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 2025-05-26.

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 4 4 6% 8% 50%	118202
	Night	6% 7% 3 6% 4 4 4 4 6% 7% 50%	119995
Transparency	Day	29% 7% 13% 26% 19% 5%	117945
	Night	31% 6% 13% 27% 18% 5%	119842
Seeing	Day	10% 8% 23% 34% 20% 5%	16614
	Night	9% 6% 24% 43% 17% 2	26751

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	5% 9% 4 10% 5% 5% 6% 4 7% 6% 38%	3638
	11th to 20th	Day	2 8% 3 10% 6% 6% 5% 4 6% 8% 42%	3640
	21st to end	Day	2 5% 2 6% 4 4 4 4 5% 7% 58%	4390
February	1st to 10th	Day	4 13% 6% 9% 7% 6% 4 4 5% 7% 36%	1800
	11th to 20th	Day	3 12% 5% 9% 4 4 5% 3 6% 8% 40%	1755
	21st to end	Day	3 8% 3 9% 6% 5% 4 4 8% 9% 41%	1611
March	1st to 10th	Day	3 9% 3 5% 4 4 3 3 5% 8% 53%	4632
	11th to 20th	Day	4 9% 3 6% 4 5% 5% 4 6% 8% 47%	5000
	21st to end	Day	11% 15% 4 8% 4 4 4 4 6% 9% 31%	5661
April	1st to 10th	Day	9% 11% 3 7% 4 4 4 4 6% 7% 41%	2400
	11th to 20th	Day	10% 10% 3 7% 4 5% 5% 4 7% 9% 37%	2371
	21st to end	Day	14% 8% 2 6% 4 4 5% 5% 6% 8% 37%	2524
May	1st to 10th	Day	18% 9% 3 6% 5% 4 5% 4 6% 7% 35%	4533
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 47%	4528
	21st to end	Day	12% 6% 2 5% 4 3 6% 5% 7% 9% 41%	4897
June	1st to 10th	Day	8% 4 1 4 4 5% 5% 6% 8% 10% 44%	2660
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 5% 9% 9% 46%	2660
	21st to end	Day	4 3 1 4 3 4 4 6% 7% 8% 58%	2660
July	1st to 10th	Day	4 2 1 3 3 4 5% 5% 8% 8% 58%	4430
	11th to 20th	Day	1 1 1 2 3 3 5% 6% 8% 9% 61%	4430
	21st to end	Day	2 2 1 4 3 4 5% 4 7% 9% 60%	4802
August	1st to 10th	Day	7% 4 2 5% 4 4 5% 5% 7% 9% 47%	4168
	11th to 20th	Day	3 3 1 5% 3 4 5% 5% 7% 8% 56%	4028
	21st to end	Day	3 2 1 4 2 3 3 3 6% 8% 65%	4222
September	1st to 10th	Day	6% 3 1 3 2 3 4 3 5% 6% 64%	2280
	11th to 20th	Day	7% 3 1 3 2 3 3 3 5% 6% 63%	2280
	21st to end	Day	10% 4 1 3 2 3 3 3 5% 5% 62%	2018
October	1st to 10th	Day	6% 5% 1 4 1 2 2 2 3 4 69%	3170
	11th to 20th	Day	10% 5% 2 4 3 2 3 2 4 5% 61%	3150
	21st to end	Day	7% 7% 2 5% 4 3 4 3 5% 7% 52%	3291
November	1st to 10th	Day	5% 8% 3 7% 4 3 5% 4 5% 8% 49%	1723
	11th to 20th	Day	5% 7% 3 4 5% 4 5% 4 6% 7% 51%	1600
	21st to end	Day	6% 7% 2 5% 4 4 4 4 6% 8% 50%	1600
December	1st to 10th	Day	2 6% 2 6% 4 4 4 3 5% 7% 56%	2620
	11th to 20th	Day	3 6% 2 5% 3 3 4 4 6% 9% 54%	3026
	21st to end	Day	3 8% 3 7% 5% 5% 5% 4 6% 8% 47%	4004
January	1st to 10th	Night	3 13% 5% 9% 4 5% 4 3 5% 7% 40%	6327
	11th to 20th	Night	2 10% 4 12% 8% 6% 5% 4 5% 5% 38%	6340
	21st to end	Night	3 7% 3 8% 4 3 4 4 5% 6% 53%	6629
February	1st to 10th	Night	3 11% 5% 11% 7% 6% 6% 5% 5% 6% 35%	2600
	11th to 20th	Night	4 12% 5% 10% 6% 5% 5% 5% 6% 6% 38%	2532
	21st to end	Night	2 8% 4 9% 6% 5% 4 4 6% 8% 44%	2018
March	1st to 10th	Night	4 8% 3 6% 4 3 4 4 6% 8% 48%	5246
