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 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	14% 6% 2 6% 4 4 4 4 6% 8% 42%	109024
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 45%	110311
Transparency	Day	37% 7% 12% 23% 16% 4	112668
	Night	37% 6% 11% 23% 18% 4	114807
Seeing	Day	18% 11% 26% 25% 15% 6%	15411
	Night	17% 8% 29% 31% 13% 1	25147

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	15% 5% 2 4 3 4 5% 3 4 5% 49%	3873
	11th to 20th	Day	13% 8% 2 7% 4 3 4 3 4 5% 48%	4060
	21st to end	Day	15% 7% 2 6% 4 4 4 3 4 5% 45%	4466
February	1st to 10th	Day	13% 8% 2 7% 5% 4 5% 3 6% 6% 40%	1800
	11th to 20th	Day	18% 7% 2 6% 3 4 4 3 5% 6% 40%	1775
	21st to end	Day	18% 8% 2 6% 5% 3 3 3 5% 6% 41%	1664
March	1st to 10th	Day	21% 7% 3 6% 5% 5% 3 4 4 7% 37%	3545
	11th to 20th	Day	10% 5% 2 6% 4 4 4 3 6% 6% 50%	3590
	21st to end	Day	20% 8% 2 6% 4 3 3 4 4 7% 38%	3895
April	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	14% 6% 1 5% 5% 3 3 3 5% 6% 47%	2254
	21st to end	Day	18% 6% 2 6% 4 5% 4 4 6% 5% 41%	2280
May	1st to 10th	Day	21% 8% 3 6% 4 5% 5% 4 7% 8% 30%	4077
	11th to 20th	Day	23% 7% 2 6% 4 4 5% 4 6% 8% 32%	4162
	21st to end	Day	11% 4 2 6% 4 5% 4 4 6% 10% 44%	4823
June	1st to 10th	Day	11% 5% 2 5% 5% 5% 4 5% 8% 9% 42%	2660
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 5% 7% 9% 41%	2660
	21st to end	Day	8% 4 1 5% 5% 5% 5% 4 8% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 5% 7% 5% 8% 11% 41%	4440
	11th to 20th	Day	8% 5% 2 6% 4 4 6% 5% 8% 12% 38%	4152
	21st to end	Day	11% 6% 2 7% 6% 6% 5% 5% 7% 8% 36%	4466
August	1st to 10th	Day	15% 5% 2 5% 5% 5% 6% 5% 8% 10% 33%	4104
	11th to 20th	Day	9% 4 2 6% 6% 5% 6% 5% 7% 9% 40%	3922
	21st to end	Day	10% 4 2 5% 4 4 4 3 6% 8% 49%	4180
September	1st to 10th	Day	14% 6% 2 5% 5% 5% 3 3 5% 6% 46%	2280
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 8% 48%	2263
	21st to end	Day	22% 6% 3 6% 4 3 4 3 5% 7% 37%	1990
October	1st to 10th	Day	17% 5% 2 5% 3 3 4 3 5% 7% 46%	1900
	11th to 20th	Day	16% 4 2 5% 3 4 4 3 5% 5% 50%	1900
	21st to end	Day	14% 5% 2 5% 3 3 4 3 5% 6% 51%	2090
November	1st to 10th	Day	11% 6% 2 4 4 3 4 3 4 6% 53%	1918
	11th to 20th	Day	14% 6% 2 4 4 4 4 3 5% 6% 49%	1745
	21st to end	Day	15% 5% 2 5% 3 4 5% 4 4 6% 49%	1600
December	1st to 10th	Day	12% 4 2 5% 3 3 4 4 5% 6% 52%	2600
	11th to 20th	Day	19% 8% 3 4 3 3 3 3 4 7% 44%	2990
	21st to end	Day	18% 8% 2 7% 4 4 4 4 5% 7% 38%	3960
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 4 5% 6% 52%	6055
	11th to 20th	Night	14% 7% 2 4 3 2 2 3 4 5% 54%	5900
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 49%	6490
February	1st to 10th	Night	15% 7% 2 6% 4 4 4 5% 5% 6% 40%	2600
	11th to 20th	Night	20% 7% 2 6% 3 3 3 2 5% 5% 44%	2509
	21st to end	Night	16% 8% 2 6% 4 4 4 3 4 5% 43%	1963
March	1st to 10th	Night	20% 6% 2 6% 4 4 4 4 6% 6% 38%	3528
