Selu Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Selu Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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% 7% 3 6% 6% 4 5% 5% 6% 8% 38%	74941
	Night	12% 7% 3 5% 5% 3 5% 5% 6% 8% 41%	74826
Transparency	Day	43% 8% 12% 20% 14% 2	74663
	Night	41% 7% 13% 22% 14% 3	74623
Seeing	Day	19% 13% 23% 22% 16% 7%	14101
	Night	17% 11% 23% 29% 17% 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	15% 8% 3 6% 6% 3 5% 4 5% 8% 38%	1706
	11th to 20th	Day	15% 10% 3 6% 5% 3 5% 5% 5% 6% 35%	1647
	21st to end	Day	13% 10% 3 7% 5% 3 4 5% 4 6% 40%	1782
February	1st to 10th	Day	11% 8% 4 7% 7% 4 6% 6% 5% 6% 35%	1620
	11th to 20th	Day	16% 7% 4 8% 6% 2 6% 6% 5% 6% 34%	1654
	21st to end	Day	18% 8% 3 6% 7% 3 4 5% 5% 6% 35%	1522
March	1st to 10th	Day	13% 5% 3 4 5% 3 5% 4 5% 7% 46%	1980
	11th to 20th	Day	17% 8% 4 5% 5% 4 4 5% 5% 7% 35%	1980
	21st to end	Day	16% 9% 3 6% 6% 4 5% 4 5% 7% 36%	2178
April	1st to 10th	Day	10% 6% 3 5% 6% 3 4 4 5% 8% 46%	2016
	11th to 20th	Day	13% 7% 4 4 5% 3 4 5% 6% 7% 43%	2134
	21st to end	Day	15% 7% 2 6% 6% 3 5% 5% 6% 6% 39%	2321
May	1st to 10th	Day	13% 7% 3 7% 6% 4 5% 7% 6% 8% 34%	2340
	11th to 20th	Day	16% 7% 3 6% 7% 4 5% 5% 6% 8% 32%	2301
	21st to end	Day	12% 6% 3 6% 6% 4 7% 6% 7% 10% 34%	2531
June	1st to 10th	Day	10% 6% 2 5% 5% 5% 6% 6% 6% 10% 39%	2340
	11th to 20th	Day	8% 5% 3 6% 6% 4 6% 6% 8% 10% 38%	2340
	21st to end	Day	9% 5% 3 5% 6% 4 6% 6% 7% 11% 37%	2340
July	1st to 10th	Day	9% 6% 3 6% 6% 5% 7% 7% 7% 10% 34%	2340
	11th to 20th	Day	7% 5% 3 6% 6% 5% 7% 8% 8% 11% 35%	2340
	21st to end	Day	10% 6% 3 6% 8% 5% 8% 7% 8% 9% 30%	2547
August	1st to 10th	Day	13% 5% 2 6% 7% 4 7% 7% 7% 11% 31%	2327
	11th to 20th	Day	13% 6% 2 6% 6% 4 7% 8% 8% 10% 30%	2303
	21st to end	Day	10% 6% 3 6% 6% 4 5% 6% 7% 9% 38%	2426
September	1st to 10th	Day	12% 5% 2 5% 7% 3 6% 5% 6% 7% 42%	2078
	11th to 20th	Day	14% 6% 2 5% 5% 4 5% 5% 6% 8% 38%	1980
	21st to end	Day	20% 7% 2 5% 6% 3 5% 4 5% 7% 36%	1980
October	1st to 10th	Day	16% 6% 2 4 4 3 3 3 4 6% 48%	1839
	11th to 20th	Day	13% 6% 3 6% 5% 3 4 5% 4 6% 45%	1800
	21st to end	Day	14% 6% 2 6% 4 3 4 5% 4 7% 46%	1980
November	1st to 10th	Day	12% 5% 2 4 4 4 3 4 4 7% 50%	1857
	11th to 20th	Day	10% 7% 2 5% 5% 3 5% 5% 5% 7% 46%	1715
	21st to end	Day	13% 6% 2 5% 4 4 4 4 4 7% 46%	1710
December	1st to 10th	Day	15% 10% 3 6% 6% 3 6% 5% 6% 6% 34%	1710
	11th to 20th	Day	14% 9% 2 6% 4 3 4 4 5% 8% 42%	2153
	21st to end	Day	16% 12% 3 6% 6% 3 5% 6% 7% 7% 30%	3124
January	1st to 10th	Night	13% 8% 3 6% 6% 4 5% 5% 5% 8% 37%	2459
	11th to 20th	Night	15% 8% 4 7% 6% 3 5% 6% 5% 7% 34%	2386
	21st to end	Night	13% 9% 3 4 6% 4 5% 5% 6% 7% 38%	2574
February	1st to 10th	Night	14% 8% 3 6% 5% 3 5% 5% 6% 8% 37%	2340
	11th to 20th	Night	19% 7% 3 6% 6% 4 5% 5% 5% 6% 34%	2189
	21st to end	Night	14% 7% 4 6% 5% 3 5% 6% 6% 7% 38%	1732
