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 2025-01-07.

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% 7% 2 6% 4 4 4 4 5% 7% 40%	104897
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 39%	104149
Transparency	Day	42% 5% 12% 23% 15% 2	108448
	Night	45% 5% 11% 22% 15% 2	108687
Seeing	Day	19% 4 18% 33% 21% 5%	15201
	Night	21% 6% 26% 30% 15% 2	24118

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% 3 3 4 2 5% 7% 43%	2565
	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	10% 6% 3 10% 5% 5% 5% 5% 6% 9% 35%	4440
	11th to 20th	Day	8% 6% 2 6% 4 5% 5% 4 7% 11% 43%	4440
	21st to end	Day	15% 7% 2 6% 5% 4 5% 4 6% 8% 40%	4814
August	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 7% 39%	4112
	11th to 20th	Day	14% 9% 3 7% 4 4 5% 3 5% 8% 38%	4045
	21st to end	Day	7% 3 1 4 3 4 5% 5% 7% 10% 49%	4244
September	1st to 10th	Day	12% 4 2 5% 4 4 4 3 5% 7% 50%	2249
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 4 4 56%	2090
	21st to end	Day	15% 7% 2 6% 4 3 3 3 5% 6% 47%	2090
October	1st to 10th	Day	19% 7% 2 6% 4 4 4 3 5% 5% 43%	2090
	11th to 20th	Day	15% 5% 2 6% 3 4 4 3 4 7% 47%	1850
	21st to end	Day	20% 6% 2 6% 4 3 4 3 4 6% 42%	1881
November	1st to 10th	Day	16% 6% 2 5% 3 2 3 3 4 6% 49%	1761
	11th to 20th	Day	18% 7% 2 4 3 3 3 3 4 7% 48%	1800
	21st to end	Day	20% 7% 2 5% 4 3 3 3 5% 6% 43%	1800
December	1st to 10th	Day	17% 8% 2 4 3 2 3 3 5% 6% 47%	2681
	11th to 20th	Day	21% 6% 1 4 3 2 3 3 4 6% 47%	2986
	21st to end	Day	22% 10% 2 6% 4 3 4 3 4 5% 36%	3960
January	1st to 10th	Night	19% 7% 2 4 4 3 3 3 5% 7% 44%	4514
	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	11% 7% 3 8% 5% 6% 5% 5% 7% 9% 35%	2520
	11th to 20th	Night	10% 6% 2 7% 5% 6% 6% 5% 7% 9% 39%	2520
	21st to end	Night	15% 8% 3 7% 5% 4 4 4 6% 7% 38%	2739
August	1st to 10th	Night	17% 7% 3 8% 5% 5% 5% 4 6% 6% 34%	2784
	11th to 20th	Night	12% 8% 3 8% 5% 6% 6% 5% 7% 8% 33%	2782
	21st to end	Night	10% 5% 1 4 3 4 4 3 6% 8% 50%	3407
September	1st to 10th	Night	12% 5% 2 6% 3 4 4 4 5% 7% 48%	1930
	11th to 20th	Night	14% 6% 2 5% 4 3 2 3 4 6% 52%	2090
	21st to end	Night	15% 6% 2 5% 3 4 3 4 5% 8% 45%	2090
October	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 5% 6% 41%	2090
	11th to 20th	Night	15% 5% 2 6% 4 4 4 3 5% 7% 45%	2325
	21st to end	Night	22% 8% 3 5% 4 3 3 3 5% 6% 39%	2717
November	1st to 10th	Night	14% 6% 2 6% 4 4 3 4 5% 6% 48%	2537
	11th to 20th	Night	17% 6% 2 6% 4 3 4 3 4 6% 46%	2600
	21st to end	Night	16% 8% 2 4 4 4 4 3 5% 7% 42%	2600
December	1st to 10th	Night	19% 8% 2 5% 4 4 4 3 5% 5% 41%	4497
	11th to 20th	Night	21% 8% 3 5% 3 3 3 3 4 6% 40%	5208
	21st to end	Night	20% 9% 3 6% 4 4 4 3 4 6% 38%	6996

