Menlo School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Menlo School clear sky chart from 2005-11-06 to 2026-06-16.

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 4 3 5% 7% 61%	146133
	Night	6% 5% 2 5% 4 4 4 3 5% 7% 55%	143369
Transparency	Day	21% 6% 10% 21% 26% 17%	141067
	Night	26% 6% 11% 20% 23% 15%	139424
Seeing	Day	6% 9% 24% 32% 21% 8%	17523
	Night	8% 5% 15% 35% 30% 6%	26914

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 4 4 6% 9% 42%	5100
	11th to 20th	Day	6% 5% 2 6% 4 4 3 3 4 6% 57%	5340
	21st to end	Day	10% 8% 3 7% 5% 5% 5% 4 6% 7% 40%	5843
February	1st to 10th	Day	10% 7% 3 6% 4 5% 3 4 6% 6% 45%	1890
	11th to 20th	Day	11% 8% 3 7% 4 5% 4 4 7% 7% 41%	1943
	21st to end	Day	9% 6% 3 6% 5% 5% 4 3 5% 8% 45%	1730
March	1st to 10th	Day	8% 7% 3 9% 6% 6% 6% 5% 7% 8% 34%	4991
	11th to 20th	Day	6% 5% 2 6% 4 4 5% 4 7% 8% 49%	5110
	21st to end	Day	7% 6% 2 7% 5% 5% 6% 5% 7% 10% 39%	5686
April	1st to 10th	Day	9% 7% 2 5% 4 3 4 4 6% 7% 48%	2520
	11th to 20th	Day	8% 5% 2 4 4 3 4 3 5% 7% 56%	2489
	21st to end	Day	5% 5% 2 4 3 3 4 3 5% 9% 58%	2520
May	1st to 10th	Day	3 3 2 5% 3 3 3 2 4 6% 68%	7262
	11th to 20th	Day	2 2 1 2 1 2 1 1 3 5% 81%	7707
	21st to end	Day	2 3 1 3 2 3 3 3 4 7% 69%	8992
June	1st to 10th	Day	4 4 2 3 3 3 3 3 4 7% 64%	2940
	11th to 20th	Day	1 2 1 2 1 2 2 2 4 6% 77%	2871
	21st to end	Day	2 2 1 3 2 2 2 2 3 5% 76%	2800
July	1st to 10th	Day	1 1 2 1 2 3 2 3 4 82%	6287
	11th to 20th	Day	1 1 1 2 3 3 3 2 4 6% 75%	5880
	21st to end	Day	1 1 4 3 3 4 3 4 7% 70%	6402
August	1st to 10th	Day	1 2 1 2 2 2 3 3 5% 7% 73%	5777
	11th to 20th	Day	2 1 3 3 2 2 2 3 5% 76%	5500
	21st to end	Day	1 1 3 2 4 3 3 4 7% 71%	5984
September	1st to 10th	Day	1 2 1 3 2 2 2 2 3 5% 75%	2400
	11th to 20th	Day	2 4 2 4 3 3 3 3 4 8% 64%	2400
	21st to end	Day	3 2 1 3 1 2 2 2 4 5% 75%	2118
October	1st to 10th	Day	2 2 1 3 3 3 2 2 4 6% 72%	3280
	11th to 20th	Day	3 3 1 4 4 3 3 2 5% 6% 65%	3261
	21st to end	Day	3 3 1 4 3 4 3 3 6% 8% 61%	3558
November	1st to 10th	Day	5% 5% 2 5% 4 3 3 4 6% 8% 56%	2033
	11th to 20th	Day	7% 8% 3 6% 3 5% 5% 4 5% 7% 47%	1890
	21st to end	Day	8% 5% 2 5% 3 3 3 4 5% 8% 53%	1729
December	1st to 10th	Day	5% 3 1 6% 5% 4 4 3 6% 10% 54%	2700
	11th to 20th	Day	11% 9% 3 10% 6% 5% 6% 4 8% 8% 31%	3108
	21st to end	Day	12% 10% 4 9% 5% 6% 7% 4 7% 8% 28%	4092
January	1st to 10th	Night	10% 8% 3 7% 4 5% 4 4 6% 8% 41%	7798
	11th to 20th	Night	7% 6% 3 7% 4 4 4 3 5% 7% 52%	7650
	21st to end	Night	9% 9% 3 9% 6% 5% 5% 4 5% 7% 39%	8371
February	1st to 10th	Night	10% 7% 3 6% 4 5% 4 3 5% 5% 48%	2730
	11th to 20th	Night	9% 6% 3 7% 6% 5% 5% 5% 5% 8% 42%	2563
	21st to end	Night	8% 5% 2 6% 5% 4 4 3 5% 7% 49%	2076
March	1st to 10th	Night	9% 9% 3 9% 5% 5% 5% 4 6% 6% 37%	5106
	11th to 20th	Night	9% 7% 3 6% 4 4 4 2 4 6% 50%	5110
