Sheldrake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sheldrake clear sky chart from 2005-11-06 to 2026-06-16.

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	18% 10% 3 8% 5% 4 4 3 5% 6% 32%	138003
	Night	17% 12% 4 8% 5% 4 4 3 5% 6% 31%	134290
Transparency	Day	51% 6% 13% 21% 8% 1	141398
	Night	54% 6% 11% 19% 9% 1	138990
Seeing	Day	23% 8% 24% 30% 13% 2	18605
	Night	24% 8% 25% 29% 13% 2	26096

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% 20% 5% 11% 5% 3 3 2 4 6% 29%	4093
	11th to 20th	Day	10% 21% 7% 13% 5% 4 4 3 5% 7% 21%	4084
	21st to end	Day	4 9% 3 10% 7% 6% 6% 4 7% 9% 36%	4804
February	1st to 10th	Day	8% 13% 5% 10% 6% 6% 5% 3 4 7% 33%	1890
	11th to 20th	Day	10% 17% 4 9% 6% 5% 5% 3 5% 6% 30%	1843
	21st to end	Day	8% 14% 5% 10% 6% 5% 5% 5% 5% 8% 31%	1557
March	1st to 10th	Day	16% 13% 4 9% 5% 4 4 3 5% 6% 30%	4814
	11th to 20th	Day	19% 17% 4 11% 5% 4 4 3 4 5% 23%	5090
	21st to end	Day	19% 18% 5% 9% 5% 4 4 3 4 4 26%	5587
April	1st to 10th	Day	20% 13% 4 8% 5% 5% 5% 3 5% 5% 27%	2555
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 5% 6% 36%	2702
	21st to end	Day	23% 10% 3 6% 4 3 3 3 5% 6% 33%	2730
May	1st to 10th	Day	31% 10% 3 7% 4 3 3 3 4 5% 26%	8236
	11th to 20th	Day	19% 7% 3 7% 5% 4 5% 3 5% 7% 35%	8375
	21st to end	Day	25% 9% 3 7% 4 4 4 3 4 6% 32%	9716
June	1st to 10th	Day	24% 7% 3 7% 5% 3 4 4 5% 6% 32%	3150
	11th to 20th	Day	23% 6% 2 6% 4 4 5% 3 6% 7% 33%	3075
	21st to end	Day	24% 8% 2 6% 4 4 4 4 6% 7% 31%	3000
July	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	6820
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 7% 8% 31%	6820
	21st to end	Day	13% 5% 2 5% 4 4 4 4 6% 8% 44%	7393
August	1st to 10th	Day	14% 5% 2 6% 5% 4 5% 4 7% 9% 40%	4418
	11th to 20th	Day	22% 9% 3 8% 5% 5% 5% 4 6% 6% 28%	4175
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 5% 7% 41%	4675
September	1st to 10th	Day	19% 6% 2 6% 5% 5% 5% 4 6% 6% 37%	2410
	11th to 20th	Day	25% 6% 2 4 4 3 3 3 5% 6% 39%	2302
	21st to end	Day	21% 7% 1 5% 4 3 4 4 5% 6% 39%	2155
October	1st to 10th	Day	18% 8% 3 7% 4 4 4 3 6% 7% 37%	2000
	11th to 20th	Day	25% 8% 3 7% 4 3 4 4 6% 6% 31%	2000
	21st to end	Day	26% 10% 3 8% 4 3 3 2 5% 5% 31%	1959
November	1st to 10th	Day	25% 10% 2 6% 3 3 5% 3 5% 6% 31%	1645
	11th to 20th	Day	23% 15% 3 7% 5% 4 3 3 4 5% 27%	1680
	21st to end	Day	21% 15% 5% 7% 6% 4 3 3 6% 4 26%	1659
December	1st to 10th	Day	11% 15% 5% 10% 6% 5% 5% 4 6% 6% 27%	2350
	11th to 20th	Day	22% 10% 4 9% 4 4 4 3 5% 7% 27%	2688
	21st to end	Day	13% 12% 5% 8% 4 4 3 3 5% 6% 37%	3553
January	1st to 10th	Night	14% 20% 6% 11% 5% 4 4 3 4 5% 25%	8831
	11th to 20th	Night	11% 17% 6% 12% 5% 5% 5% 3 5% 7% 25%	8831
	21st to end	Night	4 10% 4 11% 6% 5% 5% 3 5% 9% 38%	9378
February	1st to 10th	Night	8% 13% 6% 9% 6% 5% 4 3 6% 7% 33%	2730
	11th to 20th	Night	12% 17% 5% 9% 5% 5% 5% 3 4 5% 31%	2663
