Frog Island Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Frog Island Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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% 9% 3 8% 5% 5% 5% 4 6% 8% 37%	127531
	Night	13% 10% 3 8% 5% 5% 5% 4 5% 7% 36%	124596
Transparency	Day	27% 40% 8% 16% 8% 1	130986
	Night	30% 39% 9% 14% 8% 1	129228
Seeing	Day	15% 10% 30% 28% 13% 3	17636
	Night	17% 8% 31% 30% 12% 1	27690

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	7% 11% 6% 13% 6% 5% 5% 4 6% 7% 29%	3697
	11th to 20th	Day	13% 11% 4 8% 6% 6% 7% 5% 6% 9% 26%	3669
	21st to end	Day	12% 13% 4 10% 6% 5% 5% 4 6% 6% 28%	4070
February	1st to 10th	Day	9% 10% 4 10% 7% 6% 6% 4 7% 7% 31%	1661
	11th to 20th	Day	10% 12% 4 8% 6% 7% 6% 5% 6% 6% 31%	1937
	21st to end	Day	11% 9% 4 12% 6% 5% 6% 5% 8% 7% 27%	1650
March	1st to 10th	Day	12% 8% 3 6% 4 4 4 4 6% 8% 40%	4478
	11th to 20th	Day	10% 9% 4 11% 7% 5% 6% 4 6% 7% 31%	4692
	21st to end	Day	20% 12% 4 9% 6% 5% 5% 4 5% 6% 26%	5914
April	1st to 10th	Day	15% 11% 3 9% 7% 5% 5% 4 5% 7% 28%	2400
	11th to 20th	Day	17% 9% 4 7% 5% 5% 5% 4 5% 6% 33%	2418
	21st to end	Day	17% 8% 2 7% 4 4 6% 5% 5% 8% 35%	2600
May	1st to 10th	Day	12% 7% 3 7% 4 4 4 3 5% 7% 45%	5994
	11th to 20th	Day	11% 9% 3 7% 5% 4 5% 4 5% 7% 40%	6319
	21st to end	Day	11% 8% 3 7% 5% 4 5% 4 6% 8% 40%	6949
June	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 4 7% 8% 42%	2800
	11th to 20th	Day	13% 6% 2 7% 5% 4 4 5% 7% 7% 40%	2800
	21st to end	Day	13% 5% 2 7% 4 4 5% 5% 8% 9% 39%	2800
July	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 6% 10% 45%	6359
	11th to 20th	Day	6% 4 2 5% 4 3 4 4 6% 10% 52%	6360
	21st to end	Day	7% 5% 2 5% 4 4 4 4 7% 9% 48%	6926
August	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 10% 42%	4563
	11th to 20th	Day	11% 4 2 5% 3 4 4 4 6% 7% 50%	4186
	21st to end	Day	11% 6% 3 7% 4 4 4 4 5% 7% 46%	4331
September	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	2400
	11th to 20th	Day	10% 5% 2 5% 5% 4 5% 4 7% 8% 44%	2247
	21st to end	Day	12% 7% 2 6% 5% 5% 4 5% 6% 8% 39%	2200
October	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 4 6% 9% 38%	2165
	11th to 20th	Day	19% 10% 4 8% 5% 5% 5% 4 6% 7% 26%	1877
	21st to end	Day	18% 11% 4 8% 5% 5% 5% 4 6% 7% 27%	1980
November	1st to 10th	Day	13% 10% 3 8% 5% 4 6% 4 5% 7% 35%	1852
	11th to 20th	Day	15% 11% 4 11% 6% 5% 5% 4 5% 6% 27%	1890
	21st to end	Day	16% 13% 4 9% 5% 4 6% 5% 6% 7% 24%	1689
December	1st to 10th	Day	12% 14% 4 11% 7% 7% 6% 5% 6% 8% 21%	2572
	11th to 20th	Day	14% 15% 5% 12% 7% 6% 6% 4 5% 6% 22%	3016
	21st to end	Day	22% 20% 5% 12% 6% 4 5% 3 4 3 16%	4070
January	1st to 10th	Night	10% 12% 5% 14% 7% 5% 4 3 5% 6% 29%	6488
	11th to 20th	Night	9% 14% 5% 13% 7% 6% 6% 4 5% 7% 23%	6456
	21st to end	Night	9% 13% 5% 10% 5% 5% 5% 4 6% 7% 30%	7150
February	1st to 10th	Night	10% 13% 5% 9% 6% 4 5% 5% 5% 7% 30%	2736
