Shadowlands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shadowlands Observatory clear sky chart from 2005-11-06 to 2024-08-30.

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 3 5% 7% 46%	95592
	Night	13% 6% 2 5% 4 3 3 3 5% 7% 50%	93515
Transparency	Day	38% 6% 10% 24% 19% 4	95366
	Night	34% 5% 10% 24% 21% 5%	93347
Seeing	Day	14% 8% 21% 26% 23% 7%	15178
	Night	14% 7% 23% 36% 18% 2	24951

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% 9% 3 6% 3 4 4 2 2 4 47%	2847
	11th to 20th	Day	11% 8% 2 6% 4 3 3 2 4 5% 52%	2850
	21st to end	Day	23% 9% 2 4 3 3 2 2 3 4 45%	3135
February	1st to 10th	Day	20% 9% 3 6% 3 2 3 2 3 4 44%	1710
	11th to 20th	Day	17% 9% 2 6% 4 3 3 3 5% 5% 44%	1686
	21st to end	Day	19% 9% 3 6% 4 4 3 3 4 4 40%	1727
March	1st to 10th	Day	18% 8% 3 5% 4 3 3 2 3 5% 46%	3436
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 4 6% 47%	3480
	21st to end	Day	17% 8% 3 7% 4 4 4 3 4 6% 41%	3814
April	1st to 10th	Day	13% 6% 3 7% 4 4 4 3 4 6% 46%	2194
	11th to 20th	Day	15% 7% 3 6% 5% 3 4 3 4 5% 45%	2254
	21st to end	Day	17% 8% 2 7% 5% 4 4 3 5% 5% 40%	2280
May	1st to 10th	Day	18% 8% 3 7% 4 4 4 3 5% 6% 37%	4005
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 4 6% 8% 42%	4082
	21st to end	Day	15% 7% 3 7% 5% 4 6% 3 6% 7% 37%	4477
June	1st to 10th	Day	8% 6% 3 6% 4 5% 4 4 7% 9% 43%	2470
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 7% 8% 46%	2470
	21st to end	Day	4 3 2 4 4 5% 6% 5% 8% 9% 52%	2470
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 5% 7% 11% 43%	4120
	11th to 20th	Day	4 3 1 5% 3 4 4 5% 7% 8% 56%	4120
	21st to end	Day	7% 6% 3 7% 6% 6% 6% 6% 8% 10% 36%	4465
August	1st to 10th	Day	6% 3 2 6% 6% 5% 6% 5% 8% 9% 45%	2458
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 8% 8% 45%	2372
	21st to end	Day	6% 4 1 6% 4 4 6% 5% 6% 8% 50%	2485
September	1st to 10th	Day	11% 5% 2 4 4 4 4 3 5% 7% 52%	2160
	11th to 20th	Day	12% 5% 2 4 3 4 5% 3 5% 8% 48%	2017
	21st to end	Day	8% 4 1 4 4 4 5% 4 4 5% 57%	1949
October	1st to 10th	Day	8% 5% 2 5% 4 3 4 3 5% 7% 55%	1800
	11th to 20th	Day	14% 4 2 5% 3 2 3 3 3 5% 57%	1800
	21st to end	Day	18% 5% 2 5% 2 2 3 3 4 5% 50%	1980
November	1st to 10th	Day	12% 5% 1 4 3 3 3 3 4 6% 57%	1858
	11th to 20th	Day	11% 6% 1 5% 3 3 3 3 4 6% 55%	1900
	21st to end	Day	17% 7% 3 5% 3 2 3 3 4 5% 49%	1715
December	1st to 10th	Day	16% 6% 2 6% 3 3 3 2 5% 6% 48%	1710
	11th to 20th	Day	13% 9% 4 7% 4 3 4 3 4 6% 43%	2161
	21st to end	Day	17% 13% 4 8% 4 3 4 3 4 6% 35%	3135
January	1st to 10th	Night	14% 9% 3 6% 4 4 4 3 4 5% 44%	4108
	11th to 20th	Night	12% 6% 2 4 3 2 2 2 3 4 59%	4120
	21st to end	Night	23% 7% 2 4 3 2 2 2 3 5% 47%	4532
February	1st to 10th	Night	18% 8% 2 6% 4 3 2 2 5% 5% 44%	2470
	11th to 20th	Night	16% 7% 3 5% 3 2 3 3 5% 5% 47%	2389
	21st to end	Night	17% 6% 2 6% 4 3 3 3 4 6% 47%	1732
