Little Grass Valley Reservoir
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Little Grass Valley Reservoir clear sky chart from 2005-11-06 to 2025-09-19.

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% 9% 2 4 2 2 3 2 3 5% 63%	135070
	Night	7% 10% 2 4 3 3 3 3 4 5% 57%	131376
Transparency	Day	26% 3 6% 18% 26% 20%	129991
	Night	31% 4 7% 17% 24% 18%	127439
Seeing	Day	16% 15% 30% 25% 11% 3	16194
	Night	16% 8% 18% 33% 22% 3	25419

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	9% 17% 3 5% 4 4 3 3 5% 7% 41%	3639
	11th to 20th	Day	6% 15% 2 6% 4 3 3 2 2 4 53%	3866
	21st to end	Day	9% 16% 3 6% 3 3 4 2 3 5% 46%	4510
February	1st to 10th	Day	15% 13% 3 4 3 3 3 3 5% 7% 41%	1800
	11th to 20th	Day	10% 17% 3 5% 3 3 3 4 5% 5% 42%	1870
	21st to end	Day	9% 19% 4 4 3 3 4 3 4 6% 42%	1650
March	1st to 10th	Day	9% 26% 8% 6% 3 3 3 2 3 5% 31%	4606
	11th to 20th	Day	7% 19% 5% 6% 3 2 3 2 4 5% 44%	5000
	21st to end	Day	9% 28% 4 6% 3 3 3 3 4 5% 31%	5722
April	1st to 10th	Day	11% 16% 2 4 4 4 4 4 5% 6% 40%	2400
	11th to 20th	Day	9% 12% 2 5% 3 3 4 4 5% 6% 47%	2369
	21st to end	Day	9% 9% 2 4 3 3 3 3 5% 7% 52%	2400
May	1st to 10th	Day	5% 6% 1 3 2 2 2 2 3 5% 68%	5735
	11th to 20th	Day	3 7% 2 4 2 2 2 2 3 5% 68%	6277
	21st to end	Day	4 3 1 2 2 2 2 2 3 5% 73%	6902
June	1st to 10th	Day	7% 5% 1 4 3 3 3 3 6% 7% 58%	2800
	11th to 20th	Day	3 3 1 3 2 2 3 2 4 5% 74%	2800
	21st to end	Day	3 1 1 2 1 2 2 1 3 4 81%	2800
July	1st to 10th	Day	1 1 1 1 1 1 1 2 92%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 88%	6320
	21st to end	Day	1 1 1 2 2 2 2 2 3 4 81%	6579
August	1st to 10th	Day	2 2 1 2 1 1 2 2 3 3 82%	5777
	11th to 20th	Day	1 1 1 1 1 2 2 90%	5476
	21st to end	Day	2 2 1 2 1 2 2 1 3 3 81%	5984
September	1st to 10th	Day	2 1 1 1 1 2 1 2 4 83%	2400
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 5% 72%	2377
	21st to end	Day	4 3 1 2 1 1 2 3 3 4 75%	2052
October	1st to 10th	Day	4 3 1 3 2 2 2 2 4 5% 71%	3180
	11th to 20th	Day	8% 4 1 2 2 2 3 2 4 6% 65%	3161
	21st to end	Day	6% 7% 2 5% 3 3 3 3 4 6% 57%	3448
November	1st to 10th	Day	12% 9% 1 5% 3 4 2 3 5% 5% 51%	1769
	11th to 20th	Day	12% 8% 2 5% 4 4 4 4 5% 7% 46%	1800
	21st to end	Day	9% 12% 2 4 3 4 4 3 4 5% 50%	1630
December	1st to 10th	Day	9% 6% 2 3 3 3 4 4 5% 6% 55%	2620
	11th to 20th	Day	16% 17% 4 6% 4 4 5% 3 6% 7% 28%	3028
	21st to end	Day	16% 15% 3 6% 4 3 4 4 6% 8% 32%	4004
January	1st to 10th	Night	6% 13% 4 7% 4 4 5% 4 6% 7% 40%	6306
	11th to 20th	Night	7% 13% 3 5% 3 3 3 3 4 5% 52%	6100
	21st to end	Night	9% 16% 3 6% 4 4 4 3 3 5% 44%	6468
February	1st to 10th	Night	15% 11% 3 5% 3 3 3 3 5% 5% 44%	2600
	11th to 20th	Night	9% 15% 2 4 4 4 5% 3 6% 6% 42%	2416
	21st to end	Night	12% 15% 3 5% 3 3 3 2 4 6% 43%	1980
March	1st to 10th	Night	9% 27% 8% 6% 3 2 3 2 4 5% 32%	5271
