Cupertino
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cupertino clear sky chart from 2005-11-06 to 2024-03-06.

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	6% 4 2 4 4 2 3 4 4 7% 59%	78703
	Night	7% 5% 2 4 4 3 4 4 5% 7% 55%	80851
Transparency	Day	27% 3 8% 21% 25% 17%	78423
	Night	31% 3 9% 20% 22% 15%	80653
Seeing	Day	7% 10% 24% 31% 20% 7%	14477
	Night	8% 5% 14% 36% 31% 7%	23976

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	12% 7% 3 7% 6% 3 5% 5% 6% 8% 38%	2728
	11th to 20th	Day	11% 9% 4 9% 6% 4 5% 5% 6% 6% 34%	2860
	21st to end	Day	15% 9% 4 7% 6% 4 5% 5% 6% 8% 30%	3146
February	1st to 10th	Day	11% 7% 3 5% 5% 3 5% 4 6% 6% 46%	1710
	11th to 20th	Day	11% 5% 3 5% 6% 3 5% 4 5% 7% 45%	1764
	21st to end	Day	10% 6% 3 5% 6% 3 5% 3 5% 8% 47%	1570
March	1st to 10th	Day	10% 7% 3 6% 7% 3 5% 6% 7% 8% 37%	1994
	11th to 20th	Day	10% 6% 3 6% 6% 4 6% 6% 6% 9% 38%	1980
	21st to end	Day	9% 6% 3 5% 6% 3 4 5% 6% 8% 46%	2214
April	1st to 10th	Day	10% 6% 3 6% 5% 3 4 4 6% 9% 46%	2160
	11th to 20th	Day	8% 5% 2 4 4 2 3 3 4 7% 57%	2129
	21st to end	Day	6% 4 2 3 3 2 4 3 4 8% 62%	2160
May	1st to 10th	Day	4 4 2 4 3 1 3 3 4 8% 64%	2204
	11th to 20th	Day	5% 3 1 3 3 2 2 3 4 7% 68%	2342
	21st to end	Day	5% 4 2 3 3 2 3 3 4 7% 63%	2723
June	1st to 10th	Day	4 4 2 4 3 2 3 3 4 7% 65%	2520
	11th to 20th	Day	2 2 1 1 1 1 1 2 3 6% 80%	2520
	21st to end	Day	2 2 1 2 2 1 2 2 2 4 81%	2520
July	1st to 10th	Day	1 2 1 1 1 1 2 2 3 5% 82%	2520
	11th to 20th	Day	1 1 1 2 2 1 2 2 3 5% 79%	2342
	21st to end	Day	1 1 1 2 1 1 2 2 4 5% 81%	2546
August	1st to 10th	Day	1 3 1 3 2 2 2 3 3 5% 76%	2328
	11th to 20th	Day	1 1 1 1 1 2 2 3 6% 80%	2170
	21st to end	Day	2 1 2 2 1 2 2 3 7% 79%	2376
September	1st to 10th	Day	1 2 1 1 2 1 1 2 2 4 84%	2160
	11th to 20th	Day	2 3 1 3 3 2 3 4 4 7% 69%	2160
	21st to end	Day	2 2 2 2 2 2 2 3 5% 78%	1915
October	1st to 10th	Day	3 3 1 3 3 2 3 3 3 6% 69%	1800
	11th to 20th	Day	4 4 1 3 2 1 3 4 5% 7% 65%	1800
	21st to end	Day	4 4 1 3 3 3 4 4 5% 6% 63%	1980
November	1st to 10th	Day	6% 4 3 4 4 3 5% 5% 5% 7% 55%	1828
	11th to 20th	Day	6% 5% 3 5% 5% 3 4 5% 5% 6% 52%	1710
	21st to end	Day	8% 4 2 4 5% 2 4 4 5% 8% 54%	1582
December	1st to 10th	Day	8% 5% 2 6% 6% 4 5% 6% 5% 9% 44%	1520
	11th to 20th	Day	13% 8% 3 7% 8% 4 5% 5% 7% 7% 32%	1928
	21st to end	Day	9% 8% 5% 8% 7% 4 5% 6% 6% 7% 35%	2794
January	1st to 10th	Night	11% 9% 3 5% 5% 3 4 4 6% 7% 43%	4220
	11th to 20th	Night	11% 10% 5% 8% 7% 5% 5% 5% 6% 7% 32%	4110
	21st to end	Night	14% 10% 4 7% 7% 3 5% 5% 5% 7% 32%	4521
February	1st to 10th	Night	9% 6% 4 5% 4 3 5% 4 4 6% 50%	2470
	11th to 20th	Night	9% 6% 2 4 5% 3 5% 5% 6% 8% 46%	2301
	21st to end	Night	9% 6% 2 5% 5% 3 5% 4 4 7% 52%	1884
March	1st to 10th	Night	12% 7% 4 6% 5% 4 4 6% 6% 7% 41%	2080
