Chanticlear Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chanticlear Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	11% 9% 4 9% 6% 5% 5% 4 6% 7% 33%	131204
	Night	11% 10% 4 9% 5% 5% 4 4 6% 7% 36%	129657
Transparency	Day	46% 6% 11% 20% 14% 3	134849
	Night	45% 6% 11% 21% 14% 3	134081
Seeing	Day	25% 17% 25% 19% 10% 5%	16818
	Night	22% 16% 21% 25% 14% 2	26963

By Month

Percentage Cloud Cover SE (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% 14% 5% 11% 5% 4 4 3 5% 7% 31%	5258
	11th to 20th	Day	7% 13% 8% 14% 6% 5% 4 3 4 3 35%	5280
	21st to end	Day	11% 12% 5% 10% 6% 4 3 3 4 5% 37%	5780
February	1st to 10th	Day	11% 12% 4 11% 6% 5% 5% 5% 5% 6% 32%	1890
	11th to 20th	Day	16% 11% 4 10% 5% 5% 4 4 5% 6% 30%	1927
	21st to end	Day	16% 11% 4 10% 5% 4 4 3 5% 5% 33%	1660
March	1st to 10th	Day	15% 10% 6% 9% 5% 4 4 4 5% 6% 32%	4935
	11th to 20th	Day	9% 9% 4 7% 4 4 4 3 5% 6% 45%	4980
	21st to end	Day	12% 9% 3 8% 5% 5% 5% 4 5% 7% 37%	5465
April	1st to 10th	Day	11% 11% 3 7% 6% 4 4 4 6% 7% 37%	2210
	11th to 20th	Day	12% 10% 3 7% 5% 4 4 4 5% 6% 39%	2411
	21st to end	Day	16% 8% 3 7% 6% 4 5% 4 5% 7% 35%	2600
May	1st to 10th	Day	13% 9% 3 9% 6% 7% 7% 6% 8% 8% 24%	5862
	11th to 20th	Day	17% 9% 3 9% 6% 6% 6% 5% 7% 7% 25%	5861
	21st to end	Day	12% 7% 3 8% 6% 6% 7% 5% 8% 9% 29%	6433
June	1st to 10th	Day	11% 7% 2 7% 6% 7% 6% 4 8% 9% 34%	2600
	11th to 20th	Day	8% 7% 2 7% 7% 6% 7% 5% 8% 10% 34%	2600
	21st to end	Day	9% 7% 2 6% 5% 6% 7% 5% 7% 9% 35%	2600
July	1st to 10th	Day	6% 5% 3 7% 6% 6% 6% 5% 9% 10% 37%	5900
	11th to 20th	Day	7% 6% 3 8% 6% 7% 7% 6% 9% 11% 29%	5900
	21st to end	Day	10% 9% 4 10% 8% 7% 8% 5% 8% 8% 23%	6424
August	1st to 10th	Day	12% 6% 3 8% 7% 6% 7% 6% 9% 10% 25%	4234
	11th to 20th	Day	8% 6% 3 8% 7% 7% 7% 6% 8% 11% 29%	4174
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 7% 9% 46%	4458
September	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 7% 41%	2250
	11th to 20th	Day	13% 8% 3 8% 4 5% 5% 4 7% 8% 35%	2200
	21st to end	Day	18% 8% 3 8% 5% 5% 5% 4 6% 6% 32%	2200
October	1st to 10th	Day	14% 6% 2 6% 5% 4 3 3 5% 6% 46%	2075
	11th to 20th	Day	12% 10% 3 6% 3 4 4 3 5% 5% 46%	2000
	21st to end	Day	14% 12% 3 6% 4 4 4 3 4 6% 43%	2200
November	1st to 10th	Day	11% 6% 2 7% 4 3 4 3 6% 7% 46%	2009
	11th to 20th	Day	10% 10% 3 6% 5% 5% 4 4 6% 9% 39%	1890
	21st to end	Day	11% 11% 3 8% 4 4 4 4 4 6% 40%	1890
December	1st to 10th	Day	10% 12% 4 11% 6% 4 5% 3 5% 6% 35%	3020
	11th to 20th	Day	14% 13% 5% 10% 5% 5% 4 4 5% 7% 28%	3463
	21st to end	Day	12% 12% 6% 10% 7% 6% 6% 5% 6% 6% 25%	4565
January	1st to 10th	Night	10% 14% 6% 14% 6% 5% 4 3 5% 6% 26%	7635
	11th to 20th	Night	10% 12% 5% 12% 6% 4 3 2 4 6% 35%	7680
	21st to end	Night	12% 11% 4 10% 6% 5% 4 4 6% 7% 33%	8401
February	1st to 10th	Night	12% 11% 4 10% 5% 5% 4 4 6% 7% 33%	2730
	11th to 20th	Night	16% 11% 5% 11% 5% 5% 5% 3 5% 6% 29%	2466
