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 2026-10-14.

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 5% 2 5% 3 3 3 3 4 6% 64%	158024
	Night	3 6% 2 5% 3 3 4 3 5% 7% 59%	151156
Transparency	Day	20% 3 7% 21% 27% 22%	152965
	Night	23% 4 10% 23% 24% 15%	147201
Seeing	Day	10% 15% 25% 25% 18% 7%	17551
	Night	6% 8% 16% 41% 25% 3	26778

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 9% 4 9% 6% 5% 5% 3 6% 8% 41%	4722
	11th to 20th	Day	2 5% 2 7% 4 3 4 3 4 6% 59%	4902
	21st to end	Day	4 10% 3 7% 4 4 4 4 6% 9% 44%	5843
February	1st to 10th	Day	4 9% 3 7% 4 4 4 5% 6% 7% 47%	1890
	11th to 20th	Day	3 9% 4 8% 4 5% 5% 3 5% 9% 45%	1997
	21st to end	Day	2 9% 3 6% 5% 5% 5% 4 6% 7% 48%	1730
March	1st to 10th	Day	3 14% 6% 13% 6% 4 4 3 5% 8% 33%	4618
	11th to 20th	Day	2 7% 2 7% 4 3 4 4 6% 8% 53%	5110
	21st to end	Day	3 12% 4 10% 4 4 4 4 5% 6% 42%	5910
April	1st to 10th	Day	4 10% 2 7% 5% 4 5% 4 7% 8% 45%	2520
	11th to 20th	Day	3 7% 2 5% 5% 4 4 4 6% 6% 53%	2489
	21st to end	Day	4 7% 2 5% 3 4 4 3 4 8% 55%	2520
May	1st to 10th	Day	2 5% 2 4 2 2 3 2 3 5% 69%	7428
	11th to 20th	Day	1 3 1 3 2 2 3 2 4 6% 73%	8160
	21st to end	Day	3 4 1 4 3 2 3 2 4 6% 67%	8992
June	1st to 10th	Day	3 3 1 4 4 4 4 3 6% 7% 61%	2940
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 5% 76%	2940
	21st to end	Day	2 2 2 2 2 2 2 3 4 79%	2940
July	1st to 10th	Day	1 1 1 2 1 2 2 89%	8219
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 4 77%	8220
	21st to end	Day	1 1 2 2 2 3 2 3 4 78%	8680
August	1st to 10th	Day	1 1 2 1 2 2 2 2 3 84%	7547
	11th to 20th	Day	1 1 1 2 2 3 5% 84%	7063
	21st to end	Day	3 3 1 3 3 3 3 2 3 5% 71%	7750
September	1st to 10th	Day	2 1 1 2 1 1 2 2 3 4 81%	2520
	11th to 20th	Day	2 2 1 2 2 3 3 2 3 5% 75%	2515
	21st to end	Day	3 4 1 2 1 2 2 2 3 4 75%	2293
October	1st to 10th	Day	2 3 1 3 2 2 2 2 4 7% 72%	3380
	11th to 20th	Day	3 5% 1 3 2 2 2 2 3 6% 70%	3292
	21st to end	Day	2 4 2 5% 3 3 4 3 6% 7% 62%	3502
November	1st to 10th	Day	4 6% 2 6% 4 4 4 4 4 7% 56%	1854
	11th to 20th	Day	3 6% 2 5% 4 4 6% 5% 7% 9% 49%	1890
	21st to end	Day	1 7% 2 6% 4 3 4 5% 6% 8% 53%	1748
December	1st to 10th	Day	2 6% 2 5% 3 3 4 3 6% 9% 58%	2700
	11th to 20th	Day	8% 13% 4 9% 5% 6% 5% 5% 7% 9% 30%	3108
	21st to end	Day	4 12% 5% 11% 5% 5% 5% 5% 7% 11% 29%	4092
January	1st to 10th	Night	4 8% 3 9% 6% 5% 6% 4 7% 11% 36%	8172
	11th to 20th	Night	2 5% 3 7% 4 3 4 3 5% 8% 57%	8049
	21st to end	Night	4 10% 4 8% 5% 5% 5% 4 5% 7% 44%	8371
February	1st to 10th	Night	2 8% 3 7% 5% 4 5% 4 6% 7% 48%	2730
	11th to 20th	Night	2 7% 2 7% 5% 5% 5% 5% 8% 9% 45%	2510
	21st to end	Night	2 7% 3 7% 5% 4 5% 5% 8% 8% 47%	2076
March	1st to 10th	Night	2 13% 6% 11% 5% 4 4 4 5% 10% 37%	5479
	11th to 20th	Night	3 9% 3 8% 4 4 4 3 6% 7% 50%	5110
