Elk Grove
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Elk Grove clear sky chart from 2005-11-06 to 2025-12-03.

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	7% 4 1 4 3 2 3 2 4 5% 66%	135602
	Night	8% 4 1 4 3 3 3 2 4 5% 63%	132510
Transparency	Day	22% 4 8% 19% 25% 23%	130540
	Night	24% 5% 9% 19% 23% 20%	128556
Seeing	Day	8% 4 14% 27% 30% 17%	16639
	Night	10% 3 14% 41% 29% 3	25965

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	18% 8% 2 7% 5% 5% 5% 4 6% 7% 35%	3639
	11th to 20th	Day	14% 8% 3 7% 5% 3 3 2 3 4 48%	4082
	21st to end	Day	18% 8% 3 7% 4 4 4 3 4 6% 39%	4510
February	1st to 10th	Day	17% 6% 3 6% 4 4 3 4 4 5% 44%	1800
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 4 5% 7% 45%	1878
	21st to end	Day	14% 5% 2 6% 4 4 3 4 4 7% 49%	1650
March	1st to 10th	Day	12% 8% 3 9% 6% 4 5% 4 5% 7% 37%	4711
	11th to 20th	Day	14% 5% 2 5% 3 4 4 4 5% 7% 47%	5000
	21st to end	Day	11% 6% 3 6% 5% 4 5% 4 6% 8% 41%	5661
April	1st to 10th	Day	12% 7% 2 5% 3 3 4 4 6% 7% 49%	2400
	11th to 20th	Day	8% 5% 1 4 3 3 4 3 5% 6% 58%	2369
	21st to end	Day	6% 3 1 3 3 2 3 3 5% 8% 63%	2400
May	1st to 10th	Day	4 2 1 2 1 1 2 1 3 4 79%	5528
	11th to 20th	Day	2 2 1 2 1 1 2 2 3 5% 80%	6223
	21st to end	Day	2 2 1 1 1 1 2 2 3 6% 81%	6902
June	1st to 10th	Day	3 3 1 3 2 2 3 3 4 7% 70%	2800
	11th to 20th	Day	1 1 1 1 2 2 2 3 4 82%	2800
	21st to end	Day	2 1 1 2 1 1 1 2 3 3 84%	2800
July	1st to 10th	Day	1 1 2 3 92%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	6233
	21st to end	Day	1 2 1 2 2 2 2 2 3 4 81%	6402
August	1st to 10th	Day	1 1 1 1 1 1 1 3 4 84%	5777
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 4 88%	5453
	21st to end	Day	1 1 1 1 1 1 2 2 4 84%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 86%	2400
	11th to 20th	Day	2 2 1 3 1 2 2 2 3 6% 75%	2400
	21st to end	Day	3 2 1 2 2 2 2 2 3 4 79%	2151
October	1st to 10th	Day	1 1 1 3 2 1 2 2 4 6% 76%	3280
	11th to 20th	Day	4 2 1 3 2 2 2 2 4 5% 72%	3261
	21st to end	Day	4 3 1 4 4 3 3 3 5% 6% 63%	3558
November	1st to 10th	Day	8% 4 1 4 3 3 4 4 5% 8% 56%	1925
	11th to 20th	Day	12% 6% 2 5% 5% 4 4 3 6% 6% 47%	1890
	21st to end	Day	12% 6% 2 5% 3 4 4 3 5% 8% 48%	1748
December	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 6% 11% 47%	2636
	11th to 20th	Day	24% 10% 5% 12% 6% 5% 4 3 5% 6% 21%	3028
	21st to end	Day	23% 9% 3 9% 6% 5% 5% 4 7% 7% 22%	4004
January	1st to 10th	Night	9% 6% 2 6% 5% 4 4 4 6% 9% 43%	6306
	11th to 20th	Night	15% 5% 2 6% 4 4 3 3 4 4 51%	5880
	21st to end	Night	15% 6% 3 7% 4 4 4 3 3 6% 45%	6468
February	1st to 10th	Night	18% 6% 2 4 3 3 3 3 4 5% 49%	2600
	11th to 20th	Night	12% 4 2 5% 4 3 3 4 6% 6% 49%	2409
	21st to end	Night	13% 4 2 4 3 3 3 3 5% 6% 54%	1980
March	1st to 10th	Night	13% 8% 2 7% 4 4 4 3 5% 7% 42%	5166
