Chico Community Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chico Community Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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% 67%	134917
	Night	12% 4 1 3 2 2 3 2 4 5% 61%	131189
Transparency	Day	22% 4 7% 18% 25% 25%	129823
	Night	26% 4 8% 17% 22% 22%	127273
Seeing	Day	11% 7% 18% 28% 27% 10%	16183
	Night	12% 4 17% 36% 27% 3	25368

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	23% 6% 2 4 3 3 3 3 5% 6% 42%	3639
	11th to 20th	Day	20% 6% 2 5% 3 2 2 1 2 4 54%	3988
	21st to end	Day	23% 5% 2 4 2 2 2 2 4 5% 48%	4510
February	1st to 10th	Day	22% 5% 1 3 3 3 3 4 4 5% 47%	1800
	11th to 20th	Day	18% 6% 2 5% 3 3 4 3 4 6% 46%	1821
	21st to end	Day	18% 5% 2 4 2 3 3 3 4 7% 49%	1650
March	1st to 10th	Day	23% 9% 3 6% 3 4 3 3 4 5% 37%	4738
	11th to 20th	Day	22% 5% 1 4 3 3 3 2 4 6% 46%	5000
	21st to end	Day	19% 7% 2 6% 4 4 4 4 5% 7% 38%	5634
April	1st to 10th	Day	15% 6% 2 4 4 3 3 3 5% 8% 47%	2400
	11th to 20th	Day	13% 4 1 4 3 3 3 4 5% 5% 57%	2369
	21st to end	Day	7% 4 2 3 3 3 3 3 5% 8% 62%	2400
May	1st to 10th	Day	5% 2 1 2 2 1 2 1 3 6% 75%	5743
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 77%	6277
	21st to end	Day	2 1 1 1 1 1 2 2 4 5% 79%	6902
June	1st to 10th	Day	5% 3 1 2 3 2 3 3 5% 8% 65%	2800
	11th to 20th	Day	2 1 1 2 2 1 2 2 3 6% 79%	2800
	21st to end	Day	2 1 1 1 1 1 2 2 2 3 85%	2800
July	1st to 10th	Day	1 1 1 1 2 3 92%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 90%	6320
	21st to end	Day	1 1 1 1 1 2 3 4 84%	6413
August	1st to 10th	Day	1 1 1 1 1 1 2 2 3 4 82%	5777
	11th to 20th	Day	1 1 1 1 1 1 3 92%	5585
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	5981
September	1st to 10th	Day	1 1 1 1 2 1 2 3 87%	2317
	11th to 20th	Day	3 2 1 2 2 1 2 2 3 5% 76%	2280
	21st to end	Day	3 1 1 1 1 1 2 3 5% 80%	2027
October	1st to 10th	Day	3 2 1 2 2 2 2 2 3 4 77%	3180
	11th to 20th	Day	6% 3 1 3 2 2 2 3 4 6% 70%	3161
	21st to end	Day	7% 3 1 4 3 4 4 4 6% 7% 57%	3448
November	1st to 10th	Day	14% 3 1 3 3 3 3 3 6% 6% 54%	1816
	11th to 20th	Day	16% 4 2 5% 3 3 3 3 5% 7% 49%	1770
	21st to end	Day	15% 5% 1 4 3 3 4 3 5% 5% 53%	1600
December	1st to 10th	Day	12% 2 1 3 2 3 3 3 5% 6% 60%	2620
	11th to 20th	Day	32% 6% 2 5% 3 3 4 3 5% 7% 30%	3028
	21st to end	Day	33% 6% 2 4 3 3 3 3 4 6% 31%	4004
January	1st to 10th	Night	15% 6% 2 6% 4 3 4 4 6% 7% 42%	6306
	11th to 20th	Night	20% 4 1 4 2 3 2 2 3 4 54%	5985
	21st to end	Night	21% 6% 2 5% 3 3 3 2 3 5% 47%	6468
February	1st to 10th	Night	21% 5% 2 4 3 3 3 3 4 6% 48%	2600
	11th to 20th	Night	17% 6% 2 4 4 3 4 3 5% 6% 47%	2464
	21st to end	Night	17% 5% 2 4 3 3 4 3 4 6% 49%	1980
March	1st to 10th	Night	20% 8% 3 7% 4 4 3 3 4 6% 37%	5139
	11th to 20th	Night	23% 7% 2 4 2 2 2 3 4 5% 47%	5000
