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 2024-01-27.

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	12% 6% 3 5% 5% 3 4 4 5% 7% 46%	75111
	Night	12% 5% 2 4 4 2 3 4 5% 7% 51%	74964
Transparency	Day	38% 6% 10% 24% 19% 4	74835
	Night	33% 5% 10% 24% 22% 6%	74760
Seeing	Day	14% 8% 21% 26% 23% 8%	14276
	Night	14% 6% 23% 36% 18% 2	24254

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% 7% 2 5% 4 3 4 2 3 4 50%	1707
	11th to 20th	Day	14% 6% 3 4 5% 3 3 4 5% 5% 47%	1710
	21st to end	Day	16% 7% 2 4 4 2 3 3 4 4 50%	1836
February	1st to 10th	Day	20% 8% 4 5% 4 1 3 3 3 4 45%	1620
	11th to 20th	Day	17% 8% 3 6% 4 2 3 4 5% 5% 43%	1596
	21st to end	Day	20% 9% 3 5% 5% 3 4 3 4 5% 40%	1628
March	1st to 10th	Day	16% 7% 3 4 5% 2 3 3 5% 5% 47%	1980
	11th to 20th	Day	20% 7% 3 5% 5% 2 5% 3 3 5% 42%	1980
	21st to end	Day	20% 8% 3 6% 6% 2 4 3 4 6% 38%	2178
April	1st to 10th	Day	13% 6% 3 6% 5% 3 4 4 4 6% 46%	2078
	11th to 20th	Day	14% 6% 3 6% 6% 2 4 4 4 6% 45%	2134
	21st to end	Day	17% 7% 3 7% 6% 3 4 4 4 5% 40%	2160
May	1st to 10th	Day	16% 7% 4 5% 4 3 4 4 5% 6% 42%	2290
	11th to 20th	Day	12% 5% 3 6% 6% 3 5% 6% 6% 8% 41%	2302
	21st to end	Day	11% 6% 3 6% 6% 3 7% 5% 6% 9% 39%	2531
June	1st to 10th	Day	8% 6% 3 6% 5% 3 5% 5% 6% 10% 43%	2340
	11th to 20th	Day	6% 3 3 6% 6% 4 6% 6% 7% 8% 46%	2340
	21st to end	Day	4 2 2 3 4 3 6% 6% 7% 9% 53%	2340
July	1st to 10th	Day	8% 5% 3 6% 6% 4 6% 7% 7% 12% 37%	2340
	11th to 20th	Day	4 3 1 4 5% 3 6% 7% 7% 9% 51%	2340
	21st to end	Day	6% 4 2 5% 6% 4 6% 7% 8% 11% 42%	2546
August	1st to 10th	Day	6% 3 2 5% 7% 3 6% 7% 8% 9% 43%	2328
	11th to 20th	Day	10% 5% 2 5% 5% 3 5% 5% 7% 9% 45%	2246
	21st to end	Day	6% 4 2 5% 5% 4 5% 6% 7% 8% 49%	2376
September	1st to 10th	Day	11% 5% 2 4 4 3 4 4 5% 7% 52%	2160
	11th to 20th	Day	12% 4 2 4 4 3 5% 5% 5% 8% 48%	2017
	21st to end	Day	8% 4 1 3 5% 3 5% 5% 4 5% 57%	1949
October	1st to 10th	Day	8% 5% 2 4 4 3 4 5% 5% 7% 55%	1800
	11th to 20th	Day	14% 4 2 4 3 2 3 4 3 5% 57%	1800
	21st to end	Day	18% 5% 3 5% 3 2 3 3 4 5% 50%	1980
November	1st to 10th	Day	12% 4 2 4 3 2 3 4 3 6% 57%	1858
	11th to 20th	Day	11% 6% 2 4 3 2 3 3 4 6% 56%	1900
	21st to end	Day	17% 7% 3 5% 3 1 3 3 4 5% 50%	1715
December	1st to 10th	Day	16% 6% 3 5% 4 2 3 3 5% 5% 48%	1710
	11th to 20th	Day	14% 8% 4 7% 5% 3 4 4 4 5% 43%	2161
	21st to end	Day	17% 13% 4 6% 5% 3 3 4 4 6% 35%	3135
January	1st to 10th	Night	16% 6% 2 4 3 2 3 3 4 5% 53%	2458
	11th to 20th	Night	15% 6% 3 4 3 2 3 3 4 5% 52%	2470
	21st to end	Night	16% 6% 2 4 4 2 2 3 3 5% 54%	2665
February	1st to 10th	Night	19% 8% 2 5% 4 2 2 3 4 5% 45%	2340
	11th to 20th	Night	16% 7% 4 4 4 2 3 4 5% 5% 46%	2259
	21st to end	Night	18% 6% 3 6% 4 2 3 3 3 6% 46%	1632
March	1st to 10th	Night	16% 5% 2 6% 5% 3 3 4 4 6% 48%	1980
	11th to 20th	Night	18% 6% 3 4 4 2 4 4 5% 6% 44%	1980
	21st to end	Night	18% 6% 3 5% 4 3 4 3 4 5% 45%	2178
April	1st to 10th	Night	12% 5% 3 5% 3 3 3 4 4 8% 50%	1880
