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 2024-07-02.

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	13% 10% 4 8% 5% 5% 5% 4 6% 7% 33%	89697
	Night	12% 10% 4 8% 5% 5% 4 4 5% 7% 36%	89455
Transparency	Day	49% 4 10% 20% 14% 2	89470
	Night	47% 4 10% 21% 14% 3	89285
Seeing	Day	24% 17% 24% 19% 11% 5%	14654
	Night	21% 15% 21% 26% 15% 2	24772

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	12% 16% 4 15% 5% 4 3 3 5% 5% 28%	2836
	11th to 20th	Day	11% 14% 9% 14% 6% 6% 4 3 4 3 25%	2840
	21st to end	Day	13% 18% 6% 9% 4 4 4 3 5% 5% 28%	3124
February	1st to 10th	Day	11% 12% 4 11% 6% 4 5% 5% 5% 6% 31%	1710
	11th to 20th	Day	15% 11% 4 11% 5% 5% 5% 4 5% 6% 30%	1783
	21st to end	Day	17% 12% 4 10% 5% 4 4 3 4 5% 32%	1579
March	1st to 10th	Day	20% 11% 5% 8% 4 3 4 4 5% 6% 31%	3435
	11th to 20th	Day	11% 9% 4 7% 4 4 5% 3 5% 6% 43%	3480
	21st to end	Day	15% 11% 3 7% 4 4 4 4 5% 7% 36%	3815
April	1st to 10th	Day	11% 10% 3 7% 6% 4 4 4 6% 7% 37%	2099
	11th to 20th	Day	12% 10% 3 7% 6% 4 4 4 5% 6% 39%	2289
	21st to end	Day	17% 8% 3 7% 5% 4 5% 4 5% 7% 35%	2470
May	1st to 10th	Day	16% 11% 3 10% 6% 6% 6% 5% 6% 6% 25%	4082
	11th to 20th	Day	17% 10% 3 9% 5% 5% 6% 4 7% 6% 27%	4081
	21st to end	Day	9% 6% 2 7% 6% 6% 7% 5% 9% 9% 33%	4476
June	1st to 10th	Day	12% 7% 2 7% 7% 7% 6% 4 7% 9% 33%	2470
	11th to 20th	Day	8% 7% 2 7% 6% 6% 7% 5% 7% 10% 34%	2470
	21st to end	Day	9% 7% 2 6% 6% 6% 7% 5% 7% 8% 35%	2470
July	1st to 10th	Day	10% 8% 4 8% 7% 6% 5% 5% 8% 9% 31%	2353
	11th to 20th	Day	9% 8% 3 8% 6% 7% 6% 5% 8% 10% 30%	2340
	21st to end	Day	11% 8% 3 9% 7% 6% 7% 6% 8% 9% 27%	2547
August	1st to 10th	Day	11% 7% 3 8% 6% 7% 7% 6% 9% 11% 26%	2327
	11th to 20th	Day	13% 8% 3 9% 7% 7% 7% 5% 7% 8% 26%	2303
	21st to end	Day	9% 6% 3 8% 6% 7% 6% 5% 8% 9% 33%	2462
September	1st to 10th	Day	11% 7% 3 6% 6% 5% 6% 5% 7% 7% 39%	2026
	11th to 20th	Day	13% 8% 3 8% 4 5% 5% 4 7% 8% 35%	1980
	21st to end	Day	18% 8% 3 7% 4 5% 4 4 6% 6% 35%	1980
October	1st to 10th	Day	15% 7% 2 6% 5% 4 3 3 5% 6% 44%	1868
	11th to 20th	Day	13% 11% 3 6% 3 4 4 3 5% 5% 43%	1800
	21st to end	Day	14% 12% 3 6% 4 3 4 3 4 6% 42%	1980
November	1st to 10th	Day	12% 6% 2 7% 4 3 4 3 5% 7% 47%	1815
	11th to 20th	Day	10% 11% 3 6% 5% 5% 4 4 5% 8% 39%	1710
	21st to end	Day	12% 10% 2 8% 4 4 4 4 4 6% 41%	1710
December	1st to 10th	Day	15% 15% 5% 10% 6% 4 4 4 5% 6% 27%	1710
	11th to 20th	Day	13% 13% 5% 9% 5% 4 3 4 5% 7% 33%	2153
	21st to end	Day	13% 14% 6% 9% 6% 6% 5% 4 6% 6% 24%	3124
January	1st to 10th	Night	11% 17% 8% 14% 5% 5% 4 3 4 4 24%	4109
	11th to 20th	Night	13% 13% 5% 10% 7% 4 3 3 4 6% 32%	4120
	21st to end	Night	14% 15% 5% 8% 6% 6% 4 4 6% 7% 25%	4532
