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 2025-03-27.

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	5% 4 2 4 3 3 3 3 4 7% 61%	112984
	Night	6% 5% 2 5% 4 4 4 3 5% 6% 57%	115189
Transparency	Day	24% 3 8% 20% 26% 20%	112743
	Night	28% 3 9% 19% 23% 17%	115020
Seeing	Day	7% 10% 25% 32% 20% 6%	15881
	Night	8% 5% 16% 35% 30% 6%	25358

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	9% 6% 2 6% 3 4 4 4 5% 8% 47%	3914
	11th to 20th	Day	7% 6% 3 7% 4 4 4 3 4 4 52%	4100
	21st to end	Day	11% 9% 3 7% 5% 5% 4 4 5% 7% 41%	4510
February	1st to 10th	Day	10% 8% 3 6% 5% 4 4 4 5% 6% 45%	1800
	11th to 20th	Day	10% 7% 3 6% 5% 4 5% 4 6% 7% 43%	1856
	21st to end	Day	9% 7% 2 7% 5% 5% 4 4 4 6% 46%	1650
March	1st to 10th	Day	8% 9% 3 9% 7% 7% 7% 5% 7% 8% 29%	3488
	11th to 20th	Day	6% 5% 2 5% 4 4 5% 3 6% 8% 52%	3600
	21st to end	Day	8% 7% 3 7% 6% 6% 6% 5% 7% 9% 36%	3950
April	1st to 10th	Day	10% 7% 2 6% 4 3 3 4 6% 8% 47%	2280
	11th to 20th	Day	8% 6% 2 5% 3 3 3 3 5% 7% 56%	2249
	21st to end	Day	5% 4 2 4 3 3 3 3 6% 8% 60%	2280
May	1st to 10th	Day	5% 3 1 3 1 2 2 2 3 6% 73%	3889
	11th to 20th	Day	3 2 1 2 2 2 2 1 2 5% 79%	4148
	21st to end	Day	3 4 2 3 2 2 2 2 3 5% 71%	4813
June	1st to 10th	Day	4 4 2 3 3 2 3 3 5% 7% 64%	2660
	11th to 20th	Day	2 2 1 1 1 1 2 2 3 7% 79%	2660
	21st to end	Day	2 2 1 2 2 2 2 1 3 4 79%	2660
July	1st to 10th	Day	1 2 1 1 2 1 2 3 85%	4420
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 6% 78%	4111
	21st to end	Day	1 2 2 2 2 2 3 6% 81%	4455
August	1st to 10th	Day	1 2 1 3 3 3 3 3 5% 7% 69%	4091
	11th to 20th	Day	2 2 2 2 2 2 3 5% 82%	3812
	21st to end	Day	1 1 3 3 2 3 2 3 6% 76%	4180
September	1st to 10th	Day	1 2 1 3 2 2 2 2 3 5% 79%	2280
	11th to 20th	Day	2 3 1 4 4 3 3 2 4 7% 68%	2280
	21st to end	Day	2 2 1 3 2 2 1 2 3 6% 77%	2018
October	1st to 10th	Day	2 2 1 3 2 3 3 2 4 6% 73%	3180
	11th to 20th	Day	3 3 1 4 3 3 3 2 4 6% 67%	3161
	21st to end	Day	2 3 1 3 3 3 3 3 5% 8% 64%	3448
November	1st to 10th	Day	5% 5% 2 4 4 3 5% 3 4 8% 56%	1929
	11th to 20th	Day	6% 7% 2 5% 5% 4 4 4 5% 7% 51%	1800
	21st to end	Day	9% 5% 2 5% 3 4 4 3 5% 8% 52%	1660
December	1st to 10th	Day	5% 3 1 5% 5% 4 4 3 6% 10% 53%	2620
	11th to 20th	Day	9% 8% 3 9% 6% 6% 5% 5% 8% 9% 31%	3028
	21st to end	Day	10% 9% 4 9% 5% 6% 7% 5% 7% 7% 30%	4004
January	1st to 10th	Night	7% 7% 3 6% 4 4 4 4 6% 8% 47%	6028
	11th to 20th	Night	8% 8% 3 8% 5% 4 3 3 4 4 48%	5880
	21st to end	Night	10% 9% 3 8% 6% 5% 4 4 4 5% 42%	6468
February	1st to 10th	Night	10% 8% 2 5% 4 4 4 4 4 5% 49%	2600
	11th to 20th	Night	9% 7% 2 5% 6% 5% 5% 4 6% 8% 44%	2429
	21st to end	Night	7% 6% 2 6% 5% 4 3 3 6% 6% 51%	1980
March	1st to 10th	Night	9% 9% 4 8% 6% 6% 6% 4 6% 6% 35%	3602
	11th to 20th	Night	8% 6% 3 7% 4 4 3 3 3 6% 54%	3600
