The Stellar Shed
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Stellar Shed clear sky chart from 2005-11-06 to 2024-01-15.

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% 8% 3 6% 5% 3 5% 5% 5% 7% 39%	74772
	Night	13% 8% 4 6% 5% 3 4 4 5% 7% 41%	75028
Transparency	Day	44% 7% 13% 22% 13% 2	74474
	Night	43% 6% 12% 22% 14% 2	74846
Seeing	Day	15% 10% 29% 26% 15% 5%	14634
	Night	16% 9% 31% 30% 12% 1	24610

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	11% 12% 4 9% 8% 4 6% 6% 6% 7% 27%	1517
	11th to 20th	Day	14% 15% 6% 10% 9% 5% 4 3 5% 5% 23%	1504
	21st to end	Day	15% 14% 8% 10% 8% 3 4 3 3 4 26%	1782
February	1st to 10th	Day	16% 13% 5% 8% 7% 3 5% 4 5% 6% 29%	1620
	11th to 20th	Day	15% 9% 4 7% 6% 4 4 4 5% 6% 35%	1656
	21st to end	Day	14% 12% 6% 10% 5% 3 3 3 5% 6% 34%	1480
March	1st to 10th	Day	16% 11% 4 7% 5% 3 4 4 4 6% 35%	1828
	11th to 20th	Day	14% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	1980
	21st to end	Day	20% 11% 5% 9% 6% 3 4 4 5% 6% 27%	2281
April	1st to 10th	Day	14% 11% 4 7% 6% 3 4 5% 4 6% 35%	2160
	11th to 20th	Day	17% 9% 3 6% 6% 2 4 4 4 7% 37%	2134
	21st to end	Day	20% 8% 4 6% 6% 3 4 4 5% 7% 33%	2160
May	1st to 10th	Day	16% 8% 4 6% 6% 3 6% 5% 6% 9% 31%	2374
	11th to 20th	Day	19% 7% 3 7% 5% 3 4 5% 6% 7% 35%	2479
	21st to end	Day	13% 7% 3 6% 7% 4 6% 6% 6% 8% 34%	2725
June	1st to 10th	Day	8% 5% 2 5% 5% 3 5% 5% 6% 9% 45%	2520
	11th to 20th	Day	9% 5% 2 5% 5% 3 6% 4 5% 9% 46%	2520
	21st to end	Day	10% 6% 2 5% 5% 3 5% 6% 6% 9% 41%	2520
July	1st to 10th	Day	7% 4 2 5% 4 3 4 5% 7% 10% 49%	2520
	11th to 20th	Day	8% 4 1 5% 5% 4 5% 5% 6% 10% 47%	2520
	21st to end	Day	6% 4 2 5% 5% 4 5% 7% 6% 11% 47%	2708
August	1st to 10th	Day	7% 5% 2 5% 5% 4 6% 7% 8% 10% 41%	2328
	11th to 20th	Day	8% 5% 3 4 5% 3 5% 6% 6% 8% 47%	2204
	21st to end	Day	8% 4 2 5% 5% 2 4 4 6% 9% 51%	2376
September	1st to 10th	Day	12% 5% 2 5% 4 3 5% 4 5% 8% 48%	2160
	11th to 20th	Day	10% 4 2 4 4 3 4 4 4 8% 54%	2156
	21st to end	Day	9% 5% 2 5% 5% 3 5% 6% 6% 8% 48%	1936
October	1st to 10th	Day	13% 7% 3 5% 4 2 3 4 5% 6% 48%	1800
	11th to 20th	Day	17% 8% 3 6% 5% 2 3 4 4 6% 43%	1800
	21st to end	Day	20% 8% 2 4 4 3 4 4 4 6% 41%	1922
November	1st to 10th	Day	11% 8% 2 5% 5% 3 4 5% 5% 7% 46%	1672
	11th to 20th	Day	16% 9% 3 6% 5% 3 3 4 6% 7% 37%	1689
	21st to end	Day	17% 10% 4 6% 6% 3 4 3 3 6% 38%	1520
December	1st to 10th	Day	14% 12% 5% 9% 5% 3 5% 4 4 6% 33%	1520
	11th to 20th	Day	13% 11% 4 7% 6% 4 5% 4 5% 6% 34%	1918
	21st to end	Day	22% 19% 7% 11% 6% 2 3 4 4 4 17%	2783
January	1st to 10th	Night	11% 11% 5% 8% 7% 4 4 3 4 6% 36%	2648
	11th to 20th	Night	14% 12% 8% 9% 7% 3 4 4 4 6% 29%	2545
	21st to end	Night	13% 11% 5% 10% 7% 4 5% 4 4 6% 31%	2574
February	1st to 10th	Night	15% 12% 5% 8% 6% 3 5% 4 4 6% 32%	2340
	11th to 20th	Night	13% 10% 3 6% 5% 3 4 5% 5% 7% 38%	2189
