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 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 5% 2 5% 3 3 3 3 4 6% 63%	101256
	Night	3 6% 2 6% 4 3 4 3 5% 7% 57%	97867
Transparency	Day	22% 3 8% 21% 25% 21%	101002
	Night	25% 3 11% 23% 22% 15%	97710
Seeing	Day	10% 15% 26% 25% 17% 7%	15025
	Night	6% 8% 16% 41% 25% 3	24294

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 9% 5% 11% 6% 5% 5% 3 6% 8% 38%	2539
	11th to 20th	Day	4 8% 4 13% 6% 5% 7% 5% 6% 6% 36%	2618
	21st to end	Day	7% 14% 5% 10% 6% 5% 5% 4 5% 7% 32%	3146
February	1st to 10th	Day	4 9% 3 7% 5% 4 4 5% 6% 7% 47%	1710
	11th to 20th	Day	3 8% 4 8% 4 5% 5% 3 5% 9% 47%	1801
	21st to end	Day	2 9% 3 6% 6% 5% 5% 4 6% 7% 47%	1570
March	1st to 10th	Day	3 15% 7% 16% 6% 5% 5% 4 6% 7% 27%	3138
	11th to 20th	Day	2 8% 2 5% 3 3 4 3 5% 7% 58%	3490
	21st to end	Day	3 12% 4 9% 5% 4 5% 4 6% 7% 40%	4031
April	1st to 10th	Day	4 10% 3 7% 5% 4 5% 4 7% 7% 45%	2280
	11th to 20th	Day	3 8% 2 5% 5% 4 4 4 6% 6% 53%	2249
	21st to end	Day	4 7% 2 5% 3 4 4 3 3 8% 57%	2280
May	1st to 10th	Day	3 5% 2 4 3 3 3 2 3 5% 67%	4004
	11th to 20th	Day	2 3 1 3 2 2 3 2 4 6% 71%	4377
	21st to end	Day	4 4 1 4 3 3 3 3 4 6% 65%	4813
June	1st to 10th	Day	3 3 1 4 4 4 4 3 6% 7% 60%	2660
	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	3 4 1 3 3 3 3 3 3 6% 69%	1900
	11th to 20th	Day	2 4 1 3 3 3 2 3 4 6% 69%	1900
	21st to end	Day	3 4 2 4 2 3 3 3 5% 6% 65%	2060
November	1st to 10th	Day	5% 6% 2 6% 4 4 4 4 4 6% 54%	1683
	11th to 20th	Day	3 5% 2 5% 4 4 6% 5% 7% 9% 51%	1710
	21st to end	Day	2 7% 2 6% 4 3 4 5% 6% 8% 52%	1582
December	1st to 10th	Day	4 10% 4 7% 4 4 4 3 7% 8% 46%	1520
	11th to 20th	Day	10% 10% 4 9% 5% 6% 5% 5% 6% 8% 32%	1928
	21st to end	Day	3 7% 4 9% 6% 5% 6% 6% 7% 12% 35%	2794
January	1st to 10th	Night	2 7% 3 9% 6% 6% 7% 5% 8% 10% 38%	4406
	11th to 20th	Night	4 9% 5% 14% 6% 6% 6% 5% 6% 7% 33%	4327
	21st to end	Night	8% 15% 4 9% 6% 6% 5% 4 6% 7% 31%	4521
February	1st to 10th	Night	2 8% 3 7% 5% 4 5% 5% 6% 7% 49%	2470
	11th to 20th	Night	2 7% 2 7% 4 4 5% 5% 8% 8% 47%	2266
	21st to end	Night	2 7% 3 7% 5% 4 5% 5% 8% 8% 47%	1884
March	1st to 10th	Night	1 14% 8% 13% 6% 4 4 4 6% 9% 31%	3719
	11th to 20th	Night	2 8% 2 6% 3 3 4 3 6% 6% 57%	3490
	21st to end	Night	3 13% 4 11% 6% 5% 6% 5% 7% 8% 33%	3622
April	1st to 10th	Night	2 8% 2 6% 5% 4 5% 5% 7% 7% 48%	1900
	11th to 20th	Night	2 8% 1 5% 4 4 4 5% 5% 9% 54%	1879
	21st to end	Night	4 5% 1 3 3 3 3 4 6% 8% 60%	1900
May	1st to 10th	Night	3 5% 2 5% 3 3 3 2 4 4 64%	2952
	11th to 20th	Night	2 4 1 3 2 2 3 3 4 5% 72%	2516
	21st to end	Night	3 4 2 3 2 2 3 3 5% 6% 68%	2762
June	1st to 10th	Night	3 4 1 5% 3 3 3 3 6% 6% 61%	1520
	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	3 4 1 4 2 3 3 2 3 4 72%	2280
	11th to 20th	Night	2 2 1 3 2 3 2 2 5% 6% 72%	2280
	21st to end	Night	3 4 2 4 2 2 3 2 5% 6% 68%	2536
November	1st to 10th	Night	3 7% 3 6% 3 3 4 4 6% 9% 53%	2430
	11th to 20th	Night	2 6% 2 4 4 4 5% 4 6% 8% 54%	2470
	21st to end	Night	2 4 2 6% 4 4 5% 5% 6% 8% 54%	2593
December	1st to 10th	Night	4 8% 3 8% 5% 3 4 3 6% 9% 47%	2660
	11th to 20th	Night	7% 12% 4 8% 5% 6% 5% 4 7% 10% 30%	3276
	21st to end	Night	5% 9% 4 10% 6% 6% 6% 5% 8% 10% 30%	4862

