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 2024-01-16.

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	17% 6% 2 5% 4 3 4 4 5% 7% 42%	74812
	Night	18% 7% 3 5% 4 3 4 4 5% 6% 42%	74974
Transparency	Day	42% 6% 12% 22% 16% 2	74517
	Night	43% 6% 12% 21% 15% 2	74803
Seeing	Day	22% 17% 31% 20% 8% 2	13931
	Night	21% 13% 32% 25% 8% 1	23087

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% 6% 1 4 5% 3 4 5% 5% 7% 45%	1517
	11th to 20th	Day	18% 8% 3 5% 5% 2 4 5% 6% 6% 38%	1480
	21st to end	Day	15% 7% 3 5% 4 2 3 5% 5% 8% 43%	1629
February	1st to 10th	Day	19% 7% 2 5% 5% 3 5% 5% 5% 6% 38%	1696
	11th to 20th	Day	16% 6% 2 5% 5% 3 5% 4 4 7% 43%	1748
	21st to end	Day	22% 8% 2 5% 5% 3 4 4 5% 6% 36%	1480
March	1st to 10th	Day	16% 8% 2 5% 4 3 4 3 4 8% 42%	1797
	11th to 20th	Day	16% 6% 3 5% 5% 3 5% 5% 5% 8% 39%	1844
	21st to end	Day	20% 7% 3 6% 5% 3 3 4 4 4 40%	2376
April	1st to 10th	Day	19% 6% 3 4 4 3 4 4 4 7% 40%	2160
	11th to 20th	Day	20% 6% 2 4 5% 3 3 4 4 6% 43%	2134
	21st to end	Day	21% 5% 2 5% 5% 3 4 3 5% 6% 41%	2260
May	1st to 10th	Day	23% 8% 3 5% 4 3 4 3 5% 7% 36%	2340
	11th to 20th	Day	20% 6% 2 4 4 3 4 4 6% 7% 41%	2384
	21st to end	Day	16% 5% 3 5% 4 2 4 4 5% 7% 44%	2725
June	1st to 10th	Day	19% 5% 2 5% 4 4 5% 4 7% 10% 35%	2520
	11th to 20th	Day	17% 6% 3 4 4 3 5% 5% 5% 9% 39%	2520
	21st to end	Day	13% 5% 2 5% 6% 3 5% 5% 5% 9% 42%	2520
July	1st to 10th	Day	14% 5% 3 4 4 3 5% 5% 6% 10% 43%	2520
	11th to 20th	Day	12% 5% 3 4 5% 3 5% 6% 6% 9% 42%	2520
	21st to end	Day	13% 5% 3 5% 5% 3 5% 6% 6% 9% 41%	2744
August	1st to 10th	Day	12% 5% 2 5% 4 3 6% 6% 6% 10% 42%	2403
	11th to 20th	Day	14% 5% 2 4 4 3 5% 4 6% 9% 43%	2142
	21st to end	Day	13% 5% 3 5% 5% 3 4 5% 7% 8% 43%	2376
September	1st to 10th	Day	14% 5% 2 5% 5% 3 4 5% 5% 7% 45%	2160
	11th to 20th	Day	13% 5% 2 3 3 3 4 3 4 7% 52%	2030
	21st to end	Day	19% 6% 2 4 5% 3 4 4 5% 6% 43%	1980
October	1st to 10th	Day	23% 6% 2 4 5% 2 3 4 4 6% 41%	1963
	11th to 20th	Day	15% 5% 2 3 4 2 4 4 5% 8% 47%	1782
	21st to end	Day	19% 7% 2 5% 4 3 5% 4 6% 5% 41%	1782
November	1st to 10th	Day	16% 5% 3 4 5% 3 4 3 3 6% 49%	1671
	11th to 20th	Day	20% 5% 2 3 3 3 3 3 5% 6% 48%	1710
	21st to end	Day	17% 5% 3 4 4 2 4 4 5% 6% 46%	1701
December	1st to 10th	Day	20% 5% 2 6% 5% 4 4 4 5% 6% 39%	1520
	11th to 20th	Day	23% 6% 2 4 3 2 3 2 3 6% 46%	1906
	21st to end	Day	21% 11% 3 6% 6% 3 3 4 4 6% 35%	2772
January	1st to 10th	Night	16% 7% 2 5% 3 3 4 4 4 7% 46%	2649
	11th to 20th	Night	21% 8% 2 5% 4 3 4 4 4 6% 40%	2591
	21st to end	Night	15% 8% 3 5% 4 2 3 4 4 6% 45%	2724