March	1st to 10th	Night	12% 6% 2 6% 5% 3 4 5% 6% 6% 45%	1980
	11th to 20th	Night	16% 8% 3 5% 5% 3 5% 6% 7% 8% 34%	1980
	21st to end	Night	14% 7% 3 5% 6% 3 5% 5% 5% 7% 40%	2178
April	1st to 10th	Night	11% 7% 2 4 6% 3 5% 5% 6% 9% 41%	1943
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 7% 47%	1775
	21st to end	Night	14% 6% 2 4 5% 3 4 4 5% 6% 45%	1654
May	1st to 10th	Night	12% 7% 3 5% 6% 3 5% 5% 4 8% 42%	1620
	11th to 20th	Night	14% 6% 3 6% 7% 4 5% 4 5% 9% 37%	1592
	21st to end	Night	11% 6% 3 6% 5% 3 4 6% 7% 10% 39%	1744
June	1st to 10th	Night	8% 4 2 4 6% 4 6% 6% 6% 12% 42%	1620
	11th to 20th	Night	9% 5% 2 5% 4 3 5% 5% 6% 9% 46%	1620
	21st to end	Night	7% 3 3 5% 4 4 5% 5% 8% 11% 45%	1620
July	1st to 10th	Night	9% 5% 2 5% 6% 4 5% 5% 7% 9% 43%	1620
	11th to 20th	Night	5% 3 3 4 5% 3 5% 6% 8% 11% 46%	1620
	21st to end	Night	5% 4 3 5% 5% 5% 6% 7% 9% 11% 41%	1771
August	1st to 10th	Night	9% 5% 3 7% 6% 3 5% 7% 6% 11% 38%	1611
	11th to 20th	Night	10% 5% 3 6% 6% 4 6% 6% 6% 9% 40%	1598
	21st to end	Night	9% 5% 2 5% 6% 4 6% 6% 6% 10% 41%	1911
September	1st to 10th	Night	9% 6% 3 5% 5% 3 5% 5% 6% 8% 46%	1878
	11th to 20th	Night	12% 7% 2 6% 7% 3 4 4 5% 7% 43%	1980
	21st to end	Night	16% 6% 3 5% 5% 3 5% 5% 5% 6% 41%	1980
October	1st to 10th	Night	14% 6% 3 5% 6% 3 4 3 4 7% 46%	2120
	11th to 20th	Night	13% 7% 2 4 5% 4 4 4 5% 6% 46%	2160
	21st to end	Night	12% 6% 3 5% 4 3 4 5% 4 7% 47%	2376
November	1st to 10th	Night	13% 6% 3 5% 4 2 4 4 4 5% 50%	2221
	11th to 20th	Night	11% 6% 3 5% 4 3 4 4 5% 7% 48%	2460
	21st to end	Night	12% 6% 3 5% 4 3 4 6% 4 7% 46%	2470
December	1st to 10th	Night	15% 9% 3 7% 5% 4 4 5% 5% 7% 35%	2470
	11th to 20th	Night	14% 8% 4 7% 5% 3 5% 5% 6% 7% 37%	3042
	21st to end	Night	17% 11% 3 7% 6% 4 6% 5% 6% 6% 28%	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	44% 6% 10% 21% 14% 4	1707
	11th to 20th	Day	50% 5% 9% 20% 14% 2	1647
	21st to end	Day	46% 5% 9% 22% 16% 3	1782
February	1st to 10th	Day	49% 5% 11% 20% 14% 2	1620
	11th to 20th	Day	50% 6% 10% 19% 13% 3	1663
	21st to end	Day	50% 5% 10% 14% 16% 4	1522
March	1st to 10th	Day	39% 4 9% 26% 19% 4	1980
	11th to 20th	Day	49% 7% 9% 19% 13% 4	1980
	21st to end	Day	48% 7% 10% 21% 11% 3	2178
April	1st to 10th	Day	37% 6% 12% 24% 16% 5%	2016
	11th to 20th	Day	40% 5% 12% 22% 17% 5%	2134
	21st to end	Day	45% 8% 11% 23% 11% 3	2321
May	1st to 10th	Day	47% 8% 14% 17% 12% 2	2340
	11th to 20th	Day	49% 8% 14% 21% 7% 2	2305
	21st to end	Day	44% 10% 17% 21% 8%	2531
June	1st to 10th	Day	39% 10% 17% 22% 12%	2340
	11th to 20th	Day	39% 12% 16% 18% 14% 2	2340
	21st to end	Day	39% 11% 15% 21% 12% 2	2340
July	1st to 10th	Day	43% 12% 14% 19% 10% 2	2275
	11th to 20th	Day	39% 13% 19% 17% 12% 1	2271
	21st to end	Day	45% 12% 16% 17% 9% 1	2405
August	1st to 10th	Day	45% 12% 16% 19% 7%	2314
	11th to 20th	Day	45% 12% 17% 18% 9%	2303
