Murillo Family Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Murillo Family Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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 3 1 3 2 2 2 2 4 6% 70%	136626
	Night	6% 5% 1 3 2 2 2 2 3 5% 68%	132632
Transparency	Day	15% 3 7% 14% 27% 34%	131713
	Night	18% 4 7% 15% 26% 31%	128520
Seeing	Day	7% 11% 29% 34% 15% 5%	17314
	Night	7% 4 17% 35% 30% 7%	27249

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	6% 3 2 3 2 2 2 2 3 4 72%	4188
	11th to 20th	Day	6% 3 1 2 1 1 2 2 3 6% 72%	4339
	21st to end	Day	10% 4 2 5% 3 4 4 3 6% 8% 53%	5016
February	1st to 10th	Day	10% 5% 2 4 3 2 2 3 5% 4 60%	2000
	11th to 20th	Day	11% 6% 1 3 3 2 3 2 4 7% 58%	1950
	21st to end	Day	10% 9% 2 4 4 3 3 2 4 6% 54%	1650
March	1st to 10th	Day	15% 9% 2 5% 3 4 3 4 5% 7% 44%	4518
	11th to 20th	Day	9% 6% 2 4 3 3 3 3 6% 7% 53%	4731
	21st to end	Day	10% 6% 2 5% 3 4 4 3 5% 7% 51%	5837
April	1st to 10th	Day	7% 5% 1 3 2 3 3 2 4 8% 61%	2400
	11th to 20th	Day	5% 5% 2 3 3 3 4 3 3 6% 63%	2369
	21st to end	Day	5% 6% 2 4 2 2 2 2 4 6% 65%	2400
May	1st to 10th	Day	5% 6% 2 3 2 2 2 2 4 6% 66%	5745
	11th to 20th	Day	2 4 1 3 3 3 3 3 5% 7% 66%	5841
	21st to end	Day	2 4 1 2 2 2 3 3 4 7% 68%	6423
June	1st to 10th	Day	3 3 1 3 2 2 3 2 5% 8% 70%	2600
	11th to 20th	Day	1 2 1 2 2 1 1 2 3 5% 80%	2647
	21st to end	Day	1 2 1 2 2 1 1 2 2 3 84%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 3 90%	6319
	11th to 20th	Day	1 1 2 1 1 2 2 3 4 83%	5907
	21st to end	Day	1 1 1 1 2 2 2 2 4 84%	6195
August	1st to 10th	Day	1 1 1 1 1 2 1 3 4 86%	5346
	11th to 20th	Day	1 1 1 1 1 1 3 92%	5364
	21st to end	Day	2 1 1 1 2 2 2 2 3 83%	5984
September	1st to 10th	Day	2 1 1 2 2 2 2 2 3 5% 78%	2400
	11th to 20th	Day	1 1 1 1 2 1 1 2 3 4 82%	2205
	21st to end	Day	2 1 1 2 2 2 3 2 3 3 80%	2200
October	1st to 10th	Day	2 2 1 2 2 2 1 2 2 4 81%	3600
	11th to 20th	Day	3 2 1 2 1 1 1 2 3 6% 78%	3495
	21st to end	Day	3 3 1 2 2 2 2 2 4 5% 72%	3375
November	1st to 10th	Day	3 3 1 2 2 3 3 2 4 5% 72%	1854
	11th to 20th	Day	5% 4 1 3 2 2 3 3 4 5% 67%	1890
	21st to end	Day	6% 3 1 4 3 3 4 3 5% 7% 60%	1890
December	1st to 10th	Day	4 3 1 3 2 2 3 2 5% 8% 67%	3040
	11th to 20th	Day	9% 4 1 4 3 3 5% 4 5% 8% 54%	3499
	21st to end	Day	9% 5% 2 5% 4 4 4 4 5% 9% 50%	4609
January	1st to 10th	Night	6% 4 1 3 2 2 2 2 3 5% 70%	5997
	11th to 20th	Night	4 5% 1 4 2 3 2 2 4 6% 67%	5765
	21st to end	Night	13% 6% 2 5% 3 3 3 3 5% 7% 51%	5984
February	1st to 10th	Night	11% 7% 1 4 3 2 2 3 4 5% 58%	2400
	11th to 20th	Night	9% 6% 2 4 3 2 2 3 3 5% 61%	2338
	21st to end	Night	12% 9% 2 4 3 3 3 3 5% 5% 54%	1980
March	1st to 10th	Night	16% 15% 2 5% 3 3 2 3 4 5% 42%	5376
	11th to 20th	Night	8% 10% 3 6% 3 4 4 3 4 7% 48%	5268