	21st to end	Night	8% 10% 3 8% 5% 4 5% 4 5% 7% 42%	5529
April	1st to 10th	Night	7% 7% 2 6% 5% 4 3 3 8% 8% 48%	2100
	11th to 20th	Night	6% 6% 2 6% 5% 3 4 4 5% 7% 54%	2079
	21st to end	Night	4 4 1 4 3 3 3 3 5% 9% 60%	2100
May	1st to 10th	Night	3 5% 2 5% 4 3 3 2 3 5% 64%	5687
	11th to 20th	Night	1 2 1 2 3 3 3 3 5% 6% 73%	5161
	21st to end	Night	2 3 2 4 3 3 4 2 3 5% 70%	5181
June	1st to 10th	Night	3 4 1 5% 3 4 2 3 6% 8% 62%	1680
	11th to 20th	Night	1 1 1 3 1 1 2 2 3 6% 78%	1640
	21st to end	Night	1 3 1 3 2 2 2 2 2 5% 76%	1600
July	1st to 10th	Night	2 1 2 1 1 1 1 3 5% 84%	3655
	11th to 20th	Night	1 1 1 2 4 3 3 2 4 9% 72%	4100
	21st to end	Night	1 1 5% 5% 8% 6% 4 6% 8% 55%	4472
August	1st to 10th	Night	1 2 1 3 3 2 3 2 4 9% 70%	4035
	11th to 20th	Night	2 1 3 3 4 3 2 4 7% 71%	4361
	21st to end	Night	1 1 1 4 3 5% 4 2 4 8% 67%	5015
September	1st to 10th	Night	2 2 1 3 4 2 3 2 4 7% 70%	2000
	11th to 20th	Night	3 5% 2 5% 3 3 3 3 5% 7% 61%	2000
	21st to end	Night	1 3 1 3 3 2 3 3 4 5% 73%	2277
October	1st to 10th	Night	2 2 1 3 2 2 3 3 4 6% 72%	3920
	11th to 20th	Night	4 4 2 5% 3 4 4 3 4 6% 62%	3897
	21st to end	Night	3 4 2 4 3 3 3 3 5% 6% 63%	4253
November	1st to 10th	Night	6% 6% 2 4 3 3 4 3 5% 7% 57%	2483
	11th to 20th	Night	8% 7% 2 6% 4 4 3 3 5% 8% 49%	2730
	21st to end	Night	6% 5% 2 5% 4 5% 4 3 5% 7% 54%	2888
December	1st to 10th	Night	5% 4 2 5% 3 4 4 4 5% 7% 57%	4700
	11th to 20th	Night	13% 10% 3 8% 5% 5% 6% 4 7% 10% 28%	5316
	21st to end	Night	13% 10% 3 9% 6% 6% 6% 5% 6% 6% 30%	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	35% 9% 17% 22% 14% 3	5101
	11th to 20th	Day	28% 5% 7% 13% 27% 20%	5340
	21st to end	Day	39% 7% 8% 16% 17% 13%	5843
February	1st to 10th	Day	38% 6% 9% 19% 18% 10%	1890
	11th to 20th	Day	40% 7% 12% 17% 18% 6%	1952
	21st to end	Day	38% 6% 11% 22% 17% 6%	1730
March	1st to 10th	Day	42% 10% 10% 18% 18% 2	4991
	11th to 20th	Day	29% 8% 15% 27% 12% 8%	5110
	21st to end	Day	35% 12% 17% 23% 11% 2	5686
April	1st to 10th	Day	35% 7% 14% 21% 19% 4	2520
	11th to 20th	Day	28% 7% 16% 24% 20% 5%	2492
	21st to end	Day	24% 7% 14% 29% 22% 4	2520
May	1st to 10th	Day	17% 4 8% 26% 32% 12%	7262
	11th to 20th	Day	8% 3 8% 29% 39% 13%	7710
	21st to end	Day	12% 6% 11% 31% 36% 4	8994
June	1st to 10th	Day	19% 7% 14% 25% 26% 8%	2940
	11th to 20th	Day	9% 4 11% 23% 36% 17%	2871
	21st to end	Day	10% 3 7% 16% 34% 30%	2800
July	1st to 10th	Day	4 2 4 12% 23% 54%	6219
	11th to 20th	Day	6% 3 6% 10% 20% 54%	5811
	21st to end	Day	8% 4 6% 20% 33% 29%	6217
August	1st to 10th	Day	12% 4 7% 15% 32% 31%	4206
	11th to 20th	Day	7% 3 5% 13% 44% 29%	3982
	21st to end	Day	9% 4 7% 16% 37% 27%	4311
September	1st to 10th	Day	10% 3 5% 15% 29% 38%	2400
	11th to 20th	Day	19% 5% 10% 24% 27% 15%	2400
	21st to end	Day	11% 4 8% 22% 33% 22%	2118
