Folsom Lake State Recration Area
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Folsom Lake State Recration Area clear sky chart from 2005-11-06 to 2025-07-21.

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	9% 3 1 3 2 2 2 2 4 5% 65%	124105
	Night	11% 4 1 4 3 2 3 2 4 5% 61%	123881
Transparency	Day	22% 4 8% 18% 25% 23%	123857
	Night	26% 4 8% 19% 23% 20%	123714
Seeing	Day	10% 5% 18% 31% 29% 8%	16081
	Night	12% 3 13% 38% 30% 5%	25378

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	21% 6% 2 3 3 3 3 3 5% 7% 44%	3639
	11th to 20th	Day	17% 6% 2 6% 3 3 2 2 2 3 53%	4007
	21st to end	Day	20% 6% 2 6% 3 3 3 3 4 6% 44%	4510
February	1st to 10th	Day	21% 5% 2 5% 3 3 3 4 4 5% 45%	1800
	11th to 20th	Day	17% 5% 2 5% 4 4 4 3 5% 6% 46%	1884
	21st to end	Day	17% 6% 2 5% 3 3 2 3 4 5% 50%	1650
March	1st to 10th	Day	16% 9% 3 7% 4 4 5% 3 5% 6% 36%	4665
	11th to 20th	Day	17% 5% 2 5% 3 3 3 3 5% 6% 47%	5000
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 5% 8% 39%	5698
April	1st to 10th	Day	14% 6% 2 4 3 3 4 4 5% 6% 49%	2400
	11th to 20th	Day	11% 4 1 4 3 3 3 3 4 7% 56%	2369
	21st to end	Day	8% 4 1 3 2 2 3 3 4 8% 63%	2400
May	1st to 10th	Day	5% 2 1 2 1 1 1 2 2 5% 77%	5553
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 4 80%	6277
	21st to end	Day	2 1 1 2 1 1 1 2 3 5% 81%	6902
June	1st to 10th	Day	5% 3 1 3 3 2 2 3 4 6% 69%	2800
	11th to 20th	Day	1 1 1 2 1 2 2 3 5% 81%	2800
	21st to end	Day	2 1 1 1 1 1 1 1 3 3 85%	2800
July	1st to 10th	Day	1 1 1 1 1 2 93%	4560
	11th to 20th	Day	1 1 1 1 1 1 2 3 3 85%	4560
	21st to end	Day	1 1 1 1 1 2 3 89%	4464
August	1st to 10th	Day	2 2 1 2 1 2 1 2 4 5% 80%	4091
	11th to 20th	Day	1 1 1 1 1 1 2 3 89%	3770
	21st to end	Day	2 1 1 1 1 1 1 2 2 89%	4180
September	1st to 10th	Day	2 1 1 2 2 3 89%	2280
	11th to 20th	Day	3 2 2 1 2 3 2 4 4 76%	2280
	21st to end	Day	3 2 1 2 1 2 2 1 2 4 80%	2052
October	1st to 10th	Day	2 2 1 3 2 2 2 2 4 6% 75%	3180
	11th to 20th	Day	4 2 1 2 2 2 3 2 3 5% 74%	3161
	21st to end	Day	5% 3 2 4 3 3 3 3 4 7% 65%	3448
November	1st to 10th	Day	11% 3 1 4 3 3 3 3 4 8% 57%	1807
	11th to 20th	Day	13% 4 1 4 4 4 4 3 4 7% 52%	1800
	21st to end	Day	13% 4 1 4 3 2 4 4 5% 7% 54%	1666
December	1st to 10th	Day	9% 3 1 4 3 3 3 3 5% 8% 58%	2620
	11th to 20th	Day	28% 7% 2 6% 4 4 4 3 5% 7% 32%	3028
	21st to end	Day	26% 8% 2 6% 4 4 4 4 7% 7% 28%	4004
January	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 5% 9% 45%	6306
	11th to 20th	Night	16% 5% 2 4 3 3 3 2 4 4 54%	5955
	21st to end	Night	18% 6% 2 6% 4 3 3 3 4 5% 46%	6468
February	1st to 10th	Night	20% 6% 2 4 3 3 2 3 3 5% 50%	2600
	11th to 20th	Night	15% 5% 2 4 3 4 4 3 5% 8% 47%	2402
	21st to end	Night	16% 5% 1 4 4 2 4 3 4 5% 52%	1980
March	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 4 6% 38%	5214
