Sheridan
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sheridan clear sky chart from 2005-11-06 to 2024-03-08.

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	6% 6% 2 5% 4 3 5% 5% 6% 8% 49%	78519
	Night	6% 7% 3 5% 5% 3 4 5% 5% 8% 49%	81109
Transparency	Day	30% 7% 14% 26% 17% 5%	78222
	Night	31% 6% 14% 26% 17% 5%	80928
Seeing	Day	10% 8% 23% 34% 20% 5%	14886
	Night	9% 6% 23% 43% 17% 2	25126

By Month

Percentage Cloud Cover SW (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	3 7% 3 7% 6% 4 6% 7% 7% 7% 43%	2538
	11th to 20th	Day	3 6% 3 6% 5% 4 6% 6% 7% 9% 45%	2540
	21st to end	Day	2 5% 2 5% 4 3 4 5% 6% 8% 57%	3054
February	1st to 10th	Day	4 12% 7% 8% 7% 4 5% 4 5% 7% 37%	1710
	11th to 20th	Day	4 11% 5% 7% 6% 3 5% 5% 6% 9% 41%	1665
	21st to end	Day	4 7% 4 8% 6% 4 6% 5% 7% 9% 40%	1532
March	1st to 10th	Day	4 7% 3 6% 6% 4 5% 4 6% 8% 46%	1919
	11th to 20th	Day	5% 7% 3 6% 5% 3 5% 6% 6% 9% 44%	1980
	21st to end	Day	10% 10% 3 7% 5% 3 5% 5% 6% 9% 35%	2250
April	1st to 10th	Day	9% 10% 3 6% 5% 3 5% 5% 6% 7% 42%	2160
	11th to 20th	Day	10% 9% 4 6% 5% 3 5% 5% 7% 10% 35%	2131
	21st to end	Day	15% 8% 3 6% 5% 3 5% 5% 5% 8% 38%	2271
May	1st to 10th	Day	12% 7% 3 5% 6% 3 5% 6% 7% 8% 38%	2498
	11th to 20th	Day	13% 6% 2 5% 5% 3 5% 5% 7% 10% 40%	2478
	21st to end	Day	17% 6% 2 5% 5% 2 6% 6% 8% 10% 33%	2724
June	1st to 10th	Day	8% 3 2 3 5% 4 6% 7% 8% 10% 44%	2520
	11th to 20th	Day	6% 4 2 5% 4 3 6% 6% 8% 10% 45%	2520
	21st to end	Day	4 3 1 3 3 3 4 6% 7% 9% 58%	2520
July	1st to 10th	Day	4 2 1 2 3 2 6% 6% 9% 9% 57%	2520
	11th to 20th	Day	1 1 1 2 3 3 5% 6% 7% 8% 63%	2520
	21st to end	Day	3 2 1 3 3 3 4 6% 7% 9% 59%	2742
August	1st to 10th	Day	4 3 1 3 5% 3 5% 6% 7% 9% 53%	2376
	11th to 20th	Day	4 2 1 4 3 2 4 6% 6% 9% 58%	2299
	21st to end	Day	3 2 1 3 3 2 4 5% 6% 9% 62%	2405
September	1st to 10th	Day	7% 3 1 2 3 2 4 4 5% 6% 64%	2160
	11th to 20th	Day	8% 3 1 2 2 2 3 4 4 6% 64%	2160
	21st to end	Day	10% 4 1 3 3 2 3 3 4 5% 61%	1913
October	1st to 10th	Day	11% 8% 2 5% 3 3 4 4 5% 5% 52%	1800
	11th to 20th	Day	10% 5% 2 4 4 2 4 4 5% 6% 53%	1800
	21st to end	Day	7% 8% 3 5% 5% 3 4 5% 5% 7% 49%	1899
November	1st to 10th	Day	5% 8% 3 6% 4 2 5% 5% 4 8% 49%	1635
	11th to 20th	Day	5% 6% 3 4 5% 3 5% 5% 7% 7% 50%	1520