	11th to 20th	Night	11% 7% 1 4 3 4 4 3 5% 7% 49%	3590
	21st to end	Night	13% 8% 2 6% 3 4 3 3 5% 6% 47%	3874
April	1st to 10th	Night	15% 8% 2 6% 5% 3 4 3 5% 6% 43%	1900
	11th to 20th	Night	12% 5% 2 5% 5% 4 3 3 6% 6% 49%	1875
	21st to end	Night	16% 6% 2 5% 3 4 5% 3 6% 7% 43%	1900
May	1st to 10th	Night	22% 9% 3 7% 4 4 4 4 5% 8% 31%	2845
	11th to 20th	Night	25% 8% 2 6% 4 4 5% 3 5% 6% 32%	2727
	21st to end	Night	11% 5% 2 6% 4 4 4 4 6% 10% 44%	2732
June	1st to 10th	Night	11% 5% 2 5% 5% 4 3 5% 8% 9% 43%	1520
	11th to 20th	Night	12% 6% 1 5% 4 5% 6% 4 7% 8% 42%	1520
	21st to end	Night	9% 4 2 5% 5% 4 6% 5% 6% 9% 45%	1520
July	1st to 10th	Night	8% 4 2 6% 4 6% 6% 5% 8% 11% 39%	2520
	11th to 20th	Night	8% 5% 2 7% 6% 5% 5% 5% 6% 11% 38%	2802
	21st to end	Night	10% 8% 3 8% 5% 5% 5% 5% 8% 8% 34%	3087
August	1st to 10th	Night	16% 5% 3 7% 6% 5% 6% 5% 7% 9% 31%	2825
	11th to 20th	Night	11% 6% 4 9% 5% 5% 5% 4 6% 8% 37%	2873
	21st to end	Night	11% 6% 2 5% 4 4 4 3 5% 8% 49%	3476
September	1st to 10th	Night	16% 6% 1 5% 3 4 5% 4 5% 7% 44%	1900
	11th to 20th	Night	10% 6% 2 7% 3 3 4 4 4 7% 49%	1913
	21st to end	Night	21% 7% 2 6% 3 4 4 3 5% 6% 40%	2190
October	1st to 10th	Night	19% 6% 2 5% 4 3 3 3 4 5% 46%	2280
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 4 6% 51%	2280
	21st to end	Night	14% 4 2 4 3 3 4 3 5% 6% 51%	2508
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 5% 54%	2377
	11th to 20th	Night	14% 7% 2 4 3 3 4 4 6% 6% 48%	2651
	21st to end	Night	13% 5% 1 5% 3 4 4 4 5% 7% 49%	2800
December	1st to 10th	Night	10% 5% 2 4 3 3 3 3 5% 7% 53%	4580
	11th to 20th	Night	20% 6% 2 5% 4 4 3 4 5% 6% 42%	5205
	21st to end	Night	17% 6% 2 6% 4 4 4 4 5% 7% 39%	6996

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 7% 25% 20% 8%	3874
	11th to 20th	Day	38% 5% 8% 21% 18% 11%	4060
	21st to end	Day	41% 5% 9% 26% 16% 3	4466
February	1st to 10th	Day	42% 6% 12% 23% 14% 2	1800
	11th to 20th	Day	43% 5% 11% 21% 17% 3	1784
	21st to end	Day	44% 5% 13% 18% 16% 4	1664
March	1st to 10th	Day	46% 6% 10% 20% 14% 5%	3545
	11th to 20th	Day	31% 6% 10% 26% 23% 5%	3590
	21st to end	Day	44% 7% 11% 19% 15% 4	3895
April	1st to 10th	Day	36% 7% 12% 20% 18% 6%	2280
	11th to 20th	Day	36% 6% 11% 21% 20% 6%	2254
	21st to end	Day	42% 8% 11% 22% 14% 4	2280
May	1st to 10th	Day	48% 10% 11% 19% 10% 1	4115
	11th to 20th	Day	47% 10% 12% 22% 8% 1	4166
	21st to end	Day	32% 10% 16% 22% 18% 2	4837
June	1st to 10th	Day	36% 9% 15% 24% 16% 1	2660
	11th to 20th	Day	36% 10% 14% 24% 13% 2	2660
	21st to end	Day	31% 10% 17% 24% 17% 1	2660
July	1st to 10th	Day	33% 12% 19% 20% 11% 5%	4370
	11th to 20th	Day	35% 10% 18% 27% 10% 1	4082
	21st to end	Day	43% 8% 10% 22% 16% 1	4324
August	1st to 10th	Day	42% 8% 19% 24% 7%	4091
	11th to 20th	Day	36% 9% 13% 27% 14% 1	3961
	21st to end	Day	32% 7% 15% 28% 17% 2	4180