	11th to 20th	Night	10% 6% 3 6% 5% 3 5% 4 4 8% 44%	1980
	21st to end	Night	9% 6% 2 5% 5% 3 4 4 5% 6% 51%	2142
April	1st to 10th	Night	9% 5% 3 6% 5% 3 4 4 6% 8% 47%	1800
	11th to 20th	Night	8% 5% 2 4 4 3 3 4 4 7% 56%	1779
	21st to end	Night	4 2 1 3 2 2 4 6% 4 9% 62%	1800
May	1st to 10th	Night	3 5% 2 4 4 2 3 4 4 8% 62%	1753
	11th to 20th	Night	3 4 2 3 3 2 3 4 6% 9% 61%	1550
	21st to end	Night	3 4 2 4 3 2 3 4 4 9% 61%	1552
June	1st to 10th	Night	3 3 1 3 3 2 3 2 5% 8% 65%	1440
	11th to 20th	Night	1 2 1 2 1 1 2 2 3 7% 78%	1440
	21st to end	Night	1 3 1 2 2 1 3 2 2 4 79%	1440
July	1st to 10th	Night	3 1 1 1 1 2 2 2 5% 83%	1440
	11th to 20th	Night	1 1 1 1 3 1 2 2 3 7% 77%	1612
	21st to end	Night	2 2 3 3 2 3 3 5% 8% 69%	1773
August	1st to 10th	Night	2 4 1 3 3 2 4 2 4 8% 67%	1611
	11th to 20th	Night	2 1 3 2 2 2 2 5% 9% 72%	1728
	21st to end	Night	1 2 1 2 2 1 2 3 4 8% 73%	1980
September	1st to 10th	Night	1 2 1 2 2 1 1 2 3 8% 77%	1800
	11th to 20th	Night	3 4 1 4 3 1 3 2 3 6% 69%	1800
	21st to end	Night	2 3 1 2 2 1 1 2 3 6% 77%	2040
October	1st to 10th	Night	4 3 1 3 2 2 2 3 3 6% 70%	2160
	11th to 20th	Night	4 4 2 5% 4 3 2 3 4 6% 64%	2160
	21st to end	Night	3 3 2 4 3 2 3 4 4 7% 64%	2376
November	1st to 10th	Night	5% 5% 2 5% 4 2 3 3 6% 8% 57%	2248
	11th to 20th	Night	6% 6% 2 5% 4 2 3 4 6% 8% 53%	2470
	21st to end	Night	6% 4 2 3 4 3 4 3 5% 8% 58%	2593
December	1st to 10th	Night	8% 6% 3 5% 5% 3 4 5% 5% 7% 49%	2660
	11th to 20th	Night	14% 8% 3 7% 7% 4 5% 6% 6% 7% 33%	3276
	21st to end	Night	10% 7% 3 6% 6% 4 6% 6% 7% 7% 36%	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	45% 4 11% 28% 8% 3	2728
	11th to 20th	Day	52% 3 12% 15% 10% 8%	2860
	21st to end	Day	52% 4 9% 16% 14% 5%	3146
February	1st to 10th	Day	39% 4 6% 23% 18% 11%	1710
	11th to 20th	Day	39% 3 8% 20% 22% 7%	1773
	21st to end	Day	39% 4 7% 24% 19% 6%	1570
March	1st to 10th	Day	46% 3 10% 22% 15% 4	1994
	11th to 20th	Day	45% 3 9% 21% 18% 4	1980
	21st to end	Day	39% 5% 12% 22% 18% 4	2214
April	1st to 10th	Day	38% 3 13% 21% 21% 5%	2160
	11th to 20th	Day	30% 3 13% 27% 23% 6%	2132
	21st to end	Day	24% 2 12% 31% 24% 6%	2160
May	1st to 10th	Day	22% 3 9% 28% 31% 6%	2204
	11th to 20th	Day	19% 1 9% 33% 29% 9%	2345
	21st to end	Day	24% 2 12% 33% 24% 5%	2724
June	1st to 10th	Day	21% 2 11% 28% 27% 10%	2520
	11th to 20th	Day	10% 2 8% 24% 38% 19%	2520
	21st to end	Day	10% 2 3 15% 34% 36%	2520
July	1st to 10th	Day	7% 2 1 11% 28% 51%	2451
	11th to 20th	Day	10% 2 4 12% 28% 44%	2272
	21st to end	Day	7% 2 1 11% 36% 42%	2405
August	1st to 10th	Day	13% 2 2 13% 35% 35%	2312
	11th to 20th	Day	8% 2 1 15% 38% 37%	2170
	21st to end	Day	9% 1 1 16% 38% 34%	2376
September	1st to 10th	Day	9% 1 1 11% 33% 45%	2160
	11th to 20th	Day	16% 3 3 24% 34% 19%	2160
	21st to end	Day	12% 2 4 24% 36% 23%	1915
