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 2026-09-29.

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	12% 7% 3 7% 5% 4 5% 4 5% 6% 43%	159010
	Night	14% 8% 3 7% 5% 4 4 3 5% 6% 42%	149740
Transparency	Day	43% 2 14% 24% 14% 4	153782
	Night	45% 2 14% 23% 13% 3	145968
Seeing	Day	16% 7% 19% 26% 21% 11%	17344
	Night	18% 4 18% 37% 21% 2	25954

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	31% 15% 5% 11% 7% 6% 5% 3 4 4 8%	4722
	11th to 20th	Day	16% 7% 2 8% 6% 5% 5% 4 5% 6% 35%	4740
	21st to end	Day	22% 9% 3 9% 6% 5% 4 3 4 4 29%	5841
February	1st to 10th	Day	21% 14% 5% 12% 6% 6% 5% 5% 6% 6% 16%	1890
	11th to 20th	Day	22% 14% 4 11% 7% 6% 5% 5% 5% 5% 18%	1845
	21st to end	Day	20% 15% 4 10% 7% 6% 5% 4 5% 6% 19%	1614
March	1st to 10th	Day	14% 11% 4 10% 7% 7% 7% 5% 7% 7% 20%	4928
	11th to 20th	Day	22% 14% 4 9% 5% 5% 5% 3 3 4 27%	5110
	21st to end	Day	20% 12% 4 12% 8% 7% 7% 4 5% 6% 13%	5651
April	1st to 10th	Day	19% 13% 4 11% 7% 6% 5% 4 7% 6% 19%	2520
	11th to 20th	Day	17% 11% 4 9% 6% 5% 6% 4 5% 5% 27%	2514
	21st to end	Day	15% 9% 3 9% 6% 6% 6% 5% 6% 7% 26%	2730
May	1st to 10th	Day	9% 6% 2 5% 4 4 4 4 6% 8% 47%	7940
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 5% 6% 9% 42%	8160
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 8% 42%	8992
June	1st to 10th	Day	14% 9% 3 8% 5% 5% 4 4 6% 7% 35%	2940
	11th to 20th	Day	10% 7% 3 8% 6% 6% 6% 5% 7% 8% 33%	2940
	21st to end	Day	6% 4 2 6% 4 4 5% 4 5% 8% 53%	3141
July	1st to 10th	Day	2 2 1 2 2 2 3 2 3 6% 74%	8345
	11th to 20th	Day	2 2 1 2 1 2 2 2 3 4 79%	8220
	21st to end	Day	3 2 1 2 2 2 3 2 3 5% 74%	8814
August	1st to 10th	Day	2 2 1 3 2 3 3 2 4 6% 73%	7547
	11th to 20th	Day	4 3 1 4 3 3 4 3 6% 8% 61%	7563
	21st to end	Day	5% 5% 2 4 3 3 3 3 5% 9% 58%	8036
September	1st to 10th	Day	5% 3 1 5% 3 3 4 3 5% 7% 61%	2520
	11th to 20th	Day	8% 6% 2 5% 4 5% 5% 4 6% 8% 47%	2478
	21st to end	Day	13% 5% 2 6% 4 5% 4 3 4 6% 47%	2213
October	1st to 10th	Day	10% 6% 2 5% 4 4 3 3 5% 6% 51%	3280
	11th to 20th	Day	13% 7% 3 9% 5% 7% 7% 5% 7% 7% 31%	3261
	21st to end	Day	20% 9% 4 9% 5% 5% 5% 4 5% 8% 26%	3541
November	1st to 10th	Day	21% 13% 5% 11% 6% 5% 5% 4 5% 7% 17%	1714
	11th to 20th	Day	21% 13% 5% 11% 7% 5% 5% 4 5% 5% 18%	1680
	21st to end	Day	20% 12% 4 11% 7% 6% 6% 4 5% 6% 21%	1680
December	1st to 10th	Day	17% 8% 3 10% 4 5% 6% 4 5% 6% 33%	2700
	11th to 20th	Day	26% 13% 5% 13% 7% 6% 5% 4 4 5% 11%	3108
	21st to end	Day	30% 15% 6% 13% 6% 6% 6% 4 4 4 7%	4092
January	1st to 10th	Night	30% 13% 5% 11% 6% 6% 5% 3 5% 5% 13%	8172
	11th to 20th	Night	14% 9% 4 8% 5% 4 4 3 4 4 41%	8220
	21st to end	Night	23% 9% 3 9% 5% 4 4 2 3 4 33%	8371
February	1st to 10th	Night	17% 13% 4 10% 7% 5% 5% 4 6% 6% 22%	2730
	11th to 20th	Night	20% 11% 3 8% 5% 5% 5% 5% 5% 6% 26%	2662
	21st to end	Night	18% 12% 4 9% 7% 5% 5% 4 5% 6% 24%	2191