	21st to end	Night	10% 15% 5% 8% 6% 5% 4 3 5% 6% 34%	2249
March	1st to 10th	Night	16% 12% 4 8% 5% 4 4 3 4 6% 34%	5244
	11th to 20th	Night	21% 20% 6% 9% 4 4 3 3 4 4 22%	5089
	21st to end	Night	20% 17% 5% 9% 5% 4 4 3 3 4 26%	5587
April	1st to 10th	Night	21% 12% 3 8% 5% 5% 4 4 5% 6% 27%	2062
	11th to 20th	Night	20% 12% 4 6% 4 3 3 3 4 6% 34%	1868
	21st to end	Night	24% 11% 2 5% 3 4 4 3 4 6% 34%	1890
May	1st to 10th	Night	31% 11% 3 6% 3 3 3 2 4 5% 27%	4692
	11th to 20th	Night	19% 8% 2 7% 5% 4 4 4 5% 6% 36%	4559
	21st to end	Night	25% 9% 3 7% 4 4 5% 4 5% 6% 28%	4491
June	1st to 10th	Night	26% 6% 2 5% 5% 4 4 4 5% 6% 33%	1470
	11th to 20th	Night	23% 7% 3 6% 4 4 4 4 6% 8% 31%	1435
	21st to end	Night	25% 8% 2 6% 6% 4 5% 3 5% 7% 29%	1400
July	1st to 10th	Night	18% 8% 3 6% 4 4 4 4 6% 7% 35%	3160
	11th to 20th	Night	16% 8% 3 8% 5% 5% 5% 5% 7% 8% 30%	3160
	21st to end	Night	12% 6% 3 7% 4 5% 4 5% 7% 9% 39%	3446
August	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 4 6% 8% 35%	2711
	11th to 20th	Night	20% 8% 3 8% 5% 5% 4 3 6% 8% 31%	2872
	21st to end	Night	17% 6% 2 8% 5% 5% 4 3 5% 6% 39%	3223
September	1st to 10th	Night	23% 6% 2 6% 3 4 4 3 4 6% 38%	1990
	11th to 20th	Night	23% 6% 2 6% 3 3 4 4 4 6% 40%	2095
	21st to end	Night	20% 8% 2 6% 3 3 3 3 5% 6% 40%	2245
October	1st to 10th	Night	22% 7% 3 7% 5% 3 4 3 5% 6% 35%	2400
	11th to 20th	Night	28% 8% 3 7% 4 4 4 3 4 6% 30%	2400
	21st to end	Night	25% 10% 3 7% 4 4 3 3 4 6% 31%	2879
November	1st to 10th	Night	25% 11% 2 6% 4 4 4 3 5% 6% 31%	2873
	11th to 20th	Night	24% 14% 4 7% 5% 4 4 3 4 5% 27%	2940
	21st to end	Night	23% 16% 4 7% 4 4 4 3 5% 5% 25%	2959
December	1st to 10th	Night	14% 14% 5% 10% 6% 5% 5% 3 4 5% 29%	5060
	11th to 20th	Night	20% 11% 4 9% 4 4 4 4 5% 5% 29%	5741
	21st to end	Night	13% 14% 4 8% 5% 4 4 3 5% 7% 34%	7667

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	60% 4 7% 19% 10% 1	4094
	11th to 20th	Day	63% 8% 12% 14% 3	4110
	21st to end	Day	45% 9% 14% 17% 13% 2	4804
February	1st to 10th	Day	51% 9% 10% 17% 11% 2	1890
	11th to 20th	Day	55% 6% 11% 17% 10% 1	1851
	21st to end	Day	50% 6% 12% 21% 10% 1	1557
March	1st to 10th	Day	55% 5% 10% 15% 14% 1	4818
	11th to 20th	Day	63% 4 10% 14% 8% 2	5090
	21st to end	Day	62% 3 8% 16% 9% 2	5587
April	1st to 10th	Day	56% 4 11% 15% 13% 2	2555
	11th to 20th	Day	47% 4 12% 23% 12% 2	2702
	21st to end	Day	52% 4 13% 18% 11% 1	2730
May	1st to 10th	Day	59% 4 10% 18% 7% 1	8249
	11th to 20th	Day	47% 5% 13% 26% 8%	8379
	21st to end	Day	52% 4 11% 21% 10% 1	9717
June	1st to 10th	Day	51% 6% 12% 21% 9% 2	3150
	11th to 20th	Day	49% 6% 14% 20% 10% 1	3075
	21st to end	Day	51% 8% 11% 21% 9%	3000
July	1st to 10th	Day	44% 8% 13% 28% 7%	6745
	11th to 20th	Day	47% 8% 19% 21% 6%	6739
	21st to end	Day	37% 7% 18% 29% 10%	7181
August	1st to 10th	Day	38% 10% 20% 27% 4	4404
