Camp Sherman Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Camp Sherman Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	4 5% 2 6% 5% 5% 5% 5% 6% 7% 50%	135678
	Night	4 6% 2 6% 5% 5% 5% 5% 7% 8% 47%	131020
Transparency	Day	29% 7% 13% 25% 18% 9%	130472
	Night	31% 8% 16% 24% 15% 6%	127221
Seeing	Day	13% 17% 27% 23% 14% 5%	15816
	Night	10% 13% 23% 34% 18% 3	24786

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% 10% 4 13% 9% 9% 9% 7% 8% 9% 16%	3639
	11th to 20th	Day	6% 8% 3 6% 6% 6% 6% 5% 7% 7% 40%	3640
	21st to end	Day	6% 9% 3 8% 5% 5% 5% 5% 7% 7% 40%	4344
February	1st to 10th	Day	5% 9% 5% 10% 8% 6% 7% 6% 8% 9% 27%	1800
	11th to 20th	Day	4 11% 3 7% 6% 6% 6% 6% 9% 9% 33%	1755
	21st to end	Day	3 7% 4 9% 7% 6% 7% 8% 9% 10% 30%	1633
March	1st to 10th	Day	4 7% 3 7% 6% 6% 7% 7% 10% 11% 32%	4606
	11th to 20th	Day	3 10% 3 9% 7% 6% 6% 6% 6% 6% 37%	5000
	21st to end	Day	3 8% 3 11% 8% 8% 9% 7% 9% 10% 24%	5697
April	1st to 10th	Day	5% 11% 3 10% 7% 8% 6% 5% 8% 9% 28%	2400
	11th to 20th	Day	4 9% 4 10% 7% 5% 5% 5% 7% 8% 37%	2369
	21st to end	Day	2 7% 3 9% 6% 6% 6% 6% 7% 9% 41%	2493
May	1st to 10th	Day	7% 6% 2 5% 4 4 5% 5% 6% 6% 50%	6291
	11th to 20th	Day	3 5% 2 6% 5% 5% 6% 5% 7% 8% 48%	6277
	21st to end	Day	4 5% 2 5% 4 4 5% 4 7% 10% 50%	6902
June	1st to 10th	Day	6% 7% 3 7% 5% 5% 5% 4 7% 7% 44%	2800
	11th to 20th	Day	4 5% 2 6% 5% 4 5% 5% 7% 8% 49%	2800
	21st to end	Day	3 3 1 3 3 3 3 4 6% 6% 65%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 2 3 4 83%	6319
	11th to 20th	Day	1 1 1 1 2 2 1 3 3 85%	6320
	21st to end	Day	2 1 2 1 1 2 2 4 5% 80%	6880
August	1st to 10th	Day	2 2 1 2 2 2 2 3 5% 6% 73%	5892
	11th to 20th	Day	1 1 1 2 2 2 3 3 4 7% 73%	5796
	21st to end	Day	5% 3 1 3 2 2 3 3 4 7% 66%	5994
September	1st to 10th	Day	2 2 1 3 2 3 3 2 4 6% 74%	2400
	11th to 20th	Day	3 4 1 5% 5% 4 5% 4 6% 6% 58%	2400
	21st to end	Day	5% 5% 2 4 3 3 4 4 5% 7% 58%	2130
October	1st to 10th	Day	3 4 2 4 4 3 4 4 4 6% 62%	3211
	11th to 20th	Day	3 4 2 5% 4 4 6% 6% 8% 10% 46%	3161
	21st to end	Day	4 7% 2 7% 6% 6% 7% 6% 8% 9% 38%	3323
November	1st to 10th	Day	4 8% 3 9% 7% 7% 7% 6% 8% 9% 32%	1754
	11th to 20th	Day	3 7% 3 10% 9% 9% 8% 7% 10% 10% 25%	1600
	21st to end	Day	4 7% 3 8% 6% 6% 8% 9% 9% 8% 33%	1600
December	1st to 10th	Day	3 5% 2 6% 4 4 5% 5% 8% 8% 50%	2620
	11th to 20th	Day	6% 10% 4 13% 7% 7% 7% 7% 9% 11% 19%	3028
	21st to end	Day	4 10% 5% 11% 8% 8% 8% 7% 9% 9% 22%	4004
January	1st to 10th	Night	5% 10% 4 12% 7% 7% 9% 8% 9% 11% 18%	6306
	11th to 20th	Night	5% 7% 3 8% 5% 5% 7% 5% 6% 8% 40%	6320
	21st to end	Night	7% 10% 3 8% 6% 5% 5% 4 5% 7% 40%	6624
February	1st to 10th	Night	4 10% 4 10% 7% 6% 6% 6% 8% 9% 31%	2600
	11th to 20th	Night	4 11% 3 8% 5% 5% 5% 5% 8% 10% 35%	2532
	21st to end	Night	3 8% 4 10% 6% 5% 6% 6% 9% 10% 33%	1996
