Portola
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portola clear sky chart from 2005-11-06 to 2025-06-10.

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	2 6% 2 5% 3 3 3 3 4 6% 62%	123520
	Night	3 6% 2 6% 4 3 4 3 5% 7% 57%	123524
Transparency	Day	22% 3 8% 22% 27% 19%	123267
	Night	24% 4 11% 24% 24% 14%	123364
Seeing	Day	10% 15% 26% 25% 17% 7%	15748
	Night	6% 8% 16% 41% 25% 3	25104

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	2 8% 4 9% 6% 5% 5% 4 7% 9% 44%	3639
	11th to 20th	Day	3 6% 3 9% 4 4 5% 3 4 5% 54%	3760
	21st to end	Day	5% 11% 4 8% 5% 4 4 3 5% 6% 45%	4510
February	1st to 10th	Day	4 10% 3 7% 4 4 4 5% 6% 7% 47%	1800
	11th to 20th	Day	3 9% 4 8% 4 5% 5% 3 5% 9% 46%	1899
	21st to end	Day	2 9% 3 6% 5% 5% 5% 4 6% 7% 49%	1650
March	1st to 10th	Day	3 14% 6% 13% 6% 4 4 4 5% 8% 32%	4518
	11th to 20th	Day	2 8% 2 7% 4 3 4 4 6% 8% 52%	5000
	21st to end	Day	3 13% 4 10% 5% 4 4 4 5% 6% 42%	5783
April	1st to 10th	Day	4 10% 2 7% 5% 4 5% 4 7% 7% 45%	2400
	11th to 20th	Day	3 7% 2 5% 5% 4 4 4 6% 6% 53%	2369
	21st to end	Day	4 7% 2 5% 3 4 4 3 4 8% 56%	2400
May	1st to 10th	Day	2 5% 2 4 2 2 2 2 3 5% 71%	5735
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 7% 68%	6277
	21st to end	Day	3 3 1 3 2 2 2 2 4 5% 72%	6902
June	1st to 10th	Day	3 3 1 4 4 4 4 3 6% 7% 61%	2789
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 5% 76%	2660
	21st to end	Day	2 2 2 2 2 2 2 4 4 79%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 2 90%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 80%	4420
	21st to end	Day	1 1 1 2 1 1 2 2 4 84%	4652
August	1st to 10th	Day	2 1 1 3 2 3 3 3 4 5% 75%	4091
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	3760
	21st to end	Day	1 1 2 1 1 1 1 2 3 87%	4180
September	1st to 10th	Day	2 1 1 1 1 1 2 1 3 3 84%	2280
	11th to 20th	Day	2 2 1 2 2 3 3 2 4 5% 75%	2275
	21st to end	Day	3 3 1 2 1 2 2 2 3 5% 76%	2075
October	1st to 10th	Day	2 3 1 3 2 2 2 2 4 7% 73%	3180
	11th to 20th	Day	3 5% 1 3 2 2 2 2 3 6% 70%	3161
	21st to end	Day	2 4 2 5% 3 3 4 3 6% 7% 62%	3394
November	1st to 10th	Day	4 6% 2 6% 4 4 4 4 5% 6% 55%	1764
	11th to 20th	Day	2 5% 2 5% 4 4 6% 5% 7% 9% 51%	1800
	21st to end	Day	2 7% 2 7% 4 3 4 5% 6% 8% 51%	1665
December	1st to 10th	Day	2 6% 2 5% 3 3 4 3 6% 9% 58%	2620
	11th to 20th	Day	8% 13% 4 8% 4 6% 5% 5% 7% 9% 30%	3028
	21st to end	Day	4 12% 5% 11% 5% 5% 5% 5% 7% 11% 29%	4004
January	1st to 10th	Night	1 6% 3 8% 6% 5% 6% 5% 8% 13% 39%	6306
	11th to 20th	Night	3 6% 4 10% 4 4 4 3 4 6% 51%	6188
	21st to end	Night	5% 11% 3 7% 5% 5% 4 4 5% 6% 45%	6468
February	1st to 10th	Night	2 9% 3 7% 5% 4 5% 4 6% 7% 48%	2600
	11th to 20th	Night	2 7% 2 7% 5% 4 5% 5% 9% 9% 46%	2388
	21st to end	Night	2 7% 3 7% 5% 4 5% 5% 8% 8% 48%	1980
March	1st to 10th	Night	2 13% 6% 11% 5% 4 4 4 5% 10% 36%	5359
