Mountain View
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain View clear sky chart from 2005-11-06 to 2026-07-31.

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	4 4 2 4 3 3 3 3 4 7% 63%	146736
	Night	5% 5% 2 6% 4 4 3 3 5% 7% 58%	143793
Transparency	Day	23% 3 7% 22% 28% 18%	141669
	Night	28% 3 9% 21% 24% 16%	139849
Seeing	Day	7% 10% 25% 32% 20% 7%	17704
	Night	8% 5% 16% 35% 29% 6%	27031

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	10% 7% 2 6% 4 4 5% 4 6% 8% 44%	5081
	11th to 20th	Day	6% 5% 2 6% 4 4 3 3 4 6% 58%	5340
	21st to end	Day	9% 8% 3 7% 5% 5% 5% 4 6% 8% 41%	5843
February	1st to 10th	Day	10% 7% 3 6% 5% 4 4 4 5% 6% 46%	1890
	11th to 20th	Day	10% 7% 3 7% 5% 5% 5% 3 6% 7% 42%	1951
	21st to end	Day	9% 7% 2 7% 5% 5% 4 4 5% 7% 46%	1730
March	1st to 10th	Day	8% 7% 3 8% 6% 6% 7% 5% 7% 8% 35%	4972
	11th to 20th	Day	6% 5% 2 6% 4 4 5% 4 7% 10% 48%	5110
	21st to end	Day	7% 6% 3 7% 5% 6% 6% 5% 7% 10% 39%	5692
April	1st to 10th	Day	9% 6% 2 6% 4 3 3 4 6% 8% 48%	2520
	11th to 20th	Day	7% 5% 2 5% 3 3 3 3 6% 7% 56%	2489
	21st to end	Day	5% 5% 2 4 3 3 3 3 5% 8% 59%	2520
May	1st to 10th	Day	3 3 1 4 3 2 2 2 3 6% 70%	7224
	11th to 20th	Day	2 1 2 2 1 1 1 2 5% 82%	7724
	21st to end	Day	2 3 1 3 2 2 2 2 3 6% 74%	8992
June	1st to 10th	Day	4 4 2 3 3 2 3 3 4 7% 66%	2940
	11th to 20th	Day	2 1 1 1 1 1 2 2 3 6% 80%	2940
	21st to end	Day	2 1 1 2 2 2 2 1 3 4 79%	2940
July	1st to 10th	Day	1 2 2 2 2 1 2 3 85%	6459
	11th to 20th	Day	1 1 1 3 2 2 2 2 3 6% 78%	6011
	21st to end	Day	1 1 3 2 2 2 2 4 8% 77%	6533
August	1st to 10th	Day	1 1 1 2 2 2 2 2 4 6% 77%	5777
	11th to 20th	Day	2 1 2 2 2 2 1 2 5% 80%	5491
	21st to end	Day	1 1 3 3 2 3 2 4 7% 74%	5984
September	1st to 10th	Day	1 2 1 3 2 2 2 2 3 5% 78%	2400
	11th to 20th	Day	2 4 2 4 3 3 3 2 4 7% 67%	2400
	21st to end	Day	2 2 1 3 2 2 1 2 3 6% 76%	2124
October	1st to 10th	Day	2 2 1 3 2 2 3 2 4 6% 73%	3280
	11th to 20th	Day	3 3 1 4 3 3 3 2 4 6% 67%	3261
	21st to end	Day	2 3 1 3 3 3 3 3 5% 8% 64%	3558
November	1st to 10th	Day	5% 5% 2 4 4 3 5% 3 4 8% 56%	2027
	11th to 20th	Day	6% 7% 2 5% 6% 5% 4 5% 5% 7% 49%	1890
	21st to end	Day	8% 5% 2 5% 3 4 4 3 5% 8% 53%	1743
December	1st to 10th	Day	5% 3 1 5% 5% 4 4 3 6% 10% 54%	2700
	11th to 20th	Day	9% 8% 3 9% 6% 6% 5% 5% 8% 9% 32%	3108
	21st to end	Day	11% 10% 4 9% 5% 6% 7% 5% 7% 7% 30%	4092
January	1st to 10th	Night	9% 8% 3 6% 5% 5% 4 3 6% 8% 43%	7816
	11th to 20th	Night	6% 6% 3 6% 4 4 3 3 4 6% 54%	7650
	21st to end	Night	8% 8% 3 8% 6% 6% 4 4 4 7% 41%	8371
February	1st to 10th	Night	9% 8% 2 5% 5% 4 4 4 4 5% 49%	2730
	11th to 20th	Night	9% 7% 2 6% 6% 5% 5% 4 6% 8% 43%	2554
	21st to end	Night	7% 6% 2 6% 5% 4 4 3 5% 7% 50%	2076
March	1st to 10th	Night	9% 8% 3 8% 6% 5% 5% 4 6% 7% 39%	5138