February	1st to 10th	Night	17% 10% 3 6% 5% 2 4 3 5% 5% 40%	2264
	11th to 20th	Night	18% 7% 3 5% 5% 3 3 4 4 7% 41%	2098
	21st to end	Night	22% 7% 2 5% 4 3 4 4 4 5% 41%	1776
March	1st to 10th	Night	19% 5% 3 4 4 2 4 4 5% 6% 45%	2150
	11th to 20th	Night	17% 7% 3 5% 4 3 5% 5% 5% 7% 40%	2111
	21st to end	Night	18% 8% 3 4 5% 2 4 4 4 5% 42%	1980
April	1st to 10th	Night	19% 8% 3 5% 5% 2 5% 3 5% 7% 38%	1800
	11th to 20th	Night	20% 4 3 5% 5% 3 4 4 4 6% 42%	1775
	21st to end	Night	22% 6% 2 5% 5% 3 3 4 4 4 41%	1698
May	1st to 10th	Night	23% 8% 3 4 5% 3 5% 5% 5% 7% 32%	1620
	11th to 20th	Night	22% 7% 2 4 4 3 5% 4 5% 7% 37%	1524
	21st to end	Night	19% 7% 3 4 5% 2 4 4 6% 8% 38%	1550
June	1st to 10th	Night	20% 5% 2 5% 6% 3 5% 5% 4 6% 40%	1440
	11th to 20th	Night	18% 6% 3 3 5% 2 4 4 6% 9% 40%	1440
	21st to end	Night	14% 6% 2 4 5% 3 4 5% 5% 9% 43%	1440
July	1st to 10th	Night	13% 5% 3 5% 6% 4 5% 6% 5% 9% 39%	1440
	11th to 20th	Night	14% 5% 2 5% 5% 4 4 6% 7% 10% 38%	1440
	21st to end	Night	14% 5% 3 4 5% 3 6% 5% 7% 9% 40%	1574
August	1st to 10th	Night	9% 5% 2 6% 5% 3 5% 6% 8% 9% 42%	1517
	11th to 20th	Night	11% 6% 3 6% 6% 4 5% 5% 7% 8% 37%	1755
	21st to end	Night	13% 5% 2 5% 4 3 4 5% 5% 8% 45%	1980
September	1st to 10th	Night	15% 6% 3 5% 5% 4 5% 4 5% 6% 43%	1800
	11th to 20th	Night	15% 5% 2 4 3 2 3 3 4 5% 54%	1930
	21st to end	Night	18% 6% 3 4 5% 3 6% 4 4 5% 43%	1980
October	1st to 10th	Night	21% 8% 2 5% 5% 2 4 3 4 5% 41%	1993
	11th to 20th	Night	16% 5% 2 4 5% 2 4 4 4 5% 48%	2178
	21st to end	Night	22% 7% 2 5% 3 3 4 4 5% 5% 40%	2574
November	1st to 10th	Night	14% 6% 2 5% 4 3 4 4 4 6% 48%	2407
	11th to 20th	Night	20% 6% 3 4 3 2 3 3 4 5% 47%	2470
	21st to end	Night	16% 5% 2 4 4 2 3 4 4 8% 46%	2474
December	1st to 10th	Night	22% 7% 2 5% 4 2 5% 4 5% 6% 39%	2660
	11th to 20th	Night	21% 7% 2 4 4 3 4 4 4 6% 41%	3288
	21st to end	Night	23% 9% 3 6% 4 3 4 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	39% 5% 11% 27% 14% 4	1518
	11th to 20th	Day	46% 6% 14% 18% 13% 2	1480
	21st to end	Day	40% 5% 12% 24% 16% 4	1629
February	1st to 10th	Day	47% 6% 13% 20% 13% 2	1696
	11th to 20th	Day	42% 5% 12% 23% 16% 3	1757
	21st to end	Day	49% 5% 9% 19% 14% 3	1480
March	1st to 10th	Day	42% 4 14% 21% 14% 4	1800
	11th to 20th	Day	43% 5% 13% 22% 15% 2	1844
	21st to end	Day	48% 5% 8% 16% 18% 5%	2376
April	1st to 10th	Day	43% 6% 11% 19% 16% 4	2160
	11th to 20th	Day	42% 6% 10% 21% 19% 3	2134
	21st to end	Day	45% 6% 10% 20% 16% 4	2260
May	1st to 10th	Day	49% 6% 10% 20% 13% 3	2340
	11th to 20th	Day	42% 6% 11% 20% 17% 3	2388
	21st to end	Day	40% 8% 15% 21% 14% 2	2725
