San Ysidro
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Ysidro clear sky chart from 2005-11-06 to 2024-11-02.

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	3 3 1 3 3 3 4 4 6% 8% 61%	100849
	Night	4 4 2 4 3 4 4 4 5% 7% 59%	98349
Transparency	Day	19% 5% 9% 23% 27% 16%	100612
	Night	24% 5% 9% 22% 26% 15%	98175
Seeing	Day	6% 9% 23% 31% 23% 8%	15028
	Night	6% 6% 22% 43% 21% 2	25177

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	3 6% 2 3 2 3 3 4 5% 7% 62%	2858
	11th to 20th	Day	2 4 1 3 3 2 3 3 6% 7% 67%	2860
	21st to end	Day	4 5% 2 5% 4 4 4 4 6% 7% 55%	3146
February	1st to 10th	Day	2 6% 2 5% 4 3 3 3 5% 6% 60%	1710
	11th to 20th	Day	3 6% 2 5% 4 4 4 3 5% 7% 58%	1820
	21st to end	Day	2 7% 2 4 2 3 3 4 5% 7% 61%	1570
March	1st to 10th	Day	2 3 1 3 3 3 4 3 5% 7% 66%	3383
	11th to 20th	Day	5% 7% 3 6% 5% 4 4 4 6% 7% 50%	3490
	21st to end	Day	3 4 1 4 3 4 5% 4 5% 7% 59%	3900
April	1st to 10th	Day	2 3 1 3 3 3 4 3 4 7% 66%	2280
	11th to 20th	Day	3 3 1 4 4 3 4 3 4 6% 64%	2251
	21st to end	Day	3 4 1 3 3 3 4 3 5% 7% 65%	2280
May	1st to 10th	Day	2 2 1 3 3 2 3 3 5% 7% 69%	3800
	11th to 20th	Day	4 3 1 4 4 4 5% 5% 7% 9% 54%	3974
	21st to end	Day	2 2 1 2 2 2 3 2 4 5% 76%	4753
June	1st to 10th	Day	2 2 1 2 3 3 4 4 7% 9% 63%	2660
	11th to 20th	Day	2 2 2 2 2 3 3 5% 6% 73%	2660
	21st to end	Day	5% 3 1 4 3 3 3 4 6% 8% 58%	2660
July	1st to 10th	Day	2 2 1 3 3 4 5% 4 6% 7% 62%	4216
	11th to 20th	Day	1 2 1 3 2 4 6% 7% 11% 12% 51%	4110
	21st to end	Day	3 2 1 4 4 5% 6% 7% 10% 12% 46%	4454
August	1st to 10th	Day	2 2 1 4 4 5% 6% 6% 10% 12% 46%	4067
	11th to 20th	Day	2 2 1 3 3 4 4 3 7% 10% 61%	3726
	21st to end	Day	4 3 1 4 4 5% 6% 6% 9% 11% 46%	4180
September	1st to 10th	Day	6% 3 1 4 3 4 5% 4 7% 8% 56%	2280
	11th to 20th	Day	5% 2 1 4 3 4 4 4 5% 7% 61%	2280
	21st to end	Day	4 2 1 3 3 3 3 3 4 5% 70%	2035
October	1st to 10th	Day	6% 3 1 3 2 3 4 3 5% 7% 62%	1900
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 4 76%	1900
	21st to end	Day	4 2 1 2 2 2 1 2 3 5% 75%	2090
November	1st to 10th	Day	2 3 1 1 2 2 3 2 5% 6% 73%	1869
	11th to 20th	Day	2 2 1 2 2 2 3 2 4 6% 72%	1715
	21st to end	Day	5% 5% 1 4 3 2 2 3 4 7% 64%	1691
December	1st to 10th	Day	4 4 2 3 2 2 4 4 4 6% 65%	1520
	11th to 20th	Day	4 4 2 5% 3 4 4 3 6% 9% 56%	1926
	21st to end	Day	4 5% 2 4 3 2 3 3 5% 7% 62%	2835
January	1st to 10th	Night	2 6% 3 8% 4 3 3 3 4 5% 59%	4097
	11th to 20th	Night	4 4 1 3 2 3 4 3 4 5% 67%	4110
	21st to end	Night	5% 6% 2 5% 4 3 4 3 5% 8% 54%	4521
February	1st to 10th	Night	2 5% 2 5% 3 3 3 3 3 7% 65%	2470
	11th to 20th	Night	5% 6% 2 4 3 3 3 3 5% 7% 59%	2244
	21st to end	Night	4 5% 1 3 2 2 3 2 5% 6% 67%	1884
March	1st to 10th	Night	3 4 1 4 3 2 3 2 4 5% 69%	3475
	11th to 20th	Night	9% 9% 3 6% 4 4 4 3 4 7% 48%	3490
