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 2024-01-17.

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	16% 6% 2 5% 5% 3 4 4 5% 7% 42%	74848
	Night	18% 7% 3 5% 5% 3 4 4 5% 7% 40%	74966
Transparency	Day	42% 5% 12% 23% 16% 3	74553
	Night	45% 5% 11% 22% 15% 2	74795
Seeing	Day	19% 4 19% 32% 21% 5%	13937
	Night	21% 6% 26% 30% 15% 2	22898

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	16% 8% 3 5% 5% 2 3 4 5% 7% 42%	1517
	11th to 20th	Day	20% 7% 3 6% 5% 2 4 4 6% 8% 36%	1488
	21st to end	Day	15% 7% 2 5% 4 3 4 5% 5% 8% 41%	1584
February	1st to 10th	Day	20% 8% 3 5% 4 3 5% 5% 5% 8% 34%	1648
	11th to 20th	Day	15% 7% 3 5% 5% 3 4 5% 6% 9% 39%	1748
	21st to end	Day	15% 10% 3 6% 5% 3 4 5% 5% 6% 37%	1480
March	1st to 10th	Day	17% 8% 2 4 5% 3 5% 5% 5% 9% 37%	1797
	11th to 20th	Day	17% 6% 3 6% 5% 2 4 4 5% 7% 41%	1923
	21st to end	Day	17% 8% 2 5% 5% 3 4 3 5% 6% 43%	2254
April	1st to 10th	Day	18% 7% 3 5% 4 2 4 4 6% 7% 39%	2160
	11th to 20th	Day	16% 6% 3 4 5% 3 4 5% 5% 7% 41%	2187
	21st to end	Day	19% 6% 2 5% 6% 3 5% 4 5% 7% 37%	2340
May	1st to 10th	Day	22% 8% 3 5% 5% 2 4 4 4 8% 35%	2340
	11th to 20th	Day	17% 7% 3 5% 5% 3 5% 3 5% 7% 41%	2315
	21st to end	Day	16% 6% 2 5% 4 3 4 5% 5% 8% 42%	2725
June	1st to 10th	Day	21% 7% 3 5% 5% 3 5% 5% 6% 7% 35%	2520
	11th to 20th	Day	15% 5% 3 5% 5% 3 5% 5% 5% 8% 42%	2520
	21st to end	Day	17% 6% 2 5% 5% 3 5% 6% 6% 8% 37%	2520
July	1st to 10th	Day	13% 5% 2 6% 4 3 5% 5% 6% 9% 42%	2520
	11th to 20th	Day	10% 6% 3 5% 5% 4 4 6% 6% 9% 43%	2520
	21st to end	Day	10% 5% 2 5% 5% 3 6% 5% 6% 8% 44%	2744
August	1st to 10th	Day	9% 5% 2 4 6% 4 5% 6% 6% 8% 44%	2334
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 5% 6% 9% 41%	2304
	21st to end	Day	13% 4 1 3 3 2 4 6% 7% 9% 48%	2412
September	1st to 10th	Day	12% 5% 2 4 5% 3 5% 4 5% 7% 48%	2131
	11th to 20th	Day	14% 5% 2 4 3 2 3 3 4 4 55%	1980
	21st to end	Day	15% 6% 2 5% 4 2 3 4 4 6% 49%	1980
October	1st to 10th	Day	19% 6% 2 5% 5% 2 4 4 4 5% 43%	1980
	11th to 20th	Day	15% 5% 1 5% 5% 3 4 4 4 7% 46%	1753
	21st to end	Day	21% 7% 2 5% 4 2 4 4 4 5% 41%	1782
November	1st to 10th	Day	17% 6% 2 5% 3 2 3 4 3 6% 49%	1671
	11th to 20th	Day	19% 6% 2 3 3 2 3 4 3 7% 46%	1710
	21st to end	Day	20% 6% 3 5% 4 2 3 4 4 7% 43%	1710
December	1st to 10th	Day	20% 7% 3 5% 4 2 4 4 5% 7% 38%	1573
	11th to 20th	Day	24% 6% 2 4 3 2 4 4 4 5% 43%	1906
	21st to end	Day	25% 9% 3 5% 6% 2 3 4 4 6% 33%	2772
January	1st to 10th	Night	17% 8% 3 4 5% 3 3 4 4 7% 40%	2649
	11th to 20th	Night	19% 10% 3 5% 5% 4 5% 4 5% 7% 32%	2616
	21st to end	Night	15% 8% 4 5% 5% 4 5% 4 6% 7% 37%	2772
February	1st to 10th	Night	17% 9% 3 5% 6% 3 5% 5% 5% 7% 35%	2306
	11th to 20th	Night	15% 9% 4 5% 5% 3 4 4 5% 6% 38%	2098
	21st to end	Night	18% 8% 2 5% 4 2 4 4 6% 6% 40%	1776
March	1st to 10th	Night	16% 9% 3 6% 5% 3 4 3 5% 6% 40%	2150
