Mountain View Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain View Observatory 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	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	138218
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	134804
Transparency	Day	38% 7% 12% 23% 17% 4	141819
	Night	38% 6% 11% 23% 18% 4	139275
Seeing	Day	18% 12% 26% 25% 15% 5%	17224
	Night	17% 9% 29% 31% 13% 1	26801

By Month

Percentage Cloud Cover SE (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	14% 5% 2 5% 4 4 5% 4 5% 6% 46%	5021
	11th to 20th	Day	13% 8% 2 6% 3 3 3 3 3 4 51%	5280
	21st to end	Day	14% 5% 2 6% 5% 4 5% 4 5% 6% 45%	5780
February	1st to 10th	Day	13% 7% 2 7% 5% 4 5% 3 6% 6% 42%	1890
	11th to 20th	Day	18% 7% 2 6% 3 4 4 3 5% 6% 40%	1866
	21st to end	Day	17% 7% 2 6% 5% 3 4 4 5% 6% 40%	1745
March	1st to 10th	Day	18% 6% 2 6% 4 5% 3 4 4 7% 42%	5045
	11th to 20th	Day	11% 5% 2 6% 4 3 4 3 6% 6% 50%	5090
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 5% 7% 40%	5600
April	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 7% 45%	2520
	11th to 20th	Day	14% 6% 2 5% 5% 3 4 3 6% 7% 47%	2494
	21st to end	Day	17% 7% 2 6% 4 5% 3 4 6% 6% 41%	2520
May	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 7% 8% 35%	7611
	11th to 20th	Day	18% 5% 2 6% 4 4 5% 4 6% 9% 37%	7763
	21st to end	Day	22% 5% 2 6% 3 4 3 4 5% 8% 38%	9045
June	1st to 10th	Day	10% 5% 2 5% 5% 4 4 5% 8% 9% 43%	2940
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 5% 7% 9% 41%	2940
	21st to end	Day	9% 4 1 5% 4 5% 5% 4 8% 10% 44%	2940
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	6500
	11th to 20th	Day	8% 6% 3 7% 5% 6% 7% 7% 10% 11% 30%	6077
	21st to end	Day	9% 5% 2 6% 5% 6% 6% 5% 7% 10% 39%	6554
August	1st to 10th	Day	15% 5% 2 5% 5% 5% 6% 5% 8% 10% 34%	4234
	11th to 20th	Day	9% 4 2 6% 6% 5% 6% 5% 7% 9% 39%	4048
	21st to end	Day	10% 4 2 5% 4 4 4 3 6% 9% 49%	4312
September	1st to 10th	Day	13% 6% 2 5% 5% 5% 4 3 5% 6% 47%	2400
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 8% 48%	2382
	21st to end	Day	22% 6% 2 6% 4 3 5% 3 5% 7% 36%	2095
October	1st to 10th	Day	16% 5% 2 5% 3 3 4 3 5% 7% 48%	2000
	11th to 20th	Day	16% 4 2 5% 3 4 4 4 5% 5% 50%	2000
	21st to end	Day	14% 5% 2 5% 3 3 4 3 5% 6% 51%	2200
November	1st to 10th	Day	11% 6% 2 4 4 3 4 3 5% 6% 53%	2016
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 6% 49%	1832
	21st to end	Day	15% 5% 2 5% 3 4 4 4 4 5% 49%	1680
December	1st to 10th	Day	12% 5% 2 5% 3 3 4 4 5% 6% 51%	2680
	11th to 20th	Day	18% 8% 3 4 3 3 3 3 4 7% 44%	3070
	21st to end	Day	18% 8% 2 7% 4 4 4 4 5% 7% 38%	4048
January	1st to 10th	Night	11% 6% 2 5% 4 4 5% 5% 6% 6% 47%	7855
	11th to 20th	Night	13% 7% 2 4 3 3 3 3 4 5% 53%	7680
	21st to end	Night	13% 6% 2 5% 4 4 4 3 4 5% 50%	8401
February	1st to 10th	Night	15% 7% 2 7% 4 4 4 5% 5% 6% 41%	2730
	11th to 20th	Night	19% 7% 2 6% 3 3 3 2 5% 5% 43%	2638
