California Institute of Technology
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the California Institute of Technology clear sky chart from 2005-11-06 to 2024-02-20.

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	6% 3 1 3 3 2 3 3 4 6% 67%	78931
	Night	6% 4 1 3 3 2 3 3 4 6% 66%	80269
Transparency	Day	20% 5% 7% 17% 27% 25%	78656
	Night	22% 6% 7% 17% 24% 23%	80065
Seeing	Day	7% 9% 27% 37% 16% 4	14604
	Night	6% 3 15% 37% 33% 7%	24557

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	7% 3 1 2 3 1 2 3 4 6% 68%	2539
	11th to 20th	Day	10% 4 1 3 2 2 2 4 5% 9% 58%	2817
	21st to end	Day	14% 5% 2 3 4 2 3 4 4 6% 53%	3498
February	1st to 10th	Day	10% 3 2 3 3 1 2 2 3 6% 63%	1900
	11th to 20th	Day	10% 3 1 3 2 2 3 4 4 6% 62%	1841
	21st to end	Day	10% 5% 1 3 4 2 3 3 4 7% 58%	1480
March	1st to 10th	Day	9% 5% 2 5% 5% 3 4 4 6% 7% 51%	1800
	11th to 20th	Day	8% 5% 2 2 3 2 3 3 6% 7% 58%	1847
	21st to end	Day	6% 4 1 3 4 3 4 4 5% 7% 58%	2376
April	1st to 10th	Day	6% 4 2 3 3 2 3 3 5% 8% 61%	2160
	11th to 20th	Day	5% 4 1 3 3 2 4 3 4 6% 67%	2129
	21st to end	Day	6% 4 1 3 2 2 2 3 3 6% 68%	2160
May	1st to 10th	Day	7% 5% 2 3 3 2 2 4 4 7% 62%	2218
	11th to 20th	Day	5% 4 2 3 4 2 3 3 4 7% 64%	2301
	21st to end	Day	6% 4 2 3 3 2 3 3 4 6% 64%	2530
June	1st to 10th	Day	5% 4 1 2 4 2 3 2 4 8% 65%	2348
	11th to 20th	Day	3 3 1 2 2 2 2 2 3 6% 74%	2520
	21st to end	Day	2 1 1 2 2 1 2 3 3 5% 78%	2520
July	1st to 10th	Day	2 2 1 2 2 2 3 2 3 6% 76%	2520
	11th to 20th	Day	2 1 1 2 3 1 2 2 2 5% 79%	2489
	21st to end	Day	2 1 1 2 2 1 3 4 5% 8% 72%	2380
August	1st to 10th	Day	2 1 1 2 2 1 2 2 3 4 81%	2149
	11th to 20th	Day	2 1 2 1 2 2 2 4 84%	2122
	21st to end	Day	1 1 1 2 1 3 3 3 5% 80%	2376
September	1st to 10th	Day	3 2 1 1 2 1 2 2 3 5% 79%	2160
	11th to 20th	Day	2 1 1 2 1 1 2 3 5% 82%	2026
	21st to end	Day	2 2 1 1 2 1 2 2 4 5% 78%	1980
October	1st to 10th	Day	4 2 1 2 3 2 3 3 4 5% 72%	1980
	11th to 20th	Day	4 2 1 2 2 1 2 2 2 4 77%	1866
	21st to end	Day	4 3 1 2 2 1 2 3 3 5% 74%	1940
November	1st to 10th	Day	4 2 1 3 3 2 2 3 4 5% 72%	1674
	11th to 20th	Day	5% 3 1 2 3 1 3 3 4 6% 70%	1710
	21st to end	Day	5% 2 2 3 4 3 4 3 4 7% 64%	1710
December	1st to 10th	Day	8% 3 2 4 3 2 3 4 4 6% 60%	1710
	11th to 20th	Day	17% 5% 2 4 4 2 3 4 5% 9% 45%	2169
	21st to end	Day	14% 6% 2 4 5% 3 4 4 5% 7% 47%	2986
January	1st to 10th	Night	8% 4 1 3 2 2 2 3 3 5% 67%	4406
	11th to 20th	Night	7% 4 2 4 3 2 3 3 4 8% 59%	4127
	21st to end	Night	14% 6% 2 3 3 2 3 4 4 6% 54%	4180
February	1st to 10th	Night	10% 4 2 4 3 2 2 2 3 5% 64%	2280
	11th to 20th	Night	9% 3 2 3 3 2 3 3 4 5% 63%	2211
	21st to end	Night	12% 4 1 3 3 2 4 4 3 4 59%	1776
March	1st to 10th	Night	10% 5% 2 3 2 2 4 4 5% 8% 54%	2160
	11th to 20th	Night	8% 4 2 3 3 2 3 4 6% 7% 58%	2108