September	1st to 10th	Day	38% 8% 12% 18% 20% 5%	2280
	11th to 20th	Day	34% 6% 9% 27% 20% 3	2263
	21st to end	Day	45% 8% 11% 18% 16% 2	1990
October	1st to 10th	Day	36% 5% 8% 18% 28% 5%	3160
	11th to 20th	Day	25% 4 8% 27% 25% 10%	3140
	21st to end	Day	21% 4 10% 30% 25% 10%	3428
November	1st to 10th	Day	32% 5% 11% 24% 23% 5%	1918
	11th to 20th	Day	35% 6% 10% 21% 21% 6%	1745
	21st to end	Day	36% 5% 11% 26% 20% 2	1600
December	1st to 10th	Day	33% 5% 11% 26% 20% 5%	2600
	11th to 20th	Day	41% 4 14% 26% 12% 3	2986
	21st to end	Day	42% 7% 15% 20% 15% 1	3960
January	1st to 10th	Night	32% 5% 7% 24% 26% 5%	6055
	11th to 20th	Night	33% 3 8% 24% 23% 9%	5900
	21st to end	Night	38% 4 8% 27% 19% 3	6490
February	1st to 10th	Night	42% 6% 13% 22% 15% 2	2600
	11th to 20th	Night	43% 4 9% 22% 18% 6%	2511
	21st to end	Night	42% 4 11% 21% 16% 5%	1963
March	1st to 10th	Night	43% 7% 10% 23% 14% 3	3528
	11th to 20th	Night	30% 7% 10% 24% 24% 4	3590
	21st to end	Night	37% 6% 9% 23% 19% 5%	3874
April	1st to 10th	Night	40% 6% 11% 18% 21% 4	1900
	11th to 20th	Night	34% 7% 11% 22% 20% 6%	1875
	21st to end	Night	37% 8% 12% 25% 13% 5%	1900
May	1st to 10th	Night	51% 8% 11% 18% 10% 3	2872
	11th to 20th	Night	50% 9% 12% 20% 8% 1	2728
	21st to end	Night	33% 10% 16% 25% 13% 3	2739
June	1st to 10th	Night	34% 9% 13% 26% 16% 2	1520
	11th to 20th	Night	36% 10% 14% 24% 14% 2	1520
	21st to end	Night	33% 8% 17% 20% 18% 3	1520
July	1st to 10th	Night	36% 11% 21% 17% 13% 3	2480
	11th to 20th	Night	38% 9% 20% 21% 10% 1	2750
	21st to end	Night	42% 9% 14% 19% 14% 1	2988
August	1st to 10th	Night	46% 9% 15% 22% 7% 1	2814
	11th to 20th	Night	43% 8% 11% 23% 14% 1	2900
	21st to end	Night	33% 6% 13% 29% 17% 1	3476
September	1st to 10th	Night	38% 8% 11% 19% 18% 6%	1900
	11th to 20th	Night	34% 6% 10% 26% 21% 3	1913
	21st to end	Night	45% 7% 10% 19% 16% 3	2190
October	1st to 10th	Night	41% 4 9% 20% 20% 5%	3800
	11th to 20th	Night	24% 3 11% 22% 31% 9%	3779
	21st to end	Night	21% 4 9% 32% 26% 8%	4120
November	1st to 10th	Night	33% 4 9% 25% 22% 6%	2377
	11th to 20th	Night	35% 5% 7% 24% 22% 7%	2651
	21st to end	Night	34% 5% 11% 24% 21% 4	2800
December	1st to 10th	Night	32% 4 12% 28% 22% 3	4580
	11th to 20th	Night	43% 4 12% 19% 17% 5%	5208
	21st to end	Night	41% 8% 15% 21% 15% 1	6996

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	27% 23% 31% 16% 3	260
	11th to 20th	Day	29% 24% 28% 17% 2	300
	21st to end	Day	20% 23% 39% 14% 3 1	330
February	1st to 10th	Day	23% 25% 35% 13% 4	300
	11th to 20th	Day	35% 24% 32% 7% 2	322
	21st to end	Day	25% 21% 30% 21% 3	414
March	1st to 10th	Day	20% 20% 34% 21% 4	500
	11th to 20th	Day	23% 20% 32% 17% 6% 1	500
	21st to end	Day	22% 19% 33% 19% 6% 1	530
April	1st to 10th	Day	22% 22% 26% 23% 6% 1	460
	11th to 20th	Day	17% 17% 26% 22% 16% 3	452
	21st to end	Day	22% 12% 30% 22% 12% 1	460