March	1st to 10th	Night	13% 9% 4 10% 8% 8% 7% 6% 6% 6% 24%	5182
	11th to 20th	Night	20% 10% 3 9% 6% 5% 4 4 4 5% 31%	5110
	21st to end	Night	23% 12% 3 10% 7% 5% 6% 4 6% 7% 17%	5553
April	1st to 10th	Night	17% 11% 3 9% 7% 6% 5% 4 5% 6% 28%	2100
	11th to 20th	Night	13% 9% 4 8% 7% 5% 4 4 6% 7% 33%	2049
	21st to end	Night	12% 9% 3 8% 4 4 5% 5% 7% 7% 36%	1890
May	1st to 10th	Night	11% 5% 1 5% 3 3 3 3 5% 7% 54%	4989
	11th to 20th	Night	8% 7% 3 6% 4 4 4 3 6% 8% 46%	4700
	21st to end	Night	6% 4 2 5% 4 4 5% 5% 7% 9% 48%	5181
June	1st to 10th	Night	12% 5% 3 7% 5% 5% 4 3 5% 6% 45%	1680
	11th to 20th	Night	7% 6% 3 6% 5% 4 4 4 6% 7% 50%	1676
	21st to end	Night	5% 5% 2 5% 3 3 3 2 4 6% 61%	1470
July	1st to 10th	Night	1 1 1 1 1 1 1 3 5% 84%	4560
	11th to 20th	Night	2 1 1 2 1 1 2 2 3 3 83%	4740
	21st to end	Night	2 1 1 2 2 2 2 2 4 5% 78%	5263
August	1st to 10th	Night	2 2 1 3 2 2 3 2 4 5% 77%	5275
	11th to 20th	Night	6% 3 1 3 3 3 3 2 5% 6% 66%	5292
	21st to end	Night	5% 3 1 3 2 3 2 2 4 7% 67%	6198
September	1st to 10th	Night	5% 3 1 2 2 3 3 2 5% 6% 68%	2100
	11th to 20th	Night	7% 5% 2 5% 4 4 3 4 5% 6% 55%	2137
	21st to end	Night	8% 5% 1 5% 3 4 4 3 5% 7% 55%	2363
October	1st to 10th	Night	7% 3 2 5% 3 3 4 3 5% 6% 60%	3920
	11th to 20th	Night	13% 6% 2 6% 4 5% 5% 4 6% 9% 40%	3897
	21st to end	Night	20% 9% 3 8% 4 5% 4 4 5% 6% 31%	4267
November	1st to 10th	Night	22% 8% 4 9% 6% 6% 5% 4 6% 6% 25%	2800
	11th to 20th	Night	24% 12% 4 10% 5% 5% 5% 4 5% 5% 21%	2940
	21st to end	Night	18% 11% 4 9% 6% 6% 5% 4 5% 5% 28%	2940
December	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 6% 6% 40%	4700
	11th to 20th	Night	31% 15% 5% 10% 6% 5% 4 3 4 4 13%	5316
	21st to end	Night	28% 13% 5% 11% 7% 5% 5% 3 4 4 15%	7106

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 5% 8% 4	4723
	11th to 20th	Day	50% 2 9% 18% 15% 6%	4740
	21st to end	Day	59% 2 7% 14% 15% 2	5841
February	1st to 10th	Day	69% 4 10% 12% 4	1890
	11th to 20th	Day	68% 4 10% 11% 5% 2	1854
	21st to end	Day	67% 2 11% 11% 7% 1	1614
March	1st to 10th	Day	62% 1 17% 16% 5%	4928
	11th to 20th	Day	62% 2 7% 20% 8%	5110
	21st to end	Day	71% 2 14% 9% 3	5651
April	1st to 10th	Day	63% 4 14% 16% 4	2520
	11th to 20th	Day	57% 3 11% 16% 10% 2	2517
	21st to end	Day	55% 5% 15% 17% 8% 1	2730
May	1st to 10th	Day	36% 1 12% 35% 15% 1	7940
	11th to 20th	Day	40% 2 21% 28% 9% 1	8163
	21st to end	Day	40% 3 27% 27% 5%	8994
June	1st to 10th	Day	48% 3 17% 23% 8% 1	2940
	11th to 20th	Day	46% 3 19% 24% 7% 1	2940
	21st to end	Day	30% 3 19% 31% 13% 4	3141
July	1st to 10th	Day	15% 1 15% 35% 18% 16%	8277
	11th to 20th	Day	13% 1 6% 22% 45% 12%	8145
	21st to end	Day	15% 1 16% 40% 25% 3	8616
August	1st to 10th	Day	14% 1 8% 33% 27% 17%	5976
	11th to 20th	Day	22% 1 20% 42% 12% 3	5962
	21st to end	Day	24% 1 18% 34% 18% 4	6303