	11th to 20th	Night	3 9% 3 8% 4 4 4 3 6% 7% 49%	5000
	21st to end	Night	2 14% 4 10% 6% 5% 5% 4 6% 7% 35%	5192
April	1st to 10th	Night	2 7% 2 6% 5% 4 5% 5% 7% 8% 48%	2000
	11th to 20th	Night	2 8% 2 5% 4 3 4 4 5% 8% 55%	1979
	21st to end	Night	5% 6% 1 3 3 3 3 4 6% 8% 60%	2000
May	1st to 10th	Night	3 4 2 4 3 3 3 2 4 4 70%	4234
	11th to 20th	Night	2 4 1 4 2 3 3 2 4 5% 69%	3616
	21st to end	Night	2 3 1 3 2 2 2 2 3 5% 75%	3972
June	1st to 10th	Night	3 4 1 4 3 3 3 3 6% 7% 62%	1595
	11th to 20th	Night	2 2 1 3 1 2 2 2 4 6% 76%	1520
	21st to end	Night	2 3 1 2 3 2 2 2 2 3 77%	1520
July	1st to 10th	Night	1 1 1 1 1 2 2 90%	2540
	11th to 20th	Night	2 2 1 2 2 1 2 2 2 3 80%	2540
	21st to end	Night	1 1 1 1 1 2 1 3 4 83%	2886
August	1st to 10th	Night	3 3 1 3 2 2 2 2 4 6% 74%	2848
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 87%	3087
	21st to end	Night	2 2 1 1 1 1 1 1 1 3 86%	3498
September	1st to 10th	Night	2 1 1 1 1 2 1 2 4 84%	1900
	11th to 20th	Night	3 3 1 2 2 2 2 2 3 5% 75%	1905
	21st to end	Night	1 3 1 2 2 2 2 2 3 5% 78%	2102
October	1st to 10th	Night	2 2 1 3 2 2 3 2 3 3 77%	3800
	11th to 20th	Night	2 2 1 4 2 2 2 2 3 6% 75%	3777
	21st to end	Night	2 6% 2 4 2 3 4 3 5% 8% 62%	4171
November	1st to 10th	Night	3 6% 2 6% 3 3 4 4 6% 8% 54%	2535
	11th to 20th	Night	2 6% 2 5% 4 4 5% 4 7% 8% 54%	2600
	21st to end	Night	2 5% 2 7% 4 4 4 4 5% 9% 54%	2730
December	1st to 10th	Night	2 5% 2 5% 3 3 3 3 5% 8% 61%	4560
	11th to 20th	Night	5% 13% 4 9% 6% 6% 6% 5% 8% 12% 27%	5176
	21st to end	Night	5% 11% 5% 10% 6% 5% 6% 4 8% 10% 28%	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	32% 7% 17% 30% 14% 1	3639
	11th to 20th	Day	28% 4 11% 27% 24% 5%	3760
	21st to end	Day	38% 4 9% 19% 23% 8%	4510
February	1st to 10th	Day	34% 5% 12% 25% 19% 6%	1800
	11th to 20th	Day	33% 3 13% 28% 19% 3	1909
	21st to end	Day	31% 4 14% 28% 17% 6%	1650
March	1st to 10th	Day	46% 3 9% 22% 15% 4	4518
	11th to 20th	Day	23% 5% 14% 38% 19% 2	5000
	21st to end	Day	35% 5% 14% 29% 16% 2	5783
April	1st to 10th	Day	33% 5% 16% 26% 17% 3	2400
	11th to 20th	Day	27% 5% 12% 30% 22% 5%	2372
	21st to end	Day	27% 3 13% 30% 24% 3	2400
May	1st to 10th	Day	18% 2 5% 28% 39% 8%	5735
	11th to 20th	Day	16% 3 8% 24% 39% 10%	6280
	21st to end	Day	16% 2 5% 27% 42% 8%	6904
June	1st to 10th	Day	22% 3 8% 25% 32% 10%	2789
	11th to 20th	Day	13% 1 6% 22% 38% 20%	2660
	21st to end	Day	11% 2 3 15% 37% 33%	2660
July	1st to 10th	Day	4 1 1 5% 15% 73%	4351
	11th to 20th	Day	10% 2 2 7% 19% 61%	4345
	21st to end	Day	7% 2 1 6% 27% 56%	4505
August	1st to 10th	Day	12% 2 2 8% 23% 53%	4076
	11th to 20th	Day	6% 1 1 6% 33% 53%	3796
	21st to end	Day	6% 1 1 10% 43% 39%	4179
September	1st to 10th	Day	8% 1 1 10% 32% 48%	2280
	11th to 20th	Day	13% 2 4 20% 37% 24%	2275
	21st to end	Day	13% 2 4 22% 36% 24%	2075
