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 2024-03-08.

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% 4 3 4 4 5% 6% 59%	78608
	Night	3 6% 3 5% 5% 3 4 4 5% 7% 54%	80968
Transparency	Day	25% 3 9% 22% 24% 17%	78317
	Night	26% 4 12% 24% 21% 12%	80780
Seeing	Day	10% 16% 26% 25% 17% 6%	14073
	Night	6% 8% 16% 41% 25% 3	23515

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% 10% 7% 4 6% 4 6% 8% 38%	2539
	11th to 20th	Day	4 8% 4 12% 7% 4 7% 6% 6% 6% 36%	2618
	21st to end	Day	7% 14% 5% 8% 7% 4 5% 5% 5% 7% 32%	3146
February	1st to 10th	Day	4 9% 3 6% 5% 3 4 6% 6% 7% 47%	1710
	11th to 20th	Day	3 8% 4 7% 5% 3 5% 4 5% 9% 47%	1801
	21st to end	Day	2 8% 3 6% 6% 4 6% 5% 6% 7% 47%	1570
March	1st to 10th	Day	4 10% 5% 9% 6% 4 6% 6% 6% 8% 35%	1871
	11th to 20th	Day	4 10% 4 6% 5% 3 5% 6% 7% 8% 41%	1980
	21st to end	Day	3 10% 3 6% 5% 3 5% 5% 6% 8% 44%	2294
April	1st to 10th	Day	5% 9% 3 7% 5% 3 5% 5% 7% 8% 44%	2160
	11th to 20th	Day	3 6% 3 5% 5% 3 4 5% 6% 7% 53%	2129
	21st to end	Day	5% 6% 3 4 4 3 4 3 3 7% 57%	2160
May	1st to 10th	Day	4 5% 2 4 4 3 3 4 4 7% 60%	2273
	11th to 20th	Day	3 5% 2 4 4 3 5% 4 5% 8% 57%	2477
	21st to end	Day	7% 6% 3 5% 5% 3 5% 5% 5% 6% 50%	2723
June	1st to 10th	Day	4 3 2 3 4 3 5% 3 6% 8% 60%	2520
	11th to 20th	Day	1 2 1 2 2 2 2 3 4 5% 76%	2520
	21st to end	Day	2 2 1 2 2 1 2 2 4 4 78%	2520
July	1st to 10th	Day	1 1 1 1 1 1 2 2 3 3 83%	2520
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 4 85%	2520
	21st to end	Day	2 1 1 2 3 2 2 3 3 4 77%	2665
August	1st to 10th	Day	2 1 1 2 2 2 3 3 3 5% 77%	2328
	11th to 20th	Day	1 1 1 1 1 2 3 2 4 83%	2146
	21st to end	Day	1 1 1 1 1 1 2 2 3 88%	2376
September	1st to 10th	Day	2 1 1 1 1 1 2 2 3 3 84%	2160
	11th to 20th	Day	2 1 1 2 2 2 3 3 3 5% 76%	2156
	21st to end	Day	3 3 1 2 2 1 2 2 4 5% 74%	1966
October	1st to 10th	Day	3 4 2 3 3 2 3 3 3 5% 68%	1800
	11th to 20th	Day	2 4 2 2 3 2 3 4 4 6% 69%	1800
	21st to end	Day	3 4 2 3 3 2 3 4 5% 6% 65%	1952
November	1st to 10th	Day	5% 5% 3 5% 5% 3 5% 4 4 7% 55%	1674
	11th to 20th	Day	3 5% 2 4 5% 3 6% 6% 6% 9% 52%	1710
	21st to end	Day	2 6% 3 5% 5% 2 5% 6% 6% 8% 52%	1582
December	1st to 10th	Day	4 9% 4 6% 4 3 5% 4 7% 9% 46%	1520
	11th to 20th	Day	10% 9% 4 8% 6% 4 7% 6% 5% 8% 32%	1928
	21st to end	Day	3 7% 4 8% 7% 4 6% 7% 7% 12% 35%	2794
January	1st to 10th	Night	2 6% 3 7% 8% 4 7% 7% 7% 10% 38%	4406
	11th to 20th	Night	4 9% 5% 12% 8% 4 6% 6% 6% 8% 33%	4327
	21st to end	Night	8% 14% 4 8% 7% 5% 5% 6% 6% 7% 31%	4521
February	1st to 10th	Night	2 7% 3 6% 6% 3 5% 6% 6% 8% 49%	2470
	11th to 20th	Night	2 6% 3 6% 5% 3 5% 6% 8% 9% 47%	2266
	21st to end	Night	2 6% 3 6% 6% 3 5% 5% 8% 8% 47%	1884
March	1st to 10th	Night	2 10% 4 7% 7% 4 6% 5% 7% 8% 40%	2245
	11th to 20th	Night	3 8% 3 6% 4 3 5% 5% 7% 10% 44%	1980
	21st to end	Night	2 8% 3 6% 6% 3 6% 6% 6% 9% 46%	2062
April	1st to 10th	Night	3 7% 2 5% 6% 3 6% 6% 7% 8% 47%	1800
	11th to 20th	Night	2 7% 2 5% 3 3 4 5% 6% 9% 54%	1779
	21st to end	Night	5% 5% 2 3 3 1 3 5% 6% 9% 60%	1800
May	1st to 10th	Night	6% 5% 1 3 3 3 4 4 4 5% 62%	1684