	21st to end	Night	7% 4 1 3 3 3 4 4 5% 7% 59%	1980
April	1st to 10th	Night	7% 4 2 4 3 3 3 3 3 7% 61%	1800
	11th to 20th	Night	5% 6% 1 2 2 2 3 3 3 6% 67%	1779
	21st to end	Night	4 4 2 2 4 3 3 3 4 6% 66%	1800
May	1st to 10th	Night	5% 6% 1 4 5% 2 2 4 4 7% 61%	1739
	11th to 20th	Night	4 5% 1 3 4 3 3 3 4 7% 63%	1594
	21st to end	Night	4 4 2 3 3 2 2 2 3 5% 70%	1746
June	1st to 10th	Night	2 4 1 3 4 2 4 3 5% 7% 65%	1605
	11th to 20th	Night	2 3 1 2 3 2 2 2 3 4 75%	1440
	21st to end	Night	2 2 1 2 2 1 2 3 4 5% 75%	1440
July	1st to 10th	Night	1 3 1 1 2 1 2 3 2 5% 79%	1440
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 6% 74%	1466
	21st to end	Night	3 1 1 2 3 1 3 4 5% 7% 70%	1935
August	1st to 10th	Night	1 3 2 2 2 1 2 2 3 5% 77%	1790
	11th to 20th	Night	1 2 1 1 2 1 2 2 3 6% 79%	1780
	21st to end	Night	1 1 2 2 1 2 3 4 5% 80%	1980
September	1st to 10th	Night	3 2 1 3 2 2 2 2 2 4 77%	1800
	11th to 20th	Night	2 2 2 2 1 2 2 2 3 83%	1931
	21st to end	Night	1 2 2 2 1 1 2 2 5% 81%	1980
October	1st to 10th	Night	3 3 1 2 3 3 3 2 3 4 72%	1980
	11th to 20th	Night	5% 2 1 2 2 2 2 3 3 4 76%	2091
	21st to end	Night	4 3 1 2 2 1 2 2 3 4 75%	2415
November	1st to 10th	Night	4 3 1 2 3 2 2 3 3 6% 72%	2405
	11th to 20th	Night	4 3 1 2 2 2 3 2 3 6% 72%	2470
	21st to end	Night	6% 3 2 3 3 2 4 4 5% 7% 62%	2470
December	1st to 10th	Night	8% 3 1 4 4 2 3 3 4 7% 61%	2470
	11th to 20th	Night	17% 4 3 4 4 3 3 4 4 7% 48%	3041
	21st to end	Night	13% 7% 2 4 4 2 4 4 5% 6% 49%	4654

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	19% 4 6% 23% 29% 19%	2539
	11th to 20th	Day	26% 5% 16% 20% 17% 17%	2817
	21st to end	Day	33% 5% 6% 19% 21% 17%	3498
February	1st to 10th	Day	26% 3 6% 14% 24% 27%	1900
	11th to 20th	Day	23% 4 6% 16% 27% 24%	1851
	21st to end	Day	26% 3 9% 18% 25% 19%	1480
March	1st to 10th	Day	30% 5% 10% 18% 18% 20%	1800
	11th to 20th	Day	24% 6% 11% 20% 23% 17%	1847
	21st to end	Day	23% 6% 12% 22% 22% 15%	2376
April	1st to 10th	Day	23% 5% 12% 26% 22% 12%	2160
	11th to 20th	Day	18% 4 10% 21% 29% 18%	2132
	21st to end	Day	18% 4 8% 26% 31% 13%	2160
May	1st to 10th	Day	23% 7% 10% 20% 28% 13%	2218
	11th to 20th	Day	21% 6% 7% 22% 31% 12%	2304
	21st to end	Day	22% 6% 8% 27% 29% 8%	2530
June	1st to 10th	Day	21% 6% 9% 19% 30% 16%	2348
	11th to 20th	Day	15% 4 6% 13% 32% 31%	2520
	21st to end	Day	14% 4 4 8% 31% 39%	2520
July	1st to 10th	Day	13% 5% 5% 9% 22% 45%	2451
	11th to 20th	Day	12% 5% 6% 17% 31% 29%	2414
	21st to end	Day	12% 6% 8% 22% 33% 19%	2248
August	1st to 10th	Day	11% 3 3 13% 32% 39%	2135
	11th to 20th	Day	8% 4 3 12% 37% 36%	2122
	21st to end	Day	9% 4 5% 10% 36% 36%	2376
September	1st to 10th	Day	10% 3 5% 9% 34% 39%	2160
	11th to 20th	Day	9% 4 2 12% 36% 37%	2026
	21st to end	Day	10% 4 5% 14% 34% 33%	1980