September	1st to 10th	Day	23% 3 15% 30% 20% 10%	2520
	11th to 20th	Day	36% 3 15% 25% 16% 5%	2478
	21st to end	Day	38% 3 14% 25% 16% 5%	2213
October	1st to 10th	Day	35% 2 9% 29% 23% 2	3280
	11th to 20th	Day	51% 3 16% 17% 13% 1	3261
	21st to end	Day	58% 1 9% 20% 11% 1	3541
November	1st to 10th	Day	66% 4 11% 15% 4	1714
	11th to 20th	Day	67% 3 11% 12% 6% 1	1680
	21st to end	Day	64% 3 10% 12% 10% 1	1680
December	1st to 10th	Day	53% 2 11% 22% 12%	2700
	11th to 20th	Day	76% 2 11% 10% 1	3108
	21st to end	Day	82% 2 7% 6% 3	4092
January	1st to 10th	Night	76% 1 9% 10% 4 1	8172
	11th to 20th	Night	48% 1 8% 20% 16% 6%	8220
	21st to end	Night	57% 1 6% 13% 19% 3	8371
February	1st to 10th	Night	62% 5% 12% 15% 5%	2730
	11th to 20th	Night	58% 3 15% 15% 7% 2	2664
	21st to end	Night	60% 4 13% 14% 8% 1	2191
March	1st to 10th	Night	59% 2 12% 20% 7%	5182
	11th to 20th	Night	57% 2 11% 22% 7% 1	5110
	21st to end	Night	66% 3 15% 14% 2	5553
April	1st to 10th	Night	57% 3 17% 16% 6%	2100
	11th to 20th	Night	49% 4 17% 18% 9% 3	2049
	21st to end	Night	46% 5% 16% 24% 8% 2	1890
May	1st to 10th	Night	32% 1 12% 40% 14% 1	4987
	11th to 20th	Night	39% 2 17% 31% 9% 1	4700
	21st to end	Night	33% 3 33% 27% 4	5182
June	1st to 10th	Night	41% 3 17% 31% 8% 1	1680
	11th to 20th	Night	33% 2 21% 33% 10% 1	1676
	21st to end	Night	26% 2 21% 35% 12% 4	1470
July	1st to 10th	Night	7% 1 16% 35% 23% 17%	4520
	11th to 20th	Night	10% 1 10% 19% 43% 18%	4692
	21st to end	Night	12% 1 18% 38% 26% 4	5149
August	1st to 10th	Night	8% 1 7% 32% 39% 14%	4169
	11th to 20th	Night	19% 1 22% 41% 13% 4	4169
	21st to end	Night	19% 1 20% 37% 19% 4	4856
September	1st to 10th	Night	19% 3 15% 31% 22% 10%	2100
	11th to 20th	Night	31% 3 15% 27% 16% 8%	2137
	21st to end	Night	29% 3 17% 26% 19% 7%	2363
October	1st to 10th	Night	27% 2 10% 37% 22% 2	3920
	11th to 20th	Night	42% 2 23% 21% 11% 1	3897
	21st to end	Night	54% 2 11% 21% 10% 1	4267
November	1st to 10th	Night	58% 5% 12% 18% 7%	2800
	11th to 20th	Night	64% 3 12% 13% 6% 2	2940
	21st to end	Night	57% 4 12% 13% 12% 2	2940
December	1st to 10th	Night	45% 3 9% 27% 15%	4700
	11th to 20th	Night	76% 2 8% 10% 4	5316
	21st to end	Night	73% 2 11% 11% 2	7106

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	35% 9% 20% 20% 10% 4	220
	11th to 20th	Day	25% 5% 16% 30% 15% 9%	220
	21st to end	Day	25% 6% 17% 26% 18% 8%	471
February	1st to 10th	Day	26% 9% 22% 23% 14% 6%	430
	11th to 20th	Day	31% 7% 23% 24% 12% 4	425
	21st to end	Day	25% 16% 23% 22% 12% 3	386
March	1st to 10th	Day	26% 7% 26% 25% 14% 3	540
	11th to 20th	Day	29% 8% 21% 24% 14% 4	540
	21st to end	Day	27% 11% 21% 26% 14% 2	594
April	1st to 10th	Day	24% 12% 18% 20% 18% 7%	540
	11th to 20th	Day	23% 9% 25% 23% 13% 7%	532
	21st to end	Day	19% 9% 25% 25% 16% 6%	540
May	1st to 10th	Day	14% 10% 23% 23% 21% 8%	543
