Four Corners Algonquin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Four Corners Algonquin Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	16% 12% 4 9% 5% 5% 4 3 5% 6% 30%	127736
	Night	15% 14% 5% 11% 6% 5% 4 3 5% 6% 28%	122258
Transparency	Day	54% 6% 10% 19% 10% 1	131291
	Night	56% 6% 10% 17% 9% 1	126793
Seeing	Day	24% 18% 32% 18% 7% 2	15987
	Night	22% 16% 31% 21% 8% 1	24792

By Month

Percentage Cloud Cover NE (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	11% 17% 7% 16% 9% 8% 6% 3 5% 4 15%	3597
	11th to 20th	Day	10% 19% 7% 18% 9% 6% 5% 3 4 4 13%	3570
	21st to end	Day	12% 19% 11% 15% 7% 4 4 3 3 4 18%	4442
February	1st to 10th	Day	10% 18% 7% 15% 11% 5% 4 4 4 4 19%	1800
	11th to 20th	Day	11% 15% 7% 14% 8% 6% 5% 3 4 6% 23%	1753
	21st to end	Day	14% 18% 5% 10% 8% 5% 5% 4 4 5% 20%	1537
March	1st to 10th	Day	16% 16% 5% 12% 6% 4 3 3 5% 5% 24%	4798
	11th to 20th	Day	10% 15% 6% 12% 6% 5% 5% 4 5% 7% 25%	4980
	21st to end	Day	15% 15% 6% 10% 4 4 3 2 4 4 34%	5543
April	1st to 10th	Day	20% 15% 5% 8% 4 4 4 3 4 6% 28%	2400
	11th to 20th	Day	21% 13% 3 7% 4 3 4 2 4 5% 33%	2418
	21st to end	Day	21% 13% 4 8% 4 4 3 3 4 4 33%	2600
May	1st to 10th	Day	20% 10% 4 8% 5% 5% 4 3 5% 6% 31%	6196
	11th to 20th	Day	16% 11% 3 7% 4 4 4 3 4 7% 36%	6318
	21st to end	Day	21% 10% 3 7% 5% 4 4 3 4 6% 33%	6949
June	1st to 10th	Day	21% 9% 3 7% 5% 6% 4 4 5% 7% 30%	2800
	11th to 20th	Day	15% 7% 3 6% 4 5% 5% 4 6% 8% 36%	2859
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 5% 5% 7% 32%	3000
July	1st to 10th	Day	16% 6% 2 7% 5% 5% 5% 5% 6% 9% 34%	6617
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 5% 7% 44%	6360
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	6911
August	1st to 10th	Day	15% 7% 3 8% 6% 5% 5% 5% 6% 8% 32%	4234
	11th to 20th	Day	18% 8% 2 6% 4 5% 5% 4 7% 7% 34%	4174
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	4451
September	1st to 10th	Day	16% 8% 3 6% 4 4 4 3 6% 8% 38%	2400
	11th to 20th	Day	14% 7% 3 6% 4 3 4 3 5% 7% 43%	2375
	21st to end	Day	20% 9% 4 6% 6% 4 4 4 5% 6% 33%	2102
October	1st to 10th	Day	19% 11% 3 7% 5% 3 5% 3 5% 7% 34%	2000
	11th to 20th	Day	22% 16% 4 8% 6% 5% 4 3 5% 5% 23%	2000
	21st to end	Day	26% 17% 5% 9% 5% 4 3 2 4 6% 19%	2160
November	1st to 10th	Day	16% 16% 5% 9% 5% 4 3 3 5% 7% 27%	1646
	11th to 20th	Day	16% 17% 7% 12% 6% 5% 5% 3 4 6% 21%	1600
	21st to end	Day	18% 22% 8% 13% 6% 5% 4 3 3 4 14%	1600
December	1st to 10th	Day	15% 25% 10% 15% 6% 5% 4 3 4 3 10%	2600
	11th to 20th	Day	15% 22% 8% 14% 7% 4 3 2 3 3 19%	2986
	21st to end	Day	19% 20% 9% 14% 6% 3 3 2 3 4 16%	3960
January	1st to 10th	Night	12% 18% 9% 17% 9% 7% 4 3 4 4 14%	6349
	11th to 20th	Night	8% 16% 6% 21% 12% 8% 5% 3 4 4 12%	6315
	21st to end	Night	11% 17% 9% 19% 6% 7% 6% 3 4 4 15%	6513
February	1st to 10th	Night	10% 17% 7% 13% 8% 6% 5% 4 5% 6% 18%	2600
