The Star Deck Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Star Deck 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	15% 8% 4 6% 6% 3 5% 5% 5% 7% 36%	74988
	Night	15% 9% 4 7% 5% 3 4 4 5% 7% 38%	75073
Transparency	Day	47% 4 11% 22% 13% 2	74696
	Night	48% 3 10% 22% 14% 2	74886
Seeing	Day	18% 8% 24% 25% 17% 8%	14435
	Night	19% 7% 27% 31% 15% 2	24390

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	17% 13% 6% 9% 8% 4 5% 5% 5% 5% 23%	1517
	11th to 20th	Day	19% 13% 7% 12% 7% 4 4 4 5% 5% 21%	1520
	21st to end	Day	16% 16% 8% 10% 6% 4 3 4 5% 6% 21%	1735
February	1st to 10th	Day	19% 14% 6% 10% 7% 3 4 4 4 4 25%	1782
	11th to 20th	Day	16% 12% 4 10% 7% 4 6% 4 4 6% 27%	1748
	21st to end	Day	18% 14% 5% 9% 6% 2 3 4 3 6% 30%	1480
March	1st to 10th	Day	18% 11% 4 7% 5% 3 4 4 4 7% 32%	1800
	11th to 20th	Day	19% 10% 4 7% 6% 3 5% 4 5% 7% 31%	1925
	21st to end	Day	21% 11% 4 8% 7% 3 4 3 4 6% 27%	2248
April	1st to 10th	Day	15% 10% 3 7% 6% 4 4 5% 4 7% 35%	2160
	11th to 20th	Day	18% 8% 4 5% 4 4 4 4 4 6% 37%	2134
	21st to end	Day	20% 7% 3 7% 5% 4 4 4 5% 7% 34%	2329
May	1st to 10th	Day	19% 9% 3 6% 7% 4 5% 4 5% 6% 32%	2340
	11th to 20th	Day	19% 7% 4 6% 6% 4 5% 5% 5% 6% 33%	2301
	21st to end	Day	10% 6% 3 6% 6% 4 6% 6% 7% 9% 37%	2552
June	1st to 10th	Day	11% 6% 2 5% 5% 4 5% 6% 5% 8% 42%	2520
	11th to 20th	Day	9% 4 2 5% 5% 4 5% 5% 7% 10% 44%	2520
	21st to end	Day	10% 5% 3 5% 6% 3 5% 6% 7% 10% 41%	2520
July	1st to 10th	Day	11% 5% 2 6% 6% 3 5% 7% 6% 10% 39%	2520
	11th to 20th	Day	8% 4 2 4 6% 3 5% 6% 8% 10% 45%	2520
	21st to end	Day	9% 4 2 4 6% 3 6% 6% 7% 10% 43%	2640
August	1st to 10th	Day	8% 5% 2 5% 5% 4 5% 5% 7% 10% 44%	2327
	11th to 20th	Day	10% 5% 2 5% 5% 3 6% 6% 7% 11% 38%	2230
	21st to end	Day	7% 5% 2 5% 5% 3 5% 5% 6% 9% 47%	2376
September	1st to 10th	Day	12% 6% 3 5% 5% 3 4 5% 5% 7% 45%	2116
	11th to 20th	Day	11% 5% 3 5% 4 3 4 6% 5% 8% 47%	1980
	21st to end	Day	14% 5% 2 4 4 3 3 4 5% 7% 49%	1980
October	1st to 10th	Day	13% 4 2 5% 6% 3 4 4 5% 7% 47%	1980
	11th to 20th	Day	15% 6% 2 4 5% 3 4 6% 6% 6% 43%	1846
	21st to end	Day	21% 9% 3 6% 5% 4 4 4 4 6% 34%	1782
November	1st to 10th	Day	9% 5% 3 4 5% 3 3 4 5% 6% 51%	1672
	11th to 20th	Day	16% 10% 5% 7% 5% 2 4 5% 5% 6% 35%	1710
	21st to end	Day	21% 10% 4 6% 5% 3 4 4 4 6% 34%	1710
December	1st to 10th	Day	21% 14% 5% 8% 6% 3 4 4 4 5% 25%	1710
	11th to 20th	Day	16% 16% 7% 9% 5% 3 4 3 4 5% 27%	1975
	21st to end	Day	23% 20% 10% 9% 6% 3 4 3 3 4 15%	2783
January	1st to 10th	Night	18% 12% 5% 9% 7% 4 4 4 5% 5% 27%	2648
	11th to 20th	Night	19% 14% 7% 10% 7% 4 4 3 3 4 23%	2660
	21st to end	Night	18% 14% 6% 9% 6% 3 4 4 4 4 26%	2762
February	1st to 10th	Night	22% 12% 6% 8% 6% 3 3 3 4 5% 26%	2174
	11th to 20th	Night	15% 11% 5% 10% 6% 3 5% 4 5% 6% 31%	2098
	21st to end	Night	20% 13% 5% 8% 5% 4 4 3 4 5% 29%	1776