	21st to end	Day	40% 12% 17% 17% 11% 3	2426
September	1st to 10th	Day	39% 9% 12% 21% 16% 3	2078
	11th to 20th	Day	42% 8% 10% 23% 15% 2	1980
	21st to end	Day	48% 6% 11% 18% 15% 2	1980
October	1st to 10th	Day	38% 5% 11% 25% 17% 4	1839
	11th to 20th	Day	39% 6% 11% 23% 19% 3	1800
	21st to end	Day	39% 6% 8% 26% 17% 4	1980
November	1st to 10th	Day	33% 5% 10% 23% 25% 5%	1857
	11th to 20th	Day	37% 5% 9% 23% 22% 5%	1715
	21st to end	Day	39% 4 10% 22% 21% 3	1710
December	1st to 10th	Day	50% 7% 11% 16% 14% 3	1710
	11th to 20th	Day	40% 4 11% 19% 22% 4	2150
	21st to end	Day	48% 8% 10% 21% 11% 1	3124
January	1st to 10th	Night	45% 7% 11% 18% 15% 4	2459
	11th to 20th	Night	49% 6% 9% 18% 15% 3	2386
	21st to end	Night	43% 6% 11% 24% 15% 2	2574
February	1st to 10th	Night	44% 5% 14% 22% 14% 1	2340
	11th to 20th	Night	51% 5% 8% 19% 13% 4	2191
	21st to end	Night	46% 7% 10% 21% 11% 4	1732
March	1st to 10th	Night	39% 6% 10% 23% 18% 4	1980
	11th to 20th	Night	47% 8% 11% 21% 12% 1	1980
	21st to end	Night	44% 7% 12% 21% 14% 3	2178
April	1st to 10th	Night	39% 6% 14% 21% 16% 4	1943
	11th to 20th	Night	36% 6% 12% 21% 20% 5%	1775
	21st to end	Night	39% 6% 13% 27% 13% 2	1654
May	1st to 10th	Night	41% 8% 15% 23% 12% 1	1620
	11th to 20th	Night	43% 8% 17% 22% 8% 1	1593
	21st to end	Night	38% 9% 21% 25% 7%	1745
June	1st to 10th	Night	34% 12% 18% 24% 12% 1	1620
	11th to 20th	Night	34% 11% 18% 24% 12% 1	1620
	21st to end	Night	30% 13% 17% 25% 12% 2	1620
July	1st to 10th	Night	35% 14% 16% 20% 12% 4	1575
	11th to 20th	Night	29% 12% 23% 21% 12% 3	1566
	21st to end	Night	31% 15% 20% 25% 9%	1672
August	1st to 10th	Night	38% 12% 21% 19% 9%	1600
	11th to 20th	Night	38% 10% 18% 22% 10% 1	1598
	21st to end	Night	35% 10% 17% 25% 10% 2	1911
September	1st to 10th	Night	36% 10% 14% 21% 16% 4	1878
	11th to 20th	Night	39% 6% 12% 24% 17% 1	1980
	21st to end	Night	43% 7% 10% 23% 14% 2	1980
October	1st to 10th	Night	38% 6% 12% 23% 18% 3	2120
	11th to 20th	Night	38% 6% 9% 25% 18% 4	2160
	21st to end	Night	36% 4 9% 26% 19% 6%	2376
November	1st to 10th	Night	37% 5% 9% 21% 23% 5%	2221
	11th to 20th	Night	36% 5% 8% 22% 22% 7%	2460
	21st to end	Night	37% 5% 9% 26% 18% 4	2470
December	1st to 10th	Night	49% 6% 10% 21% 12% 2	2470
	11th to 20th	Night	46% 5% 10% 21% 16% 2	3044
	21st to end	Night	51% 7% 12% 17% 12% 1	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	34% 25% 23% 11% 7%	270
	11th to 20th	Day	33% 23% 31% 12% 2	249
	21st to end	Day	36% 24% 25% 12% 4	264
February	1st to 10th	Day	30% 25% 31% 11% 3	240
	11th to 20th	Day	40% 30% 23% 6% 1	240
	21st to end	Day	35% 23% 29% 11% 2	240
March	1st to 10th	Day	25% 23% 28% 19% 5%	420
	11th to 20th	Day	28% 22% 26% 17% 7% 1	420
	21st to end	Day	26% 18% 30% 20% 5%	462
April	1st to 10th	Day	22% 22% 24% 22% 10% 1	420
	11th to 20th	Day	22% 17% 26% 19% 12% 4	412