	21st to end	Night	8% 12% 3 8% 5% 6% 5% 5% 6% 5% 38%	3839
April	1st to 10th	Night	7% 7% 2 6% 5% 4 4 4 6% 7% 48%	1900
	11th to 20th	Night	6% 6% 2 5% 4 3 4 3 4 7% 56%	1879
	21st to end	Night	4 3 1 4 3 3 4 3 5% 8% 62%	1900
May	1st to 10th	Night	3 5% 2 3 2 3 3 2 4 5% 69%	3084
	11th to 20th	Night	2 3 1 3 3 3 4 2 4 5% 72%	2748
	21st to end	Night	1 3 1 4 2 2 2 2 4 5% 73%	2762
June	1st to 10th	Night	3 3 1 4 4 2 2 3 4 9% 64%	1520
	11th to 20th	Night	1 1 3 2 2 1 2 4 5% 79%	1520
	21st to end	Night	1 2 1 3 2 3 2 1 2 4 78%	1520
July	1st to 10th	Night	2 1 2 1 1 1 1 2 3 87%	2540
	11th to 20th	Night	1 1 4 2 2 2 2 4 8% 74%	2853
	21st to end	Night	1 1 4 3 2 3 3 5% 9% 69%	3109
August	1st to 10th	Night	2 3 2 6% 5% 4 3 2 5% 7% 62%	2848
	11th to 20th	Night	2 1 2 2 2 2 2 4 5% 79%	3031
	21st to end	Night	1 2 1 5% 4 3 3 2 4 5% 71%	3498
September	1st to 10th	Night	1 1 1 4 2 2 2 2 4 7% 74%	1900
	11th to 20th	Night	3 4 2 4 3 3 3 2 4 6% 66%	1900
	21st to end	Night	2 2 1 3 2 2 2 2 3 6% 75%	2159
October	1st to 10th	Night	2 2 1 2 2 2 3 2 4 6% 75%	3800
	11th to 20th	Night	4 4 2 5% 3 2 3 2 4 6% 67%	3777
	21st to end	Night	3 4 2 4 2 3 3 3 5% 6% 65%	4121
November	1st to 10th	Night	6% 5% 2 5% 3 3 3 3 6% 8% 56%	2369
	11th to 20th	Night	6% 6% 3 5% 4 4 3 3 5% 8% 53%	2600
	21st to end	Night	7% 5% 2 4 4 4 4 3 5% 7% 56%	2737
December	1st to 10th	Night	5% 4 2 4 3 4 4 3 5% 7% 58%	4560
	11th to 20th	Night	11% 9% 3 8% 6% 6% 6% 4 7% 10% 30%	5176
	21st to end	Night	11% 8% 3 9% 6% 6% 6% 5% 7% 7% 31%	6952

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	36% 4 17% 23% 16% 4	3914
	11th to 20th	Day	37% 3 8% 14% 20% 17%	4100
	21st to end	Day	43% 3 7% 14% 17% 16%	4510
February	1st to 10th	Day	40% 4 7% 21% 18% 10%	1800
	11th to 20th	Day	40% 3 11% 19% 21% 7%	1865
	21st to end	Day	39% 5% 7% 23% 19% 7%	1650
March	1st to 10th	Day	52% 4 12% 16% 14% 3	3488
	11th to 20th	Day	31% 3 12% 29% 16% 10%	3600
	21st to end	Day	46% 4 14% 23% 11% 2	3950
April	1st to 10th	Day	37% 4 13% 21% 20% 4	2280
	11th to 20th	Day	30% 3 14% 26% 22% 5%	2252
	21st to end	Day	24% 4 12% 31% 24% 5%	2280
May	1st to 10th	Day	17% 2 8% 32% 28% 13%	3889
	11th to 20th	Day	11% 1 5% 28% 40% 14%	4151
	21st to end	Day	16% 2 9% 34% 34% 4	4814
June	1st to 10th	Day	21% 3 11% 27% 28% 10%	2660
	11th to 20th	Day	10% 2 7% 24% 38% 19%	2660
	21st to end	Day	11% 2 3 14% 34% 37%	2660
July	1st to 10th	Day	6% 1 1 7% 16% 69%	4351
	11th to 20th	Day	10% 2 3 7% 20% 59%	4041
	21st to end	Day	5% 2 1 9% 38% 44%	4315
August	1st to 10th	Day	15% 1 2 14% 34% 34%	4076
	11th to 20th	Day	6% 1 1 11% 49% 31%	3850
	21st to end	Day	10% 2 4 13% 40% 32%	4179
September	1st to 10th	Day	9% 1 3 11% 33% 44%	2280
	11th to 20th	Day	17% 3 5% 25% 32% 18%	2280
	21st to end	Day	11% 2 4 23% 37% 22%	2018
