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	6% 6% 2 5% 4 4 5% 4 7% 8% 50%	158836
	Night	5% 7% 2 6% 4 4 4 4 6% 8% 49%	150189
Transparency	Day	28% 7% 14% 26% 19% 6%	153629
	Night	30% 6% 14% 27% 17% 5%	146419
Seeing	Day	10% 8% 23% 34% 20% 5%	18570
	Night	9% 6% 24% 43% 17% 2	28500

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	4 8% 4 9% 5% 5% 6% 5% 7% 7% 40%	4720
	11th to 20th	Day	2 8% 3 9% 6% 6% 5% 4 6% 7% 44%	4740
	21st to end	Day	1 5% 2 7% 4 5% 5% 4 6% 8% 53%	5682
February	1st to 10th	Day	3 13% 6% 9% 7% 6% 4 4 5% 7% 37%	1890
	11th to 20th	Day	3 11% 5% 9% 4 5% 5% 4 6% 8% 41%	1845
	21st to end	Day	3 7% 3 8% 6% 5% 4 4 8% 9% 42%	1689
March	1st to 10th	Day	3 9% 3 5% 4 4 3 3 5% 8% 53%	4735
	11th to 20th	Day	4 9% 3 6% 4 5% 5% 4 6% 7% 47%	5110
	21st to end	Day	11% 14% 4 8% 4 4 4 4 6% 9% 31%	5786
April	1st to 10th	Day	9% 11% 3 6% 4 4 5% 4 6% 7% 42%	2520
	11th to 20th	Day	9% 10% 3 7% 4 5% 5% 4 6% 9% 38%	2491
	21st to end	Day	14% 8% 3 6% 4 4 5% 5% 6% 8% 37%	2650
May	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 6% 7% 45%	8160
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 47%	8191
	21st to end	Day	10% 5% 2 5% 4 4 6% 5% 7% 9% 43%	9026
June	1st to 10th	Day	7% 4 1 4 4 5% 5% 6% 8% 10% 45%	2940
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 5% 9% 9% 46%	2940
	21st to end	Day	4 3 1 4 3 3 4 6% 7% 9% 56%	2940
July	1st to 10th	Day	3 2 1 3 3 4 5% 5% 9% 11% 54%	8249
	11th to 20th	Day	4 2 1 3 2 3 5% 5% 7% 8% 61%	8250
	21st to end	Day	4 3 1 4 3 4 6% 6% 9% 10% 49%	8957
August	1st to 10th	Day	4 2 1 3 3 3 4 4 6% 8% 60%	7695
	11th to 20th	Day	3 3 1 4 3 4 5% 5% 8% 9% 54%	7565
	21st to end	Day	4 4 2 5% 4 4 5% 4 7% 8% 54%	7830
September	1st to 10th	Day	6% 3 1 3 2 3 4 3 5% 6% 65%	2520
	11th to 20th	Day	8% 3 1 3 2 3 3 3 5% 7% 61%	2520
	21st to end	Day	9% 4 1 3 2 3 3 3 5% 6% 62%	2210
October	1st to 10th	Day	6% 5% 1 3 1 2 2 2 3 4 69%	3270
	11th to 20th	Day	10% 5% 2 4 3 2 3 2 4 5% 61%	3250
	21st to end	Day	7% 7% 2 5% 4 3 4 4 5% 7% 53%	3396
November	1st to 10th	Day	5% 8% 3 7% 4 3 5% 4 5% 8% 49%	1811
	11th to 20th	Day	4 7% 3 5% 5% 4 5% 4 6% 7% 50%	1680
	21st to end	Day	6% 7% 2 5% 4 4 4 4 6% 8% 50%	1680
December	1st to 10th	Day	2 6% 2 6% 5% 4 4 3 5% 7% 55%	2700
	11th to 20th	Day	3 6% 2 5% 3 3 4 4 6% 9% 54%	3106
	21st to end	Day	3 8% 3 6% 5% 5% 5% 4 6% 8% 47%	4092
January	1st to 10th	Night	3 12% 5% 9% 5% 5% 5% 4 6% 7% 39%	8201
	11th to 20th	Night	3 9% 4 11% 7% 6% 5% 3 5% 6% 40%	8250
	21st to end	Night	3 8% 3 9% 5% 4 4 4 5% 6% 49%	8573
February	1st to 10th	Night	3 11% 5% 11% 7% 6% 6% 5% 5% 6% 36%	2730
	11th to 20th	Night	3 12% 5% 9% 5% 5% 5% 4 6% 6% 40%	2662
	21st to end	Night	2 7% 4 9% 6% 5% 4 4 6% 8% 44%	2116
March	1st to 10th	Night	4 8% 3 6% 4 3 4 4 6% 8% 49%	5363
	11th to 20th	Night	7% 8% 2 6% 4 3 4 3 5% 8% 50%	5110