March	1st to 10th	Night	17% 9% 4 7% 6% 3 4 4 4 6% 37%	2160
	11th to 20th	Night	16% 8% 4 7% 5% 3 4 4 5% 6% 37%	2034
	21st to end	Night	21% 9% 3 7% 5% 3 4 4 5% 7% 32%	2123
April	1st to 10th	Night	14% 8% 3 5% 5% 3 5% 5% 5% 7% 39%	1800
	11th to 20th	Night	16% 8% 3 5% 6% 3 3 3 4 8% 40%	1775
	21st to end	Night	19% 7% 3 6% 5% 4 4 3 6% 6% 36%	1630
May	1st to 10th	Night	18% 8% 4 7% 6% 3 5% 5% 6% 7% 31%	1620
	11th to 20th	Night	17% 7% 2 5% 5% 3 5% 4 5% 7% 40%	1592
	21st to end	Night	9% 5% 2 7% 5% 3 6% 5% 7% 10% 41%	1719
June	1st to 10th	Night	9% 7% 2 6% 5% 3 5% 5% 5% 7% 49%	1440
	11th to 20th	Night	9% 6% 3 5% 4 3 4 4 5% 10% 46%	1440
	21st to end	Night	7% 2 2 4 6% 4 4 7% 8% 11% 45%	1440
July	1st to 10th	Night	9% 5% 2 5% 5% 3 5% 6% 7% 9% 45%	1440
	11th to 20th	Night	8% 4 1 4 4 2 5% 5% 6% 8% 54%	1440
	21st to end	Night	9% 5% 2 5% 5% 3 4 5% 5% 8% 52%	1678
August	1st to 10th	Night	7% 4 2 5% 5% 3 5% 5% 6% 9% 50%	1611
	11th to 20th	Night	6% 4 3 6% 5% 3 5% 4 6% 8% 50%	1668
	21st to end	Night	6% 4 2 4 5% 3 4 6% 6% 8% 53%	1980
September	1st to 10th	Night	11% 6% 2 4 5% 2 4 5% 5% 8% 47%	1824
	11th to 20th	Night	8% 4 2 4 4 3 3 5% 6% 7% 54%	1980
	21st to end	Night	11% 5% 2 4 4 3 4 4 5% 6% 50%	1980
October	1st to 10th	Night	10% 4 2 4 5% 2 4 3 4 7% 54%	1980
	11th to 20th	Night	16% 7% 3 4 5% 3 4 3 5% 6% 44%	2110
	21st to end	Night	19% 8% 3 6% 4 3 4 5% 4 6% 39%	2574
November	1st to 10th	Night	13% 7% 3 5% 4 2 3 4 5% 6% 49%	2406
	11th to 20th	Night	14% 9% 4 7% 5% 3 4 3 5% 7% 39%	2470
	21st to end	Night	19% 11% 4 7% 6% 3 4 3 4 6% 32%	2470
December	1st to 10th	Night	17% 12% 6% 9% 6% 3 4 5% 5% 6% 28%	2470
	11th to 20th	Night	18% 14% 5% 8% 6% 3 3 4 4 5% 31%	3217
	21st to end	Night	23% 19% 7% 8% 6% 3 5% 3 4 5% 17%	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	63% 2 7% 18% 9%	1518
	11th to 20th	Day	66% 3 11% 15% 6%	1520
	21st to end	Day	64% 3 10% 18% 4 1	1735
February	1st to 10th	Day	63% 2 8% 16% 9% 1	1782
	11th to 20th	Day	59% 3 10% 19% 9% 1	1758
	21st to end	Day	58% 1 9% 21% 9% 2	1480
March	1st to 10th	Day	54% 3 10% 22% 10% 1	1800
	11th to 20th	Day	54% 3 14% 18% 10% 1	1925
	21st to end	Day	60% 3 12% 19% 6% 1	2248
April	1st to 10th	Day	49% 4 11% 21% 14% 1	2160
	11th to 20th	Day	48% 3 11% 24% 11% 4	2134
	21st to end	Day	52% 3 11% 21% 11% 3	2329
May	1st to 10th	Day	52% 4 12% 22% 9% 1	2340
	11th to 20th	Day	51% 4 13% 18% 13% 1	2305
	21st to end	Day	42% 7% 14% 21% 15% 2	2552
June	1st to 10th	Day	40% 5% 13% 25% 15% 3	2520
	11th to 20th	Day	35% 5% 13% 26% 20% 2	2520
	21st to end	Day	39% 5% 16% 21% 15% 4	2520
July	1st to 10th	Day	40% 4 14% 24% 15% 3	2451
	11th to 20th	Day	33% 5% 15% 28% 17% 2	2445
	21st to end	Day	35% 6% 16% 24% 15% 3	2493
August	1st to 10th	Day	34% 5% 14% 31% 15% 2	2314
	11th to 20th	Day	38% 6% 13% 27% 15% 1	2230
