El Cajon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the El Cajon clear sky chart from 2005-11-06 to 2025-12-03.

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% 4 1 3 2 3 3 3 5% 6% 66%	135507
	Night	7% 8% 2 5% 3 2 3 2 5% 6% 58%	132839
Transparency	Day	20% 3 4 17% 28% 28%	130604
	Night	28% 3 4 15% 23% 26%	128715
Seeing	Day	8% 6% 25% 36% 18% 7%	17222
	Night	10% 2 8% 35% 35% 10%	27604

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% 3 1 3 3 3 2 2 3 5% 68%	4098
	11th to 20th	Day	5% 4 2 3 2 2 2 3 4 5% 67%	4373
	21st to end	Day	11% 5% 2 5% 3 3 4 4 7% 10% 47%	5016
February	1st to 10th	Day	9% 5% 3 4 4 3 3 4 4 5% 57%	2000
	11th to 20th	Day	9% 7% 2 4 3 3 4 4 5% 7% 53%	1950
	21st to end	Day	11% 6% 3 5% 4 4 5% 3 5% 5% 49%	1650
March	1st to 10th	Day	10% 6% 2 6% 4 4 5% 5% 8% 10% 41%	4518
	11th to 20th	Day	10% 5% 2 4 3 4 3 3 4 7% 55%	4780
	21st to end	Day	10% 6% 2 5% 3 3 3 2 5% 7% 54%	5744
April	1st to 10th	Day	6% 5% 2 6% 4 3 3 3 5% 7% 56%	2400
	11th to 20th	Day	4 4 2 4 3 3 4 3 5% 6% 62%	2369
	21st to end	Day	7% 5% 1 3 2 2 2 3 5% 7% 63%	2400
May	1st to 10th	Day	5% 4 2 4 2 2 2 2 4 5% 67%	5700
	11th to 20th	Day	3 5% 2 5% 3 3 4 3 6% 8% 60%	5841
	21st to end	Day	3 6% 2 4 2 3 3 2 5% 6% 64%	6423
June	1st to 10th	Day	4 7% 2 4 3 2 3 3 4 7% 62%	2600
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 6% 73%	2600
	21st to end	Day	1 3 1 4 2 2 2 2 4 6% 72%	2600
July	1st to 10th	Day	1 2 1 2 1 1 2 1 3 5% 82%	5879
	11th to 20th	Day	1 3 2 3 2 2 3 2 5% 7% 70%	5880
	21st to end	Day	1 2 1 2 1 2 2 2 4 6% 78%	6104
August	1st to 10th	Day	1 1 1 1 1 1 2 1 3 5% 82%	5346
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 5% 82%	5364
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 75%	5984
September	1st to 10th	Day	3 2 1 3 2 2 2 3 4 6% 71%	2375
	11th to 20th	Day	3 3 1 3 1 2 2 2 4 5% 75%	2200
	21st to end	Day	3 3 1 2 1 2 3 2 4 5% 73%	2200
October	1st to 10th	Day	2 3 1 2 2 2 2 2 3 5% 78%	3600
	11th to 20th	Day	3 4 2 3 1 2 3 2 3 4 72%	3579
	21st to end	Day	3 3 1 3 1 2 3 3 5% 6% 71%	3413
November	1st to 10th	Day	5% 3 1 4 2 2 3 2 4 7% 68%	1854
	11th to 20th	Day	5% 3 1 3 3 3 3 3 5% 5% 65%	1890
	21st to end	Day	6% 4 2 4 3 3 3 4 5% 7% 59%	1890
December	1st to 10th	Day	4 2 1 2 2 2 2 2 4 8% 69%	2968
	11th to 20th	Day	8% 4 1 4 4 4 5% 4 6% 8% 52%	3409
	21st to end	Day	8% 5% 2 6% 4 5% 5% 5% 8% 10% 44%	4510
January	1st to 10th	Night	5% 5% 1 4 3 3 3 3 4 5% 65%	5867
	11th to 20th	Night	6% 6% 2 4 3 3 3 3 4 6% 61%	5616
	21st to end	Night	15% 7% 2 4 3 3 3 3 5% 6% 49%	5984
February	1st to 10th	Night	9% 6% 2 6% 3 3 2 3 5% 6% 56%	2400
	11th to 20th	Night	10% 7% 3 4 4 3 4 4 3 5% 52%	2338
	21st to end	Night	11% 7% 2 5% 3 3 3 3 6% 5% 50%	1980
March	1st to 10th	Night	20% 12% 2 6% 3 2 2 2 4 6% 40%	5376