	21st to end	Day	6% 6% 3 4 5% 3 3 5% 6% 8% 51%	1520
December	1st to 10th	Day	3 6% 2 5% 6% 5% 5% 6% 6% 9% 46%	1520
	11th to 20th	Day	3 5% 2 5% 4 2 4 5% 5% 9% 58%	1926
	21st to end	Day	4 7% 3 4 4 3 5% 5% 6% 9% 51%	2794
January	1st to 10th	Night	3 9% 5% 8% 5% 3 5% 5% 6% 8% 43%	4417
	11th to 20th	Night	3 7% 3 10% 10% 4 6% 5% 5% 6% 40%	4430
	21st to end	Night	4 8% 3 7% 5% 3 5% 5% 5% 7% 47%	4640
February	1st to 10th	Night	4 10% 5% 10% 7% 4 6% 6% 5% 7% 36%	2470
	11th to 20th	Night	4 11% 5% 8% 7% 4 5% 5% 5% 6% 39%	2402
	21st to end	Night	2 8% 4 9% 7% 4 5% 5% 6% 8% 43%	1921
March	1st to 10th	Night	3 8% 4 6% 4 2 5% 5% 7% 10% 46%	2199
	11th to 20th	Night	6% 8% 3 6% 5% 3 3 6% 5% 8% 46%	1980
	21st to end	Night	10% 12% 3 6% 4 3 4 5% 5% 8% 40%	2106
April	1st to 10th	Night	10% 9% 3 5% 5% 3 4 5% 5% 8% 42%	1800
	11th to 20th	Night	10% 8% 4 6% 6% 3 6% 6% 6% 9% 37%	1777
	21st to end	Night	14% 6% 3 5% 4 3 4 4 5% 9% 43%	1688
May	1st to 10th	Night	11% 6% 2 4 4 2 5% 5% 5% 8% 47%	1459
	11th to 20th	Night	14% 6% 3 4 5% 3 4 4 5% 7% 45%	1417
	21st to end	Night	17% 5% 3 4 4 3 5% 6% 7% 11% 36%	1552
June	1st to 10th	Night	10% 5% 2 4 3 3 5% 5% 6% 10% 48%	1440
	11th to 20th	Night	9% 4 1 5% 4 3 4 6% 7% 8% 50%	1440
	21st to end	Night	5% 4 1 3 4 1 4 5% 6% 7% 60%	1440
July	1st to 10th	Night	4 1 1 2 3 2 4 5% 8% 11% 60%	1440
	11th to 20th	Night	2 2 1 2 2 2 3 5% 5% 9% 65%	1440
	21st to end	Night	2 2 2 2 2 5% 4 6% 8% 67%	1576
August	1st to 10th	Night	4 3 1 3 4 3 4 6% 6% 11% 56%	1560
	11th to 20th	Night	5% 2 2 3 1 3 4 6% 8% 66%	1602
	21st to end	Night	3 2 1 3 4 2 4 4 5% 8% 65%	1949
September	1st to 10th	Night	6% 3 1 3 3 2 3 4 4 7% 65%	1800
	11th to 20th	Night	7% 3 1 3 3 2 4 3 5% 8% 62%	1800
	21st to end	Night	10% 4 2 3 2 2 3 3 4 7% 61%	2043
October	1st to 10th	Night	13% 9% 2 4 3 2 3 4 4 7% 49%	2160
	11th to 20th	Night	11% 6% 2 4 4 3 3 3 4 6% 55%	2160
	21st to end	Night	6% 7% 2 5% 3 2 3 5% 5% 8% 54%	2438
November	1st to 10th	Night	6% 7% 4 5% 4 3 4 4 5% 9% 48%	2439
	11th to 20th	Night	5% 8% 3 6% 5% 3 4 4 5% 8% 49%	2660
	21st to end	Night	5% 9% 3 5% 4 3 4 4 5% 7% 52%	2660
December	1st to 10th	Night	4 7% 4 7% 6% 4 6% 6% 6% 8% 42%	2660
	11th to 20th	Night	2 6% 2 5% 4 3 5% 4 6% 8% 53%	3271
	21st to end	Night	4 8% 3 5% 4 3 4 5% 5% 8% 50%	4873

