Feather River Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Feather River Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	10% 3 1 3 2 2 2 2 3 5% 66%	136288
	Night	13% 4 1 3 2 2 3 2 4 5% 61%	133128
Transparency	Day	22% 3 6% 18% 25% 25%	131203
	Night	27% 4 7% 18% 22% 22%	129200
Seeing	Day	11% 6% 19% 29% 26% 8%	16656
	Night	13% 3 14% 35% 30% 5%	26013

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	25% 5% 2 4 3 3 3 3 4 6% 42%	3719
	11th to 20th	Day	19% 6% 2 4 3 2 2 2 2 3 55%	4070
	21st to end	Day	23% 5% 2 4 2 3 3 2 4 6% 47%	4609
February	1st to 10th	Day	23% 4 1 4 3 3 3 3 4 6% 45%	1801
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 5% 6% 45%	1845
	21st to end	Day	19% 5% 1 3 2 3 3 3 5% 7% 48%	1650
March	1st to 10th	Day	22% 10% 3 6% 4 3 3 3 4 5% 37%	4709
	11th to 20th	Day	21% 6% 2 5% 3 3 3 3 4 6% 46%	5000
	21st to end	Day	20% 7% 2 5% 4 4 4 3 5% 8% 37%	5658
April	1st to 10th	Day	15% 5% 2 5% 4 3 3 3 5% 7% 48%	2400
	11th to 20th	Day	12% 4 1 4 3 3 2 3 5% 6% 56%	2369
	21st to end	Day	7% 4 1 3 3 3 3 3 4 7% 61%	2400
May	1st to 10th	Day	5% 2 1 2 1 1 2 2 3 6% 74%	5694
	11th to 20th	Day	4 2 1 2 2 2 2 2 3 5% 77%	6277
	21st to end	Day	2 1 2 1 1 2 2 3 5% 80%	6902
June	1st to 10th	Day	5% 3 1 3 2 3 3 3 5% 6% 66%	2800
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 5% 79%	2800
	21st to end	Day	2 1 1 1 1 1 1 2 3 85%	2800
July	1st to 10th	Day	1 1 1 2 2 92%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 90%	6320
	21st to end	Day	1 1 1 1 1 1 2 2 3 4 85%	6432
August	1st to 10th	Day	1 1 1 1 1 2 2 3 4 83%	5777
	11th to 20th	Day	1 1 1 1 1 1 2 92%	5536
	21st to end	Day	1 1 2 1 1 1 1 2 3 86%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 87%	2400
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 4 78%	2400
	21st to end	Day	4 2 1 2 1 1 2 1 3 5% 79%	2142
October	1st to 10th	Day	3 2 1 3 2 2 2 2 3 5% 76%	3280
	11th to 20th	Day	7% 2 1 2 1 2 2 2 4 6% 69%	3261
	21st to end	Day	7% 3 1 3 3 3 5% 3 5% 7% 59%	3558
November	1st to 10th	Day	14% 4 1 3 2 3 3 4 4 8% 54%	1901
	11th to 20th	Day	17% 4 1 5% 3 3 4 3 5% 7% 49%	1890
	21st to end	Day	14% 4 1 3 3 2 3 3 4 6% 55%	1685
December	1st to 10th	Day	11% 3 1 3 3 3 3 3 5% 6% 58%	2700
	11th to 20th	Day	32% 6% 2 5% 3 3 3 4 4 8% 30%	3108
	21st to end	Day	31% 6% 2 5% 3 3 4 3 5% 7% 30%	4092
January	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 5% 8% 42%	6446
	11th to 20th	Night	19% 4 2 4 2 3 3 2 3 4 55%	6122
	21st to end	Night	21% 6% 2 5% 3 3 3 3 3 5% 46%	6611
February	1st to 10th	Night	21% 5% 2 3 3 3 2 3 4 6% 48%	2600
	11th to 20th	Night	17% 5% 2 5% 3 3 4 3 5% 6% 47%	2441
	21st to end	Night	19% 5% 2 4 3 3 3 3 4 5% 49%	1980
March	1st to 10th	Night	23% 9% 2 6% 4 3 3 3 4 6% 36%	5170
	11th to 20th	Night	24% 7% 2 4 2 2 2 3 3 5% 46%	5000