	11th to 20th	Night	10% 12% 3 9% 6% 6% 5% 4 7% 7% 32%	2348
	21st to end	Night	12% 12% 4 10% 5% 6% 5% 4 6% 6% 29%	1980
March	1st to 10th	Night	14% 8% 3 6% 5% 4 3 3 5% 8% 41%	5362
	11th to 20th	Night	11% 10% 4 8% 5% 5% 5% 4 7% 9% 32%	5266
	21st to end	Night	21% 9% 3 7% 5% 5% 5% 4 5% 7% 30%	5059
April	1st to 10th	Night	18% 12% 4 8% 5% 4 5% 3 5% 6% 30%	2000
	11th to 20th	Night	20% 9% 3 7% 4 4 4 4 5% 5% 35%	1931
	21st to end	Night	16% 9% 3 7% 5% 4 5% 3 5% 8% 36%	1800
May	1st to 10th	Night	15% 7% 2 6% 3 3 4 3 4 7% 46%	3948
	11th to 20th	Night	13% 9% 2 8% 5% 5% 4 4 4 7% 38%	3596
	21st to end	Night	17% 7% 2 6% 3 4 5% 3 5% 7% 41%	3949
June	1st to 10th	Night	11% 6% 2 6% 4 5% 4 4 6% 7% 45%	1600
	11th to 20th	Night	15% 6% 3 6% 5% 6% 4 3 6% 7% 38%	1600
	21st to end	Night	15% 7% 2 5% 4 3 4 4 5% 8% 44%	1600
July	1st to 10th	Night	11% 4 2 6% 4 4 5% 4 6% 8% 46%	3620
	11th to 20th	Night	7% 4 2 4 4 3 3 3 5% 8% 57%	3620
	21st to end	Night	6% 4 2 6% 3 4 4 3 6% 8% 53%	3948
August	1st to 10th	Night	10% 5% 2 7% 4 5% 5% 4 5% 8% 46%	2597
	11th to 20th	Night	12% 4 2 5% 3 4 5% 3 6% 7% 51%	2861
	21st to end	Night	11% 5% 2 7% 4 4 4 3 5% 6% 49%	3551
September	1st to 10th	Night	9% 6% 2 6% 3 4 3 3 5% 7% 51%	2000
	11th to 20th	Night	11% 6% 2 5% 4 4 5% 4 6% 5% 49%	2132
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 5% 8% 41%	2200
October	1st to 10th	Night	15% 8% 2 7% 4 4 4 4 5% 7% 40%	2233
	11th to 20th	Night	16% 9% 3 8% 5% 5% 5% 4 6% 6% 34%	2522
	21st to end	Night	20% 11% 3 7% 4 4 4 4 6% 7% 31%	2860
November	1st to 10th	Night	15% 9% 3 7% 5% 5% 5% 4 5% 7% 36%	2666
	11th to 20th	Night	15% 13% 3 8% 6% 5% 5% 4 5% 6% 30%	2730
	21st to end	Night	15% 13% 4 8% 5% 5% 5% 4 5% 7% 28%	2930
December	1st to 10th	Night	15% 10% 4 9% 5% 5% 5% 4 6% 8% 30%	4764
	11th to 20th	Night	13% 14% 5% 13% 7% 5% 5% 3 5% 5% 25%	5343
	21st to end	Night	25% 16% 5% 10% 5% 4 4 3 4 5% 20%	7150

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	37% 30% 10% 17% 6%	3698
	11th to 20th	Day	35% 31% 11% 14% 8% 1	3700
	21st to end	Day	40% 31% 9% 10% 10%	4070
February	1st to 10th	Day	18% 63% 6% 8% 5%	1661
	11th to 20th	Day	21% 64% 5% 6% 5%	1947
	21st to end	Day	18% 63% 4 9% 5% 1	1650
March	1st to 10th	Day	25% 31% 14% 13% 13% 3	4480
	11th to 20th	Day	36% 32% 11% 14% 6% 1	4692
	21st to end	Day	42% 33% 6% 12% 7%	5914
April	1st to 10th	Day	22% 63% 3 8% 4	2400
	11th to 20th	Day	20% 63% 3 8% 5% 1	2418
	21st to end	Day	17% 63% 4 8% 7% 1	2600
May	1st to 10th	Day	24% 31% 8% 22% 14% 1	6003
	11th to 20th	Day	30% 32% 8% 20% 10% 1	6323
	21st to end	Day	30% 32% 7% 16% 13% 2	6949
June	1st to 10th	Day	13% 63% 6% 11% 6% 1	2800
	11th to 20th	Day	19% 63% 6% 7% 5% 1	2800
	21st to end	Day	15% 62% 7% 11% 5%	2800
July	1st to 10th	Day	26% 28% 12% 24% 9%	6291
	11th to 20th	Day	20% 28% 11% 31% 9%	6285
	21st to end	Day	23% 28% 10% 25% 13%	6728