October	1st to 10th	Day	14% 1 4 14% 36% 30%	3180
	11th to 20th	Day	16% 2 6% 26% 37% 12%	3161
	21st to end	Day	18% 4 9% 29% 29% 11%	3394
November	1st to 10th	Day	28% 4 11% 25% 24% 10%	1764
	11th to 20th	Day	25% 6% 15% 28% 23% 4	1800
	21st to end	Day	26% 5% 12% 26% 24% 7%	1665
December	1st to 10th	Day	23% 4 10% 30% 28% 5%	2620
	11th to 20th	Day	46% 7% 11% 23% 12% 1	3028
	21st to end	Day	43% 7% 17% 22% 10% 1	4004
January	1st to 10th	Night	29% 9% 21% 28% 12%	6306
	11th to 20th	Night	28% 3 14% 27% 24% 4	6188
	21st to end	Night	37% 4 10% 17% 27% 5%	6468
February	1st to 10th	Night	30% 7% 11% 28% 19% 6%	2600
	11th to 20th	Night	28% 5% 15% 31% 20% 2	2389
	21st to end	Night	27% 6% 15% 29% 17% 6%	1980
March	1st to 10th	Night	39% 5% 12% 28% 11% 4	5359
	11th to 20th	Night	29% 5% 14% 34% 16% 2	5000
	21st to end	Night	41% 7% 16% 27% 8% 1	5192
April	1st to 10th	Night	28% 6% 18% 29% 16% 3	2000
	11th to 20th	Night	24% 4 17% 31% 18% 5%	1979
	21st to end	Night	22% 5% 16% 36% 18% 3	2000
May	1st to 10th	Night	17% 2 10% 34% 33% 4	4231
	11th to 20th	Night	18% 3 10% 26% 35% 8%	3616
	21st to end	Night	15% 1 8% 34% 37% 5%	3972
June	1st to 10th	Night	21% 4 13% 27% 27% 8%	1595
	11th to 20th	Night	12% 2 9% 26% 34% 18%	1520
	21st to end	Night	14% 2 3 17% 33% 31%	1520
July	1st to 10th	Night	4 1 1 5% 17% 72%	2500
	11th to 20th	Night	12% 1 2 7% 18% 60%	2492
	21st to end	Night	8% 1 2 7% 31% 51%	2795
August	1st to 10th	Night	14% 1 2 8% 30% 45%	2839
	11th to 20th	Night	6% 1 1 7% 39% 45%	3117
	21st to end	Night	8% 1 1 13% 40% 37%	3498
September	1st to 10th	Night	9% 1 2 8% 33% 47%	1900
	11th to 20th	Night	13% 2 5% 21% 35% 24%	1905
	21st to end	Night	12% 2 4 22% 36% 24%	2102
October	1st to 10th	Night	13% 1 5% 14% 34% 31%	3800
	11th to 20th	Night	13% 2 6% 28% 40% 11%	3777
	21st to end	Night	20% 4 9% 30% 27% 11%	4171
November	1st to 10th	Night	25% 5% 12% 28% 21% 10%	2535
	11th to 20th	Night	24% 5% 14% 29% 24% 5%	2600
	21st to end	Night	25% 5% 12% 27% 23% 8%	2730
December	1st to 10th	Night	20% 4 11% 29% 26% 9%	4560
	11th to 20th	Night	40% 8% 15% 23% 12% 1	5176
	21st to end	Night	44% 7% 16% 22% 9% 1	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	21% 18% 19% 26% 15% 2	200
	11th to 20th	Day	14% 19% 21% 27% 14% 5%	255
	21st to end	Day	11% 16% 28% 23% 18% 5%	440
February	1st to 10th	Day	19% 20% 25% 21% 13% 3	400
	11th to 20th	Day	18% 20% 26% 26% 10% 1	470
	21st to end	Day	15% 21% 27% 25% 11% 1	414
March	1st to 10th	Day	20% 27% 23% 19% 9% 2	500
	11th to 20th	Day	21% 22% 23% 20% 12% 2	500
	21st to end	Day	17% 25% 27% 19% 8% 3	550
April	1st to 10th	Day	14% 23% 28% 21% 11% 3	500
	11th to 20th	Day	16% 23% 24% 21% 11% 5%	492
	21st to end	Day	12% 20% 25% 21% 16% 6%	500
May	1st to 10th	Day	16% 19% 28% 21% 11% 6%	503
