Thimble Islands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thimble Islands Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	18% 7% 2 6% 4 4 4 3 5% 7% 40%	126544
	Night	19% 8% 2 6% 4 4 4 3 5% 6% 41%	123502
Transparency	Day	43% 6% 12% 21% 15% 3	130086
	Night	43% 5% 11% 21% 16% 3	128046
Seeing	Day	22% 17% 32% 20% 8% 2	16340
	Night	22% 14% 32% 24% 8% 1	25335

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	13% 7% 2 5% 4 4 4 3 5% 6% 46%	3597
	11th to 20th	Day	14% 9% 2 6% 4 3 4 3 5% 5% 45%	3570
	21st to end	Day	19% 7% 3 7% 4 4 4 3 4 4 41%	4075
February	1st to 10th	Day	18% 8% 2 6% 5% 5% 5% 4 5% 6% 39%	1885
	11th to 20th	Day	16% 7% 2 5% 4 5% 4 3 5% 7% 42%	1948
	21st to end	Day	21% 9% 2 5% 4 3 3 4 5% 6% 37%	1650
March	1st to 10th	Day	23% 8% 2 5% 4 3 3 2 4 5% 41%	4476
	11th to 20th	Day	13% 9% 2 6% 4 4 4 3 5% 7% 42%	4662
	21st to end	Day	20% 7% 3 6% 4 4 5% 4 5% 6% 36%	5970
April	1st to 10th	Day	20% 8% 2 5% 4 4 4 3 5% 7% 38%	2400
	11th to 20th	Day	20% 7% 2 5% 4 3 4 3 4 6% 42%	2374
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 6% 41%	2512
May	1st to 10th	Day	30% 8% 2 5% 3 3 4 3 5% 8% 28%	5886
	11th to 20th	Day	22% 9% 3 6% 4 4 4 3 5% 6% 35%	6084
	21st to end	Day	20% 7% 3 6% 4 4 5% 4 6% 7% 36%	6949
June	1st to 10th	Day	18% 6% 2 6% 4 4 5% 3 7% 9% 36%	2800
	11th to 20th	Day	17% 6% 2 5% 3 4 4 4 5% 9% 40%	2800
	21st to end	Day	13% 6% 2 5% 5% 5% 4 4 6% 9% 41%	2800
July	1st to 10th	Day	15% 7% 3 7% 5% 4 5% 4 6% 9% 35%	6360
	11th to 20th	Day	13% 7% 3 6% 5% 4 5% 4 7% 10% 36%	6360
	21st to end	Day	15% 7% 3 6% 4 4 4 4 5% 7% 42%	6926
August	1st to 10th	Day	18% 7% 3 7% 4 4 5% 4 5% 8% 34%	4371
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 9% 44%	3867
	21st to end	Day	8% 4 2 6% 5% 4 5% 4 8% 9% 45%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 6% 47%	2400
	11th to 20th	Day	12% 5% 2 4 3 3 4 3 4 7% 53%	2254
	21st to end	Day	19% 7% 3 5% 4 4 5% 3 5% 6% 40%	2200
October	1st to 10th	Day	21% 6% 2 5% 3 3 3 3 4 6% 44%	2181
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 5% 8% 48%	1980
	21st to end	Day	18% 6% 2 5% 3 4 4 3 6% 6% 43%	1980
November	1st to 10th	Day	15% 5% 2 5% 4 3 3 3 3 6% 51%	1779
	11th to 20th	Day	19% 5% 2 3 3 3 3 2 5% 6% 49%	1800
	21st to end	Day	18% 6% 3 4 4 3 4 3 5% 6% 46%	1790
December	1st to 10th	Day	16% 6% 2 7% 4 4 3 3 4 6% 46%	2600
	11th to 20th	Day	22% 7% 2 4 3 2 3 2 3 5% 47%	2986
	21st to end	Day	20% 10% 2 6% 4 3 3 2 4 5% 40%	3960
January	1st to 10th	Night	14% 6% 2 4 3 4 4 3 5% 6% 49%	6349
	11th to 20th	Night	15% 8% 3 7% 4 3 3 3 4 4 47%	6315
	21st to end	Night	18% 9% 2 7% 5% 4 4 3 4 5% 38%	6878
February	1st to 10th	Night	16% 10% 2 7% 4 3 4 3 5% 5% 41%	2515
	11th to 20th	Night	18% 8% 2 5% 4 3 3 3 5% 7% 41%	2338
	21st to end	Night	20% 8% 2 6% 4 5% 3 3 3 5% 40%	1980
March	1st to 10th	Night	22% 9% 3 5% 3 3 3 2 4 5% 40%	5364