	11th to 20th	Night	7% 8% 2 6% 4 3 4 3 5% 8% 51%	5000
	21st to end	Night	9% 14% 4 9% 5% 4 4 3 6% 8% 33%	5306
April	1st to 10th	Night	10% 11% 2 6% 4 4 4 4 6% 9% 41%	2000
	11th to 20th	Night	10% 8% 3 7% 5% 4 6% 4 5% 9% 38%	1977
	21st to end	Night	13% 7% 3 6% 4 4 4 3 6% 9% 42%	1874
May	1st to 10th	Night	17% 9% 2 5% 4 4 4 4 5% 6% 40%	2654
	11th to 20th	Night	12% 5% 2 5% 4 4 4 4 6% 8% 46%	2597
	21st to end	Night	13% 6% 2 5% 4 4 4 5% 6% 9% 42%	2806
June	1st to 10th	Night	9% 6% 2 5% 3 3 4 4 7% 9% 48%	1520
	11th to 20th	Night	8% 4 1 5% 3 3 4 5% 7% 7% 52%	1520
	21st to end	Night	5% 4 1 4 3 2 4 4 7% 7% 60%	1520
July	1st to 10th	Night	3 2 1 3 2 3 4 4 7% 9% 63%	2540
	11th to 20th	Night	2 2 1 3 3 4 5% 6% 8% 10% 56%	2540
	21st to end	Night	2 2 1 4 3 4 5% 4 7% 10% 58%	2761
August	1st to 10th	Night	7% 4 2 4 4 4 4 5% 6% 9% 51%	2768
	11th to 20th	Night	4 2 1 4 3 4 5% 4 9% 10% 53%	2811
	21st to end	Night	2 2 1 3 3 2 3 3 5% 8% 68%	3443
September	1st to 10th	Night	5% 3 1 3 3 3 3 3 4 7% 65%	1900
	11th to 20th	Night	7% 3 1 4 3 3 3 3 5% 8% 61%	1900
	21st to end	Night	9% 4 1 3 2 2 3 2 4 7% 63%	2157
October	1st to 10th	Night	8% 5% 1 3 2 2 2 2 3 5% 69%	3800
	11th to 20th	Night	11% 5% 2 4 2 3 2 2 4 6% 59%	3778
	21st to end	Night	5% 7% 2 5% 3 3 3 3 5% 7% 56%	4235
November	1st to 10th	Night	6% 8% 3 6% 4 5% 4 3 6% 8% 48%	2571
	11th to 20th	Night	5% 9% 3 6% 4 4 4 3 5% 7% 50%	2800
	21st to end	Night	4 9% 2 5% 4 4 4 3 5% 7% 52%	2800
December	1st to 10th	Night	3 6% 3 6% 4 4 4 4 6% 7% 51%	4570
	11th to 20th	Night	2 6% 3 6% 4 5% 5% 5% 7% 8% 50%	5181
	21st to end	Night	4 8% 3 7% 4 4 5% 4 6% 8% 47%	6974

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% 7% 12% 26% 13% 1	3639
	11th to 20th	Day	36% 8% 17% 21% 14% 2	3640
	21st to end	Day	23% 5% 12% 34% 22% 3	4390
February	1st to 10th	Day	45% 7% 14% 22% 10% 2	1800
	11th to 20th	Day	40% 6% 13% 23% 13% 4	1764
	21st to end	Day	36% 6% 13% 25% 15% 5%	1611
March	1st to 10th	Day	27% 5% 9% 27% 27% 4	4632
	11th to 20th	Day	31% 7% 15% 31% 14% 1	5000
	21st to end	Day	47% 9% 15% 24% 5%	5661
April	1st to 10th	Day	38% 7% 15% 24% 13% 2	2400
	11th to 20th	Day	41% 7% 16% 22% 13% 1	2374
	21st to end	Day	42% 8% 15% 22% 12% 1	2524
May	1st to 10th	Day	46% 8% 11% 25% 9% 2	4533
	11th to 20th	Day	32% 9% 14% 30% 13% 2	4532
	21st to end	Day	33% 10% 18% 32% 6%	4897
June	1st to 10th	Day	29% 12% 22% 23% 13% 2	2660
	11th to 20th	Day	29% 9% 18% 25% 16% 3	2660
	21st to end	Day	20% 8% 13% 26% 24% 9%	2660
July	1st to 10th	Day	20% 7% 10% 29% 24% 10%	4361
	11th to 20th	Day	14% 6% 9% 23% 30% 17%	4355
	21st to end	Day	17% 6% 9% 21% 32% 16%	4651
August	1st to 10th	Day	29% 6% 10% 20% 22% 12%	4153
	11th to 20th	Day	22% 6% 9% 26% 30% 7%	4067
	21st to end	Day	16% 5% 10% 22% 35% 11%	4221
September	1st to 10th	Day	21% 4 7% 23% 30% 15%	2280
	11th to 20th	Day	21% 5% 11% 28% 27% 8%	2280