October	1st to 10th	Day	18% 2 6% 19% 29% 26%	1800
	11th to 20th	Day	19% 3 9% 20% 23% 25%	1800
	21st to end	Day	23% 2 8% 22% 25% 20%	1980
November	1st to 10th	Day	30% 4 7% 22% 24% 13%	1828
	11th to 20th	Day	31% 5% 9% 23% 23% 9%	1710
	21st to end	Day	31% 3 8% 18% 30% 10%	1582
December	1st to 10th	Day	38% 4 11% 20% 17% 11%	1520
	11th to 20th	Day	53% 3 10% 15% 14% 5%	1928
	21st to end	Day	49% 4 12% 20% 12% 3	2794
January	1st to 10th	Night	42% 3 17% 24% 11% 3	4220
	11th to 20th	Night	53% 4 14% 12% 9% 8%	4110
	21st to end	Night	54% 4 10% 13% 13% 7%	4521
February	1st to 10th	Night	38% 2 7% 21% 20% 12%	2470
	11th to 20th	Night	36% 3 9% 20% 24% 7%	2303
	21st to end	Night	34% 3 10% 30% 17% 5%	1884
March	1st to 10th	Night	45% 3 12% 20% 15% 5%	2080
	11th to 20th	Night	40% 4 8% 23% 20% 6%	1980
	21st to end	Night	36% 4 13% 22% 18% 6%	2142
April	1st to 10th	Night	35% 4 15% 22% 18% 5%	1800
	11th to 20th	Night	31% 3 11% 28% 19% 8%	1779
	21st to end	Night	21% 4 12% 33% 24% 7%	1800
May	1st to 10th	Night	23% 2 9% 27% 26% 12%	1753
	11th to 20th	Night	20% 3 9% 33% 27% 8%	1551
	21st to end	Night	21% 2 16% 30% 26% 5%	1552
June	1st to 10th	Night	19% 3 11% 28% 28% 11%	1440
	11th to 20th	Night	10% 2 7% 21% 35% 25%	1440
	21st to end	Night	13% 1 2 17% 34% 33%	1440
July	1st to 10th	Night	8% 1 1 10% 30% 51%	1400
	11th to 20th	Night	8% 3 3 11% 32% 43%	1559
	21st to end	Night	11% 2 2 13% 34% 37%	1674
August	1st to 10th	Night	18% 2 1 13% 32% 33%	1602
	11th to 20th	Night	11% 2 1 12% 35% 39%	1728
	21st to end	Night	11% 2 2 13% 39% 34%	1980
September	1st to 10th	Night	9% 2 2 11% 29% 48%	1800
	11th to 20th	Night	17% 4 3 22% 34% 21%	1800
	21st to end	Night	13% 2 4 21% 34% 27%	2040
October	1st to 10th	Night	18% 2 8% 19% 26% 28%	2160
	11th to 20th	Night	23% 3 6% 21% 24% 24%	2160
	21st to end	Night	22% 2 5% 22% 27% 22%	2376
November	1st to 10th	Night	27% 3 8% 25% 21% 16%	2248
	11th to 20th	Night	30% 4 9% 25% 23% 9%	2470
	21st to end	Night	27% 3 9% 22% 24% 14%	2593
December	1st to 10th	Night	36% 5% 10% 19% 18% 13%	2660
	11th to 20th	Night	52% 4 10% 15% 16% 4	3276
	21st to end	Night	48% 4 15% 17% 12% 4	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% 14% 29% 20% 14% 1	291
	11th to 20th	Day	15% 11% 28% 29% 13% 4	370
	21st to end	Day	14% 13% 28% 27% 15% 4	407
February	1st to 10th	Day	15% 17% 29% 23% 12% 5%	370
	11th to 20th	Day	16% 17% 31% 23% 10% 2	413
	21st to end	Day	12% 16% 33% 25% 12% 2	382
March	1st to 10th	Day	18% 16% 33% 24% 9% 1	441
	11th to 20th	Day	16% 17% 27% 23% 13% 4	420
	21st to end	Day	12% 19% 33% 26% 8% 2	462
April	1st to 10th	Day	12% 16% 33% 25% 11% 4	420
	11th to 20th	Day	13% 18% 30% 24% 12% 4	412
	21st to end	Day	6% 21% 27% 28% 12% 6%	420
May	1st to 10th	Day	8% 13% 31% 29% 14% 4	423
