Useppa Island Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Useppa Island Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	6% 4 2 5% 4 4 5% 5% 7% 11% 47%	131032
	Night	6% 5% 2 7% 5% 5% 5% 5% 7% 10% 43%	129831
Transparency	Day	19% 40% 15% 16% 8% 3	134790
	Night	23% 40% 11% 15% 9% 3	134143
Seeing	Day	7% 2 8% 22% 33% 28%	17291
	Night	8% 2 12% 31% 33% 13%	27408

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	6% 5% 2 5% 3 3 4 3 6% 9% 54%	5258
	11th to 20th	Day	16% 8% 3 7% 4 4 5% 4 5% 8% 36%	5280
	21st to end	Day	11% 6% 2 6% 4 4 4 3 5% 7% 47%	6390
February	1st to 10th	Day	8% 5% 2 5% 4 4 4 5% 6% 7% 51%	2100
	11th to 20th	Day	5% 3 2 4 3 4 3 4 7% 8% 59%	1998
	21st to end	Day	6% 4 1 3 3 3 4 3 7% 9% 58%	1744
March	1st to 10th	Day	5% 4 2 6% 4 4 5% 5% 6% 8% 53%	4935
	11th to 20th	Day	3 3 1 4 3 3 3 3 6% 7% 66%	4980
	21st to end	Day	6% 3 2 5% 4 3 4 4 6% 8% 56%	5465
April	1st to 10th	Day	4 3 1 3 3 3 3 3 8% 11% 59%	2200
	11th to 20th	Day	5% 3 1 4 3 3 4 4 7% 9% 56%	2176
	21st to end	Day	4 3 1 3 3 3 4 3 7% 10% 59%	2345
May	1st to 10th	Day	1 1 1 2 2 3 4 3 5% 9% 69%	5721
	11th to 20th	Day	6% 3 1 4 3 3 4 4 6% 10% 56%	5861
	21st to end	Day	2 2 1 3 3 3 3 4 6% 11% 62%	6433
June	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 5% 9% 11% 34%	2600
	11th to 20th	Day	7% 5% 2 6% 4 5% 6% 6% 9% 14% 36%	2600
	21st to end	Day	5% 6% 3 7% 6% 5% 7% 7% 10% 13% 32%	2600
July	1st to 10th	Day	4 4 2 7% 6% 6% 7% 6% 10% 14% 33%	5900
	11th to 20th	Day	3 3 2 6% 6% 7% 8% 7% 11% 16% 32%	5900
	21st to end	Day	2 3 1 5% 4 5% 7% 7% 11% 16% 38%	6424
August	1st to 10th	Day	4 3 2 9% 7% 7% 7% 8% 10% 16% 28%	4143
	11th to 20th	Day	3 3 2 6% 5% 5% 7% 6% 10% 15% 40%	3759
	21st to end	Day	5% 4 2 7% 5% 6% 8% 6% 12% 14% 30%	3949
September	1st to 10th	Day	5% 5% 2 6% 5% 8% 7% 7% 11% 13% 31%	2200
	11th to 20th	Day	8% 6% 2 6% 6% 7% 7% 5% 9% 11% 34%	2200
	21st to end	Day	6% 6% 3 7% 6% 6% 6% 5% 9% 13% 34%	2200
October	1st to 10th	Day	7% 6% 2 8% 5% 5% 6% 4 7% 9% 40%	2200
	11th to 20th	Day	6% 4 2 6% 5% 5% 4 4 6% 9% 49%	2045
	21st to end	Day	7% 5% 1 5% 4 4 5% 4 7% 8% 50%	2200
November	1st to 10th	Day	6% 5% 2 4 3 4 4 4 6% 8% 55%	2057
	11th to 20th	Day	6% 5% 2 4 4 4 4 4 6% 9% 52%	2100
	21st to end	Day	5% 4 1 5% 3 3 4 5% 7% 10% 52%	2021
December	1st to 10th	Day	4 3 1 4 3 3 3 3 5% 7% 64%	3020
	11th to 20th	Day	7% 6% 3 6% 4 4 5% 4 7% 8% 47%	3463
	21st to end	Day	9% 7% 3 8% 5% 6% 5% 4 6% 9% 37%	4565
January	1st to 10th	Night	5% 4 2 5% 4 4 4 4 7% 8% 52%	7635
	11th to 20th	Night	14% 9% 3 9% 6% 5% 5% 4 6% 7% 32%	7680
	21st to end	Night	10% 5% 2 7% 4 4 4 4 6% 8% 45%	7787
February	1st to 10th	Night	8% 6% 2 7% 5% 5% 4 3 5% 7% 49%	2520
	11th to 20th	Night	7% 4 2 5% 3 3 4 5% 6% 7% 52%	2399
	21st to end	Night	5% 4 1 4 4 4 4 4 5% 8% 57%	1885
