Rancho Cordova
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rancho Cordova clear sky chart from 2005-11-06 to 2025-07-06.

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 3 2 4 5% 65%	123878
	Night	11% 4 1 4 3 2 3 2 4 5% 61%	123814
Transparency	Day	23% 4 7% 19% 25% 22%	123631
	Night	26% 4 9% 20% 23% 19%	123647
Seeing	Day	9% 5% 16% 29% 30% 12%	16040
	Night	11% 2 14% 39% 30% 4	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	20% 6% 2 3 4 4 4 4 5% 8% 40%	3639
	11th to 20th	Day	16% 7% 2 6% 4 3 3 2 3 3 51%	4031
	21st to end	Day	19% 7% 2 6% 4 4 4 3 4 6% 42%	4510
February	1st to 10th	Day	20% 6% 2 6% 3 3 3 3 4 6% 44%	1800
	11th to 20th	Day	16% 6% 2 5% 3 4 5% 4 4 7% 45%	1885
	21st to end	Day	16% 5% 2 6% 3 4 3 3 5% 6% 48%	1650
March	1st to 10th	Day	16% 8% 3 8% 5% 4 5% 4 5% 6% 36%	4683
	11th to 20th	Day	17% 5% 1 5% 3 3 4 3 5% 7% 48%	5000
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 41%	5695
April	1st to 10th	Day	13% 7% 2 4 3 4 4 3 5% 7% 49%	2400
	11th to 20th	Day	10% 5% 2 4 2 3 3 3 5% 6% 57%	2369
	21st to end	Day	7% 4 1 3 2 2 3 3 5% 7% 63%	2400
May	1st to 10th	Day	5% 2 1 2 1 1 1 2 2 4 78%	5519
	11th to 20th	Day	2 1 1 2 1 1 2 2 3 4 80%	6272
	21st to end	Day	2 1 2 1 1 1 1 3 5% 81%	6902
June	1st to 10th	Day	4 3 1 3 3 2 2 2 5% 7% 69%	2800
	11th to 20th	Day	1 2 2 2 2 2 2 5% 82%	2800
	21st to end	Day	2 1 1 2 1 1 2 3 3 84%	2800
July	1st to 10th	Day	1 1 1 1 1 2 93%	4491
	11th to 20th	Day	1 1 2 1 2 1 2 3 4 84%	4402
	21st to end	Day	1 1 1 1 1 2 3 87%	4455
August	1st to 10th	Day	2 2 1 2 1 1 2 2 4 5% 79%	4091
	11th to 20th	Day	1 1 1 1 1 1 3 4 88%	3767
	21st to end	Day	2 1 1 1 1 1 1 2 3 87%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 88%	2280
	11th to 20th	Day	3 2 1 2 1 2 3 2 3 5% 75%	2280
	21st to end	Day	3 1 1 2 1 1 2 1 3 4 80%	2053
October	1st to 10th	Day	1 2 1 3 2 2 2 2 4 5% 76%	3180
	11th to 20th	Day	4 2 1 3 2 2 3 2 3 5% 73%	3161
	21st to end	Day	4 3 1 5% 3 3 3 3 4 6% 64%	3448
November	1st to 10th	Day	10% 4 1 3 2 3 3 3 6% 9% 55%	1813
	11th to 20th	Day	13% 4 2 4 4 4 4 3 5% 7% 51%	1800
	21st to end	Day	13% 4 1 4 3 3 3 4 5% 7% 53%	1670
December	1st to 10th	Day	9% 4 1 4 4 3 3 2 4 9% 56%	2620
	11th to 20th	Day	28% 7% 2 7% 4 4 4 3 5% 8% 28%	3028
	21st to end	Day	25% 8% 2 6% 5% 5% 6% 4 7% 8% 24%	4004
January	1st to 10th	Night	12% 6% 2 5% 3 4 4 4 6% 8% 46%	6306
	11th to 20th	Night	16% 4 2 5% 3 3 3 2 3 4 54%	5941
	21st to end	Night	17% 6% 2 5% 4 3 3 2 4 5% 47%	6468
February	1st to 10th	Night	20% 6% 1 4 3 3 3 3 4 4 51%	2600
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 5% 8% 48%	2402
	21st to end	Night	15% 6% 2 3 3 3 3 3 4 6% 52%	1980
March	1st to 10th	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	5209
	11th to 20th	Night	20% 6% 2 4 3 2 3 3 4 5% 49%	5000