	21st to end	Day	33% 5% 12% 26% 17% 7%	2376
September	1st to 10th	Day	38% 4 11% 26% 17% 5%	2116
	11th to 20th	Day	36% 4 11% 26% 21% 3	1980
	21st to end	Day	35% 3 9% 27% 25% 1	1980
October	1st to 10th	Day	38% 3 11% 25% 21% 3	1980
	11th to 20th	Day	40% 3 9% 26% 20% 2	1846
	21st to end	Day	53% 2 9% 22% 13% 1	1782
November	1st to 10th	Day	35% 2 12% 30% 18% 4	1672
	11th to 20th	Day	49% 2 9% 22% 14% 4	1710
	21st to end	Day	53% 2 9% 23% 11% 2	1710
December	1st to 10th	Day	62% 3 9% 18% 8%	1710
	11th to 20th	Day	61% 2 7% 22% 8% 1	1972
	21st to end	Day	75% 1 7% 10% 6%	2783
January	1st to 10th	Night	60% 3 7% 20% 10% 1	2648
	11th to 20th	Night	67% 2 8% 16% 6% 1	2660
	21st to end	Night	62% 2 9% 18% 8% 1	2762
February	1st to 10th	Night	62% 3 11% 17% 7% 1	2174
	11th to 20th	Night	55% 3 11% 19% 11% 1	2099
	21st to end	Night	59% 2 9% 17% 10% 2	1776
March	1st to 10th	Night	50% 3 9% 24% 13% 1	2160
	11th to 20th	Night	49% 3 11% 22% 12% 3	2034
	21st to end	Night	53% 3 10% 23% 8% 2	2123
April	1st to 10th	Night	45% 2 11% 25% 14% 2	1800
	11th to 20th	Night	45% 2 8% 22% 18% 5%	1775
	21st to end	Night	48% 3 10% 22% 13% 4	1630
May	1st to 10th	Night	51% 5% 10% 21% 10% 2	1620
	11th to 20th	Night	44% 4 13% 23% 14% 2	1593
	21st to end	Night	37% 6% 16% 23% 15% 3	1720
June	1st to 10th	Night	36% 3 11% 26% 21% 3	1440
	11th to 20th	Night	35% 4 12% 25% 21% 3	1440
	21st to end	Night	31% 8% 14% 26% 18% 4	1440
July	1st to 10th	Night	35% 5% 15% 23% 16% 6%	1400
	11th to 20th	Night	28% 4 15% 29% 21% 3	1392
	21st to end	Night	31% 3 14% 31% 18% 3	1585
August	1st to 10th	Night	32% 4 14% 30% 18% 2	1600
	11th to 20th	Night	32% 4 13% 29% 19% 2	1668
	21st to end	Night	29% 5% 13% 26% 22% 6%	1980
September	1st to 10th	Night	35% 3 11% 24% 19% 7%	1824
	11th to 20th	Night	29% 4 9% 30% 25% 3	1980
	21st to end	Night	35% 3 9% 25% 24% 4	1980
October	1st to 10th	Night	32% 3 11% 26% 25% 4	1980
	11th to 20th	Night	42% 3 7% 27% 20% 2	2110
	21st to end	Night	47% 2 8% 25% 15% 2	2574
November	1st to 10th	Night	38% 2 9% 25% 22% 3	2406
	11th to 20th	Night	46% 2 8% 23% 17% 3	2470
	21st to end	Night	55% 2 9% 23% 11%	2470
December	1st to 10th	Night	57% 3 9% 20% 10% 1	2470
	11th to 20th	Night	57% 1 8% 21% 12% 1	3219
	21st to end	Night	73% 2 8% 11% 6%	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	24% 16% 31% 23% 6% 1	270
	11th to 20th	Day	24% 16% 38% 16% 5% 1	270
	21st to end	Day	29% 15% 30% 22% 5%	282
February	1st to 10th	Day	31% 10% 37% 19% 4	240
	11th to 20th	Day	23% 19% 33% 19% 5%	240
	21st to end	Day	29% 9% 39% 20% 3	198
March	1st to 10th	Day	21% 17% 38% 21% 3	240
	11th to 20th	Day	27% 10% 34% 18% 10% 1	365
	21st to end	Day	30% 13% 32% 17% 8% 1	462
April	1st to 10th	Day	18% 12% 33% 25% 10% 2	420
	11th to 20th	Day	22% 12% 29% 23% 11% 4	412
	21st to end	Day	26% 11% 34% 20% 7% 2	420