	11th to 20th	Night	9% 24% 5% 4 2 2 2 2 3 4 44%	5000
	21st to end	Night	10% 25% 4 7% 3 4 4 3 4 5% 30%	5253
April	1st to 10th	Night	10% 14% 3 6% 4 3 3 4 5% 5% 44%	2000
	11th to 20th	Night	10% 11% 3 5% 3 3 3 3 5% 6% 48%	1979
	21st to end	Night	8% 9% 1 4 3 3 3 3 5% 7% 55%	2000
May	1st to 10th	Night	4 8% 2 4 2 2 3 2 3 5% 64%	4248
	11th to 20th	Night	4 5% 2 3 3 3 2 2 4 4 68%	3616
	21st to end	Night	4 3 1 3 2 2 2 2 2 4 76%	3972
June	1st to 10th	Night	6% 5% 1 4 3 4 3 3 4 6% 61%	1600
	11th to 20th	Night	3 3 1 2 2 2 3 2 3 5% 75%	1600
	21st to end	Night	3 2 1 3 2 1 2 2 3 4 78%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 2 91%	3640
	11th to 20th	Night	2 1 1 1 1 1 1 1 1 2 87%	3640
	21st to end	Night	1 1 1 2 2 2 2 3 4 5% 77%	4281
August	1st to 10th	Night	3 2 1 2 2 2 2 2 3 3 79%	4035
	11th to 20th	Night	1 1 1 1 1 1 2 3 90%	4373
	21st to end	Night	2 2 1 2 1 2 2 2 3 3 80%	5015
September	1st to 10th	Night	2 1 1 2 1 1 2 2 3 3 82%	2000
	11th to 20th	Night	3 3 1 3 2 2 2 1 3 5% 76%	1980
	21st to end	Night	3 3 1 2 1 2 2 2 3 5% 75%	2122
October	1st to 10th	Night	4 4 1 3 2 1 1 1 2 3 77%	3800
	11th to 20th	Night	4 3 1 3 2 2 2 2 4 4 73%	3777
	21st to end	Night	7% 6% 1 3 2 3 4 4 5% 7% 58%	4121
November	1st to 10th	Night	11% 10% 2 4 3 3 3 4 4 7% 49%	2530
	11th to 20th	Night	9% 8% 2 4 3 4 4 4 6% 8% 49%	2600
	21st to end	Night	10% 9% 2 5% 3 3 3 3 4 6% 52%	2765
December	1st to 10th	Night	9% 6% 2 3 2 3 3 3 4 6% 59%	4560
	11th to 20th	Night	15% 19% 4 6% 4 4 5% 4 6% 7% 26%	5176
	21st to end	Night	16% 18% 4 7% 4 3 4 3 5% 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	43% 7% 10% 19% 18% 3	3639
	11th to 20th	Day	38% 3 5% 16% 24% 14%	3866
	21st to end	Day	42% 4 6% 13% 20% 14%	4510
February	1st to 10th	Day	44% 4 10% 18% 16% 9%	1800
	11th to 20th	Day	44% 4 8% 19% 20% 5%	1879
	21st to end	Day	45% 3 8% 23% 15% 7%	1650
March	1st to 10th	Day	56% 5% 5% 12% 17% 5%	4606
	11th to 20th	Day	44% 4 8% 26% 16% 2	5000
	21st to end	Day	54% 6% 9% 20% 9% 1	5722
April	1st to 10th	Day	44% 5% 11% 22% 15% 3	2400
	11th to 20th	Day	37% 7% 11% 23% 17% 6%	2372
	21st to end	Day	31% 5% 14% 26% 20% 3	2400
May	1st to 10th	Day	20% 3 9% 23% 36% 9%	5735
	11th to 20th	Day	20% 3 9% 25% 35% 8%	6280
	21st to end	Day	16% 3 5% 33% 38% 5%	6904
June	1st to 10th	Day	25% 5% 10% 24% 28% 9%	2800
	11th to 20th	Day	14% 3 7% 21% 36% 18%	2800
	21st to end	Day	10% 2 3 16% 39% 30%	2800
July	1st to 10th	Day	3 1 1 7% 20% 67%	6251
	11th to 20th	Day	6% 1 2 6% 17% 69%	6245
	21st to end	Day	9% 3 4 14% 30% 41%	6389
August	1st to 10th	Day	13% 3 2 9% 23% 50%	4206
	11th to 20th	Day	5% 1 1 8% 44% 40%	3957
	21st to end	Day	8% 2 1 12% 43% 34%	4311
September	1st to 10th	Day	9% 2 2 11% 31% 46%	2400
	11th to 20th	Day	16% 3 5% 21% 34% 21%	2377
	21st to end	Day	15% 3 5% 21% 33% 25%	2052