	11th to 20th	Night	11% 16% 6% 14% 7% 6% 4 3 5% 5% 24%	2533
	21st to end	Night	10% 18% 5% 12% 8% 6% 4 3 5% 6% 22%	2092
March	1st to 10th	Night	15% 15% 5% 13% 5% 5% 4 3 4 5% 26%	5025
	11th to 20th	Night	9% 14% 6% 11% 5% 5% 6% 4 7% 8% 26%	4979
	21st to end	Night	16% 13% 5% 10% 6% 4 4 4 5% 5% 29%	5387
April	1st to 10th	Night	19% 16% 4 10% 4 2 3 3 4 5% 30%	2000
	11th to 20th	Night	18% 11% 5% 6% 4 4 3 3 4 5% 36%	1949
	21st to end	Night	19% 11% 4 9% 5% 4 4 3 4 6% 30%	1800
May	1st to 10th	Night	20% 10% 2 6% 4 4 4 4 6% 7% 32%	3743
	11th to 20th	Night	15% 8% 2 5% 4 4 4 3 5% 7% 42%	3575
	21st to end	Night	19% 9% 3 8% 5% 4 4 3 5% 6% 36%	3926
June	1st to 10th	Night	17% 9% 2 8% 4 5% 5% 3 7% 6% 35%	1600
	11th to 20th	Night	14% 7% 2 6% 4 3 4 4 5% 8% 44%	1532
	21st to end	Night	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	1400
July	1st to 10th	Night	15% 8% 2 6% 5% 4 5% 4 5% 8% 37%	3335
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 46%	3600
	21st to end	Night	10% 6% 2 5% 3 3 4 4 5% 7% 50%	3926
August	1st to 10th	Night	11% 5% 2 5% 3 4 4 4 6% 9% 45%	2914
	11th to 20th	Night	16% 8% 3 5% 4 4 4 3 4 6% 42%	2875
	21st to end	Night	15% 4 2 5% 3 4 4 3 7% 9% 45%	3412
September	1st to 10th	Night	11% 7% 2 5% 5% 4 4 3 6% 8% 46%	2000
	11th to 20th	Night	13% 8% 2 6% 3 3 3 3 5% 6% 49%	2022
	21st to end	Night	18% 9% 3 6% 4 3 4 3 5% 6% 39%	2297
October	1st to 10th	Night	18% 10% 3 6% 4 3 4 3 5% 6% 39%	2400
	11th to 20th	Night	20% 14% 3 8% 5% 5% 4 3 4 6% 28%	2400
	21st to end	Night	23% 16% 5% 9% 5% 4 4 3 5% 5% 23%	2680
November	1st to 10th	Night	16% 16% 5% 8% 5% 4 4 3 5% 5% 28%	2695
	11th to 20th	Night	15% 17% 5% 10% 6% 6% 4 3 5% 6% 25%	2800
	21st to end	Night	18% 21% 7% 12% 7% 4 5% 3 4 5% 16%	2800
December	1st to 10th	Night	16% 23% 8% 15% 7% 4 4 3 4 4 10%	4580
	11th to 20th	Night	15% 20% 8% 14% 6% 4 4 3 3 4 18%	5208
	21st to end	Night	20% 21% 7% 14% 8% 4 4 2 3 4 13%	6996

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	72% 5% 7% 10% 6%	3598
	11th to 20th	Day	74% 5% 5% 8% 7% 1	3600
	21st to end	Day	71% 3 6% 10% 8% 2	4442
February	1st to 10th	Day	68% 6% 7% 12% 7% 1	1800
	11th to 20th	Day	64% 5% 9% 12% 9% 1	1762
	21st to end	Day	65% 6% 6% 13% 9% 1	1537
March	1st to 10th	Day	62% 5% 10% 8% 14% 2	4801
	11th to 20th	Day	57% 6% 13% 15% 7% 2	4980
	21st to end	Day	55% 4 6% 18% 14% 3	5543
April	1st to 10th	Day	59% 5% 7% 18% 9% 2	2400
	11th to 20th	Day	54% 5% 7% 20% 12% 1	2418
	21st to end	Day	55% 5% 7% 17% 13% 3	2600
May	1st to 10th	Day	52% 7% 9% 19% 9% 5%	6206
	11th to 20th	Day	47% 6% 10% 21% 15% 1	6322
	21st to end	Day	52% 6% 10% 18% 12% 3	6949
June	1st to 10th	Day	52% 7% 10% 19% 10% 1	2800
	11th to 20th	Day	45% 7% 11% 23% 12% 2	2859
	21st to end	Day	49% 8% 12% 19% 10% 1	3000
July	1st to 10th	Day	45% 6% 14% 25% 9% 1	6548
	11th to 20th	Day	38% 6% 12% 33% 11%	6285