	11th to 20th	Day	11% 18% 29% 20% 14% 8%	528
	21st to end	Day	10% 15% 25% 24% 17% 9%	573
June	1st to 10th	Day	9% 13% 26% 24% 16% 11%	527
	11th to 20th	Day	9% 16% 30% 24% 14% 6%	490
	21st to end	Day	3 11% 31% 27% 18% 11%	490
July	1st to 10th	Day	2 8% 33% 26% 20% 11%	486
	11th to 20th	Day	1 8% 30% 29% 20% 12%	484
	21st to end	Day	2 5% 29% 31% 20% 13%	531
August	1st to 10th	Day	4 7% 29% 25% 21% 13%	487
	11th to 20th	Day	2 9% 26% 28% 22% 13%	477
	21st to end	Day	2 11% 27% 29% 22% 9%	539
September	1st to 10th	Day	3 10% 27% 29% 23% 8%	490
	11th to 20th	Day	4 14% 25% 28% 22% 6%	490
	21st to end	Day	7% 10% 28% 29% 21% 5%	490
October	1st to 10th	Day	7% 13% 24% 26% 22% 9%	490
	11th to 20th	Day	8% 13% 23% 26% 25% 6%	490
	21st to end	Day	9% 17% 20% 29% 20% 4	509
November	1st to 10th	Day	13% 11% 16% 28% 24% 8%	300
	11th to 20th	Day	10% 10% 19% 28% 25% 8%	300
	21st to end	Day	8% 16% 19% 30% 20% 7%	233
December	1st to 10th	Day	13% 14% 20% 23% 19% 14%	200
	11th to 20th	Day	12% 15% 23% 22% 23% 7%	200
	21st to end	Day	18% 16% 19% 27% 17% 3	220
January	1st to 10th	Night	9% 11% 21% 37% 18% 4	800
	11th to 20th	Night	6% 12% 19% 44% 17% 3	800
	21st to end	Night	8% 7% 21% 41% 20% 3	880
February	1st to 10th	Night	10% 11% 20% 41% 16% 2	800
	11th to 20th	Night	8% 12% 17% 42% 16% 4	784
	21st to end	Night	9% 14% 19% 41% 15% 1	660
March	1st to 10th	Night	9% 15% 23% 35% 16% 2	800
	11th to 20th	Night	10% 14% 20% 33% 22% 2	800
	21st to end	Night	8% 19% 21% 35% 17% 1	880
April	1st to 10th	Night	6% 12% 23% 39% 19% 1	800
	11th to 20th	Night	10% 12% 18% 38% 19% 2	791
	21st to end	Night	8% 10% 19% 39% 21% 3	800
May	1st to 10th	Night	8% 12% 20% 37% 21% 2	669
	11th to 20th	Night	5% 6% 13% 50% 24% 2	394
	21st to end	Night	5% 7% 10% 42% 34% 2	433
June	1st to 10th	Night	5% 2 15% 44% 33% 2	399
	11th to 20th	Night	2 4 15% 47% 31% 1	380
	21st to end	Night	3 1 9% 51% 33% 2	380
July	1st to 10th	Night	2 7% 55% 34% 2	370
	11th to 20th	Night	1 1 8% 55% 35% 1	368
	21st to end	Night	3 3 43% 46% 4	492
August	1st to 10th	Night	4 1 2 47% 42% 4	567
	11th to 20th	Night	2 1 9% 46% 36% 5%	725
	21st to end	Night	2 1 10% 49% 34% 3	836
September	1st to 10th	Night	1 1 13% 44% 37% 4	760
	11th to 20th	Night	3 3 10% 41% 35% 7%	760
	21st to end	Night	3 4 12% 42% 31% 8%	760
October	1st to 10th	Night	5% 5% 15% 40% 30% 5%	760
	11th to 20th	Night	5% 7% 11% 40% 32% 5%	760
	21st to end	Night	6% 7% 15% 38% 25% 9%	836
November	1st to 10th	Night	7% 8% 17% 36% 28% 5%	780
	11th to 20th	Night	6% 9% 19% 43% 19% 4	800
	21st to end	Night	6% 11% 17% 39% 24% 3	800
December	1st to 10th	Night	9% 10% 17% 41% 19% 5%	800
	11th to 20th	Night	11% 11% 18% 39% 19% 2	800
	21st to end	Night	11% 12% 24% 39% 13% 1	880

Page generated at 2025-06-07 07:28:17. Version ac07

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