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 5% 5% 7% 36%	2034
	21st to end	Night	19% 8% 3 6% 5% 2 4 2 4 5% 43%	2100
April	1st to 10th	Night	20% 8% 2 6% 6% 3 3 4 5% 6% 37%	1800
	11th to 20th	Night	19% 7% 2 5% 4 2 4 4 5% 7% 40%	1721
	21st to end	Night	21% 8% 3 6% 5% 2 4 5% 5% 5% 36%	1620
May	1st to 10th	Night	21% 9% 3 6% 6% 3 5% 4 5% 6% 32%	1620
	11th to 20th	Night	18% 7% 3 7% 5% 3 5% 5% 5% 8% 34%	1593
	21st to end	Night	19% 8% 3 6% 4 2 3 5% 6% 7% 38%	1550
June	1st to 10th	Night	21% 7% 3 6% 6% 3 4 4 5% 7% 35%	1440
	11th to 20th	Night	18% 8% 2 5% 4 2 3 5% 5% 9% 37%	1440
	21st to end	Night	17% 7% 2 4 5% 3 5% 5% 6% 8% 37%	1440
July	1st to 10th	Night	16% 6% 2 5% 5% 3 5% 5% 5% 8% 40%	1440
	11th to 20th	Night	13% 6% 2 5% 5% 4 6% 6% 6% 9% 38%	1440
	21st to end	Night	13% 5% 3 4 4 3 5% 5% 6% 8% 43%	1574
August	1st to 10th	Night	9% 5% 2 4 4 4 5% 6% 7% 8% 45%	1584
	11th to 20th	Night	12% 6% 3 6% 6% 4 5% 6% 7% 8% 37%	1596
	21st to end	Night	14% 5% 2 4 4 2 4 5% 6% 8% 45%	1939
September	1st to 10th	Night	13% 5% 2 5% 4 3 4 5% 5% 7% 47%	1828
	11th to 20th	Night	13% 6% 2 4 4 3 2 4 4 6% 52%	1980
	21st to end	Night	14% 5% 2 4 4 3 3 5% 5% 8% 47%	1980
October	1st to 10th	Night	20% 7% 2 5% 4 3 3 3 5% 6% 41%	1980
	11th to 20th	Night	15% 5% 3 5% 5% 3 4 5% 5% 7% 43%	2202
	21st to end	Night	23% 8% 3 5% 4 2 3 4 5% 6% 37%	2574
November	1st to 10th	Night	15% 5% 3 5% 4 3 4 4 4 6% 47%	2407
	11th to 20th	Night	18% 6% 2 5% 5% 2 4 3 4 6% 45%	2470
	21st to end	Night	17% 7% 3 4 4 3 5% 4 5% 7% 43%	2470
December	1st to 10th	Night	21% 9% 3 5% 5% 3 4 4 5% 5% 36%	2605
	11th to 20th	Night	20% 7% 2 4 4 3 3 4 5% 9% 38%	3288
	21st to end	Night	24% 9% 3 5% 4 2 3 4 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	43% 5% 10% 25% 15% 2	1518
	11th to 20th	Day	47% 7% 12% 20% 13% 2	1488
	21st to end	Day	42% 5% 11% 26% 15% 1	1584
February	1st to 10th	Day	49% 6% 12% 19% 12% 2	1648
	11th to 20th	Day	43% 5% 13% 22% 14% 3	1757
	21st to end	Day	47% 5% 11% 20% 14% 3	1480
March	1st to 10th	Day	44% 6% 12% 19% 15% 4	1800
	11th to 20th	Day	42% 4 11% 21% 16% 6%	1923
	21st to end	Day	44% 3 9% 19% 18% 7%	2254
April	1st to 10th	Day	44% 5% 9% 19% 19% 4	2160
	11th to 20th	Day	41% 5% 11% 23% 16% 4	2187
	21st to end	Day	47% 5% 9% 19% 18% 4	2340
May	1st to 10th	Day	49% 4 9% 21% 15% 2	2340
	11th to 20th	Day	44% 5% 9% 21% 17% 3	2319
	21st to end	Day	41% 6% 15% 21% 14% 2	2725
June	1st to 10th	Day	48% 6% 11% 23% 11% 1	2520
	11th to 20th	Day	40% 6% 13% 21% 18% 3	2520
	21st to end	Day	44% 8% 12% 23% 13% 1	2520
July	1st to 10th	Day	37% 7% 12% 24% 17% 2	2450
	11th to 20th	Day	38% 6% 14% 28% 13% 1	2445
	21st to end	Day	36% 5% 16% 26% 14% 3	2591
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	44% 4 12% 23% 15% 2	2649
	11th to 20th	Night	52% 7% 12% 19% 8% 2	2616
	21st to end	Night	46% 4 10% 20% 18% 2	2772