	21st to end	Night	23% 9% 2 6% 3 4 4 3 5% 7% 35%	5315
April	1st to 10th	Night	12% 7% 2 4 4 3 4 4 5% 7% 49%	2000
	11th to 20th	Night	11% 5% 1 4 2 2 3 3 5% 7% 57%	1979
	21st to end	Night	8% 4 1 2 3 2 3 2 4 7% 64%	2000
May	1st to 10th	Night	4 2 1 3 2 2 2 2 3 6% 72%	4277
	11th to 20th	Night	3 2 1 2 2 3 3 3 4 5% 72%	3616
	21st to end	Night	2 1 1 1 1 2 2 2 3 4 82%	3972
June	1st to 10th	Night	5% 3 1 3 3 2 4 3 5% 7% 65%	1600
	11th to 20th	Night	2 1 1 2 2 2 2 4 6% 78%	1600
	21st to end	Night	2 1 1 2 2 2 2 2 3 3 81%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 92%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 89%	3640
	21st to end	Night	1 1 1 1 2 2 2 3 4 82%	4422
August	1st to 10th	Night	2 2 1 2 1 1 1 2 3 3 82%	4035
	11th to 20th	Night	1 1 1 1 1 2 2 90%	4323
	21st to end	Night	2 1 1 2 1 2 1 2 2 3 83%	5015
September	1st to 10th	Night	2 1 1 1 1 2 1 2 3 87%	2000
	11th to 20th	Night	4 2 1 3 2 2 2 2 3 5% 76%	2000
	21st to end	Night	3 1 1 1 1 1 2 2 3 4 80%	2253
October	1st to 10th	Night	4 2 1 3 2 2 2 2 2 3 78%	3920
	11th to 20th	Night	4 2 1 2 1 2 2 2 4 4 76%	3897
	21st to end	Night	10% 2 1 2 2 3 3 3 5% 7% 63%	4253
November	1st to 10th	Night	15% 4 1 3 2 2 3 3 5% 7% 54%	2616
	11th to 20th	Night	15% 4 1 3 3 3 3 4 6% 7% 52%	2730
	21st to end	Night	15% 4 1 3 3 4 3 2 3 6% 55%	2933
December	1st to 10th	Night	12% 3 1 3 2 2 2 3 5% 7% 61%	4700
	11th to 20th	Night	32% 6% 2 4 4 3 4 4 6% 7% 29%	5316
	21st to end	Night	32% 7% 2 5% 4 3 3 3 4 5% 32%	7106

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	44% 6% 10% 23% 9% 8%	3719
	11th to 20th	Day	38% 3 4 16% 23% 16%	4070
	21st to end	Day	40% 4 7% 14% 19% 15%	4609
February	1st to 10th	Day	40% 3 9% 19% 19% 9%	1801
	11th to 20th	Day	39% 5% 9% 19% 21% 7%	1854
	21st to end	Day	37% 3 9% 25% 18% 8%	1650
March	1st to 10th	Day	48% 5% 6% 20% 17% 3	4709
	11th to 20th	Day	40% 5% 9% 24% 14% 8%	5000
	21st to end	Day	43% 7% 12% 25% 11% 2	5658
April	1st to 10th	Day	35% 5% 10% 24% 20% 5%	2400
	11th to 20th	Day	29% 4 12% 23% 23% 9%	2372
	21st to end	Day	22% 5% 12% 30% 26% 6%	2400
May	1st to 10th	Day	13% 2 7% 23% 34% 21%	5694
	11th to 20th	Day	12% 3 6% 26% 39% 14%	6280
	21st to end	Day	9% 2 8% 30% 39% 11%	6904
June	1st to 10th	Day	18% 4 10% 25% 27% 14%	2800
	11th to 20th	Day	9% 3 7% 19% 33% 29%	2800
	21st to end	Day	7% 2 3 13% 36% 40%	2800
July	1st to 10th	Day	3 1 1 7% 21% 67%	6251
	11th to 20th	Day	4 1 2 5% 14% 74%	6245
	21st to end	Day	6% 2 3 17% 28% 44%	6246
August	1st to 10th	Day	10% 1 3 7% 27% 52%	4206
	11th to 20th	Day	4 1 2 7% 41% 45%	4007
	21st to end	Day	6% 1 2 10% 40% 42%	4311
September	1st to 10th	Day	6% 1 2 9% 27% 55%	2400
	11th to 20th	Day	13% 2 5% 20% 38% 22%	2400
	21st to end	Day	11% 2 5% 18% 32% 33%	2142