August	1st to 10th	Day	23% 39% 12% 19% 8%	4548
	11th to 20th	Day	23% 38% 11% 21% 7%	4225
	21st to end	Day	26% 38% 11% 18% 7%	4330
September	1st to 10th	Day	18% 60% 8% 11% 4	2400
	11th to 20th	Day	15% 60% 8% 12% 5%	2247
	21st to end	Day	16% 59% 7% 12% 4	2200
October	1st to 10th	Day	16% 37% 8% 22% 18%	3529
	11th to 20th	Day	30% 38% 6% 18% 8%	3028
	21st to end	Day	33% 38% 10% 14% 5%	3180
November	1st to 10th	Day	23% 56% 6% 10% 4 1	1852
	11th to 20th	Day	26% 55% 7% 9% 4	1890
	21st to end	Day	28% 55% 6% 7% 3	1689
December	1st to 10th	Day	39% 39% 10% 9% 3	2572
	11th to 20th	Day	43% 32% 9% 9% 7%	3017
	21st to end	Day	55% 29% 3 8% 4	4070
January	1st to 10th	Night	38% 30% 7% 18% 7%	6488
	11th to 20th	Night	44% 30% 11% 8% 8%	6500
	21st to end	Night	37% 30% 11% 14% 7%	7150
February	1st to 10th	Night	21% 62% 4 8% 5%	2736
	11th to 20th	Night	19% 62% 4 11% 5% 1	2349
	21st to end	Night	18% 63% 4 10% 5% 1	1980
March	1st to 10th	Night	29% 30% 15% 15% 11% 1	5373
	11th to 20th	Night	34% 32% 14% 13% 6%	5267
	21st to end	Night	38% 32% 9% 14% 6%	5059
April	1st to 10th	Night	22% 62% 3 6% 6% 1	2000
	11th to 20th	Night	21% 63% 3 8% 4 2	1931
	21st to end	Night	18% 62% 4 8% 6% 1	1800
May	1st to 10th	Night	25% 31% 8% 21% 12% 2	3955
	11th to 20th	Night	30% 32% 8% 17% 11% 1	3597
	21st to end	Night	33% 30% 6% 19% 7% 5%	3950
June	1st to 10th	Night	14% 63% 4 12% 7%	1600
	11th to 20th	Night	20% 63% 4 7% 5% 1	1600
	21st to end	Night	16% 60% 4 13% 7% 1	1600
July	1st to 10th	Night	29% 28% 12% 22% 8% 2	3580
	11th to 20th	Night	20% 26% 13% 30% 10% 1	3572
	21st to end	Night	21% 27% 11% 21% 19% 1	3832
August	1st to 10th	Night	25% 37% 10% 15% 12% 1	2588
	11th to 20th	Night	24% 38% 10% 21% 7%	2888
	21st to end	Night	24% 38% 10% 17% 11%	3551
September	1st to 10th	Night	17% 58% 8% 12% 5%	2000
	11th to 20th	Night	16% 59% 6% 13% 6%	2132
	21st to end	Night	18% 58% 7% 11% 6%	2200
October	1st to 10th	Night	22% 36% 6% 25% 10% 1	3649
	11th to 20th	Night	25% 37% 9% 18% 12%	4110
	21st to end	Night	32% 37% 10% 16% 7%	4610
November	1st to 10th	Night	24% 54% 6% 11% 4	2666
	11th to 20th	Night	22% 56% 7% 10% 5% 1	2730
	21st to end	Night	27% 55% 6% 8% 3 1	2930
December	1st to 10th	Night	33% 37% 15% 12% 3	4764
	11th to 20th	Night	43% 33% 7% 11% 7%	5341
	21st to end	Night	49% 30% 6% 8% 7%	7150

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	14% 12% 37% 27% 11%	330
	11th to 20th	Day	19% 13% 33% 25% 8% 1	330
	21st to end	Day	17% 8% 35% 33% 7% 1	363
February	1st to 10th	Day	14% 13% 43% 24% 5% 1	300
	11th to 20th	Day	12% 18% 40% 26% 4 1	300
	21st to end	Day	17% 14% 49% 17% 3	249
March	1st to 10th	Day	13% 14% 44% 22% 7%	300
	11th to 20th	Day	14% 15% 37% 25% 7% 2	368
	21st to end	Day	21% 14% 31% 24% 9% 1	550
April	1st to 10th	Day	22% 11% 34% 25% 6% 1	500
	11th to 20th	Day	19% 15% 30% 23% 10% 2	492
	21st to end	Day	18% 13% 32% 26% 9% 2	500