February	1st to 10th	Night	49% 6% 12% 18% 13% 2	2306
	11th to 20th	Night	48% 5% 9% 22% 14% 3	2100
	21st to end	Night	44% 5% 9% 24% 14% 4	1776
March	1st to 10th	Night	47% 4 8% 21% 16% 4	2160
	11th to 20th	Night	49% 4 9% 17% 17% 4	2035
	21st to end	Night	47% 3 8% 16% 21% 5%	2100
April	1st to 10th	Night	48% 5% 8% 18% 16% 4	1800
	11th to 20th	Night	44% 4 9% 21% 19% 3	1721
	21st to end	Night	50% 5% 8% 17% 16% 3	1620
May	1st to 10th	Night	52% 4 8% 19% 13% 3	1620
	11th to 20th	Night	48% 7% 9% 18% 14% 4	1594
	21st to end	Night	46% 7% 13% 19% 13% 2	1551
June	1st to 10th	Night	50% 5% 10% 21% 13% 1	1440
	11th to 20th	Night	44% 7% 12% 19% 16% 2	1440
	21st to end	Night	44% 7% 13% 22% 13% 1	1440
July	1st to 10th	Night	42% 6% 13% 23% 15% 1	1400
	11th to 20th	Night	40% 6% 15% 28% 10%	1392
	21st to end	Night	37% 5% 14% 30% 12% 2	1486
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	18% 11% 33% 28% 9% 1	172
	21st to end	Day	18% 5% 37% 27% 13% 1	176
February	1st to 10th	Day	27% 8% 38% 24% 4	338
	11th to 20th	Day	22% 6% 40% 27% 5%	416
	21st to end	Day	21% 9% 27% 34% 8%	346
March	1st to 10th	Day	21% 10% 29% 30% 10%	420
	11th to 20th	Day	19% 5% 33% 35% 8%	420
	21st to end	Day	21% 5% 27% 31% 11% 3	462
April	1st to 10th	Day	23% 5% 25% 30% 14% 3	420
	11th to 20th	Day	18% 3 20% 40% 18% 1	412
	21st to end	Day	26% 3 16% 35% 18% 2	420
May	1st to 10th	Day	25% 3 17% 39% 13% 3	423
	11th to 20th	Day	18% 3 15% 36% 23% 4	447
	21st to end	Day	20% 1 9% 35% 27% 7%	485
June	1st to 10th	Day	20% 2 9% 34% 29% 6%	450
	11th to 20th	Day	16% 3 11% 39% 26% 5%	450
	21st to end	Day	18% 2 8% 38% 25% 10%	450
July	1st to 10th	Day	16% 2 4 31% 35% 11%	445
	11th to 20th	Day	13% 3 5% 30% 41% 9%	444
	21st to end	Day	9% 2 6% 34% 43% 6%	488
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% 20% 4	748
	21st to end	Night	19% 12% 41% 22% 6%	792
February	1st to 10th	Night	24% 13% 37% 23% 3	720
	11th to 20th	Night	22% 10% 46% 19% 4	703
	21st to end	Night	22% 9% 38% 25% 6%	592
March	1st to 10th	Night	23% 10% 36% 27% 4	720
	11th to 20th	Night	22% 7% 35% 27% 9% 1	720
	21st to end	Night	23% 7% 30% 29% 10% 1	792
April	1st to 10th	Night	24% 5% 29% 32% 10% 1	720
	11th to 20th	Night	24% 2 24% 36% 13% 1	657
	21st to end	Night	25% 3 20% 34% 15% 2	540
May	1st to 10th	Night	25% 2 19% 37% 14% 3	540
	11th to 20th	Night	22% 2 18% 38% 17% 2	532
	21st to end	Night	24% 1 10% 38% 24% 3	389
June	1st to 10th	Night	26% 2 9% 39% 20% 4	360
	11th to 20th	Night	21% 2 10% 40% 25% 2	360
	21st to end	Night	19% 3 6% 43% 25% 3	360
July	1st to 10th	Night	20% 1 7% 36% 31% 4	350
	11th to 20th	Night	13% 2 3 41% 36% 5%	348
	21st to end	Night	14% 1 6% 38% 36% 5%	394
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 2024-01-14 09:54:59. Version ac07

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