	11th to 20th	Night	17% 10% 3 5% 4 4 4 3 4 6% 40%	5293
	21st to end	Night	21% 9% 3 6% 4 3 4 3 4 5% 37%	4962
April	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 5% 7% 37%	2000
	11th to 20th	Night	21% 5% 2 5% 5% 4 3 3 5% 6% 41%	1975
	21st to end	Night	20% 6% 2 6% 4 4 4 3 4 4 42%	1886
May	1st to 10th	Night	28% 11% 4 7% 4 3 4 3 5% 6% 26%	4054
	11th to 20th	Night	24% 9% 3 6% 4 4 4 3 5% 7% 31%	3852
	21st to end	Night	24% 9% 3 6% 4 3 4 3 5% 7% 33%	3926
June	1st to 10th	Night	20% 6% 2 6% 5% 4 5% 4 5% 6% 39%	1600
	11th to 20th	Night	18% 7% 2 5% 4 3 4 3 6% 9% 40%	1600
	21st to end	Night	14% 7% 2 5% 5% 4 4 4 5% 9% 42%	1600
July	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 5% 8% 34%	3600
	11th to 20th	Night	21% 7% 3 7% 4 4 4 4 6% 8% 31%	3600
	21st to end	Night	15% 7% 2 5% 3 4 5% 4 6% 7% 42%	3926
August	1st to 10th	Night	23% 7% 3 8% 4 4 5% 4 5% 8% 29%	2746
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	3164
	21st to end	Night	9% 4 2 4 3 3 4 3 5% 8% 54%	3586
September	1st to 10th	Night	14% 6% 2 5% 4 5% 5% 3 5% 7% 44%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 6% 54%	2145
	21st to end	Night	18% 7% 3 6% 3 5% 5% 3 4 6% 41%	2200
October	1st to 10th	Night	19% 7% 2 6% 4 3 4 2 4 6% 43%	2215
	11th to 20th	Night	16% 5% 2 5% 4 3 4 3 4 5% 49%	2420
	21st to end	Night	21% 7% 2 5% 3 3 4 3 5% 6% 42%	2860
November	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 6% 49%	2564
	11th to 20th	Night	19% 6% 2 4 3 2 3 3 4 5% 48%	2600
	21st to end	Night	16% 6% 2 5% 4 3 4 3 4 7% 46%	2605
December	1st to 10th	Night	17% 7% 2 4 3 3 4 2 4 4 48%	4580
	11th to 20th	Night	27% 7% 2 4 3 3 3 2 3 4 42%	5208
	21st to end	Night	20% 9% 3 6% 3 4 3 2 4 4 42%	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	39% 3 8% 28% 18% 3	3598
	11th to 20th	Day	41% 4 11% 21% 15% 7%	3600
	21st to end	Day	46% 5% 9% 22% 16% 2	4075
February	1st to 10th	Day	46% 6% 13% 20% 14% 2	1885
	11th to 20th	Day	42% 4 11% 23% 16% 3	1957
	21st to end	Day	47% 5% 11% 19% 14% 3	1650
March	1st to 10th	Day	46% 4 11% 16% 13% 10%	4479
	11th to 20th	Day	39% 7% 15% 27% 11% 2	4662
	21st to end	Day	47% 5% 8% 15% 19% 6%	5970
April	1st to 10th	Day	45% 6% 11% 19% 16% 4	2400
	11th to 20th	Day	43% 5% 11% 20% 18% 3	2374
	21st to end	Day	44% 6% 10% 21% 16% 4	2512
May	1st to 10th	Day	55% 6% 16% 13% 8% 1	5897
	11th to 20th	Day	48% 7% 13% 18% 12% 2	6088
	21st to end	Day	46% 8% 11% 21% 13% 1	6949
June	1st to 10th	Day	43% 7% 13% 24% 12% 1	2800
	11th to 20th	Day	42% 7% 15% 22% 13% 1	2800
	21st to end	Day	40% 9% 13% 23% 14% 1	2800
July	1st to 10th	Day	46% 8% 10% 20% 14% 1	6290
	11th to 20th	Day	42% 7% 17% 22% 10% 1	6285
	21st to end	Day	41% 6% 11% 23% 19% 1	6728
August	1st to 10th	Day	47% 8% 14% 21% 9% 1	4357
	11th to 20th	Day	38% 7% 14% 30% 11% 1	3903
	21st to end	Day	34% 8% 13% 27% 17% 2	4312
