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	14% 8% 3 8% 5% 5% 5% 4 5% 6% 37%	118504
	Night	16% 9% 3 7% 5% 4 4 3 5% 6% 38%	119546
Transparency	Day	48% 2 13% 20% 13% 4	118249
	Night	49% 2 13% 21% 12% 3	119390
Seeing	Day	17% 7% 20% 26% 21% 10%	15551
	Night	18% 4 18% 36% 22% 2	24361

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	29% 14% 5% 11% 6% 6% 6% 3 4 5% 9%	3639
	11th to 20th	Day	21% 9% 3 9% 6% 5% 5% 4 5% 6% 26%	3640
	21st to end	Day	22% 10% 3 9% 6% 5% 4 3 4 3 31%	4508
February	1st to 10th	Day	21% 14% 5% 13% 6% 6% 5% 5% 5% 6% 15%	1800
	11th to 20th	Day	22% 13% 4 11% 7% 6% 4 5% 5% 5% 18%	1755
	21st to end	Day	19% 15% 4 11% 7% 6% 5% 4 5% 6% 19%	1539
March	1st to 10th	Day	14% 11% 4 10% 7% 7% 7% 5% 7% 7% 20%	4821
	11th to 20th	Day	22% 14% 4 9% 5% 5% 5% 3 3 4 27%	5000
	21st to end	Day	20% 12% 4 12% 8% 7% 7% 4 5% 6% 13%	5529
April	1st to 10th	Day	19% 13% 4 11% 7% 6% 5% 4 6% 6% 19%	2400
	11th to 20th	Day	17% 12% 4 9% 6% 5% 5% 4 5% 4 28%	2393
	21st to end	Day	15% 10% 3 9% 6% 6% 6% 5% 7% 7% 25%	2600
May	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 4 6% 6% 36%	4418
	11th to 20th	Day	8% 6% 2 5% 4 3 4 4 5% 8% 52%	4517
	21st to end	Day	12% 7% 2 8% 6% 6% 7% 6% 9% 8% 28%	4886
June	1st to 10th	Day	15% 9% 4 8% 5% 4 5% 4 6% 7% 32%	2660
	11th to 20th	Day	11% 8% 3 8% 7% 6% 6% 6% 7% 8% 30%	2660
	21st to end	Day	6% 4 2 6% 4 4 4 3 5% 7% 54%	2841
July	1st to 10th	Day	3 2 1 3 2 2 3 2 3 4 74%	4490
	11th to 20th	Day	3 2 1 2 2 2 2 3 3 4 76%	4420
	21st to end	Day	4 2 1 2 2 2 2 2 3 5% 75%	4734
August	1st to 10th	Day	1 3 1 3 3 3 3 3 4 6% 70%	4091
	11th to 20th	Day	5% 3 2 5% 4 4 4 4 7% 8% 55%	4026
	21st to end	Day	5% 6% 2 4 3 4 3 3 5% 8% 57%	4340
September	1st to 10th	Day	4 3 1 4 2 3 4 3 5% 7% 64%	2280
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 4 6% 8% 45%	2242
	21st to end	Day	13% 5% 2 6% 4 5% 4 3 4 6% 47%	2020
October	1st to 10th	Day	10% 5% 2 5% 4 4 3 3 5% 6% 51%	3180
	11th to 20th	Day	13% 7% 3 9% 5% 7% 7% 5% 7% 7% 31%	3161
	21st to end	Day	20% 9% 3 9% 5% 5% 5% 4 5% 8% 26%	3431
November	1st to 10th	Day	20% 14% 5% 11% 7% 5% 5% 4 5% 6% 17%	1631
	11th to 20th	Day	21% 13% 5% 11% 7% 5% 5% 4 5% 5% 18%	1600
	21st to end	Day	19% 12% 4 12% 7% 6% 6% 4 5% 6% 21%	1600
December	1st to 10th	Day	16% 8% 3 10% 4 5% 6% 4 5% 6% 33%	2620
	11th to 20th	Day	26% 13% 5% 13% 7% 6% 5% 4 4 5% 11%	3028
	21st to end	Day	30% 15% 6% 13% 6% 6% 6% 4 4 4 6%	4004
January	1st to 10th	Night	30% 13% 5% 11% 6% 6% 5% 3 5% 4 13%	6306
	11th to 20th	Night	18% 12% 5% 9% 5% 4 4 3 4 5% 33%	6320
	21st to end	Night	24% 9% 3 8% 5% 4 3 2 3 4 34%	6468
February	1st to 10th	Night	17% 13% 4 11% 8% 5% 5% 5% 6% 6% 21%	2600
	11th to 20th	Night	20% 11% 3 8% 5% 5% 5% 5% 5% 6% 26%	2532
	21st to end	Night	18% 12% 4 10% 7% 5% 5% 4 5% 6% 25%	2090
