Bell Isle State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bell Isle State Park clear sky chart from 2005-11-06 to 2024-01-12.

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	13% 5% 2 5% 5% 3 5% 5% 5% 8% 43%	74673
	Night	14% 6% 2 5% 5% 3 4 4 5% 7% 44%	75048
Transparency	Day	39% 5% 12% 24% 17% 3	74384
	Night	39% 5% 11% 23% 17% 4	74856
Seeing	Day	17% 10% 28% 26% 15% 4	14007
	Night	17% 10% 28% 29% 13% 2	23651

By Month

Percentage Cloud Cover SE (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	14% 6% 3 5% 4 3 4 4 5% 6% 45%	1579
	11th to 20th	Day	16% 8% 2 5% 5% 2 4 4 4 6% 44%	1635
	21st to end	Day	15% 7% 2 5% 4 2 3 4 5% 6% 45%	1782
February	1st to 10th	Day	13% 8% 2 6% 6% 3 5% 5% 6% 8% 38%	1620
	11th to 20th	Day	20% 6% 2 3 4 2 5% 3 4 6% 46%	1681
	21st to end	Day	17% 6% 3 6% 5% 4 5% 6% 4 6% 37%	1480
March	1st to 10th	Day	14% 6% 2 4 4 2 3 4 4 6% 50%	1916
	11th to 20th	Day	17% 5% 2 4 4 3 4 4 5% 7% 43%	1980
	21st to end	Day	16% 6% 2 5% 5% 4 5% 4 4 6% 42%	2233
April	1st to 10th	Day	12% 5% 2 4 5% 2 4 4 4 7% 51%	2160
	11th to 20th	Day	11% 5% 3 5% 5% 3 4 5% 5% 7% 49%	2134
	21st to end	Day	16% 5% 2 4 5% 3 4 5% 5% 8% 43%	2160
May	1st to 10th	Day	14% 6% 2 5% 5% 4 4 5% 5% 10% 39%	2203
	11th to 20th	Day	17% 5% 2 5% 5% 3 5% 4 5% 9% 39%	2402
	21st to end	Day	13% 6% 2 4 5% 3 5% 5% 6% 9% 42%	2725
June	1st to 10th	Day	11% 4 2 5% 5% 3 5% 5% 7% 10% 43%	2520
	11th to 20th	Day	10% 5% 2 4 6% 3 5% 6% 7% 11% 41%	2520
	21st to end	Day	8% 4 2 5% 6% 4 7% 6% 6% 11% 41%	2520
July	1st to 10th	Day	8% 5% 3 6% 5% 4 6% 5% 7% 9% 41%	2520
	11th to 20th	Day	6% 4 2 5% 7% 4 6% 7% 9% 11% 40%	2425
	21st to end	Day	7% 4 2 6% 7% 5% 7% 8% 8% 11% 35%	2547
August	1st to 10th	Day	9% 5% 3 6% 6% 4 7% 6% 8% 10% 35%	2327
	11th to 20th	Day	10% 6% 3 7% 6% 5% 6% 6% 7% 9% 37%	2146
	21st to end	Day	12% 5% 3 5% 5% 3 5% 5% 6% 10% 41%	2376
September	1st to 10th	Day	13% 6% 2 4 5% 4 5% 5% 6% 7% 43%	2160
	11th to 20th	Day	12% 6% 3 6% 5% 3 4 5% 5% 7% 44%	2160
	21st to end	Day	20% 7% 3 5% 5% 3 4 4 5% 6% 39%	1916
October	1st to 10th	Day	15% 6% 3 6% 5% 3 3 3 4 7% 46%	1800
	11th to 20th	Day	17% 5% 2 4 3 2 4 4 4 6% 51%	1800
	21st to end	Day	14% 5% 2 4 5% 2 4 4 4 7% 49%	1980
November	1st to 10th	Day	14% 5% 2 4 4 2 3 3 4 6% 52%	1758
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 5% 6% 50%	1710
	21st to end	Day	15% 4 2 5% 4 2 4 4 4 5% 52%	1596
December	1st to 10th	Day	15% 7% 3 5% 4 3 4 4 5% 7% 43%	1520
	11th to 20th	Day	16% 7% 2 4 4 2 3 3 3 4 51%	1910
	21st to end	Day	16% 6% 2 5% 5% 2 4 5% 5% 6% 43%	2772
January	1st to 10th	Night	16% 6% 2 5% 5% 3 3 4 4 6% 47%	2583
	11th to 20th	Night	16% 6% 3 5% 5% 3 4 5% 4 7% 40%	2365
	21st to end	Night	16% 7% 2 5% 3 2 4 4 4 6% 47%	2574
February	1st to 10th	Night	16% 8% 2 5% 5% 3 3 5% 6% 6% 41%	2340
	11th to 20th	Night	19% 7% 3 4 5% 2 4 4 4 5% 43%	2164
