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 2026-01-17.

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% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	127462
	Night	12% 7% 3 7% 5% 4 5% 4 6% 8% 40%	124242
Transparency	Day	41% 8% 12% 21% 14% 3	131094
	Night	40% 8% 12% 22% 15% 3	128679
Seeing	Day	19% 13% 23% 22% 16% 7%	16819
	Night	17% 12% 23% 29% 17% 2	26940

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	13% 8% 4 7% 4 4 4 3 4 7% 42%	4146
	11th to 20th	Day	10% 9% 4 8% 5% 4 4 4 6% 6% 41%	4114
	21st to end	Day	14% 9% 3 6% 5% 5% 4 3 4 5% 42%	4466
February	1st to 10th	Day	11% 9% 3 9% 6% 5% 5% 5% 5% 6% 36%	1800
	11th to 20th	Day	17% 7% 3 9% 5% 4 5% 5% 5% 6% 33%	1843
	21st to end	Day	17% 9% 3 7% 6% 4 3 4 5% 5% 37%	1698
March	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 5% 7% 35%	4935
	11th to 20th	Day	10% 6% 3 5% 4 4 4 3 5% 7% 48%	4980
	21st to end	Day	14% 8% 2 7% 5% 4 4 4 5% 6% 40%	5465
April	1st to 10th	Day	10% 7% 3 6% 5% 5% 4 3 6% 7% 44%	2240
	11th to 20th	Day	13% 7% 3 5% 5% 4 5% 4 6% 7% 43%	2374
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 4 6% 6% 39%	2579
May	1st to 10th	Day	11% 7% 3 8% 6% 6% 7% 5% 8% 10% 29%	5862
	11th to 20th	Day	19% 7% 3 8% 5% 6% 5% 5% 7% 9% 27%	5861
	21st to end	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 10% 35%	6433
June	1st to 10th	Day	9% 6% 2 6% 5% 6% 5% 5% 6% 10% 39%	2600
	11th to 20th	Day	7% 5% 2 7% 6% 6% 6% 5% 8% 11% 38%	2600
	21st to end	Day	8% 6% 2 5% 6% 6% 6% 4 7% 11% 38%	2600
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 6% 6% 8% 11% 38%	5900
	11th to 20th	Day	7% 6% 3 8% 6% 7% 7% 6% 9% 12% 30%	5900
	21st to end	Day	10% 7% 3 9% 7% 7% 7% 6% 8% 10% 25%	6424
August	1st to 10th	Day	14% 7% 3 8% 6% 6% 6% 5% 7% 11% 28%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 6% 7% 6% 9% 10% 33%	4174
	21st to end	Day	6% 4 2 6% 4 5% 4 4 6% 11% 47%	4393
September	1st to 10th	Day	11% 6% 2 6% 6% 4 5% 4 6% 7% 44%	2308
	11th to 20th	Day	14% 7% 2 6% 5% 4 6% 4 6% 8% 38%	2200
	21st to end	Day	19% 8% 2 7% 5% 4 5% 4 6% 6% 33%	2200
October	1st to 10th	Day	15% 6% 2 5% 3 4 3 3 4 6% 49%	2043
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 6% 47%	2000
	21st to end	Day	14% 7% 2 6% 4 3 4 4 4 6% 46%	2200
November	1st to 10th	Day	11% 5% 2 5% 4 5% 3 3 5% 8% 49%	2057
	11th to 20th	Day	10% 7% 2 6% 4 4 4 4 6% 7% 46%	1895
	21st to end	Day	13% 7% 2 6% 4 5% 4 4 4 7% 45%	1890
December	1st to 10th	Day	10% 8% 2 7% 5% 5% 5% 4 5% 6% 44%	3020
	11th to 20th	Day	16% 10% 3 6% 4 4 5% 4 6% 8% 36%	3463
	21st to end	Day	15% 10% 3 7% 5% 5% 6% 4 7% 7% 31%	4565
January	1st to 10th	Night	12% 9% 4 8% 5% 4 4 4 6% 6% 38%	6019
	11th to 20th	Night	13% 7% 3 10% 6% 4 4 3 4 5% 41%	5979
	21st to end	Night	16% 9% 2 5% 4 3 3 3 5% 6% 43%	6490
February	1st to 10th	Night	14% 8% 3 6% 5% 4 5% 4 6% 8% 38%	2600
	11th to 20th	Night	18% 7% 3 7% 5% 5% 5% 4 5% 6% 34%	2440