	11th to 20th	Night	9% 6% 3 7% 4 4 3 3 4 7% 50%	5110
	21st to end	Night	8% 10% 3 7% 4 5% 5% 4 5% 6% 43%	5523
April	1st to 10th	Night	7% 6% 2 6% 5% 4 4 4 6% 7% 50%	2100
	11th to 20th	Night	5% 6% 2 6% 4 3 5% 3 5% 7% 54%	2079
	21st to end	Night	4 3 1 4 3 3 4 3 5% 8% 61%	2100
May	1st to 10th	Night	2 4 2 4 3 3 3 2 4 5% 67%	5741
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 5% 76%	5140
	21st to end	Night	1 3 1 4 3 2 2 2 4 6% 73%	5181
June	1st to 10th	Night	3 3 1 4 4 2 2 3 4 8% 64%	1680
	11th to 20th	Night	1 1 3 2 2 1 2 4 5% 80%	1680
	21st to end	Night	1 2 1 3 2 2 2 1 2 4 78%	1680
July	1st to 10th	Night	1 3 2 1 1 1 2 3 86%	3720
	11th to 20th	Night	1 1 1 7% 3 2 2 2 3 7% 72%	4183
	21st to end	Night	1 1 4 3 2 3 3 5% 10% 68%	4562
August	1st to 10th	Night	1 2 1 5% 3 3 2 2 3 6% 72%	4035
	11th to 20th	Night	2 1 4 3 2 2 2 3 4 76%	4369
	21st to end	Night	1 1 6% 4 3 3 2 4 6% 69%	5015
September	1st to 10th	Night	1 2 1 4 2 2 2 2 4 7% 73%	2000
	11th to 20th	Night	3 4 2 4 3 3 4 2 3 6% 66%	2000
	21st to end	Night	2 2 1 3 2 2 2 2 3 6% 74%	2273
October	1st to 10th	Night	2 2 1 2 2 2 3 2 4 6% 74%	3920
	11th to 20th	Night	4 4 2 5% 3 2 3 2 4 6% 67%	3897
	21st to end	Night	3 4 2 4 3 3 3 3 5% 6% 65%	4253
November	1st to 10th	Night	5% 5% 3 5% 3 3 3 3 5% 8% 56%	2491
	11th to 20th	Night	8% 6% 3 5% 4 4 3 3 5% 8% 51%	2730
	21st to end	Night	6% 5% 2 5% 4 4 4 3 5% 7% 56%	2874
December	1st to 10th	Night	5% 4 2 4 3 4 5% 3 5% 7% 58%	4700
	11th to 20th	Night	11% 9% 3 8% 6% 6% 6% 4 7% 10% 30%	5316
	21st to end	Night	11% 9% 3 9% 6% 6% 6% 5% 6% 7% 31%	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	38% 4 17% 24% 14% 3	5082
	11th to 20th	Day	30% 2 6% 13% 26% 21%	5340
	21st to end	Day	40% 3 7% 18% 20% 12%	5843
February	1st to 10th	Day	39% 4 7% 21% 18% 11%	1890
	11th to 20th	Day	41% 3 11% 19% 20% 6%	1960
	21st to end	Day	39% 5% 8% 23% 18% 6%	1730
March	1st to 10th	Day	45% 4 9% 19% 21% 2	4972
	11th to 20th	Day	31% 3 14% 30% 14% 8%	5110
	21st to end	Day	41% 5% 15% 27% 11% 2	5692
April	1st to 10th	Day	35% 4 13% 23% 21% 4	2520
	11th to 20th	Day	29% 3 14% 26% 21% 6%	2492
	21st to end	Day	25% 3 12% 31% 23% 5%	2520
May	1st to 10th	Day	18% 2 5% 29% 34% 12%	7224
	11th to 20th	Day	8% 1 5% 32% 41% 13%	7727
	21st to end	Day	13% 2 7% 36% 38% 4	8994
June	1st to 10th	Day	20% 3 11% 27% 29% 10%	2940
	11th to 20th	Day	9% 2 7% 22% 40% 20%	2940
	21st to end	Day	10% 1 3 14% 36% 36%	2940
July	1st to 10th	Day	6% 1 1 10% 24% 59%	6391
	11th to 20th	Day	10% 2 2 6% 24% 57%	5941
	21st to end	Day	8% 2 3 20% 36% 32%	6348
August	1st to 10th	Day	15% 1 2 14% 35% 33%	4206
	11th to 20th	Day	6% 1 1 11% 49% 31%	3973
	21st to end	Day	10% 2 4 13% 40% 31%	4311
September	1st to 10th	Day	9% 2 3 12% 32% 42%	2400
	11th to 20th	Day	18% 3 5% 25% 32% 18%	2400
	21st to end	Day	12% 2 4 23% 36% 22%	2124
