Sacramento
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sacramento clear sky chart from 2005-11-06 to 2025-08-05.

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	7% 4 1 4 3 2 3 2 4 6% 65%	129592
	Night	9% 4 1 4 3 3 3 2 4 6% 62%	127366
Transparency	Day	22% 4 7% 19% 25% 24%	129302
	Night	24% 5% 8% 19% 23% 21%	127170
Seeing	Day	8% 5% 17% 28% 28% 14%	16185
	Night	9% 4 18% 40% 26% 3	25468

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	19% 8% 3 7% 5% 5% 5% 4 5% 6% 35%	3639
	11th to 20th	Day	15% 9% 3 6% 3 3 3 2 2 4 49%	4082
	21st to end	Day	18% 8% 3 7% 4 4 4 3 4 6% 40%	4510
February	1st to 10th	Day	19% 5% 3 6% 4 3 3 3 4 5% 44%	1800
	11th to 20th	Day	14% 5% 2 5% 4 4 5% 3 5% 8% 45%	1866
	21st to end	Day	14% 6% 2 5% 4 3 4 3 4 8% 48%	1650
March	1st to 10th	Day	12% 9% 3 8% 5% 4 5% 3 5% 7% 37%	4731
	11th to 20th	Day	14% 5% 2 5% 4 3 3 4 5% 8% 47%	5000
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 41%	5655
April	1st to 10th	Day	12% 7% 2 4 3 3 4 4 5% 7% 48%	2400
	11th to 20th	Day	9% 3 1 4 3 3 4 3 5% 7% 57%	2369
	21st to end	Day	5% 4 1 2 2 3 3 3 5% 8% 63%	2400
May	1st to 10th	Day	4 2 1 2 1 1 2 2 2 5% 78%	5571
	11th to 20th	Day	2 2 1 2 1 1 2 2 3 6% 79%	6223
	21st to end	Day	2 1 2 2 1 1 2 3 6% 80%	6902
June	1st to 10th	Day	3 3 1 3 2 2 2 3 4 6% 69%	2800
	11th to 20th	Day	1 1 1 1 1 2 2 4 5% 82%	2800
	21st to end	Day	2 1 2 1 1 2 1 3 4 83%	2800
July	1st to 10th	Day	1 1 1 1 1 3 92%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	6232
	21st to end	Day	1 2 1 2 2 1 2 2 3 5% 80%	6402
August	1st to 10th	Day	1 1 1 2 1 1 2 2 4 6% 77%	4144
	11th to 20th	Day	1 1 1 1 1 1 1 3 4 87%	3796
	21st to end	Day	2 2 1 1 1 1 1 2 3 87%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 87%	2280
	11th to 20th	Day	2 2 1 3 2 2 2 2 4 6% 74%	2280
	21st to end	Day	3 2 1 2 1 1 2 2 2 5% 79%	2038
October	1st to 10th	Day	1 1 1 2 2 2 2 2 4 6% 76%	3180
	11th to 20th	Day	4 2 1 3 2 2 2 2 4 6% 72%	3161
	21st to end	Day	4 3 2 4 3 3 3 3 4 6% 64%	3448
November	1st to 10th	Day	9% 5% 1 3 3 3 3 3 5% 8% 57%	1835
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 7% 47%	1800
	21st to end	Day	13% 6% 2 5% 3 3 3 3 6% 7% 50%	1647
December	1st to 10th	Day	8% 4 2 6% 4 4 4 4 7% 10% 48%	2620
	11th to 20th	Day	26% 10% 5% 12% 6% 5% 3 3 4 5% 21%	3028
	21st to end	Day	25% 8% 3 8% 5% 5% 5% 4 6% 7% 24%	4004
January	1st to 10th	Night	10% 6% 2 7% 5% 5% 4 3 6% 9% 42%	6306
	11th to 20th	Night	15% 5% 2 6% 4 3 3 3 4 5% 51%	5880
	21st to end	Night	16% 7% 3 7% 4 3 4 3 4 6% 46%	6468
February	1st to 10th	Night	18% 6% 2 4 3 4 3 3 4 4 49%	2600
	11th to 20th	Night	13% 5% 2 5% 3 4 4 4 5% 9% 48%	2420
	21st to end	Night	13% 4 2 4 3 3 3 3 4 6% 54%	1980
March	1st to 10th	Night	14% 7% 3 7% 4 4 4 3 4 7% 42%	5160
