San Clemente Island (North)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Clemente Island (North) clear sky chart from 2005-11-06 to 2024-01-17.

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	5% 6% 2 5% 4 3 4 5% 7% 12% 46%	75117
	Night	6% 7% 2 4 4 2 3 4 5% 9% 52%	74713
Transparency	Day	29% 5% 8% 17% 23% 19%	74845
	Night	28% 4 7% 16% 23% 22%	74507
Seeing	Day	7% 7% 17% 32% 28% 9%	14690
	Night	9% 6% 13% 27% 32% 13%	24716

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	6% 6% 3 4 6% 3 4 4 7% 9% 47%	1568
	11th to 20th	Day	7% 4 1 3 4 3 4 5% 5% 8% 55%	1755
	21st to end	Day	9% 6% 3 5% 4 2 4 3 3 7% 55%	1980
February	1st to 10th	Day	8% 5% 3 4 5% 3 4 4 5% 9% 50%	1800
	11th to 20th	Day	7% 4 2 4 5% 3 4 5% 6% 10% 51%	1750
	21st to end	Day	8% 4 2 5% 4 3 4 4 6% 9% 52%	1480
March	1st to 10th	Day	6% 4 2 4 4 3 5% 5% 8% 11% 48%	1800
	11th to 20th	Day	6% 8% 3 6% 4 3 4 5% 7% 12% 43%	1865
	21st to end	Day	5% 6% 2 4 4 2 4 4 5% 10% 53%	2376
April	1st to 10th	Day	7% 5% 2 4 4 2 4 5% 6% 10% 51%	2160
	11th to 20th	Day	5% 5% 2 5% 3 2 3 4 5% 9% 56%	2129
	21st to end	Day	6% 6% 2 3 3 2 3 4 6% 12% 54%	2160
May	1st to 10th	Day	5% 8% 3 5% 5% 2 4 4 6% 9% 48%	2160
	11th to 20th	Day	6% 9% 3 5% 5% 3 3 4 7% 13% 43%	2274
	21st to end	Day	7% 10% 3 6% 4 2 4 4 6% 12% 42%	2530
June	1st to 10th	Day	8% 11% 4 8% 6% 3 5% 5% 7% 10% 32%	2340
	11th to 20th	Day	5% 8% 3 5% 5% 3 4 5% 7% 14% 39%	2474
	21st to end	Day	4 8% 3 6% 6% 3 4 6% 8% 17% 36%	2520
July	1st to 10th	Day	5% 10% 3 5% 5% 3 5% 6% 9% 17% 31%	2520
	11th to 20th	Day	3 6% 3 5% 5% 2 5% 6% 11% 20% 35%	2363
	21st to end	Day	2 5% 2 4 5% 3 6% 7% 10% 19% 37%	2350
August	1st to 10th	Day	2 6% 2 5% 4 3 4 5% 9% 21% 41%	2149
	11th to 20th	Day	3 5% 2 3 3 3 4 5% 7% 22% 43%	2122
	21st to end	Day	2 3 2 4 4 3 5% 6% 8% 17% 46%	2376
September	1st to 10th	Day	3 5% 3 4 4 3 4 5% 8% 12% 48%	2160
	11th to 20th	Day	3 6% 2 4 4 3 4 5% 8% 15% 47%	2044
	21st to end	Day	2 4 2 4 4 2 4 4 6% 14% 53%	1980
October	1st to 10th	Day	4 4 2 3 3 2 4 4 6% 11% 55%	1980
	11th to 20th	Day	5% 6% 2 4 3 2 4 4 5% 9% 55%	1848
	21st to end	Day	4 4 2 5% 4 3 5% 4 7% 9% 54%	1980
November	1st to 10th	Day	3 4 2 4 4 3 5% 5% 7% 12% 50%	1697
	11th to 20th	Day	4 6% 2 5% 4 2 5% 5% 6% 10% 51%	1710
	21st to end	Day	4 3 2 5% 5% 4 5% 5% 7% 11% 51%	1710
December	1st to 10th	Day	7% 6% 3 4 4 4 5% 4 7% 10% 47%	1710
	11th to 20th	Day	11% 7% 3 6% 5% 3 5% 5% 6% 7% 42%	2169
	21st to end	Day	8% 8% 3 7% 6% 5% 6% 5% 6% 11% 37%	3128
January	1st to 10th	Night	7% 5% 2 4 4 3 3 5% 5% 9% 53%	2594
	11th to 20th	Night	5% 4 2 4 4 3 4 4 5% 8% 57%	2337
	21st to end	Night	8% 5% 2 3 4 2 3 4 4 8% 57%	2376
February	1st to 10th	Night	7% 4 2 6% 4 2 4 3 5% 7% 56%	2160
	11th to 20th	Night	7% 6% 2 4 4 2 4 4 5% 7% 54%	2098
	21st to end	Night	9% 6% 2 5% 4 3 3 5% 5% 8% 51%	1776
March	1st to 10th	Night	6% 5% 2 4 4 2 4 5% 7% 9% 52%	2160
