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 2024-11-06.

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%	101238
	Night	19% 8% 2 6% 4 4 4 3 5% 6% 40%	97829
Transparency	Day	42% 6% 12% 22% 15% 2	104798
	Night	43% 6% 12% 22% 15% 2	102357
Seeing	Day	22% 17% 32% 20% 8% 2	15059
	Night	21% 14% 32% 25% 8% 1	24080

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% 7% 2 5% 4 4 4 3 4 5% 46%	2517
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 5% 4 39%	2520
	21st to end	Day	23% 10% 3 7% 4 4 4 3 4 5% 33%	2845
February	1st to 10th	Day	18% 8% 2 6% 4 5% 5% 3 5% 6% 39%	1790
	11th to 20th	Day	15% 7% 2 5% 4 5% 4 3 5% 7% 43%	1848
	21st to end	Day	21% 9% 2 6% 5% 3 4 4 5% 5% 36%	1570
March	1st to 10th	Day	27% 9% 2 6% 4 4 3 2 3 5% 35%	3116
	11th to 20th	Day	10% 6% 2 5% 5% 5% 5% 4 6% 8% 45%	3260
	21st to end	Day	21% 6% 3 6% 3 4 4 4 5% 6% 38%	4166
April	1st to 10th	Day	19% 8% 2 5% 5% 4 4 3 5% 7% 39%	2280
	11th to 20th	Day	20% 7% 2 5% 4 3 4 3 4 6% 42%	2254
	21st to end	Day	21% 6% 2 6% 4 4 4 3 5% 6% 41%	2386
May	1st to 10th	Day	28% 10% 2 6% 3 3 4 2 5% 7% 31%	4106
	11th to 20th	Day	23% 7% 2 6% 4 4 4 3 5% 7% 34%	4234
	21st to end	Day	16% 5% 2 5% 3 3 3 3 6% 8% 46%	4837
June	1st to 10th	Day	18% 6% 2 5% 4 5% 5% 3 7% 9% 36%	2660
	11th to 20th	Day	16% 6% 2 5% 3 4 5% 4 6% 9% 40%	2660
	21st to end	Day	13% 6% 2 5% 5% 5% 4 4 6% 8% 41%	2660
July	1st to 10th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 9% 36%	4440
	11th to 20th	Day	14% 6% 2 5% 5% 4 5% 4 6% 9% 40%	4440
	21st to end	Day	17% 8% 3 7% 4 4 5% 4 6% 7% 35%	4814
August	1st to 10th	Day	19% 7% 3 7% 4 4 5% 4 5% 8% 34%	4237
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 9% 44%	3746
	21st to end	Day	8% 4 2 6% 5% 5% 5% 4 8% 9% 45%	4180
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 6% 47%	2280
	11th to 20th	Day	13% 5% 2 4 3 4 4 3 4 7% 52%	2142
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 6% 40%	2090
October	1st to 10th	Day	22% 6% 2 5% 4 3 3 3 4 6% 42%	2072
	11th to 20th	Day	14% 6% 2 4 3 3 3 3 5% 8% 49%	1881
	21st to end	Day	18% 7% 2 5% 3 4 4 3 6% 6% 42%	1881
November	1st to 10th	Day	15% 5% 2 5% 4 3 3 3 3 5% 50%	1717
	11th to 20th	Day	20% 6% 2 3 3 3 3 3 5% 6% 47%	1710
	21st to end	Day	17% 6% 2 5% 4 3 4 3 5% 6% 46%	1701
December	1st to 10th	Day	20% 5% 2 7% 5% 4 4 4 5% 5% 39%	1520
	11th to 20th	Day	23% 7% 2 4 3 2 3 2 3 5% 46%	1906
	21st to end	Day	20% 11% 3 7% 5% 4 3 3 4 6% 35%	2772
January	1st to 10th	Night	17% 6% 2 4 3 3 3 3 5% 6% 46%	4429
	11th to 20th	Night	18% 9% 3 6% 4 3 3 3 4 4 44%	4440
	21st to end	Night	25% 11% 3 7% 5% 3 3 3 3 4 33%	4808
February	1st to 10th	Night	16% 10% 2 7% 4 3 4 3 5% 5% 41%	2390
	11th to 20th	Night	18% 8% 2 6% 4 3 3 3 4 6% 42%	2218
	21st to end	Night	21% 8% 2 5% 4 4 3 3 3 5% 41%	1884
March	1st to 10th	Night	26% 11% 3 5% 3 3 3 2 4 5% 35%	3724