	21st to end	Night	10% 12% 2 6% 3 3 3 2 4 5% 50%	5121
April	1st to 10th	Night	8% 6% 2 4 3 4 3 3 5% 7% 56%	2000
	11th to 20th	Night	7% 7% 2 3 3 2 3 3 4 5% 61%	1979
	21st to end	Night	7% 8% 2 4 3 2 2 3 3 6% 61%	2000
May	1st to 10th	Night	9% 6% 2 3 2 2 2 2 3 4 64%	4238
	11th to 20th	Night	3 5% 2 4 2 2 2 2 3 5% 68%	4074
	21st to end	Night	2 6% 2 2 2 3 2 3 3 5% 70%	4474
June	1st to 10th	Night	1 5% 2 2 1 2 2 2 4 6% 73%	1800
	11th to 20th	Night	2 2 1 2 1 1 2 2 3 4 80%	1743
	21st to end	Night	1 1 1 1 1 1 1 2 2 3 86%	1600
July	1st to 10th	Night	1 1 1 2 93%	3640
	11th to 20th	Night	1 1 1 1 1 1 2 2 4 87%	4052
	21st to end	Night	1 1 1 1 1 2 1 1 2 3 86%	4671
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	4486
	11th to 20th	Night	1 1 2 3 92%	4506
	21st to end	Night	1 1 2 1 1 2 2 3 3 84%	5015
September	1st to 10th	Night	2 1 1 3 2 2 2 2 3 4 80%	2000
	11th to 20th	Night	2 2 1 2 1 2 1 2 2 4 82%	2193
	21st to end	Night	3 3 1 1 1 1 2 2 3 4 81%	2200
October	1st to 10th	Night	3 2 1 1 1 1 2 1 2 3 84%	3600
	11th to 20th	Night	3 4 2 1 2 2 2 4 9% 70%	3658
	21st to end	Night	3 3 1 3 2 3 2 2 3 4 75%	4427
November	1st to 10th	Night	4 3 1 2 2 2 3 3 3 5% 71%	2665
	11th to 20th	Night	6% 4 2 3 1 2 3 2 4 6% 67%	2730
	21st to end	Night	7% 5% 1 3 3 3 4 3 5% 6% 59%	2730
December	1st to 10th	Night	5% 4 1 3 2 2 3 3 4 6% 68%	4370
	11th to 20th	Night	9% 5% 2 4 3 3 4 4 6% 8% 52%	4941
	21st to end	Night	7% 6% 2 5% 3 3 4 4 6% 11% 49%	6611

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	17% 3 4 19% 34% 23%	4189
	11th to 20th	Day	15% 3 10% 17% 20% 35%	4339
	21st to end	Day	27% 5% 7% 11% 25% 25%	5016
February	1st to 10th	Day	26% 3 6% 12% 25% 27%	2000
	11th to 20th	Day	27% 3 5% 16% 26% 23%	1960
	21st to end	Day	30% 4 8% 15% 20% 22%	1650
March	1st to 10th	Day	31% 6% 10% 14% 22% 18%	4518
	11th to 20th	Day	28% 5% 10% 22% 21% 13%	4731
	21st to end	Day	26% 6% 12% 27% 19% 10%	5837
April	1st to 10th	Day	24% 4 10% 21% 26% 16%	2400
	11th to 20th	Day	20% 4 8% 19% 28% 22%	2372
	21st to end	Day	19% 4 10% 19% 34% 14%	2400
May	1st to 10th	Day	18% 3 8% 18% 30% 22%	5745
	11th to 20th	Day	13% 4 10% 22% 36% 15%	5844
	21st to end	Day	10% 4 8% 18% 44% 16%	6424
June	1st to 10th	Day	13% 3 9% 21% 33% 21%	2600
	11th to 20th	Day	8% 2 5% 12% 28% 45%	2647
	21st to end	Day	8% 1 4 7% 24% 56%	2800
July	1st to 10th	Day	4 1 2 4 13% 77%	6251
	11th to 20th	Day	7% 2 4 8% 22% 57%	5837
	21st to end	Day	6% 2 4 13% 24% 52%	6016
August	1st to 10th	Day	7% 2 3 10% 37% 42%	3890
	11th to 20th	Day	4 1 3 12% 25% 55%	3880
	21st to end	Day	5% 1 3 6% 24% 61%	4311
September	1st to 10th	Day	10% 2 5% 9% 29% 45%	2400
	11th to 20th	Day	7% 1 4 11% 31% 46%	2205
	21st to end	Day	10% 2 4 12% 29% 43%	2200