October	1st to 10th	Day	14% 3 6% 12% 28% 37%	3280
	11th to 20th	Day	17% 5% 10% 24% 29% 15%	3261
	21st to end	Day	19% 5% 9% 25% 29% 14%	3558
November	1st to 10th	Day	28% 5% 12% 23% 21% 11%	2033
	11th to 20th	Day	34% 7% 10% 22% 20% 7%	1890
	21st to end	Day	30% 5% 11% 18% 27% 9%	1729
December	1st to 10th	Day	26% 5% 8% 25% 30% 7%	2700
	11th to 20th	Day	45% 8% 13% 15% 14% 5%	3108
	21st to end	Day	50% 10% 13% 16% 10% 2	4092
January	1st to 10th	Night	36% 10% 18% 21% 13% 3	7798
	11th to 20th	Night	31% 6% 8% 13% 27% 15%	7650
	21st to end	Night	41% 6% 8% 14% 18% 12%	8371
February	1st to 10th	Night	37% 6% 10% 18% 19% 11%	2730
	11th to 20th	Night	38% 7% 11% 18% 19% 6%	2565
	21st to end	Night	34% 7% 13% 27% 15% 4	2076
March	1st to 10th	Night	42% 10% 12% 18% 16% 2	5106
	11th to 20th	Night	33% 8% 12% 24% 16% 7%	5110
	21st to end	Night	40% 11% 14% 21% 12% 2	5529
April	1st to 10th	Night	32% 8% 17% 22% 17% 4	2100
	11th to 20th	Night	29% 8% 14% 24% 18% 7%	2079
	21st to end	Night	20% 9% 14% 32% 20% 5%	2100
May	1st to 10th	Night	20% 6% 9% 23% 31% 11%	5685
	11th to 20th	Night	9% 4 13% 28% 31% 15%	5162
	21st to end	Night	13% 7% 13% 27% 37% 3	5182
June	1st to 10th	Night	19% 8% 13% 24% 27% 8%	1680
	11th to 20th	Night	9% 6% 9% 22% 33% 21%	1640
	21st to end	Night	12% 4 7% 16% 33% 28%	1600
July	1st to 10th	Night	4 2 4 11% 22% 56%	3615
	11th to 20th	Night	5% 5% 6% 15% 15% 53%	4046
	21st to end	Night	8% 5% 9% 22% 31% 24%	4345
August	1st to 10th	Night	15% 5% 8% 17% 28% 28%	2929
	11th to 20th	Night	9% 3 6% 12% 43% 27%	3148
	21st to end	Night	10% 5% 9% 16% 34% 25%	3608
September	1st to 10th	Night	12% 5% 7% 14% 25% 37%	2000
	11th to 20th	Night	19% 6% 11% 24% 27% 14%	2000
	21st to end	Night	13% 5% 8% 21% 29% 25%	2277
October	1st to 10th	Night	13% 3 7% 11% 25% 40%	3920
	11th to 20th	Night	18% 4 10% 25% 25% 18%	3897
	21st to end	Night	21% 4 9% 22% 27% 18%	4253
November	1st to 10th	Night	26% 6% 12% 24% 19% 12%	2483
	11th to 20th	Night	32% 6% 12% 24% 18% 8%	2730
	21st to end	Night	28% 6% 12% 21% 21% 12%	2888
December	1st to 10th	Night	26% 6% 8% 24% 26% 10%	4700
	11th to 20th	Night	46% 7% 12% 15% 15% 4	5316
	21st to end	Night	50% 9% 15% 15% 8% 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	19% 14% 25% 24% 13% 4	351
	11th to 20th	Day	12% 8% 22% 32% 21% 5%	430
	21st to end	Day	11% 9% 27% 29% 19% 5%	473
February	1st to 10th	Day	13% 16% 25% 25% 15% 6%	430
	11th to 20th	Day	17% 17% 28% 25% 12% 2	477
	21st to end	Day	9% 15% 28% 34% 13% 2	446
March	1st to 10th	Day	15% 16% 31% 27% 10% 2	540
	11th to 20th	Day	15% 14% 25% 25% 17% 4	540
	21st to end	Day	11% 16% 34% 25% 11% 3	594
April	1st to 10th	Day	11% 15% 32% 26% 13% 4	540
	11th to 20th	Day	10% 17% 29% 25% 14% 5%	532
	21st to end	Day	6% 16% 29% 30% 14% 5%	540
May	1st to 10th	Day	4 13% 34% 29% 15% 6%	543