	11th to 20th	Night	12% 6% 2 5% 4 3 4 4 5% 6% 50%	1775
	21st to end	Night	15% 6% 3 5% 4 2 4 5% 4 6% 46%	1800
May	1st to 10th	Night	12% 5% 2 5% 5% 3 4 4 5% 7% 48%	1666
	11th to 20th	Night	13% 5% 3 4 4 3 5% 4 5% 8% 45%	1593
	21st to end	Night	7% 4 2 5% 5% 3 5% 5% 6% 11% 46%	1745
June	1st to 10th	Night	4 4 1 3 4 2 4 5% 7% 8% 57%	1620
	11th to 20th	Night	4 3 2 3 3 2 3 4 5% 11% 60%	1620
	21st to end	Night	3 2 1 3 3 2 4 4 5% 9% 64%	1620
July	1st to 10th	Night	4 4 2 4 6% 3 5% 5% 8% 12% 48%	1620
	11th to 20th	Night	3 3 1 3 3 3 4 5% 5% 11% 60%	1620
	21st to end	Night	3 2 2 4 4 3 5% 6% 6% 11% 53%	1772
August	1st to 10th	Night	3 2 1 4 5% 4 5% 6% 7% 10% 53%	1611
	11th to 20th	Night	6% 4 2 4 5% 3 5% 6% 6% 9% 50%	1654
	21st to end	Night	5% 4 1 5% 5% 3 4 5% 6% 9% 52%	1980
September	1st to 10th	Night	7% 4 2 4 4 2 4 3 5% 8% 58%	1800
	11th to 20th	Night	9% 4 1 5% 5% 3 3 5% 7% 8% 51%	1938
	21st to end	Night	7% 3 1 4 4 2 4 4 5% 7% 59%	1992
October	1st to 10th	Night	7% 3 2 3 3 2 4 4 5% 7% 60%	2160
	11th to 20th	Night	13% 4 2 3 3 2 3 3 3 5% 59%	2160
	21st to end	Night	18% 4 2 4 3 2 2 3 3 4 54%	2376
November	1st to 10th	Night	11% 5% 2 3 3 2 3 3 3 5% 59%	2220
	11th to 20th	Night	12% 5% 1 3 4 2 3 3 4 6% 58%	2280
	21st to end	Night	18% 5% 2 4 3 2 3 3 3 6% 51%	2462
December	1st to 10th	Night	16% 5% 2 4 4 3 3 3 4 6% 49%	2470
	11th to 20th	Night	16% 7% 3 5% 4 3 4 4 4 5% 46%	3036
	21st to end	Night	18% 9% 4 6% 5% 3 3 4 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	40% 4 7% 24% 20% 5%	1708
	11th to 20th	Day	39% 4 8% 25% 22% 2	1710
	21st to end	Day	39% 3 7% 26% 21% 4	1836
February	1st to 10th	Day	45% 3 10% 21% 16% 4	1620
	11th to 20th	Day	45% 3 7% 22% 18% 4	1605
	21st to end	Day	49% 3 6% 15% 19% 8%	1628
March	1st to 10th	Day	40% 3 7% 20% 19% 10%	1980
	11th to 20th	Day	47% 4 7% 22% 15% 5%	1980
	21st to end	Day	48% 4 8% 25% 13% 2	2178
April	1st to 10th	Day	40% 4 10% 21% 20% 5%	2078
	11th to 20th	Day	41% 4 9% 23% 17% 5%	2134
	21st to end	Day	46% 6% 7% 22% 18% 2	2160
May	1st to 10th	Day	44% 5% 10% 21% 19% 2	2290
	11th to 20th	Day	39% 7% 14% 24% 15% 1	2306
	21st to end	Day	42% 8% 12% 24% 14% 1	2531
June	1st to 10th	Day	36% 8% 14% 26% 15% 2	2340
	11th to 20th	Day	33% 7% 15% 25% 18% 2	2340
	21st to end	Day	25% 8% 13% 28% 22% 4	2340
July	1st to 10th	Day	36% 10% 15% 25% 13% 1	2276
	11th to 20th	Day	26% 9% 13% 28% 20% 3	2271
	21st to end	Day	32% 10% 16% 29% 11% 1	2405
August	1st to 10th	Day	33% 9% 14% 29% 13% 1	2314
	11th to 20th	Day	33% 8% 13% 26% 18% 2	2246
	21st to end	Day	30% 9% 14% 27% 16% 4	2376
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	36% 3 8% 22% 25% 6%	2458
	11th to 20th	Night	36% 3 10% 23% 23% 4	2470
	21st to end	Night	35% 3 6% 26% 25% 6%	2665
February	1st to 10th	Night	42% 4 8% 24% 17% 5%	2340
	11th to 20th	Night	40% 3 7% 24% 20% 6%	2260
	21st to end	Night	42% 3 7% 19% 21% 9%	1632
March	1st to 10th	Night	40% 3 8% 17% 21% 11%	1980