	21st to end	Night	13% 8% 3 8% 5% 4 5% 4 6% 7% 38%	1930
March	1st to 10th	Night	17% 8% 3 6% 5% 4 5% 3 4 5% 40%	4918
	11th to 20th	Night	11% 6% 3 5% 3 3 3 4 5% 8% 49%	4980
	21st to end	Night	13% 8% 3 7% 4 4 5% 4 5% 6% 42%	5467
April	1st to 10th	Night	11% 8% 2 5% 5% 5% 5% 4 7% 8% 40%	2158
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 6% 8% 46%	1975
	21st to end	Night	13% 6% 2 5% 4 5% 4 4 6% 6% 45%	1837
May	1st to 10th	Night	13% 8% 3 7% 5% 4 5% 4 6% 10% 36%	4033
	11th to 20th	Night	18% 7% 3 8% 6% 6% 5% 3 6% 9% 30%	4032
	21st to end	Night	17% 6% 2 6% 4 4 4 4 6% 9% 35%	4421
June	1st to 10th	Night	8% 5% 2 5% 5% 5% 5% 5% 7% 11% 42%	1800
	11th to 20th	Night	9% 6% 2 6% 4 4 6% 4 7% 9% 44%	1800
	21st to end	Night	7% 4 2 5% 5% 5% 5% 4 8% 11% 45%	1800
July	1st to 10th	Night	7% 4 2 6% 5% 6% 6% 5% 9% 10% 40%	4058
	11th to 20th	Night	6% 6% 3 7% 5% 6% 6% 6% 9% 12% 35%	4057
	21st to end	Night	8% 8% 3 9% 6% 6% 6% 5% 8% 10% 31%	4427
August	1st to 10th	Night	13% 7% 3 9% 5% 4 5% 4 6% 11% 32%	2915
	11th to 20th	Night	8% 6% 2 7% 6% 6% 5% 5% 7% 10% 38%	2875
	21st to end	Night	6% 4 1 5% 4 5% 4 4 6% 10% 51%	3471
September	1st to 10th	Night	8% 6% 3 6% 4 4 4 4 6% 8% 47%	2088
	11th to 20th	Night	12% 7% 2 7% 5% 4 4 3 5% 7% 41%	2200
	21st to end	Night	17% 6% 3 7% 4 5% 5% 4 6% 6% 38%	2200
October	1st to 10th	Night	13% 7% 3 6% 5% 4 3 3 4 7% 47%	2356
	11th to 20th	Night	12% 7% 2 5% 4 5% 3 3 5% 6% 47%	2400
	21st to end	Night	12% 6% 2 5% 4 4 4 3 4 7% 48%	2640
November	1st to 10th	Night	12% 7% 3 6% 4 3 4 3 5% 5% 49%	2461
	11th to 20th	Night	10% 7% 2 6% 4 4 4 3 5% 7% 47%	2720
	21st to end	Night	11% 7% 2 6% 4 4 5% 5% 5% 7% 45%	2730
December	1st to 10th	Night	11% 7% 2 7% 4 4 4 4 6% 7% 44%	4380
	11th to 20th	Night	15% 10% 4 9% 5% 5% 4 4 5% 7% 31%	4952
	21st to end	Night	16% 10% 3 8% 5% 5% 5% 4 6% 7% 30%	6633

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	41% 4 9% 22% 22% 2	4147
	11th to 20th	Day	41% 6% 8% 18% 23% 4	4114
	21st to end	Day	44% 5% 8% 23% 18% 2	4466
February	1st to 10th	Day	48% 5% 10% 20% 15% 2	1800
	11th to 20th	Day	50% 6% 9% 19% 14% 3	1852
	21st to end	Day	49% 5% 10% 15% 17% 4	1698
March	1st to 10th	Day	45% 6% 9% 19% 14% 7%	4935
	11th to 20th	Day	34% 6% 7% 22% 23% 8%	4980
	21st to end	Day	44% 6% 9% 22% 16% 3	5465
April	1st to 10th	Day	39% 6% 12% 23% 16% 5%	2240
	11th to 20th	Day	40% 5% 11% 23% 16% 4	2374
	21st to end	Day	44% 7% 11% 24% 11% 3	2579
May	1st to 10th	Day	47% 9% 14% 19% 9% 1	5897
	11th to 20th	Day	51% 9% 12% 20% 7% 1	5865
	21st to end	Day	41% 10% 15% 17% 14% 3	6447
June	1st to 10th	Day	38% 11% 17% 22% 13%	2600
	11th to 20th	Day	38% 12% 16% 19% 14% 2	2600
	21st to end	Day	37% 11% 16% 21% 13% 2	2600
July	1st to 10th	Day	37% 13% 17% 20% 10% 3	5835
	11th to 20th	Day	41% 15% 20% 18% 5% 1	5831
	21st to end	Day	49% 13% 15% 16% 7%	6240
August	1st to 10th	Day	47% 12% 17% 19% 5%	4221