	11th to 20th	Night	14% 9% 2 5% 3 3 3 3 4 6% 47%	5207
	21st to end	Night	12% 15% 3 6% 2 3 3 2 4 6% 45%	5226
April	1st to 10th	Night	10% 8% 3 5% 3 2 3 3 5% 8% 50%	2000
	11th to 20th	Night	9% 10% 3 6% 3 3 2 2 3 6% 53%	1979
	21st to end	Night	8% 10% 3 7% 3 3 3 2 5% 6% 50%	2000
May	1st to 10th	Night	10% 11% 3 5% 3 2 2 1 3 5% 54%	4284
	11th to 20th	Night	7% 20% 6% 8% 3 3 2 2 4 5% 41%	4074
	21st to end	Night	6% 20% 5% 6% 3 2 3 2 4 5% 45%	4474
June	1st to 10th	Night	6% 13% 5% 6% 4 3 2 3 6% 9% 42%	1800
	11th to 20th	Night	3 8% 3 6% 3 3 2 2 5% 9% 56%	1800
	21st to end	Night	2 6% 2 5% 2 2 3 3 6% 8% 61%	1800
July	1st to 10th	Night	4 2 3 2 1 2 2 5% 6% 73%	4100
	11th to 20th	Night	6% 3 6% 3 2 4 3 6% 7% 61%	4100
	21st to end	Night	2 6% 2 4 2 2 2 2 6% 6% 67%	4779
August	1st to 10th	Night	1 2 1 2 1 1 2 1 5% 7% 78%	4486
	11th to 20th	Night	1 5% 2 3 2 2 2 2 5% 7% 69%	4506
	21st to end	Night	3 3 2 3 1 2 2 2 4 6% 72%	5015
September	1st to 10th	Night	4 5% 2 5% 3 3 3 3 5% 6% 63%	2025
	11th to 20th	Night	5% 6% 2 4 2 2 2 2 3 5% 67%	2200
	21st to end	Night	4 5% 2 4 1 2 2 2 4 7% 67%	2200
October	1st to 10th	Night	3 3 1 3 2 2 2 2 3 5% 74%	3600
	11th to 20th	Night	6% 13% 2 3 1 2 2 2 4 5% 60%	3579
	21st to end	Night	4 5% 1 3 2 2 3 2 4 5% 67%	4374
November	1st to 10th	Night	5% 5% 2 3 2 2 2 2 4 6% 65%	2665
	11th to 20th	Night	5% 5% 2 4 2 3 4 3 4 5% 64%	2730
	21st to end	Night	7% 5% 2 5% 4 3 4 3 5% 8% 55%	2730
December	1st to 10th	Night	5% 3 1 3 2 3 4 3 5% 6% 65%	4266
	11th to 20th	Night	9% 6% 2 5% 3 3 4 4 6% 8% 50%	4811
	21st to end	Night	8% 6% 2 6% 4 4 4 4 7% 10% 46%	6468

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	20% 2 3 18% 24% 32%	4099
	11th to 20th	Day	20% 3 6% 20% 21% 30%	4373
	21st to end	Day	31% 4 5% 10% 27% 23%	5016
February	1st to 10th	Day	29% 3 3 14% 21% 29%	2000
	11th to 20th	Day	30% 3 5% 17% 23% 21%	1960
	21st to end	Day	36% 5% 4 13% 20% 21%	1650
March	1st to 10th	Day	33% 6% 4 23% 20% 13%	4518
	11th to 20th	Day	28% 4 6% 26% 26% 10%	4780
	21st to end	Day	30% 3 7% 32% 17% 10%	5744
April	1st to 10th	Day	28% 4 6% 23% 27% 13%	2400
	11th to 20th	Day	22% 4 5% 20% 29% 20%	2372
	21st to end	Day	21% 3 5% 23% 34% 13%	2400
May	1st to 10th	Day	21% 3 4 21% 38% 13%	5700
	11th to 20th	Day	21% 4 4 19% 39% 13%	5844
	21st to end	Day	21% 3 3 17% 43% 13%	6424
June	1st to 10th	Day	23% 3 4 20% 31% 19%	2600
	11th to 20th	Day	15% 3 2 10% 29% 41%	2600
	21st to end	Day	15% 3 2 7% 24% 50%	2600
July	1st to 10th	Day	7% 1 1 7% 18% 65%	5816
	11th to 20th	Day	14% 2 3 16% 44% 21%	5811
	21st to end	Day	9% 2 4 23% 29% 32%	5929
August	1st to 10th	Day	10% 2 5% 21% 39% 24%	3890
	11th to 20th	Day	9% 1 4 13% 28% 45%	3880
	21st to end	Day	10% 2 2 8% 28% 49%	4311
September	1st to 10th	Day	16% 3 3 10% 32% 37%	2375
	11th to 20th	Day	13% 3 1 7% 36% 40%	2200