	21st to end	Night	9% 14% 4 9% 5% 4 4 3 6% 8% 34%	5423
April	1st to 10th	Night	10% 10% 2 6% 4 4 4 4 6% 9% 41%	2100
	11th to 20th	Night	9% 8% 3 7% 5% 4 6% 4 5% 9% 38%	2077
	21st to end	Night	13% 6% 3 6% 4 4 5% 3 6% 9% 41%	1968
May	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 5% 7% 49%	4784
	11th to 20th	Night	12% 7% 3 8% 4 4 4 4 5% 7% 43%	4701
	21st to end	Night	9% 5% 2 5% 4 4 5% 5% 6% 9% 46%	5181
June	1st to 10th	Night	9% 6% 2 5% 4 3 5% 4 7% 9% 48%	1680
	11th to 20th	Night	8% 4 1 5% 4 4 5% 5% 7% 7% 51%	1680
	21st to end	Night	6% 4 1 4 3 2 4 4 7% 7% 59%	1680
July	1st to 10th	Night	3 2 1 4 3 4 5% 4 8% 11% 55%	4740
	11th to 20th	Night	4 3 1 4 3 4 5% 5% 8% 9% 54%	4740
	21st to end	Night	4 3 1 5% 3 4 5% 4 8% 10% 53%	5152
August	1st to 10th	Night	4 3 1 3 3 3 4 4 6% 9% 59%	5111
	11th to 20th	Night	3 3 1 4 3 4 6% 5% 9% 11% 50%	5291
	21st to end	Night	4 4 2 5% 2 3 4 3 7% 10% 57%	6384
September	1st to 10th	Night	5% 3 1 3 3 2 4 3 4 7% 65%	2100
	11th to 20th	Night	7% 3 1 4 3 4 3 3 5% 8% 59%	2100
	21st to end	Night	9% 4 1 3 2 2 3 2 4 7% 63%	2361
October	1st to 10th	Night	8% 5% 1 3 2 2 2 2 3 5% 69%	3920
	11th to 20th	Night	11% 5% 1 4 2 3 2 2 4 6% 59%	3898
	21st to end	Night	5% 7% 2 5% 3 3 3 3 5% 7% 56%	4371
November	1st to 10th	Night	6% 8% 3 6% 3 5% 4 3 6% 8% 48%	2703
	11th to 20th	Night	4 8% 3 7% 4 4 4 3 6% 8% 49%	2940
	21st to end	Night	4 9% 2 5% 4 4 4 3 5% 7% 51%	2940
December	1st to 10th	Night	3 6% 3 7% 5% 4 4 4 6% 7% 50%	4710
	11th to 20th	Night	2 6% 3 6% 4 5% 5% 5% 7% 8% 50%	5321
	21st to end	Night	4 8% 3 7% 4 4 5% 4 6% 8% 47%	7128

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	37% 8% 16% 27% 12% 1	4723
	11th to 20th	Day	35% 8% 16% 25% 13% 3	4740
	21st to end	Day	26% 6% 13% 32% 20% 3	5682
February	1st to 10th	Day	44% 7% 14% 22% 11% 2	1890
	11th to 20th	Day	40% 6% 13% 23% 14% 4	1854
	21st to end	Day	35% 6% 12% 26% 15% 5%	1689
March	1st to 10th	Day	27% 5% 9% 27% 27% 4	4735
	11th to 20th	Day	31% 8% 15% 31% 14% 1	5110
	21st to end	Day	46% 9% 15% 24% 6%	5786
April	1st to 10th	Day	38% 7% 15% 25% 13% 1	2520
	11th to 20th	Day	40% 7% 16% 23% 14% 1	2494
	21st to end	Day	42% 8% 15% 21% 12% 1	2650
May	1st to 10th	Day	36% 7% 14% 26% 15% 2	8160
	11th to 20th	Day	30% 8% 13% 27% 18% 4	8195
	21st to end	Day	32% 9% 14% 34% 11%	9027
June	1st to 10th	Day	29% 11% 21% 24% 14% 2	2940
	11th to 20th	Day	29% 9% 19% 24% 16% 3	2940
	21st to end	Day	20% 9% 15% 26% 23% 8%	2940
July	1st to 10th	Day	18% 8% 14% 28% 22% 10%	8181
	11th to 20th	Day	17% 6% 10% 23% 28% 16%	8175
	21st to end	Day	23% 8% 14% 22% 23% 9%	8759
August	1st to 10th	Day	21% 5% 8% 17% 28% 22%	6097
	11th to 20th	Day	26% 7% 14% 23% 26% 5%	5964
	21st to end	Day	19% 6% 14% 26% 27% 8%	6145
September	1st to 10th	Day	20% 4 8% 24% 31% 15%	2520
	11th to 20th	Day	22% 5% 11% 28% 25% 7%	2520
