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 2025-02-25.

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	23% 11% 3 8% 5% 4 4 3 5% 6% 28%	108882
	Night	22% 11% 3 8% 5% 4 4 3 5% 6% 29%	109924
Transparency	Day	31% 45% 7% 10% 6% 1	112474
	Night	32% 43% 7% 11% 6%	114487
Seeing	Day	27% 11% 27% 26% 9% 1	17782
	Night	27% 11% 28% 25% 8% 1	25620

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% 6% 5% 3 5% 5% 27%	3616
	11th to 20th	Day	15% 14% 7% 12% 6% 5% 6% 3 5% 5% 22%	3591
	21st to end	Day	15% 11% 4 8% 5% 5% 5% 4 6% 7% 31%	4449
February	1st to 10th	Day	22% 12% 4 9% 7% 4 4 4 4 5% 25%	1800
	11th to 20th	Day	18% 13% 4 9% 6% 5% 4 3 4 5% 27%	1753
	21st to end	Day	14% 15% 4 6% 5% 5% 5% 4 5% 6% 31%	1525
March	1st to 10th	Day	23% 15% 4 8% 5% 5% 3 2 4 4 28%	3324
	11th to 20th	Day	17% 17% 5% 11% 6% 4 4 3 4 6% 24%	3480
	21st to end	Day	31% 13% 3 9% 5% 4 3 2 3 4 22%	3879
April	1st to 10th	Day	26% 12% 4 7% 5% 4 4 3 4 5% 27%	2280
	11th to 20th	Day	24% 10% 3 7% 4 4 4 2 5% 6% 31%	2254
	21st to end	Day	24% 11% 3 6% 4 4 4 3 4 6% 30%	2468
May	1st to 10th	Day	28% 13% 3 7% 5% 4 4 3 5% 6% 20%	4315
	11th to 20th	Day	26% 12% 3 7% 4 4 4 3 4 5% 28%	4398
	21st to end	Day	22% 11% 3 7% 5% 4 5% 4 7% 8% 24%	4836
June	1st to 10th	Day	28% 11% 2 7% 6% 5% 4 4 4 7% 22%	2660
	11th to 20th	Day	24% 9% 3 7% 5% 4 4 4 5% 6% 31%	2660
	21st to end	Day	34% 11% 3 6% 4 4 4 3 5% 6% 21%	2660
July	1st to 10th	Day	27% 11% 3 6% 4 4 4 4 5% 8% 24%	4440
	11th to 20th	Day	27% 14% 3 7% 3 3 4 4 5% 6% 24%	4440
	21st to end	Day	23% 11% 3 7% 5% 4 4 3 5% 7% 29%	4795
August	1st to 10th	Day	28% 12% 3 6% 3 4 4 3 5% 7% 24%	4105
	11th to 20th	Day	21% 8% 2 7% 4 5% 4 4 7% 8% 30%	4045
	21st to end	Day	12% 6% 2 5% 4 5% 5% 3 5% 8% 43%	4258
September	1st to 10th	Day	19% 7% 2 6% 5% 4 4 3 6% 7% 37%	2280
	11th to 20th	Day	21% 7% 2 6% 3 3 4 4 5% 7% 37%	2280
	21st to end	Day	21% 7% 2 6% 4 5% 4 3 5% 7% 36%	1983
October	1st to 10th	Day	20% 6% 2 7% 5% 4 5% 4 6% 7% 35%	1900
	11th to 20th	Day	21% 7% 3 6% 5% 4 5% 3 5% 7% 34%	1900
	21st to end	Day	22% 9% 3 7% 5% 4 3 3 5% 6% 31%	2050
November	1st to 10th	Day	23% 8% 4 7% 5% 5% 4 3 5% 7% 29%	1670
	11th to 20th	Day	24% 10% 3 10% 5% 4 4 2 5% 5% 28%	1600
	21st to end	Day	20% 9% 4 9% 6% 5% 5% 4 6% 7% 26%	1600
December	1st to 10th	Day	21% 11% 4 9% 5% 4 4 2 5% 6% 28%	2610
	11th to 20th	Day	31% 10% 3 6% 4 4 4 3 4 5% 28%	2996
	21st to end	Day	21% 11% 6% 10% 4 3 3 3 4 5% 29%	3982
January	1st to 10th	Night	14% 14% 4 8% 5% 5% 5% 4 5% 6% 28%	6349
	11th to 20th	Night	18% 12% 5% 14% 6% 5% 4 3 5% 6% 21%	6314
	21st to end	Night	13% 12% 5% 10% 6% 5% 5% 4 5% 7% 29%	6490
February	1st to 10th	Night	22% 12% 3 9% 5% 5% 5% 4 6% 4 25%	2600
	11th to 20th	Night	19% 11% 4 10% 5% 5% 5% 4 5% 7% 25%	2533
