Mendocino
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mendocino clear sky chart from 2005-11-06 to 2025-10-19.

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	11% 7% 2 5% 3 3 3 3 5% 8% 49%	135261
	Night	14% 10% 3 6% 3 3 3 3 4 6% 47%	131897
Transparency	Day	32% 3 9% 23% 20% 12%	130113
	Night	39% 3 9% 20% 18% 11%	128052
Seeing	Day	15% 11% 25% 28% 16% 5%	16620
	Night	16% 5% 14% 28% 28% 8%	25748

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% 11% 3 7% 3 4 3 3 5% 7% 34%	3984
	11th to 20th	Day	21% 6% 2 6% 3 2 3 2 3 4 45%	4100
	21st to end	Day	23% 7% 2 5% 4 3 3 2 4 6% 41%	4510
February	1st to 10th	Day	22% 8% 3 5% 4 3 4 3 4 5% 40%	1800
	11th to 20th	Day	19% 9% 3 6% 4 4 4 3 6% 8% 34%	1755
	21st to end	Day	19% 8% 2 6% 3 4 3 3 5% 7% 40%	1661
March	1st to 10th	Day	24% 12% 3 6% 4 3 3 2 4 7% 32%	4952
	11th to 20th	Day	21% 6% 2 6% 4 4 3 3 5% 8% 38%	5000
	21st to end	Day	20% 11% 3 6% 5% 5% 5% 4 6% 8% 27%	5486
April	1st to 10th	Day	16% 8% 2 5% 4 4 4 3 7% 8% 38%	2360
	11th to 20th	Day	12% 8% 3 5% 4 4 4 3 6% 7% 43%	2369
	21st to end	Day	9% 7% 2 5% 3 3 4 3 6% 10% 47%	2480
May	1st to 10th	Day	7% 6% 1 4 2 2 3 2 4 7% 62%	5880
	11th to 20th	Day	5% 4 1 4 3 2 3 2 5% 8% 62%	5840
	21st to end	Day	4 6% 2 3 2 2 2 2 4 7% 65%	6893
June	1st to 10th	Day	9% 10% 3 5% 3 3 3 3 6% 9% 46%	2800
	11th to 20th	Day	3 5% 2 3 3 3 2 2 5% 9% 65%	2800
	21st to end	Day	4 8% 3 5% 4 3 2 2 3 8% 59%	2800
July	1st to 10th	Day	1 4 2 3 2 2 2 2 3 8% 70%	6216
	11th to 20th	Day	1 7% 2 5% 3 2 2 2 4 12% 58%	5880
	21st to end	Day	1 13% 3 7% 3 4 3 3 5% 11% 46%	6402
August	1st to 10th	Day	1 6% 3 5% 3 3 3 2 5% 9% 60%	5777
	11th to 20th	Day	4 9% 2 5% 3 2 2 2 4 9% 57%	5796
	21st to end	Day	2 7% 3 5% 3 3 3 3 5% 9% 57%	5998
September	1st to 10th	Day	3 6% 2 4 3 3 3 2 5% 8% 59%	2400
	11th to 20th	Day	6% 7% 2 4 3 3 4 3 5% 10% 53%	2299
	21st to end	Day	4 4 2 4 3 3 3 2 5% 10% 60%	2159
October	1st to 10th	Day	5% 3 1 3 2 3 3 2 4 6% 69%	3280
	11th to 20th	Day	11% 7% 2 5% 4 4 4 4 6% 7% 45%	3242
	21st to end	Day	11% 6% 3 7% 5% 4 4 3 5% 8% 44%	3448
November	1st to 10th	Day	14% 7% 3 6% 5% 3 4 3 6% 7% 44%	1959
	11th to 20th	Day	17% 7% 2 6% 3 3 4 3 5% 10% 39%	1683
	21st to end	Day	13% 7% 2 5% 3 4 5% 4 5% 7% 47%	1600
December	1st to 10th	Day	11% 7% 2 5% 4 3 4 4 6% 7% 49%	2620
	11th to 20th	Day	31% 8% 2 6% 5% 4 4 4 6% 7% 22%	3028
	21st to end	Day	28% 10% 3 8% 5% 4 4 3 5% 7% 24%	4004
January	1st to 10th	Night	21% 10% 3 7% 5% 4 5% 4 5% 7% 30%	5973
	11th to 20th	Night	19% 8% 3 6% 4 3 3 2 3 5% 44%	5880
	21st to end	Night	25% 9% 3 6% 3 3 3 2 3 4 39%	6468
February	1st to 10th	Night	20% 7% 3 6% 3 3 3 4 4 5% 41%	2600
	11th to 20th	Night	18% 11% 3 7% 5% 4 4 3 5% 6% 34%	2532
	21st to end	Night	19% 7% 2 5% 3 3 3 2 4 6% 45%	1962
March	1st to 10th	Night	23% 15% 4 6% 4 4 4 3 4 6% 29%	4942