	21st to end	Night	3 4 2 4 3 4 4 3 5% 7% 62%	3748
April	1st to 10th	Night	4 4 2 4 3 3 3 3 4 7% 63%	1900
	11th to 20th	Night	5% 4 1 4 3 3 4 3 4 5% 64%	1877
	21st to end	Night	4 3 2 4 3 3 3 3 4 6% 66%	1900
May	1st to 10th	Night	4 5% 2 4 3 3 3 3 4 5% 64%	3170
	11th to 20th	Night	5% 6% 3 6% 4 4 5% 4 6% 7% 51%	2914
	21st to end	Night	3 2 1 4 3 3 3 2 3 5% 71%	2829
June	1st to 10th	Night	2 3 1 3 3 4 5% 4 7% 8% 60%	1520
	11th to 20th	Night	2 2 1 4 3 2 3 3 4 4 71%	1520
	21st to end	Night	5% 4 2 6% 4 4 5% 4 8% 7% 52%	1520
July	1st to 10th	Night	2 3 1 5% 4 4 5% 5% 7% 8% 56%	2748
	11th to 20th	Night	2 4 1 6% 5% 6% 8% 7% 10% 12% 39%	2860
	21st to end	Night	6% 4 2 6% 6% 6% 7% 7% 10% 11% 36%	3109
August	1st to 10th	Night	3 4 3 9% 7% 8% 8% 7% 10% 11% 30%	2870
	11th to 20th	Night	4 4 2 5% 5% 6% 7% 6% 9% 12% 42%	3113
	21st to end	Night	4 3 2 7% 6% 7% 8% 7% 10% 12% 34%	3487
September	1st to 10th	Night	4 3 1 5% 4 5% 5% 5% 7% 7% 52%	1900
	11th to 20th	Night	6% 3 1 4 4 3 4 3 5% 8% 59%	1900
	21st to end	Night	4 3 1 3 3 3 3 3 4 6% 66%	2140
October	1st to 10th	Night	7% 4 2 4 3 3 3 2 4 5% 62%	2280
	11th to 20th	Night	3 1 1 3 2 2 3 2 3 5% 75%	2280
	21st to end	Night	4 2 1 3 2 2 2 2 3 4 74%	2508
November	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 77%	2244
	11th to 20th	Night	2 2 1 2 2 2 3 2 4 6% 75%	2443
	21st to end	Night	7% 5% 2 3 2 3 2 3 4 5% 65%	2488
December	1st to 10th	Night	5% 4 1 3 3 3 2 3 4 6% 67%	2660
	11th to 20th	Night	5% 4 2 4 3 3 4 3 6% 8% 58%	3271
	21st to end	Night	6% 5% 2 4 3 3 3 3 3 6% 62%	4859

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	21% 5% 9% 37% 26% 3	2859
	11th to 20th	Day	16% 4 17% 39% 22% 2	2860
	21st to end	Day	27% 4 9% 30% 25% 4	3146
February	1st to 10th	Day	23% 5% 8% 28% 27% 9%	1710
	11th to 20th	Day	26% 3 6% 27% 28% 10%	1830
	21st to end	Day	22% 3 5% 20% 34% 16%	1570
March	1st to 10th	Day	15% 5% 4 22% 38% 15%	3383
	11th to 20th	Day	28% 7% 6% 27% 24% 8%	3490
	21st to end	Day	21% 7% 4 23% 34% 11%	3900
April	1st to 10th	Day	17% 4 5% 19% 35% 21%	2280
	11th to 20th	Day	19% 5% 3 18% 31% 23%	2254
	21st to end	Day	17% 5% 5% 22% 33% 18%	2280
May	1st to 10th	Day	12% 5% 5% 16% 36% 26%	3800
	11th to 20th	Day	21% 9% 8% 21% 26% 17%	3977
	21st to end	Day	10% 5% 5% 11% 25% 45%	4753
June	1st to 10th	Day	14% 6% 7% 19% 28% 27%	2660
	11th to 20th	Day	12% 3 4 12% 25% 44%	2660
	21st to end	Day	22% 5% 6% 19% 23% 26%	2660
July	1st to 10th	Day	19% 6% 10% 14% 21% 31%	4152
	11th to 20th	Day	18% 7% 19% 32% 18% 6%	4041
	21st to end	Day	24% 9% 19% 31% 13% 4	4314
August	1st to 10th	Day	23% 9% 23% 22% 16% 7%	4053
	11th to 20th	Day	16% 5% 13% 24% 25% 16%	3762
	21st to end	Day	26% 8% 18% 23% 19% 5%	4179
September	1st to 10th	Day	23% 6% 9% 24% 24% 14%	2280
	11th to 20th	Day	21% 5% 10% 18% 26% 20%	2280