	21st to end	Night	13% 14% 4 8% 7% 4 4 4 5% 6% 30%	2022
March	1st to 10th	Night	23% 11% 4 8% 6% 5% 5% 3 4 5% 25%	3514
	11th to 20th	Night	23% 12% 5% 9% 4 4 5% 3 6% 6% 24%	3479
	21st to end	Night	32% 11% 4 9% 4 3 3 2 3 4 25%	3753
April	1st to 10th	Night	27% 11% 3 6% 5% 4 4 4 4 5% 27%	1900
	11th to 20th	Night	23% 9% 3 6% 5% 4 4 3 5% 5% 33%	1875
	21st to end	Night	25% 10% 3 6% 4 4 3 3 5% 6% 31%	1710
May	1st to 10th	Night	26% 14% 5% 8% 5% 4 4 3 4 5% 21%	2632
	11th to 20th	Night	26% 12% 4 6% 4 4 4 3 4 6% 28%	2506
	21st to end	Night	27% 15% 4 6% 3 3 3 3 5% 6% 25%	2749
June	1st to 10th	Night	32% 12% 3 6% 4 3 4 3 4 5% 25%	1520
	11th to 20th	Night	25% 10% 3 4 5% 4 3 3 5% 6% 32%	1520
	21st to end	Night	35% 13% 3 7% 4 4 3 3 4 5% 20%	1520
July	1st to 10th	Night	29% 13% 5% 8% 4 4 3 3 4 6% 21%	2530
	11th to 20th	Night	28% 21% 3 7% 3 3 3 2 3 4 22%	2530
	21st to end	Night	26% 10% 3 5% 3 3 4 3 4 6% 32%	2766
August	1st to 10th	Night	31% 10% 2 6% 3 3 3 3 4 6% 29%	2825
	11th to 20th	Night	21% 13% 3 6% 4 3 4 4 6% 8% 29%	2782
	21st to end	Night	16% 5% 1 5% 2 4 4 3 4 6% 51%	3396
September	1st to 10th	Night	19% 8% 2 4 3 3 4 3 5% 8% 41%	1900
	11th to 20th	Night	20% 8% 2 5% 3 3 3 3 5% 6% 42%	1900
	21st to end	Night	21% 10% 3 6% 4 4 3 3 4 6% 36%	2191
October	1st to 10th	Night	19% 6% 3 6% 4 4 4 3 5% 7% 39%	2280
	11th to 20th	Night	21% 7% 2 5% 4 4 3 3 6% 6% 39%	2280
	21st to end	Night	26% 8% 2 5% 4 4 3 3 4 6% 33%	2546
November	1st to 10th	Night	22% 9% 3 6% 4 4 4 3 6% 6% 33%	2623
	11th to 20th	Night	22% 9% 3 8% 5% 4 4 3 6% 7% 29%	2800
	21st to end	Night	23% 10% 3 8% 5% 4 4 3 5% 6% 30%	2800
December	1st to 10th	Night	18% 9% 4 9% 6% 4 4 4 5% 6% 31%	4580
	11th to 20th	Night	28% 11% 3 6% 3 3 3 3 4 6% 30%	5213
	21st to end	Night	25% 11% 3 7% 4 4 4 3 5% 5% 29%	6996

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% 30% 14% 11% 5% 1	3617
	11th to 20th	Day	40% 30% 13% 13% 4	3620
	21st to end	Day	36% 32% 8% 9% 12% 4	4449
February	1st to 10th	Day	25% 62% 3 4 5% 1	1800
	11th to 20th	Day	22% 61% 4 6% 5% 1	1761
	21st to end	Day	19% 64% 3 6% 6% 1	1525
March	1st to 10th	Day	37% 41% 6% 9% 6% 1	3328
	11th to 20th	Day	34% 46% 9% 7% 3 1	3480
	21st to end	Day	44% 41% 5% 5% 5% 1	3879
April	1st to 10th	Day	22% 65% 3 4 5% 2	2280
	11th to 20th	Day	20% 67% 3 7% 3	2254
	21st to end	Day	21% 65% 4 4 5% 1	2468
May	1st to 10th	Day	38% 46% 6% 7% 2	4330
	11th to 20th	Day	34% 43% 8% 11% 4	4402
	21st to end	Day	31% 48% 10% 8% 3	4836
June	1st to 10th	Day	20% 67% 3 7% 3	2660
	11th to 20th	Day	17% 66% 5% 7% 4 1	2660
	21st to end	Day	23% 64% 3 6% 4	2660
July	1st to 10th	Day	41% 39% 5% 7% 7% 1	4370
	11th to 20th	Day	42% 40% 5% 8% 5%	4364
	21st to end	Day	35% 39% 7% 11% 8%	4642
August	1st to 10th	Day	41% 40% 5% 9% 4	4092
	11th to 20th	Day	32% 42% 12% 13% 2	4084
