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 2024-11-05.

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 3 2 3 2 4 5% 61%	105209
	Night	8% 10% 2 4 3 3 3 3 4 5% 56%	102538
Transparency	Day	27% 4 6% 18% 25% 21%	104957
	Night	31% 4 7% 18% 23% 18%	102378
Seeing	Day	16% 15% 30% 26% 12% 3	15078
	Night	16% 8% 18% 33% 23% 3	24328

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	10% 23% 3 5% 4 4 3 3 4 6% 35%	2539
	11th to 20th	Day	9% 21% 3 8% 6% 4 4 3 3 5% 34%	2696
	21st to end	Day	12% 20% 4 7% 4 4 5% 3 4 6% 32%	3146
February	1st to 10th	Day	15% 13% 3 4 3 3 3 3 5% 7% 41%	1710
	11th to 20th	Day	10% 17% 3 5% 3 3 3 4 4 5% 43%	1774
	21st to end	Day	10% 20% 4 4 3 3 4 3 4 5% 41%	1570
March	1st to 10th	Day	10% 30% 10% 7% 3 3 3 2 3 5% 25%	3200
	11th to 20th	Day	8% 15% 2 4 2 2 2 2 3 5% 54%	3490
	21st to end	Day	11% 25% 4 7% 4 4 4 4 4 5% 29%	3996
April	1st to 10th	Day	12% 16% 2 4 4 4 3 4 5% 6% 40%	2280
	11th to 20th	Day	9% 13% 2 5% 3 3 4 4 5% 6% 46%	2249
	21st to end	Day	9% 9% 2 4 3 3 3 3 5% 7% 52%	2280
May	1st to 10th	Day	7% 7% 1 3 2 2 2 2 3 4 66%	4004
	11th to 20th	Day	3 5% 1 3 2 2 2 2 3 4 72%	4377
	21st to end	Day	6% 5% 1 3 2 2 3 2 4 5% 66%	4813
June	1st to 10th	Day	7% 5% 1 4 3 3 3 3 6% 7% 57%	2660
	11th to 20th	Day	3 3 1 3 3 2 3 2 3 5% 73%	2660
	21st to end	Day	3 1 1 2 1 2 2 2 3 4 80%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 91%	4420
	11th to 20th	Day	2 1 1 2 1 1 2 1 2 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	4580
August	1st to 10th	Day	2 3 1 3 2 2 2 2 4 4 75%	4091
	11th to 20th	Day	1 1 1 1 2 1 2 3 89%	3797
	21st to end	Day	2 2 1 1 1 1 1 2 2 87%	4180
September	1st to 10th	Day	2 1 1 1 1 2 1 2 3 85%	2280
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 5% 72%	2280
	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% 10% 2 5% 3 4 2 3 5% 5% 50%	1715
	11th to 20th	Day	12% 7% 1 5% 4 4 4 4 5% 7% 47%	1710
	21st to end	Day	9% 11% 2 4 3 4 5% 3 4 5% 50%	1549
December	1st to 10th	Day	16% 10% 2 4 3 3 4 4 5% 6% 43%	1520
	11th to 20th	Day	21% 15% 4 7% 5% 4 4 2 5% 5% 28%	1928
	21st to end	Day	10% 11% 3 6% 4 3 4 5% 6% 9% 38%	2794
January	1st to 10th	Night	9% 15% 4 6% 4 3 4 3 5% 7% 39%	4406
	11th to 20th	Night	11% 19% 4 7% 4 4 4 4 5% 6% 34%	4265
	21st to end	Night	12% 22% 3 8% 5% 5% 4 3 4 5% 29%	4521
February	1st to 10th	Night	14% 11% 3 5% 4 3 3 3 6% 6% 45%	2470
	11th to 20th	Night	10% 15% 2 4 3 4 5% 3 6% 6% 43%	2292
	21st to end	Night	12% 15% 3 5% 3 3 3 2 4 6% 42%	1884
March	1st to 10th	Night	10% 31% 10% 6% 3 2 2 2 3 4 27%	3657