October	1st to 10th	Day	16% 2 4 14% 33% 30%	3180
	11th to 20th	Day	20% 3 6% 19% 39% 13%	3161
	21st to end	Day	29% 4 6% 24% 24% 13%	3448
November	1st to 10th	Day	35% 4 7% 21% 20% 12%	1769
	11th to 20th	Day	37% 4 11% 20% 21% 7%	1800
	21st to end	Day	37% 4 7% 19% 21% 12%	1630
December	1st to 10th	Day	29% 4 7% 20% 26% 14%	2620
	11th to 20th	Day	55% 5% 7% 15% 14% 4	3028
	21st to end	Day	49% 7% 12% 17% 13% 3	4004
January	1st to 10th	Night	42% 6% 13% 18% 18% 3	6306
	11th to 20th	Night	36% 4 7% 13% 27% 13%	6100
	21st to end	Night	43% 5% 6% 10% 17% 19%	6468
February	1st to 10th	Night	41% 4 7% 19% 20% 9%	2600
	11th to 20th	Night	42% 5% 7% 19% 20% 6%	2418
	21st to end	Night	42% 5% 8% 21% 17% 7%	1980
March	1st to 10th	Night	55% 5% 5% 15% 14% 6%	5271
	11th to 20th	Night	45% 3 5% 22% 21% 3	5000
	21st to end	Night	55% 8% 9% 17% 9% 1	5253
April	1st to 10th	Night	41% 7% 10% 22% 17% 3	2000
	11th to 20th	Night	35% 5% 12% 22% 20% 6%	1979
	21st to end	Night	28% 5% 13% 32% 18% 3	2000
May	1st to 10th	Night	23% 5% 8% 25% 31% 8%	4245
	11th to 20th	Night	20% 4 8% 25% 34% 8%	3616
	21st to end	Night	15% 3 7% 30% 37% 7%	3972
June	1st to 10th	Night	24% 6% 12% 26% 25% 9%	1600
	11th to 20th	Night	14% 3 10% 21% 35% 18%	1600
	21st to end	Night	13% 2 4 16% 33% 32%	1600
July	1st to 10th	Night	4 1 1 7% 21% 66%	3600
	11th to 20th	Night	7% 1 2 6% 16% 68%	3592
	21st to end	Night	10% 3 4 14% 26% 43%	4159
August	1st to 10th	Night	15% 2 3 8% 21% 51%	2929
	11th to 20th	Night	5% 2 2 10% 39% 43%	3160
	21st to end	Night	8% 1 3 13% 35% 40%	3608
September	1st to 10th	Night	9% 3 2 10% 30% 47%	2000
	11th to 20th	Night	14% 2 6% 19% 37% 22%	1980
	21st to end	Night	13% 3 5% 21% 33% 26%	2122
October	1st to 10th	Night	16% 2 4 11% 31% 37%	3800
	11th to 20th	Night	15% 3 5% 22% 41% 13%	3777
	21st to end	Night	24% 4 7% 24% 25% 15%	4121
November	1st to 10th	Night	35% 5% 8% 20% 19% 14%	2530
	11th to 20th	Night	32% 5% 12% 21% 21% 9%	2600
	21st to end	Night	34% 4 7% 21% 22% 12%	2765
December	1st to 10th	Night	27% 4 6% 18% 25% 20%	4560
	11th to 20th	Night	55% 5% 5% 13% 19% 3	5176
	21st to end	Night	54% 6% 10% 14% 12% 4	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	44% 12% 22% 17% 5% 1	200
	11th to 20th	Day	30% 15% 26% 23% 6%	296
	21st to end	Day	29% 14% 24% 20% 12% 1	440
February	1st to 10th	Day	33% 15% 26% 19% 8% 1	400
	11th to 20th	Day	30% 21% 28% 16% 4	454
	21st to end	Day	26% 18% 31% 19% 6%	414
March	1st to 10th	Day	38% 24% 21% 12% 4 1	500
	11th to 20th	Day	33% 19% 23% 16% 7% 2	500
	21st to end	Day	31% 23% 26% 14% 5%	550
April	1st to 10th	Day	26% 21% 31% 16% 7% 1	500
	11th to 20th	Day	23% 23% 27% 20% 6% 1	492
	21st to end	Day	15% 22% 30% 22% 9% 2	500
May	1st to 10th	Day	18% 24% 30% 20% 6% 3	503
	11th to 20th	Day	16% 17% 32% 23% 9% 4	528