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 4 6% 52%	5000
	21st to end	Night	15% 6% 2 5% 3 3 4 3 5% 8% 44%	5319
April	1st to 10th	Night	9% 5% 3 4 3 3 3 3 5% 6% 55%	2000
	11th to 20th	Night	7% 4 1 4 3 3 4 3 5% 6% 60%	1979
	21st to end	Night	5% 3 1 3 3 2 3 2 5% 8% 66%	2000
May	1st to 10th	Night	2 2 1 2 1 2 2 2 3 5% 78%	4406
	11th to 20th	Night	1 1 1 2 1 2 2 2 3 4 79%	3672
	21st to end	Night	1 1 1 1 1 1 1 2 4 85%	3972
June	1st to 10th	Night	3 3 1 2 3 2 2 3 5% 6% 71%	1600
	11th to 20th	Night	1 1 1 2 2 1 2 3 3 85%	1600
	21st to end	Night	2 1 1 1 1 1 2 1 2 3 86%	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 1 1 2 88%	3727
	21st to end	Night	2 1 3 2 2 2 2 3 5% 79%	4472
August	1st to 10th	Night	2 2 1 2 2 2 2 2 3 6% 76%	2884
	11th to 20th	Night	1 1 1 1 1 1 2 3 89%	3048
	21st to end	Night	2 2 1 1 1 1 1 2 3 87%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 4 87%	1900
	11th to 20th	Night	3 2 1 3 2 2 2 2 3 4 76%	1900
	21st to end	Night	1 1 2 2 1 2 1 3 5% 81%	2137
October	1st to 10th	Night	3 1 1 2 2 2 2 2 3 4 78%	3800
	11th to 20th	Night	3 1 1 2 2 2 2 2 3 6% 77%	3777
	21st to end	Night	5% 2 1 3 3 3 4 3 4 6% 67%	4121
November	1st to 10th	Night	9% 4 2 3 2 2 3 3 5% 8% 58%	2462
	11th to 20th	Night	9% 4 2 5% 3 3 3 3 6% 7% 57%	2600
	21st to end	Night	12% 4 2 4 3 3 3 2 4 5% 58%	2750
December	1st to 10th	Night	10% 3 2 4 3 3 4 3 5% 8% 57%	4560
	11th to 20th	Night	22% 8% 3 8% 5% 5% 5% 4 7% 8% 26%	5176
	21st to end	Night	23% 7% 3 9% 6% 6% 5% 4 5% 6% 27%	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	47% 8% 13% 17% 11% 4	3639
	11th to 20th	Day	40% 5% 7% 17% 17% 14%	4082
	21st to end	Day	43% 6% 8% 13% 20% 9%	4510
February	1st to 10th	Day	42% 4 10% 20% 16% 7%	1800
	11th to 20th	Day	37% 6% 12% 21% 18% 6%	1875
	21st to end	Day	36% 5% 9% 24% 20% 6%	1650
March	1st to 10th	Day	44% 7% 9% 20% 18% 3	4731
	11th to 20th	Day	34% 6% 12% 26% 13% 8%	5000
	21st to end	Day	37% 9% 14% 26% 12% 2	5655
April	1st to 10th	Day	34% 6% 12% 23% 18% 7%	2400
	11th to 20th	Day	27% 6% 12% 24% 22% 9%	2372
	21st to end	Day	20% 5% 14% 28% 27% 7%	2400
May	1st to 10th	Day	12% 2 6% 26% 31% 24%	5571
	11th to 20th	Day	9% 3 6% 27% 41% 15%	6226
	21st to end	Day	8% 3 8% 28% 40% 13%	6904
June	1st to 10th	Day	16% 4 9% 28% 29% 14%	2800
	11th to 20th	Day	8% 3 7% 21% 34% 27%	2800
	21st to end	Day	7% 2 3 12% 37% 39%	2800
July	1st to 10th	Day	3 1 1 7% 19% 69%	6251
	11th to 20th	Day	5% 1 1 6% 14% 72%	6157
	21st to end	Day	8% 3 4 14% 26% 45%	6217
August	1st to 10th	Day	9% 2 2 8% 26% 53%	4129
	11th to 20th	Day	4 1 2 10% 40% 43%	3833
	21st to end	Day	7% 1 3 10% 38% 42%	4179
September	1st to 10th	Day	5% 1 2 8% 30% 53%	2280
	11th to 20th	Day	12% 3 7% 21% 39% 18%	2280
	21st to end	Day	11% 2 5% 18% 35% 29%	2038