	11th to 20th	Night	7% 16% 2 3 2 2 3 2 4 5% 53%	3490
	21st to end	Night	12% 25% 4 6% 3 3 5% 4 5% 5% 28%	3657
April	1st to 10th	Night	10% 14% 3 6% 4 3 3 3 4 5% 44%	1900
	11th to 20th	Night	10% 11% 3 5% 3 3 2 3 5% 6% 47%	1879
	21st to end	Night	8% 8% 1 4 3 3 3 3 5% 7% 56%	1900
May	1st to 10th	Night	6% 11% 2 4 3 2 2 2 3 4 60%	2966
	11th to 20th	Night	4 3 1 3 3 3 2 2 3 5% 70%	2516
	21st to end	Night	6% 5% 1 3 2 2 2 2 3 6% 69%	2762
June	1st to 10th	Night	7% 5% 1 4 4 4 3 3 4 6% 60%	1520
	11th to 20th	Night	3 3 1 2 2 2 3 2 3 5% 74%	1520
	21st to end	Night	3 2 1 3 2 2 2 2 2 4 77%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 1 2 91%	2540
	11th to 20th	Night	3 2 1 2 1 1 2 1 2 3 82%	2540
	21st to end	Night	1 1 1 2 1 1 2 2 2 3 85%	2974
August	1st to 10th	Night	4 3 1 3 2 3 2 3 4 5% 71%	2848
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 88%	3035
	21st to end	Night	2 1 1 1 1 1 1 1 2 2 87%	3498
September	1st to 10th	Night	2 1 2 1 1 2 2 3 3 84%	1900
	11th to 20th	Night	4 3 1 3 2 2 2 1 3 5% 76%	1900
	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 6% 49%	2464
	11th to 20th	Night	10% 7% 2 4 3 4 4 4 5% 8% 50%	2470
	21st to end	Night	10% 8% 2 5% 3 3 3 3 5% 7% 52%	2626
December	1st to 10th	Night	15% 10% 2 4 3 3 3 3 5% 7% 45%	2660
	11th to 20th	Night	22% 17% 3 5% 3 3 3 3 5% 6% 30%	3276
	21st to end	Night	12% 15% 3 7% 4 4 4 4 5% 6% 34%	4862

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	50% 6% 7% 21% 14% 2	2539
	11th to 20th	Day	54% 5% 7% 15% 12% 6%	2696
	21st to end	Day	54% 5% 8% 17% 13% 3	3146
February	1st to 10th	Day	43% 4 10% 19% 16% 8%	1710
	11th to 20th	Day	44% 4 8% 19% 21% 5%	1783
	21st to end	Day	46% 3 8% 22% 15% 6%	1570
March	1st to 10th	Day	62% 6% 5% 12% 12% 4	3200
	11th to 20th	Day	35% 3 7% 31% 20% 3	3490
	21st to end	Day	55% 7% 10% 18% 8% 2	3996
April	1st to 10th	Day	44% 5% 11% 21% 15% 3	2280
	11th to 20th	Day	38% 7% 11% 23% 17% 5%	2252
	21st to end	Day	31% 5% 14% 26% 20% 3	2280
May	1st to 10th	Day	24% 3 12% 24% 29% 8%	4004
	11th to 20th	Day	17% 3 6% 22% 42% 10%	4380
	21st to end	Day	21% 5% 7% 40% 25% 3	4814
June	1st to 10th	Day	26% 5% 10% 23% 26% 9%	2660
	11th to 20th	Day	15% 3 8% 22% 35% 17%	2660
	21st to end	Day	10% 2 3 16% 38% 30%	2660
July	1st to 10th	Day	4 1 1 6% 16% 72%	4351
	11th to 20th	Day	8% 2 3 8% 19% 61%	4345
	21st to end	Day	6% 1 2 6% 29% 56%	4435
August	1st to 10th	Day	13% 3 2 9% 22% 50%	4076
	11th to 20th	Day	5% 1 1 8% 44% 41%	3834
	21st to end	Day	7% 2 1 12% 43% 35%	4179