	21st to end	Day	24% 4 9% 29% 26% 7%	2018
October	1st to 10th	Day	20% 4 8% 18% 35% 15%	3170
	11th to 20th	Day	27% 4 10% 26% 27% 7%	3150
	21st to end	Day	30% 5% 13% 34% 14% 4	3291
November	1st to 10th	Day	32% 7% 12% 23% 21% 5%	1723
	11th to 20th	Day	29% 7% 14% 28% 19% 3	1600
	21st to end	Day	30% 5% 15% 29% 18% 4	1600
December	1st to 10th	Day	26% 5% 19% 30% 17% 2	2620
	11th to 20th	Day	24% 6% 22% 34% 13% 2	3024
	21st to end	Day	33% 5% 12% 30% 15% 4	4004
January	1st to 10th	Night	41% 6% 12% 30% 10% 1	6327
	11th to 20th	Night	45% 6% 15% 19% 13% 3	6340
	21st to end	Night	29% 6% 14% 28% 19% 4	6629
February	1st to 10th	Night	46% 7% 14% 21% 10% 3	2600
	11th to 20th	Night	43% 6% 14% 21% 13% 3	2533
	21st to end	Night	36% 6% 13% 24% 15% 6%	2018
March	1st to 10th	Night	29% 6% 10% 25% 25% 5%	5246
	11th to 20th	Night	29% 8% 17% 26% 19% 2	5000
	21st to end	Night	45% 11% 18% 18% 7%	5306
April	1st to 10th	Night	38% 9% 16% 22% 12% 3	2000
	11th to 20th	Night	40% 7% 14% 27% 11% 1	1977
	21st to end	Night	37% 7% 15% 26% 13% 1	1874
May	1st to 10th	Night	40% 8% 11% 25% 15% 2	2654
	11th to 20th	Night	32% 9% 15% 32% 11% 2	2598
	21st to end	Night	33% 9% 19% 30% 8% 1	2806
June	1st to 10th	Night	30% 8% 21% 24% 14% 2	1520
	11th to 20th	Night	28% 8% 19% 23% 17% 5%	1520
	21st to end	Night	20% 6% 14% 29% 26% 6%	1520
July	1st to 10th	Night	16% 6% 14% 34% 22% 8%	2500
	11th to 20th	Night	17% 7% 10% 27% 29% 11%	2492
	21st to end	Night	16% 7% 10% 23% 29% 15%	2674
August	1st to 10th	Night	27% 6% 13% 23% 20% 11%	2760
	11th to 20th	Night	21% 6% 11% 32% 21% 8%	2838
	21st to end	Night	15% 4 10% 25% 32% 15%	3443
September	1st to 10th	Night	19% 4 8% 24% 30% 16%	1900
	11th to 20th	Night	22% 5% 11% 25% 27% 10%	1900
	21st to end	Night	23% 3 9% 27% 29% 8%	2157
October	1st to 10th	Night	21% 2 9% 22% 28% 17%	3800
	11th to 20th	Night	27% 4 9% 28% 24% 8%	3778
	21st to end	Night	27% 5% 13% 33% 14% 8%	4235
November	1st to 10th	Night	32% 6% 14% 25% 18% 5%	2571
	11th to 20th	Night	32% 5% 12% 30% 18% 4	2800
	21st to end	Night	30% 5% 16% 27% 17% 5%	2800
December	1st to 10th	Night	29% 6% 18% 27% 18% 3	4570
	11th to 20th	Night	28% 6% 17% 35% 11% 2	5182
	21st to end	Night	32% 6% 11% 33% 13% 5%	6974

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% 7% 18% 40% 21% 7%	300
	11th to 20th	Day	13% 8% 22% 34% 17% 7%	300
	21st to end	Day	9% 13% 27% 32% 15% 4	330
February	1st to 10th	Day	10% 10% 37% 35% 8% 1	300
	11th to 20th	Day	9% 11% 26% 40% 13% 2	300
	21st to end	Day	6% 13% 25% 39% 15% 2	249
March	1st to 10th	Day	8% 12% 28% 36% 13% 3	354
	11th to 20th	Day	11% 14% 30% 31% 11% 4	500
	21st to end	Day	18% 12% 27% 28% 12% 3	550
April	1st to 10th	Day	17% 13% 31% 28% 9% 2	500
	11th to 20th	Day	18% 20% 28% 25% 9% 1	492
	21st to end	Day	19% 12% 27% 26% 14% 2	500