June	1st to 10th	Day	44% 8% 13% 23% 11% 1	2520
	11th to 20th	Day	42% 8% 14% 22% 13% 1	2520
	21st to end	Day	39% 10% 13% 24% 14% 1	2520
July	1st to 10th	Day	39% 9% 12% 22% 17% 1	2450
	11th to 20th	Day	37% 9% 14% 26% 13% 2	2445
	21st to end	Day	38% 8% 15% 20% 17% 3	2591
August	1st to 10th	Day	37% 10% 14% 24% 14% 1	2389
	11th to 20th	Day	38% 8% 15% 24% 14% 1	2142
	21st to end	Day	38% 8% 11% 22% 19% 2	2376
September	1st to 10th	Day	39% 8% 12% 21% 18% 2	2160
	11th to 20th	Day	33% 6% 12% 23% 24% 3	2030
	21st to end	Day	42% 6% 9% 20% 21% 3	1980
October	1st to 10th	Day	45% 5% 9% 21% 16% 3	1963
	11th to 20th	Day	36% 5% 13% 27% 17% 1	1782
	21st to end	Day	44% 7% 10% 21% 16% 3	1782
November	1st to 10th	Day	38% 5% 13% 20% 21% 3	1671
	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	40% 4 12% 24% 16% 4	2649
	11th to 20th	Night	47% 6% 12% 22% 11% 2	2591
	21st to end	Night	40% 5% 11% 22% 19% 3	2724
February	1st to 10th	Night	46% 5% 12% 22% 12% 2	2264
	11th to 20th	Night	44% 5% 11% 20% 16% 4	2100
	21st to end	Night	47% 4 8% 23% 15% 4	1776
March	1st to 10th	Night	40% 5% 13% 21% 17% 4	2160
	11th to 20th	Night	43% 6% 11% 20% 16% 4	2112
	21st to end	Night	44% 4 10% 18% 19% 5%	1980
April	1st to 10th	Night	47% 5% 10% 20% 16% 4	1800
	11th to 20th	Night	44% 5% 10% 20% 19% 2	1775
	21st to end	Night	47% 5% 9% 20% 14% 5%	1698
May	1st to 10th	Night	51% 8% 10% 15% 14% 2	1620
	11th to 20th	Night	46% 7% 11% 18% 15% 3	1525
	21st to end	Night	44% 7% 17% 19% 12% 1	1551
June	1st to 10th	Night	45% 7% 11% 18% 16% 2	1440
	11th to 20th	Night	42% 8% 14% 25% 11% 1	1440
	21st to end	Night	38% 9% 13% 25% 14%	1440
July	1st to 10th	Night	41% 10% 13% 19% 15% 2	1400
	11th to 20th	Night	39% 9% 16% 26% 9% 1	1392
	21st to end	Night	38% 8% 16% 23% 13% 2	1486
August	1st to 10th	Night	35% 10% 17% 21% 15% 2	1507
	11th to 20th	Night	43% 8% 14% 23% 10% 1	1755
	21st to end	Night	36% 8% 13% 23% 19% 1	1980
September	1st to 10th	Night	41% 7% 10% 24% 16% 2	1800
	11th to 20th	Night	34% 5% 11% 22% 25% 3	1930
	21st to end	Night	43% 6% 8% 19% 21% 3	1980
October	1st to 10th	Night	47% 5% 9% 22% 14% 3	1993
	11th to 20th	Night	39% 5% 12% 27% 15% 1	2178
	21st to end	Night	46% 5% 12% 20% 15% 1	2574
November	1st to 10th	Night	37% 6% 12% 21% 20% 4	2407
	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	25% 35% 24% 14% 1 1	170
	21st to end	Day	21% 29% 36% 9% 3 2	196
February	1st to 10th	Day	29% 22% 34% 13% 1	316
	11th to 20th	Day	25% 27% 35% 12% 1	416
	21st to end	Day	30% 25% 29% 12% 3	346
March	1st to 10th	Day	24% 24% 35% 15% 2	420
	11th to 20th	Day	23% 26% 34% 12% 3 1	420