	11th to 20th	Night	42% 5% 9% 19% 18% 7%	1980
	21st to end	Night	43% 4 8% 24% 18% 4	2178
April	1st to 10th	Night	34% 4 10% 22% 22% 8%	1880
	11th to 20th	Night	35% 5% 8% 22% 20% 9%	1775
	21st to end	Night	38% 6% 9% 22% 24% 2	1800
May	1st to 10th	Night	35% 6% 12% 23% 18% 5%	1666
	11th to 20th	Night	36% 7% 13% 24% 18% 2	1594
	21st to end	Night	31% 8% 17% 27% 15% 1	1746
June	1st to 10th	Night	22% 8% 16% 31% 20% 3	1620
	11th to 20th	Night	19% 7% 14% 34% 23% 3	1620
	21st to end	Night	18% 5% 13% 30% 28% 6%	1620
July	1st to 10th	Night	25% 9% 18% 26% 20% 2	1575
	11th to 20th	Night	17% 8% 14% 32% 25% 3	1566
	21st to end	Night	23% 7% 16% 34% 19% 2	1673
August	1st to 10th	Night	25% 8% 14% 35% 17% 1	1601
	11th to 20th	Night	29% 6% 14% 26% 22% 3	1654
	21st to end	Night	28% 7% 13% 27% 22% 4	1980
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	23% 19% 31% 18% 8% 1	282
February	1st to 10th	Day	27% 15% 35% 17% 5% 1	240
	11th to 20th	Day	28% 20% 34% 16% 2	240
	21st to end	Day	38% 19% 26% 14% 3	198
March	1st to 10th	Day	23% 20% 29% 22% 6% 1	240
	11th to 20th	Day	30% 17% 34% 17% 3	240
	21st to end	Day	28% 19% 33% 14% 5%	264
April	1st to 10th	Day	20% 16% 34% 20% 9% 1	338
	11th to 20th	Day	20% 15% 30% 26% 9%	412
	21st to end	Day	18% 17% 31% 23% 9% 2	420
May	1st to 10th	Day	18% 11% 31% 30% 10%	553
	11th to 20th	Day	15% 8% 25% 29% 20% 3	625
	21st to end	Day	12% 5% 26% 31% 20% 6%	679
June	1st to 10th	Day	10% 4 14% 27% 34% 11%	630
	11th to 20th	Day	6% 3 13% 36% 32% 10%	630
	21st to end	Day	4 3 14% 37% 32% 10%	630
July	1st to 10th	Day	7% 1 10% 27% 39% 15%	621
	11th to 20th	Day	4 1 8% 30% 38% 19%	618
	21st to end	Day	5% 2 7% 25% 40% 21%	684
August	1st to 10th	Day	8% 2 8% 31% 40% 11%	626
	11th to 20th	Day	11% 2 7% 31% 36% 11%	560
	21st to end	Day	6% 1 5% 24% 43% 21%	495
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%	1025
February	1st to 10th	Night	23% 11% 29% 33% 4	900
	11th to 20th	Night	21% 9% 37% 29% 4	865
	21st to end	Night	22% 12% 36% 28% 2	448
March	1st to 10th	Night	19% 9% 30% 37% 5%	540
	11th to 20th	Night	21% 10% 31% 31% 7%	540
	21st to end	Night	18% 9% 34% 31% 8%	594
April	1st to 10th	Night	16% 9% 34% 34% 6%	540
	11th to 20th	Night	14% 10% 33% 35% 9%	533
	21st to end	Night	18% 6% 35% 30% 10%	540
May	1st to 10th	Night	14% 5% 28% 41% 11% 1	540
	11th to 20th	Night	14% 6% 23% 37% 18% 2	531
	21st to end	Night	9% 3 22% 37% 26% 3	584
June	1st to 10th	Night	6% 3 9% 39% 41% 2	540
	11th to 20th	Night	4 1 10% 51% 32% 2	540
	21st to end	Night	4 1 11% 38% 40% 6%	540
July	1st to 10th	Night	5% 1 6% 36% 45% 7%	525
	11th to 20th	Night	3 1 6% 36% 44% 9%	522
	21st to end	Night	5% 3 35% 47% 10%	591
August	1st to 10th	Night	4 1 6% 41% 43% 5%	537
	11th to 20th	Night	8% 1 8% 35% 42% 5%	534
	21st to end	Night	6% 2 4 36% 46% 7%	594
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 2024-01-24 08:37:26. Version ac07

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