	21st to end	Night	19% 6% 2 5% 3 3 4 4 5% 7% 42%	5280
April	1st to 10th	Night	11% 6% 3 4 3 3 4 4 4 6% 52%	2000
	11th to 20th	Night	9% 4 2 4 3 3 3 3 5% 7% 58%	1979
	21st to end	Night	7% 3 1 3 2 2 3 3 4 7% 66%	2000
May	1st to 10th	Night	4 3 1 2 2 2 2 1 3 5% 76%	4473
	11th to 20th	Night	2 1 1 2 1 2 2 2 4 4 79%	3617
	21st to end	Night	2 1 1 1 1 2 1 3 4 84%	3972
June	1st to 10th	Night	4 3 1 3 2 2 2 3 5% 5% 71%	1600
	11th to 20th	Night	1 1 1 2 2 2 1 2 2 4 84%	1600
	21st to end	Night	2 2 1 1 1 1 1 3 2 85%	1600
July	1st to 10th	Night	1 1 1 1 1 2 94%	2580
	11th to 20th	Night	1 2 1 1 1 2 2 1 2 4 83%	2555
	21st to end	Night	1 1 1 1 1 1 1 3 4 86%	3109
August	1st to 10th	Night	3 2 1 2 2 2 2 2 3 5% 77%	2848
	11th to 20th	Night	1 1 1 1 1 1 3 3 88%	3079
	21st to end	Night	3 1 1 1 1 1 1 2 1 89%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 3 88%	1900
	11th to 20th	Night	4 2 1 2 2 2 1 2 3 4 78%	1900
	21st to end	Night	2 1 1 1 2 1 2 2 5% 81%	2124
October	1st to 10th	Night	3 2 1 2 2 1 2 2 3 4 78%	3800
	11th to 20th	Night	3 1 1 2 2 2 2 1 3 5% 77%	3777
	21st to end	Night	6% 3 1 3 3 3 3 2 4 5% 68%	4121
November	1st to 10th	Night	11% 4 1 4 2 2 3 3 4 8% 58%	2483
	11th to 20th	Night	10% 4 1 5% 2 3 3 3 5% 8% 56%	2600
	21st to end	Night	13% 4 1 3 3 3 2 2 4 6% 58%	2725
December	1st to 10th	Night	10% 3 1 3 2 3 3 3 4 6% 62%	4560
	11th to 20th	Night	25% 6% 3 6% 4 3 4 4 7% 9% 30%	5176
	21st to end	Night	25% 7% 3 8% 5% 4 5% 4 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	42% 11% 13% 20% 10% 4	3639
	11th to 20th	Day	41% 3 7% 17% 18% 14%	4031
	21st to end	Day	44% 5% 8% 13% 25% 5%	4510
February	1st to 10th	Day	43% 3 9% 21% 16% 7%	1800
	11th to 20th	Day	39% 5% 12% 21% 18% 6%	1894
	21st to end	Day	37% 3 9% 24% 21% 6%	1650
March	1st to 10th	Day	47% 5% 10% 19% 18% 2	4683
	11th to 20th	Day	36% 5% 12% 27% 13% 8%	5000
	21st to end	Day	39% 6% 16% 26% 11% 2	5695
April	1st to 10th	Day	34% 5% 12% 25% 17% 7%	2400
	11th to 20th	Day	29% 4 13% 25% 22% 7%	2372
	21st to end	Day	21% 4 14% 29% 26% 6%	2400
May	1st to 10th	Day	13% 2 6% 27% 31% 21%	5519
	11th to 20th	Day	10% 3 5% 29% 39% 14%	6275
	21st to end	Day	9% 2 8% 29% 40% 12%	6904
June	1st to 10th	Day	17% 3 10% 28% 29% 13%	2800
	11th to 20th	Day	8% 2 7% 21% 36% 27%	2800
	21st to end	Day	8% 2 2 13% 37% 39%	2800
July	1st to 10th	Day	3 1 1 6% 15% 74%	4422
	11th to 20th	Day	7% 2 2 7% 17% 65%	4327
	21st to end	Day	5% 1 1 7% 25% 61%	4315
August	1st to 10th	Day	10% 2 2 7% 24% 56%	4076
	11th to 20th	Day	4 1 1 8% 38% 48%	3803
	21st to end	Day	7% 2 2 10% 37% 43%	4179
September	1st to 10th	Day	5% 1 2 9% 30% 54%	2280
	11th to 20th	Day	13% 4 4 21% 38% 21%	2280
	21st to end	Day	11% 2 4 19% 34% 30%	2053