September	1st to 10th	Day	38% 7% 12% 23% 19% 2	2400
	11th to 20th	Day	32% 5% 12% 23% 24% 2	2254
	21st to end	Day	44% 6% 9% 19% 19% 2	2200
October	1st to 10th	Day	36% 5% 10% 20% 24% 6%	3558
	11th to 20th	Day	28% 4 9% 24% 31% 4	3208
	21st to end	Day	32% 5% 13% 25% 22% 3	3180
November	1st to 10th	Day	37% 5% 13% 21% 22% 2	1779
	11th to 20th	Day	37% 4 12% 22% 22% 3	1800
	21st to end	Day	40% 4 13% 27% 12% 3	1790
December	1st to 10th	Day	40% 4 15% 27% 13% 1	2600
	11th to 20th	Day	41% 3 15% 18% 20% 3	2986
	21st to end	Day	48% 4 8% 20% 15% 5%	3960
January	1st to 10th	Night	36% 4 9% 28% 20% 3	6349
	11th to 20th	Night	41% 4 12% 21% 16% 6%	6360
	21st to end	Night	48% 4 9% 19% 16% 4	6878
February	1st to 10th	Night	45% 5% 12% 24% 12% 2	2515
	11th to 20th	Night	43% 5% 10% 20% 18% 4	2340
	21st to end	Night	47% 4 8% 22% 15% 4	1980
March	1st to 10th	Night	47% 4 11% 17% 14% 7%	5374
	11th to 20th	Night	44% 6% 15% 21% 11% 4	5294
	21st to end	Night	49% 3 7% 12% 24% 5%	4962
April	1st to 10th	Night	47% 5% 10% 19% 15% 4	2000
	11th to 20th	Night	45% 5% 10% 19% 20% 2	1975
	21st to end	Night	45% 5% 9% 21% 16% 4	1886
May	1st to 10th	Night	59% 6% 14% 11% 9% 1	4060
	11th to 20th	Night	52% 8% 13% 15% 10% 3	3853
	21st to end	Night	50% 6% 13% 21% 9% 1	3927
June	1st to 10th	Night	45% 7% 11% 19% 17% 2	1600
	11th to 20th	Night	41% 8% 14% 25% 12% 1	1600
	21st to end	Night	39% 9% 13% 24% 14% 1	1600
July	1st to 10th	Night	49% 7% 12% 20% 11% 1	3560
	11th to 20th	Night	49% 7% 15% 19% 9% 1	3552
	21st to end	Night	40% 6% 13% 23% 17% 1	3809
August	1st to 10th	Night	53% 8% 13% 16% 9% 1	2736
	11th to 20th	Night	42% 6% 13% 29% 8% 1	3193
	21st to end	Night	27% 6% 15% 31% 19% 1	3586
September	1st to 10th	Night	40% 6% 10% 25% 18% 1	2000
	11th to 20th	Night	34% 5% 11% 23% 25% 3	2145
	21st to end	Night	44% 6% 9% 19% 19% 3	2200
October	1st to 10th	Night	39% 4 11% 24% 19% 3	3618
	11th to 20th	Night	26% 4 10% 26% 31% 4	3931
	21st to end	Night	32% 4 14% 27% 21% 2	4610
November	1st to 10th	Night	36% 6% 12% 22% 21% 4	2564
	11th to 20th	Night	40% 4 10% 22% 20% 4	2600
	21st to end	Night	39% 4 13% 29% 13% 2	2605
December	1st to 10th	Night	41% 3 12% 29% 14% 1	4580
	11th to 20th	Night	48% 4 11% 17% 18% 2	5208
	21st to end	Night	46% 4 10% 19% 17% 4	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	28% 27% 29% 15% 3	200
	11th to 20th	Day	28% 35% 25% 13% 1 1	200
	21st to end	Day	25% 27% 32% 9% 4 2	245
February	1st to 10th	Day	28% 25% 35% 12% 1	385
	11th to 20th	Day	29% 27% 32% 11% 1	496
	21st to end	Day	29% 27% 29% 12% 4	414
March	1st to 10th	Day	28% 24% 32% 14% 2	500
	11th to 20th	Day	24% 27% 33% 12% 3 1	500
	21st to end	Day	32% 21% 33% 11% 3 1	550
April	1st to 10th	Day	29% 27% 29% 10% 5%	500
	11th to 20th	Day	29% 21% 31% 15% 4 1	492
	21st to end	Day	29% 17% 33% 15% 4 1	500
