Sea Ranch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sea Ranch clear sky chart from 2005-11-06 to 2025-11-18.

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% 4 1 4 3 3 3 3 5% 7% 59%	135508
	Night	12% 5% 2 4 3 3 3 3 4 6% 54%	132308
Transparency	Day	17% 37% 5% 14% 16% 12%	130372
	Night	21% 37% 4 12% 14% 11%	128436
Seeing	Day	14% 9% 25% 27% 19% 6%	16776
	Night	15% 6% 17% 28% 26% 8%	25968

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	16% 8% 2 6% 4 3 4 4 6% 7% 39%	4006
	11th to 20th	Day	20% 6% 3 5% 3 3 3 3 3 5% 47%	4100
	21st to end	Day	21% 7% 2 5% 3 3 3 3 5% 5% 43%	4510
February	1st to 10th	Day	22% 7% 2 6% 4 3 3 3 4 5% 42%	1800
	11th to 20th	Day	18% 6% 1 6% 4 3 4 4 7% 8% 38%	1755
	21st to end	Day	18% 6% 2 5% 3 3 4 3 5% 6% 44%	1676
March	1st to 10th	Day	20% 10% 3 6% 4 4 3 4 5% 7% 34%	4952
	11th to 20th	Day	18% 5% 2 5% 3 3 4 3 6% 8% 45%	5000
	21st to end	Day	21% 10% 2 5% 4 4 5% 4 6% 8% 30%	5486
April	1st to 10th	Day	16% 7% 2 5% 5% 3 4 4 6% 8% 41%	2373
	11th to 20th	Day	9% 6% 3 5% 4 4 4 3 5% 8% 48%	2369
	21st to end	Day	8% 3 1 4 3 3 4 4 7% 10% 54%	2430
May	1st to 10th	Day	4 2 1 3 2 2 3 2 4 6% 72%	5880
	11th to 20th	Day	3 2 1 2 2 2 3 2 5% 7% 72%	5840
	21st to end	Day	3 2 2 1 1 1 2 3 6% 79%	6876
June	1st to 10th	Day	5% 5% 1 3 3 3 4 3 6% 9% 58%	2800
	11th to 20th	Day	2 2 1 2 2 2 2 3 5% 6% 74%	2800
	21st to end	Day	3 1 1 1 2 2 2 2 4 5% 77%	2800
July	1st to 10th	Day	1 1 1 2 1 2 2 2 3 6% 81%	6192
	11th to 20th	Day	3 1 3 3 3 5% 4 5% 7% 67%	5880
	21st to end	Day	2 1 3 2 2 3 2 5% 7% 73%	6402
August	1st to 10th	Day	1 1 1 2 1 2 2 2 5% 7% 77%	5777
	11th to 20th	Day	1 2 1 2 2 2 2 3 4 6% 76%	5752
	21st to end	Day	1 2 1 3 3 3 4 4 6% 8% 65%	5984
September	1st to 10th	Day	1 2 1 3 1 2 2 3 3 8% 75%	2400
	11th to 20th	Day	4 3 1 4 3 3 3 3 5% 8% 65%	2325
	21st to end	Day	4 3 1 2 2 2 3 2 4 7% 71%	2144
October	1st to 10th	Day	4 2 1 3 2 2 2 2 3 5% 73%	3280
	11th to 20th	Day	10% 2 1 3 2 2 3 3 5% 7% 62%	3261
	21st to end	Day	8% 5% 2 5% 3 3 4 4 6% 9% 51%	3558
November	1st to 10th	Day	11% 5% 2 4 3 3 4 4 6% 9% 49%	2059
	11th to 20th	Day	16% 7% 2 6% 3 3 4 3 5% 7% 45%	1789
	21st to end	Day	13% 4 1 4 4 4 3 4 5% 8% 50%	1600
December	1st to 10th	Day	12% 5% 1 4 3 3 4 4 7% 9% 49%	2620
	11th to 20th	Day	29% 9% 2 5% 4 5% 4 4 6% 7% 24%	3028
	21st to end	Day	26% 9% 3 6% 4 4 4 4 6% 7% 25%	4004
January	1st to 10th	Night	16% 7% 2 5% 4 4 4 4 7% 8% 39%	5940
	11th to 20th	Night	17% 7% 3 6% 4 3 3 3 3 4 48%	5880
	21st to end	Night	22% 8% 3 5% 3 3 3 3 3 5% 43%	6468
February	1st to 10th	Night	20% 6% 2 4 3 3 3 3 5% 6% 44%	2600
	11th to 20th	Night	17% 8% 2 5% 3 4 4 4 6% 7% 40%	2532
	21st to end	Night	16% 7% 1 4 2 3 4 2 5% 6% 49%	1947
March	1st to 10th	Night	18% 12% 3 7% 5% 4 5% 4 5% 6% 32%	4942
	11th to 20th	Night	21% 4 2 4 3 3 3 2 3 6% 48%	5000