	11th to 20th	Night	12% 7% 2 6% 4 5% 5% 3 5% 7% 43%	3695
	21st to end	Night	22% 8% 3 5% 4 3 3 3 5% 5% 39%	3466
April	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 7% 38%	1900
	11th to 20th	Night	20% 5% 3 6% 5% 4 3 4 5% 6% 41%	1875
	21st to end	Night	21% 7% 2 6% 4 4 4 3 4 4 42%	1792
May	1st to 10th	Night	24% 11% 4 7% 4 4 4 4 5% 6% 28%	2834
	11th to 20th	Night	24% 8% 3 7% 4 4 4 4 5% 7% 29%	2688
	21st to end	Night	19% 8% 3 4 3 3 4 3 5% 7% 41%	2738
June	1st to 10th	Night	20% 6% 2 6% 4 4 5% 4 5% 6% 39%	1520
	11th to 20th	Night	17% 6% 2 4 4 3 4 3 6% 9% 41%	1520
	21st to end	Night	14% 7% 2 5% 4 4 4 4 5% 9% 41%	1520
July	1st to 10th	Night	14% 9% 4 7% 5% 5% 5% 5% 6% 8% 33%	2520
	11th to 20th	Night	19% 6% 2 6% 4 4 4 4 6% 9% 35%	2520
	21st to end	Night	20% 9% 3 6% 4 4 5% 4 6% 7% 33%	2739
August	1st to 10th	Night	23% 7% 3 8% 4 4 5% 4 6% 8% 28%	2661
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	3065
	21st to end	Night	9% 4 2 4 3 3 4 3 5% 8% 54%	3476
September	1st to 10th	Night	14% 6% 2 5% 4 5% 5% 3 5% 6% 43%	1900
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 5% 54%	2038
	21st to end	Night	18% 7% 3 6% 3 5% 5% 3 4 5% 41%	2090
October	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 4 6% 41%	2104
	11th to 20th	Night	15% 5% 2 5% 4 3 4 3 4 5% 50%	2299
	21st to end	Night	21% 7% 2 5% 3 4 4 3 5% 5% 41%	2717
November	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 4 6% 49%	2483
	11th to 20th	Night	20% 6% 2 4 3 2 3 3 4 5% 47%	2470
	21st to end	Night	16% 6% 2 5% 4 3 4 3 4 7% 46%	2474
December	1st to 10th	Night	22% 7% 2 5% 4 4 5% 3 5% 5% 39%	2660
	11th to 20th	Night	21% 7% 2 4 4 4 4 3 4 6% 41%	3288
	21st to end	Night	23% 10% 3 6% 4 4 3 3 4 5% 37%	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	40% 4 11% 30% 12% 3	2518
	11th to 20th	Day	47% 5% 12% 23% 11% 1	2520
	21st to end	Day	52% 6% 9% 21% 10% 2	2845
February	1st to 10th	Day	46% 6% 12% 20% 13% 2	1790
	11th to 20th	Day	41% 4 11% 24% 16% 3	1857
	21st to end	Day	49% 5% 10% 19% 13% 3	1570
March	1st to 10th	Day	52% 5% 13% 17% 11% 2	3119
	11th to 20th	Day	32% 8% 16% 30% 13% 2	3260
	21st to end	Day	45% 5% 8% 17% 20% 5%	4166
April	1st to 10th	Day	44% 6% 11% 19% 16% 4	2280
	11th to 20th	Day	43% 6% 10% 20% 18% 3	2254
	21st to end	Day	44% 6% 10% 21% 16% 4	2386
May	1st to 10th	Day	53% 6% 15% 14% 11% 1	4120
	11th to 20th	Day	48% 7% 15% 18% 10% 2	4238
	21st to end	Day	36% 7% 13% 24% 18% 2	4837
June	1st to 10th	Day	43% 7% 13% 24% 12% 1	2660
	11th to 20th	Day	41% 7% 15% 23% 13% 1	2660
	21st to end	Day	40% 9% 13% 23% 14% 1	2660
July	1st to 10th	Day	45% 9% 10% 20% 15% 1	4370
	11th to 20th	Day	40% 7% 16% 22% 13% 1	4365
	21st to end	Day	48% 7% 12% 17% 15% 1	4661
August	1st to 10th	Day	48% 8% 15% 21% 8%	4223
	11th to 20th	Day	38% 7% 14% 30% 10% 1	3782
	21st to end	Day	34% 8% 13% 27% 16% 2	4180