	21st to end	Day	31% 20% 33% 12% 4 1	462
April	1st to 10th	Day	25% 27% 31% 10% 6%	420
	11th to 20th	Day	27% 20% 31% 16% 5% 1	412
	21st to end	Day	30% 17% 33% 15% 4 1	420
May	1st to 10th	Day	30% 20% 30% 13% 6% 1	423
	11th to 20th	Day	23% 17% 36% 17% 6% 1	447
	21st to end	Day	21% 12% 28% 26% 10% 2	485
June	1st to 10th	Day	20% 13% 32% 23% 10% 2	450
	11th to 20th	Day	18% 15% 33% 24% 9% 1	450
	21st to end	Day	15% 12% 32% 28% 11% 2	450
July	1st to 10th	Day	17% 6% 30% 28% 16% 3	445
	11th to 20th	Day	13% 5% 29% 35% 14% 3	444
	21st to end	Day	13% 10% 34% 32% 9% 2	488
August	1st to 10th	Day	13% 8% 30% 34% 11% 5%	446
	11th to 20th	Day	15% 5% 32% 27% 16% 3	439
	21st to end	Day	12% 10% 32% 27% 13% 6%	495
September	1st to 10th	Day	19% 9% 27% 23% 17% 4	450
	11th to 20th	Day	16% 10% 32% 24% 12% 7%	450
	21st to end	Day	22% 11% 34% 21% 10% 3	450
October	1st to 10th	Day	29% 11% 26% 20% 11% 4	450
	11th to 20th	Day	17% 19% 30% 24% 8% 1	441
	21st to end	Day	24% 19% 32% 17% 7% 1	396
November	1st to 10th	Day	17% 15% 32% 23% 9% 3	365
	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% 32% 18% 3	741
	21st to end	Night	20% 23% 34% 20% 3	792
February	1st to 10th	Night	23% 21% 34% 21% 2	720
	11th to 20th	Night	24% 22% 37% 15% 2	703
	21st to end	Night	27% 19% 36% 16% 2	592
March	1st to 10th	Night	24% 19% 35% 20% 3	720
	11th to 20th	Night	23% 16% 37% 20% 4 1	720
	21st to end	Night	21% 20% 36% 19% 4	792
April	1st to 10th	Night	25% 18% 34% 20% 3	720
	11th to 20th	Night	22% 14% 36% 23% 4	711
	21st to end	Night	23% 15% 36% 20% 6%	618
May	1st to 10th	Night	25% 13% 32% 22% 7%	540
	11th to 20th	Night	24% 12% 37% 22% 6%	463
	21st to end	Night	22% 5% 25% 37% 10% 1	389
June	1st to 10th	Night	23% 4 31% 31% 11% 1	360
	11th to 20th	Night	21% 4 28% 37% 10%	360
	21st to end	Night	17% 3 30% 39% 10% 1	360
July	1st to 10th	Night	15% 4 24% 39% 17% 1	350
	11th to 20th	Night	13% 2 20% 47% 16% 1	348
	21st to end	Night	14% 4 27% 40% 13% 1	394
August	1st to 10th	Night	10% 4 29% 41% 15% 1	443
	11th to 20th	Night	15% 4 22% 41% 17% 1	691
	21st to end	Night	15% 4 23% 35% 18% 3	792
September	1st to 10th	Night	18% 4 29% 33% 14% 3	720
	11th to 20th	Night	17% 6% 27% 29% 18% 3	720
	21st to end	Night	22% 6% 27% 28% 16% 1	720
October	1st to 10th	Night	23% 10% 28% 24% 13% 3	720
	11th to 20th	Night	18% 14% 32% 26% 9% 1	720
	21st to end	Night	27% 11% 36% 19% 7%	792
November	1st to 10th	Night	16% 15% 34% 23% 11% 1	740
	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 2024-01-13 01:38:13. Version ac07

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