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	2566
	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	44% 7% 12% 21% 15% 1	4370
	11th to 20th	Day	35% 6% 17% 27% 14% 1	4365
	21st to end	Day	42% 6% 14% 24% 13% 2	4661
August	1st to 10th	Day	43% 7% 16% 24% 9% 1	4098
	11th to 20th	Day	44% 6% 14% 26% 10% 1	4084
	21st to end	Day	27% 6% 16% 32% 18% 1	4244
September	1st to 10th	Day	35% 6% 11% 26% 21% 1	2249
	11th to 20th	Day	33% 4 9% 29% 23% 3	2090
	21st to end	Day	39% 4 9% 25% 21% 2	2090
October	1st to 10th	Day	41% 4 10% 24% 19% 2	3480
	11th to 20th	Day	30% 4 7% 32% 24% 3	3053
	21st to end	Day	32% 3 10% 31% 21% 2	3081
November	1st to 10th	Day	38% 3 13% 28% 16% 2	1761
	11th to 20th	Day	39% 3 9% 22% 24% 3	1800
	21st to end	Day	43% 5% 15% 23% 14% 1	1800
December	1st to 10th	Day	40% 4 14% 30% 12% 1	2681
	11th to 20th	Day	41% 4 13% 17% 19% 6%	2986
	21st to end	Day	49% 6% 8% 20% 13% 5%	3960
January	1st to 10th	Night	41% 4 13% 28% 12% 3	4514
	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	44% 7% 13% 21% 15% 1	2480
	11th to 20th	Night	39% 7% 15% 24% 15%	2472
	21st to end	Night	45% 5% 12% 23% 14% 1	2651
August	1st to 10th	Night	50% 6% 12% 20% 11% 1	2774
	11th to 20th	Night	46% 7% 15% 20% 11%	2809
	21st to end	Night	31% 6% 15% 26% 22% 1	3407
September	1st to 10th	Night	35% 5% 11% 27% 21% 1	1930
	11th to 20th	Night	34% 4 9% 27% 24% 2	2090
	21st to end	Night	38% 5% 9% 26% 20% 2	2090
October	1st to 10th	Night	43% 4 15% 23% 13% 2	3480
	11th to 20th	Night	29% 4 9% 28% 28% 2	3861
	21st to end	Night	35% 3 11% 31% 19% 2	4467
November	1st to 10th	Night	38% 4 11% 26% 18% 3	2537
	11th to 20th	Night	41% 4 9% 22% 21% 3	2600
	21st to end	Night	43% 4 14% 25% 13% 2	2600
December	1st to 10th	Night	46% 4 10% 29% 11% 1	4497
	11th to 20th	Night	46% 4 12% 16% 20% 1	5208
	21st to end	Night	48% 5% 10% 21% 16% 1	6996

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% 7% 28% 28% 10% 1	192
	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	11% 2 6% 33% 42% 6%	532
August	1st to 10th	Day	12% 2 5% 32% 41% 9%	486
	11th to 20th	Day	15% 2 5% 33% 33% 13%	479
	21st to end	Day	12% 1 6% 36% 35% 10%	539
September	1st to 10th	Day	15% 1 10% 29% 31% 13%	490
	11th to 20th	Day	20% 1 13% 26% 27% 13%	490
	21st to end	Day	16% 3 15% 36% 23% 8%	490
October	1st to 10th	Day	22% 3 17% 28% 23% 8%	490
	11th to 20th	Day	16% 4 22% 32% 21% 5%	413
	21st to end	Day	25% 3 24% 34% 14% 1	429
November	1st to 10th	Day	15% 5% 27% 31% 19% 3	395
	11th to 20th	Day	23% 8% 28% 33% 9%	400
	21st to end	Day	24% 8% 30% 26% 11% 1	400
December	1st to 10th	Day	21% 4 31% 33% 9% 2	255
	11th to 20th	Day	24% 6% 36% 30% 6%	200
	21st to end	Day	25% 6% 30% 26% 12% 1	220
January	1st to 10th	Night	22% 13% 40% 21% 4	785
	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	13% 1 4 40% 39% 4	539
	11th to 20th	Night	12% 2 9% 38% 33% 6%	562
	21st to end	Night	16% 1 8% 34% 34% 7%	793
September	1st to 10th	Night	14% 1 13% 36% 30% 6%	760
	11th to 20th	Night	17% 3 15% 32% 24% 9%	760
	21st to end	Night	16% 1 16% 34% 26% 7%	760
October	1st to 10th	Night	22% 4 20% 29% 20% 4	760
	11th to 20th	Night	17% 5% 26% 33% 16% 2	760
	21st to end	Night	26% 5% 27% 28% 13% 1	836
November	1st to 10th	Night	17% 4 30% 35% 12% 2	780
	11th to 20th	Night	20% 9% 36% 27% 8% 2	800
	21st to end	Night	20% 13% 33% 27% 6% 1	800
December	1st to 10th	Night	25% 8% 38% 23% 5% 1	800
	11th to 20th	Night	24% 12% 37% 23% 4	800
	21st to end	Night	26% 11% 34% 24% 4	880

Page generated at 2025-01-04 03:24:04. Version ac07

Copyright © 2025 Attilla Danko.