	21st to end	Night	18% 11% 3 7% 4 4 4 3 5% 7% 34%	5489
April	1st to 10th	Night	13% 7% 2 5% 4 4 3 3 5% 8% 45%	2025
	11th to 20th	Night	9% 6% 2 5% 4 4 4 3 5% 6% 51%	1979
	21st to end	Night	7% 4 1 3 3 3 3 3 5% 8% 60%	1965
May	1st to 10th	Night	4 3 1 3 1 2 2 2 3 6% 73%	4100
	11th to 20th	Night	4 2 1 4 2 2 2 2 3 7% 69%	4073
	21st to end	Night	2 2 1 2 1 1 2 1 3 5% 80%	3981
June	1st to 10th	Night	5% 7% 2 4 2 4 3 3 5% 8% 56%	1600
	11th to 20th	Night	3 2 1 2 2 2 2 2 4 6% 75%	1600
	21st to end	Night	2 3 1 2 2 2 2 2 3 4 76%	1600
July	1st to 10th	Night	1 1 1 2 2 3 3 2 4 4 77%	3768
	11th to 20th	Night	1 6% 3 7% 5% 4 4 3 3 4 61%	4100
	21st to end	Night	1 5% 2 4 2 2 3 3 3 6% 70%	4472
August	1st to 10th	Night	1 2 1 2 1 2 3 2 4 5% 77%	4035
	11th to 20th	Night	1 2 3 2 2 3 3 6% 5% 73%	4097
	21st to end	Night	2 2 1 4 4 3 4 4 5% 5% 65%	5015
September	1st to 10th	Night	1 1 1 2 2 2 2 2 3 7% 76%	2000
	11th to 20th	Night	4 4 2 4 2 2 2 2 4 7% 67%	2070
	21st to end	Night	4 4 2 3 2 2 2 2 4 5% 70%	2255
October	1st to 10th	Night	3 3 1 3 2 2 2 2 3 4 74%	3920
	11th to 20th	Night	7% 6% 2 4 2 2 2 2 4 6% 64%	3897
	21st to end	Night	12% 3 1 3 2 3 3 3 5% 7% 57%	4253
November	1st to 10th	Night	13% 5% 2 4 3 3 4 4 5% 8% 49%	2460
	11th to 20th	Night	16% 5% 1 4 4 4 4 4 6% 8% 43%	2757
	21st to end	Night	14% 5% 2 5% 3 3 3 3 4 7% 51%	2800
December	1st to 10th	Night	13% 5% 1 4 3 3 3 3 6% 8% 52%	4560
	11th to 20th	Night	30% 7% 2 5% 4 4 4 4 6% 7% 27%	5176
	21st to end	Night	35% 7% 2 5% 3 3 3 2 3 5% 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	31% 28% 12% 15% 8% 6%	4006
	11th to 20th	Day	34% 25% 5% 11% 13% 12%	4100
	21st to end	Day	32% 27% 5% 10% 11% 15%	4510
February	1st to 10th	Day	22% 61% 3 7% 5% 2	1800
	11th to 20th	Day	22% 60% 3 7% 6% 2	1764
	21st to end	Day	15% 61% 4 11% 6% 3	1676
March	1st to 10th	Day	39% 28% 4 15% 14% 1	4952
	11th to 20th	Day	27% 29% 12% 16% 9% 7%	5000
	21st to end	Day	39% 30% 10% 15% 5% 1	5486
April	1st to 10th	Day	16% 61% 4 13% 5% 1	2373
	11th to 20th	Day	13% 62% 6% 10% 7% 2	2372
	21st to end	Day	11% 62% 6% 12% 8% 1	2430
May	1st to 10th	Day	9% 28% 9% 17% 24% 13%	5880
	11th to 20th	Day	8% 29% 5% 24% 25% 8%	5843
	21st to end	Day	4 27% 4 33% 31% 1	6878
June	1st to 10th	Day	6% 61% 3 14% 13% 4	2800
	11th to 20th	Day	4 61% 3 11% 16% 6%	2800
	21st to end	Day	3 59% 1 6% 18% 12%	2800
July	1st to 10th	Day	4 25% 1 8% 21% 41%	6126
	11th to 20th	Day	7% 27% 1 5% 14% 45%	5811
	21st to end	Day	7% 27% 4 13% 27% 23%	6217
August	1st to 10th	Day	9% 35% 1 6% 19% 30%	4206
	11th to 20th	Day	7% 36% 1 15% 27% 14%	4163
	21st to end	Day	9% 36% 3 15% 20% 16%	4311
September	1st to 10th	Day	6% 57% 1 9% 17% 10%	2400
	11th to 20th	Day	8% 57% 3 15% 14% 4	2325
	21st to end	Day	9% 56% 2 10% 17% 6%	2144