March	1st to 10th	Night	5% 4 2 6% 4 4 4 4 6% 8% 54%	4918
	11th to 20th	Night	4 2 2 4 3 3 4 4 6% 9% 59%	4980
	21st to end	Night	7% 5% 2 6% 4 4 5% 3 7% 8% 49%	5467
April	1st to 10th	Night	4 3 1 4 4 4 5% 4 7% 10% 55%	2200
	11th to 20th	Night	3 4 2 5% 4 4 4 4 6% 9% 54%	2173
	21st to end	Night	4 3 1 5% 4 4 4 4 7% 9% 54%	2052
May	1st to 10th	Night	3 3 1 6% 4 5% 5% 5% 7% 11% 50%	4201
	11th to 20th	Night	6% 4 1 5% 4 3 3 4 6% 8% 57%	4032
	21st to end	Night	3 3 1 4 4 5% 4 4 6% 10% 55%	4421
June	1st to 10th	Night	10% 7% 2 6% 6% 5% 6% 5% 8% 9% 35%	1800
	11th to 20th	Night	5% 6% 3 6% 6% 6% 6% 6% 9% 12% 35%	1800
	21st to end	Night	5% 5% 2 8% 6% 7% 7% 7% 10% 14% 30%	1800
July	1st to 10th	Night	4 5% 3 9% 8% 8% 9% 7% 10% 13% 24%	4058
	11th to 20th	Night	4 6% 3 8% 8% 9% 10% 7% 10% 12% 23%	4057
	21st to end	Night	3 6% 3 9% 6% 7% 8% 7% 10% 13% 28%	4427
August	1st to 10th	Night	5% 5% 3 10% 7% 6% 6% 5% 10% 13% 29%	3000
	11th to 20th	Night	4 4 2 7% 6% 7% 8% 7% 11% 17% 29%	3256
	21st to end	Night	7% 5% 2 9% 6% 7% 8% 8% 10% 14% 24%	3949
September	1st to 10th	Night	4 5% 3 9% 7% 7% 7% 7% 10% 15% 27%	2200
	11th to 20th	Night	8% 6% 3 8% 7% 6% 7% 6% 8% 11% 29%	2200
	21st to end	Night	8% 7% 3 9% 6% 7% 6% 6% 9% 10% 29%	2200
October	1st to 10th	Night	9% 6% 2 7% 5% 5% 6% 5% 9% 10% 37%	2200
	11th to 20th	Night	6% 6% 3 7% 4 5% 5% 4 6% 9% 45%	2354
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 7% 8% 46%	2640
November	1st to 10th	Night	7% 6% 2 6% 4 4 4 4 6% 8% 50%	2461
	11th to 20th	Night	7% 5% 2 5% 5% 4 4 3 5% 8% 51%	2520
	21st to end	Night	7% 4 2 6% 4 4 3 4 6% 9% 50%	2594
December	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 8% 62%	4380
	11th to 20th	Night	7% 6% 2 5% 4 4 4 3 5% 8% 52%	4952
	21st to end	Night	8% 8% 3 8% 5% 5% 6% 5% 7% 9% 35%	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	20% 22% 8% 24% 22% 4	5259
	11th to 20th	Day	40% 23% 8% 14% 9% 5%	5280
	21st to end	Day	29% 24% 6% 24% 12% 5%	6390
February	1st to 10th	Day	8% 60% 7% 11% 9% 5%	2100
	11th to 20th	Day	7% 59% 4 14% 12% 5%	2008
	21st to end	Day	10% 63% 6% 9% 9% 3	1744
March	1st to 10th	Day	23% 34% 10% 16% 14% 4	4935
	11th to 20th	Day	12% 31% 10% 16% 16% 15%	4980
	21st to end	Day	20% 30% 8% 22% 13% 6%	5465
April	1st to 10th	Day	6% 62% 7% 15% 7% 3	2200
	11th to 20th	Day	8% 62% 8% 7% 9% 5%	2176
	21st to end	Day	7% 63% 5% 11% 13% 1	2345
May	1st to 10th	Day	9% 31% 20% 28% 10% 2	5754
	11th to 20th	Day	16% 31% 14% 27% 10% 1	5865
	21st to end	Day	9% 30% 19% 33% 9%	6447
June	1st to 10th	Day	15% 65% 12% 7% 1	2600
	11th to 20th	Day	15% 67% 12% 6%	2600
	21st to end	Day	11% 66% 17% 7%	2600
July	1st to 10th	Day	27% 34% 33% 6%	5835
	11th to 20th	Day	27% 35% 30% 8%	5831
	21st to end	Day	20% 36% 29% 15% 1	6240
August	1st to 10th	Day	27% 43% 22% 5% 3	4130
	11th to 20th	Day	15% 43% 26% 13% 2	3795