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	38% 5% 15% 31% 10% 1	2539
	11th to 20th	Day	40% 5% 15% 27% 10% 4	2618
	21st to end	Day	48% 3 12% 24% 10% 2	3146
February	1st to 10th	Day	34% 4 13% 26% 18% 5%	1710
	11th to 20th	Day	32% 3 14% 28% 19% 3	1811
	21st to end	Day	32% 4 14% 28% 17% 4	1570
March	1st to 10th	Day	50% 4 11% 21% 13% 1	3138
	11th to 20th	Day	23% 3 11% 37% 23% 2	3490
	21st to end	Day	36% 4 16% 29% 13% 2	4031
April	1st to 10th	Day	34% 5% 16% 26% 16% 3	2280
	11th to 20th	Day	27% 5% 12% 30% 22% 4	2252
	21st to end	Day	27% 3 13% 30% 24% 3	2280
May	1st to 10th	Day	20% 2 6% 33% 32% 6%	4004
	11th to 20th	Day	15% 2 7% 21% 44% 12%	4380
	21st to end	Day	22% 2 6% 35% 31% 3	4814
June	1st to 10th	Day	23% 3 8% 26% 31% 9%	2660
	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	18% 2 6% 20% 34% 20%	1900
	11th to 20th	Day	16% 2 8% 25% 33% 17%	1900
	21st to end	Day	18% 2 10% 29% 26% 15%	2060
November	1st to 10th	Day	29% 3 10% 24% 24% 10%	1683
	11th to 20th	Day	25% 6% 14% 28% 23% 4	1710
	21st to end	Day	25% 5% 13% 26% 24% 6%	1582
December	1st to 10th	Day	33% 4 12% 26% 21% 3	1520
	11th to 20th	Day	47% 5% 10% 22% 13% 2	1928
	21st to end	Day	38% 5% 16% 26% 14% 1	2794
January	1st to 10th	Night	34% 5% 22% 29% 10% 1	4406
	11th to 20th	Night	39% 5% 18% 22% 12% 4	4327
	21st to end	Night	48% 4 13% 22% 12% 2	4521
February	1st to 10th	Night	29% 6% 11% 29% 19% 5%	2470
	11th to 20th	Night	28% 4 15% 31% 20% 2	2267
	21st to end	Night	28% 6% 16% 29% 17% 4	1884
March	1st to 10th	Night	42% 4 16% 26% 10% 2	3719
	11th to 20th	Night	24% 4 10% 40% 20% 2	3490
	21st to end	Night	43% 4 16% 26% 10% 1	3622
April	1st to 10th	Night	29% 6% 18% 28% 15% 3	1900
	11th to 20th	Night	24% 4 18% 31% 17% 4	1879
	21st to end	Night	21% 5% 17% 36% 18% 3	1900
May	1st to 10th	Night	21% 2 12% 33% 28% 4	2952
	11th to 20th	Night	15% 2 10% 23% 39% 11%	2516
	21st to end	Night	18% 1 11% 42% 24% 4	2762
June	1st to 10th	Night	22% 4 13% 26% 27% 8%	1520
	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	18% 2 7% 21% 31% 21%	2280
	11th to 20th	Night	14% 2 8% 25% 35% 17%	2280
	21st to end	Night	17% 2 10% 29% 29% 13%	2536
November	1st to 10th	Night	26% 5% 11% 27% 21% 10%	2430
	11th to 20th	Night	24% 5% 13% 28% 25% 5%	2470
	21st to end	Night	25% 5% 12% 28% 22% 8%	2593
December	1st to 10th	Night	30% 5% 14% 25% 21% 5%	2660
	11th to 20th	Night	40% 6% 14% 22% 16% 2	3276
	21st to end	Night	44% 4 15% 23% 12% 2	4862