	21st to end	Night	12% 11% 5% 10% 6% 5% 5% 3 5% 5% 32%	1970
March	1st to 10th	Night	16% 12% 3 7% 4 4 3 3 5% 6% 36%	4918
	11th to 20th	Night	11% 8% 3 7% 3 3 4 3 5% 6% 48%	4980
	21st to end	Night	10% 9% 4 7% 5% 5% 5% 4 6% 7% 38%	5467
April	1st to 10th	Night	10% 11% 4 7% 5% 5% 4 4 6% 8% 36%	2190
	11th to 20th	Night	10% 9% 3 5% 5% 4 4 4 6% 8% 42%	1954
	21st to end	Night	14% 8% 3 7% 5% 5% 5% 3 5% 8% 37%	1800
May	1st to 10th	Night	15% 10% 3 8% 6% 5% 5% 4 6% 8% 29%	4033
	11th to 20th	Night	16% 10% 3 8% 6% 5% 4 4 7% 8% 28%	4032
	21st to end	Night	16% 11% 4 8% 5% 5% 4 4 6% 7% 31%	4421
June	1st to 10th	Night	8% 6% 3 7% 4 6% 6% 5% 6% 10% 40%	1800
	11th to 20th	Night	7% 6% 3 6% 6% 5% 5% 5% 8% 9% 40%	1800
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 5% 7% 12% 41%	1800
July	1st to 10th	Night	6% 5% 2 7% 6% 5% 6% 5% 8% 11% 39%	4058
	11th to 20th	Night	6% 6% 2 7% 6% 6% 6% 5% 8% 12% 36%	4057
	21st to end	Night	8% 9% 4 11% 7% 7% 7% 5% 7% 9% 26%	4427
August	1st to 10th	Night	12% 7% 3 8% 6% 5% 6% 4 7% 9% 34%	2915
	11th to 20th	Night	10% 8% 3 7% 6% 5% 5% 5% 8% 7% 36%	2875
	21st to end	Night	5% 4 2 5% 4 4 4 3 6% 9% 53%	3406
September	1st to 10th	Night	9% 7% 2 7% 4 4 4 4 6% 7% 46%	2145
	11th to 20th	Night	13% 8% 2 6% 4 4 4 4 5% 7% 45%	2200
	21st to end	Night	14% 11% 2 7% 4 5% 4 4 5% 6% 39%	2200
October	1st to 10th	Night	13% 8% 2 5% 3 3 3 2 4 6% 50%	2325
	11th to 20th	Night	10% 10% 3 7% 4 3 4 3 4 6% 46%	2400
	21st to end	Night	12% 11% 3 6% 4 4 4 2 3 7% 44%	2640
November	1st to 10th	Night	10% 9% 3 7% 4 4 4 3 5% 5% 46%	2507
	11th to 20th	Night	11% 10% 3 7% 4 4 4 4 6% 6% 41%	2730
	21st to end	Night	9% 12% 4 8% 4 4 4 4 5% 6% 40%	2730
December	1st to 10th	Night	10% 9% 5% 12% 5% 4 4 4 7% 7% 34%	4380
	11th to 20th	Night	12% 11% 7% 13% 6% 5% 5% 4 4 6% 27%	4952
	21st to end	Night	12% 11% 7% 11% 5% 6% 5% 5% 6% 8% 24%	6633

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	50% 4 11% 17% 17% 1	5259
	11th to 20th	Day	52% 4 4 14% 21% 5%	5280
	21st to end	Day	49% 3 7% 23% 16% 2	5780
February	1st to 10th	Day	51% 4 7% 20% 16% 1	1890
	11th to 20th	Day	53% 4 7% 19% 14% 3	1936
	21st to end	Day	52% 5% 8% 15% 15% 5%	1660
March	1st to 10th	Day	51% 5% 6% 18% 14% 6%	4935
	11th to 20th	Day	40% 3 6% 21% 23% 7%	4980
	21st to end	Day	46% 4 8% 23% 16% 2	5465
April	1st to 10th	Day	45% 4 9% 22% 15% 4	2210
	11th to 20th	Day	45% 3 9% 24% 16% 4	2411
	21st to end	Day	50% 3 12% 22% 11% 2	2600
May	1st to 10th	Day	53% 6% 13% 18% 9% 1	5897
	11th to 20th	Day	56% 5% 11% 21% 7%	5865
	21st to end	Day	48% 5% 14% 18% 12% 3	6447
June	1st to 10th	Day	44% 7% 14% 21% 14%	2600
	11th to 20th	Day	43% 9% 14% 20% 13% 1	2600
	21st to end	Day	41% 8% 14% 22% 13% 2	2600
July	1st to 10th	Day	39% 9% 19% 20% 11% 2	5835
	11th to 20th	Day	44% 11% 19% 21% 5% 1	5831
	21st to end	Day	53% 9% 13% 16% 8%	6240
August	1st to 10th	Day	47% 9% 19% 20% 6%	4221
