Casa Becca del Norte Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Casa Becca del Norte Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	10% 9% 3 7% 4 4 4 4 5% 7% 43%	130676
	Night	11% 10% 3 7% 4 4 4 3 5% 7% 41%	128725
Transparency	Day	40% 7% 12% 22% 15% 5%	130402
	Night	43% 6% 12% 21% 13% 4	128576
Seeing	Day	17% 12% 26% 27% 14% 4	16186
	Night	19% 5% 16% 36% 22% 3	25290

By Month

Percentage Cloud Cover NW (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	17% 20% 8% 14% 5% 4 4 4 4 6% 14%	3639
	11th to 20th	Day	13% 10% 3 8% 6% 4 4 3 4 7% 38%	3616
	21st to end	Day	21% 17% 3 7% 5% 4 4 2 3 5% 29%	4004
February	1st to 10th	Day	14% 15% 5% 11% 7% 5% 4 4 6% 6% 25%	1600
	11th to 20th	Day	13% 15% 4 9% 5% 5% 5% 4 6% 7% 28%	1845
	21st to end	Day	16% 15% 4 8% 5% 4 5% 3 6% 5% 28%	1650
March	1st to 10th	Day	8% 13% 3 7% 6% 6% 5% 4 6% 8% 33%	4518
	11th to 20th	Day	11% 12% 3 6% 4 4 4 3 4 7% 43%	4661
	21st to end	Day	14% 14% 4 9% 7% 6% 5% 4 6% 8% 23%	5962
April	1st to 10th	Day	14% 11% 4 10% 7% 6% 6% 5% 6% 7% 25%	2400
	11th to 20th	Day	11% 10% 3 9% 6% 5% 5% 5% 6% 7% 34%	2438
	21st to end	Day	11% 10% 4 8% 6% 5% 5% 4 5% 8% 34%	2600
May	1st to 10th	Day	6% 6% 3 7% 5% 5% 5% 5% 7% 8% 43%	6306
	11th to 20th	Day	8% 8% 3 9% 6% 6% 6% 5% 7% 10% 31%	6277
	21st to end	Day	9% 7% 3 7% 5% 5% 5% 5% 7% 9% 38%	6995
June	1st to 10th	Day	17% 7% 3 6% 5% 4 4 4 5% 6% 39%	3000
	11th to 20th	Day	12% 8% 3 8% 7% 6% 5% 4 6% 7% 35%	3000
	21st to end	Day	7% 5% 2 5% 4 5% 5% 4 5% 8% 51%	3000
July	1st to 10th	Day	3 3 1 3 3 2 3 3 5% 6% 69%	6459
	11th to 20th	Day	2 2 1 2 2 2 3 2 3 4 77%	6320
	21st to end	Day	4 2 1 2 2 2 3 3 4 6% 70%	6874
August	1st to 10th	Day	3 3 1 3 2 3 4 3 5% 7% 66%	4541
	11th to 20th	Day	2 3 1 4 3 3 3 3 5% 8% 64%	4326
	21st to end	Day	4 3 2 4 3 4 5% 3 5% 7% 60%	4389
September	1st to 10th	Day	4 3 1 4 3 2 3 3 5% 5% 66%	2400
	11th to 20th	Day	7% 5% 1 5% 5% 5% 4 4 6% 8% 50%	2304
	21st to end	Day	9% 6% 2 6% 4 4 4 4 6% 7% 49%	2200
October	1st to 10th	Day	7% 5% 2 5% 4 3 3 3 5% 8% 54%	3423
	11th to 20th	Day	10% 6% 2 5% 5% 5% 5% 5% 7% 8% 43%	3070
	21st to end	Day	16% 8% 2 6% 3 4 5% 4 5% 7% 41%	3200
November	1st to 10th	Day	22% 13% 4 7% 4 4 4 3 5% 6% 27%	1854
	11th to 20th	Day	22% 14% 4 9% 5% 4 5% 3 3 5% 26%	1739
	21st to end	Day	16% 21% 6% 10% 5% 6% 3 3 4 5% 22%	1512
December	1st to 10th	Day	13% 11% 4 9% 4 4 4 4 5% 9% 33%	2304
	11th to 20th	Day	21% 22% 6% 10% 6% 3 3 3 3 6% 17%	2642
	21st to end	Day	19% 25% 8% 11% 4 4 4 3 5% 5% 12%	3608
January	1st to 10th	Night	22% 20% 6% 11% 5% 5% 4 3 3 6% 17%	6306
	11th to 20th	Night	12% 12% 2 8% 5% 4 4 3 6% 8% 36%	6282
	21st to end	Night	18% 18% 5% 7% 4 3 4 3 4 4 31%	6952
February	1st to 10th	Night	15% 17% 4 9% 5% 4 4 3 4 6% 28%	2800
	11th to 20th	Night	12% 14% 4 10% 5% 5% 4 4 5% 6% 32%	2438
	21st to end	Night	12% 11% 4 7% 6% 5% 5% 4 6% 8% 33%	1980
