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	15% 9% 4 8% 6% 4 5% 5% 5% 6% 33%	78743
	Night	16% 9% 4 7% 6% 3 5% 4 5% 6% 35%	80815
Transparency	Day	51% 3 13% 19% 11% 2	78448
	Night	51% 3 13% 20% 11% 3	80631
Seeing	Day	17% 7% 20% 26% 20% 10%	13959
	Night	18% 4 18% 36% 22% 2	22859

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	25% 14% 7% 12% 9% 5% 7% 5% 4 5% 7%	2539
	11th to 20th	Day	29% 10% 4 9% 8% 4 6% 5% 4 4 16%	2540
	21st to end	Day	28% 13% 5% 10% 9% 4 6% 5% 4 4 13%	3144
February	1st to 10th	Day	22% 13% 5% 12% 6% 4 6% 6% 5% 6% 16%	1710
	11th to 20th	Day	22% 12% 4 10% 8% 4 6% 5% 5% 5% 18%	1665
	21st to end	Day	20% 13% 5% 10% 8% 4 6% 5% 5% 5% 19%	1464
March	1st to 10th	Day	20% 13% 5% 9% 8% 4 8% 5% 6% 5% 17%	1998
	11th to 20th	Day	22% 13% 5% 9% 8% 4 5% 4 4 6% 20%	1980
	21st to end	Day	20% 13% 5% 10% 9% 5% 7% 6% 5% 6% 14%	2196
April	1st to 10th	Day	19% 12% 5% 10% 8% 4 6% 5% 6% 6% 18%	2160
	11th to 20th	Day	19% 11% 4 9% 7% 4 6% 6% 5% 4 24%	2150
	21st to end	Day	16% 9% 4 8% 8% 5% 6% 6% 7% 7% 24%	2340
May	1st to 10th	Day	12% 9% 3 7% 6% 3 5% 6% 6% 8% 34%	2434
	11th to 20th	Day	13% 9% 3 7% 7% 4 5% 6% 6% 8% 31%	2477
	21st to end	Day	15% 6% 3 7% 7% 4 7% 6% 7% 8% 30%	2723
June	1st to 10th	Day	15% 8% 4 8% 6% 3 5% 5% 6% 8% 32%	2520
	11th to 20th	Day	12% 7% 3 8% 8% 4 7% 7% 7% 8% 29%	2520
	21st to end	Day	6% 4 2 5% 4 3 5% 4 5% 7% 55%	2691
July	1st to 10th	Day	4 4 2 5% 4 2 4 4 5% 6% 60%	2562
	11th to 20th	Day	5% 3 1 3 3 2 3 4 5% 6% 64%	2520
	21st to end	Day	4 2 1 2 2 2 2 2 3 5% 75%	2709
August	1st to 10th	Day	2 3 1 3 4 2 4 4 4 8% 66%	2328
	11th to 20th	Day	4 3 1 3 4 1 3 4 5% 6% 65%	2298
	21st to end	Day	5% 4 2 3 3 2 4 4 5% 10% 58%	2471
September	1st to 10th	Day	4 3 1 3 3 2 3 4 4 7% 65%	2160
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 5% 6% 8% 46%	2124
	21st to end	Day	13% 5% 2 5% 5% 4 4 4 4 5% 47%	1914
October	1st to 10th	Day	13% 7% 3 6% 7% 4 4 5% 5% 6% 39%	1800
	11th to 20th	Day	15% 8% 4 7% 6% 4 5% 6% 7% 7% 32%	1800
	21st to end	Day	19% 11% 4 9% 6% 4 5% 3 4 6% 31%	1976
November	1st to 10th	Day	20% 13% 6% 10% 8% 3 5% 6% 5% 6% 17%	1548
	11th to 20th	Day	21% 13% 5% 9% 8% 4 6% 5% 5% 6% 19%	1520
	21st to end	Day	19% 11% 5% 10% 8% 3 7% 5% 5% 6% 22%	1520
December	1st to 10th	Day	20% 10% 5% 11% 6% 4 6% 5% 5% 6% 23%	1520
	11th to 20th	Day	21% 12% 5% 12% 9% 4 7% 6% 5% 6% 14%	1928
	21st to end	Day	25% 15% 7% 13% 8% 5% 6% 5% 4 4 8%	2794
January	1st to 10th	Night	25% 13% 6% 10% 9% 5% 6% 5% 5% 5% 11%	4406
	11th to 20th	Night	25% 13% 6% 10% 7% 4 5% 4 4 4 18%	4420
	21st to end	Night	31% 12% 5% 9% 7% 4 4 4 3 4 16%	4521
February	1st to 10th	Night	18% 12% 5% 10% 8% 4 6% 5% 6% 6% 20%	2470
	11th to 20th	Night	19% 10% 4 7% 6% 4 6% 6% 5% 7% 27%	2402
	21st to end	Night	17% 11% 5% 8% 9% 4 6% 5% 5% 6% 24%	1989