March	1st to 10th	Night	3 7% 2 7% 6% 5% 7% 7% 10% 11% 34%	5272
	11th to 20th	Night	2 6% 3 7% 6% 6% 6% 5% 7% 9% 43%	5000
	21st to end	Night	4 7% 3 9% 7% 8% 8% 7% 10% 11% 26%	5277
April	1st to 10th	Night	5% 8% 3 8% 7% 6% 6% 7% 9% 9% 34%	2000
	11th to 20th	Night	3 7% 3 6% 5% 5% 7% 6% 7% 11% 40%	1979
	21st to end	Night	4 6% 2 7% 6% 5% 7% 6% 8% 9% 40%	1904
May	1st to 10th	Night	9% 6% 2 4 3 3 4 4 5% 7% 53%	3668
	11th to 20th	Night	4 3 1 6% 4 5% 6% 6% 9% 9% 47%	3616
	21st to end	Night	5% 4 2 5% 4 4 5% 5% 6% 8% 53%	3972
June	1st to 10th	Night	8% 7% 2 7% 5% 4 5% 4 6% 8% 45%	1600
	11th to 20th	Night	4 4 2 3 5% 4 6% 4 5% 8% 54%	1600
	21st to end	Night	4 3 1 3 3 3 3 3 5% 6% 67%	1600
July	1st to 10th	Night	1 2 1 2 2 2 3 2 3 3 80%	3640
	11th to 20th	Night	1 1 1 1 1 2 1 2 3 85%	3640
	21st to end	Night	2 2 1 2 1 2 2 2 4 3 79%	3970
August	1st to 10th	Night	1 2 1 3 3 3 3 3 6% 7% 69%	3898
	11th to 20th	Night	2 2 1 3 2 3 3 2 4 6% 73%	4052
	21st to end	Night	5% 2 1 3 2 2 3 2 4 6% 69%	5005
September	1st to 10th	Night	2 1 2 2 2 2 3 4 5% 77%	2000
	11th to 20th	Night	3 3 1 3 3 4 5% 4 6% 8% 62%	2000
	21st to end	Night	5% 4 1 4 2 3 4 3 6% 7% 61%	2265
October	1st to 10th	Night	2 4 2 4 3 3 3 3 5% 6% 66%	3836
	11th to 20th	Night	3 3 2 5% 4 5% 5% 5% 7% 10% 51%	3777
	21st to end	Night	4 5% 2 7% 6% 6% 7% 6% 9% 10% 39%	4241
November	1st to 10th	Night	4 7% 3 8% 7% 6% 8% 6% 8% 9% 33%	2542
	11th to 20th	Night	4 7% 3 8% 8% 8% 7% 6% 9% 10% 31%	2800
	21st to end	Night	2 6% 3 7% 6% 6% 6% 7% 8% 11% 37%	2800
December	1st to 10th	Night	3 4 2 6% 4 4 4 4 6% 9% 53%	4560
	11th to 20th	Night	6% 10% 4 11% 7% 7% 8% 6% 8% 10% 21%	5176
	21st to end	Night	7% 9% 4 10% 7% 6% 7% 6% 9% 9% 28%	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	54% 12% 15% 15% 3	3639
	11th to 20th	Day	35% 11% 18% 22% 13% 3	3640
	21st to end	Day	40% 7% 13% 19% 15% 7%	4344
February	1st to 10th	Day	47% 11% 17% 18% 7%	1800
	11th to 20th	Day	40% 11% 17% 22% 9% 1	1764
	21st to end	Day	40% 12% 21% 19% 7% 1	1633
March	1st to 10th	Day	35% 9% 14% 30% 12% 1	4606
	11th to 20th	Day	40% 9% 12% 26% 12%	5000
	21st to end	Day	45% 11% 21% 17% 5%	5697
April	1st to 10th	Day	48% 8% 16% 19% 7% 1	2400
	11th to 20th	Day	41% 8% 15% 23% 11% 3	2372
	21st to end	Day	36% 7% 20% 25% 10% 2	2493
May	1st to 10th	Day	28% 5% 12% 33% 19% 2	6291
	11th to 20th	Day	27% 9% 15% 29% 19% 2	6280
	21st to end	Day	26% 6% 23% 36% 8%	6904
June	1st to 10th	Day	35% 6% 14% 30% 12% 2	2800
	11th to 20th	Day	28% 5% 15% 33% 15% 3	2800
	21st to end	Day	18% 4 14% 33% 23% 8%	2800
July	1st to 10th	Day	8% 2 5% 18% 29% 38%	6251
	11th to 20th	Day	7% 1 4 10% 25% 52%	6245
	21st to end	Day	9% 3 5% 28% 35% 20%	6681
August	1st to 10th	Day	13% 3 5% 20% 37% 22%	4285
	11th to 20th	Day	13% 3 7% 35% 31% 10%	4195
	21st to end	Day	16% 3 7% 29% 31% 14%	4321
September	1st to 10th	Day	13% 3 8% 27% 33% 17%	2400