March	1st to 10th	Night	6% 10% 3 7% 5% 5% 5% 4 6% 7% 41%	5376
	11th to 20th	Night	11% 11% 2 7% 5% 4 3 2 4 5% 45%	5337
	21st to end	Night	13% 11% 3 9% 6% 6% 7% 5% 8% 8% 24%	5011
April	1st to 10th	Night	10% 9% 3 8% 6% 5% 6% 5% 6% 8% 33%	2000
	11th to 20th	Night	10% 8% 3 7% 6% 5% 5% 3 5% 7% 41%	1912
	21st to end	Night	8% 9% 3 7% 5% 4 5% 5% 7% 7% 40%	1800
May	1st to 10th	Night	7% 6% 2 5% 4 4 6% 4 7% 8% 47%	3650
	11th to 20th	Night	9% 8% 3 7% 5% 5% 5% 4 6% 8% 40%	3616
	21st to end	Night	8% 5% 2 7% 5% 4 5% 5% 6% 9% 44%	3862
June	1st to 10th	Night	10% 7% 2 5% 4 5% 4 4 7% 9% 43%	1400
	11th to 20th	Night	9% 6% 2 7% 5% 6% 5% 6% 8% 8% 40%	1400
	21st to end	Night	6% 3 1 5% 4 3 5% 5% 6% 7% 56%	1400
July	1st to 10th	Night	3 2 2 1 2 2 2 4 6% 74%	3496
	11th to 20th	Night	3 1 1 2 1 2 2 2 3 4 79%	3640
	21st to end	Night	2 2 1 3 2 2 2 2 4 7% 74%	3969
August	1st to 10th	Night	3 2 1 3 2 2 3 2 4 7% 70%	2609
	11th to 20th	Night	2 2 1 4 3 3 4 4 5% 9% 63%	2725
	21st to end	Night	3 3 1 4 4 4 4 4 5% 7% 61%	3527
September	1st to 10th	Night	3 2 1 4 2 3 4 3 5% 7% 65%	2000
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 5% 9% 53%	2095
	21st to end	Night	9% 5% 2 5% 4 3 4 3 5% 7% 54%	2200
October	1st to 10th	Night	6% 4 2 4 3 3 3 3 5% 8% 59%	3774
	11th to 20th	Night	11% 5% 2 5% 3 3 4 5% 6% 8% 48%	4080
	21st to end	Night	14% 7% 2 5% 4 4 5% 4 5% 7% 43%	4600
November	1st to 10th	Night	20% 12% 3 7% 5% 5% 5% 4 6% 7% 27%	2665
	11th to 20th	Night	20% 13% 3 8% 5% 4 4 4 5% 6% 27%	2877
	21st to end	Night	19% 14% 5% 11% 6% 5% 4 3 4 5% 25%	3085
December	1st to 10th	Night	10% 10% 3 9% 5% 5% 4 3 4 9% 36%	4921
	11th to 20th	Night	19% 20% 6% 12% 6% 4 4 3 4 5% 16%	5550
	21st to end	Night	21% 20% 6% 13% 6% 4 4 3 4 5% 15%	7390

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	71% 8% 7% 11% 2	3639
	11th to 20th	Day	45% 7% 20% 20% 6% 2	3640
	21st to end	Day	60% 5% 11% 18% 7%	4004
February	1st to 10th	Day	59% 9% 13% 10% 6% 2	1600
	11th to 20th	Day	56% 7% 14% 15% 7% 1	1854
	21st to end	Day	56% 7% 12% 15% 9% 1	1650
March	1st to 10th	Day	47% 8% 10% 17% 17% 1	4518
	11th to 20th	Day	43% 5% 17% 26% 8%	4661
	21st to end	Day	55% 12% 14% 14% 5%	5962
April	1st to 10th	Day	56% 7% 14% 17% 5% 1	2400
	11th to 20th	Day	48% 6% 14% 19% 12% 2	2438
	21st to end	Day	47% 6% 13% 21% 10% 2	2600
May	1st to 10th	Day	36% 6% 10% 26% 22% 1	6306
	11th to 20th	Day	43% 10% 12% 25% 10% 1	6280
	21st to end	Day	38% 10% 16% 27% 9%	6996
June	1st to 10th	Day	45% 6% 9% 21% 17% 3	3000
	11th to 20th	Day	47% 6% 11% 20% 14% 3	3000
	21st to end	Day	31% 6% 12% 27% 20% 4	3000
July	1st to 10th	Day	17% 3 8% 21% 28% 22%	6390
	11th to 20th	Day	13% 3 6% 24% 26% 28%	6245
	21st to end	Day	16% 3 6% 30% 29% 16%	6681
August	1st to 10th	Day	18% 5% 7% 29% 30% 12%	4524
	11th to 20th	Day	18% 4 10% 41% 23% 4	4368