October	1st to 10th	Day	12% 2 5% 12% 26% 44%	3180
	11th to 20th	Day	15% 3 6% 24% 32% 19%	3161
	21st to end	Day	20% 3 8% 20% 34% 15%	3448
November	1st to 10th	Day	26% 5% 11% 26% 23% 10%	1835
	11th to 20th	Day	36% 5% 12% 23% 18% 5%	1800
	21st to end	Day	32% 7% 11% 22% 20% 9%	1647
December	1st to 10th	Day	30% 7% 11% 28% 18% 6%	2620
	11th to 20th	Day	65% 5% 7% 12% 9% 2	3028
	21st to end	Day	55% 9% 12% 15% 8% 1	4004
January	1st to 10th	Night	34% 10% 18% 22% 11% 4	6306
	11th to 20th	Night	35% 7% 9% 17% 21% 11%	5880
	21st to end	Night	40% 5% 10% 12% 23% 11%	6468
February	1st to 10th	Night	38% 5% 8% 21% 20% 9%	2600
	11th to 20th	Night	33% 5% 10% 23% 21% 7%	2422
	21st to end	Night	30% 6% 9% 25% 23% 7%	1980
March	1st to 10th	Night	38% 8% 12% 19% 19% 4	5160
	11th to 20th	Night	34% 5% 13% 23% 16% 9%	5000
	21st to end	Night	36% 7% 14% 26% 14% 3	5319
April	1st to 10th	Night	29% 6% 14% 21% 22% 8%	2000
	11th to 20th	Night	24% 5% 12% 27% 21% 11%	1979
	21st to end	Night	17% 5% 12% 30% 28% 7%	2000
May	1st to 10th	Night	9% 4 9% 27% 29% 22%	4403
	11th to 20th	Night	9% 2 7% 30% 37% 15%	3672
	21st to end	Night	7% 2 7% 33% 38% 14%	3972
June	1st to 10th	Night	15% 4 11% 27% 30% 13%	1600
	11th to 20th	Night	7% 2 7% 20% 34% 30%	1600
	21st to end	Night	7% 2 3 11% 34% 42%	1600
July	1st to 10th	Night	3 1 1 7% 20% 68%	3600
	11th to 20th	Night	6% 1 1 5% 12% 76%	3679
	21st to end	Night	9% 2 3 16% 25% 45%	4345
August	1st to 10th	Night	12% 2 1 8% 26% 51%	2875
	11th to 20th	Night	4 1 2 8% 39% 47%	3077
	21st to end	Night	8% 1 2 9% 35% 47%	3498
September	1st to 10th	Night	5% 1 3 7% 30% 54%	1900
	11th to 20th	Night	12% 2 5% 19% 38% 23%	1900
	21st to end	Night	9% 1 5% 18% 32% 35%	2137
October	1st to 10th	Night	12% 3 4 10% 20% 51%	3800
	11th to 20th	Night	11% 3 6% 28% 29% 23%	3777
	21st to end	Night	19% 4 6% 19% 31% 20%	4121
November	1st to 10th	Night	24% 5% 10% 24% 22% 15%	2462
	11th to 20th	Night	26% 4 12% 27% 22% 8%	2600
	21st to end	Night	28% 6% 8% 22% 25% 11%	2750
December	1st to 10th	Night	25% 6% 9% 28% 23% 9%	4560
	11th to 20th	Night	54% 7% 9% 16% 12% 2	5176
	21st to end	Night	53% 9% 11% 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	37% 5% 13% 23% 19% 4	200
	11th to 20th	Day	18% 7% 18% 23% 23% 11%	395
	21st to end	Day	16% 6% 18% 22% 22% 16%	440
February	1st to 10th	Day	21% 8% 17% 19% 22% 15%	400
	11th to 20th	Day	16% 10% 23% 23% 19% 10%	452
	21st to end	Day	12% 12% 20% 29% 21% 7%	414
March	1st to 10th	Day	16% 11% 23% 28% 17% 5%	500
	11th to 20th	Day	18% 11% 21% 21% 17% 11%	500
	21st to end	Day	16% 11% 23% 24% 19% 7%	550
April	1st to 10th	Day	13% 9% 23% 26% 18% 11%	500
	11th to 20th	Day	9% 7% 26% 28% 16% 13%	492
	21st to end	Day	4 9% 18% 32% 22% 15%	500
May	1st to 10th	Day	5% 6% 26% 30% 19% 13%	503
