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 2025-02-21.

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% 5% 2 5% 3 3 3 3 5% 7% 60%	112626
	Night	6% 6% 2 5% 4 4 4 3 5% 7% 54%	114797
Transparency	Day	23% 6% 10% 19% 24% 18%	112386
	Night	27% 7% 11% 18% 21% 15%	114627
Seeing	Day	7% 9% 24% 32% 21% 7%	15744
	Night	8% 5% 15% 35% 31% 6%	25214

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% 7% 2 6% 4 4 4 3 6% 9% 44%	3933
	11th to 20th	Day	8% 6% 3 7% 5% 5% 3 3 4 5% 51%	4100
	21st to end	Day	12% 9% 3 7% 5% 5% 4 4 5% 6% 40%	4510
February	1st to 10th	Day	11% 8% 3 6% 4 5% 3 4 6% 6% 44%	1800
	11th to 20th	Day	10% 8% 3 6% 4 4 3 4 7% 7% 42%	1848
	21st to end	Day	10% 6% 3 6% 5% 5% 4 3 5% 7% 46%	1571
March	1st to 10th	Day	9% 9% 4 10% 7% 7% 7% 5% 7% 7% 29%	3399
	11th to 20th	Day	6% 5% 2 5% 4 4 4 3 6% 7% 53%	3490
	21st to end	Day	9% 7% 2 7% 5% 6% 6% 5% 7% 9% 36%	3876
April	1st to 10th	Day	10% 8% 3 5% 4 3 4 4 6% 7% 47%	2280
	11th to 20th	Day	8% 5% 2 4 4 3 3 3 4 7% 57%	2249
	21st to end	Day	4 5% 2 3 3 3 3 3 5% 9% 59%	2280
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 4 5% 72%	3914
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 5% 76%	4131
	21st to end	Day	3 5% 2 4 2 2 2 2 4 6% 67%	4813
June	1st to 10th	Day	5% 4 2 3 3 3 2 3 5% 7% 63%	2660
	11th to 20th	Day	1 2 1 2 2 2 2 2 5% 6% 76%	2660
	21st to end	Day	2 2 1 3 2 2 2 2 3 5% 76%	2660
July	1st to 10th	Day	2 1 2 1 1 2 2 3 5% 82%	4401
	11th to 20th	Day	1 2 1 2 2 2 3 2 4 6% 74%	4110
	21st to end	Day	1 1 2 2 2 2 2 3 7% 76%	4455
August	1st to 10th	Day	1 2 1 3 3 2 4 4 6% 9% 65%	4091
	11th to 20th	Day	2 1 2 2 2 1 2 3 5% 79%	3821
	21st to end	Day	2 1 3 3 3 3 2 4 6% 72%	4180
September	1st to 10th	Day	1 2 1 3 2 2 2 2 2 5% 77%	2280
	11th to 20th	Day	2 4 2 4 3 3 3 3 4 8% 64%	2280
	21st to end	Day	2 2 1 3 1 2 2 2 4 5% 75%	2012
October	1st to 10th	Day	2 2 1 3 3 3 2 2 4 6% 72%	3180
	11th to 20th	Day	3 3 1 4 4 3 3 2 5% 6% 65%	3161
	21st to end	Day	3 3 1 4 3 3 3 3 6% 8% 61%	3448
November	1st to 10th	Day	5% 5% 2 4 4 3 3 4 5% 8% 56%	1934
	11th to 20th	Day	6% 7% 3 6% 3 5% 5% 4 5% 7% 49%	1800
	21st to end	Day	9% 6% 2 5% 3 3 3 4 5% 8% 52%	1647
December	1st to 10th	Day	5% 3 2 6% 5% 4 4 3 6% 10% 53%	2620
	11th to 20th	Day	11% 9% 3 9% 6% 5% 6% 5% 8% 8% 30%	3028
	21st to end	Day	12% 10% 4 9% 5% 6% 7% 4 7% 8% 28%	4004
January	1st to 10th	Night	8% 7% 2 6% 4 5% 4 4 6% 9% 44%	6023
	11th to 20th	Night	10% 8% 4 8% 4 5% 4 3 4 5% 46%	5880
	21st to end	Night	11% 9% 3 9% 5% 4 4 4 4 6% 41%	6468
February	1st to 10th	Night	11% 7% 3 6% 4 5% 4 3 5% 5% 48%	2600
	11th to 20th	Night	8% 7% 3 7% 6% 5% 4 5% 5% 8% 43%	2438
	21st to end	Night	9% 5% 2 6% 5% 4 4 3 5% 7% 50%	1884
March	1st to 10th	Night	10% 10% 4 9% 6% 5% 5% 4 6% 6% 34%	3471
	11th to 20th	Night	8% 8% 3 5% 4 4 3 2 4 6% 53%	3490