	11th to 20th	Night	15% 5% 1 4 4 3 3 2 4 5% 52%	5000
	21st to end	Night	15% 6% 2 5% 4 3 4 3 5% 7% 46%	5314
April	1st to 10th	Night	9% 5% 2 4 4 3 4 3 4 7% 55%	2000
	11th to 20th	Night	7% 4 2 4 3 3 3 4 5% 6% 60%	1979
	21st to end	Night	5% 3 1 3 2 2 3 3 4 6% 67%	2000
May	1st to 10th	Night	2 2 1 2 2 1 2 2 2 5% 79%	4450
	11th to 20th	Night	1 1 2 1 2 2 1 3 5% 80%	3667
	21st to end	Night	1 1 1 1 1 2 2 2 4 85%	3972
June	1st to 10th	Night	4 2 1 3 2 2 2 3 3 6% 72%	1600
	11th to 20th	Night	1 1 1 1 1 2 1 3 3 86%	1600
	21st to end	Night	2 1 1 1 1 1 2 1 2 3 85%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 93%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 88%	3726
	21st to end	Night	1 3 1 2 1 2 2 2 3 5% 80%	4472
August	1st to 10th	Night	1 1 1 2 1 1 1 1 3 4 83%	4035
	11th to 20th	Night	1 1 1 1 1 1 2 2 90%	4413
	21st to end	Night	2 2 1 1 1 1 1 2 3 84%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 1 3 4 87%	2000
	11th to 20th	Night	3 2 1 2 2 2 3 2 3 4 78%	2000
	21st to end	Night	2 1 1 2 2 1 2 1 3 5% 81%	2244
October	1st to 10th	Night	3 1 1 2 2 2 2 2 3 4 78%	3920
	11th to 20th	Night	3 2 1 3 2 2 2 2 3 5% 76%	3897
	21st to end	Night	5% 2 1 4 2 3 3 3 4 5% 67%	4253
November	1st to 10th	Night	8% 4 1 3 3 2 3 3 5% 8% 59%	2591
	11th to 20th	Night	9% 4 2 5% 3 3 3 4 5% 7% 55%	2730
	21st to end	Night	11% 4 2 5% 3 3 3 3 4 6% 56%	2867
December	1st to 10th	Night	9% 3 2 4 3 3 4 3 5% 7% 58%	4588
	11th to 20th	Night	21% 9% 3 7% 6% 5% 5% 4 7% 8% 26%	5176
	21st to end	Night	22% 7% 3 9% 6% 6% 5% 4 6% 6% 26%	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	46% 9% 14% 18% 10% 3	3639
	11th to 20th	Day	40% 6% 7% 17% 17% 12%	4082
	21st to end	Day	44% 6% 8% 13% 24% 5%	4510
February	1st to 10th	Day	42% 5% 9% 21% 16% 7%	1800
	11th to 20th	Day	37% 7% 13% 20% 17% 6%	1887
	21st to end	Day	35% 6% 10% 23% 20% 6%	1650
March	1st to 10th	Day	44% 8% 9% 20% 17% 2	4711
	11th to 20th	Day	34% 6% 13% 27% 12% 8%	5000
	21st to end	Day	36% 9% 17% 25% 11% 2	5661
April	1st to 10th	Day	34% 5% 12% 24% 18% 7%	2400
	11th to 20th	Day	27% 6% 12% 25% 22% 8%	2372
	21st to end	Day	20% 4 15% 28% 27% 7%	2400
May	1st to 10th	Day	12% 3 6% 27% 31% 21%	5528
	11th to 20th	Day	9% 3 6% 29% 40% 13%	6226
	21st to end	Day	7% 3 8% 29% 40% 12%	6904
June	1st to 10th	Day	16% 4 11% 27% 29% 13%	2800
	11th to 20th	Day	7% 3 7% 21% 36% 27%	2800
	21st to end	Day	8% 2 3 12% 37% 39%	2800
July	1st to 10th	Day	2 1 2 7% 19% 70%	6251
	11th to 20th	Day	5% 1 2 5% 14% 73%	6158
	21st to end	Day	9% 2 3 15% 27% 44%	6217
August	1st to 10th	Day	9% 2 2 7% 27% 53%	4206
	11th to 20th	Day	4 1 2 9% 40% 44%	3946
	21st to end	Day	6% 1 3 10% 38% 41%	4311
September	1st to 10th	Day	6% 2 3 10% 31% 50%	2400
	11th to 20th	Day	12% 3 7% 21% 38% 19%	2400