March	1st to 10th	Night	16% 10% 6% 9% 8% 4 7% 6% 4 6% 25%	2118
	11th to 20th	Night	19% 9% 4 8% 7% 5% 5% 5% 6% 7% 25%	1980
	21st to end	Night	20% 10% 4 9% 8% 4 5% 6% 5% 6% 22%	2160
April	1st to 10th	Night	18% 10% 4 8% 8% 4 6% 4 4 6% 26%	1800
	11th to 20th	Night	15% 8% 4 8% 7% 4 5% 5% 6% 7% 31%	1754
	21st to end	Night	13% 8% 3 8% 5% 3 5% 6% 7% 7% 35%	1620
May	1st to 10th	Night	8% 6% 3 7% 6% 3 5% 5% 6% 6% 45%	1524
	11th to 20th	Night	11% 6% 3 7% 7% 3 6% 5% 7% 8% 39%	1416
	21st to end	Night	10% 6% 3 6% 6% 3 6% 6% 7% 9% 39%	1552
June	1st to 10th	Night	12% 5% 3 6% 6% 3 5% 4 5% 7% 43%	1440
	11th to 20th	Night	8% 5% 4 5% 6% 3 4 5% 6% 8% 46%	1437
	21st to end	Night	5% 5% 2 5% 4 2 3 3 4 6% 63%	1260
July	1st to 10th	Night	3 2 1 3 3 2 3 3 3 6% 73%	1386
	11th to 20th	Night	3 3 1 2 2 2 2 3 3 6% 73%	1440
	21st to end	Night	2 1 1 1 1 2 1 3 4 83%	1604
August	1st to 10th	Night	1 2 2 2 1 2 3 5% 7% 73%	1611
	11th to 20th	Night	3 2 1 2 3 2 2 2 3 5% 75%	1602
	21st to end	Night	4 3 1 3 3 3 2 3 4 7% 66%	1885
September	1st to 10th	Night	4 2 1 2 2 2 3 3 5% 6% 70%	1800
	11th to 20th	Night	7% 4 2 4 5% 4 4 5% 5% 6% 54%	1831
	21st to end	Night	9% 5% 2 4 4 4 4 5% 5% 7% 53%	2045
October	1st to 10th	Night	11% 4 3 5% 4 3 4 4 5% 5% 50%	2160
	11th to 20th	Night	14% 6% 3 5% 5% 3 4 4 5% 8% 43%	2160
	21st to end	Night	17% 8% 3 6% 4 3 5% 4 5% 6% 39%	2378
November	1st to 10th	Night	22% 7% 4 8% 7% 4 5% 5% 5% 6% 26%	2526
	11th to 20th	Night	23% 12% 4 8% 7% 4 5% 6% 5% 5% 22%	2660
	21st to end	Night	18% 9% 4 7% 6% 4 6% 5% 5% 5% 29%	2660
December	1st to 10th	Night	19% 10% 4 8% 7% 4 5% 5% 6% 5% 26%	2660
	11th to 20th	Night	22% 12% 4 9% 8% 4 6% 5% 4 6% 19%	3276
	21st to end	Night	22% 13% 5% 10% 8% 4 6% 4 5% 4 19%	4862

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	81% 2 6% 8% 2 1	2539
	11th to 20th	Day	71% 2 11% 11% 3 2	2540
	21st to end	Day	75% 3 8% 9% 4	3144
February	1st to 10th	Day	69% 4 11% 12% 4	1710
	11th to 20th	Day	68% 4 11% 11% 5% 1	1674
	21st to end	Day	67% 3 12% 11% 6% 1	1464
March	1st to 10th	Day	66% 3 13% 14% 4	1998
	11th to 20th	Day	66% 3 12% 14% 5% 1	1980
	21st to end	Day	70% 3 14% 11% 2	2196
April	1st to 10th	Day	65% 4 13% 14% 4	2160
	11th to 20th	Day	60% 4 10% 15% 8% 3	2153
	21st to end	Day	56% 5% 16% 15% 8% 1	2340
May	1st to 10th	Day	47% 3 16% 24% 9% 1	2434
	11th to 20th	Day	49% 4 16% 21% 8% 1	2480
	21st to end	Day	49% 6% 21% 21% 4	2724
June	1st to 10th	Day	50% 3 18% 22% 6% 1	2520
	11th to 20th	Day	50% 4 21% 19% 6%	2520
	21st to end	Day	29% 3 21% 30% 12% 4	2691
July	1st to 10th	Day	26% 3 16% 27% 22% 6%	2493
	11th to 20th	Day	21% 2 11% 31% 28% 7%	2445
	21st to end	Day	13% 1 9% 34% 35% 7%	2558
August	1st to 10th	Day	19% 2 16% 36% 21% 5%	2312
	11th to 20th	Day	20% 2 16% 33% 22% 7%	2298
	21st to end	Day	23% 3 13% 35% 22% 4	2471