	11th to 20th	Day	42% 8% 16% 22% 10% 1	4213
	21st to end	Day	31% 7% 18% 29% 13% 1	4458
September	1st to 10th	Day	41% 6% 12% 20% 17% 4	2250
	11th to 20th	Day	45% 5% 8% 24% 17% 1	2200
	21st to end	Day	52% 4 11% 19% 13% 2	2200
October	1st to 10th	Day	29% 5% 9% 20% 30% 6%	3374
	11th to 20th	Day	31% 3 9% 24% 17% 16%	3240
	21st to end	Day	32% 2 8% 30% 15% 14%	3538
November	1st to 10th	Day	37% 3 9% 22% 24% 4	2009
	11th to 20th	Day	42% 3 7% 23% 21% 4	1890
	21st to end	Day	44% 4 8% 22% 18% 3	1890
December	1st to 10th	Day	48% 5% 8% 19% 18% 2	3020
	11th to 20th	Day	54% 4 9% 15% 16% 2	3460
	21st to end	Day	56% 5% 9% 18% 10% 1	4565
January	1st to 10th	Night	56% 4 10% 18% 10% 2	7636
	11th to 20th	Night	49% 5% 5% 16% 20% 5%	7680
	21st to end	Night	48% 4 8% 22% 15% 2	8401
February	1st to 10th	Night	49% 4 11% 19% 15% 1	2730
	11th to 20th	Night	56% 4 7% 18% 11% 4	2467
	21st to end	Night	53% 4 8% 19% 11% 5%	1970
March	1st to 10th	Night	48% 3 5% 23% 16% 4	4918
	11th to 20th	Night	37% 3 7% 24% 22% 7%	4980
	21st to end	Night	45% 4 10% 23% 15% 2	5467
April	1st to 10th	Night	46% 5% 12% 20% 15% 3	2190
	11th to 20th	Night	40% 4 11% 24% 17% 4	1954
	21st to end	Night	45% 5% 14% 24% 12% 1	1800
May	1st to 10th	Night	52% 7% 14% 19% 8%	4060
	11th to 20th	Night	53% 6% 14% 21% 4	4033
	21st to end	Night	51% 5% 14% 21% 6% 2	4429
June	1st to 10th	Night	37% 8% 20% 21% 14% 1	1800
	11th to 20th	Night	37% 8% 20% 22% 12% 1	1800
	21st to end	Night	34% 8% 16% 25% 14% 2	1800
July	1st to 10th	Night	36% 12% 19% 23% 10% 1	4015
	11th to 20th	Night	38% 12% 22% 22% 5% 1	4006
	21st to end	Night	49% 11% 13% 18% 8%	4297
August	1st to 10th	Night	46% 7% 19% 22% 6%	2904
	11th to 20th	Night	42% 8% 16% 25% 8%	2902
	21st to end	Night	27% 6% 15% 37% 14% 1	3406
September	1st to 10th	Night	37% 7% 14% 20% 17% 4	2145
	11th to 20th	Night	40% 5% 10% 24% 19% 1	2200
	21st to end	Night	47% 5% 10% 23% 14% 1	2200
October	1st to 10th	Night	32% 5% 8% 21% 30% 3	3806
	11th to 20th	Night	32% 4 8% 21% 19% 15%	3899
	21st to end	Night	26% 2 7% 31% 20% 13%	4252
November	1st to 10th	Night	40% 3 9% 20% 23% 4	2507
	11th to 20th	Night	42% 3 8% 21% 20% 5%	2730
	21st to end	Night	44% 3 8% 23% 18% 4	2730
December	1st to 10th	Night	45% 6% 10% 25% 12% 1	4380
	11th to 20th	Night	56% 6% 9% 16% 11% 2	4954
	21st to end	Night	56% 5% 10% 16% 12% 1	6633

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	45% 28% 20% 6% 1 1	330
	11th to 20th	Day	47% 28% 19% 5%	330
	21st to end	Day	44% 29% 20% 6% 1	363
February	1st to 10th	Day	37% 32% 23% 6% 2	330
	11th to 20th	Day	53% 27% 17% 3	315
	21st to end	Day	47% 26% 24% 2 1	259
March	1st to 10th	Day	34% 25% 23% 15% 3	500
	11th to 20th	Day	30% 32% 24% 10% 3 1	500
	21st to end	Day	32% 27% 25% 13% 3	550
April	1st to 10th	Day	32% 26% 22% 12% 7% 1	500
	11th to 20th	Day	30% 22% 23% 15% 8% 2	492
	21st to end	Day	27% 20% 30% 15% 6% 2	500