September	1st to 10th	Day	38% 7% 12% 22% 19% 2	2280
	11th to 20th	Day	33% 5% 12% 23% 24% 3	2142
	21st to end	Day	44% 6% 9% 19% 20% 2	2090
October	1st to 10th	Day	37% 5% 10% 19% 23% 6%	3449
	11th to 20th	Day	27% 4 9% 24% 32% 4	3109
	21st to end	Day	32% 5% 13% 25% 22% 3	3081
November	1st to 10th	Day	38% 5% 13% 20% 21% 3	1717
	11th to 20th	Day	39% 4 12% 21% 20% 3	1710
	21st to end	Day	40% 5% 13% 27% 13% 3	1701
December	1st to 10th	Day	47% 6% 14% 17% 15% 1	1520
	11th to 20th	Day	42% 4 16% 18% 18% 2	1906
	21st to end	Day	52% 5% 11% 22% 10% 1	2772
January	1st to 10th	Night	39% 4 12% 30% 12% 4	4429
	11th to 20th	Night	45% 5% 12% 23% 13% 3	4440
	21st to end	Night	55% 4 9% 19% 11% 2	4808
February	1st to 10th	Night	45% 5% 12% 23% 12% 2	2390
	11th to 20th	Night	44% 5% 10% 20% 17% 4	2220
	21st to end	Night	47% 4 8% 23% 15% 4	1884
March	1st to 10th	Night	53% 4 10% 19% 12% 2	3734
	11th to 20th	Night	37% 8% 15% 22% 15% 3	3696
	21st to end	Night	47% 3 9% 12% 24% 6%	3466
April	1st to 10th	Night	46% 5% 10% 19% 15% 3	1900
	11th to 20th	Night	45% 5% 10% 19% 19% 1	1875
	21st to end	Night	46% 5% 9% 21% 15% 5%	1792
May	1st to 10th	Night	56% 7% 13% 13% 9% 1	2840
	11th to 20th	Night	53% 9% 13% 13% 9% 2	2689
	21st to end	Night	41% 7% 16% 23% 12% 1	2739
June	1st to 10th	Night	44% 7% 11% 19% 17% 2	1520
	11th to 20th	Night	40% 8% 14% 26% 12% 1	1520
	21st to end	Night	39% 9% 13% 25% 14%	1520
July	1st to 10th	Night	48% 9% 12% 17% 12% 1	2480
	11th to 20th	Night	43% 8% 16% 23% 10% 1	2472
	21st to end	Night	49% 7% 13% 14% 16% 2	2651
August	1st to 10th	Night	54% 9% 13% 15% 9% 1	2651
	11th to 20th	Night	43% 6% 13% 29% 8% 1	3094
	21st to end	Night	27% 6% 15% 32% 19% 1	3476
September	1st to 10th	Night	40% 7% 10% 24% 18% 1	1900
	11th to 20th	Night	34% 5% 11% 23% 25% 3	2038
	21st to end	Night	44% 6% 9% 19% 20% 3	2090
October	1st to 10th	Night	40% 5% 11% 24% 18% 3	3507
	11th to 20th	Night	25% 4 10% 26% 31% 4	3810
	21st to end	Night	32% 4 14% 26% 21% 3	4467
November	1st to 10th	Night	37% 6% 12% 22% 20% 4	2483
	11th to 20th	Night	41% 4 10% 23% 19% 4	2470
	21st to end	Night	39% 4 13% 29% 13% 2	2474
December	1st to 10th	Night	47% 5% 12% 20% 15% 1	2660
	11th to 20th	Night	44% 6% 14% 17% 17% 2	3288
	21st to end	Night	50% 5% 12% 22% 11% 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	24% 26% 31% 17% 3	180
	11th to 20th	Day	26% 34% 25% 13% 1 1	180
	21st to end	Day	22% 27% 35% 10% 5% 2	220
February	1st to 10th	Day	27% 24% 35% 13% 1	350
	11th to 20th	Day	26% 27% 34% 11% 1	456
	21st to end	Day	30% 24% 29% 13% 4	382
March	1st to 10th	Day	26% 23% 34% 14% 3	460
	11th to 20th	Day	23% 28% 33% 12% 3 1	460
	21st to end	Day	31% 21% 33% 11% 3 1	506
April	1st to 10th	Day	26% 27% 30% 10% 5%	460
	11th to 20th	Day	28% 20% 31% 15% 4 1	452
	21st to end	Day	29% 17% 33% 15% 4 2	460