	21st to end	Day	24% 5% 12% 32% 24% 4	4388
September	1st to 10th	Day	20% 4 10% 32% 25% 9%	2400
	11th to 20th	Day	30% 6% 12% 26% 21% 4	2304
	21st to end	Day	34% 6% 18% 27% 12% 3	2200
October	1st to 10th	Day	30% 7% 16% 36% 12%	3423
	11th to 20th	Day	38% 8% 15% 21% 17%	3070
	21st to end	Day	43% 8% 17% 20% 11% 1	3200
November	1st to 10th	Day	58% 8% 14% 15% 4 1	1854
	11th to 20th	Day	61% 8% 12% 13% 4 2	1739
	21st to end	Day	65% 7% 11% 13% 4	1512
December	1st to 10th	Day	49% 10% 20% 17% 4	2304
	11th to 20th	Day	71% 6% 11% 8% 3	2644
	21st to end	Day	73% 7% 10% 7% 2	3608
January	1st to 10th	Night	69% 7% 8% 13% 2	6306
	11th to 20th	Night	45% 8% 17% 19% 10% 1	6320
	21st to end	Night	57% 7% 12% 17% 6%	6952
February	1st to 10th	Night	57% 6% 14% 13% 7% 2	2800
	11th to 20th	Night	52% 7% 15% 16% 9% 1	2440
	21st to end	Night	50% 6% 15% 16% 12% 1	1980
March	1st to 10th	Night	40% 7% 14% 21% 18% 1	5376
	11th to 20th	Night	42% 4 15% 27% 12%	5338
	21st to end	Night	53% 11% 13% 18% 5%	5011
April	1st to 10th	Night	47% 6% 15% 23% 9% 1	2000
	11th to 20th	Night	44% 4 10% 24% 16% 2	1912
	21st to end	Night	40% 6% 12% 26% 14% 2	1800
May	1st to 10th	Night	32% 6% 9% 25% 25% 2	3647
	11th to 20th	Night	37% 8% 12% 28% 14% 1	3616
	21st to end	Night	32% 11% 15% 32% 9% 1	3862
June	1st to 10th	Night	37% 6% 9% 24% 19% 5%	1400
	11th to 20th	Night	38% 7% 13% 20% 18% 4	1400
	21st to end	Night	25% 6% 10% 28% 25% 7%	1400
July	1st to 10th	Night	13% 3 8% 20% 32% 24%	3456
	11th to 20th	Night	11% 2 5% 27% 21% 35%	3592
	21st to end	Night	12% 3 4 29% 31% 21%	3857
August	1st to 10th	Night	16% 3 6% 27% 28% 20%	2601
	11th to 20th	Night	19% 5% 6% 36% 29% 6%	2749
	21st to end	Night	22% 5% 12% 28% 27% 6%	3527
September	1st to 10th	Night	18% 5% 6% 30% 29% 12%	2000
	11th to 20th	Night	29% 6% 11% 25% 24% 6%	2095
	21st to end	Night	30% 4 15% 30% 16% 5%	2200
October	1st to 10th	Night	25% 6% 13% 40% 17% 1	3774
	11th to 20th	Night	34% 8% 13% 23% 21% 1	4080
	21st to end	Night	41% 7% 15% 27% 9% 1	4600
November	1st to 10th	Night	56% 8% 15% 15% 5% 1	2665
	11th to 20th	Night	58% 6% 16% 13% 5% 2	2877
	21st to end	Night	62% 7% 13% 13% 5%	3085
December	1st to 10th	Night	46% 10% 17% 23% 4	4921
	11th to 20th	Night	70% 7% 12% 8% 2	5547
	21st to end	Night	70% 7% 10% 9% 3	7390

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% 11% 18% 27% 15% 3	200
	11th to 20th	Day	27% 6% 20% 32% 12% 5%	200
	21st to end	Day	32% 3 24% 27% 11% 2	220
February	1st to 10th	Day	25% 11% 31% 26% 7% 2	200
	11th to 20th	Day	25% 13% 28% 25% 8% 2	440
	21st to end	Day	26% 10% 30% 24% 10%	414
March	1st to 10th	Day	24% 17% 27% 19% 10% 2	500
	11th to 20th	Day	24% 12% 23% 24% 16% 2	500
	21st to end	Day	24% 19% 26% 21% 9% 1	550
April	1st to 10th	Day	23% 14% 31% 22% 7% 2	500
	11th to 20th	Day	22% 22% 27% 18% 9% 3	492
	21st to end	Day	17% 16% 27% 26% 11% 3	500