	11th to 20th	Day	54% 8% 15% 18% 4	4214
	21st to end	Day	40% 8% 18% 27% 8%	4675
September	1st to 10th	Day	45% 8% 17% 22% 7%	2410
	11th to 20th	Day	45% 7% 18% 23% 7%	2302
	21st to end	Day	44% 7% 15% 25% 9%	2155
October	1st to 10th	Day	50% 5% 15% 23% 6% 1	3260
	11th to 20th	Day	49% 6% 17% 23% 6%	3240
	21st to end	Day	49% 4 15% 17% 14%	3140
November	1st to 10th	Day	53% 6% 14% 17% 9%	1645
	11th to 20th	Day	60% 6% 11% 16% 6%	1680
	21st to end	Day	61% 7% 10% 15% 7% 1	1659
December	1st to 10th	Day	57% 8% 11% 15% 7% 1	2350
	11th to 20th	Day	57% 7% 13% 17% 5% 1	2688
	21st to end	Day	49% 6% 12% 21% 9% 3	3553
January	1st to 10th	Night	63% 5% 7% 17% 7% 1	8831
	11th to 20th	Night	59% 7% 10% 16% 7% 1	8880
	21st to end	Night	44% 8% 13% 22% 12%	9378
February	1st to 10th	Night	51% 8% 10% 16% 12% 2	2730
	11th to 20th	Night	56% 5% 9% 16% 12% 1	2666
	21st to end	Night	52% 6% 8% 19% 14% 1	2249
March	1st to 10th	Night	53% 4 9% 14% 17% 3	5253
	11th to 20th	Night	67% 4 6% 12% 10% 1	5090
	21st to end	Night	63% 3 6% 12% 12% 3	5587
April	1st to 10th	Night	57% 5% 9% 14% 13% 2	2062
	11th to 20th	Night	51% 4 8% 21% 13% 3	1868
	21st to end	Night	52% 4 11% 14% 17% 2	1890
May	1st to 10th	Night	59% 6% 8% 17% 7% 2	4697
	11th to 20th	Night	48% 5% 12% 21% 13% 1	4560
	21st to end	Night	55% 5% 10% 21% 9% 1	4492
June	1st to 10th	Night	50% 5% 12% 21% 11% 1	1470
	11th to 20th	Night	51% 6% 13% 18% 10% 1	1435
	21st to end	Night	56% 6% 10% 19% 8% 1	1400
July	1st to 10th	Night	46% 6% 13% 23% 9% 2	3125
	11th to 20th	Night	49% 7% 16% 23% 5%	3118
	21st to end	Night	40% 7% 18% 27% 8%	3343
August	1st to 10th	Night	46% 9% 15% 25% 6%	2701
	11th to 20th	Night	51% 8% 16% 21% 4	2899
	21st to end	Night	45% 6% 15% 24% 9%	3223
September	1st to 10th	Night	46% 6% 14% 25% 9% 1	1990
	11th to 20th	Night	46% 6% 17% 24% 7%	2095
	21st to end	Night	45% 5% 12% 25% 12%	2245
October	1st to 10th	Night	52% 4 12% 24% 7% 1	3920
	11th to 20th	Night	53% 4 15% 21% 7%	3899
	21st to end	Night	52% 4 10% 24% 9% 1	4653
November	1st to 10th	Night	54% 8% 12% 17% 9% 1	2873
	11th to 20th	Night	60% 7% 11% 16% 6%	2940
	21st to end	Night	63% 6% 11% 14% 7%	2959
December	1st to 10th	Night	57% 9% 10% 15% 8%	5060
	11th to 20th	Night	57% 6% 11% 18% 7%	5742
	21st to end	Night	50% 6% 16% 19% 7% 2	7667

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	17% 15% 29% 23% 15% 1	220
	11th to 20th	Day	21% 14% 24% 25% 12% 4	220
	21st to end	Day	13% 9% 34% 29% 12% 3	258
February	1st to 10th	Day	15% 10% 34% 25% 12% 4	430
	11th to 20th	Day	15% 12% 29% 29% 12% 2	426
	21st to end	Day	16% 11% 28% 29% 13% 3	355
March	1st to 10th	Day	18% 11% 32% 29% 9% 2	521
	11th to 20th	Day	25% 10% 26% 27% 11% 1	540
	21st to end	Day	24% 12% 25% 28% 10% 1	594
April	1st to 10th	Day	26% 14% 31% 22% 5%	540
	11th to 20th	Day	21% 10% 29% 28% 11% 1	532
	21st to end	Day	27% 8% 22% 28% 13% 2	540
