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 2024-09-29.

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% 59%	100730
	Night	6% 6% 2 5% 4 4 4 3 5% 7% 54%	97721
Transparency	Day	24% 6% 10% 19% 23% 18%	100490
	Night	28% 7% 11% 18% 20% 15%	97551
Seeing	Day	7% 9% 24% 32% 21% 7%	15305
	Night	8% 5% 15% 34% 31% 7%	24638

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	12% 9% 3 7% 5% 4 4 4 7% 8% 37%	2735
	11th to 20th	Day	12% 8% 4 10% 7% 6% 5% 4 5% 6% 33%	2860
	21st to end	Day	16% 11% 4 8% 6% 5% 5% 4 6% 6% 30%	3146
February	1st to 10th	Day	11% 7% 2 6% 4 5% 3 5% 6% 6% 45%	1710
	11th to 20th	Day	11% 8% 3 7% 4 4 3 4 7% 7% 44%	1753
	21st to end	Day	10% 6% 3 6% 5% 5% 4 3 5% 7% 46%	1570
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% 76%	1993
October	1st to 10th	Day	3 3 1 4 4 3 2 2 3 7% 68%	1800
	11th to 20th	Day	5% 3 2 4 3 3 3 2 5% 7% 63%	1800
	21st to end	Day	5% 5% 2 4 4 3 3 3 5% 7% 60%	1980
November	1st to 10th	Day	6% 5% 2 5% 4 3 4 4 5% 8% 54%	1836
	11th to 20th	Day	6% 7% 3 6% 3 5% 5% 4 5% 7% 50%	1710
	21st to end	Day	8% 6% 2 5% 3 3 3 4 5% 8% 53%	1565
December	1st to 10th	Day	8% 5% 2 8% 6% 5% 4 4 6% 8% 44%	1520
	11th to 20th	Day	14% 10% 3 10% 6% 5% 4 4 7% 6% 31%	1928
	21st to end	Day	12% 11% 4 8% 5% 5% 6% 4 6% 8% 33%	2794
January	1st to 10th	Night	11% 8% 3 7% 4 5% 5% 4 6% 8% 40%	4214
	11th to 20th	Night	14% 10% 5% 10% 5% 6% 5% 4 5% 6% 30%	4110
	21st to end	Night	16% 12% 4 9% 6% 5% 4 4 4 6% 30%	4521
February	1st to 10th	Night	10% 7% 3 5% 4 5% 4 3 5% 5% 49%	2470
	11th to 20th	Night	8% 7% 3 6% 5% 5% 4 5% 5% 8% 44%	2313
	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 5% 74%	2150
October	1st to 10th	Night	4 4 1 4 3 2 3 3 4 6% 66%	2160
	11th to 20th	Night	5% 4 2 5% 3 3 3 3 4 6% 61%	2160
	21st to end	Night	4 5% 3 4 3 3 3 3 4 6% 62%	2376
November	1st to 10th	Night	6% 6% 2 4 3 3 4 3 5% 7% 56%	2240
	11th to 20th	Night	7% 6% 2 5% 4 4 3 4 6% 8% 51%	2470
	21st to end	Night	6% 5% 2 4 4 4 4 3 5% 7% 56%	2612
December	1st to 10th	Night	9% 7% 3 7% 4 4 4 4 5% 7% 46%	2660
	11th to 20th	Night	17% 11% 4 8% 5% 4 4 3 5% 7% 31%	3276
	21st to end	Night	12% 9% 3 8% 6% 5% 6% 4 7% 7% 34%	4862