	21st to end	Night	2 14% 4 10% 6% 5% 5% 4 6% 7% 36%	5306
April	1st to 10th	Night	2 8% 2 6% 5% 4 5% 5% 7% 8% 48%	2100
	11th to 20th	Night	2 8% 2 5% 4 3 4 5% 5% 8% 54%	2079
	21st to end	Night	5% 6% 2 3 3 3 3 4 6% 8% 58%	2100
May	1st to 10th	Night	3 4 2 6% 3 3 3 2 3 4 66%	5499
	11th to 20th	Night	2 4 1 3 2 2 3 2 4 5% 73%	4700
	21st to end	Night	2 4 2 3 2 2 3 2 4 5% 71%	5181
June	1st to 10th	Night	3 4 1 4 3 3 3 3 6% 6% 63%	1680
	11th to 20th	Night	1 2 3 1 2 2 2 4 5% 77%	1680
	21st to end	Night	2 2 1 3 3 2 2 2 2 3 78%	1680
July	1st to 10th	Night	1 1 1 1 1 1 2 3 89%	4740
	11th to 20th	Night	3 3 1 3 2 2 2 2 2 3 77%	4740
	21st to end	Night	2 1 1 2 2 2 3 2 3 5% 77%	5390
August	1st to 10th	Night	2 2 1 1 1 1 1 1 2 4 84%	5275
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 84%	5819
	21st to end	Night	3 3 1 3 2 3 3 3 4 6% 70%	6499
September	1st to 10th	Night	3 1 2 1 2 2 1 3 4 81%	2100
	11th to 20th	Night	3 3 1 2 2 2 2 2 3 5% 76%	2105
	21st to end	Night	2 3 1 2 2 2 2 2 3 5% 77%	2324
October	1st to 10th	Night	2 2 1 3 2 2 3 2 3 3 77%	4040
	11th to 20th	Night	2 3 1 4 2 2 2 2 4 6% 74%	3933
	21st to end	Night	2 6% 2 4 2 3 4 3 5% 8% 62%	4305
November	1st to 10th	Night	3 6% 3 6% 3 3 4 4 6% 9% 54%	2665
	11th to 20th	Night	3 6% 2 5% 4 4 5% 4 7% 8% 52%	2730
	21st to end	Night	2 4 2 6% 4 4 4 4 6% 9% 54%	2867
December	1st to 10th	Night	2 5% 2 5% 3 2 3 3 5% 8% 61%	4700
	11th to 20th	Night	5% 13% 4 9% 6% 5% 6% 5% 8% 12% 28%	5316
	21st to end	Night	5% 12% 5% 10% 6% 6% 6% 4 8% 10% 28%	7106

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	35% 7% 16% 30% 11%	4723
	11th to 20th	Day	24% 3 9% 23% 30% 11%	4902
	21st to end	Day	34% 4 10% 20% 24% 8%	5843
February	1st to 10th	Day	33% 4 12% 25% 19% 6%	1890
	11th to 20th	Day	34% 4 13% 28% 19% 3	2007
	21st to end	Day	31% 5% 14% 28% 17% 6%	1730
March	1st to 10th	Day	45% 3 9% 23% 16% 4	4618
	11th to 20th	Day	23% 4 13% 38% 19% 2	5110
	21st to end	Day	35% 5% 14% 29% 16% 2	5910
April	1st to 10th	Day	33% 5% 16% 26% 17% 3	2520
	11th to 20th	Day	27% 5% 12% 29% 22% 5%	2492
	21st to end	Day	27% 3 13% 30% 24% 3	2520
May	1st to 10th	Day	19% 3 5% 25% 39% 9%	7428
	11th to 20th	Day	13% 3 6% 21% 43% 14%	8163
	21st to end	Day	19% 2 5% 27% 40% 7%	8994
June	1st to 10th	Day	21% 3 8% 25% 33% 10%	2940
	11th to 20th	Day	12% 1 6% 20% 38% 23%	2940
	21st to end	Day	10% 2 3 15% 38% 33%	2940
July	1st to 10th	Day	5% 1 1 6% 21% 65%	8151
	11th to 20th	Day	12% 2 2 6% 15% 63%	8145
	21st to end	Day	10% 3 3 10% 25% 49%	8485
August	1st to 10th	Day	9% 1 2 7% 23% 58%	5976
	11th to 20th	Day	6% 2 2 18% 34% 39%	5567
	21st to end	Day	14% 2 2 17% 38% 27%	6077
September	1st to 10th	Day	9% 1 2 11% 32% 44%	2520
	11th to 20th	Day	13% 2 4 19% 38% 24%	2515
	21st to end	Day	14% 2 4 21% 36% 23%	2293
October	1st to 10th	Day	14% 2 4 14% 36% 30%	3380
