Star Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star Island clear sky chart from 2005-11-06 to 2024-07-31.

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	17% 7% 2 6% 4 4 4 4 5% 7% 40%	89721
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 38%	90121
Transparency	Day	43% 6% 12% 22% 15% 2	89440
	Night	46% 5% 11% 21% 14% 2	89956
Seeing	Day	19% 4 18% 33% 21% 5%	14683
	Night	21% 6% 26% 30% 15% 2	23504

By Month

Percentage Cloud Cover NE (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% 8% 3 5% 3 3 4 2 5% 7% 44%	2517
	11th to 20th	Day	17% 6% 3 8% 4 3 3 3 5% 6% 41%	2520
	21st to end	Day	21% 9% 2 7% 4 4 4 4 4 6% 35%	2772
February	1st to 10th	Day	19% 8% 3 5% 4 4 4 4 6% 7% 35%	1739
	11th to 20th	Day	14% 7% 2 6% 4 3 4 4 6% 8% 40%	1848
	21st to end	Day	15% 11% 3 7% 4 4 4 4 6% 5% 38%	1570
March	1st to 10th	Day	25% 11% 2 6% 4 4 4 3 4 6% 30%	3116
	11th to 20th	Day	16% 7% 3 5% 3 3 4 3 6% 7% 43%	3381
	21st to end	Day	25% 8% 2 5% 3 4 4 3 5% 5% 35%	3960
April	1st to 10th	Day	18% 8% 2 6% 4 3 4 4 5% 7% 38%	2280
	11th to 20th	Day	16% 7% 2 5% 4 3 5% 5% 6% 7% 41%	2311
	21st to end	Day	19% 7% 2 6% 5% 4 4 3 5% 7% 38%	2470
May	1st to 10th	Day	20% 10% 4 7% 5% 4 4 4 5% 8% 32%	4106
	11th to 20th	Day	21% 8% 3 8% 5% 4 4 3 5% 6% 34%	4109
	21st to end	Day	16% 6% 2 5% 3 3 4 4 5% 8% 44%	4837
June	1st to 10th	Day	20% 7% 2 6% 5% 4 5% 4 6% 7% 35%	2660
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 8% 41%	2660
	21st to end	Day	17% 7% 2 7% 4 4 6% 4 6% 7% 36%	2660
July	1st to 10th	Day	12% 6% 2 7% 4 4 4 4 6% 9% 42%	2660
	11th to 20th	Day	9% 7% 2 6% 4 5% 4 5% 6% 9% 43%	2660
	21st to end	Day	11% 5% 2 5% 5% 4 5% 5% 7% 8% 44%	2887
August	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 8% 44%	2334
	11th to 20th	Day	11% 7% 3 7% 4 4 5% 4 6% 9% 41%	2304
	21st to end	Day	12% 4 1 4 3 4 5% 4 7% 8% 48%	2412
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 6% 7% 48%	2131
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 4 4 55%	1980
	21st to end	Day	15% 6% 2 5% 3 3 4 3 4 6% 49%	1980
October	1st to 10th	Day	19% 7% 2 6% 4 3 4 3 4 5% 43%	1980
	11th to 20th	Day	15% 5% 1 6% 4 4 4 4 4 7% 46%	1753
	21st to end	Day	21% 7% 2 6% 4 3 4 3 4 6% 41%	1782
November	1st to 10th	Day	17% 7% 2 5% 3 2 3 3 3 6% 49%	1671
	11th to 20th	Day	19% 7% 2 4 3 3 3 3 4 7% 46%	1710
	21st to end	Day	19% 7% 2 5% 4 3 3 3 5% 6% 43%	1710
December	1st to 10th	Day	20% 7% 3 6% 4 3 3 3 5% 7% 38%	1573
	11th to 20th	Day	24% 6% 2 4 3 3 3 4 4 5% 42%	1906
	21st to end	Day	25% 10% 2 7% 4 3 3 3 4 6% 32%	2772
January	1st to 10th	Night	20% 7% 2 4 4 3 3 3 5% 7% 44%	4429
	11th to 20th	Night	15% 9% 3 8% 5% 5% 4 3 5% 6% 38%	4440
	21st to end	Night	23% 11% 3 7% 4 5% 4 3 5% 5% 30%	4884
February	1st to 10th	Night	16% 9% 3 7% 4 4 5% 4 6% 6% 36%	2435
	11th to 20th	Night	15% 10% 3 6% 5% 4 4 4 5% 6% 39%	2218