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	35% 8% 14% 29% 13% 1	2539
	11th to 20th	Day	30% 9% 20% 25% 13% 3	2540
	21st to end	Day	24% 5% 16% 36% 18% 2	3054
February	1st to 10th	Day	45% 6% 14% 23% 10% 2	1710
	11th to 20th	Day	40% 7% 13% 23% 12% 4	1674
	21st to end	Day	37% 6% 13% 25% 15% 5%	1532
March	1st to 10th	Day	34% 6% 15% 26% 18% 2	1919
	11th to 20th	Day	33% 7% 13% 28% 15% 3	1980
	21st to end	Day	44% 8% 14% 26% 7%	2250
April	1st to 10th	Day	38% 7% 16% 24% 13% 2	2160
	11th to 20th	Day	43% 7% 17% 21% 11% 1	2134
	21st to end	Day	42% 8% 15% 22% 12% 1	2271
May	1st to 10th	Day	40% 8% 15% 26% 9% 1	2498
	11th to 20th	Day	37% 9% 19% 24% 10% 1	2482
	21st to end	Day	41% 11% 20% 21% 6% 1	2724
June	1st to 10th	Day	30% 12% 22% 22% 13% 2	2520
	11th to 20th	Day	30% 9% 18% 24% 16% 4	2520
	21st to end	Day	20% 8% 14% 27% 23% 9%	2520
July	1st to 10th	Day	19% 8% 12% 30% 24% 7%	2451
	11th to 20th	Day	16% 7% 11% 27% 24% 17%	2445
	21st to end	Day	19% 6% 10% 22% 28% 15%	2590
August	1st to 10th	Day	25% 5% 10% 25% 23% 11%	2360
	11th to 20th	Day	21% 6% 9% 29% 26% 10%	2299
	21st to end	Day	17% 5% 10% 23% 30% 15%	2405
September	1st to 10th	Day	21% 4 7% 22% 30% 15%	2160
	11th to 20th	Day	21% 5% 11% 28% 27% 8%	2160
	21st to end	Day	25% 4 9% 30% 26% 5%	1913
October	1st to 10th	Day	34% 6% 13% 22% 19% 6%	1800
	11th to 20th	Day	31% 5% 13% 29% 18% 5%	1800
	21st to end	Day	33% 5% 15% 27% 15% 5%	1899
November	1st to 10th	Day	33% 7% 12% 23% 21% 5%	1635
	11th to 20th	Day	29% 7% 15% 27% 19% 3	1520
	21st to end	Day	30% 5% 14% 30% 18% 4	1520
December	1st to 10th	Day	32% 7% 16% 26% 16% 4	1520
	11th to 20th	Day	24% 5% 20% 35% 14% 2	1924
	21st to end	Day	29% 4 13% 31% 17% 6%	2794
January	1st to 10th	Night	35% 8% 13% 31% 12% 2	4417
	11th to 20th	Night	42% 5% 17% 19% 13% 4	4430
	21st to end	Night	33% 7% 17% 28% 13% 2	4640
February	1st to 10th	Night	45% 6% 15% 21% 9% 3	2470
	11th to 20th	Night	43% 6% 14% 22% 13% 3	2403
	21st to end	Night	37% 6% 12% 24% 15% 6%	1921
March	1st to 10th	Night	30% 6% 18% 25% 18% 3	2199
	11th to 20th	Night	34% 7% 14% 24% 17% 4	1980
	21st to end	Night	42% 7% 14% 24% 11% 1	2106
April	1st to 10th	Night	38% 9% 17% 21% 12% 4	1800
	11th to 20th	Night	41% 8% 15% 26% 10% 1	1777
	21st to end	Night	37% 6% 15% 27% 14% 1	1688
May	1st to 10th	Night	34% 6% 16% 30% 13% 1	1459
	11th to 20th	Night	36% 9% 16% 25% 12% 2	1418
	21st to end	Night	38% 9% 23% 26% 3 1	1552
June	1st to 10th	Night	30% 8% 21% 24% 14% 2	1440
	11th to 20th	Night	29% 9% 19% 22% 15% 5%	1440
	21st to end	Night	21% 6% 14% 29% 26% 5%	1440
July	1st to 10th	Night	16% 7% 16% 31% 25% 5%	1400
	11th to 20th	Night	15% 5% 10% 30% 25% 14%	1392
	21st to end	Night	14% 5% 9% 27% 28% 16%	1488
August	1st to 10th	Night	19% 5% 11% 32% 23% 9%	1552
	11th to 20th	Night	16% 5% 10% 30% 29% 10%	1602
	21st to end	Night	17% 4 12% 23% 28% 16%	1949
September	1st to 10th	Night	20% 4 8% 24% 28% 16%	1800
	11th to 20th	Night	22% 5% 11% 25% 27% 11%	1800
	21st to end	Night	25% 3 9% 28% 29% 6%	2043
October	1st to 10th	Night	36% 4 12% 23% 20% 5%	2160
	11th to 20th	Night	31% 4 10% 29% 19% 7%	2160
	21st to end	Night	27% 5% 16% 29% 17% 5%	2438
November	1st to 10th	Night	33% 6% 14% 24% 18% 5%	2439
	11th to 20th	Night	33% 5% 12% 30% 17% 3	2660
	21st to end	Night	30% 5% 15% 27% 17% 6%	2660
December	1st to 10th	Night	37% 6% 15% 22% 16% 4	2660
	11th to 20th	Night	26% 6% 17% 36% 14% 2	3272
	21st to end	Night	30% 5% 13% 29% 16% 7%	4873