	11th to 20th	Day	38% 12% 18% 22% 9% 1	4213
	21st to end	Day	30% 10% 18% 27% 13% 2	4393
September	1st to 10th	Day	37% 9% 12% 20% 18% 4	2308
	11th to 20th	Day	42% 8% 10% 24% 15% 2	2200
	21st to end	Day	50% 7% 11% 17% 14% 1	2200
October	1st to 10th	Day	31% 5% 10% 21% 26% 7%	3329
	11th to 20th	Day	28% 4 8% 27% 19% 15%	3240
	21st to end	Day	28% 5% 9% 28% 18% 12%	3538
November	1st to 10th	Day	33% 5% 11% 23% 24% 4	2057
	11th to 20th	Day	36% 5% 9% 23% 22% 4	1895
	21st to end	Day	41% 4 11% 22% 20% 2	1890
December	1st to 10th	Day	40% 7% 10% 19% 21% 4	3020
	11th to 20th	Day	44% 6% 12% 17% 17% 3	3460
	21st to end	Day	48% 9% 11% 20% 11% 1	4565
January	1st to 10th	Night	43% 6% 9% 20% 18% 3	6019
	11th to 20th	Night	42% 6% 8% 18% 21% 6%	5979
	21st to end	Night	41% 4 8% 25% 19% 3	6490
February	1st to 10th	Night	44% 5% 13% 22% 15% 1	2600
	11th to 20th	Night	51% 5% 8% 19% 14% 4	2442
	21st to end	Night	45% 7% 10% 22% 12% 5%	1930
March	1st to 10th	Night	43% 7% 7% 23% 14% 5%	4918
	11th to 20th	Night	33% 5% 9% 23% 23% 7%	4980
	21st to end	Night	42% 6% 10% 22% 17% 3	5467
April	1st to 10th	Night	40% 7% 14% 20% 16% 3	2158
	11th to 20th	Night	36% 6% 12% 22% 19% 5%	1975
	21st to end	Night	39% 7% 14% 26% 13% 2	1837
May	1st to 10th	Night	43% 12% 16% 20% 9% 1	4060
	11th to 20th	Night	50% 10% 14% 21% 6%	4033
	21st to end	Night	43% 10% 16% 22% 7% 2	4429
June	1st to 10th	Night	34% 12% 18% 23% 13% 1	1800
	11th to 20th	Night	34% 11% 18% 25% 12% 1	1800
	21st to end	Night	30% 13% 17% 25% 13% 2	1800
July	1st to 10th	Night	34% 16% 17% 21% 11% 2	4015
	11th to 20th	Night	38% 15% 22% 19% 5% 1	4006
	21st to end	Night	44% 14% 14% 19% 8%	4297
August	1st to 10th	Night	44% 10% 21% 19% 6%	2904
	11th to 20th	Night	37% 11% 19% 24% 9% 1	2902
	21st to end	Night	27% 9% 15% 35% 12% 1	3471
September	1st to 10th	Night	35% 9% 15% 20% 17% 4	2088
	11th to 20th	Night	40% 6% 12% 24% 17% 1	2200
	21st to end	Night	45% 8% 11% 22% 13% 2	2200
October	1st to 10th	Night	35% 5% 12% 19% 25% 4	3850
	11th to 20th	Night	27% 5% 10% 25% 22% 12%	3899
	21st to end	Night	24% 4 10% 30% 22% 11%	4252
November	1st to 10th	Night	37% 5% 10% 22% 22% 5%	2461
	11th to 20th	Night	37% 5% 8% 23% 21% 6%	2720
	21st to end	Night	38% 5% 9% 26% 18% 4	2730
December	1st to 10th	Night	38% 6% 10% 27% 18% 1	4380
	11th to 20th	Night	51% 6% 12% 18% 11% 2	4954
	21st to end	Night	48% 9% 12% 19% 12% 1	6633

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	36% 24% 24% 10% 5%	330
	11th to 20th	Day	36% 24% 27% 11% 1	318
	21st to end	Day	33% 24% 27% 12% 4	330
February	1st to 10th	Day	26% 25% 32% 13% 5%	300
	11th to 20th	Day	43% 27% 21% 7% 1	300
	21st to end	Day	34% 22% 30% 11% 3	297
March	1st to 10th	Day	27% 21% 26% 19% 5% 1	500
	11th to 20th	Day	28% 21% 27% 17% 7% 1	500
	21st to end	Day	26% 19% 31% 19% 5%	550
April	1st to 10th	Day	22% 24% 24% 20% 8% 1	500
	11th to 20th	Day	21% 18% 28% 19% 11% 3	492