March	1st to 10th	Night	13% 9% 4 10% 8% 8% 7% 6% 6% 6% 25%	5069
	11th to 20th	Night	20% 10% 3 9% 6% 5% 4 4 4 5% 31%	5000
	21st to end	Night	23% 12% 3 10% 7% 5% 6% 4 6% 7% 17%	5433
April	1st to 10th	Night	17% 11% 3 9% 7% 6% 5% 4 4 6% 27%	2000
	11th to 20th	Night	13% 9% 3 8% 6% 5% 4 4 6% 7% 34%	1950
	21st to end	Night	12% 9% 3 8% 4 4 5% 5% 7% 7% 35%	1800
May	1st to 10th	Night	17% 7% 2 6% 4 3 3 3 4 5% 46%	2764
	11th to 20th	Night	6% 4 2 5% 3 3 3 3 6% 7% 58%	2596
	21st to end	Night	6% 5% 2 6% 6% 5% 6% 6% 8% 9% 41%	2805
June	1st to 10th	Night	13% 6% 3 7% 5% 5% 5% 3 5% 6% 42%	1520
	11th to 20th	Night	7% 6% 3 6% 5% 4 4 4 6% 8% 47%	1516
	21st to end	Night	5% 5% 2 5% 4 3 3 2 4 6% 62%	1330
July	1st to 10th	Night	2 1 2 1 1 1 1 2 5% 84%	2444
	11th to 20th	Night	3 2 1 2 1 2 1 2 2 4 80%	2540
	21st to end	Night	2 1 2 1 1 2 1 4 4 82%	2815
August	1st to 10th	Night	1 2 1 2 1 1 2 2 4 5% 79%	2848
	11th to 20th	Night	5% 3 1 4 3 2 3 3 5% 7% 65%	2812
	21st to end	Night	6% 3 1 4 3 3 2 2 4 6% 66%	3333
September	1st to 10th	Night	4 2 1 2 2 3 3 2 5% 6% 70%	1900
	11th to 20th	Night	7% 5% 2 5% 4 5% 3 4 5% 6% 54%	1933
	21st to end	Night	8% 5% 1 5% 4 4 4 3 5% 7% 55%	2159
October	1st to 10th	Night	7% 3 1 5% 3 3 4 3 5% 6% 60%	3800
	11th to 20th	Night	13% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	3777
	21st to end	Night	20% 9% 3 8% 4 5% 4 4 5% 6% 31%	4135
November	1st to 10th	Night	22% 8% 4 9% 6% 6% 5% 4 6% 6% 26%	2663
	11th to 20th	Night	24% 12% 4 9% 5% 5% 5% 4 5% 5% 22%	2800
	21st to end	Night	17% 11% 4 9% 6% 5% 6% 4 5% 5% 29%	2800
December	1st to 10th	Night	13% 8% 3 8% 5% 4 4 3 6% 6% 41%	4560
	11th to 20th	Night	31% 15% 5% 10% 6% 5% 4 3 4 4 13%	5176
	21st to end	Night	29% 13% 5% 11% 7% 5% 5% 3 4 4 15%	6952

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	79% 2 6% 9% 4	3639
	11th to 20th	Day	59% 2 10% 22% 7% 1	3640
	21st to end	Day	59% 2 8% 14% 15% 3	4508
February	1st to 10th	Day	69% 4 10% 12% 4	1800
	11th to 20th	Day	68% 4 10% 10% 5% 2	1764
	21st to end	Day	66% 3 12% 11% 7% 1	1539
March	1st to 10th	Day	61% 1 17% 16% 5%	4821
	11th to 20th	Day	62% 2 7% 20% 8%	5000
	21st to end	Day	71% 2 14% 9% 3	5529
April	1st to 10th	Day	64% 4 13% 15% 4	2400
	11th to 20th	Day	57% 4 11% 16% 10% 2	2396
	21st to end	Day	55% 5% 15% 16% 8% 1	2600
May	1st to 10th	Day	48% 2 11% 25% 13%	4418
	11th to 20th	Day	32% 2 18% 32% 15% 1	4520
	21st to end	Day	50% 5% 25% 18% 2	4887
June	1st to 10th	Day	50% 3 17% 22% 7% 1	2660
	11th to 20th	Day	49% 4 20% 21% 6%	2660
	21st to end	Day	29% 3 20% 31% 12% 4	2841
July	1st to 10th	Day	16% 2 9% 19% 24% 30%	4421
	11th to 20th	Day	13% 1 7% 22% 42% 14%	4345
	21st to end	Day	15% 2 11% 30% 37% 5%	4584
August	1st to 10th	Day	17% 1 11% 33% 26% 12%	4076
	11th to 20th	Day	27% 1 21% 33% 13% 4	4065
	21st to end	Day	27% 2 15% 28% 23% 5%	4339