	21st to end	Day	22% 44% 26% 7% 1	3949
September	1st to 10th	Day	15% 64% 16% 6%	2200
	11th to 20th	Day	14% 62% 13% 8% 2	2200
	21st to end	Day	14% 62% 11% 8% 4 1	2200
October	1st to 10th	Day	35% 39% 13% 9% 3	3590
	11th to 20th	Day	17% 38% 8% 18% 17% 2	3311
	21st to end	Day	17% 37% 6% 22% 15% 4	3538
November	1st to 10th	Day	10% 55% 10% 15% 7% 3	2057
	11th to 20th	Day	16% 56% 6% 9% 9% 4	2100
	21st to end	Day	11% 56% 6% 12% 9% 5%	2021
December	1st to 10th	Day	10% 35% 12% 20% 16% 7%	3020
	11th to 20th	Day	22% 33% 9% 14% 15% 7%	3460
	21st to end	Day	33% 30% 12% 17% 8%	4565
January	1st to 10th	Night	21% 23% 7% 25% 21% 3	7636
	11th to 20th	Night	43% 23% 9% 12% 8% 4	7680
	21st to end	Night	28% 23% 7% 23% 12% 6%	7787
February	1st to 10th	Night	12% 60% 5% 11% 9% 3	2520
	11th to 20th	Night	9% 59% 4 12% 11% 6%	2400
	21st to end	Night	10% 63% 5% 9% 11% 3	1885
March	1st to 10th	Night	22% 33% 9% 17% 13% 6%	4918
	11th to 20th	Night	14% 32% 10% 17% 14% 13%	4980
	21st to end	Night	25% 30% 8% 18% 15% 3	5467
April	1st to 10th	Night	8% 63% 6% 14% 7% 2	2200
	11th to 20th	Night	7% 64% 7% 8% 10% 4	2173
	21st to end	Night	9% 63% 6% 11% 11% 1	2052
May	1st to 10th	Night	20% 34% 17% 21% 6% 2	4230
	11th to 20th	Night	16% 31% 13% 31% 9%	4033
	21st to end	Night	14% 31% 15% 27% 11% 1	4429
June	1st to 10th	Night	14% 65% 12% 9%	1800
	11th to 20th	Night	16% 67% 11% 5%	1800
	21st to end	Night	13% 67% 13% 7% 1	1800
July	1st to 10th	Night	35% 36% 25% 4	4015
	11th to 20th	Night	37% 39% 19% 5%	4006
	21st to end	Night	34% 33% 21% 12%	4297
August	1st to 10th	Night	28% 43% 19% 6% 5%	2989
	11th to 20th	Night	21% 44% 24% 10% 1	3286
	21st to end	Night	26% 46% 24% 4	3949
September	1st to 10th	Night	18% 64% 12% 6%	2200
	11th to 20th	Night	17% 62% 11% 8% 2	2200
	21st to end	Night	17% 62% 9% 8% 4	2200
October	1st to 10th	Night	40% 40% 11% 6% 3	3590
	11th to 20th	Night	21% 38% 8% 15% 16% 1	3827
	21st to end	Night	22% 37% 5% 21% 14% 3	4252
November	1st to 10th	Night	14% 55% 9% 13% 8% 2	2461
	11th to 20th	Night	15% 56% 6% 10% 10% 4	2520
	21st to end	Night	14% 56% 5% 11% 11% 4	2594
December	1st to 10th	Night	12% 36% 10% 23% 10% 10%	4380
	11th to 20th	Night	20% 32% 9% 15% 18% 6%	4954
	21st to end	Night	34% 31% 13% 12% 9% 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	7% 3 17% 40% 26% 7%	330
	11th to 20th	Day	10% 6% 21% 28% 22% 12%	330
	21st to end	Day	12% 5% 17% 39% 24% 3	363
February	1st to 10th	Day	5% 4 13% 31% 34% 14%	330
	11th to 20th	Day	5% 3 12% 30% 39% 12%	315
	21st to end	Day	5% 3 16% 31% 35% 11%	343
March	1st to 10th	Day	4 3 17% 40% 30% 5%	500
	11th to 20th	Day	5% 3 14% 38% 31% 8%	500
	21st to end	Day	4 2 13% 41% 30% 9%	550
April	1st to 10th	Day	5% 2 15% 38% 35% 6%	500
	11th to 20th	Day	8% 2 13% 35% 33% 10%	492
	21st to end	Day	5% 1 8% 33% 39% 14%	500