February	1st to 10th	Night	12% 11% 4 9% 6% 5% 4 4 6% 7% 32%	2470
	11th to 20th	Night	16% 11% 5% 11% 5% 5% 5% 4 4 6% 29%	2279
	21st to end	Night	13% 11% 5% 11% 6% 5% 5% 3 5% 5% 31%	1875
March	1st to 10th	Night	20% 14% 3 8% 5% 4 3 3 4 5% 30%	3418
	11th to 20th	Night	11% 9% 4 8% 4 4 4 3 5% 6% 42%	3480
	21st to end	Night	13% 9% 3 6% 4 4 4 4 6% 7% 41%	3817
April	1st to 10th	Night	10% 10% 4 7% 6% 5% 4 4 5% 8% 37%	2081
	11th to 20th	Night	11% 9% 3 5% 4 4 4 4 6% 8% 42%	1855
	21st to end	Night	14% 8% 3 6% 5% 5% 5% 3 5% 8% 38%	1710
May	1st to 10th	Night	17% 12% 3 8% 6% 5% 6% 3 5% 7% 28%	2813
	11th to 20th	Night	17% 13% 3 8% 5% 4 3 3 4 6% 33%	2812
	21st to end	Night	10% 9% 3 7% 4 5% 5% 4 7% 9% 37%	3079
June	1st to 10th	Night	8% 5% 3 6% 4 6% 6% 5% 7% 11% 40%	1710
	11th to 20th	Night	7% 6% 2 6% 6% 5% 5% 5% 8% 9% 41%	1710
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 41%	1710
July	1st to 10th	Night	9% 5% 3 7% 5% 4 6% 5% 7% 10% 40%	1635
	11th to 20th	Night	7% 5% 1 5% 5% 5% 5% 5% 8% 12% 43%	1620
	21st to end	Night	6% 5% 2 6% 7% 6% 7% 5% 8% 12% 37%	1771
August	1st to 10th	Night	9% 7% 2 9% 6% 5% 5% 4 7% 8% 39%	1611
	11th to 20th	Night	12% 7% 2 6% 5% 5% 5% 5% 8% 7% 39%	1598
	21st to end	Night	8% 5% 3 7% 5% 5% 5% 4 8% 9% 42%	1876
September	1st to 10th	Night	10% 7% 2 7% 4 4 4 4 7% 7% 44%	1930
	11th to 20th	Night	13% 8% 3 6% 3 4 4 4 5% 7% 45%	1980
	21st to end	Night	13% 11% 3 7% 4 4 4 3 5% 6% 42%	1980
October	1st to 10th	Night	14% 8% 2 5% 3 3 3 2 4 6% 48%	2092
	11th to 20th	Night	11% 10% 2 7% 4 3 4 3 4 6% 44%	2160
	21st to end	Night	11% 12% 3 6% 4 4 4 3 4 7% 42%	2376
November	1st to 10th	Night	11% 9% 3 7% 3 4 4 3 5% 5% 47%	2262
	11th to 20th	Night	11% 10% 3 6% 4 4 4 4 6% 6% 42%	2470
	21st to end	Night	9% 11% 3 7% 4 4 4 4 5% 6% 41%	2470
December	1st to 10th	Night	14% 13% 5% 10% 6% 4 4 3 5% 6% 28%	2470
	11th to 20th	Night	12% 12% 5% 13% 5% 5% 5% 4 4 6% 30%	3042
	21st to end	Night	14% 12% 8% 10% 5% 5% 5% 4 5% 7% 23%	4532

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	57% 2 5% 19% 14% 3	2837
	11th to 20th	Day	62% 3 4 14% 14% 2	2840
	21st to end	Day	58% 2 5% 16% 17% 1	3124
February	1st to 10th	Day	52% 5% 7% 21% 15% 1	1710
	11th to 20th	Day	53% 4 8% 18% 14% 3	1792
	21st to end	Day	54% 4 8% 14% 15% 5%	1579
March	1st to 10th	Day	53% 4 6% 21% 13% 3	3435
	11th to 20th	Day	43% 3 7% 24% 19% 4	3480
	21st to end	Day	49% 3 8% 24% 13% 3	3815
April	1st to 10th	Day	44% 4 9% 23% 16% 4	2099
	11th to 20th	Day	45% 3 8% 24% 16% 4	2289
	21st to end	Day	50% 3 12% 22% 12% 2	2470
May	1st to 10th	Day	58% 4 10% 18% 10% 1	4120
	11th to 20th	Day	56% 3 10% 24% 7% 1	4085
	21st to end	Day	45% 4 16% 20% 13% 3	4489
June	1st to 10th	Day	46% 6% 14% 21% 13%	2470
	11th to 20th	Day	42% 9% 14% 20% 14% 1	2470