September	1st to 10th	Day	20% 3 14% 31% 21% 11%	2280
	11th to 20th	Day	37% 3 16% 23% 16% 5%	2242
	21st to end	Day	39% 3 14% 24% 15% 5%	2020
October	1st to 10th	Day	34% 2 9% 29% 24% 2	3180
	11th to 20th	Day	52% 3 16% 17% 12% 1	3161
	21st to end	Day	58% 1 9% 20% 11% 1	3431
November	1st to 10th	Day	66% 4 10% 15% 4	1631
	11th to 20th	Day	66% 3 12% 12% 6% 1	1600
	21st to end	Day	63% 3 9% 13% 11% 1	1600
December	1st to 10th	Day	52% 2 11% 22% 13%	2620
	11th to 20th	Day	76% 2 11% 10% 1	3028
	21st to end	Day	83% 2 7% 6% 3	4004
January	1st to 10th	Night	76% 1 8% 9% 5% 1	6306
	11th to 20th	Night	56% 1 9% 24% 8% 1	6320
	21st to end	Night	57% 1 7% 15% 16% 4	6468
February	1st to 10th	Night	63% 5% 12% 15% 5%	2600
	11th to 20th	Night	58% 3 15% 15% 7% 2	2534
	21st to end	Night	60% 4 13% 14% 8% 1	2090
March	1st to 10th	Night	58% 2 12% 20% 7%	5069
	11th to 20th	Night	57% 2 12% 22% 7%	5000
	21st to end	Night	67% 3 15% 14% 2	5433
April	1st to 10th	Night	59% 4 15% 16% 7%	2000
	11th to 20th	Night	48% 4 17% 19% 10% 3	1950
	21st to end	Night	47% 5% 16% 23% 7% 2	1800
May	1st to 10th	Night	42% 1 13% 31% 11% 1	2764
	11th to 20th	Night	28% 2 16% 37% 16% 2	2596
	21st to end	Night	38% 4 36% 19% 3	2805
June	1st to 10th	Night	43% 3 18% 29% 7% 1	1520
	11th to 20th	Night	35% 3 23% 31% 8%	1516
	21st to end	Night	25% 2 21% 35% 12% 4	1330
July	1st to 10th	Night	9% 1 10% 22% 26% 32%	2404
	11th to 20th	Night	12% 1 9% 20% 35% 24%	2492
	21st to end	Night	9% 1 12% 30% 40% 8%	2727
August	1st to 10th	Night	10% 1 9% 37% 36% 7%	2839
	11th to 20th	Night	22% 1 24% 36% 12% 5%	2839
	21st to end	Night	22% 2 18% 28% 25% 6%	3333
September	1st to 10th	Night	16% 3 14% 32% 24% 11%	1900
	11th to 20th	Night	32% 3 16% 26% 15% 9%	1933
	21st to end	Night	30% 3 17% 24% 19% 7%	2159
October	1st to 10th	Night	27% 2 10% 37% 22% 3	3800
	11th to 20th	Night	42% 2 23% 20% 11% 1	3777
	21st to end	Night	54% 2 11% 21% 10% 2	4135
November	1st to 10th	Night	58% 5% 12% 18% 7%	2663
	11th to 20th	Night	64% 3 12% 13% 7% 2	2800
	21st to end	Night	56% 4 12% 13% 13% 2	2800
December	1st to 10th	Night	44% 3 9% 28% 16%	4560
	11th to 20th	Night	76% 2 8% 10% 4	5176
	21st to end	Night	74% 2 11% 10% 2	6952

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% 10% 21% 21% 10% 5%	200
	11th to 20th	Day	27% 6% 18% 29% 12% 9%	200
	21st to end	Day	25% 6% 17% 25% 18% 8%	438
February	1st to 10th	Day	26% 9% 22% 23% 14% 7%	400
	11th to 20th	Day	31% 8% 23% 23% 12% 4	395
	21st to end	Day	24% 16% 23% 21% 12% 3	359
March	1st to 10th	Day	25% 7% 27% 25% 14% 3	500
	11th to 20th	Day	27% 9% 21% 24% 15% 4	500
	21st to end	Day	28% 11% 21% 24% 14% 2	550
April	1st to 10th	Day	25% 13% 19% 21% 16% 5%	500
	11th to 20th	Day	23% 10% 25% 22% 13% 7%	492
	21st to end	Day	19% 9% 26% 25% 16% 6%	500
May	1st to 10th	Day	15% 11% 24% 23% 19% 8%	503