	11th to 20th	Day	23% 5% 11% 28% 24% 8%	2400
	21st to end	Day	25% 5% 12% 26% 23% 9%	2130
October	1st to 10th	Day	22% 6% 9% 24% 28% 12%	3211
	11th to 20th	Day	26% 10% 17% 29% 16% 2	3161
	21st to end	Day	36% 8% 13% 27% 15% 1	3323
November	1st to 10th	Day	42% 8% 17% 22% 10% 1	1754
	11th to 20th	Day	43% 15% 20% 15% 8%	1600
	21st to end	Day	38% 12% 18% 18% 14% 1	1600
December	1st to 10th	Day	27% 10% 14% 25% 22% 2	2620
	11th to 20th	Day	51% 14% 17% 16% 3	3028
	21st to end	Day	48% 11% 16% 20% 4 1	4004
January	1st to 10th	Night	49% 14% 19% 15% 3	6306
	11th to 20th	Night	35% 11% 15% 24% 13% 2	6320
	21st to end	Night	42% 6% 13% 15% 17% 6%	6624
February	1st to 10th	Night	42% 13% 20% 15% 9%	2600
	11th to 20th	Night	40% 10% 20% 21% 7% 2	2534
	21st to end	Night	39% 11% 21% 21% 8% 1	1996
March	1st to 10th	Night	34% 10% 16% 30% 8% 2	5272
	11th to 20th	Night	31% 8% 19% 32% 9%	5000
	21st to end	Night	42% 13% 23% 19% 3	5277
April	1st to 10th	Night	39% 9% 25% 19% 7% 1	2000
	11th to 20th	Night	34% 10% 21% 24% 9% 2	1979
	21st to end	Night	35% 10% 21% 23% 9% 2	1904
May	1st to 10th	Night	28% 6% 12% 33% 22%	3665
	11th to 20th	Night	23% 12% 19% 32% 13% 1	3616
	21st to end	Night	25% 6% 35% 28% 6%	3972
June	1st to 10th	Night	36% 7% 16% 30% 11% 1	1600
	11th to 20th	Night	26% 7% 16% 37% 13% 2	1600
	21st to end	Night	18% 3 15% 36% 23% 6%	1600
July	1st to 10th	Night	11% 3 7% 23% 24% 33%	3600
	11th to 20th	Night	7% 2 5% 12% 26% 49%	3592
	21st to end	Night	10% 3 5% 32% 32% 18%	3857
August	1st to 10th	Night	12% 3 6% 20% 41% 17%	2831
	11th to 20th	Night	12% 3 9% 39% 28% 8%	2929
	21st to end	Night	13% 3 10% 30% 33% 11%	3598
September	1st to 10th	Night	10% 3 8% 30% 34% 14%	2000
	11th to 20th	Night	19% 6% 14% 27% 24% 9%	2000
	21st to end	Night	21% 4 13% 30% 23% 10%	2265
October	1st to 10th	Night	18% 5% 11% 26% 31% 8%	3836
	11th to 20th	Night	24% 8% 22% 29% 14% 2	3777
	21st to end	Night	33% 10% 15% 29% 12% 1	4241
November	1st to 10th	Night	41% 10% 16% 20% 11% 2	2542
	11th to 20th	Night	39% 15% 18% 20% 7%	2800
	21st to end	Night	34% 11% 20% 19% 16% 1	2800
December	1st to 10th	Night	27% 7% 13% 26% 27% 1	4560
	11th to 20th	Night	51% 13% 17% 15% 4	5176
	21st to end	Night	44% 13% 19% 20% 4 1	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	24% 16% 27% 21% 11% 2	200
	11th to 20th	Day	20% 10% 26% 26% 12% 7%	200
	21st to end	Day	17% 19% 23% 23% 13% 5%	365
February	1st to 10th	Day	21% 25% 24% 20% 9% 2	400
	11th to 20th	Day	20% 26% 31% 14% 7% 1	395
	21st to end	Day	21% 21% 29% 20% 8% 1	407
March	1st to 10th	Day	18% 25% 29% 20% 7% 1	500
	11th to 20th	Day	20% 20% 28% 21% 9% 1	500
	21st to end	Day	26% 23% 26% 17% 6% 1	550
April	1st to 10th	Day	22% 19% 29% 16% 11% 2	500
	11th to 20th	Day	18% 25% 29% 17% 6% 4	492
	21st to end	Day	16% 25% 28% 18% 9% 3	500
May	1st to 10th	Day	16% 18% 29% 22% 10% 4	503