	21st to end	Day	43% 8% 14% 21% 13% 2	2470
July	1st to 10th	Day	48% 5% 16% 18% 11% 2	2288
	11th to 20th	Day	45% 7% 17% 18% 12% 1	2271
	21st to end	Day	49% 7% 15% 20% 8% 1	2405
August	1st to 10th	Day	49% 7% 18% 19% 7%	2314
	11th to 20th	Day	53% 6% 13% 19% 9%	2303
	21st to end	Day	44% 7% 17% 18% 12% 2	2462
September	1st to 10th	Day	43% 6% 12% 21% 14% 4	2026
	11th to 20th	Day	46% 5% 7% 24% 17% 1	1980
	21st to end	Day	49% 3 10% 20% 14% 2	1980
October	1st to 10th	Day	42% 4 10% 24% 17% 4	1868
	11th to 20th	Day	43% 3 9% 23% 19% 3	1800
	21st to end	Day	45% 2 6% 25% 17% 4	1980
November	1st to 10th	Day	38% 3 8% 22% 25% 5%	1815
	11th to 20th	Day	43% 3 7% 22% 21% 5%	1710
	21st to end	Day	43% 3 9% 22% 19% 4	1710
December	1st to 10th	Day	58% 4 8% 16% 13% 2	1710
	11th to 20th	Day	49% 3 8% 16% 20% 2	2150
	21st to end	Day	59% 3 6% 19% 12% 1	3124
January	1st to 10th	Night	63% 3 5% 13% 13% 3	4109
	11th to 20th	Night	54% 3 4 17% 17% 5%	4120
	21st to end	Night	57% 3 6% 18% 14% 1	4532
February	1st to 10th	Night	49% 4 11% 19% 15% 1	2470
	11th to 20th	Night	56% 4 7% 18% 12% 4	2280
	21st to end	Night	54% 4 9% 19% 11% 4	1875
March	1st to 10th	Night	57% 3 5% 20% 13% 2	3418
	11th to 20th	Night	43% 4 7% 27% 15% 5%	3480
	21st to end	Night	43% 3 11% 24% 17% 3	3817
April	1st to 10th	Night	45% 5% 11% 21% 15% 3	2081
	11th to 20th	Night	40% 4 11% 23% 17% 4	1855
	21st to end	Night	44% 4 14% 24% 12% 1	1710
May	1st to 10th	Night	56% 4 12% 20% 8%	2840
	11th to 20th	Night	54% 3 13% 25% 5%	2813
	21st to end	Night	43% 5% 16% 26% 9% 2	3087
June	1st to 10th	Night	37% 8% 20% 22% 13% 1	1710
	11th to 20th	Night	36% 7% 20% 22% 12% 1	1710
	21st to end	Night	35% 8% 16% 25% 14% 2	1710
July	1st to 10th	Night	39% 8% 17% 21% 13% 3	1590
	11th to 20th	Night	32% 8% 22% 22% 12% 3	1566
	21st to end	Night	35% 9% 18% 27% 10%	1672
August	1st to 10th	Night	43% 6% 18% 23% 10%	1600
	11th to 20th	Night	41% 7% 17% 22% 11% 1	1598
	21st to end	Night	38% 6% 16% 27% 12% 2	1876
September	1st to 10th	Night	38% 7% 14% 21% 16% 4	1930
	11th to 20th	Night	40% 5% 10% 25% 19% 2	1980
	21st to end	Night	45% 4 9% 25% 15% 1	1980
October	1st to 10th	Night	39% 3 10% 25% 20% 4	2092
	11th to 20th	Night	41% 3 9% 25% 19% 3	2160
	21st to end	Night	43% 3 5% 25% 18% 5%	2376
November	1st to 10th	Night	39% 3 9% 20% 24% 5%	2262
	11th to 20th	Night	42% 3 8% 20% 21% 6%	2470
	21st to end	Night	43% 3 7% 24% 19% 4	2470
December	1st to 10th	Night	57% 4 8% 18% 11% 3	2470
	11th to 20th	Night	52% 6% 7% 16% 16% 2	3044
	21st to end	Night	60% 3 7% 15% 13% 1	4532

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	39% 30% 23% 7% 1 1	270
	11th to 20th	Day	42% 30% 22% 6%	270
	21st to end	Day	40% 30% 22% 7% 1	297