	11th to 20th	Day	17% 11% 21% 24% 17% 11%	528
	21st to end	Day	11% 9% 20% 25% 22% 13%	550
June	1st to 10th	Day	16% 8% 20% 26% 18% 12%	490
	11th to 20th	Day	13% 9% 24% 30% 19% 5%	490
	21st to end	Day	5% 7% 26% 28% 24% 10%	671
July	1st to 10th	Day	5% 3 16% 33% 30% 12%	530
	11th to 20th	Day	3 4 17% 28% 32% 16%	484
	21st to end	Day	2 2 14% 29% 34% 18%	531
August	1st to 10th	Day	3 3 11% 32% 30% 22%	487
	11th to 20th	Day	4 2 10% 26% 33% 25%	477
	21st to end	Day	6% 3 16% 31% 28% 16%	539
September	1st to 10th	Day	6% 3 18% 28% 28% 18%	490
	11th to 20th	Day	10% 5% 15% 28% 29% 12%	490
	21st to end	Day	14% 5% 16% 30% 26% 9%	490
October	1st to 10th	Day	14% 7% 19% 24% 22% 13%	490
	11th to 20th	Day	19% 6% 17% 23% 24% 11%	490
	21st to end	Day	24% 8% 22% 26% 13% 7%	534
November	1st to 10th	Day	25% 7% 24% 26% 11% 6%	233
	11th to 20th	Day	22% 10% 25% 20% 18% 7%	200
	21st to end	Day	23% 7% 19% 23% 18% 11%	200
December	1st to 10th	Day	25% 4 17% 27% 17% 11%	200
	11th to 20th	Day	36% 3 23% 25% 10% 3	200
	21st to end	Day	33% 6% 24% 23% 12% 2	220
January	1st to 10th	Night	30% 6% 20% 27% 14% 3	800
	11th to 20th	Night	27% 6% 18% 28% 20% 3	800
	21st to end	Night	25% 4 15% 35% 19% 3	880
February	1st to 10th	Night	23% 4 22% 32% 17% 2	800
	11th to 20th	Night	25% 5% 21% 30% 18% 1	784
	21st to end	Night	25% 7% 22% 35% 12%	660
March	1st to 10th	Night	21% 6% 21% 35% 15% 2	800
	11th to 20th	Night	23% 4 21% 34% 18% 1	800
	21st to end	Night	26% 5% 22% 31% 16% 1	880
April	1st to 10th	Night	23% 6% 20% 33% 18% 1	800
	11th to 20th	Night	17% 6% 25% 36% 14% 2	762
	21st to end	Night	16% 3 19% 42% 20% 1	600
May	1st to 10th	Night	13% 3 21% 46% 17% 1	492
	11th to 20th	Night	12% 4 18% 41% 23% 2	394
	21st to end	Night	13% 3 18% 43% 21% 2	422
June	1st to 10th	Night	16% 3 16% 43% 22% 1	380
	11th to 20th	Night	9% 3 23% 45% 20% 1	380
	21st to end	Night	7% 2 16% 49% 25%	380
July	1st to 10th	Night	4 1 11% 55% 28% 1	370
	11th to 20th	Night	4 1 18% 50% 27%	368
	21st to end	Night	3 13% 49% 33% 2	446
August	1st to 10th	Night	2 1 10% 43% 40% 4	567
	11th to 20th	Night	5% 7% 41% 44% 3	564
	21st to end	Night	5% 1 14% 44% 32% 3	736
September	1st to 10th	Night	5% 1 15% 38% 39% 2	760
	11th to 20th	Night	8% 3 10% 39% 32% 7%	760
	21st to end	Night	11% 3 14% 37% 31% 5%	760
October	1st to 10th	Night	13% 2 17% 39% 26% 4	760
	11th to 20th	Night	17% 3 14% 35% 27% 5%	760
	21st to end	Night	21% 2 23% 30% 19% 5%	836
November	1st to 10th	Night	24% 3 17% 38% 17%	780
	11th to 20th	Night	29% 7% 20% 27% 16% 2	800
	21st to end	Night	22% 4 18% 34% 18% 3	800
December	1st to 10th	Night	21% 4 17% 35% 20% 4	800
	11th to 20th	Night	30% 6% 22% 28% 13% 1	800
	21st to end	Night	32% 6% 23% 26% 12% 1	880

Page generated at 2025-05-23 06:21:03. Version ac07

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