	21st to end	Night	9% 12% 3 9% 5% 5% 5% 4 5% 6% 37%	3775
April	1st to 10th	Night	7% 7% 2 6% 6% 4 3 3 8% 8% 47%	1900
	11th to 20th	Night	6% 6% 2 5% 4 3 4 4 4 7% 55%	1879
	21st to end	Night	4 4 1 4 3 3 3 3 5% 9% 62%	1900
May	1st to 10th	Night	3 5% 2 3 3 2 3 2 4 6% 68%	3058
	11th to 20th	Night	2 3 1 3 4 3 3 3 5% 6% 66%	2755
	21st to end	Night	2 4 1 4 3 2 3 2 4 5% 70%	2762
June	1st to 10th	Night	4 4 2 5% 3 4 2 3 6% 8% 61%	1520
	11th to 20th	Night	1 1 1 3 2 1 2 2 4 7% 77%	1520
	21st to end	Night	1 3 1 3 2 2 2 2 2 5% 75%	1520
July	1st to 10th	Night	2 1 1 1 1 1 1 3 5% 83%	2555
	11th to 20th	Night	1 1 1 2 3 3 3 2 4 8% 72%	2860
	21st to end	Night	2 1 3 4 5% 4 4 6% 9% 61%	3109
August	1st to 10th	Night	2 3 1 4 4 3 4 3 5% 11% 59%	2848
	11th to 20th	Night	3 1 3 3 3 3 2 4 7% 73%	3023
	21st to end	Night	1 2 1 5% 4 4 3 2 4 7% 67%	3498
September	1st to 10th	Night	1 2 1 3 4 2 2 2 4 7% 71%	1900
	11th to 20th	Night	3 5% 2 5% 3 3 3 3 5% 7% 62%	1900
	21st to end	Night	1 2 1 3 3 2 3 2 4 6% 73%	2163
October	1st to 10th	Night	2 2 1 3 2 2 3 3 5% 6% 72%	3800
	11th to 20th	Night	4 3 1 5% 3 4 4 3 4 6% 62%	3777
	21st to end	Night	4 4 2 4 3 3 3 3 5% 6% 63%	4121
November	1st to 10th	Night	6% 6% 2 4 3 3 4 3 5% 7% 57%	2362
	11th to 20th	Night	7% 6% 2 5% 4 4 3 3 5% 8% 51%	2600
	21st to end	Night	7% 6% 2 4 4 4 4 3 5% 7% 55%	2750
December	1st to 10th	Night	5% 4 2 5% 3 4 4 4 5% 6% 57%	4560
	11th to 20th	Night	13% 10% 3 8% 5% 5% 6% 5% 7% 10% 28%	5176
	21st to end	Night	13% 10% 3 9% 6% 6% 6% 5% 6% 6% 30%	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	33% 9% 18% 20% 16% 4	3933
	11th to 20th	Day	35% 7% 8% 13% 21% 16%	4100
	21st to end	Day	42% 7% 7% 12% 15% 17%	4510
February	1st to 10th	Day	39% 6% 9% 18% 18% 10%	1800
	11th to 20th	Day	39% 6% 12% 18% 19% 6%	1857
	21st to end	Day	37% 6% 10% 23% 18% 6%	1571
March	1st to 10th	Day	49% 12% 11% 13% 12% 3	3399
	11th to 20th	Day	28% 7% 13% 26% 15% 10%	3490
	21st to end	Day	40% 13% 16% 19% 10% 2	3876
April	1st to 10th	Day	36% 7% 14% 20% 18% 4	2280
	11th to 20th	Day	28% 7% 15% 24% 21% 5%	2252
	21st to end	Day	23% 7% 14% 28% 23% 5%	2280
May	1st to 10th	Day	16% 5% 11% 30% 26% 12%	3914
	11th to 20th	Day	11% 4 10% 26% 36% 13%	4134
	21st to end	Day	17% 8% 15% 29% 29% 3	4814
June	1st to 10th	Day	21% 8% 14% 25% 24% 8%	2660
	11th to 20th	Day	10% 4 11% 24% 35% 15%	2660
	21st to end	Day	11% 4 7% 16% 32% 30%	2660
July	1st to 10th	Day	5% 2 5% 10% 15% 62%	4332
	11th to 20th	Day	8% 3 6% 11% 16% 55%	4041
	21st to end	Day	6% 2 5% 11% 36% 40%	4315
August	1st to 10th	Day	12% 4 7% 15% 31% 31%	4076
	11th to 20th	Day	7% 3 5% 12% 44% 28%	3859
	21st to end	Day	9% 4 7% 16% 36% 28%	4179
September	1st to 10th	Day	9% 3 5% 14% 29% 40%	2280
	11th to 20th	Day	18% 5% 10% 24% 27% 16%	2280
	21st to end	Day	11% 4 8% 22% 34% 22%	2012