March	1st to 10th	Night	21% 6% 2 5% 4 3 2 2 4 6% 44%	3419
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 4 6% 49%	3480
	21st to end	Night	15% 7% 3 6% 4 4 4 3 5% 7% 44%	3817
April	1st to 10th	Night	12% 5% 3 6% 3 3 3 3 4 7% 49%	1984
	11th to 20th	Night	12% 6% 2 6% 3 4 3 3 5% 6% 50%	1875
	21st to end	Night	15% 7% 2 6% 4 3 4 3 4 7% 45%	1900
May	1st to 10th	Night	14% 7% 2 6% 4 4 3 3 5% 7% 43%	2894
	11th to 20th	Night	13% 5% 2 6% 4 5% 4 3 6% 8% 43%	2823
	21st to end	Night	15% 6% 2 6% 4 4 4 3 6% 8% 42%	3090
June	1st to 10th	Night	4 4 1 4 4 3 4 4 7% 9% 56%	1710
	11th to 20th	Night	4 3 1 3 3 3 3 4 6% 11% 60%	1710
	21st to end	Night	3 2 1 3 3 3 4 3 5% 9% 64%	1710
July	1st to 10th	Night	8% 4 2 6% 4 4 4 3 6% 9% 49%	2850
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 9% 61%	2850
	21st to end	Night	5% 5% 2 7% 4 5% 5% 5% 7% 9% 45%	3098
August	1st to 10th	Night	3 2 1 5% 4 5% 5% 5% 7% 9% 54%	1701
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 9% 50%	1747
	21st to end	Night	5% 4 1 6% 4 4 4 4 7% 8% 53%	2080
September	1st to 10th	Night	7% 4 1 4 4 3 4 2 5% 8% 58%	1800
	11th to 20th	Night	9% 4 1 5% 4 3 4 4 7% 8% 51%	1938
	21st to end	Night	7% 3 1 5% 3 3 4 3 5% 7% 59%	1992
October	1st to 10th	Night	7% 3 1 4 3 3 4 3 5% 8% 60%	2160
	11th to 20th	Night	13% 4 2 4 3 3 3 2 4 5% 59%	2160
	21st to end	Night	18% 4 2 5% 2 3 2 2 3 4 54%	2376
November	1st to 10th	Night	11% 5% 2 4 3 3 3 3 4 5% 58%	2220
	11th to 20th	Night	12% 5% 1 3 3 3 2 3 4 6% 57%	2280
	21st to end	Night	18% 5% 2 5% 3 3 3 3 3 6% 51%	2462
December	1st to 10th	Night	16% 5% 2 5% 3 3 3 2 4 6% 49%	2470
	11th to 20th	Night	16% 7% 3 6% 4 4 3 3 4 5% 46%	3036
	21st to end	Night	18% 9% 4 7% 4 4 4 3 4 5% 40%	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	43% 3 5% 24% 22% 3	2848
	11th to 20th	Day	35% 5% 12% 24% 21% 3	2850
	21st to end	Day	44% 3 6% 19% 22% 6%	3135
February	1st to 10th	Day	46% 3 10% 21% 17% 4	1710
	11th to 20th	Day	44% 3 7% 22% 20% 4	1695
	21st to end	Day	48% 4 6% 16% 19% 8%	1727
March	1st to 10th	Day	41% 5% 7% 18% 18% 11%	3436
	11th to 20th	Day	38% 5% 10% 23% 19% 4	3480
	21st to end	Day	45% 4 8% 26% 15% 1	3814
April	1st to 10th	Day	40% 4 9% 21% 20% 6%	2194
	11th to 20th	Day	41% 4 9% 22% 18% 5%	2254
	21st to end	Day	46% 6% 7% 22% 18% 2	2280
May	1st to 10th	Day	46% 6% 8% 22% 16% 2	4042
	11th to 20th	Day	38% 7% 11% 26% 17% 1	4086
	21st to end	Day	45% 8% 11% 20% 15% 1	4489
June	1st to 10th	Day	36% 8% 13% 26% 15% 2	2470
	11th to 20th	Day	33% 7% 15% 25% 19% 2	2470
	21st to end	Day	26% 8% 13% 29% 21% 4	2470
July	1st to 10th	Day	33% 7% 17% 28% 13% 2	4056
	11th to 20th	Day	24% 7% 9% 36% 22% 2	4051
	21st to end	Day	39% 8% 16% 26% 10% 1	4325
August	1st to 10th	Day	32% 9% 14% 29% 14% 2	2444
	11th to 20th	Day	33% 8% 13% 27% 18% 2	2372
	21st to end	Day	29% 8% 14% 29% 16% 4	2485