May	1st to 10th	Day	16% 14% 30% 26% 12% 3	503
	11th to 20th	Day	17% 14% 27% 25% 12% 4	528
	21st to end	Day	16% 10% 25% 26% 19% 5%	613
June	1st to 10th	Day	20% 10% 25% 26% 16% 4	730
	11th to 20th	Day	16% 17% 26% 28% 11% 2	730
	21st to end	Day	9% 9% 26% 31% 19% 5%	730
July	1st to 10th	Day	5% 10% 33% 32% 16% 4	587
	11th to 20th	Day	4 14% 35% 30% 13% 4	524
	21st to end	Day	6% 7% 33% 34% 16% 4	575
August	1st to 10th	Day	7% 8% 27% 36% 17% 6%	527
	11th to 20th	Day	5% 6% 26% 33% 22% 7%	517
	21st to end	Day	9% 11% 24% 32% 17% 7%	583
September	1st to 10th	Day	7% 7% 29% 27% 23% 7%	530
	11th to 20th	Day	11% 11% 24% 32% 16% 5%	530
	21st to end	Day	16% 13% 27% 25% 14% 4	530
October	1st to 10th	Day	13% 12% 28% 29% 14% 4	530
	11th to 20th	Day	22% 10% 22% 27% 14% 5%	416
	21st to end	Day	22% 11% 27% 27% 11% 2	363
November	1st to 10th	Day	30% 9% 19% 29% 12% 2	330
	11th to 20th	Day	34% 10% 22% 17% 16% 2	250
	21st to end	Day	31% 11% 22% 20% 15% 2	220
December	1st to 10th	Day	26% 8% 21% 25% 13% 6%	204
	11th to 20th	Day	30% 13% 20% 25% 11% 2	200
	21st to end	Day	31% 7% 19% 28% 15%	220
January	1st to 10th	Night	36% 6% 17% 27% 12% 2	800
	11th to 20th	Night	29% 4 18% 33% 13% 3	800
	21st to end	Night	23% 4 18% 37% 16% 2	880
February	1st to 10th	Night	24% 6% 20% 35% 12% 2	800
	11th to 20th	Night	22% 5% 22% 32% 17% 2	784
	21st to end	Night	21% 6% 19% 33% 19% 2	660
March	1st to 10th	Night	19% 7% 20% 35% 19% 2	800
	11th to 20th	Night	21% 6% 18% 33% 21% 1	800
	21st to end	Night	20% 5% 20% 37% 16% 2	880
April	1st to 10th	Night	15% 5% 17% 40% 21% 3	800
	11th to 20th	Night	14% 7% 25% 35% 17% 2	724
	21st to end	Night	13% 7% 19% 37% 22% 2	600
May	1st to 10th	Night	12% 6% 16% 36% 27% 3	410
	11th to 20th	Night	11% 7% 16% 29% 31% 6%	394
	21st to end	Night	15% 4 10% 37% 28% 5%	433
June	1st to 10th	Night	14% 4 11% 45% 24% 3	400
	11th to 20th	Night	12% 6% 17% 40% 23% 3	400
	21st to end	Night	8% 3 12% 42% 33% 2	400
July	1st to 10th	Night	4 2 9% 42% 39% 3	390
	11th to 20th	Night	2 3 13% 47% 34% 1	388
	21st to end	Night	2 3 7% 42% 42% 3	438
August	1st to 10th	Night	4 4 10% 33% 42% 7%	398
	11th to 20th	Night	3 3 9% 35% 42% 7%	479
	21st to end	Night	4 5% 11% 38% 38% 4	825
September	1st to 10th	Night	5% 3 12% 40% 34% 6%	800
	11th to 20th	Night	8% 3 11% 33% 37% 8%	800
	21st to end	Night	13% 5% 14% 38% 26% 6%	800
October	1st to 10th	Night	16% 4 13% 38% 27% 2	800
	11th to 20th	Night	18% 5% 14% 35% 26% 2	800
	21st to end	Night	23% 3 20% 36% 15% 3	880
November	1st to 10th	Night	29% 5% 14% 38% 13% 1	820
	11th to 20th	Night	31% 7% 18% 30% 13% 1	840
	21st to end	Night	29% 5% 17% 31% 17% 2	985
December	1st to 10th	Night	24% 4 15% 36% 18% 2	1011
	11th to 20th	Night	32% 5% 17% 35% 11% 1	1000
	21st to end	Night	33% 5% 18% 34% 10% 1	1071

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