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	23% 18% 19% 24% 14% 2	180
	11th to 20th	Day	15% 21% 20% 26% 13% 4	232
	21st to end	Day	11% 17% 28% 23% 18% 4	407
February	1st to 10th	Day	17% 19% 26% 22% 14% 3	370
	11th to 20th	Day	18% 20% 26% 24% 10% 1	432
	21st to end	Day	16% 22% 27% 25% 10% 1	382
March	1st to 10th	Day	21% 28% 23% 18% 8% 2	460
	11th to 20th	Day	21% 22% 23% 20% 12% 2	460
	21st to end	Day	17% 25% 29% 20% 8% 2	506
April	1st to 10th	Day	15% 23% 28% 21% 10% 3	460
	11th to 20th	Day	17% 24% 24% 20% 10% 4	452
	21st to end	Day	12% 22% 24% 21% 16% 5%	460
May	1st to 10th	Day	16% 20% 27% 21% 11% 5%	463
	11th to 20th	Day	11% 18% 30% 20% 14% 8%	488
	21st to end	Day	11% 16% 25% 23% 16% 9%	529
June	1st to 10th	Day	9% 14% 28% 23% 15% 10%	490
	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% 21% 4	509
November	1st to 10th	Day	14% 12% 15% 27% 25% 8%	273
	11th to 20th	Day	9% 10% 20% 27% 26% 9%	270
	21st to end	Day	8% 14% 20% 32% 21% 6%	210
December	1st to 10th	Day	14% 14% 22% 23% 17% 9%	180
	11th to 20th	Day	9% 16% 25% 21% 23% 6%	180
	21st to end	Day	17% 15% 20% 27% 18% 3	198
January	1st to 10th	Night	9% 11% 20% 36% 19% 4	760
	11th to 20th	Night	6% 12% 19% 43% 17% 3	760
	21st to end	Night	8% 7% 21% 41% 20% 3	836
February	1st to 10th	Night	9% 10% 20% 42% 17% 2	760
	11th to 20th	Night	8% 12% 17% 41% 17% 4	744
	21st to end	Night	10% 15% 19% 40% 15% 1	628
March	1st to 10th	Night	9% 15% 23% 35% 16% 3	760
	11th to 20th	Night	10% 13% 19% 33% 23% 2	760
	21st to end	Night	8% 18% 22% 35% 16% 1	836
April	1st to 10th	Night	6% 13% 22% 39% 19% 1	760
	11th to 20th	Night	11% 13% 18% 37% 19% 2	751
	21st to end	Night	7% 11% 19% 39% 21% 3	760
May	1st to 10th	Night	8% 13% 20% 37% 20% 2	636
	11th to 20th	Night	5% 6% 13% 49% 25% 2	374
	21st to end	Night	5% 7% 10% 41% 34% 2	411
June	1st to 10th	Night	5% 2 16% 43% 32% 2	380
	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% 9% 17% 34% 28% 5%	748
	11th to 20th	Night	6% 8% 18% 43% 20% 4	760
	21st to end	Night	5% 10% 17% 39% 25% 3	760
December	1st to 10th	Night	9% 11% 18% 41% 17% 4	760
	11th to 20th	Night	11% 11% 18% 39% 18% 3	760
	21st to end	Night	11% 11% 24% 40% 14% 1	836

Page generated at 2024-10-30 10:19:23. Version ac07

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