	11th to 20th	Day	16% 12% 20% 23% 18% 10%	568
	21st to end	Day	10% 8% 19% 26% 22% 14%	617
June	1st to 10th	Day	15% 8% 19% 27% 18% 13%	570
	11th to 20th	Day	12% 8% 23% 29% 21% 7%	570
	21st to end	Day	6% 7% 25% 28% 24% 11%	771
July	1st to 10th	Day	4 3 16% 33% 29% 15%	614
	11th to 20th	Day	3 4 16% 28% 32% 16%	564
	21st to end	Day	2 2 13% 28% 34% 20%	619
August	1st to 10th	Day	3 3 11% 31% 28% 24%	567
	11th to 20th	Day	4 2 10% 25% 33% 27%	557
	21st to end	Day	6% 3 15% 29% 30% 18%	627
September	1st to 10th	Day	7% 3 17% 26% 28% 19%	570
	11th to 20th	Day	10% 5% 15% 27% 30% 13%	570
	21st to end	Day	14% 5% 15% 28% 27% 10%	563
October	1st to 10th	Day	15% 6% 19% 26% 22% 13%	530
	11th to 20th	Day	19% 7% 16% 24% 24% 10%	530
	21st to end	Day	24% 7% 22% 26% 13% 7%	578
November	1st to 10th	Day	26% 7% 24% 26% 12% 6%	256
	11th to 20th	Day	22% 9% 24% 21% 18% 6%	220
	21st to end	Day	25% 6% 17% 24% 17% 10%	220
December	1st to 10th	Day	28% 4 17% 25% 16% 10%	220
	11th to 20th	Day	38% 3 24% 23% 10% 3	220
	21st to end	Day	33% 6% 23% 23% 13% 2	242
January	1st to 10th	Night	30% 6% 20% 27% 14% 3	840
	11th to 20th	Night	25% 5% 17% 29% 21% 3	840
	21st to end	Night	25% 4 15% 35% 19% 3	924
February	1st to 10th	Night	23% 4 21% 33% 18% 2	840
	11th to 20th	Night	25% 5% 21% 31% 18% 1	824
	21st to end	Night	25% 7% 21% 35% 11%	692
March	1st to 10th	Night	22% 6% 22% 35% 15% 2	840
	11th to 20th	Night	24% 3 21% 34% 17% 1	840
	21st to end	Night	25% 5% 22% 32% 15% 1	924
April	1st to 10th	Night	22% 6% 20% 33% 19% 1	840
	11th to 20th	Night	17% 6% 24% 36% 15% 1	801
	21st to end	Night	16% 3 19% 43% 19% 1	630
May	1st to 10th	Night	12% 3 21% 46% 17% 1	517
	11th to 20th	Night	12% 3 19% 42% 22% 1	414
	21st to end	Night	13% 3 19% 43% 21% 2	455
June	1st to 10th	Night	15% 3 18% 43% 21% 1	420
	11th to 20th	Night	9% 3 23% 46% 20% 1	420
	21st to end	Night	7% 2 16% 49% 25%	420
July	1st to 10th	Night	4 1 11% 56% 28% 1	410
	11th to 20th	Night	4 2 19% 50% 25%	408
	21st to end	Night	2 14% 50% 32% 2	494
August	1st to 10th	Night	3 1 10% 44% 39% 4	627
	11th to 20th	Night	5% 7% 42% 42% 3	624
	21st to end	Night	5% 1 14% 45% 32% 3	814
September	1st to 10th	Night	7% 1 15% 39% 37% 2	840
	11th to 20th	Night	9% 3 11% 40% 31% 6%	840
	21st to end	Night	11% 3 13% 38% 31% 4	832
October	1st to 10th	Night	13% 2 17% 39% 26% 4	800
	11th to 20th	Night	17% 3 14% 35% 27% 5%	800
	21st to end	Night	21% 2 23% 32% 18% 5%	880
November	1st to 10th	Night	25% 4 17% 38% 17%	820
	11th to 20th	Night	29% 6% 20% 28% 15% 2	840
	21st to end	Night	23% 4 18% 34% 18% 3	840
December	1st to 10th	Night	22% 4 16% 34% 20% 4	840
	11th to 20th	Night	31% 6% 22% 27% 13% 1	840
	21st to end	Night	30% 6% 22% 27% 13% 1	924

Page generated at 2026-09-26 01:18:43. Version ac07

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