	21st to end	Night	16% 11% 4 7% 6% 3 4 3 4 5% 39%	1776
March	1st to 10th	Night	14% 8% 5% 7% 5% 3 4 4 4 5% 40%	2146
	11th to 20th	Night	14% 9% 3 7% 5% 4 4 4 5% 7% 39%	1980
	21st to end	Night	20% 10% 5% 8% 7% 4 4 4 5% 5% 29%	2073
April	1st to 10th	Night	15% 9% 4 6% 4 3 3 4 6% 7% 40%	1800
	11th to 20th	Night	17% 9% 3 5% 5% 2 3 3 5% 6% 42%	1775
	21st to end	Night	16% 6% 3 6% 6% 3 4 4 6% 8% 38%	1800
May	1st to 10th	Night	16% 8% 3 6% 5% 3 5% 4 5% 8% 37%	1583
	11th to 20th	Night	17% 5% 3 6% 5% 3 5% 3 5% 8% 39%	1416
	21st to end	Night	10% 6% 2 5% 6% 4 6% 5% 7% 8% 41%	1551
June	1st to 10th	Night	7% 5% 2 4 6% 3 4 5% 5% 10% 49%	1440
	11th to 20th	Night	8% 4 3 5% 5% 2 5% 6% 7% 9% 46%	1440
	21st to end	Night	10% 6% 2 5% 5% 3 4 5% 5% 9% 48%	1440
July	1st to 10th	Night	7% 3 2 4 5% 3 4 5% 7% 10% 51%	1440
	11th to 20th	Night	5% 3 2 3 4 3 3 5% 7% 9% 57%	1440
	21st to end	Night	5% 4 2 4 6% 3 5% 5% 7% 10% 50%	1606
August	1st to 10th	Night	4 3 2 5% 5% 3 4 6% 6% 10% 52%	1611
	11th to 20th	Night	7% 3 3 4 4 3 5% 4 6% 7% 54%	1697
	21st to end	Night	7% 4 1 4 4 2 3 3 5% 8% 59%	1980
September	1st to 10th	Night	10% 4 2 4 4 2 3 3 5% 7% 56%	1800
	11th to 20th	Night	10% 4 2 4 4 3 4 5% 5% 7% 52%	1800
	21st to end	Night	7% 4 2 5% 5% 3 4 4 6% 8% 53%	2007
October	1st to 10th	Night	11% 5% 2 5% 5% 3 3 3 4 6% 53%	2160
	11th to 20th	Night	15% 6% 2 5% 4 2 3 3 5% 6% 47%	2160
	21st to end	Night	18% 7% 3 6% 5% 3 4 4 5% 6% 40%	2433
November	1st to 10th	Night	12% 6% 2 4 4 2 3 4 4 7% 53%	2406
	11th to 20th	Night	16% 7% 3 5% 5% 3 4 4 5% 8% 41%	2489
	21st to end	Night	17% 10% 3 7% 5% 3 4 4 4 5% 40%	2660
December	1st to 10th	Night	13% 11% 5% 8% 6% 3 5% 4 4 7% 35%	2660
	11th to 20th	Night	15% 11% 4 7% 6% 3 4 5% 4 6% 35%	3279
	21st to end	Night	22% 17% 7% 10% 7% 3 4 4 3 4 19%	4884

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	55% 6% 13% 15% 10% 1	1518
	11th to 20th	Day	63% 5% 11% 16% 5%	1504
	21st to end	Day	62% 5% 11% 13% 8% 1	1782
February	1st to 10th	Day	57% 6% 15% 17% 5% 1	1620
	11th to 20th	Day	49% 7% 13% 21% 9% 1	1665
	21st to end	Day	52% 5% 12% 20% 9% 1	1480
March	1st to 10th	Day	50% 6% 14% 20% 9% 1	1828
	11th to 20th	Day	47% 7% 14% 21% 10% 1	1980
	21st to end	Day	57% 6% 12% 18% 6% 1	2281
April	1st to 10th	Day	48% 6% 12% 22% 11%	2160
	11th to 20th	Day	47% 6% 10% 23% 12% 2	2134
	21st to end	Day	50% 7% 11% 18% 13% 2	2160
May	1st to 10th	Day	49% 8% 14% 19% 10% 1	2374
	11th to 20th	Day	48% 8% 11% 21% 12%	2483
	21st to end	Day	46% 9% 14% 16% 13% 2	2725
June	1st to 10th	Day	34% 9% 16% 24% 16% 2	2520
	11th to 20th	Day	35% 8% 11% 23% 21% 3	2520
	21st to end	Day	37% 9% 15% 22% 15% 2	2520
July	1st to 10th	Day	29% 8% 16% 28% 15% 3	2451
	11th to 20th	Day	32% 9% 16% 26% 14% 3	2445
	21st to end	Day	29% 9% 18% 28% 14% 1	2557
August	1st to 10th	Day	33% 11% 18% 26% 10% 1	2314