	21st to end	Night	19% 6% 2 4 3 3 3 4 4 7% 46%	1776
March	1st to 10th	Night	14% 5% 2 4 4 3 4 4 4 6% 50%	2041
	11th to 20th	Night	19% 8% 2 4 4 2 4 4 6% 6% 41%	1980
	21st to end	Night	17% 7% 3 5% 4 3 5% 3 5% 6% 42%	2121
April	1st to 10th	Night	14% 8% 2 5% 5% 2 4 4 6% 7% 43%	1800
	11th to 20th	Night	11% 6% 2 5% 5% 2 4 4 4 8% 49%	1775
	21st to end	Night	17% 6% 2 5% 4 3 3 4 5% 7% 45%	1800
May	1st to 10th	Night	15% 6% 2 6% 5% 3 5% 5% 6% 8% 39%	1772
	11th to 20th	Night	16% 6% 3 5% 5% 3 4 4 5% 8% 40%	1487
	21st to end	Night	13% 6% 3 5% 5% 4 5% 4 6% 9% 42%	1550
June	1st to 10th	Night	13% 4 2 4 5% 4 5% 6% 6% 9% 42%	1440
	11th to 20th	Night	10% 5% 2 6% 5% 3 6% 6% 7% 11% 39%	1440
	21st to end	Night	8% 4 2 6% 5% 3 6% 6% 7% 10% 42%	1440
July	1st to 10th	Night	8% 5% 2 6% 6% 4 5% 6% 6% 11% 40%	1440
	11th to 20th	Night	5% 3 2 4 5% 4 5% 6% 7% 12% 46%	1530
	21st to end	Night	7% 4 3 6% 7% 3 7% 6% 7% 11% 40%	1771
August	1st to 10th	Night	7% 4 2 7% 5% 4 6% 7% 7% 11% 38%	1611
	11th to 20th	Night	10% 6% 3 7% 5% 4 5% 6% 8% 9% 39%	1735
	21st to end	Night	9% 5% 3 5% 7% 4 5% 6% 6% 10% 40%	1980
September	1st to 10th	Night	12% 5% 2 5% 4 3 4 5% 6% 7% 47%	1800
	11th to 20th	Night	10% 5% 2 6% 4 3 5% 4 5% 8% 49%	1800
	21st to end	Night	19% 8% 3 5% 4 3 3 5% 5% 6% 38%	2039
October	1st to 10th	Night	14% 5% 3 5% 4 3 4 4 5% 6% 47%	2160
	11th to 20th	Night	16% 5% 2 4 4 3 4 3 3 6% 49%	2160
	21st to end	Night	15% 6% 2 4 4 2 3 3 5% 6% 50%	2376
November	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 4 6% 50%	2320
	11th to 20th	Night	17% 5% 3 4 3 2 4 4 4 6% 48%	2470
	21st to end	Night	12% 4 2 3 5% 3 5% 4 5% 7% 50%	2579
December	1st to 10th	Night	17% 8% 2 6% 4 3 4 5% 4 6% 42%	2660
	11th to 20th	Night	17% 6% 3 4 4 3 4 3 4 6% 45%	3285
	21st to end	Night	14% 7% 3 5% 4 3 4 4 5% 8% 43%	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	41% 5% 8% 26% 16% 4	1580
	11th to 20th	Day	43% 4 10% 23% 17% 3	1635
	21st to end	Day	40% 3 10% 25% 18% 4	1782
February	1st to 10th	Day	45% 4 10% 26% 12% 3	1620
	11th to 20th	Day	42% 4 7% 23% 21% 4	1690
	21st to end	Day	47% 4 11% 18% 15% 5%	1480
March	1st to 10th	Day	36% 3 11% 24% 21% 4	1916
	11th to 20th	Day	42% 3 10% 21% 18% 6%	1980
	21st to end	Day	43% 6% 10% 19% 19% 4	2233
April	1st to 10th	Day	34% 5% 11% 23% 21% 7%	2160
	11th to 20th	Day	36% 4 11% 22% 21% 6%	2134
	21st to end	Day	41% 5% 10% 22% 16% 6%	2160
May	1st to 10th	Day	41% 6% 13% 22% 15% 3	2203
	11th to 20th	Day	43% 6% 14% 23% 12% 3	2406
	21st to end	Day	40% 6% 14% 26% 13% 1	2725
June	1st to 10th	Day	36% 6% 15% 27% 13% 2	2520
	11th to 20th	Day	35% 8% 16% 23% 16% 2	2520
	21st to end	Day	36% 7% 15% 24% 17% 2	2520
July	1st to 10th	Day	38% 8% 14% 22% 15% 3	2450
	11th to 20th	Day	33% 9% 18% 27% 11% 1	2351
	21st to end	Day	38% 9% 16% 20% 16% 1	2405
August	1st to 10th	Day	40% 9% 15% 23% 11% 1	2314