May	1st to 10th	Day	14% 10% 21% 29% 20% 6%	679
	11th to 20th	Day	17% 7% 22% 29% 20% 5%	725
	21st to end	Day	16% 8% 22% 27% 22% 6%	760
June	1st to 10th	Day	7% 5% 20% 33% 27% 8%	680
	11th to 20th	Day	9% 10% 24% 37% 17% 4	680
	21st to end	Day	6% 4 24% 38% 23% 5%	680
July	1st to 10th	Day	4 2 18% 38% 31% 7%	671
	11th to 20th	Day	1 4 22% 40% 30% 4	668
	21st to end	Day	4 3 20% 34% 34% 6%	739
August	1st to 10th	Day	7% 3 25% 38% 23% 6%	537
	11th to 20th	Day	5% 4 20% 38% 24% 8%	477
	21st to end	Day	4 5% 21% 37% 23% 10%	539
September	1st to 10th	Day	7% 6% 20% 36% 24% 8%	490
	11th to 20th	Day	10% 5% 18% 36% 23% 8%	490
	21st to end	Day	13% 7% 18% 30% 21% 10%	364
October	1st to 10th	Day	12% 8% 20% 26% 22% 12%	300
	11th to 20th	Day	14% 11% 20% 29% 19% 7%	300
	21st to end	Day	13% 10% 25% 32% 15% 5%	330
November	1st to 10th	Day	10% 9% 26% 31% 17% 8%	300
	11th to 20th	Day	10% 12% 24% 34% 17% 3	300
	21st to end	Day	10% 6% 19% 44% 19% 2	300
December	1st to 10th	Day	9% 8% 20% 46% 15% 3	300
	11th to 20th	Day	9% 10% 24% 43% 13% 1	300
	21st to end	Day	6% 10% 27% 39% 14% 4	330
January	1st to 10th	Night	8% 8% 26% 41% 16% 2	1000
	11th to 20th	Night	9% 10% 34% 32% 13% 3	1000
	21st to end	Night	8% 10% 31% 37% 13% 1	950
February	1st to 10th	Night	7% 8% 33% 40% 11% 2	800
	11th to 20th	Night	9% 7% 31% 40% 12% 1	784
	21st to end	Night	6% 8% 29% 42% 12% 3	660
March	1st to 10th	Night	8% 5% 30% 43% 13% 2	800
	11th to 20th	Night	9% 5% 29% 41% 14% 3	800
	21st to end	Night	15% 6% 27% 35% 16% 2	800
April	1st to 10th	Night	13% 7% 28% 41% 10% 1	600
	11th to 20th	Night	13% 8% 28% 41% 9%	593
	21st to end	Night	15% 6% 22% 42% 14% 1	600
May	1st to 10th	Night	14% 4 20% 41% 20% 2	600
	11th to 20th	Night	17% 5% 16% 40% 22% 1	591
	21st to end	Night	17% 5% 16% 41% 20% 1	634
June	1st to 10th	Night	11% 4 20% 43% 21% 2	570
	11th to 20th	Night	10% 5% 23% 46% 15% 1	570
	21st to end	Night	6% 3 16% 51% 23% 1	570
July	1st to 10th	Night	4 1 14% 49% 30% 2	555
	11th to 20th	Night	3 1 16% 57% 23%	552
	21st to end	Night	2 1 12% 54% 30% 1	624
August	1st to 10th	Night	4 2 11% 57% 25% 1	567
	11th to 20th	Night	5% 2 10% 50% 31% 2	564
	21st to end	Night	3 2 14% 50% 29% 2	627
September	1st to 10th	Night	6% 2 13% 49% 27% 3	570
	11th to 20th	Night	7% 2 14% 47% 25% 4	570
	21st to end	Night	11% 3 17% 41% 22% 6%	703
October	1st to 10th	Night	17% 4 17% 37% 20% 4	760
	11th to 20th	Night	13% 6% 23% 36% 21% 1	760
	21st to end	Night	10% 8% 23% 42% 15% 3	902
November	1st to 10th	Night	10% 5% 24% 44% 14% 2	975
	11th to 20th	Night	8% 10% 29% 40% 12% 1	1000
	21st to end	Night	10% 6% 27% 45% 12% 1	1000
December	1st to 10th	Night	7% 7% 30% 43% 11% 1	1000
	11th to 20th	Night	7% 6% 28% 45% 12% 1	1000
	21st to end	Night	10% 7% 28% 41% 12% 2	1100

Page generated at 2025-05-23 07:20:58. Version ac07

Copyright © 2025 Attilla Danko.