	11th to 20th	Day	20% 18% 26% 20% 10% 6%	528
	21st to end	Day	15% 14% 26% 25% 15% 5%	573
June	1st to 10th	Day	12% 20% 25% 22% 14% 7%	530
	11th to 20th	Day	14% 24% 28% 20% 11% 3	530
	21st to end	Day	6% 16% 25% 28% 18% 7%	530
July	1st to 10th	Day	3 12% 35% 27% 16% 7%	526
	11th to 20th	Day	2 13% 31% 26% 19% 8%	524
	21st to end	Day	3 10% 31% 28% 19% 9%	575
August	1st to 10th	Day	3 10% 29% 30% 19% 10%	527
	11th to 20th	Day	5% 8% 26% 31% 19% 12%	517
	21st to end	Day	5% 13% 27% 28% 18% 9%	583
September	1st to 10th	Day	5% 12% 23% 30% 23% 8%	530
	11th to 20th	Day	7% 13% 24% 30% 21% 5%	530
	21st to end	Day	12% 15% 22% 29% 19% 4	530
October	1st to 10th	Day	12% 17% 23% 26% 19% 3	503
	11th to 20th	Day	12% 19% 22% 24% 19% 4	490
	21st to end	Day	18% 20% 24% 18% 15% 5%	463
November	1st to 10th	Day	16% 17% 24% 25% 14% 4	295
	11th to 20th	Day	19% 21% 21% 19% 15% 6%	200
	21st to end	Day	19% 13% 24% 23% 17% 6%	200
December	1st to 10th	Day	18% 14% 22% 19% 18% 11%	200
	11th to 20th	Day	17% 23% 25% 23% 11% 2	200
	21st to end	Day	22% 18% 23% 21% 13% 2	220
January	1st to 10th	Night	17% 18% 22% 27% 14% 3	800
	11th to 20th	Night	15% 14% 23% 29% 17% 3	800
	21st to end	Night	15% 10% 23% 35% 14% 4	880
February	1st to 10th	Night	12% 17% 25% 30% 14% 2	800
	11th to 20th	Night	13% 19% 24% 30% 13% 1	784
	21st to end	Night	14% 20% 25% 33% 7%	660
March	1st to 10th	Night	13% 19% 29% 27% 12% 1	800
	11th to 20th	Night	11% 17% 24% 32% 15% 1	800
	21st to end	Night	12% 22% 28% 26% 11% 1	880
April	1st to 10th	Night	12% 19% 25% 31% 12% 1	800
	11th to 20th	Night	10% 19% 26% 29% 14% 3	791
	21st to end	Night	10% 17% 27% 31% 14% 1	704
May	1st to 10th	Night	10% 12% 23% 34% 19% 2	415
	11th to 20th	Night	10% 11% 24% 33% 21% 1	394
	21st to end	Night	8% 7% 23% 40% 20% 2	433
June	1st to 10th	Night	12% 8% 26% 34% 18% 2	400
	11th to 20th	Night	9% 11% 29% 36% 16% 1	400
	21st to end	Night	6% 8% 21% 42% 23% 1	400
July	1st to 10th	Night	2 4 21% 47% 25% 1	390
	11th to 20th	Night	1 3 20% 55% 20% 1	388
	21st to end	Night	2 4 15% 48% 30% 2	438
August	1st to 10th	Night	2 2 14% 42% 35% 5%	537
	11th to 20th	Night	3 3 12% 38% 36% 9%	594
	21st to end	Night	3 5% 21% 42% 26% 3	870
September	1st to 10th	Night	2 7% 17% 39% 32% 4	800
	11th to 20th	Night	5% 5% 18% 39% 28% 6%	800
	21st to end	Night	9% 8% 17% 35% 27% 4	800
October	1st to 10th	Night	6% 7% 24% 36% 23% 4	772
	11th to 20th	Night	8% 12% 19% 35% 21% 5%	760
	21st to end	Night	12% 13% 24% 31% 15% 5%	836
November	1st to 10th	Night	12% 12% 23% 38% 13% 1	780
	11th to 20th	Night	14% 19% 24% 30% 13% 1	800
	21st to end	Night	9% 15% 22% 36% 16% 3	800
December	1st to 10th	Night	11% 14% 22% 32% 18% 3	800
	11th to 20th	Night	14% 15% 25% 31% 13% 2	800
	21st to end	Night	16% 20% 24% 29% 9% 1	880

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