May	1st to 10th	Day	29% 5% 24% 29% 12% 2	753
	11th to 20th	Day	25% 4 28% 28% 13% 3	774
	21st to end	Day	27% 7% 23% 30% 11% 1	845
June	1st to 10th	Day	26% 3 23% 33% 14% 1	780
	11th to 20th	Day	26% 4 20% 32% 16% 3	755
	21st to end	Day	26% 5% 20% 33% 14% 2	730
July	1st to 10th	Day	20% 5% 21% 36% 17% 2	720
	11th to 20th	Day	22% 4 18% 37% 18% 1	718
	21st to end	Day	23% 4 17% 38% 15% 3	795
August	1st to 10th	Day	19% 2 16% 38% 21% 4	644
	11th to 20th	Day	20% 4 17% 34% 22% 2	519
	21st to end	Day	20% 3 22% 31% 18% 5%	583
September	1st to 10th	Day	22% 8% 22% 31% 14% 2	530
	11th to 20th	Day	28% 6% 22% 29% 12% 2	530
	21st to end	Day	23% 7% 33% 25% 10% 3	505
October	1st to 10th	Day	22% 9% 29% 27% 10% 2	420
	11th to 20th	Day	28% 6% 29% 21% 12% 3	420
	21st to end	Day	28% 11% 22% 26% 11% 3	462
November	1st to 10th	Day	30% 11% 26% 25% 7%	425
	11th to 20th	Day	33% 10% 22% 25% 8% 2	430
	21st to end	Day	31% 13% 26% 24% 5% 1	409
December	1st to 10th	Day	18% 11% 25% 31% 11% 4	220
	11th to 20th	Day	20% 17% 30% 25% 8%	220
	21st to end	Day	21% 16% 28% 24% 10% 2	242
January	1st to 10th	Night	24% 10% 31% 25% 9% 1	950
	11th to 20th	Night	22% 13% 29% 26% 8% 2	950
	21st to end	Night	16% 10% 36% 28% 9%	993
February	1st to 10th	Night	13% 10% 36% 29% 10% 2	840
	11th to 20th	Night	20% 9% 31% 28% 11% 2	823
	21st to end	Night	16% 9% 30% 28% 15% 3	692
March	1st to 10th	Night	19% 10% 30% 29% 11% 1	840
	11th to 20th	Night	21% 9% 30% 30% 10% 1	840
	21st to end	Night	22% 10% 26% 30% 10% 2	924
April	1st to 10th	Night	27% 10% 27% 26% 9%	635
	11th to 20th	Night	25% 7% 27% 29% 11% 1	624
	21st to end	Night	27% 7% 20% 30% 12% 3	630
May	1st to 10th	Night	30% 3 17% 31% 16% 2	630
	11th to 20th	Night	22% 5% 21% 33% 15% 3	589
	21st to end	Night	30% 3 19% 30% 16% 2	455
June	1st to 10th	Night	26% 2 19% 36% 16% 1	420
	11th to 20th	Night	27% 3 14% 32% 22% 1	410
	21st to end	Night	28% 4 12% 37% 19% 2	400
July	1st to 10th	Night	20% 3 16% 35% 23% 3	390
	11th to 20th	Night	22% 2 11% 38% 26% 1	388
	21st to end	Night	22% 4 11% 36% 24% 3	438
August	1st to 10th	Night	20% 2 12% 33% 27% 6%	586
	11th to 20th	Night	24% 4 9% 36% 24% 3	592
	21st to end	Night	20% 4 16% 30% 26% 4	660
September	1st to 10th	Night	24% 3 23% 31% 17% 3	600
	11th to 20th	Night	24% 5% 20% 35% 14% 3	695
	21st to end	Night	23% 6% 22% 32% 16% 1	800
October	1st to 10th	Night	26% 6% 27% 29% 11% 1	800
	11th to 20th	Night	32% 4 22% 25% 15% 2	800
	21st to end	Night	30% 9% 24% 26% 10% 1	927
November	1st to 10th	Night	30% 11% 30% 23% 6%	930
	11th to 20th	Night	32% 10% 26% 22% 7% 1	950
	21st to end	Night	33% 11% 25% 23% 8%	950
December	1st to 10th	Night	26% 11% 30% 23% 9% 1	950
	11th to 20th	Night	25% 13% 34% 23% 5% 1	950
	21st to end	Night	25% 12% 28% 25% 8% 2	1045

Page generated at 2026-06-13 02:31:38. Version ac07

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