	21st to end	Night	21% 9% 3 6% 4 3 4 4 5% 7% 35%	5341
April	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 6% 9% 47%	2000
	11th to 20th	Night	11% 4 1 4 3 3 3 2 4 7% 57%	1979
	21st to end	Night	7% 4 1 4 2 2 3 3 4 6% 64%	2000
May	1st to 10th	Night	4 3 1 4 2 2 2 2 3 5% 70%	4222
	11th to 20th	Night	3 3 1 3 2 3 2 3 4 5% 71%	3616
	21st to end	Night	2 2 1 1 1 2 2 2 3 5% 79%	3972
June	1st to 10th	Night	5% 2 1 4 3 3 3 4 5% 8% 63%	1600
	11th to 20th	Night	2 1 2 2 1 2 1 4 6% 78%	1600
	21st to end	Night	2 1 1 2 2 1 2 2 3 3 82%	1600
July	1st to 10th	Night	1 1 1 1 1 2 3 91%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 1 1 2 89%	3640
	21st to end	Night	1 1 1 2 2 4 5% 83%	4444
August	1st to 10th	Night	2 2 1 2 1 1 2 2 3 3 81%	4035
	11th to 20th	Night	1 1 1 1 1 2 2 90%	4272
	21st to end	Night	2 1 2 1 2 2 1 2 3 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	1930
	11th to 20th	Night	4 2 1 2 2 2 3 2 3 5% 76%	1900
	21st to end	Night	3 1 1 1 1 1 2 2 4 5% 80%	2150
October	1st to 10th	Night	3 2 2 2 2 2 1 3 3 80%	3800
	11th to 20th	Night	4 2 1 2 1 2 3 2 3 5% 75%	3777
	21st to end	Night	9% 2 1 3 3 3 3 3 5% 7% 63%	4121
November	1st to 10th	Night	14% 4 1 3 2 3 4 4 5% 7% 54%	2480
	11th to 20th	Night	14% 3 1 3 3 3 3 4 6% 8% 51%	2625
	21st to end	Night	15% 4 2 4 2 3 3 3 4 5% 55%	2800
December	1st to 10th	Night	13% 3 1 2 2 2 3 2 5% 7% 61%	4560
	11th to 20th	Night	33% 6% 2 4 3 4 4 4 6% 7% 27%	5176
	21st to end	Night	33% 6% 2 6% 4 3 3 3 3 4 32%	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	43% 7% 12% 22% 11% 5%	3639
	11th to 20th	Day	37% 4 5% 16% 23% 13%	3988
	21st to end	Day	39% 5% 8% 13% 19% 15%	4510
February	1st to 10th	Day	38% 5% 11% 18% 19% 9%	1800
	11th to 20th	Day	37% 6% 9% 20% 21% 7%	1830
	21st to end	Day	34% 5% 11% 26% 17% 7%	1650
March	1st to 10th	Day	45% 7% 7% 20% 17% 3	4738
	11th to 20th	Day	39% 4 9% 23% 16% 8%	5000
	21st to end	Day	40% 9% 13% 24% 12% 2	5634
April	1st to 10th	Day	34% 6% 11% 23% 19% 6%	2400
	11th to 20th	Day	28% 5% 11% 23% 23% 10%	2372
	21st to end	Day	21% 5% 13% 29% 24% 8%	2400
May	1st to 10th	Day	12% 3 8% 21% 32% 24%	5743
	11th to 20th	Day	11% 3 6% 24% 38% 19%	6280
	21st to end	Day	8% 3 9% 29% 36% 14%	6904
June	1st to 10th	Day	17% 5% 11% 25% 27% 15%	2800
	11th to 20th	Day	8% 3 8% 21% 32% 28%	2800
	21st to end	Day	7% 1 4 14% 32% 42%	2800
July	1st to 10th	Day	3 1 2 8% 22% 65%	6251
	11th to 20th	Day	5% 1 1 5% 17% 72%	6245
	21st to end	Day	6% 1 5% 17% 28% 43%	6227
August	1st to 10th	Day	10% 2 2 7% 27% 51%	4206
	11th to 20th	Day	4 1 1 10% 41% 43%	4044
	21st to end	Day	6% 1 3 13% 38% 39%	4311
September	1st to 10th	Day	5% 1 3 8% 26% 57%	2317
	11th to 20th	Day	12% 3 6% 21% 36% 22%	2280
	21st to end	Day	10% 2 6% 20% 30% 33%	2027