May	1st to 10th	Day	27% 21% 29% 13% 7% 3	503
	11th to 20th	Day	24% 17% 29% 18% 9% 4	527
	21st to end	Day	19% 13% 24% 24% 12% 9%	573
June	1st to 10th	Day	14% 12% 30% 24% 14% 6%	530
	11th to 20th	Day	11% 11% 29% 27% 16% 6%	530
	21st to end	Day	13% 11% 28% 27% 15% 6%	530
July	1st to 10th	Day	12% 7% 25% 30% 16% 11%	525
	11th to 20th	Day	9% 7% 23% 34% 16% 11%	524
	21st to end	Day	13% 6% 25% 27% 18% 11%	576
August	1st to 10th	Day	16% 9% 22% 26% 17% 10%	526
	11th to 20th	Day	13% 8% 20% 28% 20% 11%	519
	21st to end	Day	9% 7% 22% 27% 19% 15%	583
September	1st to 10th	Day	12% 10% 21% 26% 20% 12%	530
	11th to 20th	Day	17% 7% 20% 26% 21% 10%	530
	21st to end	Day	27% 11% 23% 24% 13% 3	530
October	1st to 10th	Day	20% 11% 24% 23% 17% 5%	530
	11th to 20th	Day	21% 18% 24% 22% 12% 2	530
	21st to end	Day	27% 17% 27% 19% 9% 2	583
November	1st to 10th	Day	22% 18% 31% 19% 9% 2	487
	11th to 20th	Day	33% 21% 29% 14% 3 1	330
	21st to end	Day	33% 22% 28% 13% 3	330
December	1st to 10th	Day	38% 27% 25% 8% 1	330
	11th to 20th	Day	37% 23% 28% 9% 2	330
	21st to end	Day	35% 26% 26% 10% 4	363
January	1st to 10th	Night	33% 28% 21% 15% 3	840
	11th to 20th	Night	37% 24% 23% 15% 2	840
	21st to end	Night	31% 27% 25% 15% 1	924
February	1st to 10th	Night	30% 23% 28% 19% 1	840
	11th to 20th	Night	38% 29% 22% 12%	804
	21st to end	Night	30% 27% 27% 15% 1	660
March	1st to 10th	Night	28% 23% 26% 19% 3	800
	11th to 20th	Night	30% 22% 25% 17% 6% 1	800
	21st to end	Night	26% 22% 30% 17% 5%	880
April	1st to 10th	Night	25% 21% 26% 21% 6%	800
	11th to 20th	Night	22% 20% 25% 24% 9% 1	770
	21st to end	Night	19% 16% 24% 33% 7% 1	600
May	1st to 10th	Night	21% 14% 26% 27% 12% 2	600
	11th to 20th	Night	19% 9% 20% 33% 17% 1	591
	21st to end	Night	15% 6% 18% 30% 26% 4	650
June	1st to 10th	Night	10% 6% 21% 37% 23% 4	600
	11th to 20th	Night	10% 5% 19% 32% 30% 5%	600
	21st to end	Night	11% 3 17% 36% 29% 4	600
July	1st to 10th	Night	9% 2 14% 38% 32% 4	585
	11th to 20th	Night	8% 2 14% 37% 34% 5%	582
	21st to end	Night	9% 3 14% 35% 35% 5%	657
August	1st to 10th	Night	13% 3 12% 37% 32% 4	597
	11th to 20th	Night	15% 4 11% 32% 32% 6%	594
	21st to end	Night	10% 5% 11% 33% 35% 7%	765
September	1st to 10th	Night	13% 5% 13% 32% 32% 5%	800
	11th to 20th	Night	18% 5% 14% 29% 30% 5%	800
	21st to end	Night	22% 10% 17% 29% 20% 2	800
October	1st to 10th	Night	19% 9% 16% 29% 23% 5%	800
	11th to 20th	Night	19% 15% 23% 27% 14% 2	800
	21st to end	Night	21% 17% 21% 28% 11% 2	880
November	1st to 10th	Night	19% 15% 23% 28% 13% 2	820
	11th to 20th	Night	24% 21% 21% 26% 8% 1	840
	21st to end	Night	25% 20% 23% 24% 8%	840
December	1st to 10th	Night	30% 22% 26% 18% 4	840
	11th to 20th	Night	25% 26% 22% 23% 4	840
	21st to end	Night	28% 22% 26% 21% 3	924

Page generated at 2026-02-13 01:42:17. Version ac07

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