	21st to end	Day	23% 38% 8% 18% 12% 1	4258
September	1st to 10th	Day	17% 61% 6% 8% 6% 1	2280
	11th to 20th	Day	20% 61% 4 11% 4	2280
	21st to end	Day	16% 60% 5% 12% 6%	1983
October	1st to 10th	Day	31% 39% 8% 14% 8%	3160
	11th to 20th	Day	22% 38% 8% 18% 14%	3140
	21st to end	Day	27% 37% 12% 14% 9% 2	3355
November	1st to 10th	Day	24% 58% 6% 8% 3 1	1670
	11th to 20th	Day	27% 58% 6% 6% 3 1	1600
	21st to end	Day	27% 58% 5% 6% 4	1600
December	1st to 10th	Day	35% 36% 8% 16% 4	2610
	11th to 20th	Day	40% 33% 9% 9% 9%	2995
	21st to end	Day	41% 29% 11% 12% 7%	3982
January	1st to 10th	Night	39% 31% 12% 14% 3	6349
	11th to 20th	Night	45% 32% 10% 10% 4	6360
	21st to end	Night	38% 31% 8% 11% 11% 1	6490
February	1st to 10th	Night	27% 62% 3 3 4 1	2600
	11th to 20th	Night	23% 61% 3 8% 5%	2536
	21st to end	Night	19% 63% 3 8% 5% 1	2022
March	1st to 10th	Night	36% 42% 5% 11% 5% 1	3523
	11th to 20th	Night	37% 45% 5% 8% 5%	3480
	21st to end	Night	44% 41% 5% 7% 3	3753
April	1st to 10th	Night	21% 65% 4 5% 4 1	1900
	11th to 20th	Night	18% 67% 4 7% 4 1	1875
	21st to end	Night	20% 65% 3 6% 5% 1	1710
May	1st to 10th	Night	38% 47% 5% 6% 3 1	2637
	11th to 20th	Night	34% 44% 8% 11% 3	2507
	21st to end	Night	38% 43% 8% 8% 3	2750
June	1st to 10th	Night	20% 66% 4 6% 3	1520
	11th to 20th	Night	18% 65% 3 10% 5%	1520
	21st to end	Night	23% 63% 4 7% 3	1520
July	1st to 10th	Night	48% 36% 3 7% 5% 2	2491
	11th to 20th	Night	48% 36% 4 7% 5%	2482
	21st to end	Night	35% 37% 6% 12% 9% 1	2679
August	1st to 10th	Night	39% 38% 6% 14% 3	2813
	11th to 20th	Night	33% 41% 11% 13% 2	2809
	21st to end	Night	19% 37% 10% 21% 13%	3395
September	1st to 10th	Night	17% 60% 4 12% 6% 1	1900
	11th to 20th	Night	19% 60% 4 12% 5%	1900
	21st to end	Night	17% 60% 5% 13% 5%	2191
October	1st to 10th	Night	33% 38% 9% 14% 6%	3800
	11th to 20th	Night	19% 37% 9% 19% 15%	3779
	21st to end	Night	26% 37% 10% 16% 10%	4182
November	1st to 10th	Night	24% 57% 6% 9% 3	2623
	11th to 20th	Night	26% 59% 5% 6% 4	2800
	21st to end	Night	26% 58% 7% 6% 4	2800
December	1st to 10th	Night	32% 38% 9% 16% 5%	4580
	11th to 20th	Night	38% 33% 7% 12% 9% 1	5215
	21st to end	Night	41% 29% 9% 13% 8%	6996

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% 30% 19% 2	400
	11th to 20th	Day	26% 17% 34% 17% 6% 1	400
	21st to end	Day	21% 15% 38% 23% 3	440
February	1st to 10th	Day	29% 18% 30% 18% 5%	400
	11th to 20th	Day	27% 22% 32% 16% 2	396
	21st to end	Day	20% 14% 39% 23% 4	319
March	1st to 10th	Day	21% 16% 38% 22% 4	427
	11th to 20th	Day	21% 15% 33% 25% 6%	460
	21st to end	Day	26% 15% 33% 20% 6% 1	506
April	1st to 10th	Day	32% 15% 27% 22% 4	460
	11th to 20th	Day	28% 11% 35% 20% 6%	452
	21st to end	Day	24% 9% 30% 28% 8% 1	460
May	1st to 10th	Day	32% 8% 30% 23% 5% 2	582