October	1st to 10th	Day	12% 3 3 12% 23% 47%	3180
	11th to 20th	Day	17% 3 5% 24% 30% 22%	3161
	21st to end	Day	23% 4 7% 22% 30% 14%	3448
November	1st to 10th	Day	28% 5% 11% 23% 20% 14%	1816
	11th to 20th	Day	34% 7% 11% 22% 17% 9%	1770
	21st to end	Day	31% 6% 9% 20% 21% 14%	1600
December	1st to 10th	Day	26% 4 6% 24% 28% 12%	2620
	11th to 20th	Day	55% 4 11% 16% 11% 3	3028
	21st to end	Day	53% 7% 12% 18% 8% 2	4004
January	1st to 10th	Night	39% 7% 17% 18% 13% 6%	6306
	11th to 20th	Night	34% 7% 7% 15% 23% 14%	5985
	21st to end	Night	42% 5% 7% 8% 17% 20%	6468
February	1st to 10th	Night	36% 5% 9% 21% 17% 13%	2600
	11th to 20th	Night	35% 5% 9% 20% 22% 8%	2466
	21st to end	Night	35% 5% 9% 23% 20% 8%	1980
March	1st to 10th	Night	42% 9% 11% 19% 14% 5%	5139
	11th to 20th	Night	38% 5% 9% 20% 19% 9%	5000
	21st to end	Night	43% 8% 13% 22% 12% 3	5341
April	1st to 10th	Night	31% 6% 14% 22% 19% 8%	2000
	11th to 20th	Night	25% 6% 12% 25% 20% 12%	1979
	21st to end	Night	21% 5% 13% 27% 26% 8%	2000
May	1st to 10th	Night	14% 5% 8% 20% 29% 23%	4219
	11th to 20th	Night	16% 3 7% 21% 33% 20%	3616
	21st to end	Night	9% 4 8% 28% 35% 16%	3972
June	1st to 10th	Night	18% 7% 11% 26% 26% 12%	1600
	11th to 20th	Night	9% 3 9% 18% 31% 29%	1600
	21st to end	Night	9% 2 4 13% 31% 41%	1600
July	1st to 10th	Night	3 1 2 8% 21% 66%	3600
	11th to 20th	Night	6% 1 1 4 14% 73%	3592
	21st to end	Night	6% 1 4 19% 27% 42%	4318
August	1st to 10th	Night	14% 2 2 8% 25% 50%	2929
	11th to 20th	Night	4 2 2 9% 36% 48%	3084
	21st to end	Night	7% 1 2 11% 35% 44%	3608
September	1st to 10th	Night	5% 2 2 6% 27% 58%	1930
	11th to 20th	Night	12% 3 6% 17% 36% 26%	1900
	21st to end	Night	9% 2 5% 16% 31% 37%	2150
October	1st to 10th	Night	12% 2 3 10% 18% 54%	3800
	11th to 20th	Night	14% 3 7% 23% 30% 24%	3777
	21st to end	Night	20% 3 5% 23% 28% 20%	4121
November	1st to 10th	Night	28% 6% 10% 24% 17% 16%	2480
	11th to 20th	Night	29% 7% 14% 22% 19% 10%	2625
	21st to end	Night	31% 5% 7% 21% 22% 14%	2800
December	1st to 10th	Night	25% 3 8% 24% 23% 17%	4560
	11th to 20th	Night	55% 4 9% 16% 13% 3	5176
	21st to end	Night	55% 6% 10% 14% 11% 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	41% 8% 15% 22% 15% 1	200
	11th to 20th	Day	23% 9% 19% 28% 14% 7%	353
	21st to end	Day	21% 5% 20% 23% 23% 7%	440
February	1st to 10th	Day	25% 9% 19% 25% 18% 5%	400
	11th to 20th	Day	22% 13% 22% 24% 15% 4	429
	21st to end	Day	15% 12% 25% 28% 16% 3	414
March	1st to 10th	Day	25% 17% 25% 20% 12% 2	500
	11th to 20th	Day	26% 9% 22% 22% 15% 5%	500
	21st to end	Day	21% 15% 23% 25% 13% 3	550
April	1st to 10th	Day	17% 13% 25% 24% 16% 5%	500
	11th to 20th	Day	14% 12% 24% 25% 19% 6%	492
	21st to end	Day	7% 12% 21% 30% 23% 7%	500
May	1st to 10th	Day	7% 9% 34% 25% 20% 6%	503