	21st to end	Day	34% 6% 15% 33% 12%	6713
August	1st to 10th	Day	48% 7% 13% 23% 8%	4221
	11th to 20th	Day	46% 7% 15% 26% 6% 1	4213
	21st to end	Day	38% 7% 12% 32% 10%	4451
September	1st to 10th	Day	43% 7% 14% 24% 11% 1	2400
	11th to 20th	Day	39% 7% 13% 28% 13%	2375
	21st to end	Day	50% 8% 10% 23% 9%	2102
October	1st to 10th	Day	50% 6% 11% 24% 8% 1	3260
	11th to 20th	Day	56% 5% 9% 19% 10% 1	3240
	21st to end	Day	65% 5% 11% 15% 4 1	3474
November	1st to 10th	Day	56% 7% 12% 18% 6% 1	1646
	11th to 20th	Day	65% 6% 10% 11% 7% 1	1600
	21st to end	Day	75% 5% 7% 8% 4 1	1600
December	1st to 10th	Day	79% 4 7% 7% 2	2600
	11th to 20th	Day	72% 4 4 11% 8% 1	2986
	21st to end	Day	74% 3 5% 11% 7%	3960
January	1st to 10th	Night	74% 5% 7% 8% 5%	6349
	11th to 20th	Night	74% 7% 6% 6% 6% 1	6360
	21st to end	Night	72% 4 6% 7% 9% 1	6513
February	1st to 10th	Night	66% 5% 11% 11% 6% 1	2600
	11th to 20th	Night	63% 5% 9% 16% 8% 1	2535
	21st to end	Night	61% 6% 7% 17% 7% 1	2092
March	1st to 10th	Night	60% 4 9% 12% 11% 5%	5035
	11th to 20th	Night	53% 7% 16% 15% 7% 1	4980
	21st to end	Night	54% 6% 9% 18% 12% 2	5387
April	1st to 10th	Night	57% 4 9% 17% 12% 2	2000
	11th to 20th	Night	50% 5% 7% 23% 14% 1	1949
	21st to end	Night	55% 5% 7% 16% 15% 2	1800
May	1st to 10th	Night	49% 7% 10% 20% 10% 4	3749
	11th to 20th	Night	39% 8% 13% 22% 16% 2	3576
	21st to end	Night	49% 7% 10% 21% 12% 1	3927
June	1st to 10th	Night	47% 7% 11% 21% 13% 1	1600
	11th to 20th	Night	38% 6% 14% 26% 15% 1	1532
	21st to end	Night	46% 7% 15% 20% 11%	1400
July	1st to 10th	Night	44% 7% 16% 21% 11%	3297
	11th to 20th	Night	39% 5% 12% 33% 11%	3552
	21st to end	Night	33% 6% 10% 37% 13%	3809
August	1st to 10th	Night	35% 7% 17% 27% 13% 2	2904
	11th to 20th	Night	42% 7% 17% 25% 8% 1	2902
	21st to end	Night	35% 6% 16% 31% 10% 2	3412
September	1st to 10th	Night	35% 8% 16% 28% 12% 1	2000
	11th to 20th	Night	36% 6% 14% 31% 13%	2022
	21st to end	Night	45% 7% 13% 25% 11%	2297
October	1st to 10th	Night	41% 5% 11% 27% 15%	3920
	11th to 20th	Night	52% 5% 12% 19% 12%	3899
	21st to end	Night	57% 5% 18% 15% 6%	4316
November	1st to 10th	Night	56% 7% 12% 17% 7%	2695
	11th to 20th	Night	60% 7% 11% 14% 6% 1	2800
	21st to end	Night	70% 6% 9% 10% 3	2800
December	1st to 10th	Night	77% 6% 9% 7% 2	4580
	11th to 20th	Night	71% 4 5% 10% 9%	5208
	21st to end	Night	76% 3 5% 9% 7%	6996

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	23% 26% 36% 12% 4 1	200
	11th to 20th	Day	30% 20% 32% 15% 3 2	200
	21st to end	Day	26% 21% 35% 14% 4	325
February	1st to 10th	Day	30% 30% 31% 6% 3	300
	11th to 20th	Day	27% 34% 34% 5%	300
	21st to end	Day	35% 25% 33% 7% 1	301
March	1st to 10th	Day	28% 22% 36% 12% 1	500
	11th to 20th	Day	26% 23% 34% 13% 4 1	500
	21st to end	Day	32% 22% 29% 13% 3	550
April	1st to 10th	Day	28% 23% 33% 11% 4 1	500
	11th to 20th	Day	35% 21% 22% 13% 7% 2	492