	11th to 20th	Day	31% 9% 14% 28% 16% 1	2204
	21st to end	Day	30% 7% 13% 28% 18% 3	2376
September	1st to 10th	Day	34% 8% 11% 25% 18% 4	2160
	11th to 20th	Day	29% 6% 12% 28% 22% 3	2156
	21st to end	Day	32% 7% 12% 27% 20% 2	1936
October	1st to 10th	Day	37% 6% 10% 26% 21% 1	1800
	11th to 20th	Day	43% 5% 9% 26% 16% 2	1800
	21st to end	Day	45% 5% 12% 23% 13% 2	1922
November	1st to 10th	Day	38% 6% 14% 24% 15% 3	1672
	11th to 20th	Day	46% 5% 14% 21% 12% 1	1689
	21st to end	Day	49% 5% 15% 22% 9%	1520
December	1st to 10th	Day	54% 5% 13% 20% 8% 1	1520
	11th to 20th	Day	49% 7% 13% 22% 7% 2	1915
	21st to end	Day	71% 6% 8% 10% 5%	2783
January	1st to 10th	Night	50% 5% 12% 20% 13% 1	2648
	11th to 20th	Night	57% 5% 15% 15% 8%	2545
	21st to end	Night	55% 6% 13% 17% 8% 1	2574
February	1st to 10th	Night	53% 5% 13% 17% 11% 1	2340
	11th to 20th	Night	46% 6% 12% 23% 12% 1	2190
	21st to end	Night	48% 4 11% 21% 13% 3	1776
March	1st to 10th	Night	46% 6% 12% 24% 11% 1	2146
	11th to 20th	Night	45% 6% 12% 21% 12% 3	1980
	21st to end	Night	58% 5% 11% 17% 8% 1	2073
April	1st to 10th	Night	44% 5% 11% 24% 14% 2	1800
	11th to 20th	Night	44% 5% 9% 22% 17% 4	1775
	21st to end	Night	43% 7% 12% 21% 13% 3	1800
May	1st to 10th	Night	44% 8% 13% 22% 12% 1	1583
	11th to 20th	Night	43% 7% 13% 20% 16% 1	1417
	21st to end	Night	38% 9% 14% 20% 16% 3	1552
June	1st to 10th	Night	31% 8% 15% 25% 18% 4	1440
	11th to 20th	Night	31% 9% 11% 22% 23% 5%	1440
	21st to end	Night	33% 8% 16% 20% 20% 3	1440
July	1st to 10th	Night	28% 8% 15% 29% 16% 4	1400
	11th to 20th	Night	22% 7% 16% 32% 21% 2	1392
	21st to end	Night	27% 8% 15% 28% 18% 4	1517
August	1st to 10th	Night	25% 10% 19% 29% 14% 3	1601
	11th to 20th	Night	28% 7% 12% 31% 19% 2	1697
	21st to end	Night	24% 7% 13% 29% 21% 7%	1980
September	1st to 10th	Night	29% 6% 11% 26% 24% 4	1800
	11th to 20th	Night	30% 6% 12% 27% 20% 5%	1800
	21st to end	Night	29% 6% 12% 26% 24% 3	2007
October	1st to 10th	Night	34% 5% 10% 23% 23% 4	2160
	11th to 20th	Night	37% 5% 11% 25% 19% 3	2160
	21st to end	Night	44% 6% 11% 20% 17% 2	2433
November	1st to 10th	Night	32% 6% 13% 26% 19% 4	2406
	11th to 20th	Night	42% 6% 11% 25% 14% 2	2489
	21st to end	Night	46% 5% 12% 23% 13% 1	2660
December	1st to 10th	Night	49% 6% 13% 22% 9% 1	2660
	11th to 20th	Night	49% 5% 13% 24% 8% 1	3281
	21st to end	Night	66% 8% 10% 11% 5%	4884

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	21% 16% 35% 20% 7% 1	270
	11th to 20th	Day	23% 18% 39% 15% 4 1	252
	21st to end	Day	22% 17% 38% 17% 5% 1	264
February	1st to 10th	Day	26% 18% 41% 12% 4	240
	11th to 20th	Day	28% 16% 34% 17% 4 1	240
	21st to end	Day	23% 18% 43% 14% 2 1	198
March	1st to 10th	Day	19% 21% 38% 19% 3	240
	11th to 20th	Day	21% 18% 34% 20% 6% 2	240
	21st to end	Day	27% 19% 33% 14% 5% 1	367
April	1st to 10th	Day	21% 15% 35% 22% 6%	420