September	1st to 10th	Day	32% 6% 9% 26% 19% 8%	2160
	11th to 20th	Day	33% 6% 10% 24% 22% 6%	2017
	21st to end	Day	29% 6% 9% 20% 28% 8%	1949
October	1st to 10th	Day	29% 5% 9% 22% 27% 8%	1800
	11th to 20th	Day	31% 4 6% 19% 29% 11%	1800
	21st to end	Day	38% 4 8% 22% 20% 8%	1980
November	1st to 10th	Day	30% 4 8% 26% 25% 7%	1858
	11th to 20th	Day	32% 3 6% 24% 26% 8%	1900
	21st to end	Day	38% 4 7% 19% 26% 6%	1715
December	1st to 10th	Day	38% 5% 8% 24% 21% 4	1710
	11th to 20th	Day	43% 4 6% 20% 22% 5%	2159
	21st to end	Day	51% 4 7% 19% 14% 5%	3135
January	1st to 10th	Night	43% 3 6% 20% 24% 3	4108
	11th to 20th	Night	31% 4 13% 23% 26% 3	4120
	21st to end	Night	42% 3 9% 18% 23% 4	4532
February	1st to 10th	Night	43% 4 8% 24% 17% 5%	2470
	11th to 20th	Night	39% 3 7% 23% 21% 6%	2390
	21st to end	Night	40% 3 7% 19% 21% 9%	1732
March	1st to 10th	Night	43% 5% 8% 16% 18% 11%	3419
	11th to 20th	Night	36% 4 8% 26% 19% 6%	3480
	21st to end	Night	40% 5% 10% 25% 17% 2	3817
April	1st to 10th	Night	35% 4 10% 21% 21% 9%	1984
	11th to 20th	Night	35% 5% 8% 22% 20% 10%	1875
	21st to end	Night	38% 6% 10% 22% 23% 2	1900
May	1st to 10th	Night	38% 8% 11% 24% 15% 4	2922
	11th to 20th	Night	38% 8% 12% 24% 16% 1	2824
	21st to end	Night	38% 8% 14% 23% 14% 3	3099
June	1st to 10th	Night	23% 8% 15% 31% 20% 3	1710
	11th to 20th	Night	19% 7% 14% 34% 24% 3	1710
	21st to end	Night	18% 5% 13% 31% 27% 6%	1710
July	1st to 10th	Night	30% 7% 17% 26% 17% 3	2805
	11th to 20th	Night	18% 6% 10% 38% 25% 2	2796
	21st to end	Night	33% 7% 17% 28% 15% 1	2999
August	1st to 10th	Night	24% 7% 14% 35% 17% 2	1691
	11th to 20th	Night	29% 6% 14% 27% 22% 3	1747
	21st to end	Night	27% 7% 13% 28% 22% 4	2080
September	1st to 10th	Night	26% 5% 9% 28% 24% 9%	1800
	11th to 20th	Night	29% 7% 11% 22% 25% 6%	1938
	21st to end	Night	25% 5% 8% 23% 30% 9%	1992
October	1st to 10th	Night	23% 5% 13% 23% 25% 11%	2160
	11th to 20th	Night	29% 3 6% 17% 32% 12%	2160
	21st to end	Night	36% 3 7% 20% 24% 11%	2376
November	1st to 10th	Night	29% 4 8% 23% 26% 11%	2220
	11th to 20th	Night	29% 3 7% 24% 27% 10%	2280
	21st to end	Night	36% 3 8% 19% 27% 6%	2462
December	1st to 10th	Night	38% 4 9% 22% 22% 6%	2470
	11th to 20th	Night	41% 3 6% 22% 21% 6%	3037
	21st to end	Night	47% 4 8% 20% 17% 4	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	26% 13% 29% 23% 8% 1	270
	11th to 20th	Day	24% 16% 34% 19% 6%	270
	21st to end	Day	22% 19% 32% 18% 8% 1	297
February	1st to 10th	Day	27% 14% 35% 17% 4 1	270
	11th to 20th	Day	27% 19% 33% 16% 4	270
	21st to end	Day	36% 21% 28% 12% 3	225
March	1st to 10th	Day	21% 19% 30% 23% 6% 1	270
	11th to 20th	Day	27% 17% 36% 17% 3	270
	21st to end	Day	29% 19% 33% 13% 5%	297
April	1st to 10th	Day	22% 17% 33% 19% 8% 1	374
	11th to 20th	Day	21% 14% 32% 25% 8%	452
	21st to end	Day	18% 18% 30% 23% 9% 2	460