September	1st to 10th	Day	8% 1 2 10% 32% 48%	2280
	11th to 20th	Day	16% 3 5% 21% 34% 21%	2280
	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	36% 4 7% 21% 20% 12%	1715
	11th to 20th	Day	36% 4 10% 20% 22% 8%	1710
	21st to end	Day	36% 4 8% 20% 22% 11%	1549
December	1st to 10th	Day	40% 4 8% 20% 20% 7%	1520
	11th to 20th	Day	59% 4 6% 14% 14% 4	1928
	21st to end	Day	41% 8% 14% 19% 15% 4	2794
January	1st to 10th	Night	44% 5% 13% 23% 12% 2	4406
	11th to 20th	Night	51% 6% 10% 12% 15% 6%	4265
	21st to end	Night	57% 6% 7% 14% 12% 5%	4521
February	1st to 10th	Night	40% 4 7% 20% 20% 8%	2470
	11th to 20th	Night	41% 5% 7% 20% 20% 7%	2294
	21st to end	Night	43% 4 8% 21% 17% 6%	1884
March	1st to 10th	Night	62% 6% 6% 13% 11% 3	3657
	11th to 20th	Night	35% 3 5% 27% 26% 3	3490
	21st to end	Night	55% 9% 10% 17% 8% 2	3657
April	1st to 10th	Night	42% 7% 10% 22% 16% 3	1900
	11th to 20th	Night	36% 5% 12% 22% 19% 6%	1879
	21st to end	Night	28% 5% 14% 32% 18% 4	1900
May	1st to 10th	Night	28% 6% 8% 23% 27% 7%	2966
	11th to 20th	Night	18% 5% 8% 19% 39% 12%	2516
	21st to end	Night	19% 4 10% 38% 23% 5%	2762
June	1st to 10th	Night	24% 6% 12% 25% 23% 9%	1520
	11th to 20th	Night	14% 3 10% 22% 35% 16%	1520
	21st to end	Night	13% 3 4 16% 31% 33%	1520
July	1st to 10th	Night	5% 1 1 6% 15% 71%	2500
	11th to 20th	Night	10% 2 3 8% 14% 63%	2492
	21st to end	Night	7% 1 2 6% 26% 58%	2881
August	1st to 10th	Night	16% 2 3 8% 20% 51%	2839
	11th to 20th	Night	5% 2 2 10% 39% 43%	3063
	21st to end	Night	8% 1 2 13% 35% 41%	3498
September	1st to 10th	Night	8% 2 2 9% 30% 49%	1900
	11th to 20th	Night	14% 2 6% 19% 37% 22%	1900
	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% 13%	2464
	11th to 20th	Night	31% 5% 11% 21% 22% 9%	2470
	21st to end	Night	33% 4 8% 22% 23% 11%	2626
December	1st to 10th	Night	39% 5% 8% 17% 21% 10%	2660
	11th to 20th	Night	55% 4 4 13% 20% 4	3276
	21st to end	Night	48% 6% 10% 16% 15% 4	4862

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% 13% 22% 16% 5% 1	180
	11th to 20th	Day	30% 14% 25% 24% 6%	271
	21st to end	Day	29% 14% 25% 19% 12% 1	407
February	1st to 10th	Day	31% 15% 26% 19% 8% 1	370
	11th to 20th	Day	29% 21% 28% 17% 5%	418
	21st to end	Day	27% 19% 29% 19% 6%	382
March	1st to 10th	Day	39% 23% 21% 11% 5% 1	460
	11th to 20th	Day	31% 19% 24% 17% 8% 2	460
	21st to end	Day	30% 24% 28% 13% 5%	506
April	1st to 10th	Day	26% 21% 30% 16% 7% 1	460
	11th to 20th	Day	25% 23% 25% 21% 6% 1	452
	21st to end	Day	15% 22% 30% 23% 9% 2	460
May	1st to 10th	Day	18% 24% 30% 20% 6% 2	463