	21st to end	Day	16% 3 4 23% 31% 23%	2035
October	1st to 10th	Day	21% 5% 8% 21% 23% 22%	1900
	11th to 20th	Day	15% 2 3 14% 37% 29%	1900
	21st to end	Day	14% 3 7% 23% 36% 18%	2090
November	1st to 10th	Day	13% 3 5% 24% 39% 17%	1869
	11th to 20th	Day	14% 2 7% 26% 40% 11%	1715
	21st to end	Day	22% 3 3 26% 34% 12%	1691
December	1st to 10th	Day	20% 3 9% 34% 25% 8%	1520
	11th to 20th	Day	25% 4 11% 31% 25% 5%	1924
	21st to end	Day	21% 4 8% 25% 30% 11%	2835
January	1st to 10th	Night	26% 5% 10% 32% 22% 5%	4097
	11th to 20th	Night	20% 3 10% 37% 24% 6%	4110
	21st to end	Night	27% 5% 11% 28% 25% 5%	4521
February	1st to 10th	Night	21% 3 9% 26% 30% 11%	2470
	11th to 20th	Night	25% 4 5% 27% 27% 13%	2244
	21st to end	Night	19% 3 4 21% 36% 18%	1884
March	1st to 10th	Night	18% 3 3 21% 41% 15%	3475
	11th to 20th	Night	36% 5% 6% 21% 24% 9%	3490
	21st to end	Night	21% 4 5% 27% 33% 11%	3748
April	1st to 10th	Night	21% 4 3 18% 31% 22%	1900
	11th to 20th	Night	21% 4 4 16% 28% 26%	1877
	21st to end	Night	20% 4 5% 18% 34% 19%	1900
May	1st to 10th	Night	18% 5% 6% 14% 30% 26%	3170
	11th to 20th	Night	28% 8% 9% 21% 16% 17%	2915
	21st to end	Night	13% 7% 5% 10% 20% 45%	2829
June	1st to 10th	Night	19% 6% 11% 17% 21% 27%	1520
	11th to 20th	Night	15% 4 4 11% 23% 42%	1520
	21st to end	Night	27% 6% 9% 15% 16% 27%	1520
July	1st to 10th	Night	24% 7% 11% 13% 15% 30%	2703
	11th to 20th	Night	30% 8% 19% 27% 10% 5%	2806
	21st to end	Night	35% 10% 19% 24% 11% 2	3011
August	1st to 10th	Night	41% 11% 16% 15% 10% 7%	2861
	11th to 20th	Night	30% 8% 17% 15% 16% 14%	3143
	21st to end	Night	37% 9% 18% 19% 12% 5%	3487
September	1st to 10th	Night	26% 8% 12% 21% 19% 13%	1900
	11th to 20th	Night	23% 5% 10% 17% 22% 23%	1900
	21st to end	Night	19% 4 6% 17% 29% 25%	2140
October	1st to 10th	Night	25% 4 6% 16% 23% 25%	2280
	11th to 20th	Night	14% 2 4 14% 33% 32%	2280
	21st to end	Night	15% 3 5% 19% 38% 20%	2508
November	1st to 10th	Night	11% 3 4 23% 42% 18%	2244
	11th to 20th	Night	12% 2 5% 24% 41% 15%	2443
	21st to end	Night	23% 2 4 21% 35% 15%	2488
December	1st to 10th	Night	20% 3 7% 33% 30% 7%	2660
	11th to 20th	Night	24% 4 9% 29% 28% 6%	3272
	21st to end	Night	25% 3 7% 27% 30% 8%	4859

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	6% 14% 28% 32% 18% 1	270
	11th to 20th	Day	8% 11% 30% 30% 18% 2	270
	21st to end	Day	3 9% 31% 36% 16% 5%	297
February	1st to 10th	Day	6% 14% 31% 32% 16% 1	270
	11th to 20th	Day	9% 11% 34% 34% 10%	270
	21st to end	Day	11% 19% 31% 29% 8% 2	225
March	1st to 10th	Day	7% 19% 32% 27% 15% 1	421
	11th to 20th	Day	12% 18% 31% 25% 13%	460
	21st to end	Day	15% 23% 35% 16% 10% 2	506
April	1st to 10th	Day	11% 18% 35% 23% 8% 5%	460
	11th to 20th	Day	9% 22% 37% 22% 9% 2	452