May	1st to 10th	Day	29% 20% 29% 14% 6% 1	463
	11th to 20th	Day	23% 16% 36% 17% 6% 1	487
	21st to end	Day	20% 12% 31% 25% 9% 2	529
June	1st to 10th	Day	20% 13% 33% 23% 10% 2	490
	11th to 20th	Day	17% 15% 34% 24% 9% 1	490
	21st to end	Day	16% 13% 33% 26% 11% 2	490
July	1st to 10th	Day	18% 6% 31% 27% 15% 3	485
	11th to 20th	Day	13% 6% 31% 33% 13% 3	484
	21st to end	Day	15% 10% 34% 30% 9% 2	532
August	1st to 10th	Day	14% 9% 30% 32% 10% 5%	486
	11th to 20th	Day	15% 6% 33% 27% 17% 3	479
	21st to end	Day	11% 10% 33% 27% 13% 6%	539
September	1st to 10th	Day	19% 9% 28% 23% 17% 4	490
	11th to 20th	Day	15% 9% 31% 25% 13% 7%	490
	21st to end	Day	23% 11% 34% 20% 9% 2	490
October	1st to 10th	Day	28% 11% 27% 20% 11% 3	490
	11th to 20th	Day	17% 19% 31% 23% 9% 1	480
	21st to end	Day	23% 18% 33% 17% 8% 1	429
November	1st to 10th	Day	18% 16% 32% 23% 9% 3	381
	11th to 20th	Day	24% 25% 27% 19% 5%	370
	21st to end	Day	21% 21% 29% 22% 7%	361
December	1st to 10th	Day	23% 21% 35% 15% 6%	180
	11th to 20th	Day	31% 26% 32% 9% 2	180
	21st to end	Day	27% 20% 32% 14% 6% 1	198
January	1st to 10th	Night	22% 22% 35% 18% 3	760
	11th to 20th	Night	28% 19% 33% 18% 3	760
	21st to end	Night	21% 23% 34% 20% 3	836
February	1st to 10th	Night	22% 21% 35% 21% 1	760
	11th to 20th	Night	23% 22% 38% 15% 2	743
	21st to end	Night	27% 19% 35% 16% 3	628
March	1st to 10th	Night	24% 19% 34% 20% 3	760
	11th to 20th	Night	22% 17% 37% 20% 4 1	760
	21st to end	Night	22% 21% 35% 18% 3	836
April	1st to 10th	Night	25% 17% 35% 20% 3	760
	11th to 20th	Night	23% 14% 36% 23% 4	751
	21st to end	Night	22% 15% 37% 19% 6%	652
May	1st to 10th	Night	25% 13% 32% 22% 7%	570
	11th to 20th	Night	24% 13% 36% 22% 6%	489
	21st to end	Night	22% 5% 26% 37% 10% 1	411
June	1st to 10th	Night	22% 5% 30% 32% 11% 1	380
	11th to 20th	Night	20% 4 28% 37% 11%	380
	21st to end	Night	18% 4 29% 39% 10% 1	380
July	1st to 10th	Night	15% 4 24% 40% 17% 1	370
	11th to 20th	Night	14% 2 21% 45% 16% 1	368
	21st to end	Night	15% 4 27% 40% 12% 1	416
August	1st to 10th	Night	12% 4 29% 40% 14% 1	468
	11th to 20th	Night	15% 4 23% 40% 17% 1	730
	21st to end	Night	14% 4 24% 35% 19% 3	836
September	1st to 10th	Night	18% 4 29% 33% 14% 3	760
	11th to 20th	Night	16% 6% 26% 29% 19% 3	760
	21st to end	Night	23% 6% 28% 28% 15% 1	760
October	1st to 10th	Night	22% 10% 28% 24% 13% 2	760
	11th to 20th	Night	17% 14% 33% 25% 9% 1	760
	21st to end	Night	26% 11% 35% 20% 8%	836
November	1st to 10th	Night	16% 15% 35% 22% 11% 1	764
	11th to 20th	Night	22% 17% 34% 20% 6% 1	760
	21st to end	Night	21% 19% 31% 23% 5% 1	760
December	1st to 10th	Night	26% 18% 31% 19% 6% 1	760
	11th to 20th	Night	25% 21% 30% 20% 3	760
	21st to end	Night	26% 16% 38% 18% 2	836

Page generated at 2024-11-03 13:19:03. Version ac07

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