May	1st to 10th	Day	17% 12% 31% 30% 10%	601
	11th to 20th	Day	15% 7% 25% 30% 20% 3	675
	21st to end	Day	12% 6% 27% 30% 19% 6%	734
June	1st to 10th	Day	9% 4 16% 28% 33% 10%	680
	11th to 20th	Day	6% 3 14% 36% 31% 10%	680
	21st to end	Day	4 3 14% 37% 33% 10%	680
July	1st to 10th	Day	7% 2 10% 28% 39% 14%	671
	11th to 20th	Day	4 1 8% 31% 37% 18%	668
	21st to end	Day	6% 2 8% 27% 38% 20%	739
August	1st to 10th	Day	7% 2 8% 32% 39% 11%	676
	11th to 20th	Day	12% 2 8% 31% 36% 11%	606
	21st to end	Day	6% 1 6% 23% 43% 21%	532
September	1st to 10th	Day	9% 1 7% 24% 43% 16%	450
	11th to 20th	Day	10% 2 9% 30% 37% 12%	307
	21st to end	Day	11% 4 15% 30% 24% 15%	270
October	1st to 10th	Day	4 4 21% 35% 29% 6%	270
	11th to 20th	Day	14% 11% 28% 30% 15% 2	270
	21st to end	Day	28% 14% 24% 21% 10% 2	297
November	1st to 10th	Day	18% 7% 31% 20% 20% 3	270
	11th to 20th	Day	15% 13% 34% 27% 9% 2	270
	21st to end	Day	23% 16% 21% 25% 14% 1	270
December	1st to 10th	Day	19% 17% 35% 17% 10% 3	270
	11th to 20th	Day	21% 15% 32% 23% 9%	270
	21st to end	Day	24% 16% 35% 20% 5% 1	297
January	1st to 10th	Night	19% 9% 33% 32% 6%	950
	11th to 20th	Night	19% 12% 31% 32% 6%	950
	21st to end	Night	18% 8% 35% 34% 5%	1045
February	1st to 10th	Night	22% 11% 30% 33% 4	950
	11th to 20th	Night	21% 9% 37% 30% 4	915
	21st to end	Night	21% 13% 37% 28% 1	476
March	1st to 10th	Night	20% 9% 30% 36% 5%	570
	11th to 20th	Night	20% 11% 32% 31% 6%	570
	21st to end	Night	19% 10% 33% 30% 7%	627
April	1st to 10th	Night	16% 10% 34% 33% 6%	570
	11th to 20th	Night	14% 10% 34% 33% 9%	563
	21st to end	Night	19% 6% 35% 30% 10%	570
May	1st to 10th	Night	14% 5% 28% 42% 11% 1	570
	11th to 20th	Night	14% 6% 23% 38% 17% 2	561
	21st to end	Night	10% 3 22% 37% 25% 3	617
June	1st to 10th	Night	6% 3 10% 39% 40% 2	570
	11th to 20th	Night	4 1 11% 49% 33% 2	570
	21st to end	Night	4 1 11% 38% 41% 6%	570
July	1st to 10th	Night	6% 1 7% 37% 43% 6%	555
	11th to 20th	Night	3 1 7% 38% 43% 9%	552
	21st to end	Night	5% 4 35% 46% 9%	624
August	1st to 10th	Night	3 1 7% 41% 43% 5%	567
	11th to 20th	Night	8% 1 9% 35% 42% 5%	564
	21st to end	Night	6% 2 4 36% 46% 7%	624
September	1st to 10th	Night	9% 1 6% 36% 44% 4	540
	11th to 20th	Night	11% 1 9% 38% 38% 4	540
	21st to end	Night	8% 1 14% 38% 34% 5%	552
October	1st to 10th	Night	9% 3 14% 39% 32% 4	720
	11th to 20th	Night	15% 5% 25% 40% 16% 1	720
	21st to end	Night	20% 8% 22% 33% 17% 1	792
November	1st to 10th	Night	11% 6% 25% 42% 15% 1	740
	11th to 20th	Night	13% 9% 28% 37% 12%	760
	21st to end	Night	20% 10% 25% 33% 11%	942
December	1st to 10th	Night	20% 8% 29% 35% 8%	950
	11th to 20th	Night	22% 11% 24% 39% 5%	950
	21st to end	Night	19% 12% 29% 34% 6%	1045

Page generated at 2024-08-27 08:49:42. Version ac07

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