	11th to 20th	Night	7% 8% 3 5% 4 3 3 4 6% 9% 47%	2088
	21st to end	Night	5% 5% 2 4 3 2 4 4 5% 8% 59%	1980
April	1st to 10th	Night	7% 6% 3 4 3 2 4 3 5% 8% 55%	1800
	11th to 20th	Night	6% 8% 2 4 3 2 2 2 4 7% 61%	1779
	21st to end	Night	7% 8% 3 4 3 2 2 3 5% 6% 57%	1800
May	1st to 10th	Night	7% 8% 2 5% 3 2 3 4 4 9% 54%	1800
	11th to 20th	Night	9% 13% 4 4 2 1 2 3 5% 8% 49%	1617
	21st to end	Night	7% 13% 4 4 4 1 2 3 3 7% 52%	1746
June	1st to 10th	Night	10% 16% 3 8% 5% 3 4 3 4 7% 37%	1620
	11th to 20th	Night	9% 10% 3 6% 4 2 3 3 4 12% 44%	1478
	21st to end	Night	4 14% 3 7% 5% 2 4 5% 6% 12% 39%	1440
July	1st to 10th	Night	8% 11% 4 6% 4 2 3 4 5% 12% 41%	1440
	11th to 20th	Night	5% 9% 4 4 5% 3 4 4 5% 16% 41%	1587
	21st to end	Night	4 8% 3 6% 5% 3 4 5% 8% 16% 40%	1970
August	1st to 10th	Night	2 7% 2 5% 4 3 4 4 8% 15% 47%	1790
	11th to 20th	Night	4 8% 2 4 4 2 3 3 8% 18% 46%	1780
	21st to end	Night	3 6% 3 5% 4 3 4 4 7% 14% 50%	1980
September	1st to 10th	Night	5% 7% 2 5% 4 3 4 4 6% 9% 51%	1800
	11th to 20th	Night	5% 5% 2 4 4 3 3 4 5% 12% 55%	1916
	21st to end	Night	3 4 1 4 4 2 4 4 5% 8% 61%	1980
October	1st to 10th	Night	3 4 1 3 2 2 3 3 5% 11% 63%	1980
	11th to 20th	Night	5% 6% 2 4 3 2 3 4 4 8% 60%	2108
	21st to end	Night	5% 4 1 3 2 2 3 3 4 8% 63%	2376
November	1st to 10th	Night	3 4 2 3 3 3 2 4 4 8% 62%	2381
	11th to 20th	Night	3 4 2 4 4 2 4 3 5% 7% 62%	2470
	21st to end	Night	3 6% 2 4 4 3 4 4 6% 8% 55%	2470
December	1st to 10th	Night	7% 6% 2 4 5% 2 3 4 5% 7% 56%	2470
	11th to 20th	Night	12% 7% 3 5% 5% 3 4 5% 5% 7% 44%	3041
	21st to end	Night	9% 7% 2 4 5% 3 5% 5% 6% 8% 45%	4525

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	33% 4 9% 19% 22% 13%	1568
	11th to 20th	Day	27% 6% 8% 15% 24% 20%	1755
	21st to end	Day	32% 3 8% 17% 18% 23%	1980
February	1st to 10th	Day	31% 3 6% 17% 20% 23%	1800
	11th to 20th	Day	27% 4 6% 19% 26% 18%	1760
	21st to end	Day	29% 5% 9% 20% 21% 17%	1480
March	1st to 10th	Day	28% 5% 10% 21% 18% 17%	1800
	11th to 20th	Day	34% 5% 11% 17% 21% 12%	1865
	21st to end	Day	26% 5% 12% 23% 23% 11%	2376
April	1st to 10th	Day	29% 5% 12% 22% 22% 10%	2160
	11th to 20th	Day	24% 4 8% 21% 27% 15%	2132
	21st to end	Day	25% 5% 9% 21% 28% 11%	2160
May	1st to 10th	Day	31% 4 10% 18% 23% 14%	2160
	11th to 20th	Day	32% 5% 9% 19% 24% 12%	2277
	21st to end	Day	33% 4 10% 21% 25% 6%	2530
June	1st to 10th	Day	43% 7% 9% 11% 20% 9%	2340
	11th to 20th	Day	32% 4 8% 10% 25% 21%	2474
	21st to end	Day	31% 3 7% 8% 22% 27%	2520
July	1st to 10th	Day	31% 5% 7% 12% 20% 25%	2451
	11th to 20th	Day	27% 5% 9% 18% 24% 17%	2289
	21st to end	Day	26% 5% 11% 23% 25% 11%	2220
August	1st to 10th	Day	23% 3 8% 13% 26% 26%	2135
	11th to 20th	Day	22% 4 6% 14% 32% 23%	2122
	21st to end	Day	20% 4 6% 14% 27% 28%	2376
September	1st to 10th	Day	25% 4 7% 10% 24% 30%	2160