May	1st to 10th	Day	17% 13% 35% 22% 12% 2	544
	11th to 20th	Day	15% 9% 35% 26% 11% 2	725
	21st to end	Day	14% 8% 30% 31% 13% 4	789
June	1st to 10th	Day	11% 8% 34% 32% 13% 2	730
	11th to 20th	Day	17% 7% 26% 34% 13% 2	730
	21st to end	Day	13% 6% 26% 37% 15% 2	730
July	1st to 10th	Day	8% 4 24% 37% 22% 4	721
	11th to 20th	Day	8% 5% 25% 38% 18% 6%	718
	21st to end	Day	7% 5% 25% 34% 23% 7%	794
August	1st to 10th	Day	7% 3 22% 38% 22% 7%	726
	11th to 20th	Day	11% 5% 18% 32% 26% 8%	531
	21st to end	Day	12% 8% 23% 26% 24% 7%	583
September	1st to 10th	Day	12% 7% 28% 28% 18% 7%	530
	11th to 20th	Day	11% 4 26% 32% 18% 9%	530
	21st to end	Day	13% 12% 23% 27% 19% 7%	530
October	1st to 10th	Day	17% 9% 31% 26% 15% 3	495
	11th to 20th	Day	18% 16% 35% 24% 7%	330
	21st to end	Day	24% 16% 29% 22% 7% 2	363
November	1st to 10th	Day	14% 15% 38% 25% 8% 1	330
	11th to 20th	Day	20% 19% 37% 16% 7% 1	330
	21st to end	Day	22% 17% 37% 20% 5%	330
December	1st to 10th	Day	19% 16% 36% 20% 9%	294
	11th to 20th	Day	24% 15% 39% 18% 3 1	308
	21st to end	Day	28% 16% 31% 20% 5% 1	363
January	1st to 10th	Night	17% 10% 38% 29% 6%	1050
	11th to 20th	Night	20% 10% 38% 24% 8%	1050
	21st to end	Night	17% 6% 35% 31% 10% 1	1155
February	1st to 10th	Night	16% 7% 40% 29% 7% 1	1001
	11th to 20th	Night	16% 12% 38% 28% 6% 1	793
	21st to end	Night	19% 10% 36% 30% 5%	660
March	1st to 10th	Night	18% 8% 37% 29% 8% 1	800
	11th to 20th	Night	15% 10% 41% 27% 7%	800
	21st to end	Night	19% 8% 31% 32% 9% 1	705
April	1st to 10th	Night	21% 8% 34% 29% 7% 1	600
	11th to 20th	Night	23% 7% 29% 28% 11% 1	593
	21st to end	Night	18% 9% 29% 35% 9% 1	600
May	1st to 10th	Night	19% 7% 30% 32% 12% 1	600
	11th to 20th	Night	19% 5% 29% 35% 11% 2	591
	21st to end	Night	15% 4 21% 40% 19% 2	650
June	1st to 10th	Night	12% 4 25% 43% 16% 1	600
	11th to 20th	Night	18% 4 20% 40% 16% 2	600
	21st to end	Night	17% 3 21% 40% 18% 1	600
July	1st to 10th	Night	8% 2 16% 44% 27% 2	585
	11th to 20th	Night	10% 3 19% 39% 27% 2	582
	21st to end	Night	9% 3 21% 40% 24% 4	657
August	1st to 10th	Night	8% 2 19% 39% 29% 3	597
	11th to 20th	Night	11% 1 18% 37% 29% 4	594
	21st to end	Night	12% 3 23% 36% 24% 1	660
September	1st to 10th	Night	11% 5% 24% 32% 22% 6%	600
	11th to 20th	Night	13% 5% 24% 32% 20% 5%	732
	21st to end	Night	16% 9% 24% 27% 21% 3	800
October	1st to 10th	Night	18% 8% 30% 27% 15% 2	800
	11th to 20th	Night	22% 12% 31% 26% 9%	800
	21st to end	Night	24% 11% 32% 24% 8%	880
November	1st to 10th	Night	18% 13% 35% 27% 6%	820
	11th to 20th	Night	21% 16% 35% 22% 5% 1	840
	21st to end	Night	23% 16% 34% 22% 5%	1040
December	1st to 10th	Night	21% 15% 37% 21% 6%	1050
	11th to 20th	Night	21% 14% 38% 20% 7% 1	1050
	21st to end	Night	27% 11% 35% 23% 5%	1155

Page generated at 2026-01-29 01:21:40. Version ac07

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