October	1st to 10th	Day	13% 2 4 13% 29% 38%	3280
	11th to 20th	Day	17% 2 8% 29% 29% 16%	3261
	21st to end	Day	18% 2 6% 27% 32% 15%	3558
November	1st to 10th	Day	27% 5% 9% 22% 24% 11%	2027
	11th to 20th	Day	35% 4 11% 22% 21% 8%	1890
	21st to end	Day	31% 4 8% 20% 29% 8%	1743
December	1st to 10th	Day	27% 3 6% 27% 30% 7%	2700
	11th to 20th	Day	46% 4 12% 19% 14% 5%	3108
	21st to end	Day	50% 6% 14% 17% 10% 2	4092
January	1st to 10th	Night	40% 4 17% 23% 14% 2	7816
	11th to 20th	Night	33% 2 8% 15% 28% 14%	7650
	21st to end	Night	42% 2 8% 17% 18% 12%	8371
February	1st to 10th	Night	38% 3 7% 20% 20% 12%	2730
	11th to 20th	Night	38% 3 11% 20% 21% 6%	2556
	21st to end	Night	34% 4 11% 29% 17% 5%	2076
March	1st to 10th	Night	45% 3 12% 20% 17% 2	5138
	11th to 20th	Night	36% 3 11% 25% 17% 7%	5110
	21st to end	Night	44% 4 14% 23% 12% 2	5523
April	1st to 10th	Night	34% 4 15% 25% 18% 4	2100
	11th to 20th	Night	31% 4 12% 28% 18% 7%	2079
	21st to end	Night	23% 3 12% 33% 23% 6%	2100
May	1st to 10th	Night	20% 2 10% 25% 32% 11%	5739
	11th to 20th	Night	10% 2 7% 29% 36% 16%	5141
	21st to end	Night	13% 3 10% 33% 38% 3	5182
June	1st to 10th	Night	18% 3 11% 26% 31% 10%	1680
	11th to 20th	Night	9% 2 7% 19% 37% 25%	1680
	21st to end	Night	12% 2 2 14% 35% 36%	1680
July	1st to 10th	Night	6% 1 1 9% 23% 60%	3680
	11th to 20th	Night	13% 4 2 5% 23% 53%	4130
	21st to end	Night	12% 4 4 17% 34% 29%	4435
August	1st to 10th	Night	21% 4 2 11% 34% 29%	2929
	11th to 20th	Night	8% 2 2 9% 47% 32%	3156
	21st to end	Night	13% 2 4 12% 40% 28%	3608
September	1st to 10th	Night	12% 2 4 12% 28% 43%	2000
	11th to 20th	Night	19% 3 4 25% 31% 18%	2000
	21st to end	Night	14% 2 5% 21% 33% 25%	2273
October	1st to 10th	Night	13% 2 5% 13% 27% 40%	3920
	11th to 20th	Night	20% 2 7% 28% 26% 17%	3897
	21st to end	Night	21% 2 7% 23% 30% 17%	4253
November	1st to 10th	Night	27% 4 10% 26% 20% 13%	2491
	11th to 20th	Night	32% 4 10% 25% 20% 8%	2730
	21st to end	Night	28% 3 10% 23% 22% 12%	2874
December	1st to 10th	Night	26% 4 8% 27% 26% 9%	4700
	11th to 20th	Night	45% 4 11% 19% 16% 4	5316
	21st to end	Night	51% 4 17% 16% 9% 3	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	21% 15% 28% 21% 14% 2	345
	11th to 20th	Day	12% 10% 27% 30% 16% 5%	430
	21st to end	Day	12% 10% 29% 27% 17% 4	473
February	1st to 10th	Day	13% 19% 25% 25% 13% 5%	430
	11th to 20th	Day	16% 19% 29% 24% 10% 1	482
	21st to end	Day	11% 16% 30% 31% 11% 1	446
March	1st to 10th	Day	15% 18% 32% 26% 9% 1	540
	11th to 20th	Day	15% 18% 26% 22% 16% 3	540
	21st to end	Day	11% 18% 34% 24% 10% 2	594
April	1st to 10th	Day	11% 16% 33% 25% 13% 4	540
	11th to 20th	Day	11% 17% 29% 25% 13% 4	532
	21st to end	Day	5% 19% 29% 28% 12% 6%	540
May	1st to 10th	Day	6% 12% 34% 29% 15% 5%	543