	11th to 20th	Night	3 6% 2 4 4 2 4 5% 6% 7% 57%	1416
	21st to end	Night	4 5% 3 4 4 2 5% 4 6% 7% 57%	1552
June	1st to 10th	Night	4 4 1 4 4 2 4 4 5% 7% 61%	1440
	11th to 20th	Night	2 2 1 2 2 2 3 3 4 6% 74%	1440
	21st to end	Night	2 2 1 2 3 2 3 3 2 4 77%	1440
July	1st to 10th	Night	1 1 1 1 1 2 2 3 4 83%	1440
	11th to 20th	Night	1 2 1 1 1 2 2 4 86%	1440
	21st to end	Night	2 2 1 2 3 1 2 2 3 5% 77%	1648
August	1st to 10th	Night	3 2 1 2 2 2 3 3 3 4 75%	1611
	11th to 20th	Night	2 2 1 2 1 2 2 3 4 80%	1754
	21st to end	Night	1 1 1 1 1 1 2 2 2 4 84%	1980
September	1st to 10th	Night	3 1 1 1 1 2 1 2 4 84%	1800
	11th to 20th	Night	3 2 1 1 2 2 2 3 3 5% 77%	1804
	21st to end	Night	2 3 1 1 2 2 2 3 3 5% 77%	1991
October	1st to 10th	Night	3 3 2 4 3 2 3 2 3 4 72%	2160
	11th to 20th	Night	2 2 1 3 3 2 3 3 5% 6% 71%	2160
	21st to end	Night	3 4 2 3 2 1 3 3 5% 6% 69%	2402
November	1st to 10th	Night	3 6% 3 5% 4 2 4 5% 6% 9% 54%	2405
	11th to 20th	Night	2 5% 2 4 4 3 5% 5% 6% 9% 55%	2470
	21st to end	Night	2 4 2 5% 5% 3 5% 5% 5% 9% 54%	2593
December	1st to 10th	Night	4 7% 4 6% 5% 3 4 5% 6% 9% 48%	2660
	11th to 20th	Night	7% 12% 4 7% 6% 4 6% 6% 7% 10% 31%	3276
	21st to end	Night	5% 9% 4 8% 7% 4 7% 6% 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	42% 6% 11% 24% 15% 2	1871
	11th to 20th	Day	35% 5% 14% 23% 19% 4	1980
	21st to end	Day	33% 6% 21% 25% 12% 3	2294
April	1st to 10th	Day	34% 5% 16% 24% 17% 3	2160
	11th to 20th	Day	27% 5% 11% 30% 22% 5%	2132
	21st to end	Day	27% 3 13% 30% 23% 3	2160
May	1st to 10th	Day	24% 4 8% 31% 29% 5%	2273
	11th to 20th	Day	25% 2 11% 28% 27% 6%	2480
	21st to end	Day	34% 3 11% 27% 21% 3	2724
June	1st to 10th	Day	23% 3 8% 26% 31% 9%	2520
	11th to 20th	Day	13% 1 6% 22% 37% 20%	2520
	21st to end	Day	11% 2 3 16% 37% 32%	2520
July	1st to 10th	Day	8% 1 2 10% 27% 52%	2451
	11th to 20th	Day	7% 2 2 11% 28% 51%	2445
	21st to end	Day	12% 2 2 8% 25% 50%	2517
August	1st to 10th	Day	13% 2 2 13% 27% 44%	2312
	11th to 20th	Day	8% 2 1 10% 31% 48%	2146
	21st to end	Day	5% 1 1 10% 36% 46%	2376
September	1st to 10th	Day	8% 1 1 10% 32% 48%	2160
	11th to 20th	Day	13% 2 4 20% 37% 24%	2156
	21st to end	Day	14% 2 5% 22% 35% 22%	1966
October	1st to 10th	Day	19% 2 6% 21% 33% 19%	1800
	11th to 20th	Day	16% 3 8% 24% 32% 17%	1800
	21st to end	Day	18% 2 10% 29% 25% 16%	1952
November	1st to 10th	Day	29% 3 10% 24% 24% 10%	1674
	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	37% 4 16% 27% 13% 3	2245
	11th to 20th	Night	31% 6% 14% 27% 19% 4	1980
	21st to end	Night	30% 7% 20% 27% 14% 2	2062
April	1st to 10th	Night	30% 6% 19% 27% 16% 3	1800
	11th to 20th	Night	24% 5% 18% 30% 18% 5%	1779
	21st to end	Night	21% 5% 18% 35% 17% 3	1800
May	1st to 10th	Night	24% 3 11% 30% 28% 4	1684
	11th to 20th	Night	24% 3 15% 32% 19% 7%	1416
	21st to end	Night	26% 2 19% 32% 19% 3	1552
June	1st to 10th	Night	22% 4 13% 26% 27% 8%	1440
	11th to 20th	Night	12% 2 10% 26% 33% 18%	1440
	21st to end	Night	14% 2 3 18% 33% 30%	1440
July	1st to 10th	Night	7% 2 3 9% 31% 49%	1400