	21st to end	Day	14% 2 2 13% 31% 37%	2200
October	1st to 10th	Day	12% 2 2 9% 18% 57%	3600
	11th to 20th	Day	16% 2 1 13% 27% 39%	3579
	21st to end	Day	15% 2 2 10% 33% 38%	3413
November	1st to 10th	Day	19% 2 3 16% 22% 38%	1854
	11th to 20th	Day	21% 3 4 13% 22% 38%	1890
	21st to end	Day	26% 3 5% 14% 20% 33%	1890
December	1st to 10th	Day	16% 2 4 14% 28% 37%	2968
	11th to 20th	Day	30% 4 7% 15% 24% 21%	3408
	21st to end	Day	34% 4 8% 25% 21% 9%	4510
January	1st to 10th	Night	22% 3 3 15% 27% 29%	5867
	11th to 20th	Night	25% 4 6% 18% 16% 31%	5616
	21st to end	Night	37% 3 4 10% 23% 24%	5984
February	1st to 10th	Night	30% 2 5% 13% 21% 28%	2400
	11th to 20th	Night	35% 3 5% 14% 21% 22%	2339
	21st to end	Night	35% 3 3 15% 23% 21%	1980
March	1st to 10th	Night	45% 4 6% 15% 16% 13%	5376
	11th to 20th	Night	37% 5% 5% 24% 20% 9%	5207
	21st to end	Night	42% 3 8% 20% 17% 10%	5226
April	1st to 10th	Night	33% 3 5% 22% 22% 14%	2000
	11th to 20th	Night	34% 3 4 17% 23% 19%	1979
	21st to end	Night	37% 3 5% 18% 22% 16%	2000
May	1st to 10th	Night	34% 3 5% 21% 27% 10%	4281
	11th to 20th	Night	46% 3 3 18% 22% 8%	4075
	21st to end	Night	41% 4 3 16% 24% 12%	4474
June	1st to 10th	Night	38% 4 4 14% 25% 15%	1800
	11th to 20th	Night	27% 3 1 11% 23% 34%	1800
	21st to end	Night	21% 4 2 7% 20% 46%	1800
July	1st to 10th	Night	11% 2 1 7% 20% 59%	4055
	11th to 20th	Night	21% 3 3 18% 39% 17%	4046
	21st to end	Night	18% 3 4 20% 31% 24%	4642
August	1st to 10th	Night	11% 2 5% 21% 41% 21%	3256
	11th to 20th	Night	13% 2 4 13% 25% 42%	3256
	21st to end	Night	15% 3 2 6% 26% 47%	3608
September	1st to 10th	Night	22% 4 2 9% 27% 36%	2025
	11th to 20th	Night	22% 2 2 8% 30% 36%	2200
	21st to end	Night	19% 2 2 11% 35% 31%	2200
October	1st to 10th	Night	16% 2 2 10% 19% 52%	3600
	11th to 20th	Night	29% 2 2 10% 20% 39%	3579
	21st to end	Night	20% 2 1 10% 28% 39%	4374
November	1st to 10th	Night	22% 2 4 15% 20% 38%	2665
	11th to 20th	Night	23% 3 3 13% 18% 39%	2730
	21st to end	Night	28% 3 5% 17% 16% 31%	2730
December	1st to 10th	Night	21% 2 4 15% 23% 35%	4266
	11th to 20th	Night	32% 3 5% 12% 23% 24%	4811
	21st to end	Night	32% 5% 10% 25% 18% 10%	6468

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	17% 3 23% 35% 20% 3	300
	11th to 20th	Day	14% 5% 27% 37% 15% 3	417
	21st to end	Day	13% 9% 31% 31% 12% 3	550
February	1st to 10th	Day	11% 12% 29% 31% 13% 5%	500
	11th to 20th	Day	13% 12% 33% 30% 10% 2	495
	21st to end	Day	14% 11% 37% 29% 8% 1	414
March	1st to 10th	Day	15% 15% 35% 23% 10% 2	500
	11th to 20th	Day	15% 11% 30% 32% 10% 2	500
	21st to end	Day	11% 15% 39% 23% 9% 2	550
April	1st to 10th	Day	11% 11% 35% 24% 11% 7%	500
	11th to 20th	Day	8% 13% 36% 31% 9% 3	492
	21st to end	Day	9% 12% 31% 29% 13% 6%	500
May	1st to 10th	Day	12% 11% 30% 31% 12% 5%	503