	11th to 20th	Day	8% 9% 20% 25% 25% 12%	528
	21st to end	Day	4 7% 19% 28% 30% 11%	573
June	1st to 10th	Day	4 7% 18% 29% 32% 10%	530
	11th to 20th	Day	3 7% 21% 34% 28% 7%	530
	21st to end	Day	2 3 15% 34% 33% 14%	530
July	1st to 10th	Day	1 2 12% 36% 37% 12%	526
	11th to 20th	Day	1 8% 33% 43% 14%	524
	21st to end	Day	1 7% 34% 43% 14%	575
August	1st to 10th	Day	2 1 11% 32% 39% 16%	527
	11th to 20th	Day	1 2 11% 28% 41% 18%	517
	21st to end	Day	2 1 9% 28% 45% 15%	583
September	1st to 10th	Day	1 2 10% 31% 43% 14%	503
	11th to 20th	Day	4 4 18% 28% 32% 14%	490
	21st to end	Day	3 4 19% 34% 30% 11%	490
October	1st to 10th	Day	4 7% 20% 25% 30% 15%	490
	11th to 20th	Day	8% 6% 18% 24% 31% 14%	490
	21st to end	Day	12% 7% 20% 27% 24% 10%	539
November	1st to 10th	Day	16% 7% 18% 22% 25% 11%	352
	11th to 20th	Day	15% 5% 16% 29% 23% 11%	285
	21st to end	Day	15% 6% 20% 27% 26% 8%	200
December	1st to 10th	Day	23% 6% 20% 22% 15% 15%	200
	11th to 20th	Day	28% 4 19% 26% 17% 8%	200
	21st to end	Day	24% 9% 19% 24% 20% 3	220
January	1st to 10th	Night	22% 5% 17% 35% 19% 2	800
	11th to 20th	Night	21% 5% 15% 39% 18% 2	800
	21st to end	Night	19% 4 19% 35% 21% 3	880
February	1st to 10th	Night	21% 4 17% 38% 19% 2	800
	11th to 20th	Night	18% 7% 20% 33% 20% 2	784
	21st to end	Night	18% 5% 24% 37% 14% 1	660
March	1st to 10th	Night	22% 10% 20% 30% 16% 3	800
	11th to 20th	Night	21% 7% 19% 28% 22% 2	800
	21st to end	Night	19% 9% 23% 33% 16% 1	880
April	1st to 10th	Night	12% 8% 27% 34% 18% 2	800
	11th to 20th	Night	12% 7% 23% 34% 21% 3	791
	21st to end	Night	8% 6% 22% 37% 24% 3	800
May	1st to 10th	Night	7% 7% 30% 32% 23% 1	657
	11th to 20th	Night	5% 4 23% 39% 27% 2	394
	21st to end	Night	5% 7% 19% 40% 27% 1	433
June	1st to 10th	Night	7% 3 19% 44% 27% 2	400
	11th to 20th	Night	1 5% 22% 47% 24% 1	400
	21st to end	Night	2 2 17% 46% 31% 2	400
July	1st to 10th	Night	1 1 12% 48% 35% 2	390
	11th to 20th	Night	1 1 12% 51% 34% 2	388
	21st to end	Night	1 7% 40% 48% 4	642
August	1st to 10th	Night	2 2 8% 38% 47% 3	597
	11th to 20th	Night	1 1 9% 43% 43% 5%	684
	21st to end	Night	1 1 6% 38% 51% 3	880
September	1st to 10th	Night	1 1 8% 38% 47% 4	772
	11th to 20th	Night	4 2 11% 39% 38% 7%	760
	21st to end	Night	4 3 14% 36% 36% 8%	760
October	1st to 10th	Night	5% 4 17% 35% 33% 6%	760
	11th to 20th	Night	7% 3 15% 34% 36% 5%	760
	21st to end	Night	9% 4 15% 36% 29% 7%	836
November	1st to 10th	Night	14% 6% 14% 32% 30% 4	780
	11th to 20th	Night	14% 2 19% 39% 24% 2	800
	21st to end	Night	15% 4 17% 39% 22% 3	800
December	1st to 10th	Night	21% 4 14% 35% 22% 4	800
	11th to 20th	Night	29% 4 16% 30% 20% 2	800
	21st to end	Night	25% 6% 19% 33% 16% 1	880

Page generated at 2025-08-31 22:51:04. Version ac07

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