May	1st to 10th	Day	23% 10% 29% 26% 9% 3	423
	11th to 20th	Day	22% 5% 28% 24% 15% 6%	447
	21st to end	Day	15% 4 22% 25% 22% 13%	506
June	1st to 10th	Day	13% 4 21% 32% 23% 7%	630
	11th to 20th	Day	12% 4 20% 32% 23% 8%	630
	21st to end	Day	11% 5% 23% 30% 22% 9%	630
July	1st to 10th	Day	12% 2 11% 26% 27% 22%	621
	11th to 20th	Day	9% 3 15% 33% 26% 14%	618
	21st to end	Day	11% 1 13% 28% 31% 16%	581
August	1st to 10th	Day	11% 2 9% 25% 33% 20%	446
	11th to 20th	Day	15% 4 13% 26% 29% 14%	439
	21st to end	Day	8% 3 11% 29% 33% 17%	495
September	1st to 10th	Day	12% 3 14% 27% 31% 13%	450
	11th to 20th	Day	10% 3 12% 26% 31% 18%	450
	21st to end	Day	16% 6% 20% 23% 23% 12%	450
October	1st to 10th	Day	13% 6% 21% 34% 17% 9%	450
	11th to 20th	Day	14% 12% 34% 27% 13% 1	406
	21st to end	Day	29% 11% 37% 20% 3	297
November	1st to 10th	Day	14% 14% 29% 25% 15% 3	270
	11th to 20th	Day	21% 14% 29% 21% 12% 3	270
	21st to end	Day	25% 8% 36% 24% 6% 1	270
December	1st to 10th	Day	21% 14% 40% 19% 6% 1	270
	11th to 20th	Day	21% 14% 34% 25% 4 2	270
	21st to end	Day	29% 17% 32% 18% 4 1	297
January	1st to 10th	Night	23% 12% 34% 25% 5%	950
	11th to 20th	Night	27% 11% 37% 21% 4	950
	21st to end	Night	25% 11% 33% 27% 4	918
February	1st to 10th	Night	29% 9% 33% 25% 3	720
	11th to 20th	Night	22% 13% 36% 27% 2	703
	21st to end	Night	26% 11% 37% 24% 2	592
March	1st to 10th	Night	22% 12% 33% 27% 6%	720
	11th to 20th	Night	20% 11% 36% 26% 7% 1	720
	21st to end	Night	26% 7% 31% 29% 6% 1	737
April	1st to 10th	Night	17% 10% 36% 28% 9% 1	540
	11th to 20th	Night	18% 9% 34% 30% 9% 1	533
	21st to end	Night	22% 6% 38% 25% 8% 1	540
May	1st to 10th	Night	20% 7% 32% 29% 11%	540
	11th to 20th	Night	21% 3 21% 35% 18% 2	531
	21st to end	Night	11% 3 19% 39% 27% 2	584
June	1st to 10th	Night	11% 4 18% 47% 19% 1	540
	11th to 20th	Night	11% 4 18% 44% 22% 2	540
	21st to end	Night	7% 2 21% 38% 29% 2	540
July	1st to 10th	Night	11% 2 7% 35% 37% 7%	525
	11th to 20th	Night	8% 3 11% 38% 34% 5%	522
	21st to end	Night	10% 3 10% 40% 34% 3	591
August	1st to 10th	Night	7% 2 9% 42% 36% 5%	537
	11th to 20th	Night	8% 1 10% 42% 32% 7%	534
	21st to end	Night	8% 2 11% 36% 38% 5%	594
September	1st to 10th	Night	13% 2 12% 36% 32% 5%	564
	11th to 20th	Night	9% 3 14% 34% 35% 6%	720
	21st to end	Night	13% 3 19% 34% 26% 3	720
October	1st to 10th	Night	12% 4 24% 34% 21% 5%	720
	11th to 20th	Night	19% 8% 29% 29% 14% 1	720
	21st to end	Night	22% 8% 35% 27% 7% 1	792
November	1st to 10th	Night	17% 7% 32% 30% 14% 1	740
	11th to 20th	Night	18% 13% 32% 29% 8% 1	760
	21st to end	Night	25% 9% 30% 29% 8%	760
December	1st to 10th	Night	23% 9% 37% 24% 6%	760
	11th to 20th	Night	26% 8% 33% 26% 7% 1	888
	21st to end	Night	31% 12% 30% 23% 4	1045

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