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	7% 6% 18% 40% 23% 7%	270
	11th to 20th	Day	12% 6% 21% 36% 18% 7%	270
	21st to end	Day	9% 12% 28% 32% 14% 4	297
February	1st to 10th	Day	10% 11% 34% 36% 9% 1	270
	11th to 20th	Day	10% 11% 25% 40% 12% 2	270
	21st to end	Day	5% 13% 24% 41% 15% 2	225
March	1st to 10th	Day	8% 11% 29% 37% 13% 2	310
	11th to 20th	Day	11% 14% 30% 30% 10% 5%	420
	21st to end	Day	18% 11% 28% 28% 13% 3	462
April	1st to 10th	Day	17% 14% 32% 27% 8% 2	420
	11th to 20th	Day	18% 19% 31% 24% 8%	412
	21st to end	Day	20% 12% 26% 25% 15% 2	420
May	1st to 10th	Day	14% 9% 22% 31% 19% 5%	581
	11th to 20th	Day	17% 6% 22% 29% 21% 5%	625
	21st to end	Day	17% 7% 21% 27% 23% 6%	679
June	1st to 10th	Day	7% 5% 20% 32% 28% 8%	630
	11th to 20th	Day	9% 9% 24% 37% 18% 4	630
	21st to end	Day	6% 3 23% 38% 24% 5%	630
July	1st to 10th	Day	4 2 17% 37% 32% 7%	621
	11th to 20th	Day	1 4 23% 40% 28% 4	618
	21st to end	Day	4 3 20% 33% 34% 5%	684
August	1st to 10th	Day	5% 3 25% 39% 22% 5%	495
	11th to 20th	Day	5% 4 19% 39% 24% 8%	437
	21st to end	Day	3 4 20% 38% 23% 10%	495
September	1st to 10th	Day	7% 6% 21% 36% 23% 6%	450
	11th to 20th	Day	10% 4 18% 37% 24% 8%	450
	21st to end	Day	14% 7% 18% 31% 20% 9%	331
October	1st to 10th	Day	13% 8% 19% 27% 22% 11%	270
	11th to 20th	Day	14% 12% 20% 29% 17% 7%	270
	21st to end	Day	13% 10% 26% 32% 15% 4	297
November	1st to 10th	Day	10% 8% 27% 31% 16% 8%	270
	11th to 20th	Day	9% 11% 24% 33% 19% 4	270
	21st to end	Day	10% 7% 19% 43% 19% 2	270
December	1st to 10th	Day	8% 7% 20% 46% 16% 3	270
	11th to 20th	Day	9% 10% 24% 43% 13% 1	270
	21st to end	Day	7% 9% 27% 39% 14% 4	297
January	1st to 10th	Night	7% 7% 26% 41% 17% 2	950
	11th to 20th	Night	8% 10% 33% 33% 13% 3	950
	21st to end	Night	9% 10% 30% 37% 13% 2	903
February	1st to 10th	Night	7% 7% 33% 40% 12% 2	760
	11th to 20th	Night	9% 8% 30% 40% 12% 1	744
	21st to end	Night	6% 8% 29% 43% 12% 3	628
March	1st to 10th	Night	7% 5% 31% 43% 12% 2	750
	11th to 20th	Night	9% 5% 28% 40% 15% 3	720
	21st to end	Night	16% 6% 25% 34% 17% 2	720
April	1st to 10th	Night	13% 7% 28% 41% 11% 1	540
	11th to 20th	Night	14% 7% 28% 41% 10%	533
	21st to end	Night	16% 5% 21% 42% 14% 1	540
May	1st to 10th	Night	14% 4 19% 41% 20% 1	540
	11th to 20th	Night	17% 4 14% 41% 23% 1	531
	21st to end	Night	18% 5% 15% 40% 22% 1	584
June	1st to 10th	Night	11% 4 19% 42% 22% 2	540
	11th to 20th	Night	10% 5% 22% 47% 15% 1	540
	21st to end	Night	7% 3 15% 51% 23% 1	540
July	1st to 10th	Night	4 1 13% 49% 31% 2	525
	11th to 20th	Night	3 1 16% 56% 24%	522
	21st to end	Night	2 1 11% 54% 30% 1	591
August	1st to 10th	Night	4 2 10% 58% 26% 1	537
	11th to 20th	Night	5% 1 10% 49% 32% 2	534
	21st to end	Night	3 2 13% 50% 30% 2	594
September	1st to 10th	Night	6% 2 14% 49% 26% 3	540
	11th to 20th	Night	7% 3 14% 47% 25% 4	540
	21st to end	Night	11% 3 17% 41% 22% 6%	666
October	1st to 10th	Night	18% 4 17% 37% 20% 4	720
	11th to 20th	Night	13% 7% 23% 36% 20% 1	720
	21st to end	Night	10% 8% 23% 41% 15% 3	854
November	1st to 10th	Night	10% 5% 25% 44% 14% 2	925
	11th to 20th	Night	8% 10% 29% 40% 13% 1	950
	21st to end	Night	10% 6% 27% 44% 12% 1	950
December	1st to 10th	Night	6% 7% 30% 43% 11% 1	950
	11th to 20th	Night	8% 5% 28% 45% 13% 1	950
	21st to end	Night	10% 7% 27% 41% 13% 2	1045

Page generated at 2024-03-05 05:23:58. Version ac07

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