	11th to 20th	Day	23% 4 6% 10% 32% 25%	2044
	21st to end	Day	20% 4 5% 14% 32% 24%	1980
October	1st to 10th	Day	22% 4 8% 19% 22% 24%	1980
	11th to 20th	Day	26% 4 7% 13% 26% 25%	1848
	21st to end	Day	24% 5% 6% 13% 20% 31%	1980
November	1st to 10th	Day	24% 3 9% 19% 20% 24%	1697
	11th to 20th	Day	26% 5% 8% 14% 18% 28%	1710
	21st to end	Day	25% 5% 10% 18% 19% 23%	1710
December	1st to 10th	Day	31% 5% 9% 19% 20% 15%	1710
	11th to 20th	Day	39% 6% 9% 16% 20% 10%	2168
	21st to end	Day	40% 7% 10% 20% 15% 8%	3128
January	1st to 10th	Night	28% 5% 8% 17% 25% 17%	2594
	11th to 20th	Night	25% 4 9% 17% 24% 22%	2337
	21st to end	Night	26% 4 9% 16% 18% 27%	2376
February	1st to 10th	Night	29% 4 5% 14% 23% 26%	2160
	11th to 20th	Night	28% 3 7% 16% 22% 23%	2099
	21st to end	Night	31% 4 6% 17% 24% 18%	1776
March	1st to 10th	Night	27% 4 10% 22% 20% 17%	2160
	11th to 20th	Night	33% 5% 9% 16% 23% 14%	2088
	21st to end	Night	24% 4 9% 26% 23% 14%	1980
April	1st to 10th	Night	29% 4 9% 24% 23% 12%	1800
	11th to 20th	Night	26% 3 4 22% 29% 16%	1779
	21st to end	Night	30% 3 7% 21% 27% 12%	1800
May	1st to 10th	Night	31% 3 8% 19% 26% 14%	1800
	11th to 20th	Night	34% 4 8% 19% 24% 12%	1618
	21st to end	Night	34% 3 6% 21% 27% 9%	1746
June	1st to 10th	Night	48% 5% 7% 10% 20% 11%	1620
	11th to 20th	Night	36% 4 5% 12% 22% 22%	1478
	21st to end	Night	38% 2 6% 6% 19% 28%	1440
July	1st to 10th	Night	34% 3 5% 8% 21% 30%	1400
	11th to 20th	Night	33% 3 6% 14% 25% 19%	1539
	21st to end	Night	30% 5% 9% 22% 22% 13%	1860
August	1st to 10th	Night	23% 3 7% 14% 28% 26%	1780
	11th to 20th	Night	25% 2 5% 12% 30% 26%	1780
	21st to end	Night	23% 3 5% 10% 26% 32%	1980
September	1st to 10th	Night	27% 4 6% 8% 21% 34%	1800
	11th to 20th	Night	21% 3 4 9% 30% 33%	1916
	21st to end	Night	22% 3 4 11% 32% 29%	1980
October	1st to 10th	Night	18% 3 5% 17% 27% 29%	1980
	11th to 20th	Night	23% 3 5% 14% 24% 31%	2108
	21st to end	Night	20% 3 5% 12% 25% 35%	2376
November	1st to 10th	Night	21% 3 5% 17% 22% 32%	2381
	11th to 20th	Night	22% 3 7% 15% 20% 34%	2470
	21st to end	Night	27% 3 7% 18% 19% 25%	2470
December	1st to 10th	Night	29% 3 7% 17% 23% 21%	2470
	11th to 20th	Night	39% 4 8% 17% 21% 12%	3041
	21st to end	Night	35% 6% 9% 24% 15% 10%	4525

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	14% 8% 24% 31% 18% 5%	229
	11th to 20th	Day	12% 7% 21% 32% 24% 5%	395
	21st to end	Day	9% 13% 21% 30% 22% 5%	462
February	1st to 10th	Day	12% 9% 19% 24% 27% 9%	420
	11th to 20th	Day	12% 13% 22% 30% 20% 3	415
	21st to end	Day	11% 15% 24% 31% 17% 2	346
March	1st to 10th	Day	12% 12% 31% 28% 14% 5%	420
	11th to 20th	Day	11% 9% 23% 34% 20% 5%	420
	21st to end	Day	9% 17% 26% 30% 16% 2	462
April	1st to 10th	Day	10% 10% 23% 33% 18% 6%	420
	11th to 20th	Day	9% 13% 29% 29% 17% 4	412
	21st to end	Day	5% 11% 25% 33% 18% 8%	420