	21st to end	Day	24% 4 9% 29% 27% 7%	2210
October	1st to 10th	Day	20% 4 8% 18% 34% 14%	3270
	11th to 20th	Day	27% 4 10% 26% 26% 7%	3250
	21st to end	Day	30% 5% 12% 34% 15% 4	3396
November	1st to 10th	Day	32% 7% 12% 24% 21% 4	1811
	11th to 20th	Day	29% 7% 14% 28% 19% 3	1680
	21st to end	Day	30% 5% 15% 29% 17% 3	1680
December	1st to 10th	Day	27% 5% 19% 29% 17% 2	2700
	11th to 20th	Day	25% 6% 21% 33% 13% 2	3104
	21st to end	Day	33% 5% 13% 30% 15% 4	4092
January	1st to 10th	Night	39% 7% 13% 30% 10% 1	8202
	11th to 20th	Night	42% 6% 15% 23% 11% 4	8250
	21st to end	Night	33% 6% 12% 27% 18% 4	8573
February	1st to 10th	Night	44% 7% 14% 22% 10% 3	2730
	11th to 20th	Night	42% 6% 13% 22% 14% 2	2663
	21st to end	Night	35% 6% 12% 25% 16% 6%	2116
March	1st to 10th	Night	29% 6% 10% 25% 25% 5%	5363
	11th to 20th	Night	29% 7% 17% 25% 19% 2	5110
	21st to end	Night	45% 11% 18% 18% 8%	5423
April	1st to 10th	Night	38% 8% 16% 22% 12% 3	2100
	11th to 20th	Night	39% 7% 14% 27% 12% 1	2077
	21st to end	Night	37% 7% 15% 26% 13% 1	1968
May	1st to 10th	Night	32% 7% 11% 31% 18% 1	4784
	11th to 20th	Night	37% 7% 14% 25% 16% 1	4702
	21st to end	Night	30% 9% 16% 33% 11%	5182
June	1st to 10th	Night	30% 8% 20% 24% 15% 2	1680
	11th to 20th	Night	30% 8% 18% 23% 17% 4	1680
	21st to end	Night	21% 6% 14% 29% 24% 5%	1680
July	1st to 10th	Night	19% 7% 16% 32% 19% 7%	4700
	11th to 20th	Night	20% 8% 11% 22% 25% 14%	4692
	21st to end	Night	22% 8% 14% 28% 21% 9%	5038
August	1st to 10th	Night	20% 5% 10% 19% 23% 22%	4045
	11th to 20th	Night	24% 8% 17% 26% 19% 6%	4168
	21st to end	Night	15% 5% 15% 32% 24% 10%	5000
September	1st to 10th	Night	19% 4 8% 24% 30% 15%	2100
	11th to 20th	Night	23% 5% 12% 26% 26% 9%	2100
	21st to end	Night	23% 3 9% 27% 30% 8%	2361
October	1st to 10th	Night	21% 2 9% 22% 28% 17%	3920
	11th to 20th	Night	27% 4 9% 27% 24% 8%	3898
	21st to end	Night	26% 5% 13% 34% 14% 8%	4371
November	1st to 10th	Night	32% 6% 13% 25% 19% 5%	2703
	11th to 20th	Night	32% 5% 12% 30% 17% 4	2940
	21st to end	Night	30% 5% 16% 27% 16% 5%	2940
December	1st to 10th	Night	30% 6% 18% 26% 18% 2	4710
	11th to 20th	Night	28% 6% 17% 35% 11% 2	5322
	21st to end	Night	32% 6% 11% 33% 13% 5%	7128

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	8% 8% 20% 38% 20% 6%	330
	11th to 20th	Day	16% 8% 22% 32% 15% 6%	330
	21st to end	Day	9% 14% 29% 31% 14% 4	363
February	1st to 10th	Day	10% 10% 36% 36% 8% 1	330
	11th to 20th	Day	8% 10% 26% 39% 15% 2	330
	21st to end	Day	6% 12% 25% 40% 14% 2	273
March	1st to 10th	Day	9% 12% 28% 35% 13% 4	387
	11th to 20th	Day	11% 14% 30% 31% 10% 4	540
	21st to end	Day	17% 12% 28% 28% 12% 3	594
April	1st to 10th	Day	17% 13% 31% 28% 9% 2	540
	11th to 20th	Day	17% 19% 28% 26% 9% 1	532
	21st to end	Day	19% 11% 27% 26% 14% 2	540