	11th to 20th	Night	22% 7% 2 4 2 3 3 2 3 4 49%	5000
	21st to end	Night	21% 6% 2 5% 3 3 4 4 5% 7% 40%	5275
April	1st to 10th	Night	13% 5% 2 5% 3 4 4 4 4 6% 51%	2000
	11th to 20th	Night	10% 4 1 5% 3 3 3 3 5% 7% 57%	1979
	21st to end	Night	7% 3 1 3 2 2 3 3 5% 7% 65%	2000
May	1st to 10th	Night	4 3 1 3 2 1 2 2 3 5% 75%	4433
	11th to 20th	Night	3 1 1 2 2 2 3 2 3 5% 77%	3616
	21st to end	Night	2 1 1 1 1 2 2 2 4 83%	3972
June	1st to 10th	Night	4 4 1 3 3 2 2 3 5% 6% 68%	1600
	11th to 20th	Night	1 1 1 2 1 1 1 1 3 4 84%	1600
	21st to end	Night	2 1 1 1 1 2 2 2 3 84%	1600
July	1st to 10th	Night	1 1 1 1 1 1 94%	2620
	11th to 20th	Night	2 2 2 1 1 1 1 2 3 84%	2620
	21st to end	Night	1 1 1 1 2 1 2 4 86%	3097
August	1st to 10th	Night	3 2 1 2 2 1 2 2 4 5% 77%	2848
	11th to 20th	Night	1 1 1 1 2 1 2 3 88%	3066
	21st to end	Night	3 1 1 1 1 1 1 1 2 88%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 3 89%	1900
	11th to 20th	Night	3 2 1 2 2 1 2 1 3 4 78%	1900
	21st to end	Night	2 1 1 2 2 2 2 3 4 81%	2124
October	1st to 10th	Night	3 2 1 3 2 2 2 2 3 3 78%	3800
	11th to 20th	Night	4 1 1 2 2 1 1 2 3 5% 79%	3777
	21st to end	Night	7% 2 1 3 3 3 3 2 4 5% 67%	4121
November	1st to 10th	Night	13% 4 1 3 2 2 3 3 5% 7% 57%	2492
	11th to 20th	Night	11% 4 2 4 2 3 3 3 5% 7% 56%	2600
	21st to end	Night	14% 4 2 3 2 2 3 3 4 6% 58%	2730
December	1st to 10th	Night	11% 3 1 3 2 3 3 3 4 6% 62%	4560
	11th to 20th	Night	27% 6% 2 5% 4 3 4 3 6% 9% 30%	5176
	21st to end	Night	26% 7% 3 7% 5% 5% 4 4 4 6% 30%	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	40% 6% 19% 18% 13% 5%	3639
	11th to 20th	Day	38% 5% 6% 16% 21% 14%	4007
	21st to end	Day	42% 6% 8% 12% 24% 8%	4510
February	1st to 10th	Day	42% 6% 10% 18% 17% 8%	1800
	11th to 20th	Day	39% 6% 11% 19% 19% 7%	1893
	21st to end	Day	37% 6% 11% 21% 19% 7%	1650
March	1st to 10th	Day	46% 8% 7% 18% 18% 2	4665
	11th to 20th	Day	38% 4 12% 29% 11% 7%	5000
	21st to end	Day	40% 9% 15% 24% 11% 1	5698
April	1st to 10th	Day	34% 7% 11% 25% 17% 6%	2400
	11th to 20th	Day	29% 6% 13% 25% 21% 7%	2372
	21st to end	Day	21% 6% 14% 29% 26% 4	2400
May	1st to 10th	Day	14% 2 7% 27% 32% 18%	5553
	11th to 20th	Day	11% 2 5% 29% 42% 11%	6280
	21st to end	Day	9% 3 9% 30% 40% 9%	6904
June	1st to 10th	Day	16% 5% 10% 26% 30% 12%	2800
	11th to 20th	Day	8% 3 8% 20% 35% 27%	2800
	21st to end	Day	7% 2 3 13% 36% 39%	2800
July	1st to 10th	Day	3 1 1 6% 15% 74%	4491
	11th to 20th	Day	7% 1 2 7% 16% 67%	4485
	21st to end	Day	5% 1 2 5% 24% 63%	4323
August	1st to 10th	Day	10% 2 2 7% 21% 58%	4076
	11th to 20th	Day	4 1 2 6% 36% 51%	3806
	21st to end	Day	6% 1 2 9% 37% 44%	4179
September	1st to 10th	Day	4 1 3 8% 30% 55%	2280
	11th to 20th	Day	12% 3 6% 20% 37% 21%	2280