	11th to 20th	Night	7% 1 2 12% 28% 50%	1392
	21st to end	Night	12% 1 2 9% 26% 49%	1556
August	1st to 10th	Night	15% 2 3 13% 29% 39%	1602
	11th to 20th	Night	9% 2 2 12% 32% 43%	1754
	21st to end	Night	7% 1 1 12% 34% 45%	1980
September	1st to 10th	Night	9% 1 2 9% 32% 47%	1800
	11th to 20th	Night	12% 1 5% 21% 36% 25%	1804
	21st to end	Night	13% 2 4 22% 36% 23%	1991
October	1st to 10th	Night	18% 2 7% 22% 31% 19%	2160
	11th to 20th	Night	14% 2 8% 25% 33% 18%	2160
	21st to end	Night	17% 2 11% 29% 29% 13%	2402
November	1st to 10th	Night	25% 5% 11% 27% 21% 10%	2405
	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	20% 28% 23% 18% 9% 2	449
	11th to 20th	Day	20% 22% 24% 20% 12% 2	420
	21st to end	Day	16% 25% 29% 20% 8% 2	462
April	1st to 10th	Day	15% 23% 28% 22% 9% 2	420
	11th to 20th	Day	18% 25% 25% 19% 10% 3	412
	21st to end	Day	13% 22% 24% 20% 15% 5%	420
May	1st to 10th	Day	16% 20% 27% 21% 11% 5%	423
	11th to 20th	Day	12% 18% 31% 19% 12% 8%	448
	21st to end	Day	12% 16% 24% 22% 16% 9%	485
June	1st to 10th	Day	10% 15% 27% 24% 14% 10%	450
	11th to 20th	Day	10% 17% 30% 23% 14% 6%	450
	21st to end	Day	3 11% 31% 27% 17% 11%	450
July	1st to 10th	Day	2 9% 35% 25% 19% 10%	446
	11th to 20th	Day	1 9% 31% 29% 19% 12%	444
	21st to end	Day	3 6% 29% 31% 19% 13%	487
August	1st to 10th	Day	4 8% 30% 25% 20% 13%	447
	11th to 20th	Day	2 8% 25% 28% 22% 14%	437
	21st to end	Day	2 11% 29% 29% 22% 7%	495
September	1st to 10th	Day	3 9% 28% 28% 24% 8%	450
	11th to 20th	Day	4 14% 25% 29% 22% 6%	450
	21st to end	Day	8% 11% 29% 29% 20% 4	450
October	1st to 10th	Day	7% 13% 26% 25% 22% 7%	450
	11th to 20th	Day	8% 13% 23% 26% 25% 5%	450
	21st to end	Day	10% 17% 20% 27% 22% 4	467
November	1st to 10th	Day	14% 11% 16% 26% 25% 8%	270
	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% 15% 3	750
	11th to 20th	Night	10% 13% 18% 33% 23% 2	720
	21st to end	Night	7% 19% 22% 35% 17% 1	792
April	1st to 10th	Night	6% 13% 22% 38% 19% 1	720
	11th to 20th	Night	10% 13% 18% 37% 19% 3	711
	21st to end	Night	8% 10% 19% 38% 22% 4	720
May	1st to 10th	Night	8% 12% 20% 37% 21% 2	604
	11th to 20th	Night	5% 6% 14% 48% 25% 2	354
	21st to end	Night	5% 7% 11% 40% 35% 2	389
June	1st to 10th	Night	6% 2 16% 43% 33% 2	360
	11th to 20th	Night	3 4 16% 45% 31% 1	360
	21st to end	Night	4 1 10% 48% 34% 3	360
July	1st to 10th	Night	2 8% 54% 34% 2	350
	11th to 20th	Night	1 8% 55% 35% 1	348
	21st to end	Night	3 3 44% 45% 5%	466
August	1st to 10th	Night	4 1 3 46% 42% 4	537
	11th to 20th	Night	2 1 8% 46% 37% 5%	686
	21st to end	Night	2 1 10% 50% 34% 3	792
September	1st to 10th	Night	2 1 13% 43% 38% 4	720
	11th to 20th	Night	3 3 10% 40% 36% 7%	720
	21st to end	Night	3 4 12% 41% 31% 9%	720
October	1st to 10th	Night	6% 5% 15% 40% 29% 5%	720
	11th to 20th	Night	5% 7% 10% 40% 33% 5%	720
	21st to end	Night	5% 7% 15% 37% 26% 9%	792
November	1st to 10th	Night	7% 8% 17% 35% 28% 5%	740
	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 2024-03-05 04:24:00. Version ac07

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