October	1st to 10th	Day	12% 2 4 11% 26% 45%	3180
	11th to 20th	Day	15% 3 6% 21% 34% 19%	3161
	21st to end	Day	22% 3 7% 19% 34% 15%	3448
November	1st to 10th	Day	27% 5% 9% 27% 23% 10%	1813
	11th to 20th	Day	33% 6% 12% 24% 20% 5%	1800
	21st to end	Day	32% 6% 10% 24% 19% 9%	1670
December	1st to 10th	Day	28% 4 12% 28% 20% 6%	2620
	11th to 20th	Day	56% 7% 9% 16% 10% 2	3028
	21st to end	Day	55% 9% 14% 13% 7% 2	4004
January	1st to 10th	Night	36% 6% 19% 25% 9% 5%	6306
	11th to 20th	Night	36% 3 10% 18% 22% 11%	5941
	21st to end	Night	41% 4 10% 12% 24% 10%	6468
February	1st to 10th	Night	38% 3 9% 22% 20% 8%	2600
	11th to 20th	Night	36% 4 10% 23% 21% 7%	2403
	21st to end	Night	33% 3 10% 26% 21% 7%	1980
March	1st to 10th	Night	44% 5% 11% 18% 19% 4	5209
	11th to 20th	Night	37% 6% 12% 23% 14% 9%	5000
	21st to end	Night	41% 5% 15% 23% 13% 3	5280
April	1st to 10th	Night	33% 4 12% 22% 23% 6%	2000
	11th to 20th	Night	26% 4 13% 28% 19% 10%	1979
	21st to end	Night	20% 4 13% 31% 27% 6%	2000
May	1st to 10th	Night	13% 2 11% 27% 27% 20%	4470
	11th to 20th	Night	11% 2 8% 32% 37% 10%	3617
	21st to end	Night	8% 1 9% 36% 36% 10%	3972
June	1st to 10th	Night	16% 4 12% 29% 29% 11%	1600
	11th to 20th	Night	7% 1 7% 21% 36% 27%	1600
	21st to end	Night	8% 2 2 13% 34% 41%	1600
July	1st to 10th	Night	2 1 1 6% 17% 72%	2540
	11th to 20th	Night	8% 2 2 7% 18% 63%	2507
	21st to end	Night	6% 1 1 6% 29% 57%	3011
August	1st to 10th	Night	11% 2 1 8% 29% 48%	2839
	11th to 20th	Night	4 2 1 9% 39% 45%	3109
	21st to end	Night	7% 1 2 9% 38% 43%	3498
September	1st to 10th	Night	5% 1 2 8% 31% 53%	1900
	11th to 20th	Night	12% 3 4 20% 38% 22%	1900
	21st to end	Night	9% 2 4 19% 33% 33%	2124
October	1st to 10th	Night	13% 2 3 11% 21% 50%	3800
	11th to 20th	Night	12% 2 6% 27% 30% 23%	3777
	21st to end	Night	20% 3 5% 21% 31% 21%	4121
November	1st to 10th	Night	26% 4 10% 24% 21% 15%	2483
	11th to 20th	Night	27% 4 11% 26% 23% 9%	2600
	21st to end	Night	30% 3 8% 24% 25% 11%	2725
December	1st to 10th	Night	24% 4 10% 29% 24% 10%	4560
	11th to 20th	Night	49% 6% 11% 18% 12% 3	5176
	21st to end	Night	55% 7% 13% 15% 8% 2	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	34% 7% 14% 23% 18% 4	200
	11th to 20th	Day	18% 7% 18% 26% 22% 9%	370
	21st to end	Day	18% 4 17% 27% 22% 12%	440
February	1st to 10th	Day	23% 9% 13% 21% 24% 10%	400
	11th to 20th	Day	17% 9% 24% 23% 19% 8%	462
	21st to end	Day	16% 7% 21% 28% 24% 4	414
March	1st to 10th	Day	21% 9% 22% 28% 16% 5%	500
	11th to 20th	Day	21% 11% 20% 22% 18% 9%	500
	21st to end	Day	16% 10% 23% 26% 19% 6%	550
April	1st to 10th	Day	14% 8% 22% 28% 19% 8%	500
	11th to 20th	Day	12% 6% 27% 23% 22% 9%	492
	21st to end	Day	6% 6% 19% 34% 21% 15%	500
May	1st to 10th	Day	5% 8% 25% 27% 23% 11%	503