October	1st to 10th	Day	8% 34% 1 9% 14% 34%	3280
	11th to 20th	Day	15% 36% 5% 17% 21% 7%	3261
	21st to end	Day	21% 36% 6% 15% 16% 6%	3558
November	1st to 10th	Day	12% 53% 4 16% 12% 3	2059
	11th to 20th	Day	19% 54% 6% 12% 7% 2	1789
	21st to end	Day	16% 57% 4 9% 9% 6%	1600
December	1st to 10th	Day	19% 36% 4 18% 19% 4	2620
	11th to 20th	Day	42% 32% 6% 10% 7% 2	3028
	21st to end	Day	44% 26% 8% 16% 5% 1	4004
January	1st to 10th	Night	31% 28% 16% 11% 11% 5%	5940
	11th to 20th	Night	34% 26% 5% 13% 13% 9%	5880
	21st to end	Night	35% 27% 3 6% 12% 16%	6468
February	1st to 10th	Night	22% 61% 3 6% 6% 3	2600
	11th to 20th	Night	21% 60% 4 7% 6% 3	2534
	21st to end	Night	12% 61% 4 12% 6% 4	1947
March	1st to 10th	Night	41% 28% 5% 15% 9% 3	4942
	11th to 20th	Night	27% 28% 7% 17% 14% 7%	5000
	21st to end	Night	39% 30% 10% 15% 5% 1	5489
April	1st to 10th	Night	17% 61% 3 10% 7% 1	2025
	11th to 20th	Night	13% 63% 4 11% 8% 2	1979
	21st to end	Night	11% 61% 5% 13% 8% 1	1965
May	1st to 10th	Night	11% 27% 4 17% 23% 18%	4097
	11th to 20th	Night	12% 28% 3 19% 25% 13%	4074
	21st to end	Night	5% 27% 4 29% 33% 3	3981
June	1st to 10th	Night	9% 61% 2 10% 13% 5%	1600
	11th to 20th	Night	4 61% 3 9% 16% 7%	1600
	21st to end	Night	5% 59% 2 3 15% 17%	1600
July	1st to 10th	Night	5% 27% 1 7% 19% 41%	3726
	11th to 20th	Night	18% 31% 2 3 9% 37%	4046
	21st to end	Night	10% 28% 2 9% 23% 28%	4345
August	1st to 10th	Night	9% 36% 1 4 17% 33%	2929
	11th to 20th	Night	5% 36% 1 9% 31% 17%	2961
	21st to end	Night	11% 36% 1 10% 22% 19%	3608
September	1st to 10th	Night	6% 57% 1 8% 16% 12%	2000
	11th to 20th	Night	8% 56% 3 11% 15% 6%	2070
	21st to end	Night	10% 56% 1 9% 13% 11%	2255
October	1st to 10th	Night	9% 35% 1 4 14% 37%	3920
	11th to 20th	Night	15% 36% 3 16% 20% 10%	3897
	21st to end	Night	19% 35% 3 16% 18% 9%	4253
November	1st to 10th	Night	14% 53% 5% 12% 11% 5%	2460
	11th to 20th	Night	20% 55% 6% 10% 8% 2	2757
	21st to end	Night	17% 57% 3 10% 7% 6%	2800
December	1st to 10th	Night	20% 35% 4 19% 15% 7%	4560
	11th to 20th	Night	42% 32% 4 9% 9% 3	5176
	21st to end	Night	44% 26% 7% 16% 5% 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	31% 15% 22% 17% 13% 3	359
	11th to 20th	Day	28% 10% 23% 21% 16% 4	400
	21st to end	Day	23% 10% 20% 27% 16% 3	440
February	1st to 10th	Day	26% 12% 22% 19% 17% 4	400
	11th to 20th	Day	29% 14% 24% 18% 13% 2	395
	21st to end	Day	21% 14% 28% 23% 11% 4	408
March	1st to 10th	Day	29% 14% 27% 19% 9% 1	500
	11th to 20th	Day	29% 11% 29% 19% 10% 3	500
	21st to end	Day	24% 16% 30% 20% 7% 3	550
April	1st to 10th	Day	23% 16% 29% 21% 8% 3	500
	11th to 20th	Day	16% 17% 33% 21% 10% 4	492
	21st to end	Day	14% 17% 32% 21% 11% 4	500
May	1st to 10th	Day	10% 14% 34% 26% 13% 4	503
	11th to 20th	Day	16% 13% 28% 25% 14% 3	528
	21st to end	Day	10% 12% 29% 30% 13% 5%	573