	11th to 20th	Night	23% 11% 3 5% 3 3 3 2 3 5% 39%	5000
	21st to end	Night	21% 16% 3 7% 4 4 4 3 5% 6% 28%	5489
April	1st to 10th	Night	16% 9% 3 6% 4 3 4 3 5% 8% 39%	2039
	11th to 20th	Night	13% 8% 3 6% 4 3 3 3 5% 7% 45%	1979
	21st to end	Night	9% 7% 2 3 3 3 4 3 5% 10% 53%	1915
May	1st to 10th	Night	8% 5% 2 3 2 2 2 2 4 5% 65%	4100
	11th to 20th	Night	5% 6% 2 6% 4 3 3 2 3 5% 61%	4073
	21st to end	Night	4 6% 2 3 2 1 2 1 3 4 71%	3977
June	1st to 10th	Night	8% 9% 3 6% 4 5% 3 3 4 6% 49%	1600
	11th to 20th	Night	4 4 2 4 3 3 2 1 4 6% 69%	1600
	21st to end	Night	4 9% 2 5% 2 2 2 2 3 5% 65%	1600
July	1st to 10th	Night	1 6% 1 4 3 2 2 2 3 4 71%	3729
	11th to 20th	Night	1 14% 5% 8% 3 2 3 2 3 5% 54%	4100
	21st to end	Night	1 22% 3 6% 3 3 3 2 3 6% 48%	4472
August	1st to 10th	Night	1 10% 6% 9% 3 3 3 3 3 5% 56%	4035
	11th to 20th	Night	4 15% 3 6% 3 3 3 2 3 5% 53%	4052
	21st to end	Night	4 8% 5% 9% 4 3 3 2 3 6% 53%	4987
September	1st to 10th	Night	3 8% 3 4 2 2 2 1 3 7% 66%	2000
	11th to 20th	Night	7% 9% 3 5% 3 3 4 2 4 6% 55%	2097
	21st to end	Night	5% 7% 3 5% 3 3 3 3 4 6% 60%	2240
October	1st to 10th	Night	4 5% 2 4 3 2 2 1 2 4 71%	3920
	11th to 20th	Night	11% 14% 2 5% 3 3 3 2 3 5% 47%	3873
	21st to end	Night	13% 6% 3 5% 4 4 3 3 5% 7% 48%	4121
November	1st to 10th	Night	13% 9% 3 8% 3 3 4 4 6% 7% 40%	2340
	11th to 20th	Night	16% 10% 2 6% 3 4 4 4 5% 7% 39%	2714
	21st to end	Night	15% 6% 3 6% 3 4 2 3 5% 7% 46%	2800
December	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 5% 8% 48%	4560
	11th to 20th	Night	32% 8% 2 6% 4 4 4 4 5% 6% 25%	5176
	21st to end	Night	32% 9% 3 6% 4 3 3 2 3 5% 29%	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	52% 5% 13% 16% 10% 4	3984
	11th to 20th	Day	44% 1 10% 15% 15% 15%	4100
	21st to end	Day	45% 2 8% 13% 15% 16%	4510
February	1st to 10th	Day	47% 3 9% 17% 17% 6%	1800
	11th to 20th	Day	50% 3 8% 21% 15% 3	1764
	21st to end	Day	46% 4 13% 20% 13% 4	1661
March	1st to 10th	Day	54% 4 8% 22% 12% 1	4952
	11th to 20th	Day	45% 5% 16% 18% 12% 5%	5000
	21st to end	Day	56% 5% 14% 20% 5% 1	5486
April	1st to 10th	Day	46% 3 14% 25% 11% 1	2360
	11th to 20th	Day	42% 4 14% 25% 12% 3	2372
	21st to end	Day	33% 4 17% 29% 16% 2	2480
May	1st to 10th	Day	22% 4 12% 25% 28% 9%	5880
	11th to 20th	Day	18% 3 12% 36% 26% 5%	5843
	21st to end	Day	19% 3 13% 51% 13%	6895
June	1st to 10th	Day	34% 3 11% 30% 19% 3	2800
	11th to 20th	Day	18% 3 10% 28% 34% 7%	2800
	21st to end	Day	21% 2 4 25% 35% 13%	2800
July	1st to 10th	Day	11% 2 4 18% 26% 39%	6150
	11th to 20th	Day	15% 2 5% 13% 25% 41%	5811
	21st to end	Day	21% 5% 10% 18% 28% 19%	6217
August	1st to 10th	Day	21% 2 5% 20% 30% 22%	4206
	11th to 20th	Day	17% 2 4 31% 33% 14%	4195
	21st to end	Day	18% 4 5% 27% 27% 18%	4325
September	1st to 10th	Day	17% 3 4 17% 33% 26%	2400
	11th to 20th	Day	25% 3 8% 32% 27% 5%	2299