	11th to 20th	Day	5% 5% 22% 27% 29% 12%	528
	21st to end	Day	5% 4 18% 28% 27% 18%	573
June	1st to 10th	Day	3 4 19% 29% 32% 12%	530
	11th to 20th	Day	1 5% 19% 34% 35% 6%	530
	21st to end	Day	1 1 14% 34% 38% 12%	530
July	1st to 10th	Day	1 10% 35% 43% 11%	507
	11th to 20th	Day	6% 34% 47% 12%	484
	21st to end	Day	1 7% 35% 44% 13%	531
August	1st to 10th	Day	2 1 11% 36% 40% 10%	487
	11th to 20th	Day	1 2 10% 34% 39% 14%	477
	21st to end	Day	1 1 9% 30% 46% 13%	539
September	1st to 10th	Day	1 1 9% 33% 41% 14%	490
	11th to 20th	Day	4 3 15% 33% 32% 14%	490
	21st to end	Day	3 1 15% 31% 33% 17%	490
October	1st to 10th	Day	2 4 18% 29% 30% 18%	490
	11th to 20th	Day	6% 5% 11% 25% 31% 21%	490
	21st to end	Day	7% 7% 14% 27% 29% 16%	539
November	1st to 10th	Day	12% 6% 14% 25% 28% 15%	349
	11th to 20th	Day	13% 4 13% 28% 30% 11%	300
	21st to end	Day	13% 3 17% 24% 33% 9%	235
December	1st to 10th	Day	18% 5% 15% 27% 22% 14%	200
	11th to 20th	Day	20% 5% 17% 26% 23% 10%	200
	21st to end	Day	23% 9% 20% 26% 17% 5%	220
January	1st to 10th	Night	20% 4 16% 32% 23% 6%	800
	11th to 20th	Night	15% 4 13% 40% 24% 5%	800
	21st to end	Night	19% 2 13% 38% 24% 5%	880
February	1st to 10th	Night	21% 5% 14% 35% 22% 3	800
	11th to 20th	Night	16% 4 18% 36% 21% 4	784
	21st to end	Night	17% 3 18% 41% 18% 2	660
March	1st to 10th	Night	20% 5% 18% 33% 22% 3	800
	11th to 20th	Night	20% 4 14% 34% 24% 3	800
	21st to end	Night	17% 3 20% 37% 21% 2	880
April	1st to 10th	Night	12% 4 20% 40% 24% 1	800
	11th to 20th	Night	8% 4 22% 37% 24% 4	791
	21st to end	Night	8% 3 17% 39% 31% 3	800
May	1st to 10th	Night	6% 3 23% 39% 26% 4	765
	11th to 20th	Night	5% 1 14% 46% 32% 2	395
	21st to end	Night	5% 1 17% 43% 34% 1	433
June	1st to 10th	Night	3 2 18% 45% 30% 2	400
	11th to 20th	Night	1 1 16% 52% 29% 1	400
	21st to end	Night	2 1 11% 49% 36% 1	400
July	1st to 10th	Night	1 1 7% 50% 41% 1	380
	11th to 20th	Night	1 1 6% 54% 37% 1	370
	21st to end	Night	1 3 46% 47% 3	624
August	1st to 10th	Night	2 7% 50% 39% 2	567
	11th to 20th	Night	1 1 8% 47% 41% 3	717
	21st to end	Night	2 1 4 42% 49% 3	836
September	1st to 10th	Night	1 7% 38% 49% 4	760
	11th to 20th	Night	4 2 11% 38% 39% 6%	760
	21st to end	Night	2 1 13% 38% 39% 8%	760
October	1st to 10th	Night	5% 2 9% 42% 34% 7%	760
	11th to 20th	Night	6% 2 12% 36% 40% 5%	760
	21st to end	Night	6% 2 11% 37% 33% 11%	836
November	1st to 10th	Night	12% 3 11% 35% 32% 7%	780
	11th to 20th	Night	11% 2 12% 43% 28% 5%	800
	21st to end	Night	14% 2 13% 39% 27% 5%	800
December	1st to 10th	Night	16% 3 15% 35% 24% 7%	800
	11th to 20th	Night	27% 2 12% 32% 23% 4	800
	21st to end	Night	24% 3 18% 34% 19% 3	880

Page generated at 2025-07-02 22:30:15. Version ac07

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