	21st to end	Day	31% 17% 33% 12% 5% 1	500
May	1st to 10th	Day	26% 17% 32% 18% 6% 2	503
	11th to 20th	Day	21% 19% 32% 17% 8% 2	527
	21st to end	Day	23% 18% 30% 19% 8% 2	573
June	1st to 10th	Day	23% 16% 36% 20% 4 1	530
	11th to 20th	Day	17% 16% 37% 20% 8% 2	589
	21st to end	Day	22% 15% 35% 20% 7% 1	730
July	1st to 10th	Day	14% 11% 36% 30% 8% 1	640
	11th to 20th	Day	14% 14% 35% 25% 10% 2	524
	21st to end	Day	11% 14% 37% 26% 9% 3	576
August	1st to 10th	Day	16% 10% 31% 28% 10% 5%	526
	11th to 20th	Day	17% 9% 28% 30% 12% 4	519
	21st to end	Day	16% 14% 32% 23% 10% 4	583
September	1st to 10th	Day	19% 13% 31% 20% 12% 5%	530
	11th to 20th	Day	16% 11% 23% 24% 17% 10%	530
	21st to end	Day	21% 18% 26% 20% 12% 4	530
October	1st to 10th	Day	24% 20% 25% 17% 11% 3	530
	11th to 20th	Day	29% 26% 29% 12% 4 1	530
	21st to end	Day	37% 19% 28% 13% 3	561
November	1st to 10th	Day	33% 18% 33% 13% 3	268
	11th to 20th	Day	32% 20% 34% 10% 4 1	200
	21st to end	Day	31% 25% 35% 8% 1 1	200
December	1st to 10th	Day	32% 20% 33% 15% 1	200
	11th to 20th	Day	34% 27% 30% 8% 1 1	200
	21st to end	Day	29% 27% 25% 13% 4 1	220
January	1st to 10th	Night	25% 22% 36% 14% 3	800
	11th to 20th	Night	26% 21% 31% 17% 4 1	800
	21st to end	Night	17% 20% 35% 21% 6% 1	880
February	1st to 10th	Night	21% 23% 38% 14% 3	800
	11th to 20th	Night	20% 21% 41% 14% 3	783
	21st to end	Night	23% 22% 34% 18% 2	660
March	1st to 10th	Night	22% 24% 33% 18% 3	800
	11th to 20th	Night	21% 20% 37% 16% 5% 1	800
	21st to end	Night	19% 21% 35% 19% 6%	880
April	1st to 10th	Night	27% 18% 33% 18% 4	800
	11th to 20th	Night	25% 19% 29% 18% 9%	765
	21st to end	Night	25% 14% 29% 21% 10% 1	600
May	1st to 10th	Night	24% 10% 28% 28% 9%	471
	11th to 20th	Night	21% 11% 34% 26% 8%	394
	21st to end	Night	17% 11% 35% 26% 11%	433
June	1st to 10th	Night	20% 10% 36% 28% 7%	400
	11th to 20th	Night	18% 9% 30% 33% 10%	400
	21st to end	Night	20% 11% 27% 32% 11% 1	400
July	1st to 10th	Night	13% 6% 31% 36% 14% 1	390
	11th to 20th	Night	16% 7% 29% 33% 14% 1	388
	21st to end	Night	11% 6% 35% 36% 11% 1	438
August	1st to 10th	Night	12% 5% 26% 36% 19% 2	596
	11th to 20th	Night	11% 6% 24% 35% 21% 3	594
	21st to end	Night	16% 8% 31% 28% 16% 2	771
September	1st to 10th	Night	13% 9% 31% 29% 16% 1	800
	11th to 20th	Night	18% 8% 24% 26% 19% 5%	800
	21st to end	Night	23% 11% 22% 27% 16% 2	800
October	1st to 10th	Night	24% 15% 28% 21% 11% 2	800
	11th to 20th	Night	27% 17% 31% 17% 7% 1	800
	21st to end	Night	32% 18% 27% 18% 4	880
November	1st to 10th	Night	29% 17% 29% 19% 5% 1	789
	11th to 20th	Night	25% 22% 30% 16% 7%	800
	21st to end	Night	31% 20% 29% 17% 3	800
December	1st to 10th	Night	30% 22% 32% 12% 4	800
	11th to 20th	Night	26% 24% 32% 15% 2 1	800
	21st to end	Night	33% 19% 30% 14% 3 1	880

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