	21st to end	Day	17% 2 6% 29% 31% 15%	2159
October	1st to 10th	Day	17% 3 4 21% 18% 37%	3280
	11th to 20th	Day	30% 4 9% 25% 25% 7%	3242
	21st to end	Day	35% 5% 10% 24% 18% 8%	3448
November	1st to 10th	Day	39% 3 10% 26% 14% 7%	1959
	11th to 20th	Day	43% 5% 13% 21% 12% 7%	1683
	21st to end	Day	37% 4 12% 19% 18% 10%	1600
December	1st to 10th	Day	34% 3 9% 24% 24% 7%	2620
	11th to 20th	Day	59% 5% 11% 14% 9% 2	3028
	21st to end	Day	61% 5% 9% 17% 6% 2	4004
January	1st to 10th	Night	54% 5% 16% 13% 10% 2	5973
	11th to 20th	Night	46% 3 7% 20% 15% 9%	5880
	21st to end	Night	51% 2 5% 9% 20% 13%	6468
February	1st to 10th	Night	46% 3 9% 18% 18% 6%	2600
	11th to 20th	Night	52% 4 8% 20% 13% 4	2534
	21st to end	Night	41% 4 13% 24% 14% 4	1962
March	1st to 10th	Night	58% 3 9% 21% 9% 1	4942
	11th to 20th	Night	50% 3 11% 19% 11% 6%	5000
	21st to end	Night	59% 4 12% 18% 6% 1	5489
April	1st to 10th	Night	45% 5% 13% 23% 11% 2	2039
	11th to 20th	Night	40% 3 14% 26% 14% 3	1979
	21st to end	Night	30% 4 16% 33% 15% 2	1915
May	1st to 10th	Night	24% 3 9% 26% 26% 12%	4097
	11th to 20th	Night	24% 5% 7% 30% 26% 8%	4074
	21st to end	Night	20% 2 9% 49% 19%	3977
June	1st to 10th	Night	34% 3 10% 29% 21% 3	1600
	11th to 20th	Night	18% 3 7% 28% 37% 8%	1600
	21st to end	Night	22% 2 4 17% 38% 17%	1600
July	1st to 10th	Night	12% 3 4 18% 21% 41%	3688
	11th to 20th	Night	26% 2 7% 10% 19% 35%	4046
	21st to end	Night	37% 4 4 11% 23% 21%	4345
August	1st to 10th	Night	39% 3 5% 18% 22% 13%	2929
	11th to 20th	Night	20% 4 6% 24% 29% 18%	2929
	21st to end	Night	20% 3 6% 23% 28% 20%	3593
September	1st to 10th	Night	17% 3 2 17% 29% 32%	2000
	11th to 20th	Night	29% 3 5% 25% 31% 7%	2097
	21st to end	Night	24% 3 5% 21% 27% 19%	2240
October	1st to 10th	Night	19% 2 4 17% 21% 38%	3920
	11th to 20th	Night	38% 5% 6% 17% 27% 7%	3873
	21st to end	Night	35% 4 8% 26% 17% 10%	4121
November	1st to 10th	Night	43% 5% 12% 18% 15% 7%	2340
	11th to 20th	Night	45% 5% 13% 18% 14% 5%	2714
	21st to end	Night	40% 4 11% 19% 17% 10%	2800
December	1st to 10th	Night	35% 5% 9% 24% 18% 8%	4560
	11th to 20th	Night	60% 4 7% 16% 11% 2	5176
	21st to end	Night	61% 2 12% 16% 7% 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	36% 11% 23% 17% 11% 2	350
	11th to 20th	Day	28% 9% 24% 23% 12% 5%	400
	21st to end	Day	25% 10% 23% 23% 15% 5%	440
February	1st to 10th	Day	26% 15% 21% 22% 13% 4	400
	11th to 20th	Day	27% 16% 24% 20% 12% 1	395
	21st to end	Day	21% 17% 26% 24% 10% 2	392
March	1st to 10th	Day	33% 16% 26% 17% 7% 1	500
	11th to 20th	Day	28% 13% 29% 19% 9% 2	500
	21st to end	Day	26% 17% 29% 19% 8% 2	550
April	1st to 10th	Day	22% 18% 29% 19% 10% 2	500
	11th to 20th	Day	22% 16% 29% 21% 10% 3	492
	21st to end	Day	14% 21% 31% 19% 11% 4	500
May	1st to 10th	Day	17% 13% 30% 27% 10% 3	503
	11th to 20th	Day	16% 17% 28% 23% 12% 4	528