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	41% 10% 13% 25% 8% 3	2735
	11th to 20th	Day	49% 9% 11% 12% 10% 7%	2860
	21st to end	Day	51% 9% 9% 14% 13% 5%	3146
February	1st to 10th	Day	38% 6% 9% 19% 18% 9%	1710
	11th to 20th	Day	39% 6% 11% 17% 20% 7%	1762
	21st to end	Day	37% 6% 10% 23% 18% 6%	1570
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%	1993
October	1st to 10th	Day	19% 4 8% 18% 28% 23%	1800
	11th to 20th	Day	21% 5% 11% 18% 22% 23%	1800
	21st to end	Day	23% 5% 10% 19% 23% 19%	1980
November	1st to 10th	Day	30% 6% 11% 22% 20% 12%	1836
	11th to 20th	Day	32% 7% 10% 22% 21% 7%	1710
	21st to end	Day	30% 5% 11% 17% 28% 9%	1565
December	1st to 10th	Day	37% 8% 12% 18% 16% 10%	1520
	11th to 20th	Day	52% 6% 12% 12% 13% 5%	1928
	21st to end	Day	47% 9% 12% 18% 12% 3	2794
January	1st to 10th	Night	39% 9% 18% 22% 9% 3	4214
	11th to 20th	Night	52% 9% 13% 10% 9% 7%	4110
	21st to end	Night	54% 7% 9% 11% 11% 7%	4521
February	1st to 10th	Night	37% 6% 10% 18% 19% 11%	2470
	11th to 20th	Night	37% 7% 11% 18% 21% 7%	2315
	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%	2150
October	1st to 10th	Night	18% 5% 11% 17% 25% 24%	2160
	11th to 20th	Night	23% 6% 9% 19% 20% 23%	2160
	21st to end	Night	22% 5% 10% 20% 24% 20%	2376
November	1st to 10th	Night	27% 7% 12% 24% 18% 13%	2240
	11th to 20th	Night	30% 7% 12% 23% 19% 8%	2470
	21st to end	Night	27% 6% 12% 21% 22% 13%	2612
December	1st to 10th	Night	37% 8% 10% 17% 17% 12%	2660
	11th to 20th	Night	52% 6% 10% 13% 15% 3	3276
	21st to end	Night	46% 11% 14% 15% 11% 4	4862

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	21% 13% 23% 25% 14% 3	298
	11th to 20th	Day	14% 9% 22% 32% 18% 5%	370
	21st to end	Day	13% 9% 26% 28% 20% 5%	407
February	1st to 10th	Day	14% 15% 25% 24% 17% 6%	370
	11th to 20th	Day	15% 17% 28% 25% 13% 2	407
	21st to end	Day	11% 15% 29% 30% 14% 2	382
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%	483
October	1st to 10th	Day	2 7% 23% 35% 21% 12%	450
	11th to 20th	Day	5% 8% 17% 31% 26% 13%	450
	21st to end	Day	7% 5% 24% 30% 24% 9%	495
November	1st to 10th	Day	7% 9% 23% 26% 25% 9%	432
	11th to 20th	Day	4 8% 17% 30% 27% 14%	270
	21st to end	Day	6% 5% 22% 28% 31% 6%	201
December	1st to 10th	Day	8% 13% 18% 28% 25% 8%	180
	11th to 20th	Day	11% 12% 20% 24% 23% 9%	180
	21st to end	Day	16% 15% 22% 27% 18% 3	198
January	1st to 10th	Night	17% 7% 17% 32% 22% 5%	760
	11th to 20th	Night	13% 6% 15% 35% 24% 7%	760
	21st to end	Night	17% 5% 14% 36% 23% 5%	836
February	1st to 10th	Night	12% 7% 17% 38% 22% 4	760
	11th to 20th	Night	12% 7% 20% 34% 22% 5%	744
	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%	756
October	1st to 10th	Night	4 3 11% 36% 36% 10%	720
	11th to 20th	Night	6% 4 11% 29% 39% 11%	720
	21st to end	Night	5% 5% 9% 34% 33% 13%	792
November	1st to 10th	Night	7% 4 15% 31% 34% 9%	740
	11th to 20th	Night	9% 3 12% 37% 32% 7%	760
	21st to end	Night	8% 4 14% 40% 27% 6%	760
December	1st to 10th	Night	11% 6% 17% 35% 25% 6%	760
	11th to 20th	Night	17% 6% 18% 35% 19% 4	760
	21st to end	Night	16% 9% 22% 32% 19% 2	836

Page generated at 2024-09-26 06:52:23. Version ac07

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