	21st to end	Night	15% 8% 2 6% 4 3 4 3 4 5% 44%	2058
March	1st to 10th	Night	16% 5% 2 5% 4 4 4 3 5% 6% 47%	5028
	11th to 20th	Night	11% 6% 2 4 3 4 4 4 5% 7% 50%	5090
	21st to end	Night	11% 6% 2 7% 4 4 4 4 6% 7% 45%	5572
April	1st to 10th	Night	14% 8% 2 6% 5% 3 4 3 5% 6% 44%	2100
	11th to 20th	Night	12% 5% 2 5% 5% 4 3 4 6% 6% 48%	2075
	21st to end	Night	15% 5% 2 5% 3 5% 4 3 7% 7% 44%	2100
May	1st to 10th	Night	18% 8% 3 8% 4 4 5% 4 6% 7% 33%	5297
	11th to 20th	Night	20% 6% 2 6% 4 4 5% 4 6% 8% 36%	5079
	21st to end	Night	22% 7% 3 7% 4 4 4 3 5% 7% 33%	5108
June	1st to 10th	Night	10% 5% 3 5% 5% 4 3 5% 8% 8% 44%	1680
	11th to 20th	Night	12% 6% 2 6% 5% 5% 6% 4 6% 8% 41%	1680
	21st to end	Night	9% 4 2 5% 5% 4 5% 5% 6% 9% 45%	1680
July	1st to 10th	Night	9% 5% 2 7% 4 5% 6% 5% 8% 10% 38%	3678
	11th to 20th	Night	10% 8% 4 9% 7% 6% 6% 6% 7% 10% 29%	4094
	21st to end	Night	8% 7% 3 7% 5% 5% 5% 5% 8% 9% 38%	4517
August	1st to 10th	Night	15% 5% 3 8% 5% 5% 6% 5% 7% 9% 32%	2915
	11th to 20th	Night	11% 6% 3 9% 5% 5% 5% 4 6% 8% 37%	2966
	21st to end	Night	11% 6% 2 5% 4 4 4 3 5% 8% 48%	3586
September	1st to 10th	Night	15% 6% 1 5% 3 4 5% 3 5% 7% 45%	2000
	11th to 20th	Night	10% 6% 2 7% 3 3 4 4 5% 7% 49%	2014
	21st to end	Night	21% 8% 2 6% 4 4 4 3 5% 6% 38%	2305
October	1st to 10th	Night	18% 6% 2 5% 4 3 3 3 4 5% 47%	2400
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 51%	2400
	21st to end	Night	13% 4 2 4 4 3 4 3 5% 6% 51%	2640
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 5% 54%	2499
	11th to 20th	Night	14% 7% 2 4 3 3 4 3 6% 6% 49%	2784
	21st to end	Night	13% 5% 1 5% 3 4 4 4 5% 7% 49%	2940
December	1st to 10th	Night	10% 5% 2 4 3 3 3 3 5% 7% 53%	4720
	11th to 20th	Night	20% 6% 2 5% 4 4 3 4 5% 6% 43%	5345
	21st to end	Night	17% 6% 2 6% 4 4 4 4 5% 7% 40%	7150

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 10% 24% 18% 6%	5022
	11th to 20th	Day	36% 4 6% 21% 20% 13%	5280
	21st to end	Day	40% 4 8% 30% 14% 3	5780
February	1st to 10th	Day	41% 6% 12% 24% 15% 3	1890
	11th to 20th	Day	44% 5% 10% 21% 17% 3	1875
	21st to end	Day	44% 5% 13% 19% 16% 4	1745
March	1st to 10th	Day	41% 5% 8% 20% 17% 9%	5045
	11th to 20th	Day	33% 5% 10% 26% 21% 6%	5090
	21st to end	Day	42% 7% 11% 17% 17% 7%	5600
April	1st to 10th	Day	37% 7% 11% 20% 18% 6%	2520
	11th to 20th	Day	36% 5% 12% 21% 20% 5%	2494
	21st to end	Day	42% 7% 10% 23% 14% 4	2520
May	1st to 10th	Day	44% 9% 12% 20% 13% 1	7646
	11th to 20th	Day	42% 7% 11% 27% 12% 1	7767
	21st to end	Day	44% 7% 11% 18% 17% 3	9061
June	1st to 10th	Day	35% 8% 15% 23% 18% 1	2940
	11th to 20th	Day	37% 9% 14% 24% 14% 2	2940
	21st to end	Day	32% 10% 16% 25% 17% 1	2940
July	1st to 10th	Day	33% 12% 18% 20% 14% 4	6430
	11th to 20th	Day	41% 11% 18% 22% 7% 1	6007
	21st to end	Day	37% 8% 10% 24% 20% 1	6370
August	1st to 10th	Day	42% 8% 18% 24% 7%	4221