	21st to end	Day	8% 21% 35% 25% 8% 2	460
May	1st to 10th	Day	5% 20% 35% 30% 9% 2	463
	11th to 20th	Day	7% 13% 30% 31% 15% 5%	488
	21st to end	Day	6% 9% 31% 35% 17% 2	688
June	1st to 10th	Day	3 5% 20% 41% 27% 3	680
	11th to 20th	Day	3 5% 23% 38% 26% 5%	680
	21st to end	Day	5% 4 19% 34% 30% 8%	680
July	1st to 10th	Day	4 2 13% 36% 34% 11%	553
	11th to 20th	Day	2 1 6% 30% 37% 23%	484
	21st to end	Day	3 2 5% 25% 39% 25%	531
August	1st to 10th	Day	6% 1 8% 28% 37% 21%	487
	11th to 20th	Day	3 2 9% 34% 34% 18%	477
	21st to end	Day	3 1 9% 33% 33% 20%	539
September	1st to 10th	Day	5% 2 10% 30% 36% 16%	490
	11th to 20th	Day	3 3 18% 36% 27% 12%	490
	21st to end	Day	5% 6% 18% 28% 28% 14%	357
October	1st to 10th	Day	6% 5% 18% 30% 25% 15%	300
	11th to 20th	Day	7% 9% 19% 31% 25% 10%	300
	21st to end	Day	11% 5% 19% 30% 27% 7%	330
November	1st to 10th	Day	8% 10% 28% 30% 22% 1	273
	11th to 20th	Day	4 6% 21% 37% 25% 6%	270
	21st to end	Day	12% 10% 22% 32% 19% 4	270
December	1st to 10th	Day	7% 9% 26% 36% 18% 4	270
	11th to 20th	Day	9% 12% 23% 34% 20% 2	270
	21st to end	Day	12% 13% 28% 32% 13% 1	297
January	1st to 10th	Night	5% 4 27% 45% 19% 2	760
	11th to 20th	Night	8% 8% 25% 42% 16% 2	760
	21st to end	Night	7% 6% 26% 42% 18% 2	836
February	1st to 10th	Night	6% 7% 31% 43% 13% 1	760
	11th to 20th	Night	8% 8% 30% 44% 9%	744
	21st to end	Night	7% 8% 37% 43% 5%	628
March	1st to 10th	Night	5% 8% 32% 44% 11% 1	760
	11th to 20th	Night	8% 8% 27% 41% 14%	760
	21st to end	Night	7% 11% 28% 44% 9%	794
April	1st to 10th	Night	6% 13% 34% 39% 8%	570
	11th to 20th	Night	6% 15% 29% 40% 9%	563
	21st to end	Night	5% 12% 35% 39% 10%	570
May	1st to 10th	Night	5% 9% 28% 47% 10%	570
	11th to 20th	Night	8% 6% 23% 44% 17% 1	561
	21st to end	Night	4 6% 32% 44% 15%	617
June	1st to 10th	Night	3 5% 21% 50% 21% 1	570
	11th to 20th	Night	2 4 24% 51% 19% 1	570
	21st to end	Night	5% 3 19% 44% 28% 1	570
July	1st to 10th	Night	3 2 10% 46% 35% 3	555
	11th to 20th	Night	3 1 5% 37% 48% 6%	552
	21st to end	Night	4 2 4 28% 54% 8%	624
August	1st to 10th	Night	5% 2 7% 27% 51% 9%	567
	11th to 20th	Night	6% 3 7% 37% 42% 5%	564
	21st to end	Night	3 1 6% 38% 47% 5%	627
September	1st to 10th	Night	5% 2 8% 42% 41% 2	570
	11th to 20th	Night	6% 1 10% 47% 32% 3	570
	21st to end	Night	6% 3 13% 46% 30% 2	677
October	1st to 10th	Night	8% 4 16% 45% 25% 2	760
	11th to 20th	Night	4 3 21% 48% 21% 2	760
	21st to end	Night	5% 5% 21% 43% 23% 3	836
November	1st to 10th	Night	2 2 20% 51% 23% 2	748
	11th to 20th	Night	3 5% 24% 46% 19% 3	923
	21st to end	Night	9% 8% 19% 40% 21% 2	950
December	1st to 10th	Night	7% 7% 25% 46% 14% 1	950
	11th to 20th	Night	8% 9% 25% 42% 15% 1	950
	21st to end	Night	9% 9% 28% 43% 11% 1	1031

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