June	1st to 10th	Day	8% 11% 34% 28% 14% 5%	530
	11th to 20th	Day	6% 15% 35% 28% 15% 1	530
	21st to end	Day	2 5% 29% 36% 23% 5%	530
July	1st to 10th	Day	3 6% 25% 40% 19% 6%	526
	11th to 20th	Day	2 24% 42% 25% 7%	524
	21st to end	Day	1 2 17% 44% 28% 9%	575
August	1st to 10th	Day	2 3 27% 38% 24% 7%	527
	11th to 20th	Day	2 4 24% 34% 29% 8%	517
	21st to end	Day	2 4 19% 37% 29% 9%	583
September	1st to 10th	Day	3 5% 22% 34% 28% 8%	530
	11th to 20th	Day	5% 8% 22% 33% 24% 8%	530
	21st to end	Day	6% 6% 23% 28% 28% 8%	530
October	1st to 10th	Day	9% 7% 23% 24% 28% 10%	530
	11th to 20th	Day	13% 7% 18% 24% 26% 12%	530
	21st to end	Day	13% 6% 22% 27% 25% 7%	583
November	1st to 10th	Day	16% 9% 21% 25% 22% 8%	535
	11th to 20th	Day	21% 9% 16% 22% 22% 9%	298
	21st to end	Day	19% 5% 21% 22% 23% 12%	200
December	1st to 10th	Day	23% 10% 21% 20% 16% 12%	200
	11th to 20th	Day	31% 9% 17% 20% 14% 10%	200
	21st to end	Day	31% 10% 20% 17% 14% 8%	220
January	1st to 10th	Night	25% 7% 17% 27% 19% 6%	800
	11th to 20th	Night	23% 7% 16% 28% 20% 6%	800
	21st to end	Night	22% 7% 14% 28% 24% 6%	880
February	1st to 10th	Night	25% 7% 17% 26% 22% 4	800
	11th to 20th	Night	24% 9% 20% 23% 18% 6%	784
	21st to end	Night	21% 10% 20% 31% 15% 3	660
March	1st to 10th	Night	27% 11% 17% 25% 17% 4	800
	11th to 20th	Night	25% 8% 17% 29% 18% 4	800
	21st to end	Night	22% 10% 24% 26% 15% 3	880
April	1st to 10th	Night	18% 10% 24% 26% 17% 5%	800
	11th to 20th	Night	15% 12% 25% 27% 16% 5%	791
	21st to end	Night	11% 11% 30% 24% 19% 5%	765
May	1st to 10th	Night	8% 10% 28% 29% 20% 6%	600
	11th to 20th	Night	8% 5% 24% 35% 23% 6%	591
	21st to end	Night	8% 9% 26% 29% 23% 5%	442
June	1st to 10th	Night	8% 7% 29% 29% 23% 3	400
	11th to 20th	Night	3 11% 27% 38% 20% 2	400
	21st to end	Night	4 3 20% 42% 27% 4	400
July	1st to 10th	Night	2 3 19% 34% 34% 9%	438
	11th to 20th	Night	1 1 13% 32% 40% 13%	582
	21st to end	Night	2 1 10% 29% 42% 16%	657
August	1st to 10th	Night	2 2 13% 34% 37% 13%	597
	11th to 20th	Night	1 1 13% 33% 37% 15%	613
	21st to end	Night	3 2 10% 28% 46% 12%	880
September	1st to 10th	Night	2 3 14% 29% 38% 15%	800
	11th to 20th	Night	6% 4 13% 32% 35% 9%	800
	21st to end	Night	6% 6% 12% 29% 35% 12%	800
October	1st to 10th	Night	7% 6% 14% 30% 31% 11%	800
	11th to 20th	Night	10% 5% 12% 26% 36% 12%	800
	21st to end	Night	14% 4 13% 27% 31% 11%	880
November	1st to 10th	Night	15% 6% 15% 25% 30% 9%	820
	11th to 20th	Night	20% 6% 14% 27% 27% 6%	828
	21st to end	Night	17% 6% 16% 29% 25% 7%	800
December	1st to 10th	Night	26% 4 13% 28% 22% 8%	800
	11th to 20th	Night	30% 7% 15% 26% 18% 4	800
	21st to end	Night	28% 8% 19% 25% 16% 4	880

Page generated at 2025-11-14 21:56:01. Version ac07

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