	21st to end	Night	17% 9% 2 6% 3 3 4 3 6% 6% 41%	1884
March	1st to 10th	Night	30% 10% 2 5% 3 3 3 2 5% 4 33%	3724
	11th to 20th	Night	13% 8% 3 7% 6% 5% 5% 4 6% 7% 36%	3576
	21st to end	Night	26% 8% 2 7% 4 3 4 3 5% 5% 33%	3670
April	1st to 10th	Night	20% 9% 2 7% 5% 4 3 4 5% 5% 37%	1900
	11th to 20th	Night	19% 8% 2 5% 4 3 4 4 5% 6% 40%	1817
	21st to end	Night	20% 8% 3 6% 4 4 4 4 5% 5% 37%	1710
May	1st to 10th	Night	21% 12% 4 8% 5% 4 4 3 5% 7% 27%	2834
	11th to 20th	Night	20% 9% 3 7% 5% 5% 5% 4 6% 8% 27%	2813
	21st to end	Night	17% 8% 2 7% 3 3 4 4 5% 6% 40%	2738
June	1st to 10th	Night	20% 8% 2 7% 5% 3 4 3 5% 7% 35%	1520
	11th to 20th	Night	17% 9% 2 6% 4 3 3 5% 6% 9% 37%	1520
	21st to end	Night	18% 8% 2 6% 4 4 5% 4 6% 7% 36%	1520
July	1st to 10th	Night	15% 7% 2 6% 4 5% 4 5% 5% 8% 40%	1520
	11th to 20th	Night	12% 7% 1 6% 5% 5% 5% 5% 6% 9% 39%	1520
	21st to end	Night	13% 6% 3 5% 4 5% 4 4 6% 8% 42%	1662
August	1st to 10th	Night	9% 5% 2 6% 3 5% 5% 4 7% 8% 45%	1584
	11th to 20th	Night	12% 6% 2 7% 5% 5% 6% 4 8% 8% 36%	1596
	21st to end	Night	14% 5% 1 5% 4 3 4 4 6% 8% 45%	1939
September	1st to 10th	Night	13% 5% 2 6% 3 4 4 4 5% 7% 47%	1828
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 6% 52%	1980
	21st to end	Night	14% 5% 2 5% 3 4 3 4 5% 8% 47%	1980
October	1st to 10th	Night	20% 7% 2 6% 4 4 3 3 5% 5% 41%	1980
	11th to 20th	Night	15% 5% 3 6% 4 4 4 3 6% 7% 43%	2202
	21st to end	Night	23% 8% 3 5% 4 3 3 3 5% 6% 37%	2574
November	1st to 10th	Night	15% 6% 2 6% 3 4 3 4 5% 6% 46%	2407
	11th to 20th	Night	17% 6% 2 6% 4 3 4 3 4 5% 45%	2470
	21st to end	Night	16% 8% 2 4 4 4 4 3 5% 7% 42%	2470
December	1st to 10th	Night	20% 9% 2 6% 4 3 4 3 5% 5% 36%	2605
	11th to 20th	Night	20% 7% 2 5% 3 4 3 4 5% 8% 38%	3288
	21st to end	Night	24% 9% 3 6% 4 3 3 3 4 6% 35%	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	41% 5% 10% 31% 12% 1	2518
	11th to 20th	Day	44% 5% 11% 25% 13% 1	2520
	21st to end	Day	51% 6% 9% 24% 9% 1	2772
February	1st to 10th	Day	48% 5% 11% 21% 12% 3	1739
	11th to 20th	Day	42% 6% 12% 22% 15% 3	1857
	21st to end	Day	46% 5% 11% 20% 14% 3	1570
March	1st to 10th	Day	56% 5% 9% 15% 13% 3	3119
	11th to 20th	Day	38% 6% 13% 22% 14% 6%	3381
	21st to end	Day	50% 3 6% 16% 17% 8%	3960
April	1st to 10th	Day	45% 5% 9% 19% 18% 4	2280
	11th to 20th	Day	42% 5% 11% 23% 15% 4	2311
	21st to end	Day	46% 5% 8% 20% 18% 4	2470
May	1st to 10th	Day	51% 8% 12% 18% 10% 1	4120
	11th to 20th	Day	51% 6% 14% 17% 10% 2	4113
	21st to end	Day	37% 7% 14% 23% 18% 1	4837
June	1st to 10th	Day	47% 7% 11% 24% 11% 1	2660
	11th to 20th	Day	40% 6% 13% 22% 17% 3	2660
	21st to end	Day	45% 7% 12% 22% 13% 1	2660
July	1st to 10th	Day	37% 7% 13% 24% 17% 2	2590
	11th to 20th	Day	37% 6% 14% 28% 14% 1	2585
	21st to end	Day	36% 6% 16% 25% 14% 3	2734
August	1st to 10th	Day	35% 7% 16% 26% 13% 1	2320
	11th to 20th	Day	38% 7% 15% 25% 14% 1	2304