	21st to end	Day	13% 14% 30% 27% 11% 4	573
June	1st to 10th	Day	10% 14% 30% 29% 13% 4	530
	11th to 20th	Day	9% 18% 36% 25% 10% 3	530
	21st to end	Day	3 14% 34% 32% 14% 4	530
July	1st to 10th	Day	2 7% 42% 33% 11% 5%	526
	11th to 20th	Day	1 6% 39% 33% 17% 4	524
	21st to end	Day	2 4 37% 34% 17% 7%	575
August	1st to 10th	Day	4 6% 35% 33% 16% 6%	527
	11th to 20th	Day	2 9% 30% 35% 17% 6%	517
	21st to end	Day	3 7% 34% 36% 18% 2	583
September	1st to 10th	Day	4 11% 35% 31% 16% 3	530
	11th to 20th	Day	8% 11% 32% 31% 15% 2	523
	21st to end	Day	7% 14% 34% 33% 10% 2	490
October	1st to 10th	Day	7% 17% 28% 29% 17% 2	490
	11th to 20th	Day	12% 14% 23% 32% 16% 3	490
	21st to end	Day	18% 14% 27% 25% 14% 2	539
November	1st to 10th	Day	23% 13% 19% 25% 17% 3	305
	11th to 20th	Day	18% 11% 27% 25% 16% 2	300
	21st to end	Day	22% 13% 27% 23% 13% 2	215
December	1st to 10th	Day	28% 13% 21% 21% 13% 4	200
	11th to 20th	Day	29% 15% 22% 21% 12% 3	200
	21st to end	Day	29% 19% 21% 20% 10% 1	220
January	1st to 10th	Night	28% 9% 23% 26% 12% 2	800
	11th to 20th	Night	22% 14% 28% 29% 7% 1	800
	21st to end	Night	26% 14% 22% 25% 13%	880
February	1st to 10th	Night	25% 12% 24% 30% 9% 1	800
	11th to 20th	Night	23% 12% 19% 30% 14% 2	784
	21st to end	Night	24% 11% 25% 31% 7% 2	660
March	1st to 10th	Night	32% 13% 17% 24% 13% 1	800
	11th to 20th	Night	28% 10% 17% 25% 19% 2	800
	21st to end	Night	29% 11% 18% 27% 13% 1	880
April	1st to 10th	Night	21% 10% 21% 32% 16% 1	800
	11th to 20th	Night	18% 9% 21% 33% 16% 3	791
	21st to end	Night	14% 7% 20% 36% 20% 4	800
May	1st to 10th	Night	14% 9% 25% 29% 20% 3	670
	11th to 20th	Night	10% 7% 13% 38% 28% 4	394
	21st to end	Night	14% 3 12% 35% 31% 5%	433
June	1st to 10th	Night	8% 4 15% 35% 33% 6%	400
	11th to 20th	Night	5% 5% 15% 42% 30% 4	400
	21st to end	Night	4 2 11% 39% 39% 5%	400
July	1st to 10th	Night	1 1 8% 42% 46% 3	390
	11th to 20th	Night	1 1 5% 46% 44% 4	388
	21st to end	Night	1 1 5% 34% 51% 8%	570
August	1st to 10th	Night	5% 1 5% 38% 45% 7%	597
	11th to 20th	Night	1 1 8% 42% 39% 8%	734
	21st to end	Night	2 1 6% 46% 41% 4	880
September	1st to 10th	Night	3 2 11% 45% 36% 4	800
	11th to 20th	Night	4 3 11% 39% 35% 8%	792
	21st to end	Night	5% 4 22% 34% 26% 9%	760
October	1st to 10th	Night	8% 8% 15% 38% 28% 4	760
	11th to 20th	Night	10% 7% 17% 36% 26% 4	760
	21st to end	Night	13% 8% 22% 30% 23% 5%	836
November	1st to 10th	Night	19% 9% 16% 31% 22% 3	780
	11th to 20th	Night	16% 11% 24% 33% 15% 2	800
	21st to end	Night	19% 10% 27% 27% 16% 2	800
December	1st to 10th	Night	24% 14% 21% 30% 11% 1	800
	11th to 20th	Night	30% 11% 20% 29% 9% 1	800
	21st to end	Night	30% 13% 23% 27% 7%	880

Page generated at 2025-09-15 22:49:12. Version ac07

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