October	1st to 10th	Day	13% 2 5% 11% 25% 44%	3280
	11th to 20th	Day	16% 4 4 22% 32% 22%	3261
	21st to end	Day	23% 4 6% 22% 29% 16%	3558
November	1st to 10th	Day	28% 4 8% 25% 20% 14%	1901
	11th to 20th	Day	36% 4 11% 22% 18% 8%	1890
	21st to end	Day	30% 5% 9% 21% 22% 13%	1685
December	1st to 10th	Day	26% 3 7% 24% 28% 11%	2700
	11th to 20th	Day	54% 5% 10% 15% 13% 3	3108
	21st to end	Day	53% 6% 13% 17% 9% 2	4092
January	1st to 10th	Night	39% 7% 16% 19% 12% 7%	6446
	11th to 20th	Night	35% 4 7% 15% 22% 16%	6122
	21st to end	Night	42% 3 8% 10% 18% 19%	6611
February	1st to 10th	Night	38% 4 8% 19% 19% 12%	2600
	11th to 20th	Night	36% 4 9% 21% 22% 9%	2443
	21st to end	Night	36% 4 7% 22% 21% 9%	1980
March	1st to 10th	Night	48% 6% 7% 18% 15% 6%	5170
	11th to 20th	Night	40% 5% 8% 21% 17% 9%	5000
	21st to end	Night	47% 6% 10% 22% 12% 3	5315
April	1st to 10th	Night	33% 6% 12% 22% 20% 8%	2000
	11th to 20th	Night	27% 5% 12% 24% 19% 13%	1979
	21st to end	Night	21% 4 13% 28% 25% 8%	2000
May	1st to 10th	Night	15% 3 10% 22% 28% 22%	4274
	11th to 20th	Night	14% 3 6% 24% 36% 17%	3616
	21st to end	Night	9% 2 8% 28% 39% 14%	3972
June	1st to 10th	Night	18% 6% 11% 26% 28% 12%	1600
	11th to 20th	Night	8% 4 8% 18% 32% 31%	1600
	21st to end	Night	10% 2 3 12% 31% 42%	1600
July	1st to 10th	Night	3 1 1 7% 20% 68%	3600
	11th to 20th	Night	6% 2 5% 10% 77%	3592
	21st to end	Night	7% 2 3 18% 26% 44%	4297
August	1st to 10th	Night	13% 1 2 7% 25% 51%	2929
	11th to 20th	Night	4 1 2 8% 35% 50%	3122
	21st to end	Night	7% 1 2 11% 33% 47%	3608
September	1st to 10th	Night	6% 1 2 7% 28% 56%	2000
	11th to 20th	Night	13% 2 5% 17% 38% 25%	2000
	21st to end	Night	9% 2 5% 16% 30% 38%	2253
October	1st to 10th	Night	14% 2 4 10% 18% 53%	3920
	11th to 20th	Night	14% 3 5% 24% 31% 24%	3897
	21st to end	Night	21% 3 5% 21% 31% 20%	4253
November	1st to 10th	Night	29% 4 9% 23% 19% 16%	2616
	11th to 20th	Night	30% 5% 12% 25% 18% 10%	2730
	21st to end	Night	31% 4 7% 22% 23% 13%	2933
December	1st to 10th	Night	24% 3 8% 24% 23% 17%	4700
	11th to 20th	Night	53% 4 8% 16% 15% 4	5316
	21st to end	Night	55% 6% 10% 15% 10% 3	7106

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	42% 6% 19% 20% 12% 2	220
	11th to 20th	Day	22% 7% 17% 30% 16% 7%	375
	21st to end	Day	19% 7% 20% 26% 22% 6%	473
February	1st to 10th	Day	25% 8% 18% 26% 21% 3	401
	11th to 20th	Day	22% 10% 25% 24% 16% 3	440
	21st to end	Day	18% 10% 22% 33% 12% 4	414
March	1st to 10th	Day	28% 11% 28% 19% 12% 2	500
	11th to 20th	Day	26% 10% 23% 20% 15% 5%	500
	21st to end	Day	19% 14% 26% 25% 14% 2	550
April	1st to 10th	Day	17% 10% 28% 24% 17% 4	500
	11th to 20th	Day	14% 11% 27% 24% 18% 6%	492
	21st to end	Day	7% 11% 24% 31% 22% 6%	500
May	1st to 10th	Day	7% 12% 28% 28% 19% 5%	503
	11th to 20th	Day	8% 10% 22% 28% 27% 7%	528