	21st to end	Day	31% 6% 13% 27% 20% 3	2412
September	1st to 10th	Day	36% 6% 12% 26% 20% 1	2131
	11th to 20th	Day	34% 4 8% 28% 23% 2	1980
	21st to end	Day	37% 4 9% 25% 21% 2	1980
October	1st to 10th	Day	43% 4 10% 23% 16% 3	1980
	11th to 20th	Day	37% 6% 11% 31% 13% 2	1753
	21st to end	Day	46% 4 10% 23% 15% 3	1782
November	1st to 10th	Day	39% 3 13% 27% 16% 2	1671
	11th to 20th	Day	40% 3 10% 23% 22% 2	1710
	21st to end	Day	43% 5% 15% 22% 14% 1	1710
December	1st to 10th	Day	47% 5% 14% 17% 15% 1	1573
	11th to 20th	Day	46% 4 11% 18% 16% 5%	1906
	21st to end	Day	53% 6% 10% 19% 10% 2	2772
January	1st to 10th	Night	41% 4 13% 28% 11% 3	4429
	11th to 20th	Night	47% 7% 10% 21% 12% 3	4440
	21st to end	Night	57% 4 9% 18% 10% 1	4884
February	1st to 10th	Night	47% 6% 13% 18% 13% 3	2435
	11th to 20th	Night	47% 5% 9% 21% 15% 3	2220
	21st to end	Night	44% 5% 9% 24% 13% 5%	1884
March	1st to 10th	Night	56% 3 9% 17% 12% 4	3734
	11th to 20th	Night	45% 7% 10% 18% 17% 3	3577
	21st to end	Night	54% 4 6% 14% 16% 8%	3670
April	1st to 10th	Night	49% 5% 8% 18% 16% 4	1900
	11th to 20th	Night	45% 4 9% 20% 19% 3	1817
	21st to end	Night	49% 5% 8% 17% 18% 3	1710
May	1st to 10th	Night	55% 7% 11% 16% 8% 2	2840
	11th to 20th	Night	54% 8% 13% 14% 8% 2	2814
	21st to end	Night	43% 8% 13% 19% 15% 2	2739
June	1st to 10th	Night	49% 5% 11% 21% 13% 1	1520
	11th to 20th	Night	43% 7% 12% 20% 15% 2	1520
	21st to end	Night	47% 6% 12% 21% 12% 1	1520
July	1st to 10th	Night	42% 6% 12% 23% 15% 1	1480
	11th to 20th	Night	40% 6% 14% 28% 11%	1472
	21st to end	Night	39% 5% 13% 28% 12% 2	1574
August	1st to 10th	Night	35% 7% 14% 26% 15% 2	1574
	11th to 20th	Night	43% 7% 15% 21% 14% 1	1596
	21st to end	Night	36% 6% 10% 26% 21% 1	1939
September	1st to 10th	Night	36% 5% 11% 27% 20% 1	1828
	11th to 20th	Night	34% 4 9% 27% 24% 2	1980
	21st to end	Night	37% 5% 9% 26% 20% 2	1980
October	1st to 10th	Night	45% 4 12% 23% 13% 4	1980
	11th to 20th	Night	40% 5% 12% 28% 13% 1	2202
	21st to end	Night	48% 4 10% 22% 14% 2	2574
November	1st to 10th	Night	39% 5% 11% 24% 17% 3	2407
	11th to 20th	Night	42% 4 10% 22% 19% 3	2470
	21st to end	Night	43% 4 14% 25% 13% 2	2470
December	1st to 10th	Night	50% 5% 11% 22% 12% 1	2605
	11th to 20th	Night	45% 5% 12% 18% 19% 1	3288
	21st to end	Night	51% 5% 10% 23% 10% 1	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	26% 6% 28% 28% 11% 1	180
	11th to 20th	Day	17% 11% 35% 27% 8% 1	180
	21st to end	Day	19% 5% 34% 30% 12% 1	198
February	1st to 10th	Day	25% 8% 37% 27% 4	369
	11th to 20th	Day	21% 7% 39% 28% 5%	456
	21st to end	Day	20% 10% 27% 33% 9% 1	382
March	1st to 10th	Day	23% 9% 27% 30% 10% 1	460
	11th to 20th	Day	19% 6% 32% 34% 8%	460
	21st to end	Day	23% 6% 27% 31% 10% 3	506
April	1st to 10th	Day	24% 4 25% 30% 14% 3	460
	11th to 20th	Day	19% 3 19% 41% 17% 1	452