	21st to end	Day	11% 2 4 18% 35% 30%	2052
October	1st to 10th	Day	12% 3 5% 11% 24% 46%	3180
	11th to 20th	Day	15% 4 5% 22% 35% 19%	3161
	21st to end	Day	21% 4 7% 18% 34% 16%	3448
November	1st to 10th	Day	26% 5% 11% 23% 23% 12%	1807
	11th to 20th	Day	32% 7% 12% 23% 21% 6%	1800
	21st to end	Day	30% 6% 12% 20% 21% 11%	1666
December	1st to 10th	Day	28% 4 9% 28% 23% 8%	2620
	11th to 20th	Day	53% 5% 10% 16% 13% 4	3028
	21st to end	Day	51% 9% 17% 14% 7% 2	4004
January	1st to 10th	Night	35% 7% 19% 22% 10% 7%	6306
	11th to 20th	Night	34% 6% 9% 15% 23% 13%	5955
	21st to end	Night	41% 5% 8% 11% 24% 12%	6468
February	1st to 10th	Night	39% 3 8% 21% 20% 9%	2600
	11th to 20th	Night	36% 5% 9% 22% 20% 7%	2403
	21st to end	Night	33% 6% 9% 25% 20% 7%	1980
March	1st to 10th	Night	46% 8% 7% 18% 17% 5%	5214
	11th to 20th	Night	40% 4 10% 24% 14% 8%	5000
	21st to end	Night	42% 7% 13% 22% 13% 3	5275
April	1st to 10th	Night	34% 6% 12% 21% 21% 6%	2000
	11th to 20th	Night	28% 5% 14% 26% 16% 11%	1979
	21st to end	Night	20% 5% 13% 31% 25% 6%	2000
May	1st to 10th	Night	12% 5% 12% 24% 28% 19%	4430
	11th to 20th	Night	12% 2 7% 32% 37% 10%	3616
	21st to end	Night	8% 2 10% 33% 39% 8%	3972
June	1st to 10th	Night	16% 6% 11% 29% 28% 11%	1600
	11th to 20th	Night	8% 2 7% 21% 34% 28%	1600
	21st to end	Night	8% 2 3 12% 33% 41%	1600
July	1st to 10th	Night	2 1 2 6% 16% 73%	2580
	11th to 20th	Night	9% 1 2 6% 15% 66%	2572
	21st to end	Night	5% 2 2 6% 27% 59%	3000
August	1st to 10th	Night	12% 2 2 8% 25% 51%	2839
	11th to 20th	Night	4 1 2 8% 38% 46%	3095
	21st to end	Night	8% 1 2 9% 35% 46%	3498
September	1st to 10th	Night	5% 1 2 7% 30% 54%	1900
	11th to 20th	Night	12% 3 5% 19% 39% 23%	1900
	21st to end	Night	9% 3 6% 17% 31% 35%	2124
October	1st to 10th	Night	14% 2 3 10% 19% 52%	3800
	11th to 20th	Night	12% 2 6% 26% 31% 23%	3777
	21st to end	Night	22% 3 4 19% 31% 21%	4121
November	1st to 10th	Night	26% 5% 11% 22% 20% 16%	2492
	11th to 20th	Night	27% 4 12% 25% 22% 9%	2600
	21st to end	Night	30% 4 8% 22% 23% 13%	2730
December	1st to 10th	Night	25% 4 9% 27% 25% 11%	4560
	11th to 20th	Night	51% 5% 8% 17% 15% 4	5176
	21st to end	Night	54% 7% 13% 15% 9% 3	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	36% 7% 16% 21% 17% 3	200
	11th to 20th	Day	20% 8% 15% 27% 23% 6%	359
	21st to end	Day	20% 5% 19% 26% 23% 6%	440
February	1st to 10th	Day	25% 9% 14% 23% 23% 6%	400
	11th to 20th	Day	19% 11% 23% 25% 17% 4	462
	21st to end	Day	17% 9% 23% 30% 17% 4	414
March	1st to 10th	Day	22% 13% 23% 26% 13% 3	500
	11th to 20th	Day	22% 10% 24% 23% 16% 5%	500
	21st to end	Day	17% 11% 26% 24% 18% 4	550
April	1st to 10th	Day	16% 10% 25% 24% 20% 5%	500
	11th to 20th	Day	13% 7% 28% 26% 20% 6%	492
	21st to end	Day	7% 7% 22% 31% 23% 10%	500