	11th to 20th	Day	7% 13% 31% 29% 16% 5%	568
	21st to end	Day	6% 11% 27% 31% 17% 8%	617
June	1st to 10th	Day	4 9% 28% 31% 21% 8%	570
	11th to 20th	Day	1 9% 29% 36% 20% 6%	551
	21st to end	Day	1 5% 22% 38% 25% 9%	530
July	1st to 10th	Day	2 4 16% 44% 28% 8%	526
	11th to 20th	Day	1 1 13% 41% 33% 12%	524
	21st to end	Day	1 1 15% 41% 32% 10%	575
August	1st to 10th	Day	2 3 22% 38% 24% 12%	527
	11th to 20th	Day	1 2 20% 40% 26% 11%	517
	21st to end	Day	2 17% 38% 31% 11%	583
September	1st to 10th	Day	1 4 15% 39% 31% 10%	530
	11th to 20th	Day	3 6% 23% 34% 25% 10%	530
	21st to end	Day	3 8% 18% 33% 29% 10%	530
October	1st to 10th	Day	2 7% 22% 32% 24% 13%	530
	11th to 20th	Day	5% 8% 18% 30% 27% 12%	530
	21st to end	Day	6% 5% 25% 30% 25% 10%	583
November	1st to 10th	Day	6% 9% 23% 25% 25% 12%	509
	11th to 20th	Day	6% 10% 19% 30% 23% 11%	330
	21st to end	Day	8% 5% 22% 28% 29% 8%	245
December	1st to 10th	Day	6% 10% 18% 29% 27% 10%	220
	11th to 20th	Day	10% 12% 19% 23% 25% 11%	220
	21st to end	Day	15% 19% 21% 26% 16% 2	242
January	1st to 10th	Night	17% 7% 18% 33% 21% 5%	840
	11th to 20th	Night	12% 6% 15% 36% 25% 6%	840
	21st to end	Night	15% 5% 14% 37% 23% 5%	924
February	1st to 10th	Night	13% 7% 17% 39% 20% 4	840
	11th to 20th	Night	12% 8% 22% 34% 19% 5%	824
	21st to end	Night	11% 8% 20% 41% 18% 3	692
March	1st to 10th	Night	13% 11% 20% 32% 21% 2	840
	11th to 20th	Night	13% 9% 17% 34% 24% 4	840
	21st to end	Night	11% 9% 22% 38% 18% 2	924
April	1st to 10th	Night	9% 8% 21% 38% 21% 3	840
	11th to 20th	Night	8% 11% 22% 35% 21% 4	831
	21st to end	Night	5% 5% 25% 35% 24% 5%	840
May	1st to 10th	Night	5% 4 21% 39% 26% 5%	762
	11th to 20th	Night	3 4 19% 35% 33% 5%	578
	21st to end	Night	4 6% 18% 37% 29% 5%	455
June	1st to 10th	Night	3 4 16% 43% 31% 3	420
	11th to 20th	Night	1 2 15% 46% 35% 1	410
	21st to end	Night	1 4 9% 38% 45% 3	400
July	1st to 10th	Night	1 2 7% 34% 52% 4	392
	11th to 20th	Night	1 3 30% 51% 14%	582
	21st to end	Night	1 1 6% 25% 56% 11%	657
August	1st to 10th	Night	4 4 6% 29% 46% 10%	597
	11th to 20th	Night	1 2 11% 35% 44% 8%	722
	21st to end	Night	1 1 9% 32% 46% 11%	880
September	1st to 10th	Night	2 2 10% 33% 43% 10%	800
	11th to 20th	Night	5% 4 13% 33% 38% 8%	800
	21st to end	Night	2 2 11% 32% 42% 11%	800
October	1st to 10th	Night	5% 3 11% 37% 37% 9%	800
	11th to 20th	Night	7% 3 12% 30% 37% 11%	800
	21st to end	Night	5% 5% 10% 36% 33% 12%	880
November	1st to 10th	Night	7% 4 15% 31% 34% 10%	820
	11th to 20th	Night	10% 4 13% 38% 29% 7%	840
	21st to end	Night	9% 4 14% 41% 27% 6%	840
December	1st to 10th	Night	10% 5% 16% 36% 26% 6%	840
	11th to 20th	Night	16% 6% 17% 36% 21% 4	840
	21st to end	Night	17% 10% 22% 32% 18% 2	924

Page generated at 2026-06-12 23:31:41. Version ac07

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