October	1st to 10th	Day	13% 2 4 12% 30% 39%	3180
	11th to 20th	Day	17% 2 8% 28% 28% 16%	3161
	21st to end	Day	18% 2 6% 27% 31% 15%	3448
November	1st to 10th	Day	28% 5% 9% 22% 24% 12%	1929
	11th to 20th	Day	33% 4 11% 23% 22% 8%	1800
	21st to end	Day	32% 4 8% 18% 30% 9%	1660
December	1st to 10th	Day	27% 3 6% 26% 30% 7%	2620
	11th to 20th	Day	46% 3 12% 19% 15% 4	3028
	21st to end	Day	50% 6% 15% 17% 10% 2	4004
January	1st to 10th	Night	35% 4 19% 23% 15% 3	6028
	11th to 20th	Night	41% 2 10% 15% 21% 11%	5880
	21st to end	Night	44% 3 8% 12% 18% 15%	6468
February	1st to 10th	Night	39% 3 7% 20% 20% 11%	2600
	11th to 20th	Night	38% 3 11% 19% 22% 7%	2431
	21st to end	Night	34% 4 11% 30% 17% 5%	1980
March	1st to 10th	Night	50% 3 16% 15% 14% 3	3602
	11th to 20th	Night	34% 3 9% 25% 20% 9%	3600
	21st to end	Night	50% 3 12% 21% 11% 3	3839
April	1st to 10th	Night	35% 5% 15% 23% 18% 5%	1900
	11th to 20th	Night	30% 4 12% 27% 19% 7%	1879
	21st to end	Night	22% 4 12% 32% 24% 6%	1900
May	1st to 10th	Night	20% 2 12% 27% 26% 14%	3084
	11th to 20th	Night	13% 2 7% 25% 33% 19%	2749
	21st to end	Night	15% 2 11% 35% 33% 5%	2762
June	1st to 10th	Night	19% 3 12% 25% 30% 10%	1520
	11th to 20th	Night	10% 2 8% 20% 36% 24%	1520
	21st to end	Night	12% 1 2 14% 33% 37%	1520
July	1st to 10th	Night	5% 1 1 5% 17% 70%	2500
	11th to 20th	Night	10% 4 3 7% 20% 56%	2800
	21st to end	Night	9% 5% 2 11% 35% 40%	3011
August	1st to 10th	Night	22% 4 2 11% 32% 29%	2839
	11th to 20th	Night	8% 1 2 9% 48% 33%	3059
	21st to end	Night	13% 2 4 12% 40% 28%	3498
September	1st to 10th	Night	11% 2 3 11% 29% 44%	1900
	11th to 20th	Night	19% 3 4 24% 31% 19%	1900
	21st to end	Night	13% 2 5% 21% 33% 26%	2159
October	1st to 10th	Night	12% 2 5% 12% 27% 42%	3800
	11th to 20th	Night	20% 2 7% 27% 26% 18%	3777
	21st to end	Night	21% 2 7% 23% 30% 18%	4121
November	1st to 10th	Night	27% 3 10% 26% 21% 14%	2369
	11th to 20th	Night	30% 4 10% 25% 21% 9%	2600
	21st to end	Night	29% 4 10% 22% 23% 13%	2737
December	1st to 10th	Night	26% 4 7% 26% 27% 10%	4560
	11th to 20th	Night	46% 4 11% 19% 16% 3	5176
	21st to end	Night	51% 4 17% 16% 9% 3	6952

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	20% 15% 27% 23% 13% 2	319
	11th to 20th	Day	13% 11% 29% 30% 14% 4	400
	21st to end	Day	13% 10% 29% 27% 17% 4	440
February	1st to 10th	Day	14% 20% 25% 24% 14% 5%	400
	11th to 20th	Day	15% 19% 29% 25% 11% 2	447
	21st to end	Day	11% 16% 30% 30% 11% 1	414
March	1st to 10th	Day	15% 18% 32% 25% 9% 1	500
	11th to 20th	Day	16% 19% 27% 21% 13% 3	500
	21st to end	Day	11% 18% 36% 24% 9% 2	531
April	1st to 10th	Day	12% 16% 34% 24% 11% 3	460
	11th to 20th	Day	12% 18% 29% 26% 12% 4	452
	21st to end	Day	5% 20% 30% 28% 11% 6%	460
May	1st to 10th	Day	6% 13% 35% 29% 13% 4	463
	11th to 20th	Day	7% 13% 32% 30% 14% 5%	488