	11th to 20th	Day	42% 7% 15% 21% 14% 1	2146
	21st to end	Day	38% 6% 15% 23% 14% 3	2376
September	1st to 10th	Day	39% 7% 14% 19% 17% 4	2160
	11th to 20th	Day	39% 5% 8% 25% 20% 3	2160
	21st to end	Day	46% 5% 7% 23% 16% 3	1916
October	1st to 10th	Day	39% 5% 9% 25% 20% 3	1800
	11th to 20th	Day	37% 4 8% 26% 22% 4	1800
	21st to end	Day	36% 3 6% 25% 24% 6%	1980
November	1st to 10th	Day	35% 4 9% 26% 22% 5%	1758
	11th to 20th	Day	37% 3 10% 21% 23% 6%	1710
	21st to end	Day	36% 3 8% 28% 20% 5%	1596
December	1st to 10th	Day	41% 5% 12% 25% 14% 4	1520
	11th to 20th	Day	39% 3 8% 28% 16% 6%	1906
	21st to end	Day	42% 5% 9% 29% 14% 1	2772
January	1st to 10th	Night	40% 3 9% 23% 20% 6%	2583
	11th to 20th	Night	45% 4 10% 23% 15% 3	2365
	21st to end	Night	39% 3 8% 26% 20% 4	2574
February	1st to 10th	Night	42% 4 11% 24% 16% 3	2340
	11th to 20th	Night	44% 3 7% 22% 19% 6%	2166
	21st to end	Night	41% 4 9% 22% 19% 6%	1776
March	1st to 10th	Night	37% 4 8% 27% 21% 3	2041
	11th to 20th	Night	42% 4 10% 21% 18% 4	1980
	21st to end	Night	43% 4 10% 22% 16% 6%	2121
April	1st to 10th	Night	41% 5% 9% 19% 22% 5%	1800
	11th to 20th	Night	34% 5% 11% 22% 23% 6%	1775
	21st to end	Night	40% 5% 12% 22% 16% 4	1800
May	1st to 10th	Night	42% 6% 13% 22% 15% 3	1772
	11th to 20th	Night	43% 6% 15% 20% 16% 1	1488
	21st to end	Night	40% 7% 15% 26% 12% 1	1551
June	1st to 10th	Night	37% 9% 12% 24% 16% 1	1440
	11th to 20th	Night	39% 8% 16% 22% 15% 2	1440
	21st to end	Night	36% 8% 15% 22% 17% 3	1440
July	1st to 10th	Night	37% 8% 17% 21% 13% 4	1400
	11th to 20th	Night	28% 9% 21% 29% 11% 2	1481
	21st to end	Night	36% 8% 17% 26% 13%	1672
August	1st to 10th	Night	39% 7% 19% 24% 11%	1600
	11th to 20th	Night	40% 8% 18% 21% 13% 1	1735
	21st to end	Night	38% 8% 16% 22% 14% 2	1980
September	1st to 10th	Night	37% 6% 18% 19% 16% 5%	1800
	11th to 20th	Night	34% 6% 10% 25% 22% 2	1800
	21st to end	Night	46% 6% 9% 18% 18% 3	2039
October	1st to 10th	Night	39% 4 10% 25% 18% 4	2160
	11th to 20th	Night	39% 3 7% 24% 23% 4	2160
	21st to end	Night	36% 4 8% 26% 23% 4	2376
November	1st to 10th	Night	37% 3 10% 25% 20% 5%	2320
	11th to 20th	Night	38% 4 8% 21% 20% 8%	2470
	21st to end	Night	34% 4 9% 27% 22% 5%	2579
December	1st to 10th	Night	44% 3 10% 24% 15% 3	2660
	11th to 20th	Night	41% 3 10% 22% 19% 5%	3288
	21st to end	Night	40% 4 13% 27% 14% 2	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	22% 18% 40% 15% 4	210
	11th to 20th	Day	21% 22% 38% 16% 2	245
	21st to end	Day	20% 19% 41% 18% 2	264
February	1st to 10th	Day	22% 15% 46% 12% 5%	240
	11th to 20th	Day	29% 18% 38% 14% 2	349
	21st to end	Day	25% 19% 36% 16% 4	346
March	1st to 10th	Day	18% 20% 39% 17% 5%	420
	11th to 20th	Day	23% 17% 36% 18% 5% 1	420
	21st to end	Day	18% 17% 37% 22% 5% 1	462
April	1st to 10th	Day	15% 19% 40% 18% 7% 1	420
	11th to 20th	Day	12% 16% 34% 26% 8% 3	412