	21st to end	Day	15% 15% 28% 25% 13% 4	573
June	1st to 10th	Day	11% 13% 29% 29% 15% 3	530
	11th to 20th	Day	8% 16% 34% 28% 11% 3	530
	21st to end	Day	6% 6% 29% 35% 19% 5%	530
July	1st to 10th	Day	5% 5% 25% 42% 16% 7%	526
	11th to 20th	Day	2 2 23% 43% 23% 7%	524
	21st to end	Day	3 2 17% 42% 28% 8%	575
August	1st to 10th	Day	5% 4 26% 38% 20% 7%	527
	11th to 20th	Day	5% 5% 22% 36% 26% 6%	517
	21st to end	Day	6% 3 21% 39% 23% 9%	583
September	1st to 10th	Day	7% 5% 24% 35% 22% 8%	530
	11th to 20th	Day	7% 6% 24% 36% 21% 6%	530
	21st to end	Day	6% 9% 21% 32% 23% 9%	530
October	1st to 10th	Day	9% 9% 22% 28% 24% 8%	530
	11th to 20th	Day	14% 9% 17% 26% 24% 10%	523
	21st to end	Day	16% 7% 22% 29% 19% 7%	539
November	1st to 10th	Day	15% 11% 22% 25% 20% 6%	495
	11th to 20th	Day	16% 9% 19% 27% 21% 7%	258
	21st to end	Day	19% 7% 21% 23% 23% 10%	200
December	1st to 10th	Day	23% 11% 20% 22% 16% 10%	200
	11th to 20th	Day	30% 11% 19% 21% 12% 8%	200
	21st to end	Day	28% 13% 20% 20% 15% 5%	220
January	1st to 10th	Night	27% 7% 18% 26% 18% 4	800
	11th to 20th	Night	26% 5% 15% 28% 19% 7%	800
	21st to end	Night	25% 4 14% 28% 23% 5%	880
February	1st to 10th	Night	23% 8% 15% 32% 19% 4	800
	11th to 20th	Night	22% 9% 18% 29% 18% 5%	784
	21st to end	Night	22% 8% 19% 33% 16% 2	660
March	1st to 10th	Night	29% 11% 15% 28% 14% 4	800
	11th to 20th	Night	29% 7% 12% 29% 21% 4	800
	21st to end	Night	25% 9% 19% 27% 17% 3	880
April	1st to 10th	Night	20% 8% 23% 27% 19% 4	800
	11th to 20th	Night	15% 13% 18% 29% 21% 5%	791
	21st to end	Night	12% 8% 18% 32% 24% 6%	715
May	1st to 10th	Night	11% 7% 22% 28% 25% 7%	600
	11th to 20th	Night	9% 5% 17% 30% 31% 7%	591
	21st to end	Night	12% 7% 20% 28% 29% 5%	438
June	1st to 10th	Night	11% 6% 16% 32% 31% 5%	400
	11th to 20th	Night	4 6% 21% 38% 29% 2	400
	21st to end	Night	6% 1 9% 36% 42% 6%	400
July	1st to 10th	Night	3 1 10% 30% 47% 9%	426
	11th to 20th	Night	3 4 28% 49% 16%	582
	21st to end	Night	2 4 22% 51% 21%	657
August	1st to 10th	Night	4 1 5% 28% 48% 14%	597
	11th to 20th	Night	5% 1 5% 27% 48% 15%	594
	21st to end	Night	6% 1 6% 26% 47% 14%	865
September	1st to 10th	Night	5% 1 7% 28% 44% 15%	800
	11th to 20th	Night	9% 3 10% 28% 38% 12%	800
	21st to end	Night	7% 3 10% 28% 36% 16%	800
October	1st to 10th	Night	9% 4 14% 28% 34% 12%	800
	11th to 20th	Night	13% 3 10% 25% 36% 13%	792
	21st to end	Night	14% 4 11% 29% 31% 12%	836
November	1st to 10th	Night	16% 5% 15% 26% 30% 8%	780
	11th to 20th	Night	20% 5% 14% 30% 25% 6%	800
	21st to end	Night	18% 6% 15% 32% 24% 5%	800
December	1st to 10th	Night	23% 6% 15% 28% 22% 7%	800
	11th to 20th	Night	31% 8% 14% 27% 16% 4	800
	21st to end	Night	31% 8% 18% 24% 15% 4	880

Page generated at 2025-10-15 22:50:26. Version ac07

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