May	1st to 10th	Day	21% 14% 28% 21% 13% 3	463
	11th to 20th	Day	18% 9% 28% 25% 15% 3	487
	21st to end	Day	19% 3 24% 25% 21% 8%	529
June	1st to 10th	Day	10% 7% 24% 29% 23% 7%	490
	11th to 20th	Day	12% 7% 23% 31% 17% 9%	490
	21st to end	Day	9% 3 23% 38% 20% 6%	490
July	1st to 10th	Day	11% 4 13% 33% 28% 11%	485
	11th to 20th	Day	5% 2 14% 31% 34% 13%	484
	21st to end	Day	12% 2 17% 32% 27% 10%	532
August	1st to 10th	Day	13% 3 16% 28% 28% 11%	486
	11th to 20th	Day	11% 4 18% 28% 25% 14%	479
	21st to end	Day	9% 3 15% 33% 23% 17%	539
September	1st to 10th	Day	15% 4 19% 28% 19% 15%	490
	11th to 20th	Day	13% 4 18% 28% 24% 13%	490
	21st to end	Day	27% 8% 21% 24% 15% 4	490
October	1st to 10th	Day	20% 5% 20% 29% 19% 7%	490
	11th to 20th	Day	16% 11% 24% 30% 16% 3	490
	21st to end	Day	16% 10% 34% 26% 11% 3	539
November	1st to 10th	Day	16% 10% 32% 27% 12% 4	475
	11th to 20th	Day	18% 12% 34% 23% 10% 2	345
	21st to end	Day	20% 16% 34% 22% 7% 1	200
December	1st to 10th	Day	18% 17% 40% 20% 5% 1	200
	11th to 20th	Day	20% 13% 36% 23% 9%	200
	21st to end	Day	24% 17% 31% 20% 8%	220
January	1st to 10th	Night	20% 15% 41% 22% 3	800
	11th to 20th	Night	21% 13% 37% 26% 4	800
	21st to end	Night	18% 13% 42% 25% 2	880
February	1st to 10th	Night	20% 12% 41% 25% 3	800
	11th to 20th	Night	25% 17% 40% 17% 2	783
	21st to end	Night	22% 14% 39% 22% 3	660
March	1st to 10th	Night	17% 16% 39% 25% 3	800
	11th to 20th	Night	21% 15% 36% 23% 5% 1	800
	21st to end	Night	17% 14% 40% 24% 5%	863
April	1st to 10th	Night	16% 16% 37% 26% 4	760
	11th to 20th	Night	15% 10% 35% 31% 9% 1	751
	21st to end	Night	18% 8% 32% 33% 8% 1	760
May	1st to 10th	Night	20% 7% 29% 30% 12% 1	589
	11th to 20th	Night	20% 4 29% 33% 14% 1	515
	21st to end	Night	14% 3 18% 43% 23% 1	411
June	1st to 10th	Night	10% 2 21% 45% 21% 1	380
	11th to 20th	Night	12% 4 15% 43% 24% 1	380
	21st to end	Night	9% 2 18% 49% 20% 2	380
July	1st to 10th	Night	11% 2 18% 38% 27% 4	370
	11th to 20th	Night	6% 1 9% 42% 36% 5%	529
	21st to end	Night	9% 2 15% 41% 29% 4	624
August	1st to 10th	Night	10% 2 12% 42% 32% 2	567
	11th to 20th	Night	12% 2 14% 39% 30% 2	613
	21st to end	Night	12% 3 12% 40% 29% 5%	836
September	1st to 10th	Night	18% 2 13% 33% 30% 4	760
	11th to 20th	Night	12% 2 15% 36% 31% 3	760
	21st to end	Night	24% 3 20% 31% 20% 1	760
October	1st to 10th	Night	21% 3 22% 29% 21% 4	760
	11th to 20th	Night	17% 7% 26% 34% 15% 1	760
	21st to end	Night	16% 8% 32% 31% 13% 1	836
November	1st to 10th	Night	14% 8% 31% 35% 12% 1	780
	11th to 20th	Night	18% 7% 35% 30% 9% 1	800
	21st to end	Night	15% 10% 36% 29% 9% 1	800
December	1st to 10th	Night	20% 10% 37% 26% 7%	800
	11th to 20th	Night	18% 8% 38% 31% 4	800
	21st to end	Night	20% 10% 32% 33% 4	880

Page generated at 2025-03-24 09:04:30. Version ac07

Copyright © 2025 Attilla Danko.