May	1st to 10th	Day	14% 11% 21% 30% 19% 5%	728
	11th to 20th	Day	16% 7% 23% 29% 20% 5%	775
	21st to end	Day	16% 7% 21% 28% 22% 5%	844
June	1st to 10th	Day	7% 6% 21% 33% 26% 7%	780
	11th to 20th	Day	9% 10% 24% 36% 16% 4	780
	21st to end	Day	6% 4 25% 39% 21% 5%	780
July	1st to 10th	Day	4 2 17% 37% 32% 8%	771
	11th to 20th	Day	1 4 20% 39% 31% 5%	768
	21st to end	Day	4 3 20% 34% 32% 7%	849
August	1st to 10th	Day	6% 3 26% 37% 22% 6%	622
	11th to 20th	Day	5% 4 22% 36% 24% 9%	557
	21st to end	Day	4 7% 22% 36% 22% 10%	627
September	1st to 10th	Day	6% 5% 20% 36% 24% 8%	570
	11th to 20th	Day	10% 5% 20% 35% 22% 7%	570
	21st to end	Day	13% 7% 18% 31% 21% 11%	424
October	1st to 10th	Day	12% 8% 19% 27% 22% 12%	330
	11th to 20th	Day	15% 11% 20% 30% 18% 7%	330
	21st to end	Day	12% 10% 25% 33% 15% 5%	363
November	1st to 10th	Day	10% 10% 25% 31% 17% 7%	330
	11th to 20th	Day	9% 11% 25% 34% 18% 3	330
	21st to end	Day	10% 5% 19% 44% 19% 2	330
December	1st to 10th	Day	9% 8% 23% 44% 14% 3	330
	11th to 20th	Day	10% 11% 25% 41% 12% 1	330
	21st to end	Day	7% 9% 27% 40% 14% 3	363
January	1st to 10th	Night	7% 8% 27% 40% 16% 2	1050
	11th to 20th	Night	9% 10% 34% 32% 12% 2	1050
	21st to end	Night	8% 10% 32% 36% 13% 1	997
February	1st to 10th	Night	6% 7% 34% 41% 11% 1	840
	11th to 20th	Night	8% 7% 31% 40% 12% 1	824
	21st to end	Night	6% 7% 31% 42% 12% 2	692
March	1st to 10th	Night	8% 5% 30% 43% 13% 2	840
	11th to 20th	Night	9% 5% 30% 40% 13% 3	840
	21st to end	Night	15% 6% 28% 34% 15% 2	840
April	1st to 10th	Night	13% 7% 28% 42% 10%	630
	11th to 20th	Night	13% 8% 28% 42% 9%	623
	21st to end	Night	14% 6% 23% 42% 14% 1	630
May	1st to 10th	Night	13% 4 20% 42% 19% 1	630
	11th to 20th	Night	17% 5% 16% 40% 21% 1	621
	21st to end	Night	16% 5% 17% 41% 19% 1	683
June	1st to 10th	Night	11% 4 19% 44% 20% 1	630
	11th to 20th	Night	10% 5% 24% 47% 13% 1	630
	21st to end	Night	7% 4 17% 50% 21% 1	630
July	1st to 10th	Night	4 1 14% 50% 30% 2	615
	11th to 20th	Night	4 1 16% 56% 23%	612
	21st to end	Night	2 1 12% 55% 29% 1	690
August	1st to 10th	Night	4 2 12% 57% 24% 1	627
	11th to 20th	Night	5% 2 11% 50% 29% 2	624
	21st to end	Night	3 2 13% 52% 27% 2	693
September	1st to 10th	Night	6% 2 13% 50% 26% 3	630
	11th to 20th	Night	7% 3 15% 47% 24% 4	630
	21st to end	Night	10% 3 17% 42% 23% 5%	769
October	1st to 10th	Night	17% 4 17% 39% 20% 4	800
	11th to 20th	Night	13% 7% 23% 36% 20% 1	800
	21st to end	Night	9% 8% 24% 41% 15% 3	950
November	1st to 10th	Night	10% 6% 25% 44% 13% 2	1025
	11th to 20th	Night	7% 10% 29% 40% 12% 1	1050
	21st to end	Night	10% 6% 27% 45% 12% 1	1050
December	1st to 10th	Night	6% 8% 32% 42% 11% 1	1050
	11th to 20th	Night	8% 6% 29% 44% 12% 1	1050
	21st to end	Night	9% 7% 29% 41% 12% 2	1155

Page generated at 2026-09-26 02:18:43. Version ac07

Copyright © 2026 Attilla Danko.