	21st to end	Day	12% 2 4 19% 35% 28%	2151
October	1st to 10th	Day	11% 3 5% 12% 27% 42%	3280
	11th to 20th	Day	16% 4 6% 24% 32% 18%	3261
	21st to end	Day	21% 4 8% 19% 34% 14%	3558
November	1st to 10th	Day	24% 7% 10% 27% 23% 9%	1925
	11th to 20th	Day	37% 6% 13% 22% 18% 4	1890
	21st to end	Day	34% 6% 12% 22% 19% 8%	1748
December	1st to 10th	Day	30% 6% 14% 29% 17% 5%	2636
	11th to 20th	Day	63% 6% 7% 13% 8% 2	3028
	21st to end	Day	56% 11% 12% 14% 6% 1	4004
January	1st to 10th	Night	34% 8% 19% 24% 10% 4	6306
	11th to 20th	Night	36% 6% 10% 17% 22% 10%	5880
	21st to end	Night	40% 6% 9% 13% 23% 9%	6468
February	1st to 10th	Night	37% 4 9% 21% 20% 8%	2600
	11th to 20th	Night	33% 6% 11% 22% 22% 7%	2411
	21st to end	Night	30% 6% 9% 26% 22% 6%	1980
March	1st to 10th	Night	39% 8% 11% 19% 19% 4	5166
	11th to 20th	Night	34% 5% 14% 22% 15% 9%	5000
	21st to end	Night	36% 8% 15% 24% 14% 3	5314
April	1st to 10th	Night	30% 5% 13% 22% 23% 7%	2000
	11th to 20th	Night	25% 5% 12% 27% 19% 12%	1979
	21st to end	Night	19% 5% 13% 30% 28% 6%	2000
May	1st to 10th	Night	10% 4 11% 26% 29% 20%	4447
	11th to 20th	Night	10% 2 7% 31% 39% 11%	3667
	21st to end	Night	7% 2 8% 34% 38% 11%	3972
June	1st to 10th	Night	15% 4 12% 27% 30% 12%	1600
	11th to 20th	Night	6% 2 7% 20% 37% 28%	1600
	21st to end	Night	7% 2 3 13% 33% 42%	1600
July	1st to 10th	Night	3 1 7% 23% 66%	3600
	11th to 20th	Night	6% 1 1 4 13% 75%	3678
	21st to end	Night	9% 2 3 16% 27% 44%	4345
August	1st to 10th	Night	11% 1 2 8% 28% 49%	2929
	11th to 20th	Night	4 1 1 9% 38% 46%	3188
	21st to end	Night	7% 1 2 8% 37% 45%	3608
September	1st to 10th	Night	5% 2 3 9% 31% 51%	2000
	11th to 20th	Night	12% 3 5% 21% 37% 23%	2000
	21st to end	Night	9% 2 5% 19% 33% 32%	2244
October	1st to 10th	Night	12% 3 4 11% 21% 49%	3920
	11th to 20th	Night	11% 4 6% 28% 29% 22%	3897
	21st to end	Night	19% 3 6% 20% 32% 20%	4253
November	1st to 10th	Night	23% 5% 11% 24% 23% 14%	2591
	11th to 20th	Night	27% 5% 12% 25% 23% 8%	2730
	21st to end	Night	29% 4 9% 23% 25% 10%	2867
December	1st to 10th	Night	25% 5% 10% 28% 23% 9%	4588
	11th to 20th	Night	53% 6% 11% 16% 11% 2	5176
	21st to end	Night	54% 10% 12% 15% 7% 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	33% 7% 11% 22% 22% 7%	200
	11th to 20th	Day	16% 7% 15% 25% 25% 13%	395
	21st to end	Day	15% 5% 16% 23% 25% 16%	440
February	1st to 10th	Day	20% 7% 16% 19% 22% 17%	400
	11th to 20th	Day	15% 7% 23% 22% 20% 12%	458
	21st to end	Day	12% 9% 19% 28% 24% 8%	414
March	1st to 10th	Day	17% 11% 17% 28% 21% 6%	500
	11th to 20th	Day	19% 10% 17% 25% 17% 13%	500
	21st to end	Day	14% 9% 21% 26% 20% 10%	550
April	1st to 10th	Day	14% 7% 18% 29% 19% 13%	500
	11th to 20th	Day	10% 8% 20% 29% 21% 14%	492
	21st to end	Day	6% 5% 17% 29% 26% 17%	500