	21st to end	Day	25% 15% 28% 17% 11% 4	420
May	1st to 10th	Day	20% 19% 30% 17% 11% 4	423
	11th to 20th	Day	18% 11% 29% 21% 14% 6%	447
	21st to end	Day	14% 6% 22% 26% 20% 12%	485
June	1st to 10th	Day	13% 6% 24% 28% 20% 10%	450
	11th to 20th	Day	10% 8% 23% 28% 21% 11%	450
	21st to end	Day	11% 7% 26% 27% 20% 9%	450
July	1st to 10th	Day	12% 3 16% 26% 27% 16%	445
	11th to 20th	Day	5% 2 16% 33% 28% 16%	444
	21st to end	Day	9% 2 17% 29% 29% 14%	488
August	1st to 10th	Day	16% 4 18% 23% 24% 16%	446
	11th to 20th	Day	11% 4 14% 26% 27% 18%	439
	21st to end	Day	10% 3 12% 28% 25% 21%	495
September	1st to 10th	Day	12% 4 13% 26% 29% 16%	450
	11th to 20th	Day	14% 4 12% 28% 29% 14%	450
	21st to end	Day	22% 8% 20% 26% 21% 4	450
October	1st to 10th	Day	20% 8% 16% 27% 23% 6%	450
	11th to 20th	Day	16% 14% 22% 24% 21% 2	450
	21st to end	Day	19% 14% 27% 22% 15% 4	495
November	1st to 10th	Day	18% 12% 25% 27% 14% 4	455
	11th to 20th	Day	25% 19% 18% 25% 9% 3	275
	21st to end	Day	22% 21% 24% 24% 7% 1	270
December	1st to 10th	Day	26% 26% 26% 16% 5%	270
	11th to 20th	Day	30% 18% 26% 18% 8% 1	270
	21st to end	Day	28% 17% 28% 14% 11% 1	297
January	1st to 10th	Night	24% 23% 30% 19% 3	760
	11th to 20th	Night	24% 19% 29% 25% 3	735
	21st to end	Night	21% 24% 34% 19% 2	792
February	1st to 10th	Night	22% 19% 33% 23% 3	720
	11th to 20th	Night	31% 21% 30% 17% 1	703
	21st to end	Night	25% 18% 36% 18% 2	592
March	1st to 10th	Night	18% 19% 35% 26% 2	720
	11th to 20th	Night	25% 17% 28% 23% 8%	720
	21st to end	Night	19% 14% 35% 26% 5%	792
April	1st to 10th	Night	21% 17% 29% 26% 7%	720
	11th to 20th	Night	17% 15% 28% 28% 11%	711
	21st to end	Night	17% 11% 26% 34% 10% 1	574
May	1st to 10th	Night	15% 11% 27% 30% 15% 1	540
	11th to 20th	Night	15% 7% 22% 34% 20% 2	531
	21st to end	Night	13% 2 13% 32% 36% 4	584
June	1st to 10th	Night	9% 2 15% 43% 28% 3	540
	11th to 20th	Night	10% 3 16% 33% 34% 4	540
	21st to end	Night	7% 2 12% 41% 33% 4	540
July	1st to 10th	Night	11% 2 7% 36% 39% 6%	525
	11th to 20th	Night	6% 2 8% 33% 45% 6%	522
	21st to end	Night	5% 2 8% 40% 40% 5%	591
August	1st to 10th	Night	10% 3 8% 37% 37% 5%	537
	11th to 20th	Night	10% 2 7% 34% 40% 6%	534
	21st to end	Night	11% 2 9% 32% 39% 8%	723
September	1st to 10th	Night	9% 2 11% 33% 39% 7%	720
	11th to 20th	Night	14% 3 11% 32% 34% 6%	720
	21st to end	Night	18% 5% 18% 33% 24% 1	720
October	1st to 10th	Night	17% 7% 19% 28% 26% 3	720
	11th to 20th	Night	16% 9% 24% 33% 17% 1	720
	21st to end	Night	16% 12% 24% 33% 14% 1	792
November	1st to 10th	Night	18% 9% 22% 34% 16% 2	740
	11th to 20th	Night	17% 15% 24% 34% 10%	760
	21st to end	Night	18% 15% 27% 30% 10% 1	760
December	1st to 10th	Night	23% 17% 31% 24% 6%	760
	11th to 20th	Night	18% 18% 31% 28% 6%	760
	21st to end	Night	23% 16% 28% 27% 5%	836

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