October	1st to 10th	Day	16% 4 5% 18% 24% 32%	1980
	11th to 20th	Day	13% 5% 5% 15% 28% 35%	1866
	21st to end	Day	14% 4 5% 12% 22% 43%	1940
November	1st to 10th	Day	15% 4 5% 18% 25% 33%	1674
	11th to 20th	Day	17% 3 6% 15% 20% 38%	1710
	21st to end	Day	21% 5% 8% 12% 24% 31%	1710
December	1st to 10th	Day	26% 5% 6% 17% 19% 26%	1710
	11th to 20th	Day	37% 4 8% 15% 19% 16%	2168
	21st to end	Day	36% 6% 8% 23% 14% 12%	2986
January	1st to 10th	Night	22% 5% 6% 16% 33% 19%	4406
	11th to 20th	Night	26% 7% 13% 23% 14% 17%	4127
	21st to end	Night	31% 9% 6% 18% 19% 16%	4180
February	1st to 10th	Night	27% 3 6% 15% 21% 28%	2280
	11th to 20th	Night	24% 5% 7% 16% 24% 25%	2212
	21st to end	Night	29% 5% 7% 15% 22% 22%	1776
March	1st to 10th	Night	29% 5% 8% 21% 19% 18%	2160
	11th to 20th	Night	26% 7% 10% 16% 24% 17%	2108
	21st to end	Night	24% 6% 12% 23% 23% 13%	1980
April	1st to 10th	Night	27% 6% 10% 24% 19% 14%	1800
	11th to 20th	Night	19% 7% 10% 22% 24% 18%	1779
	21st to end	Night	21% 7% 10% 23% 29% 10%	1800
May	1st to 10th	Night	24% 9% 10% 21% 24% 12%	1739
	11th to 20th	Night	23% 8% 8% 25% 25% 11%	1595
	21st to end	Night	17% 7% 10% 30% 27% 8%	1746
June	1st to 10th	Night	20% 8% 13% 19% 27% 13%	1605
	11th to 20th	Night	15% 7% 6% 15% 35% 23%	1440
	21st to end	Night	17% 5% 7% 8% 31% 33%	1440
July	1st to 10th	Night	13% 7% 4 9% 27% 40%	1400
	11th to 20th	Night	14% 7% 7% 20% 27% 26%	1418
	21st to end	Night	15% 8% 9% 23% 29% 16%	1827
August	1st to 10th	Night	14% 5% 5% 9% 36% 32%	1780
	11th to 20th	Night	11% 6% 4 13% 36% 30%	1780
	21st to end	Night	12% 7% 4 10% 35% 32%	1980
September	1st to 10th	Night	16% 5% 5% 8% 30% 37%	1800
	11th to 20th	Night	9% 7% 3 11% 33% 36%	1931
	21st to end	Night	11% 5% 4 13% 36% 30%	1980
October	1st to 10th	Night	18% 5% 4 17% 23% 32%	1980
	11th to 20th	Night	16% 6% 4 15% 26% 33%	2091
	21st to end	Night	15% 5% 5% 11% 23% 42%	2415
November	1st to 10th	Night	16% 5% 6% 15% 25% 33%	2405
	11th to 20th	Night	17% 4 7% 14% 20% 38%	2470
	21st to end	Night	23% 5% 7% 17% 18% 31%	2470
December	1st to 10th	Night	25% 7% 7% 14% 18% 28%	2470
	11th to 20th	Night	37% 4 7% 14% 21% 15%	3041
	21st to end	Night	33% 10% 10% 20% 15% 12%	4654

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	16% 4 28% 35% 14% 2	180
	11th to 20th	Day	12% 12% 33% 30% 12% 1	337
	21st to end	Day	10% 14% 30% 33% 12% 1	506
February	1st to 10th	Day	13% 11% 30% 30% 12% 3	460
	11th to 20th	Day	11% 16% 31% 32% 8% 2	452
	21st to end	Day	11% 20% 31% 29% 8%	346
March	1st to 10th	Day	13% 18% 34% 24% 10% 1	420
	11th to 20th	Day	13% 14% 34% 27% 10% 2	420
	21st to end	Day	11% 19% 34% 28% 8% 1	462
April	1st to 10th	Day	9% 14% 34% 26% 14% 2	420
	11th to 20th	Day	7% 20% 38% 25% 8% 3	412
	21st to end	Day	5% 17% 33% 28% 14% 3	420