	21st to end	Day	22% 14% 29% 19% 12% 4	500
May	1st to 10th	Day	18% 19% 31% 18% 11% 3	503
	11th to 20th	Day	19% 12% 28% 21% 14% 6%	527
	21st to end	Day	16% 8% 23% 26% 17% 10%	573
June	1st to 10th	Day	11% 6% 27% 27% 18% 10%	530
	11th to 20th	Day	8% 7% 22% 29% 22% 11%	530
	21st to end	Day	10% 6% 25% 27% 21% 11%	530
July	1st to 10th	Day	11% 3 15% 27% 26% 17%	525
	11th to 20th	Day	6% 3 16% 31% 27% 17%	524
	21st to end	Day	9% 3 17% 28% 29% 14%	576
August	1st to 10th	Day	17% 4 18% 23% 23% 14%	526
	11th to 20th	Day	10% 4 15% 27% 27% 18%	519
	21st to end	Day	9% 4 14% 27% 26% 21%	583
September	1st to 10th	Day	10% 5% 15% 27% 29% 15%	530
	11th to 20th	Day	14% 4 12% 27% 29% 14%	530
	21st to end	Day	22% 8% 21% 26% 19% 3	530
October	1st to 10th	Day	17% 8% 17% 27% 24% 8%	530
	11th to 20th	Day	15% 15% 22% 24% 21% 3	530
	21st to end	Day	20% 14% 26% 22% 14% 4	583
November	1st to 10th	Day	17% 13% 27% 27% 13% 3	535
	11th to 20th	Day	25% 18% 24% 23% 9% 2	335
	21st to end	Day	26% 21% 23% 22% 7% 1	330
December	1st to 10th	Day	25% 27% 29% 15% 4	330
	11th to 20th	Day	30% 17% 27% 18% 7% 1	330
	21st to end	Day	29% 18% 27% 15% 10% 1	363
January	1st to 10th	Night	24% 23% 30% 18% 3	840
	11th to 20th	Night	24% 20% 31% 23% 2	825
	21st to end	Night	21% 23% 34% 19% 2	880
February	1st to 10th	Night	22% 18% 34% 24% 3	800
	11th to 20th	Night	30% 22% 29% 17% 1	783
	21st to end	Night	24% 18% 37% 19% 2	660
March	1st to 10th	Night	19% 19% 34% 25% 3	800
	11th to 20th	Night	25% 17% 29% 23% 7%	800
	21st to end	Night	19% 15% 36% 25% 5%	880
April	1st to 10th	Night	21% 19% 29% 26% 6%	800
	11th to 20th	Night	17% 16% 29% 28% 10%	791
	21st to end	Night	17% 10% 28% 34% 10% 1	637
May	1st to 10th	Night	15% 11% 27% 30% 16% 1	600
	11th to 20th	Night	16% 7% 23% 34% 19% 2	591
	21st to end	Night	14% 3 15% 31% 33% 4	650
June	1st to 10th	Night	9% 2 16% 43% 27% 3	600
	11th to 20th	Night	10% 3 15% 34% 34% 4	600
	21st to end	Night	7% 2 12% 39% 36% 4	600
July	1st to 10th	Night	10% 2 7% 36% 40% 5%	585
	11th to 20th	Night	6% 2 8% 32% 45% 7%	582
	21st to end	Night	5% 2 9% 39% 40% 5%	657
August	1st to 10th	Night	11% 4 8% 36% 36% 5%	597
	11th to 20th	Night	10% 3 8% 34% 40% 5%	594
	21st to end	Night	10% 2 10% 32% 38% 8%	804
September	1st to 10th	Night	9% 2 11% 34% 37% 6%	800
	11th to 20th	Night	14% 4 11% 32% 34% 6%	800
	21st to end	Night	19% 6% 18% 33% 24% 1	800
October	1st to 10th	Night	16% 7% 18% 29% 27% 3	800
	11th to 20th	Night	15% 10% 23% 34% 17% 1	800
	21st to end	Night	16% 11% 24% 34% 14% 1	880
November	1st to 10th	Night	17% 9% 22% 35% 15% 2	820
	11th to 20th	Night	17% 15% 25% 33% 9%	840
	21st to end	Night	18% 16% 27% 30% 9% 1	840
December	1st to 10th	Night	22% 17% 33% 23% 5%	840
	11th to 20th	Night	19% 18% 31% 27% 5%	840
	21st to end	Night	23% 16% 28% 27% 5%	924

Page generated at 2026-01-14 01:14:26. Version ac07

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