	21st to end	Day	4 7% 23% 27% 29% 10%	573
June	1st to 10th	Day	5% 7% 19% 29% 30% 10%	530
	11th to 20th	Day	4 6% 22% 37% 23% 8%	530
	21st to end	Day	1 4 16% 36% 33% 10%	530
July	1st to 10th	Day	2 2 14% 38% 37% 7%	526
	11th to 20th	Day	1 11% 37% 40% 10%	524
	21st to end	Day	1 1 10% 36% 41% 11%	575
August	1st to 10th	Day	1 2 13% 34% 37% 14%	527
	11th to 20th	Day	1 2 16% 31% 35% 15%	517
	21st to end	Day	1 2 10% 32% 41% 14%	583
September	1st to 10th	Day	2 3 11% 38% 34% 12%	530
	11th to 20th	Day	5% 4 19% 27% 35% 10%	530
	21st to end	Day	3 5% 20% 30% 32% 10%	530
October	1st to 10th	Day	4 8% 21% 24% 32% 11%	530
	11th to 20th	Day	7% 6% 18% 25% 33% 11%	530
	21st to end	Day	12% 5% 19% 28% 25% 10%	583
November	1st to 10th	Day	17% 3 17% 25% 28% 10%	377
	11th to 20th	Day	16% 5% 16% 30% 21% 11%	330
	21st to end	Day	13% 5% 21% 30% 22% 9%	223
December	1st to 10th	Day	20% 6% 17% 25% 21% 10%	220
	11th to 20th	Day	29% 3 17% 20% 21% 10%	220
	21st to end	Day	27% 6% 24% 21% 19% 3	242
January	1st to 10th	Night	25% 4 15% 28% 22% 5%	840
	11th to 20th	Night	18% 3 15% 34% 24% 4	840
	21st to end	Night	20% 2 15% 35% 23% 5%	924
February	1st to 10th	Night	22% 4 14% 35% 22% 4	800
	11th to 20th	Night	17% 5% 16% 34% 23% 4	784
	21st to end	Night	20% 5% 17% 38% 17% 3	660
March	1st to 10th	Night	25% 7% 17% 30% 18% 4	800
	11th to 20th	Night	22% 6% 14% 30% 25% 3	800
	21st to end	Night	20% 6% 20% 32% 20% 2	880
April	1st to 10th	Night	13% 6% 21% 38% 21% 2	800
	11th to 20th	Night	12% 5% 19% 36% 23% 6%	791
	21st to end	Night	8% 5% 18% 37% 28% 4	800
May	1st to 10th	Night	8% 3 22% 36% 26% 4	686
	11th to 20th	Night	6% 4 18% 39% 31% 3	394
	21st to end	Night	6% 3 15% 36% 38% 2	433
June	1st to 10th	Night	6% 2 15% 38% 35% 4	400
	11th to 20th	Night	1 3 20% 41% 33% 3	400
	21st to end	Night	3 1 13% 41% 38% 4	400
July	1st to 10th	Night	1 8% 43% 44% 4	390
	11th to 20th	Night	1 1 8% 45% 43% 3	388
	21st to end	Night	1 4 34% 55% 6%	633
August	1st to 10th	Night	3 1 5% 38% 49% 5%	597
	11th to 20th	Night	1 6% 39% 47% 7%	709
	21st to end	Night	1 1 5% 35% 53% 5%	880
September	1st to 10th	Night	2 1 9% 35% 48% 6%	800
	11th to 20th	Night	5% 1 9% 34% 42% 9%	800
	21st to end	Night	3 2 12% 37% 36% 10%	800
October	1st to 10th	Night	7% 2 12% 40% 31% 8%	800
	11th to 20th	Night	7% 3 13% 34% 37% 6%	800
	21st to end	Night	9% 3 13% 35% 30% 11%	880
November	1st to 10th	Night	15% 3 13% 30% 31% 8%	820
	11th to 20th	Night	16% 2 15% 35% 28% 5%	840
	21st to end	Night	14% 2 13% 36% 29% 6%	840
December	1st to 10th	Night	20% 3 12% 33% 25% 7%	840
	11th to 20th	Night	29% 3 12% 31% 22% 4	840
	21st to end	Night	28% 3 16% 31% 19% 2	924

Page generated at 2026-01-28 22:21:44. Version ac07

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