September	1st to 10th	Day	20% 3 13% 32% 21% 11%	2160
	11th to 20th	Day	37% 4 15% 23% 17% 5%	2124
	21st to end	Day	40% 3 13% 24% 15% 5%	1914
October	1st to 10th	Day	45% 3 11% 21% 17% 3	1800
	11th to 20th	Day	50% 3 14% 19% 12% 1	1800
	21st to end	Day	57% 2 10% 16% 14% 1	1976
November	1st to 10th	Day	66% 4 10% 15% 5%	1548
	11th to 20th	Day	65% 3 12% 12% 7% 1	1520
	21st to end	Day	63% 3 9% 13% 11% 1	1520
December	1st to 10th	Day	62% 4 12% 13% 9% 1	1520
	11th to 20th	Day	70% 3 13% 13% 2	1928
	21st to end	Day	81% 2 7% 6% 4	2794
January	1st to 10th	Night	76% 2 8% 9% 4 1	4406
	11th to 20th	Night	71% 2 9% 12% 4 2	4420
	21st to end	Night	74% 2 9% 10% 6% 1	4521
February	1st to 10th	Night	64% 6% 12% 14% 5%	2470
	11th to 20th	Night	57% 3 15% 15% 7% 2	2404
	21st to end	Night	60% 4 14% 13% 7% 1	1989
March	1st to 10th	Night	60% 4 12% 19% 6%	2118
	11th to 20th	Night	57% 4 14% 17% 6% 1	1980
	21st to end	Night	62% 4 15% 17% 2	2160
April	1st to 10th	Night	59% 4 15% 15% 7%	1800
	11th to 20th	Night	51% 5% 16% 17% 8% 3	1754
	21st to end	Night	47% 5% 16% 22% 8% 2	1620
May	1st to 10th	Night	40% 2 18% 29% 8% 2	1524
	11th to 20th	Night	43% 4 19% 25% 7% 3	1416
	21st to end	Night	39% 6% 23% 26% 6%	1552
June	1st to 10th	Night	42% 3 18% 30% 6% 1	1440
	11th to 20th	Night	37% 3 23% 30% 7%	1437
	21st to end	Night	25% 2 22% 34% 12% 5%	1260
July	1st to 10th	Night	15% 3 18% 32% 25% 7%	1346
	11th to 20th	Night	14% 2 15% 28% 31% 9%	1392
	21st to end	Night	7% 1 9% 36% 34% 12%	1515
August	1st to 10th	Night	13% 1 15% 40% 24% 7%	1602
	11th to 20th	Night	14% 1 16% 38% 22% 9%	1602
	21st to end	Night	20% 2 12% 38% 23% 5%	1885
September	1st to 10th	Night	16% 3 14% 32% 24% 11%	1800
	11th to 20th	Night	31% 3 15% 26% 15% 9%	1831
	21st to end	Night	31% 3 16% 23% 20% 8%	2045
October	1st to 10th	Night	36% 3 11% 27% 18% 4	2160
	11th to 20th	Night	40% 4 14% 23% 17% 3	2160
	21st to end	Night	46% 4 12% 19% 16% 3	2378
November	1st to 10th	Night	58% 5% 12% 19% 7%	2526
	11th to 20th	Night	62% 4 12% 13% 7% 2	2660
	21st to end	Night	55% 4 12% 13% 13% 2	2660
December	1st to 10th	Night	58% 4 11% 14% 11%	2660
	11th to 20th	Night	67% 4 10% 13% 6%	3276
	21st to end	Night	69% 2 13% 13% 3	4862

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	33% 11% 22% 19% 9% 5%	180
	11th to 20th	Day	29% 7% 19% 24% 12% 8%	180
	21st to end	Day	27% 6% 17% 25% 19% 6%	405
February	1st to 10th	Day	27% 9% 21% 22% 14% 7%	370
	11th to 20th	Day	31% 8% 22% 22% 13% 4	365
	21st to end	Day	23% 17% 24% 20% 11% 4	332
March	1st to 10th	Day	26% 6% 28% 25% 12% 3	449
	11th to 20th	Day	27% 9% 21% 25% 16% 3	420
	21st to end	Day	28% 12% 19% 24% 15% 2	462
April	1st to 10th	Day	25% 15% 20% 20% 16% 5%	420
	11th to 20th	Day	26% 11% 24% 21% 11% 7%	412
	21st to end	Day	20% 10% 27% 24% 15% 5%	420
May	1st to 10th	Day	15% 11% 25% 24% 18% 8%	423