October	1st to 10th	Day	14% 3 6% 12% 28% 38%	3180
	11th to 20th	Day	17% 5% 10% 24% 28% 16%	3161
	21st to end	Day	18% 5% 9% 25% 29% 14%	3448
November	1st to 10th	Day	28% 5% 11% 23% 21% 12%	1934
	11th to 20th	Day	32% 7% 10% 23% 21% 7%	1800
	21st to end	Day	31% 5% 11% 17% 27% 9%	1647
December	1st to 10th	Day	26% 5% 8% 24% 30% 7%	2620
	11th to 20th	Day	45% 8% 13% 15% 14% 4	3028
	21st to end	Day	50% 10% 13% 16% 10% 2	4004
January	1st to 10th	Night	32% 10% 20% 21% 14% 3	6023
	11th to 20th	Night	38% 7% 10% 13% 21% 11%	5880
	21st to end	Night	43% 6% 8% 10% 18% 15%	6468
February	1st to 10th	Night	37% 6% 10% 18% 19% 11%	2600
	11th to 20th	Night	38% 7% 11% 18% 20% 6%	2440
	21st to end	Night	33% 7% 12% 29% 15% 4	1884
March	1st to 10th	Night	47% 12% 13% 13% 12% 3	3471
	11th to 20th	Night	32% 8% 11% 21% 19% 10%	3490
	21st to end	Night	46% 12% 13% 17% 10% 3	3775
April	1st to 10th	Night	34% 8% 17% 21% 16% 4	1900
	11th to 20th	Night	29% 8% 13% 25% 18% 7%	1879
	21st to end	Night	20% 9% 14% 31% 20% 6%	1900
May	1st to 10th	Night	19% 7% 11% 26% 24% 13%	3058
	11th to 20th	Night	13% 6% 17% 23% 23% 17%	2756
	21st to end	Night	15% 10% 16% 28% 28% 4	2762
June	1st to 10th	Night	20% 9% 14% 24% 25% 8%	1520
	11th to 20th	Night	10% 6% 10% 23% 32% 19%	1520
	21st to end	Night	12% 4 7% 16% 32% 28%	1520
July	1st to 10th	Night	5% 3 5% 9% 16% 63%	2515
	11th to 20th	Night	7% 5% 6% 15% 16% 51%	2806
	21st to end	Night	8% 4 8% 13% 33% 34%	3011
August	1st to 10th	Night	15% 5% 8% 17% 27% 28%	2839
	11th to 20th	Night	9% 3 6% 11% 43% 27%	3051
	21st to end	Night	11% 5% 9% 17% 33% 25%	3498
September	1st to 10th	Night	11% 5% 7% 13% 25% 39%	1900
	11th to 20th	Night	19% 6% 11% 23% 27% 14%	1900
	21st to end	Night	12% 5% 8% 21% 29% 25%	2163
October	1st to 10th	Night	13% 3 7% 11% 25% 42%	3800
	11th to 20th	Night	18% 4 11% 24% 25% 18%	3777
	21st to end	Night	21% 4 8% 22% 26% 19%	4121
November	1st to 10th	Night	26% 6% 12% 24% 19% 13%	2362
	11th to 20th	Night	30% 7% 12% 24% 19% 8%	2600
	21st to end	Night	28% 6% 11% 20% 22% 13%	2750
December	1st to 10th	Night	26% 6% 8% 24% 27% 10%	4560
	11th to 20th	Night	46% 7% 12% 15% 16% 3	5176
	21st to end	Night	49% 10% 15% 15% 8% 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% 12% 25% 26% 14% 4	325
	11th to 20th	Day	13% 8% 24% 32% 18% 5%	400
	21st to end	Day	12% 9% 27% 28% 20% 5%	440
February	1st to 10th	Day	14% 17% 25% 23% 16% 6%	400
	11th to 20th	Day	15% 17% 27% 25% 13% 2	442
	21st to end	Day	11% 15% 29% 30% 14% 2	383
March	1st to 10th	Day	16% 16% 30% 27% 10% 1	460
	11th to 20th	Day	16% 15% 26% 25% 15% 4	460
	21st to end	Day	11% 17% 35% 25% 9% 2	506
April	1st to 10th	Day	12% 15% 33% 25% 12% 3	460
	11th to 20th	Day	11% 17% 29% 25% 13% 5%	452
	21st to end	Day	5% 17% 30% 29% 13% 6%	460
May	1st to 10th	Day	5% 13% 35% 29% 13% 4	463