May	1st to 10th	Day	3 4 35% 42% 16%	503
	11th to 20th	Day	9% 2 6% 28% 37% 19%	527
	21st to end	Day	6% 2 4 17% 39% 33%	573
June	1st to 10th	Day	12% 3 2 13% 39% 31%	530
	11th to 20th	Day	7% 2 3 11% 34% 43%	530
	21st to end	Day	5% 3 1 4 28% 59%	530
July	1st to 10th	Day	4 1 1 5% 29% 62%	525
	11th to 20th	Day	4 2 1 5% 28% 61%	524
	21st to end	Day	6% 2 3 7% 27% 56%	576
August	1st to 10th	Day	6% 1 2 6% 27% 58%	526
	11th to 20th	Day	6% 1 1 3 28% 61%	519
	21st to end	Day	7% 1 2 9% 33% 48%	583
September	1st to 10th	Day	7% 2 2 6% 32% 53%	530
	11th to 20th	Day	8% 2 2 11% 32% 44%	530
	21st to end	Day	8% 1 2 12% 35% 43%	530
October	1st to 10th	Day	10% 2 6% 20% 38% 25%	530
	11th to 20th	Day	6% 2 7% 24% 40% 21%	530
	21st to end	Day	9% 2 9% 26% 36% 19%	583
November	1st to 10th	Day	7% 3 12% 27% 35% 17%	535
	11th to 20th	Day	9% 2 9% 31% 37% 13%	540
	21st to end	Day	8% 1 12% 30% 35% 15%	461
December	1st to 10th	Day	5% 1 17% 28% 35% 14%	330
	11th to 20th	Day	7% 1 15% 34% 31% 13%	330
	21st to end	Day	7% 3 11% 26% 36% 17%	363
January	1st to 10th	Night	8% 4 19% 40% 27% 2	840
	11th to 20th	Night	10% 5% 23% 37% 23% 2	840
	21st to end	Night	12% 6% 22% 40% 19%	924
February	1st to 10th	Night	11% 4 22% 38% 24% 2	840
	11th to 20th	Night	8% 3 17% 41% 28% 2	804
	21st to end	Night	7% 4 15% 44% 29% 2	660
March	1st to 10th	Night	4 3 21% 46% 23% 2	800
	11th to 20th	Night	7% 2 17% 40% 31% 2	800
	21st to end	Night	5% 3 18% 45% 27% 3	880
April	1st to 10th	Night	5% 2 14% 47% 30% 2	800
	11th to 20th	Night	5% 2 17% 48% 27% 2	791
	21st to end	Night	5% 1 13% 43% 35% 3	800
May	1st to 10th	Night	3 1 8% 42% 41% 5%	679
	11th to 20th	Night	7% 2 6% 38% 41% 5%	591
	21st to end	Night	6% 1 4 28% 48% 12%	650
June	1st to 10th	Night	11% 3 4 20% 45% 18%	600
	11th to 20th	Night	6% 1 4 11% 46% 32%	600
	21st to end	Night	6% 2 2 8% 41% 42%	600
July	1st to 10th	Night	6% 1 2 5% 34% 51%	585
	11th to 20th	Night	5% 1 2 6% 35% 51%	582
	21st to end	Night	4 1 1 7% 38% 48%	657
August	1st to 10th	Night	7% 1 3 8% 40% 41%	597
	11th to 20th	Night	6% 1 2 5% 37% 49%	624
	21st to end	Night	11% 2 2 13% 42% 30%	880
September	1st to 10th	Night	7% 1 3 11% 46% 33%	800
	11th to 20th	Night	10% 3 3 14% 43% 27%	800
	21st to end	Night	12% 2 5% 20% 39% 23%	800
October	1st to 10th	Night	12% 1 7% 34% 37% 9%	800
	11th to 20th	Night	7% 3 9% 33% 38% 11%	800
	21st to end	Night	11% 2 13% 34% 33% 7%	880
November	1st to 10th	Night	10% 2 15% 39% 30% 4	820
	11th to 20th	Night	8% 3 16% 39% 30% 4	840
	21st to end	Night	9% 4 15% 40% 30% 2	840
December	1st to 10th	Night	7% 3 16% 37% 32% 5%	840
	11th to 20th	Night	9% 4 17% 38% 28% 4	840
	21st to end	Night	7% 3 17% 38% 31% 3	924

Page generated at 2026-02-13 01:42:18. Version ac07

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