	11th to 20th	Day	37% 9% 13% 27% 14% 1	4087
	21st to end	Day	32% 7% 15% 28% 17% 2	4312
September	1st to 10th	Day	37% 8% 12% 18% 20% 5%	2400
	11th to 20th	Day	34% 6% 9% 27% 20% 3	2382
	21st to end	Day	46% 7% 11% 18% 16% 2	2095
October	1st to 10th	Day	35% 5% 8% 19% 29% 5%	3260
	11th to 20th	Day	26% 4 8% 27% 25% 10%	3240
	21st to end	Day	21% 4 10% 30% 25% 10%	3538
November	1st to 10th	Day	32% 5% 11% 24% 23% 5%	2016
	11th to 20th	Day	35% 5% 10% 21% 22% 6%	1832
	21st to end	Day	36% 5% 11% 26% 20% 3	1680
December	1st to 10th	Day	34% 5% 11% 26% 19% 5%	2680
	11th to 20th	Day	41% 4 14% 26% 12% 4	3066
	21st to end	Day	42% 7% 14% 21% 15% 1	4048
January	1st to 10th	Night	35% 5% 9% 25% 22% 4	7856
	11th to 20th	Night	34% 3 7% 22% 25% 10%	7680
	21st to end	Night	37% 3 8% 29% 19% 4	8401
February	1st to 10th	Night	42% 6% 12% 23% 16% 2	2730
	11th to 20th	Night	43% 4 9% 22% 17% 5%	2640
	21st to end	Night	42% 4 12% 21% 16% 5%	2058
March	1st to 10th	Night	37% 5% 8% 23% 18% 8%	5028
	11th to 20th	Night	31% 6% 11% 23% 22% 7%	5090
	21st to end	Night	37% 6% 10% 22% 20% 6%	5572
April	1st to 10th	Night	40% 6% 11% 18% 20% 4	2100
	11th to 20th	Night	35% 7% 11% 22% 19% 5%	2075
	21st to end	Night	36% 8% 12% 26% 13% 5%	2100
May	1st to 10th	Night	49% 7% 12% 19% 10% 3	5324
	11th to 20th	Night	44% 7% 13% 25% 11% 1	5080
	21st to end	Night	50% 7% 10% 18% 12% 3	5115
June	1st to 10th	Night	34% 9% 12% 26% 17% 2	1680
	11th to 20th	Night	38% 10% 14% 23% 14% 1	1680
	21st to end	Night	34% 8% 16% 21% 18% 3	1680
July	1st to 10th	Night	38% 11% 17% 17% 14% 2	3640
	11th to 20th	Night	48% 11% 18% 15% 7% 1	4045
	21st to end	Night	37% 9% 15% 22% 17% 1	4387
August	1st to 10th	Night	46% 9% 15% 22% 7% 1	2904
	11th to 20th	Night	43% 8% 11% 23% 14% 1	2993
	21st to end	Night	33% 6% 13% 29% 17% 1	3586
September	1st to 10th	Night	37% 8% 11% 21% 18% 6%	2000
	11th to 20th	Night	34% 6% 10% 26% 21% 3	2014
	21st to end	Night	46% 7% 10% 18% 15% 3	2305
October	1st to 10th	Night	41% 4 9% 20% 21% 5%	3920
	11th to 20th	Night	24% 3 11% 22% 31% 9%	3899
	21st to end	Night	22% 4 9% 31% 26% 8%	4252
November	1st to 10th	Night	33% 4 9% 25% 23% 6%	2499
	11th to 20th	Night	35% 5% 8% 24% 22% 7%	2784
	21st to end	Night	34% 5% 11% 24% 21% 4	2940
December	1st to 10th	Night	32% 4 12% 28% 21% 3	4720
	11th to 20th	Night	43% 4 11% 19% 17% 5%	5348
	21st to end	Night	41% 8% 15% 21% 15% 1	7150

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	26% 22% 31% 17% 3	286
	11th to 20th	Day	29% 26% 28% 16% 2	330
	21st to end	Day	21% 25% 37% 13% 3 1	363
February	1st to 10th	Day	23% 24% 35% 14% 4	330
	11th to 20th	Day	35% 24% 31% 8% 1	353
	21st to end	Day	25% 21% 30% 21% 3	446
March	1st to 10th	Day	21% 20% 34% 21% 4	540
	11th to 20th	Day	24% 21% 31% 18% 6% 1	540
	21st to end	Day	22% 20% 35% 18% 5% 1	594
April	1st to 10th	Day	24% 21% 26% 22% 6% 1	540
	11th to 20th	Day	18% 18% 28% 20% 14% 2	532