	11th to 20th	Day	16% 3 6% 26% 37% 12%	3292
	21st to end	Day	18% 4 9% 29% 29% 11%	3502
November	1st to 10th	Day	27% 4 11% 25% 24% 9%	1854
	11th to 20th	Day	27% 7% 14% 27% 22% 4	1890
	21st to end	Day	25% 5% 12% 27% 25% 7%	1748
December	1st to 10th	Day	22% 4 11% 31% 28% 5%	2700
	11th to 20th	Day	45% 7% 11% 22% 12% 2	3108
	21st to end	Day	43% 7% 17% 22% 10% 1	4092
January	1st to 10th	Night	33% 9% 20% 28% 10%	8172
	11th to 20th	Night	22% 3 12% 22% 32% 9%	8049
	21st to end	Night	37% 3 10% 19% 26% 5%	8371
February	1st to 10th	Night	29% 6% 11% 28% 20% 5%	2730
	11th to 20th	Night	29% 5% 15% 30% 19% 2	2511
	21st to end	Night	28% 6% 16% 29% 17% 5%	2076
March	1st to 10th	Night	39% 5% 12% 29% 12% 4	5479
	11th to 20th	Night	28% 5% 14% 34% 16% 2	5110
	21st to end	Night	40% 7% 16% 28% 9% 1	5306
April	1st to 10th	Night	29% 6% 18% 28% 16% 3	2100
	11th to 20th	Night	25% 5% 17% 30% 18% 5%	2079
	21st to end	Night	22% 5% 16% 36% 17% 3	2100
May	1st to 10th	Night	21% 3 10% 29% 32% 5%	5497
	11th to 20th	Night	15% 3 8% 24% 41% 9%	4700
	21st to end	Night	16% 3 8% 37% 32% 4	5182
June	1st to 10th	Night	21% 4 12% 26% 29% 8%	1680
	11th to 20th	Night	11% 2 8% 24% 34% 20%	1680
	21st to end	Night	13% 2 3 17% 34% 30%	1680
July	1st to 10th	Night	3 1 1 9% 26% 59%	4700
	11th to 20th	Night	14% 2 2 6% 19% 57%	4692
	21st to end	Night	11% 2 3 13% 24% 46%	5273
August	1st to 10th	Night	10% 1 2 7% 29% 51%	4169
	11th to 20th	Night	7% 1 3 18% 37% 33%	4582
	21st to end	Night	13% 2 3 22% 33% 26%	5092
September	1st to 10th	Night	10% 1 2 10% 34% 43%	2100
	11th to 20th	Night	13% 2 5% 20% 37% 23%	2105
	21st to end	Night	13% 2 4 22% 37% 23%	2324
October	1st to 10th	Night	13% 2 5% 14% 34% 31%	4040
	11th to 20th	Night	13% 3 6% 29% 39% 10%	3933
	21st to end	Night	20% 4 8% 31% 26% 11%	4305
November	1st to 10th	Night	24% 5% 12% 29% 21% 10%	2665
	11th to 20th	Night	26% 6% 13% 28% 23% 4	2730
	21st to end	Night	24% 6% 12% 28% 22% 7%	2867
December	1st to 10th	Night	20% 4 11% 30% 25% 9%	4700
	11th to 20th	Night	40% 8% 15% 23% 13% 2	5316
	21st to end	Night	45% 7% 16% 22% 9% 1	7106

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	20% 18% 19% 27% 14% 2	220
	11th to 20th	Day	13% 18% 20% 27% 17% 6%	278
	21st to end	Day	10% 15% 27% 22% 20% 6%	473
February	1st to 10th	Day	18% 19% 24% 22% 14% 4	430
	11th to 20th	Day	17% 20% 25% 27% 9% 1	508
	21st to end	Day	15% 20% 27% 26% 11% 1	446
March	1st to 10th	Day	19% 26% 23% 20% 9% 2	540
	11th to 20th	Day	19% 21% 22% 20% 15% 2	540
	21st to end	Day	16% 24% 28% 21% 8% 3	594
April	1st to 10th	Day	14% 21% 27% 21% 12% 4	540
	11th to 20th	Day	15% 23% 24% 21% 11% 5%	532
	21st to end	Day	12% 20% 24% 22% 16% 6%	540
May	1st to 10th	Day	14% 18% 28% 22% 12% 6%	543
	11th to 20th	Day	10% 18% 29% 21% 14% 8%	568