	11th to 20th	Day	27% 4 27% 31% 9% 2	674
	21st to end	Day	31% 7% 27% 28% 6% 1	735
June	1st to 10th	Day	33% 7% 21% 28% 10%	680
	11th to 20th	Day	26% 5% 24% 33% 10% 2	680
	21st to end	Day	41% 5% 16% 27% 10% 1	680
July	1st to 10th	Day	30% 6% 18% 32% 13% 1	670
	11th to 20th	Day	32% 6% 14% 32% 16% 1	668
	21st to end	Day	31% 5% 17% 33% 12% 1	732
August	1st to 10th	Day	25% 5% 17% 36% 14% 3	486
	11th to 20th	Day	26% 6% 19% 31% 14% 4	479
	21st to end	Day	20% 6% 21% 30% 19% 3	539
September	1st to 10th	Day	21% 5% 24% 34% 14% 2	490
	11th to 20th	Day	23% 8% 23% 27% 15% 3	490
	21st to end	Day	23% 5% 27% 27% 15% 3	433
October	1st to 10th	Day	21% 7% 29% 27% 13% 3	390
	11th to 20th	Day	23% 11% 29% 23% 13% 2	390
	21st to end	Day	25% 14% 32% 21% 7% 1	429
November	1st to 10th	Day	22% 13% 32% 24% 7% 1	395
	11th to 20th	Day	29% 19% 28% 18% 5% 1	400
	21st to end	Day	24% 21% 33% 18% 3 1	400
December	1st to 10th	Day	30% 16% 32% 19% 4	400
	11th to 20th	Day	33% 23% 26% 16% 2	400
	21st to end	Day	23% 20% 34% 18% 4 1	440
January	1st to 10th	Night	27% 18% 34% 18% 3	900
	11th to 20th	Night	28% 16% 33% 19% 4	900
	21st to end	Night	22% 19% 38% 18% 3	880
February	1st to 10th	Night	27% 17% 34% 19% 4	800
	11th to 20th	Night	25% 20% 37% 17% 2	783
	21st to end	Night	19% 13% 39% 24% 6%	647
March	1st to 10th	Night	23% 15% 39% 20% 2	760
	11th to 20th	Night	24% 13% 28% 24% 10%	760
	21st to end	Night	26% 12% 31% 24% 6%	798
April	1st to 10th	Night	31% 11% 28% 24% 5%	570
	11th to 20th	Night	27% 6% 34% 25% 7%	564
	21st to end	Night	31% 6% 27% 27% 8% 1	570
May	1st to 10th	Night	33% 5% 25% 28% 8% 1	570
	11th to 20th	Night	30% 6% 21% 29% 12% 2	562
	21st to end	Night	29% 5% 26% 30% 9% 1	616
June	1st to 10th	Night	36% 5% 18% 31% 9%	570
	11th to 20th	Night	30% 4 18% 37% 10% 1	570
	21st to end	Night	39% 4 17% 29% 10% 1	570
July	1st to 10th	Night	31% 4 15% 33% 16% 1	556
	11th to 20th	Night	35% 4 15% 32% 14% 1	552
	21st to end	Night	32% 3 15% 32% 17% 1	623
August	1st to 10th	Night	25% 4 15% 38% 16% 2	568
	11th to 20th	Night	27% 5% 17% 32% 18% 2	562
	21st to end	Night	23% 3 17% 32% 21% 3	627
September	1st to 10th	Night	21% 4 24% 32% 16% 2	570
	11th to 20th	Night	23% 5% 22% 31% 17% 2	570
	21st to end	Night	27% 6% 24% 29% 11% 2	756
October	1st to 10th	Night	21% 8% 27% 31% 11% 2	760
	11th to 20th	Night	22% 10% 30% 27% 10% 1	760
	21st to end	Night	30% 13% 29% 21% 7%	856
November	1st to 10th	Night	25% 13% 31% 25% 5% 1	880
	11th to 20th	Night	24% 18% 29% 22% 6%	900
	21st to end	Night	28% 17% 31% 19% 5%	900
December	1st to 10th	Night	30% 14% 34% 17% 5%	900
	11th to 20th	Night	30% 20% 33% 15% 3	900
	21st to end	Night	28% 17% 32% 18% 4	990

Page generated at 2025-02-22 07:51:35. Version ac07

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