	21st to end	Day	5% 12% 29% 32% 16% 6%	529
June	1st to 10th	Day	4 10% 30% 31% 18% 6%	490
	11th to 20th	Day	2 9% 31% 35% 19% 5%	490
	21st to end	Day	1 4 25% 39% 24% 7%	490
July	1st to 10th	Day	2 2 17% 43% 30% 7%	486
	11th to 20th	Day	1 1 15% 43% 32% 9%	484
	21st to end	Day	1 14% 44% 31% 10%	531
August	1st to 10th	Day	2 2 21% 40% 25% 9%	487
	11th to 20th	Day	1 2 22% 40% 25% 11%	477
	21st to end	Day	2 17% 42% 28% 10%	539
September	1st to 10th	Day	1 3 14% 40% 31% 10%	490
	11th to 20th	Day	3 7% 24% 37% 21% 8%	490
	21st to end	Day	2 8% 18% 35% 28% 9%	490
October	1st to 10th	Day	2 8% 20% 34% 24% 11%	490
	11th to 20th	Day	5% 9% 20% 29% 26% 10%	490
	21st to end	Day	6% 6% 25% 32% 24% 7%	539
November	1st to 10th	Day	7% 11% 24% 25% 25% 9%	465
	11th to 20th	Day	5% 11% 19% 31% 24% 10%	300
	21st to end	Day	9% 7% 26% 30% 23% 5%	230
December	1st to 10th	Day	6% 12% 24% 26% 23% 9%	200
	11th to 20th	Day	11% 17% 21% 21% 22% 10%	200
	21st to end	Day	15% 19% 25% 24% 14% 3	220
January	1st to 10th	Night	16% 7% 18% 32% 22% 5%	800
	11th to 20th	Night	13% 7% 16% 35% 23% 7%	800
	21st to end	Night	15% 6% 14% 36% 23% 5%	880
February	1st to 10th	Night	13% 9% 18% 35% 21% 5%	800
	11th to 20th	Night	13% 9% 20% 34% 20% 4	784
	21st to end	Night	11% 9% 22% 36% 18% 3	660
March	1st to 10th	Night	13% 12% 22% 31% 20% 3	800
	11th to 20th	Night	14% 10% 18% 33% 22% 4	800
	21st to end	Night	11% 10% 23% 36% 18% 2	862
April	1st to 10th	Night	10% 9% 23% 35% 21% 3	760
	11th to 20th	Night	9% 12% 20% 33% 20% 5%	751
	21st to end	Night	5% 7% 27% 32% 22% 6%	760
May	1st to 10th	Night	5% 4 26% 36% 24% 5%	703
	11th to 20th	Night	4 3 17% 37% 33% 6%	515
	21st to end	Night	4 4 22% 37% 29% 3	411
June	1st to 10th	Night	3 4 19% 43% 28% 3	380
	11th to 20th	Night	1 3 16% 47% 32% 1	380
	21st to end	Night	1 1 14% 39% 43% 3	380
July	1st to 10th	Night	1 1 10% 35% 51% 3	370
	11th to 20th	Night	1 3 36% 49% 12%	546
	21st to end	Night	1 4 31% 55% 10%	624
August	1st to 10th	Night	4 1 7% 32% 49% 7%	567
	11th to 20th	Night	1 1 10% 39% 42% 6%	693
	21st to end	Night	2 9% 34% 45% 10%	836
September	1st to 10th	Night	1 1 8% 37% 43% 10%	760
	11th to 20th	Night	4 3 13% 37% 36% 8%	760
	21st to end	Night	3 2 11% 33% 41% 11%	760
October	1st to 10th	Night	4 2 12% 37% 36% 9%	760
	11th to 20th	Night	6% 4 11% 31% 39% 9%	760
	21st to end	Night	5% 5% 11% 35% 31% 13%	836
November	1st to 10th	Night	7% 4 17% 30% 33% 9%	780
	11th to 20th	Night	9% 4 15% 35% 30% 7%	800
	21st to end	Night	10% 4 15% 39% 25% 6%	800
December	1st to 10th	Night	10% 6% 19% 34% 25% 6%	800
	11th to 20th	Night	18% 5% 20% 33% 21% 4	800
	21st to end	Night	15% 11% 23% 30% 18% 3	880

Page generated at 2025-03-24 06:04:28. Version ac07

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