	11th to 20th	Day	8% 14% 31% 28% 13% 7%	448
	21st to end	Day	6% 12% 26% 30% 20% 7%	485
June	1st to 10th	Day	4 10% 26% 30% 21% 8%	450
	11th to 20th	Day	3 10% 29% 34% 19% 5%	450
	21st to end	Day	1 4 23% 37% 26% 9%	450
July	1st to 10th	Day	1 2 16% 40% 31% 9%	446
	11th to 20th	Day	1 1 13% 40% 33% 12%	444
	21st to end	Day	1 12% 38% 32% 16%	487
August	1st to 10th	Day	2 2 19% 37% 29% 11%	447
	11th to 20th	Day	1 17% 39% 25% 17%	437
	21st to end	Day	1 14% 41% 31% 13%	495
September	1st to 10th	Day	2 3 14% 39% 30% 12%	450
	11th to 20th	Day	3 5% 25% 35% 23% 9%	450
	21st to end	Day	3 7% 19% 34% 28% 8%	450
October	1st to 10th	Day	3 9% 23% 31% 24% 11%	450
	11th to 20th	Day	5% 9% 20% 28% 28% 10%	450
	21st to end	Day	6% 7% 22% 32% 25% 7%	495
November	1st to 10th	Day	7% 11% 21% 26% 26% 8%	424
	11th to 20th	Day	4 10% 20% 30% 26% 11%	270
	21st to end	Day	6% 10% 23% 30% 24% 7%	210
December	1st to 10th	Day	8% 10% 23% 30% 19% 9%	180
	11th to 20th	Day	10% 14% 25% 22% 19% 9%	180
	21st to end	Day	13% 19% 26% 23% 15% 5%	198
January	1st to 10th	Night	16% 5% 16% 33% 24% 6%	760
	11th to 20th	Night	11% 7% 14% 37% 23% 8%	760
	21st to end	Night	16% 5% 14% 36% 24% 6%	836
February	1st to 10th	Night	12% 8% 15% 37% 23% 5%	760
	11th to 20th	Night	13% 8% 17% 36% 19% 6%	744
	21st to end	Night	12% 9% 19% 37% 19% 3	628
March	1st to 10th	Night	13% 10% 18% 34% 21% 4	742
	11th to 20th	Night	14% 6% 18% 34% 23% 5%	720
	21st to end	Night	10% 8% 21% 39% 20% 2	792
April	1st to 10th	Night	11% 6% 20% 38% 22% 3	720
	11th to 20th	Night	10% 11% 19% 33% 22% 4	711
	21st to end	Night	5% 8% 25% 32% 24% 7%	720
May	1st to 10th	Night	5% 3 25% 35% 27% 5%	673
	11th to 20th	Night	4 3 14% 38% 36% 6%	488
	21st to end	Night	3 5% 15% 40% 33% 4	389
June	1st to 10th	Night	4 3 17% 43% 29% 4	360
	11th to 20th	Night	1 4 13% 50% 31% 1	360
	21st to end	Night	1 9% 44% 43% 3	360
July	1st to 10th	Night	1 5% 38% 53% 3	350
	11th to 20th	Night	1 2 32% 54% 12%	515
	21st to end	Night	1 1 29% 59% 10%	591
August	1st to 10th	Night	3 3 35% 51% 7%	537
	11th to 20th	Night	1 9% 38% 47% 6%	660
	21st to end	Night	1 1 7% 34% 48% 10%	792
September	1st to 10th	Night	1 7% 36% 45% 11%	720
	11th to 20th	Night	4 1 9% 38% 40% 8%	720
	21st to end	Night	3 2 10% 35% 39% 10%	720
October	1st to 10th	Night	4 2 10% 38% 35% 11%	720
	11th to 20th	Night	5% 5% 9% 30% 40% 10%	720
	21st to end	Night	5% 5% 10% 34% 32% 15%	792
November	1st to 10th	Night	6% 4 13% 34% 34% 10%	740
	11th to 20th	Night	8% 3 13% 36% 32% 8%	760
	21st to end	Night	8% 4 14% 40% 26% 7%	760
December	1st to 10th	Night	10% 5% 16% 36% 26% 6%	760
	11th to 20th	Night	16% 5% 15% 38% 22% 4	760
	21st to end	Night	14% 9% 21% 33% 20% 3	836

Page generated at 2024-03-03 03:28:58. Version ac07

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