October	1st to 10th	Day	10% 2 4 9% 18% 57%	3600
	11th to 20th	Day	8% 1 5% 8% 33% 45%	3495
	21st to end	Day	14% 2 4 7% 30% 44%	3375
November	1st to 10th	Day	15% 2 5% 13% 31% 34%	1854
	11th to 20th	Day	19% 3 6% 12% 23% 37%	1890
	21st to end	Day	23% 3 8% 13% 22% 31%	1890
December	1st to 10th	Day	16% 3 6% 12% 26% 37%	3040
	11th to 20th	Day	26% 6% 9% 13% 29% 18%	3498
	21st to end	Day	29% 6% 10% 21% 23% 12%	4609
January	1st to 10th	Night	19% 3 5% 15% 34% 25%	5997
	11th to 20th	Night	17% 4 9% 18% 19% 32%	5765
	21st to end	Night	31% 5% 9% 9% 23% 24%	5984
February	1st to 10th	Night	29% 3 8% 15% 20% 25%	2400
	11th to 20th	Night	25% 4 8% 15% 24% 24%	2339
	21st to end	Night	31% 5% 6% 14% 20% 24%	1980
March	1st to 10th	Night	40% 5% 7% 16% 15% 16%	5376
	11th to 20th	Night	30% 9% 10% 23% 19% 9%	5268
	21st to end	Night	30% 7% 13% 20% 21% 8%	5121
April	1st to 10th	Night	26% 5% 11% 22% 20% 17%	2000
	11th to 20th	Night	22% 5% 8% 20% 26% 19%	1979
	21st to end	Night	23% 3 8% 21% 33% 12%	2000
May	1st to 10th	Night	22% 3 8% 22% 28% 16%	4235
	11th to 20th	Night	16% 3 8% 26% 34% 12%	4075
	21st to end	Night	11% 3 11% 20% 41% 13%	4474
June	1st to 10th	Night	12% 3 8% 22% 33% 23%	1800
	11th to 20th	Night	8% 2 6% 14% 29% 42%	1743
	21st to end	Night	6% 2 4 7% 24% 57%	1600
July	1st to 10th	Night	2 1 1 4 17% 75%	3600
	11th to 20th	Night	4 2 3 11% 23% 57%	4000
	21st to end	Night	6% 2 3 14% 32% 43%	4539
August	1st to 10th	Night	5% 2 3 13% 37% 41%	3256
	11th to 20th	Night	3 1 3 13% 28% 52%	3256
	21st to end	Night	4 1 2 6% 26% 60%	3608
September	1st to 10th	Night	10% 2 3 10% 29% 44%	2000
	11th to 20th	Night	8% 1 4 11% 33% 42%	2193
	21st to end	Night	10% 2 4 13% 28% 43%	2200
October	1st to 10th	Night	9% 2 3 9% 19% 58%	3600
	11th to 20th	Night	12% 2 4 10% 35% 37%	3658
	21st to end	Night	15% 2 4 7% 26% 46%	4427
November	1st to 10th	Night	17% 2 6% 12% 28% 35%	2665
	11th to 20th	Night	19% 2 5% 13% 23% 37%	2730
	21st to end	Night	25% 3 8% 14% 18% 32%	2730
December	1st to 10th	Night	18% 3 5% 9% 22% 42%	4370
	11th to 20th	Night	26% 5% 9% 14% 26% 21%	4941
	21st to end	Night	26% 6% 10% 24% 25% 10%	6611

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% 11% 27% 32% 9% 1	330
	11th to 20th	Day	12% 12% 28% 34% 13% 1	399
	21st to end	Day	12% 18% 29% 29% 11% 1	550
February	1st to 10th	Day	15% 16% 32% 23% 10% 3	500
	11th to 20th	Day	14% 16% 35% 27% 8% 1	495
	21st to end	Day	14% 19% 33% 27% 7%	414
March	1st to 10th	Day	19% 18% 31% 22% 8% 1	500
	11th to 20th	Day	17% 14% 34% 25% 7% 2	500
	21st to end	Day	13% 21% 34% 24% 7% 1	550
April	1st to 10th	Day	11% 16% 32% 27% 13% 2	500
	11th to 20th	Day	7% 23% 35% 22% 10% 3	492
	21st to end	Day	7% 19% 30% 28% 9% 7%	500
May	1st to 10th	Day	9% 15% 34% 28% 11% 3	503