	11th to 20th	Day	7% 14% 32% 29% 14% 5%	568
	21st to end	Day	5% 11% 29% 31% 17% 7%	617
June	1st to 10th	Day	4 9% 29% 32% 19% 7%	570
	11th to 20th	Day	2 9% 30% 35% 19% 5%	570
	21st to end	Day	1 5% 24% 37% 24% 9%	570
July	1st to 10th	Day	1 3 17% 42% 29% 7%	566
	11th to 20th	Day	1 1 15% 42% 31% 10%	564
	21st to end	Day	1 16% 44% 29% 10%	616
August	1st to 10th	Day	2 2 22% 39% 25% 10%	527
	11th to 20th	Day	1 2 21% 40% 25% 11%	517
	21st to end	Day	2 16% 41% 28% 12%	583
September	1st to 10th	Day	1 3 16% 40% 30% 10%	530
	11th to 20th	Day	3 7% 23% 36% 22% 9%	530
	21st to end	Day	3 8% 19% 34% 27% 9%	530
October	1st to 10th	Day	2 8% 21% 33% 24% 12%	530
	11th to 20th	Day	5% 9% 20% 30% 25% 10%	530
	21st to end	Day	6% 6% 25% 31% 24% 7%	583
November	1st to 10th	Day	7% 11% 23% 25% 25% 10%	503
	11th to 20th	Day	6% 12% 19% 32% 22% 9%	330
	21st to end	Day	8% 7% 25% 30% 23% 7%	253
December	1st to 10th	Day	5% 11% 23% 27% 25% 9%	220
	11th to 20th	Day	10% 16% 21% 21% 21% 11%	220
	21st to end	Day	16% 21% 24% 23% 14% 3	242
January	1st to 10th	Night	17% 7% 19% 32% 21% 5%	840
	11th to 20th	Night	12% 7% 15% 35% 24% 7%	840
	21st to end	Night	15% 6% 14% 37% 23% 5%	924
February	1st to 10th	Night	13% 9% 18% 36% 20% 5%	840
	11th to 20th	Night	13% 9% 21% 33% 19% 4	824
	21st to end	Night	11% 9% 23% 37% 17% 2	692
March	1st to 10th	Night	12% 12% 22% 32% 19% 3	840
	11th to 20th	Night	13% 9% 18% 33% 23% 3	840
	21st to end	Night	10% 9% 24% 37% 18% 2	924
April	1st to 10th	Night	10% 8% 23% 36% 21% 2	840
	11th to 20th	Night	9% 12% 22% 35% 19% 4	831
	21st to end	Night	6% 7% 27% 33% 22% 6%	840
May	1st to 10th	Night	5% 4 25% 37% 25% 4	777
	11th to 20th	Night	3 4 20% 36% 31% 5%	570
	21st to end	Night	4 4 24% 36% 29% 4	455
June	1st to 10th	Night	3 3 20% 43% 29% 3	420
	11th to 20th	Night	1 3 17% 47% 31% 1	420
	21st to end	Night	1 1 15% 38% 42% 3	420
July	1st to 10th	Night	1 11% 36% 49% 2	410
	11th to 20th	Night	1 3 36% 48% 11%	606
	21st to end	Night	1 5% 33% 53% 9%	687
August	1st to 10th	Night	4 1 8% 33% 48% 8%	597
	11th to 20th	Night	1 1 11% 39% 42% 6%	730
	21st to end	Night	1 9% 34% 46% 9%	880
September	1st to 10th	Night	1 2 8% 37% 42% 10%	800
	11th to 20th	Night	4 3 13% 37% 36% 8%	800
	21st to end	Night	3 2 11% 34% 40% 10%	800
October	1st to 10th	Night	4 2 12% 37% 35% 9%	800
	11th to 20th	Night	6% 4 12% 31% 38% 9%	800
	21st to end	Night	5% 5% 11% 35% 31% 13%	880
November	1st to 10th	Night	7% 4 17% 30% 34% 9%	820
	11th to 20th	Night	10% 4 15% 35% 29% 7%	840
	21st to end	Night	9% 4 15% 40% 26% 6%	840
December	1st to 10th	Night	10% 5% 19% 35% 26% 6%	840
	11th to 20th	Night	17% 5% 19% 33% 22% 3	840
	21st to end	Night	16% 12% 23% 30% 17% 2	924

Page generated at 2026-07-28 00:25:19. Version ac07

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