May	1st to 10th	Day	12% 14% 34% 27% 10% 3	423
	11th to 20th	Day	8% 12% 31% 33% 13% 3	448
	21st to end	Day	9% 11% 35% 29% 13% 2	485
June	1st to 10th	Day	4 7% 29% 40% 15% 5%	450
	11th to 20th	Day	5% 5% 26% 40% 17% 7%	450
	21st to end	Day	2 3 26% 43% 17% 8%	450
July	1st to 10th	Day	2 1 16% 50% 25% 7%	446
	11th to 20th	Day	1 1 10% 49% 26% 13%	444
	21st to end	Day	2 13% 49% 25% 11%	487
August	1st to 10th	Day	2 2 18% 51% 19% 8%	447
	11th to 20th	Day	2 2 14% 49% 24% 10%	437
	21st to end	Day	1 1 18% 50% 25% 5%	495
September	1st to 10th	Day	3 2 24% 45% 19% 6%	450
	11th to 20th	Day	2 4 30% 42% 18% 4	450
	21st to end	Day	2 6% 30% 36% 20% 6%	450
October	1st to 10th	Day	4 9% 30% 34% 19% 3	450
	11th to 20th	Day	3 10% 29% 39% 17% 3	450
	21st to end	Day	5% 8% 25% 36% 21% 5%	455
November	1st to 10th	Day	4 7% 21% 39% 23% 6%	270
	11th to 20th	Day	7% 6% 21% 39% 23% 5%	270
	21st to end	Day	5% 7% 27% 37% 18% 6%	270
December	1st to 10th	Day	10% 5% 23% 41% 19% 3	270
	11th to 20th	Day	6% 6% 27% 37% 21% 4	270
	21st to end	Day	16% 9% 28% 34% 12% 1	252
January	1st to 10th	Night	11% 3 20% 42% 20% 4	760
	11th to 20th	Night	9% 3 21% 42% 23% 2	760
	21st to end	Night	13% 4 21% 41% 18% 3	836
February	1st to 10th	Night	10% 4 19% 40% 23% 4	760
	11th to 20th	Night	9% 5% 21% 38% 22% 4	742
	21st to end	Night	12% 4 22% 40% 19% 3	592
March	1st to 10th	Night	9% 5% 23% 42% 19% 3	720
	11th to 20th	Night	9% 4 17% 38% 26% 6%	720
	21st to end	Night	8% 5% 21% 42% 22% 3	792
April	1st to 10th	Night	8% 4 15% 46% 24% 3	720
	11th to 20th	Night	5% 7% 19% 39% 27% 3	711
	21st to end	Night	4 6% 20% 37% 27% 6%	720
May	1st to 10th	Night	6% 4 20% 39% 26% 4	659
	11th to 20th	Night	5% 3 17% 39% 31% 5%	531
	21st to end	Night	4 3 22% 38% 29% 4	584
June	1st to 10th	Night	2 2 15% 38% 40% 2	525
	11th to 20th	Night	2 1 6% 32% 53% 5%	360
	21st to end	Night	2 1 4 36% 50% 6%	360
July	1st to 10th	Night	3 1 3 16% 67% 11%	350
	11th to 20th	Night	1 3 16% 67% 13%	374
	21st to end	Night	3 1 2 26% 53% 15%	753
August	1st to 10th	Night	2 1 4 29% 50% 13%	716
	11th to 20th	Night	1 1 5% 20% 55% 19%	712
	21st to end	Night	2 1 5% 23% 53% 17%	792
September	1st to 10th	Night	3 1 7% 27% 47% 15%	720
	11th to 20th	Night	2 1 8% 33% 45% 11%	720
	21st to end	Night	2 2 7% 33% 47% 9%	720
October	1st to 10th	Night	4 2 10% 42% 38% 4	720
	11th to 20th	Night	5% 2 13% 37% 36% 8%	720
	21st to end	Night	4 3 15% 35% 32% 11%	792
November	1st to 10th	Night	5% 2 12% 42% 30% 8%	740
	11th to 20th	Night	4 3 12% 44% 30% 7%	760
	21st to end	Night	6% 2 18% 44% 25% 5%	760
December	1st to 10th	Night	8% 4 18% 42% 23% 4	760
	11th to 20th	Night	12% 5% 17% 44% 19% 3	760
	21st to end	Night	10% 6% 23% 40% 17% 4	836

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