May	1st to 10th	Day	30% 20% 30% 14% 5% 1	503
	11th to 20th	Day	22% 18% 36% 17% 6% 1	527
	21st to end	Day	22% 13% 31% 24% 9% 2	573
June	1st to 10th	Day	20% 13% 34% 22% 9% 2	530
	11th to 20th	Day	19% 15% 33% 23% 8% 1	530
	21st to end	Day	16% 12% 32% 27% 10% 2	530
July	1st to 10th	Day	18% 7% 31% 27% 14% 3	525
	11th to 20th	Day	13% 6% 31% 34% 13% 3	524
	21st to end	Day	14% 10% 35% 30% 8% 2	576
August	1st to 10th	Day	14% 10% 30% 31% 10% 5%	526
	11th to 20th	Day	14% 6% 33% 28% 16% 3	519
	21st to end	Day	10% 12% 34% 27% 12% 5%	583
September	1st to 10th	Day	19% 10% 29% 23% 16% 4	530
	11th to 20th	Day	14% 9% 32% 26% 12% 7%	530
	21st to end	Day	22% 10% 35% 21% 9% 2	530
October	1st to 10th	Day	26% 11% 26% 21% 11% 4	530
	11th to 20th	Day	18% 20% 30% 22% 8% 1	519
	21st to end	Day	24% 18% 33% 16% 7% 1	462
November	1st to 10th	Day	17% 17% 33% 22% 8% 3	401
	11th to 20th	Day	24% 25% 28% 18% 5%	400
	21st to end	Day	24% 22% 28% 21% 6%	390
December	1st to 10th	Day	23% 23% 34% 15% 6%	200
	11th to 20th	Day	33% 26% 31% 10% 2	200
	21st to end	Day	27% 20% 33% 15% 6%	220
January	1st to 10th	Night	23% 23% 35% 17% 3	800
	11th to 20th	Night	28% 19% 32% 18% 2	800
	21st to end	Night	21% 24% 34% 20% 3	880
February	1st to 10th	Night	22% 21% 35% 21% 1	800
	11th to 20th	Night	25% 22% 37% 14% 2	783
	21st to end	Night	26% 20% 36% 15% 3	660
March	1st to 10th	Night	25% 20% 34% 19% 3	800
	11th to 20th	Night	23% 17% 37% 19% 4 1	800
	21st to end	Night	22% 21% 36% 18% 3	880
April	1st to 10th	Night	26% 18% 34% 19% 3	800
	11th to 20th	Night	24% 15% 36% 21% 4	791
	21st to end	Night	22% 16% 37% 20% 6%	686
May	1st to 10th	Night	26% 13% 33% 22% 7%	600
	11th to 20th	Night	23% 13% 36% 22% 6%	515
	21st to end	Night	22% 6% 26% 36% 9% 1	433
June	1st to 10th	Night	22% 6% 30% 32% 11% 1	400
	11th to 20th	Night	21% 5% 28% 36% 10%	400
	21st to end	Night	17% 5% 29% 39% 10% 1	400
July	1st to 10th	Night	16% 4 24% 39% 16% 1	390
	11th to 20th	Night	14% 3 21% 45% 16% 1	388
	21st to end	Night	15% 4 28% 40% 12% 1	438
August	1st to 10th	Night	12% 4 28% 41% 15% 1	493
	11th to 20th	Night	14% 4 23% 40% 17% 1	769
	21st to end	Night	14% 5% 25% 35% 18% 3	880
September	1st to 10th	Night	17% 5% 29% 33% 14% 3	800
	11th to 20th	Night	16% 6% 26% 30% 19% 3	800
	21st to end	Night	23% 6% 27% 29% 14% 1	800
October	1st to 10th	Night	21% 10% 28% 24% 14% 2	800
	11th to 20th	Night	18% 15% 32% 24% 9% 1	800
	21st to end	Night	26% 12% 35% 20% 7%	880
November	1st to 10th	Night	16% 15% 34% 23% 11% 1	789
	11th to 20th	Night	22% 18% 34% 20% 6% 1	800
	21st to end	Night	22% 20% 30% 23% 5% 1	800
December	1st to 10th	Night	26% 18% 30% 20% 6% 1	800
	11th to 20th	Night	27% 20% 30% 20% 3	800
	21st to end	Night	26% 16% 37% 19% 2	880

Page generated at 2025-10-31 02:02:42. Version ac07

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