	21st to end	Day	20% 13% 35% 23% 7% 1	420
May	1st to 10th	Day	19% 14% 34% 23% 9% 2	423
	11th to 20th	Day	22% 6% 29% 30% 10% 2	447
	21st to end	Day	20% 5% 21% 31% 19% 4	485
June	1st to 10th	Day	13% 4 26% 32% 20% 4	450
	11th to 20th	Day	12% 5% 21% 36% 20% 7%	450
	21st to end	Day	10% 5% 23% 37% 20% 5%	450
July	1st to 10th	Day	10% 3 19% 34% 26% 8%	445
	11th to 20th	Day	5% 1 12% 40% 33% 9%	444
	21st to end	Day	10% 2 18% 37% 25% 8%	488
August	1st to 10th	Day	11% 2 16% 34% 30% 7%	446
	11th to 20th	Day	10% 3 17% 33% 29% 9%	439
	21st to end	Day	11% 4 15% 30% 27% 13%	495
September	1st to 10th	Day	15% 4 17% 28% 22% 14%	450
	11th to 20th	Day	13% 4 19% 32% 22% 10%	450
	21st to end	Day	28% 6% 25% 23% 14% 4	450
October	1st to 10th	Day	19% 7% 22% 28% 16% 8%	450
	11th to 20th	Day	17% 9% 27% 29% 16% 2	450
	21st to end	Day	16% 11% 34% 25% 11% 3	495
November	1st to 10th	Day	19% 9% 29% 25% 14% 4	408
	11th to 20th	Day	16% 17% 32% 25% 10% 1	370
	21st to end	Day	16% 17% 30% 26% 10% 1	256
December	1st to 10th	Day	17% 12% 47% 16% 7% 1	180
	11th to 20th	Day	22% 13% 37% 20% 8%	180
	21st to end	Day	22% 12% 35% 21% 9% 1	198
January	1st to 10th	Night	19% 19% 37% 21% 4	760
	11th to 20th	Night	20% 16% 32% 26% 5%	728
	21st to end	Night	20% 17% 36% 25% 2	792
February	1st to 10th	Night	19% 13% 43% 22% 3	720
	11th to 20th	Night	23% 21% 38% 15% 3	703
	21st to end	Night	22% 17% 35% 22% 3	592
March	1st to 10th	Night	17% 18% 37% 24% 4	720
	11th to 20th	Night	23% 14% 34% 22% 6% 1	720
	21st to end	Night	20% 14% 38% 24% 5%	792
April	1st to 10th	Night	20% 15% 40% 21% 4	720
	11th to 20th	Night	13% 12% 36% 29% 9% 1	711
	21st to end	Night	20% 8% 34% 30% 7%	720
May	1st to 10th	Night	17% 11% 35% 28% 8% 1	692
	11th to 20th	Night	18% 5% 30% 35% 11% 1	426
	21st to end	Night	15% 3 19% 37% 24% 2	389
June	1st to 10th	Night	14% 2 20% 42% 19% 2	360
	11th to 20th	Night	13% 4 18% 42% 19% 3	360
	21st to end	Night	10% 2 20% 46% 19% 3	360
July	1st to 10th	Night	11% 3 13% 41% 28% 3	350
	11th to 20th	Night	3 2 11% 45% 35% 5%	437
	21st to end	Night	6% 4 16% 42% 27% 5%	591
August	1st to 10th	Night	10% 2 12% 40% 32% 4	537
	11th to 20th	Night	14% 3 12% 39% 30% 3	671
	21st to end	Night	11% 2 14% 35% 32% 6%	792
September	1st to 10th	Night	16% 2 13% 36% 28% 6%	720
	11th to 20th	Night	12% 3 17% 32% 30% 6%	720
	21st to end	Night	25% 4 21% 30% 18% 2	720
October	1st to 10th	Night	17% 6% 20% 29% 23% 5%	720
	11th to 20th	Night	18% 11% 24% 30% 15% 1	720
	21st to end	Night	19% 11% 27% 27% 14% 2	792
November	1st to 10th	Night	17% 9% 27% 31% 14% 3	740
	11th to 20th	Night	19% 14% 28% 29% 11% 1	760
	21st to end	Night	14% 15% 29% 30% 10% 1	760
December	1st to 10th	Night	22% 12% 35% 21% 8% 1	760
	11th to 20th	Night	19% 11% 39% 25% 6%	760
	21st to end	Night	19% 14% 29% 33% 5%	836

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