	11th to 20th	Day	7% 12% 31% 30% 15% 5%	488
	21st to end	Day	6% 12% 27% 31% 16% 7%	529
June	1st to 10th	Day	4 10% 29% 31% 20% 6%	490
	11th to 20th	Day	2 9% 29% 36% 19% 6%	490
	21st to end	Day	1 4 22% 40% 24% 8%	490
July	1st to 10th	Day	2 4 15% 44% 28% 7%	486
	11th to 20th	Day	1 1 14% 42% 32% 10%	484
	21st to end	Day	1 1 16% 40% 32% 10%	531
August	1st to 10th	Day	2 2 21% 40% 24% 10%	487
	11th to 20th	Day	1 2 21% 40% 26% 11%	477
	21st to end	Day	2 18% 39% 30% 10%	539
September	1st to 10th	Day	1 4 13% 40% 31% 11%	490
	11th to 20th	Day	2 7% 24% 34% 24% 9%	490
	21st to end	Day	2 8% 17% 34% 29% 10%	490
October	1st to 10th	Day	2 7% 22% 34% 23% 13%	490
	11th to 20th	Day	5% 8% 18% 30% 27% 12%	490
	21st to end	Day	6% 5% 24% 31% 24% 9%	539
November	1st to 10th	Day	6% 9% 24% 25% 25% 10%	470
	11th to 20th	Day	5% 9% 19% 30% 25% 12%	300
	21st to end	Day	9% 5% 23% 27% 30% 6%	223
December	1st to 10th	Day	7% 12% 19% 29% 25% 10%	200
	11th to 20th	Day	11% 13% 20% 24% 23% 10%	200
	21st to end	Day	14% 16% 23% 27% 16% 3	220
January	1st to 10th	Night	16% 6% 17% 34% 22% 5%	800
	11th to 20th	Night	12% 6% 16% 36% 24% 7%	800
	21st to end	Night	16% 5% 14% 37% 23% 5%	880
February	1st to 10th	Night	13% 8% 17% 37% 21% 4	800
	11th to 20th	Night	12% 7% 21% 34% 20% 5%	784
	21st to end	Night	11% 8% 20% 39% 19% 3	628
March	1st to 10th	Night	14% 11% 19% 31% 22% 3	760
	11th to 20th	Night	13% 9% 17% 33% 24% 4	760
	21st to end	Night	12% 9% 22% 37% 18% 2	836
April	1st to 10th	Night	10% 8% 21% 38% 21% 3	760
	11th to 20th	Night	9% 11% 20% 34% 21% 5%	751
	21st to end	Night	5% 6% 25% 34% 24% 6%	760
May	1st to 10th	Night	5% 4 21% 39% 26% 5%	690
	11th to 20th	Night	4 4 16% 35% 35% 6%	522
	21st to end	Night	4 7% 18% 36% 30% 5%	411
June	1st to 10th	Night	3 4 16% 43% 31% 3	380
	11th to 20th	Night	1 2 15% 47% 34% 1	380
	21st to end	Night	1 4 9% 39% 44% 3	380
July	1st to 10th	Night	1 2 6% 32% 54% 4	372
	11th to 20th	Night	1 3 31% 50% 14%	552
	21st to end	Night	1 1 6% 25% 56% 11%	624
August	1st to 10th	Night	5% 4 5% 29% 48% 10%	567
	11th to 20th	Night	1 2 11% 34% 45% 8%	685
	21st to end	Night	1 1 9% 32% 46% 11%	836
September	1st to 10th	Night	2 2 10% 32% 44% 10%	760
	11th to 20th	Night	5% 4 13% 33% 38% 8%	760
	21st to end	Night	2 2 11% 31% 43% 11%	760
October	1st to 10th	Night	4 2 11% 36% 37% 10%	760
	11th to 20th	Night	6% 3 11% 30% 39% 10%	760
	21st to end	Night	5% 5% 10% 35% 33% 13%	836
November	1st to 10th	Night	7% 4 15% 31% 33% 9%	780
	11th to 20th	Night	9% 3 13% 38% 31% 7%	800
	21st to end	Night	9% 4 15% 40% 26% 6%	800
December	1st to 10th	Night	11% 5% 17% 36% 25% 6%	800
	11th to 20th	Night	17% 6% 18% 35% 20% 4	800
	21st to end	Night	16% 9% 22% 32% 18% 2	880

Page generated at 2025-02-18 00:26:33. Version ac07

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