Day	21% 6% 22% 30% 19% 3	583
September	1st to 10th	Day	21% 6% 24% 34% 13% 2	530
	11th to 20th	Day	22% 8% 23% 28% 15% 4	530
	21st to end	Day	22% 5% 27% 27% 16% 3	467
October	1st to 10th	Day	20% 7% 30% 27% 13% 3	420
	11th to 20th	Day	22% 11% 30% 23% 12% 2	420
	21st to end	Day	26% 14% 31% 21% 7% 1	462
November	1st to 10th	Day	24% 14% 32% 23% 7% 1	425
	11th to 20th	Day	30% 19% 27% 18% 5% 1	430
	21st to end	Day	25% 21% 33% 18% 3	430
December	1st to 10th	Day	30% 17% 31% 19% 3	430
	11th to 20th	Day	33% 23% 27% 16% 2	430
	21st to end	Day	22% 22% 34% 17% 3 1	473
January	1st to 10th	Night	27% 19% 34% 17% 3	950
	11th to 20th	Night	30% 16% 32% 18% 3	950
	21st to end	Night	22% 20% 38% 17% 3	924
February	1st to 10th	Night	26% 17% 34% 19% 4	840
	11th to 20th	Night	24% 20% 36% 18% 2	823
	21st to end	Night	20% 13% 39% 23% 5%	692
March	1st to 10th	Night	24% 15% 39% 19% 2	840
	11th to 20th	Night	24% 13% 29% 24% 9%	840
	21st to end	Night	27% 12% 32% 23% 6%	882
April	1st to 10th	Night	33% 11% 28% 23% 5%	630
	11th to 20th	Night	29% 8% 32% 25% 6%	624
	21st to end	Night	30% 7% 27% 28% 7% 1	630
May	1st to 10th	Night	35% 6% 24% 27% 7% 1	630
	11th to 20th	Night	31% 7% 22% 28% 10% 2	622
	21st to end	Night	28% 6% 26% 30% 9% 1	682
June	1st to 10th	Night	34% 5% 19% 32% 10%	630
	11th to 20th	Night	30% 4 19% 36% 10% 1	630
	21st to end	Night	38% 4 18% 29% 10% 1	630
July	1st to 10th	Night	30% 4 15% 33% 16% 1	587
	11th to 20th	Night	34% 4 15% 33% 14% 1	582
	21st to end	Night	32% 4 16% 32% 17% 1	656
August	1st to 10th	Night	25% 4 14% 38% 17% 2	598
	11th to 20th	Night	26% 5% 16% 33% 18% 2	592
	21st to end	Night	23% 3 18% 33% 20% 3	660
September	1st to 10th	Night	22% 4 24% 32% 17% 2	600
	11th to 20th	Night	23% 5% 22% 31% 17% 2	600
	21st to end	Night	27% 6% 24% 30% 11% 2	796
October	1st to 10th	Night	20% 8% 27% 31% 12% 2	800
	11th to 20th	Night	22% 11% 30% 27% 10% 1	800
	21st to end	Night	30% 14% 28% 21% 6%	902
November	1st to 10th	Night	25% 14% 32% 24% 5% 1	930
	11th to 20th	Night	25% 18% 30% 21% 6%	950
	21st to end	Night	28% 17% 32% 18% 5%	950
December	1st to 10th	Night	29% 15% 34% 17% 4	950
	11th to 20th	Night	31% 20% 33% 14% 3	950
	21st to end	Night	27% 19% 32% 18% 4	1045

Page generated at 2026-06-28 03:10:45. Version ac07

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