May	1st to 10th	Day	4 6% 22% 30% 24% 13%	503
	11th to 20th	Day	5% 4 20% 29% 25% 17%	528
	21st to end	Day	4 4 14% 29% 30% 19%	573
June	1st to 10th	Day	4 3 18% 25% 33% 17%	530
	11th to 20th	Day	1 4 18% 30% 37% 10%	530
	21st to end	Day	1 13% 31% 37% 18%	530
July	1st to 10th	Day	1 1 9% 33% 41% 15%	526
	11th to 20th	Day	1 1 6% 31% 42% 20%	524
	21st to end	Day	1 1 7% 32% 41% 18%	575
August	1st to 10th	Day	1 2 10% 35% 37% 16%	527
	11th to 20th	Day	2 10% 30% 40% 18%	517
	21st to end	Day	1 1 8% 28% 40% 21%	583
September	1st to 10th	Day	1 1 9% 27% 42% 20%	530
	11th to 20th	Day	3 3 13% 25% 35% 22%	530
	21st to end	Day	2 2 11% 26% 35% 23%	530
October	1st to 10th	Day	2 4 15% 26% 28% 25%	530
	11th to 20th	Day	5% 5% 12% 21% 30% 28%	530
	21st to end	Day	7% 6% 13% 23% 32% 20%	583
November	1st to 10th	Day	9% 4 13% 20% 30% 23%	401
	11th to 20th	Day	14% 4 11% 26% 31% 14%	330
	21st to end	Day	15% 2 7% 29% 33% 15%	256
December	1st to 10th	Day	16% 3 14% 24% 24% 20%	204
	11th to 20th	Day	19% 6% 12% 24% 27% 13%	200
	21st to end	Day	20% 7% 19% 27% 20% 6%	220
January	1st to 10th	Night	18% 4 15% 35% 24% 4	800
	11th to 20th	Night	14% 4 13% 39% 25% 5%	800
	21st to end	Night	17% 3 12% 38% 26% 4	880
February	1st to 10th	Night	21% 4 14% 35% 24% 3	800
	11th to 20th	Night	14% 5% 17% 38% 24% 3	784
	21st to end	Night	14% 4 18% 43% 18% 3	660
March	1st to 10th	Night	18% 6% 18% 34% 23% 2	800
	11th to 20th	Night	19% 5% 15% 36% 23% 2	800
	21st to end	Night	15% 5% 18% 39% 21% 2	880
April	1st to 10th	Night	11% 4 20% 41% 24% 1	800
	11th to 20th	Night	7% 5% 21% 42% 22% 3	791
	21st to end	Night	6% 3 21% 40% 27% 3	800
May	1st to 10th	Night	4 3 24% 42% 25% 2	755
	11th to 20th	Night	4 1 14% 52% 28% 1	419
	21st to end	Night	4 1 18% 46% 29% 1	433
June	1st to 10th	Night	4 1 17% 49% 28% 1	400
	11th to 20th	Night	2 1 16% 57% 25%	400
	21st to end	Night	1 1 13% 53% 32% 1	400
July	1st to 10th	Night	1 7% 55% 37%	390
	11th to 20th	Night	1 5% 55% 37% 1	422
	21st to end	Night	1 4 46% 46% 2	657
August	1st to 10th	Night	3 9% 47% 39% 2	597
	11th to 20th	Night	1 1 9% 50% 37% 2	749
	21st to end	Night	2 6% 46% 44% 2	880
September	1st to 10th	Night	1 1 8% 45% 44% 2	800
	11th to 20th	Night	3 2 12% 42% 36% 5%	800
	21st to end	Night	2 1 11% 40% 40% 6%	800
October	1st to 10th	Night	5% 3 10% 41% 35% 6%	800
	11th to 20th	Night	5% 3 12% 38% 36% 6%	800
	21st to end	Night	6% 3 11% 37% 35% 9%	880
November	1st to 10th	Night	9% 3 13% 35% 33% 7%	820
	11th to 20th	Night	10% 3 13% 42% 27% 5%	840
	21st to end	Night	11% 4 13% 39% 29% 5%	840
December	1st to 10th	Night	16% 2 14% 38% 24% 7%	808
	11th to 20th	Night	25% 4 11% 33% 24% 3	800
	21st to end	Night	21% 5% 19% 33% 20% 2	880

Page generated at 2025-11-29 22:00:56. Version ac07

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