	11th to 20th	Day	7% 4 21% 27% 26% 15%	528
	21st to end	Day	4 4 18% 28% 29% 17%	573
June	1st to 10th	Day	3 4 20% 28% 32% 13%	530
	11th to 20th	Day	2 5% 19% 30% 36% 8%	530
	21st to end	Day	1 1 15% 34% 35% 14%	530
July	1st to 10th	Day	1 2 11% 37% 38% 12%	526
	11th to 20th	Day	8% 37% 38% 16%	524
	21st to end	Day	1 10% 35% 39% 15%	575
August	1st to 10th	Day	2 1 14% 35% 35% 13%	504
	11th to 20th	Day	1 1 11% 33% 37% 17%	477
	21st to end	Day	1 1 9% 28% 40% 20%	539
September	1st to 10th	Day	1 1 10% 27% 41% 19%	490
	11th to 20th	Day	3 3 14% 28% 32% 19%	490
	21st to end	Day	2 2 15% 26% 32% 22%	490
October	1st to 10th	Day	2 5% 16% 27% 27% 23%	490
	11th to 20th	Day	4 6% 14% 20% 31% 25%	490
	21st to end	Day	7% 6% 15% 23% 29% 19%	539
November	1st to 10th	Day	11% 6% 17% 20% 25% 21%	371
	11th to 20th	Day	11% 7% 15% 23% 30% 13%	300
	21st to end	Day	14% 2 13% 31% 27% 12%	223
December	1st to 10th	Day	17% 7% 16% 21% 25% 15%	200
	11th to 20th	Day	20% 7% 15% 24% 26% 10%	200
	21st to end	Day	21% 9% 21% 27% 17% 5%	220
January	1st to 10th	Night	18% 5% 17% 34% 23% 4	800
	11th to 20th	Night	16% 5% 14% 38% 24% 4	800
	21st to end	Night	18% 4 15% 35% 26% 3	880
February	1st to 10th	Night	20% 4 16% 35% 22% 3	800
	11th to 20th	Night	14% 6% 21% 35% 21% 3	784
	21st to end	Night	13% 6% 21% 39% 18% 2	660
March	1st to 10th	Night	16% 7% 21% 34% 21% 2	800
	11th to 20th	Night	17% 7% 18% 34% 23% 2	800
	21st to end	Night	15% 5% 23% 36% 20% 1	880
April	1st to 10th	Night	10% 4 24% 39% 21% 2	800
	11th to 20th	Night	7% 6% 26% 36% 21% 3	791
	21st to end	Night	5% 4 25% 41% 24% 2	800
May	1st to 10th	Night	4 3 28% 41% 21% 2	737
	11th to 20th	Night	3 1 25% 52% 19% 1	424
	21st to end	Night	3 3 25% 49% 20%	433
June	1st to 10th	Night	5% 2 24% 50% 20%	400
	11th to 20th	Night	1 2 27% 51% 19%	400
	21st to end	Night	1 1 23% 51% 23% 1	400
July	1st to 10th	Night	1 17% 59% 23%	390
	11th to 20th	Night	1 1 14% 62% 21%	423
	21st to end	Night	1 13% 56% 28% 1	657
August	1st to 10th	Night	2 2 18% 53% 24% 1	579
	11th to 20th	Night	1 2 15% 50% 31% 2	698
	21st to end	Night	1 1 12% 47% 38% 1	836
September	1st to 10th	Night	1 2 11% 46% 39% 2	760
	11th to 20th	Night	2 2 17% 43% 32% 4	760
	21st to end	Night	2 1 15% 40% 36% 6%	760
October	1st to 10th	Night	4 3 13% 41% 32% 6%	760
	11th to 20th	Night	5% 3 15% 36% 35% 5%	760
	21st to end	Night	5% 3 14% 37% 32% 9%	836
November	1st to 10th	Night	9% 3 17% 32% 34% 6%	780
	11th to 20th	Night	10% 2 16% 41% 28% 4	800
	21st to end	Night	13% 4 15% 39% 26% 4	800
December	1st to 10th	Night	16% 5% 16% 34% 24% 6%	800
	11th to 20th	Night	24% 4 13% 33% 23% 3	800
	21st to end	Night	21% 6% 21% 31% 20% 1	880

Page generated at 2025-08-01 22:32:24. Version ac07

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