	11th to 20th	Day	15% 16% 33% 23% 9% 5%	488
	21st to end	Day	14% 13% 29% 28% 11% 4	529
June	1st to 10th	Day	11% 14% 29% 29% 12% 5%	490
	11th to 20th	Day	9% 18% 36% 25% 10% 3	490
	21st to end	Day	3 13% 34% 32% 13% 4	490
July	1st to 10th	Day	2 6% 42% 33% 12% 5%	486
	11th to 20th	Day	1 6% 39% 32% 17% 5%	484
	21st to end	Day	1 4 37% 35% 17% 6%	531
August	1st to 10th	Day	4 6% 34% 33% 16% 6%	487
	11th to 20th	Day	3 10% 30% 35% 17% 6%	477
	21st to end	Day	3 7% 34% 37% 17% 2	539
September	1st to 10th	Day	3 10% 35% 32% 17% 3	490
	11th to 20th	Day	9% 12% 31% 32% 14% 2	490
	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	25% 11% 18% 25% 17% 3	287
	11th to 20th	Day	17% 10% 27% 26% 17% 2	270
	21st to end	Day	18% 14% 28% 24% 13% 2	194
December	1st to 10th	Day	29% 14% 21% 20% 11% 4	180
	11th to 20th	Day	28% 15% 21% 21% 12% 3	180
	21st to end	Day	26% 18% 23% 21% 11% 2	198
January	1st to 10th	Night	27% 9% 23% 27% 12% 2	760
	11th to 20th	Night	22% 13% 28% 30% 7% 1	760
	21st to end	Night	27% 13% 21% 25% 13%	836
February	1st to 10th	Night	23% 12% 24% 31% 10% 1	760
	11th to 20th	Night	23% 12% 19% 30% 15% 2	744
	21st to end	Night	24% 11% 23% 32% 8% 2	628
March	1st to 10th	Night	32% 13% 17% 24% 13% 1	760
	11th to 20th	Night	26% 10% 18% 25% 20% 2	760
	21st to end	Night	29% 11% 19% 26% 14% 1	836
April	1st to 10th	Night	21% 9% 21% 32% 16% 1	760
	11th to 20th	Night	19% 10% 19% 32% 17% 3	751
	21st to end	Night	14% 6% 21% 35% 20% 4	760
May	1st to 10th	Night	14% 9% 25% 28% 21% 3	637
	11th to 20th	Night	10% 7% 12% 37% 30% 4	374
	21st to end	Night	14% 3 12% 33% 32% 5%	411
June	1st to 10th	Night	9% 4 14% 33% 33% 6%	380
	11th to 20th	Night	5% 5% 14% 41% 31% 4	380
	21st to end	Night	4 2 11% 40% 39% 5%	380
July	1st to 10th	Night	1 1 6% 41% 48% 4	370
	11th to 20th	Night	1 1 5% 45% 44% 4	368
	21st to end	Night	1 1 5% 33% 52% 9%	541
August	1st to 10th	Night	5% 1 5% 37% 46% 8%	567
	11th to 20th	Night	1 1 8% 42% 40% 8%	697
	21st to end	Night	2 1 6% 44% 42% 5%	836
September	1st to 10th	Night	2 2 10% 44% 38% 4	760
	11th to 20th	Night	4 3 11% 38% 35% 8%	760
	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	760
	11th to 20th	Night	15% 10% 23% 33% 16% 2	760
	21st to end	Night	17% 10% 27% 28% 16% 2	760
December	1st to 10th	Night	24% 14% 20% 30% 11% 1	760
	11th to 20th	Night	30% 11% 20% 29% 10% 1	760
	21st to end	Night	29% 13% 23% 28% 8%	836

Page generated at 2024-11-02 23:11:24. Version ac07

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