	21st to end	Day	10% 14% 24% 24% 17% 10%	617
June	1st to 10th	Day	8% 14% 27% 24% 17% 11%	570
	11th to 20th	Day	8% 15% 29% 24% 17% 7%	570
	21st to end	Day	2 11% 29% 28% 19% 11%	570
July	1st to 10th	Day	1 9% 32% 27% 21% 11%	566
	11th to 20th	Day	2 8% 29% 29% 22% 12%	564
	21st to end	Day	2 5% 29% 29% 21% 13%	619
August	1st to 10th	Day	4 6% 27% 27% 22% 14%	567
	11th to 20th	Day	2 9% 26% 27% 23% 14%	557
	21st to end	Day	3 10% 28% 28% 22% 10%	627
September	1st to 10th	Day	3 11% 27% 28% 23% 8%	570
	11th to 20th	Day	4 15% 24% 28% 23% 7%	570
	21st to end	Day	7% 11% 26% 29% 21% 7%	570
October	1st to 10th	Day	6% 12% 24% 24% 23% 10%	570
	11th to 20th	Day	8% 13% 22% 25% 24% 6%	543
	21st to end	Day	10% 17% 19% 28% 21% 5%	551
November	1st to 10th	Day	13% 11% 15% 27% 26% 8%	330
	11th to 20th	Day	12% 10% 19% 28% 24% 8%	330
	21st to end	Day	7% 15% 18% 30% 22% 8%	256
December	1st to 10th	Day	11% 13% 19% 22% 22% 13%	220
	11th to 20th	Day	11% 16% 21% 20% 23% 8%	220
	21st to end	Day	18% 17% 18% 26% 18% 2	242
January	1st to 10th	Night	10% 11% 22% 37% 18% 4	840
	11th to 20th	Night	5% 11% 18% 44% 19% 2	840
	21st to end	Night	8% 6% 21% 41% 21% 3	924
February	1st to 10th	Night	10% 10% 20% 42% 17% 2	840
	11th to 20th	Night	8% 12% 19% 42% 16% 3	824
	21st to end	Night	9% 14% 19% 41% 15% 1	692
March	1st to 10th	Night	9% 14% 23% 36% 16% 2	840
	11th to 20th	Night	10% 13% 19% 34% 23% 2	840
	21st to end	Night	8% 18% 21% 36% 16% 1	924
April	1st to 10th	Night	7% 12% 22% 38% 20% 1	840
	11th to 20th	Night	10% 12% 19% 37% 19% 3	831
	21st to end	Night	8% 10% 18% 38% 23% 3	840
May	1st to 10th	Night	8% 12% 19% 37% 21% 2	702
	11th to 20th	Night	5% 6% 13% 51% 23% 2	414
	21st to end	Night	5% 7% 10% 42% 34% 2	455
June	1st to 10th	Night	5% 2 15% 45% 32% 1	420
	11th to 20th	Night	2 4 15% 48% 30% 1	420
	21st to end	Night	3 1 9% 50% 34% 2	420
July	1st to 10th	Night	2 8% 56% 32% 2	410
	11th to 20th	Night	1 7% 56% 35%	408
	21st to end	Night	2 1 3 44% 45% 4	544
August	1st to 10th	Night	3 1 2 46% 44% 4	627
	11th to 20th	Night	2 1 10% 48% 35% 5%	803
	21st to end	Night	2 1 10% 49% 34% 3	924
September	1st to 10th	Night	2 1 14% 44% 35% 4	840
	11th to 20th	Night	4 3 10% 42% 35% 6%	840
	21st to end	Night	3 3 11% 44% 31% 7%	840
October	1st to 10th	Night	5% 5% 15% 40% 30% 5%	840
	11th to 20th	Night	6% 6% 11% 40% 32% 5%	812
	21st to end	Night	5% 7% 15% 38% 26% 9%	880
November	1st to 10th	Night	7% 8% 17% 36% 28% 5%	820
	11th to 20th	Night	7% 9% 19% 43% 19% 4	840
	21st to end	Night	5% 11% 17% 39% 25% 3	840
December	1st to 10th	Night	8% 10% 17% 42% 19% 4	840
	11th to 20th	Night	11% 11% 18% 39% 19% 3	840
	21st to end	Night	12% 13% 24% 38% 13% 1	924

Page generated at 2026-10-11 02:06:33. Version ac07

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