	11th to 20th	Day	20% 17% 31% 20% 12% 1	412
	21st to end	Day	21% 12% 34% 22% 8% 2	420
May	1st to 10th	Day	18% 11% 36% 26% 8% 2	525
	11th to 20th	Day	20% 14% 26% 26% 12% 2	625
	21st to end	Day	15% 6% 30% 26% 15% 8%	679
June	1st to 10th	Day	7% 6% 25% 32% 21% 9%	630
	11th to 20th	Day	8% 5% 29% 33% 21% 5%	630
	21st to end	Day	10% 10% 29% 30% 17% 4	630
July	1st to 10th	Day	8% 4 18% 30% 25% 15%	621
	11th to 20th	Day	9% 3 22% 35% 22% 8%	618
	21st to end	Day	6% 1 23% 36% 28% 6%	684
August	1st to 10th	Day	6% 3 22% 36% 25% 9%	626
	11th to 20th	Day	8% 5% 17% 31% 26% 13%	518
	21st to end	Day	6% 2 19% 35% 26% 11%	495
September	1st to 10th	Day	12% 6% 18% 32% 24% 7%	450
	11th to 20th	Day	8% 2 20% 34% 26% 10%	450
	21st to end	Day	10% 9% 25% 26% 22% 8%	406
October	1st to 10th	Day	18% 8% 27% 29% 14% 4	270
	11th to 20th	Day	18% 21% 31% 19% 10%	270
	21st to end	Day	32% 13% 28% 20% 6% 1	297
November	1st to 10th	Day	12% 11% 42% 23% 9% 4	270
	11th to 20th	Day	22% 16% 30% 24% 6% 2	270
	21st to end	Day	26% 10% 36% 23% 4	270
December	1st to 10th	Day	13% 17% 40% 23% 5% 3	270
	11th to 20th	Day	16% 15% 38% 22% 9%	270
	21st to end	Day	25% 22% 34% 17% 2	297
January	1st to 10th	Night	16% 15% 39% 23% 6% 1	950
	11th to 20th	Night	19% 13% 38% 24% 6%	921
	21st to end	Night	19% 12% 38% 27% 4	990
February	1st to 10th	Night	23% 12% 39% 23% 4	900
	11th to 20th	Night	18% 15% 36% 28% 3	795
	21st to end	Night	22% 15% 39% 21% 3 1	592
March	1st to 10th	Night	20% 12% 40% 24% 5%	706
	11th to 20th	Night	17% 11% 35% 29% 8%	540
	21st to end	Night	21% 15% 36% 22% 5% 1	594
April	1st to 10th	Night	19% 14% 39% 25% 3	540
	11th to 20th	Night	20% 13% 32% 28% 7%	533
	21st to end	Night	17% 11% 34% 30% 7%	540
May	1st to 10th	Night	18% 8% 37% 29% 7% 1	540
	11th to 20th	Night	17% 6% 30% 36% 11%	531
	21st to end	Night	11% 6% 29% 35% 18% 1	584
June	1st to 10th	Night	9% 4 24% 47% 16% 1	540
	11th to 20th	Night	8% 4 26% 43% 19%	540
	21st to end	Night	10% 5% 25% 40% 18% 1	540
July	1st to 10th	Night	9% 2 11% 42% 33% 4	525
	11th to 20th	Night	4 2 22% 42% 28% 2	522
	21st to end	Night	7% 1 17% 44% 30% 1	591
August	1st to 10th	Night	5% 3 12% 46% 31% 3	537
	11th to 20th	Night	6% 1 15% 43% 29% 5%	534
	21st to end	Night	8% 16% 44% 28% 3	594
September	1st to 10th	Night	12% 4 18% 37% 26% 3	540
	11th to 20th	Night	10% 3 21% 34% 25% 6%	540
	21st to end	Night	8% 7% 24% 35% 24% 2	705
October	1st to 10th	Night	13% 6% 31% 31% 18% 2	720
	11th to 20th	Night	18% 12% 32% 28% 11%	720
	21st to end	Night	22% 11% 34% 26% 7%	792
November	1st to 10th	Night	16% 8% 40% 27% 9% 1	740
	11th to 20th	Night	18% 12% 39% 25% 6%	779
	21st to end	Night	21% 11% 36% 25% 5%	950
December	1st to 10th	Night	18% 12% 40% 24% 5% 1	950
	11th to 20th	Night	22% 11% 34% 26% 6%	950
	21st to end	Night	26% 14% 34% 23% 3	1045

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