	11th to 20th	Day	10% 8% 30% 34% 14% 4	528
	21st to end	Day	9% 9% 30% 33% 15% 4	573
June	1st to 10th	Day	5% 5% 24% 38% 21% 8%	530
	11th to 20th	Day	3 3 24% 43% 20% 8%	530
	21st to end	Day	3 2 17% 45% 24% 9%	530
July	1st to 10th	Day	3 1 11% 45% 28% 12%	526
	11th to 20th	Day	2 7% 44% 32% 14%	524
	21st to end	Day	4 6% 47% 28% 14%	575
August	1st to 10th	Day	2 1 12% 45% 26% 15%	527
	11th to 20th	Day	3 9% 48% 25% 15%	517
	21st to end	Day	3 1 13% 45% 25% 12%	583
September	1st to 10th	Day	6% 1 18% 45% 18% 12%	530
	11th to 20th	Day	6% 2 27% 41% 18% 6%	530
	21st to end	Day	5% 6% 26% 37% 18% 8%	530
October	1st to 10th	Day	5% 6% 29% 35% 19% 5%	530
	11th to 20th	Day	8% 7% 30% 34% 17% 4	530
	21st to end	Day	6% 5% 25% 33% 23% 8%	482
November	1st to 10th	Day	7% 5% 21% 36% 23% 6%	330
	11th to 20th	Day	7% 4 21% 38% 27% 3	330
	21st to end	Day	10% 5% 22% 36% 22% 5%	330
December	1st to 10th	Day	9% 2 25% 38% 22% 4	306
	11th to 20th	Day	6% 3 22% 37% 25% 7%	300
	21st to end	Day	17% 3 28% 34% 14% 4	330
January	1st to 10th	Night	11% 2 15% 39% 22% 10%	800
	11th to 20th	Night	10% 1 12% 43% 25% 9%	800
	21st to end	Night	16% 2 14% 41% 23% 5%	880
February	1st to 10th	Night	12% 2 11% 42% 26% 8%	800
	11th to 20th	Night	13% 3 12% 42% 24% 6%	784
	21st to end	Night	14% 2 15% 44% 21% 4	660
March	1st to 10th	Night	18% 3 12% 41% 22% 4	800
	11th to 20th	Night	15% 2 9% 42% 25% 8%	800
	21st to end	Night	14% 3 12% 41% 26% 4	880
April	1st to 10th	Night	13% 3 9% 44% 26% 6%	800
	11th to 20th	Night	14% 2 12% 41% 25% 6%	791
	21st to end	Night	13% 4 8% 39% 29% 8%	800
May	1st to 10th	Night	12% 3 11% 37% 31% 7%	680
	11th to 20th	Night	15% 3 10% 31% 34% 7%	591
	21st to end	Night	14% 4 9% 37% 32% 4	650
June	1st to 10th	Night	11% 3 9% 35% 35% 7%	600
	11th to 20th	Night	8% 2 6% 38% 40% 7%	600
	21st to end	Night	4 2 6% 33% 47% 9%	600
July	1st to 10th	Night	2 3 2 25% 55% 12%	585
	11th to 20th	Night	1 1 1 21% 61% 15%	582
	21st to end	Night	5% 2 1 18% 56% 19%	785
August	1st to 10th	Night	3 2 2 20% 54% 20%	796
	11th to 20th	Night	4 1 13% 57% 25%	792
	21st to end	Night	5% 1 1 14% 58% 22%	880
September	1st to 10th	Night	6% 2 1 19% 57% 16%	800
	11th to 20th	Night	9% 1 2 28% 47% 13%	800
	21st to end	Night	7% 1 4 34% 39% 15%	800
October	1st to 10th	Night	9% 1 4 38% 40% 7%	800
	11th to 20th	Night	12% 2 6% 37% 34% 9%	800
	21st to end	Night	10% 1 6% 35% 34% 14%	880
November	1st to 10th	Night	8% 1 7% 40% 33% 11%	820
	11th to 20th	Night	8% 1 8% 39% 31% 13%	840
	21st to end	Night	9% 12% 43% 26% 9%	840
December	1st to 10th	Night	12% 1 10% 44% 25% 8%	808
	11th to 20th	Night	15% 2 10% 42% 23% 7%	800
	21st to end	Night	12% 1 13% 42% 23% 8%	880

Page generated at 2025-11-29 22:00:51. Version ac07

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