	11th to 20th	Day	19% 10% 20% 23% 17% 11%	448
	21st to end	Day	12% 9% 19% 25% 22% 13%	485
June	1st to 10th	Day	16% 7% 19% 27% 17% 14%	450
	11th to 20th	Day	14% 10% 24% 29% 19% 4	450
	21st to end	Day	5% 7% 25% 30% 25% 9%	621
July	1st to 10th	Day	5% 3 16% 34% 30% 12%	488
	11th to 20th	Day	3 5% 18% 29% 31% 15%	444
	21st to end	Day	2 2 14% 30% 34% 18%	487
August	1st to 10th	Day	4 3 11% 33% 28% 22%	447
	11th to 20th	Day	3 2 10% 26% 34% 25%	437
	21st to end	Day	6% 3 17% 32% 28% 15%	495
September	1st to 10th	Day	6% 3 19% 29% 28% 16%	450
	11th to 20th	Day	9% 5% 15% 28% 30% 13%	450
	21st to end	Day	15% 5% 16% 29% 26% 9%	450
October	1st to 10th	Day	15% 7% 20% 24% 22% 13%	450
	11th to 20th	Day	20% 6% 16% 23% 24% 11%	450
	21st to end	Day	24% 8% 22% 24% 14% 8%	491
November	1st to 10th	Day	24% 7% 25% 28% 12% 5%	210
	11th to 20th	Day	20% 9% 24% 21% 19% 7%	180
	21st to end	Day	23% 8% 19% 24% 15% 11%	180
December	1st to 10th	Day	27% 4 18% 28% 14% 9%	180
	11th to 20th	Day	34% 3 22% 26% 11% 3	180
	21st to end	Day	31% 6% 24% 23% 13% 3	198
January	1st to 10th	Night	29% 7% 21% 26% 15% 3	760
	11th to 20th	Night	28% 6% 17% 27% 19% 3	760
	21st to end	Night	26% 4 15% 35% 18% 3	836
February	1st to 10th	Night	24% 4 21% 31% 18% 2	760
	11th to 20th	Night	23% 6% 22% 30% 19% 1	744
	21st to end	Night	24% 8% 22% 34% 12%	628
March	1st to 10th	Night	21% 6% 22% 34% 16% 2	750
	11th to 20th	Night	23% 4 22% 34% 17% 1	720
	21st to end	Night	25% 6% 22% 30% 17% 1	792
April	1st to 10th	Night	23% 6% 20% 31% 18% 1	720
	11th to 20th	Night	18% 7% 23% 35% 15% 2	686
	21st to end	Night	17% 3 18% 41% 19% 1	540
May	1st to 10th	Night	12% 3 20% 46% 18% 1	444
	11th to 20th	Night	12% 3 17% 41% 25% 2	354
	21st to end	Night	13% 3 18% 42% 22% 2	389
June	1st to 10th	Night	15% 3 16% 44% 23% 1	360
	11th to 20th	Night	9% 3 21% 45% 20% 1	360
	21st to end	Night	7% 2 16% 49% 26%	360
July	1st to 10th	Night	4 1 11% 54% 29% 1	350
	11th to 20th	Night	4 1 18% 49% 27%	348
	21st to end	Night	3 13% 49% 34% 1	422
August	1st to 10th	Night	3 1 9% 42% 41% 4	537
	11th to 20th	Night	5% 7% 41% 44% 3	534
	21st to end	Night	5% 1 14% 45% 32% 3	697
September	1st to 10th	Night	6% 1 14% 38% 39% 2	720
	11th to 20th	Night	8% 3 10% 39% 33% 7%	720
	21st to end	Night	11% 3 13% 37% 31% 5%	720
October	1st to 10th	Night	14% 2 17% 39% 26% 4	720
	11th to 20th	Night	17% 3 14% 34% 28% 5%	720
	21st to end	Night	21% 3 23% 30% 19% 5%	792
November	1st to 10th	Night	24% 4 17% 38% 17%	740
	11th to 20th	Night	27% 7% 20% 28% 16% 2	760
	21st to end	Night	22% 4 19% 34% 18% 3	760
December	1st to 10th	Night	22% 4 17% 35% 18% 4	760
	11th to 20th	Night	29% 6% 22% 28% 14% 1	760
	21st to end	Night	30% 6% 23% 27% 12% 1	836

Page generated at 2024-03-05 04:23:57. Version ac07

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