	21st to end	Day	24% 2 16% 38% 18% 2	460
May	1st to 10th	Day	24% 3 17% 39% 15% 3	463
	11th to 20th	Day	19% 3 15% 35% 24% 5%	487
	21st to end	Day	20% 1 9% 36% 27% 7%	529
June	1st to 10th	Day	20% 2 9% 33% 29% 7%	490
	11th to 20th	Day	15% 3 11% 38% 28% 6%	490
	21st to end	Day	18% 2 9% 38% 24% 9%	490
July	1st to 10th	Day	15% 2 4 33% 36% 11%	485
	11th to 20th	Day	12% 3 5% 31% 40% 8%	484
	21st to end	Day	10% 2 5% 33% 43% 6%	529
August	1st to 10th	Day	12% 2 5% 33% 40% 9%	446
	11th to 20th	Day	14% 2 5% 33% 33% 13%	439
	21st to end	Day	13% 1 6% 36% 35% 9%	495
September	1st to 10th	Day	16% 2 10% 28% 31% 13%	450
	11th to 20th	Day	20% 1 14% 28% 27% 10%	450
	21st to end	Day	15% 2 16% 37% 23% 8%	450
October	1st to 10th	Day	23% 3 17% 27% 22% 7%	450
	11th to 20th	Day	16% 4 22% 33% 21% 4	381
	21st to end	Day	27% 3 24% 33% 12% 1	396
November	1st to 10th	Day	16% 5% 27% 29% 19% 3	365
	11th to 20th	Day	24% 8% 27% 31% 9%	370
	21st to end	Day	24% 7% 29% 27% 12% 1	370
December	1st to 10th	Day	21% 4 31% 33% 9% 1	233
	11th to 20th	Day	24% 4 36% 31% 5%	180
	21st to end	Day	26% 6% 32% 25% 11% 1	198
January	1st to 10th	Night	22% 13% 40% 21% 4	760
	11th to 20th	Night	25% 12% 39% 21% 4	760
	21st to end	Night	20% 12% 40% 22% 6%	836
February	1st to 10th	Night	23% 12% 37% 25% 4	760
	11th to 20th	Night	21% 10% 47% 19% 3	743
	21st to end	Night	22% 9% 36% 25% 7%	628
March	1st to 10th	Night	24% 10% 35% 26% 5%	760
	11th to 20th	Night	21% 8% 35% 27% 9% 1	760
	21st to end	Night	24% 7% 30% 29% 9% 1	836
April	1st to 10th	Night	24% 4 28% 33% 10% 1	760
	11th to 20th	Night	24% 2 24% 36% 12% 1	693
	21st to end	Night	24% 3 21% 35% 15% 2	570
May	1st to 10th	Night	25% 2 19% 37% 15% 3	570
	11th to 20th	Night	22% 2 18% 37% 19% 3	562
	21st to end	Night	24% 1 10% 39% 24% 3	411
June	1st to 10th	Night	25% 2 9% 40% 20% 4	380
	11th to 20th	Night	21% 2 10% 39% 26% 2	380
	21st to end	Night	19% 3 7% 43% 24% 3	380
July	1st to 10th	Night	19% 1 6% 37% 32% 4	370
	11th to 20th	Night	13% 2 4 41% 35% 5%	368
	21st to end	Night	14% 2 6% 38% 36% 4	416
August	1st to 10th	Night	11% 1 4 41% 39% 4	510
	11th to 20th	Night	12% 2 9% 39% 33% 6%	532
	21st to end	Night	16% 1 8% 33% 34% 7%	751
September	1st to 10th	Night	14% 1 12% 36% 30% 6%	720
	11th to 20th	Night	17% 3 15% 33% 23% 8%	720
	21st to end	Night	15% 1 17% 35% 25% 7%	720
October	1st to 10th	Night	23% 4 21% 29% 19% 4	720
	11th to 20th	Night	18% 5% 25% 34% 16% 2	720
	21st to end	Night	27% 5% 27% 28% 12% 1	792
November	1st to 10th	Night	17% 4 30% 34% 12% 2	740
	11th to 20th	Night	21% 9% 34% 26% 8% 2	760
	21st to end	Night	20% 13% 33% 28% 6% 1	760
December	1st to 10th	Night	25% 8% 38% 23% 5% 1	760
	11th to 20th	Night	24% 11% 38% 22% 4	760
	21st to end	Night	27% 11% 34% 24% 3	836

Page generated at 2024-07-28 09:52:10. Version ac07

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