May	1st to 10th	Day	7% 10% 21% 31% 22% 9%	423
	11th to 20th	Day	11% 8% 24% 32% 20% 4	448
	21st to end	Day	13% 11% 17% 33% 23% 4	485
June	1st to 10th	Day	10% 6% 17% 29% 29% 10%	450
	11th to 20th	Day	7% 2 16% 31% 31% 12%	450
	21st to end	Day	6% 3 10% 39% 32% 10%	450
July	1st to 10th	Day	7% 1 7% 35% 36% 14%	446
	11th to 20th	Day	4 1 3 28% 45% 19%	444
	21st to end	Day	3 1 6% 28% 46% 16%	487
August	1st to 10th	Day	4 1 9% 36% 38% 12%	447
	11th to 20th	Day	3 1 5% 38% 39% 14%	437
	21st to end	Day	3 1 6% 39% 42% 9%	495
September	1st to 10th	Day	5% 1 10% 37% 38% 9%	450
	11th to 20th	Day	4 2 17% 41% 28% 8%	450
	21st to end	Day	2 6% 15% 31% 35% 11%	450
October	1st to 10th	Day	4 5% 16% 39% 28% 8%	450
	11th to 20th	Day	7% 4 18% 36% 26% 8%	450
	21st to end	Day	5% 5% 18% 33% 27% 13%	495
November	1st to 10th	Day	4 10% 14% 25% 34% 14%	293
	11th to 20th	Day	6% 6% 14% 32% 29% 13%	270
	21st to end	Day	4 13% 17% 26% 29% 11%	270
December	1st to 10th	Day	11% 3 21% 31% 29% 5%	270
	11th to 20th	Day	7% 9% 18% 30% 27% 9%	270
	21st to end	Day	11% 18% 21% 24% 19% 7%	289
January	1st to 10th	Night	12% 6% 12% 30% 31% 9%	760
	11th to 20th	Night	8% 5% 11% 33% 35% 9%	748
	21st to end	Night	11% 8% 15% 33% 27% 6%	792
February	1st to 10th	Night	9% 6% 15% 29% 31% 10%	720
	11th to 20th	Night	10% 8% 17% 31% 26% 7%	704
	21st to end	Night	14% 9% 19% 29% 24% 4	592
March	1st to 10th	Night	11% 9% 21% 30% 24% 5%	720
	11th to 20th	Night	10% 7% 18% 32% 24% 9%	720
	21st to end	Night	8% 10% 22% 30% 26% 5%	792
April	1st to 10th	Night	10% 7% 24% 30% 22% 7%	720
	11th to 20th	Night	12% 11% 21% 25% 24% 7%	711
	21st to end	Night	10% 9% 19% 28% 26% 8%	720
May	1st to 10th	Night	13% 8% 17% 28% 26% 9%	720
	11th to 20th	Night	16% 7% 17% 28% 24% 9%	554
	21st to end	Night	14% 9% 19% 25% 27% 7%	584
June	1st to 10th	Night	17% 5% 11% 31% 25% 11%	540
	11th to 20th	Night	16% 3 7% 24% 34% 16%	398
	21st to end	Night	11% 3 7% 25% 42% 13%	360
July	1st to 10th	Night	13% 1 7% 14% 40% 25%	350
	11th to 20th	Night	10% 2 1 15% 41% 31%	495
	21st to end	Night	8% 2 4 18% 42% 27%	788
August	1st to 10th	Night	6% 1 7% 25% 38% 23%	716
	11th to 20th	Night	9% 1 3 16% 42% 29%	712
	21st to end	Night	6% 1 3 23% 43% 23%	792
September	1st to 10th	Night	7% 2 4 22% 43% 21%	720
	11th to 20th	Night	7% 1 10% 33% 34% 14%	720
	21st to end	Night	4 5% 9% 23% 41% 17%	720
October	1st to 10th	Night	4 4 14% 30% 36% 11%	720
	11th to 20th	Night	9% 3 11% 28% 34% 15%	720
	21st to end	Night	7% 5% 10% 27% 33% 18%	792
November	1st to 10th	Night	6% 6% 11% 28% 34% 15%	740
	11th to 20th	Night	5% 4 10% 27% 40% 14%	760
	21st to end	Night	6% 6% 15% 30% 36% 8%	760
December	1st to 10th	Night	11% 5% 13% 32% 33% 7%	760
	11th to 20th	Night	13% 9% 12% 34% 28% 5%	760
	21st to end	Night	9% 12% 14% 28% 28% 8%	836

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