	11th to 20th	Day	8% 14% 33% 30% 12% 4	528
	21st to end	Day	9% 10% 33% 32% 13% 4	573
June	1st to 10th	Day	3 9% 27% 35% 19% 7%	530
	11th to 20th	Day	1 8% 34% 35% 17% 6%	530
	21st to end	Day	2 7% 26% 40% 17% 8%	530
July	1st to 10th	Day	2 2 18% 48% 20% 10%	526
	11th to 20th	Day	1 1 19% 46% 20% 13%	524
	21st to end	Day	1 3 18% 44% 20% 14%	575
August	1st to 10th	Day	2 2 26% 43% 17% 10%	527
	11th to 20th	Day	1 2 20% 44% 21% 13%	517
	21st to end	Day	2 3 21% 43% 20% 11%	583
September	1st to 10th	Day	2 3 28% 41% 20% 6%	530
	11th to 20th	Day	3 7% 29% 42% 16% 3	530
	21st to end	Day	4 9% 31% 33% 19% 4	530
October	1st to 10th	Day	3 9% 32% 35% 19% 2	530
	11th to 20th	Day	4 10% 29% 37% 17% 2	530
	21st to end	Day	4 9% 29% 37% 19% 3	475
November	1st to 10th	Day	5% 11% 21% 43% 19% 2	330
	11th to 20th	Day	8% 12% 25% 36% 15% 4	330
	21st to end	Day	7% 12% 28% 35% 14% 3	330
December	1st to 10th	Day	8% 11% 29% 35% 17% 1	330
	11th to 20th	Day	8% 8% 29% 35% 18% 3	330
	21st to end	Day	17% 14% 31% 25% 10% 2	363
January	1st to 10th	Night	13% 9% 22% 32% 18% 6%	840
	11th to 20th	Night	9% 7% 24% 38% 18% 4	824
	21st to end	Night	13% 7% 26% 34% 16% 4	880
February	1st to 10th	Night	13% 8% 23% 33% 18% 5%	800
	11th to 20th	Night	11% 6% 25% 35% 18% 4	784
	21st to end	Night	14% 9% 25% 33% 17% 3	660
March	1st to 10th	Night	14% 8% 23% 34% 17% 3	800
	11th to 20th	Night	12% 6% 21% 33% 21% 7%	800
	21st to end	Night	12% 7% 24% 36% 19% 3	880
April	1st to 10th	Night	9% 6% 18% 40% 23% 4	800
	11th to 20th	Night	8% 6% 22% 37% 23% 3	791
	21st to end	Night	8% 6% 19% 36% 25% 7%	800
May	1st to 10th	Night	8% 4 19% 39% 26% 4	660
	11th to 20th	Night	8% 3 18% 33% 32% 6%	591
	21st to end	Night	5% 4 18% 40% 28% 5%	650
June	1st to 10th	Night	2 2 14% 40% 38% 4	600
	11th to 20th	Night	2 1 12% 43% 37% 4	543
	21st to end	Night	2 4 35% 52% 7%	400
July	1st to 10th	Night	1 1 27% 58% 13%	390
	11th to 20th	Night	2 3 33% 53% 8%	562
	21st to end	Night	2 1 4 29% 53% 12%	742
August	1st to 10th	Night	2 1 5% 32% 45% 15%	796
	11th to 20th	Night	1 3 25% 54% 17%	792
	21st to end	Night	2 3 27% 51% 17%	880
September	1st to 10th	Night	2 1 5% 30% 48% 15%	800
	11th to 20th	Night	3 1 8% 33% 43% 12%	800
	21st to end	Night	3 1 13% 33% 40% 10%	800
October	1st to 10th	Night	5% 1 13% 45% 31% 5%	800
	11th to 20th	Night	5% 4 14% 36% 33% 8%	800
	21st to end	Night	6% 4 19% 35% 25% 11%	880
November	1st to 10th	Night	5% 5% 14% 41% 27% 8%	820
	11th to 20th	Night	7% 4 20% 37% 24% 8%	840
	21st to end	Night	9% 4 22% 36% 23% 6%	840
December	1st to 10th	Night	10% 7% 21% 36% 21% 5%	840
	11th to 20th	Night	12% 4 25% 36% 19% 4	840
	21st to end	Night	12% 10% 27% 31% 16% 4	924

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