	21st to end	Day	23% 12% 30% 23% 11% 1	540
May	1st to 10th	Day	20% 16% 29% 20% 11% 3	543
	11th to 20th	Day	19% 10% 28% 24% 16% 3	567
	21st to end	Day	22% 5% 24% 23% 19% 7%	617
June	1st to 10th	Day	10% 7% 26% 29% 21% 7%	570
	11th to 20th	Day	14% 8% 24% 29% 17% 8%	570
	21st to end	Day	10% 4 22% 36% 21% 7%	570
July	1st to 10th	Day	11% 4 13% 32% 28% 11%	565
	11th to 20th	Day	5% 2 15% 33% 33% 12%	564
	21st to end	Day	11% 3 18% 32% 26% 10%	616
August	1st to 10th	Day	14% 3 17% 28% 27% 11%	526
	11th to 20th	Day	12% 5% 17% 29% 25% 13%	519
	21st to end	Day	9% 3 17% 32% 23% 16%	583
September	1st to 10th	Day	14% 4 22% 28% 18% 14%	530
	11th to 20th	Day	14% 3 18% 28% 25% 12%	530
	21st to end	Day	27% 8% 21% 25% 15% 4	530
October	1st to 10th	Day	19% 5% 20% 29% 19% 8%	530
	11th to 20th	Day	17% 12% 25% 29% 15% 3	530
	21st to end	Day	17% 12% 34% 25% 10% 3	583
November	1st to 10th	Day	15% 11% 33% 26% 11% 4	513
	11th to 20th	Day	19% 13% 34% 22% 10% 2	372
	21st to end	Day	20% 17% 33% 22% 6% 1	220
December	1st to 10th	Day	19% 18% 38% 20% 5% 1	220
	11th to 20th	Day	20% 14% 37% 21% 9%	220
	21st to end	Day	26% 17% 31% 19% 7%	242
January	1st to 10th	Night	19% 14% 42% 22% 3	840
	11th to 20th	Night	21% 14% 37% 25% 3	840
	21st to end	Night	18% 14% 43% 24% 2	924
February	1st to 10th	Night	20% 12% 41% 24% 3	840
	11th to 20th	Night	25% 17% 40% 17% 2	823
	21st to end	Night	22% 14% 39% 22% 3	692
March	1st to 10th	Night	17% 16% 39% 26% 3	840
	11th to 20th	Night	21% 15% 35% 22% 5%	840
	21st to end	Night	17% 15% 40% 23% 5%	924
April	1st to 10th	Night	17% 17% 36% 26% 4	840
	11th to 20th	Night	15% 11% 35% 30% 8% 1	831
	21st to end	Night	18% 8% 32% 33% 8%	840
May	1st to 10th	Night	20% 8% 30% 31% 11% 1	651
	11th to 20th	Night	20% 5% 30% 32% 13% 1	570
	21st to end	Night	16% 3 19% 41% 21% 1	455
June	1st to 10th	Night	9% 3 22% 45% 20% 1	420
	11th to 20th	Night	12% 6% 16% 43% 22% 1	420
	21st to end	Night	10% 2 18% 47% 21% 2	420
July	1st to 10th	Night	11% 3 17% 38% 28% 4	410
	11th to 20th	Night	7% 2 9% 43% 35% 5%	587
	21st to end	Night	9% 3 15% 40% 28% 4	687
August	1st to 10th	Night	10% 3 12% 41% 32% 2	597
	11th to 20th	Night	12% 2 13% 39% 31% 2	646
	21st to end	Night	11% 3 13% 40% 28% 5%	880
September	1st to 10th	Night	17% 2 14% 33% 29% 4	800
	11th to 20th	Night	12% 2 16% 37% 30% 3	800
	21st to end	Night	25% 3 20% 32% 19% 1	800
October	1st to 10th	Night	20% 3 22% 29% 22% 4	800
	11th to 20th	Night	18% 8% 26% 34% 15% 1	800
	21st to end	Night	16% 9% 32% 30% 12% 1	880
November	1st to 10th	Night	14% 9% 31% 34% 12% 1	820
	11th to 20th	Night	18% 8% 35% 29% 8% 1	840
	21st to end	Night	15% 10% 36% 29% 8% 1	840
December	1st to 10th	Night	19% 11% 38% 25% 7%	840
	11th to 20th	Night	19% 9% 37% 31% 4	840
	21st to end	Night	21% 11% 32% 32% 4	924

Page generated at 2026-07-28 03:25:20. Version ac07

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