May	1st to 10th	Day	6% 10% 28% 28% 21% 8%	503
	11th to 20th	Day	7% 7% 23% 28% 28% 8%	528
	21st to end	Day	5% 5% 18% 30% 28% 13%	573
June	1st to 10th	Day	4 5% 18% 31% 32% 10%	530
	11th to 20th	Day	2 5% 21% 36% 31% 5%	530
	21st to end	Day	1 2 15% 35% 38% 9%	530
July	1st to 10th	Day	1 11% 41% 41% 7%	526
	11th to 20th	Day	1 7% 38% 47% 7%	524
	21st to end	Day	1 8% 40% 42% 10%	532
August	1st to 10th	Day	3 12% 40% 37% 7%	487
	11th to 20th	Day	1 1 13% 38% 37% 9%	477
	21st to end	Day	1 1 10% 33% 46% 9%	539
September	1st to 10th	Day	2 1 10% 41% 35% 10%	490
	11th to 20th	Day	5% 3 17% 36% 31% 9%	490
	21st to end	Day	3 1 18% 34% 32% 11%	490
October	1st to 10th	Day	2 6% 19% 30% 31% 13%	490
	11th to 20th	Day	7% 5% 14% 26% 36% 13%	490
	21st to end	Day	9% 6% 15% 30% 29% 11%	539
November	1st to 10th	Day	12% 8% 15% 27% 28% 11%	343
	11th to 20th	Day	13% 3 16% 32% 27% 9%	300
	21st to end	Day	15% 3 19% 25% 31% 7%	233
December	1st to 10th	Day	18% 8% 15% 26% 25% 10%	200
	11th to 20th	Day	20% 7% 21% 24% 20% 9%	200
	21st to end	Day	23% 7% 21% 27% 17% 4	220
January	1st to 10th	Night	22% 4 15% 31% 22% 7%	800
	11th to 20th	Night	17% 3 12% 40% 23% 5%	800
	21st to end	Night	20% 2 13% 36% 22% 6%	880
February	1st to 10th	Night	22% 4 14% 35% 21% 4	800
	11th to 20th	Night	17% 5% 17% 37% 20% 5%	784
	21st to end	Night	18% 4 18% 41% 17% 3	660
March	1st to 10th	Night	22% 6% 15% 33% 20% 4	800
	11th to 20th	Night	22% 4 14% 32% 25% 3	800
	21st to end	Night	19% 4 18% 36% 20% 2	880
April	1st to 10th	Night	15% 3 17% 40% 24% 2	800
	11th to 20th	Night	9% 5% 19% 35% 26% 5%	791
	21st to end	Night	8% 3 15% 41% 29% 4	800
May	1st to 10th	Night	7% 4 21% 38% 27% 3	751
	11th to 20th	Night	6% 2 13% 44% 32% 3	394
	21st to end	Night	6% 1 14% 42% 37% 1	433
June	1st to 10th	Night	5% 2 15% 43% 33% 2	400
	11th to 20th	Night	1 1 14% 50% 32% 2	400
	21st to end	Night	3 1 9% 46% 41% 2	400
July	1st to 10th	Night	1 7% 49% 42% 2	390
	11th to 20th	Night	2 4 55% 38% 1	388
	21st to end	Night	1 4 42% 49% 4	612
August	1st to 10th	Night	3 6% 46% 43% 2	567
	11th to 20th	Night	1 5% 48% 42% 3	716
	21st to end	Night	2 5% 40% 49% 3	836
September	1st to 10th	Night	2 1 6% 39% 49% 5%	760
	11th to 20th	Night	4 1 9% 38% 41% 6%	760
	21st to end	Night	2 1 12% 38% 38% 9%	760
October	1st to 10th	Night	6% 1 11% 41% 34% 8%	760
	11th to 20th	Night	6% 2 11% 35% 40% 6%	760
	21st to end	Night	7% 1 12% 38% 30% 12%	836
November	1st to 10th	Night	13% 3 10% 34% 32% 7%	780
	11th to 20th	Night	12% 2 13% 40% 27% 6%	800
	21st to end	Night	16% 1 14% 38% 24% 7%	800
December	1st to 10th	Night	17% 4 14% 35% 23% 7%	800
	11th to 20th	Night	28% 3 11% 33% 22% 4	800
	21st to end	Night	26% 4 18% 32% 17% 4	880

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