February	1st to 10th	Day	37% 30% 24% 8% 2	270
	11th to 20th	Day	50% 28% 18% 4	270
	21st to end	Day	47% 27% 23% 2 1	234
March	1st to 10th	Day	33% 25% 23% 15% 3	460
	11th to 20th	Day	30% 31% 24% 10% 3 1	460
	21st to end	Day	31% 26% 25% 14% 3	506
April	1st to 10th	Day	31% 26% 22% 13% 7% 1	460
	11th to 20th	Day	29% 21% 23% 15% 8% 2	452
	21st to end	Day	29% 20% 28% 15% 7% 2	460
May	1st to 10th	Day	28% 20% 29% 13% 7% 3	463
	11th to 20th	Day	23% 15% 30% 19% 9% 4	487
	21st to end	Day	16% 12% 24% 26% 13% 9%	529
June	1st to 10th	Day	14% 12% 29% 24% 14% 6%	490
	11th to 20th	Day	11% 11% 27% 27% 16% 7%	490
	21st to end	Day	14% 12% 30% 25% 13% 7%	490
July	1st to 10th	Day	13% 7% 22% 28% 17% 11%	449
	11th to 20th	Day	8% 7% 23% 34% 16% 12%	444
	21st to end	Day	13% 6% 26% 26% 18% 11%	488
August	1st to 10th	Day	14% 9% 21% 26% 19% 12%	446
	11th to 20th	Day	14% 7% 18% 28% 21% 13%	439
	21st to end	Day	10% 6% 23% 26% 20% 15%	495
September	1st to 10th	Day	12% 9% 18% 26% 22% 12%	450
	11th to 20th	Day	17% 6% 19% 27% 21% 11%	450
	21st to end	Day	25% 11% 22% 24% 15% 3	450
October	1st to 10th	Day	22% 11% 23% 20% 18% 6%	450
	11th to 20th	Day	22% 18% 23% 23% 13% 2	450
	21st to end	Day	25% 18% 27% 18% 9% 2	495
November	1st to 10th	Day	21% 16% 29% 21% 10% 2	413
	11th to 20th	Day	32% 19% 29% 16% 4 1	270
	21st to end	Day	27% 23% 30% 16% 3	270
December	1st to 10th	Day	38% 24% 26% 10% 1	270
	11th to 20th	Day	34% 23% 28% 11% 3	270
	21st to end	Day	34% 25% 26% 11% 4	297
January	1st to 10th	Night	31% 28% 21% 15% 3	760
	11th to 20th	Night	35% 23% 24% 16% 2	760
	21st to end	Night	30% 28% 25% 16% 2	836
February	1st to 10th	Night	30% 21% 28% 19% 1	760
	11th to 20th	Night	36% 29% 22% 12%	743
	21st to end	Night	31% 27% 26% 15% 1	628
March	1st to 10th	Night	27% 23% 27% 20% 3	760
	11th to 20th	Night	30% 22% 24% 17% 6% 1	760
	21st to end	Night	24% 22% 29% 18% 6%	836
April	1st to 10th	Night	25% 21% 26% 22% 6%	760
	11th to 20th	Night	22% 20% 24% 24% 9% 1	731
	21st to end	Night	18% 17% 24% 34% 6% 1	570
May	1st to 10th	Night	22% 13% 25% 27% 12% 2	570
	11th to 20th	Night	18% 9% 21% 33% 18% 1	561
	21st to end	Night	13% 6% 18% 31% 28% 4	617
June	1st to 10th	Night	10% 5% 20% 37% 23% 4	570
	11th to 20th	Night	10% 5% 19% 31% 31% 5%	570
	21st to end	Night	11% 4 17% 36% 28% 4	570
July	1st to 10th	Night	10% 2 14% 36% 33% 4	530
	11th to 20th	Night	8% 2 13% 37% 35% 6%	522
	21st to end	Night	8% 3 12% 35% 36% 6%	591
August	1st to 10th	Night	12% 3 11% 37% 34% 4	537
	11th to 20th	Night	16% 4 9% 32% 33% 7%	534
	21st to end	Night	10% 5% 10% 33% 35% 7%	688
September	1st to 10th	Night	13% 5% 12% 30% 34% 6%	720
	11th to 20th	Night	18% 4 14% 29% 30% 5%	720
	21st to end	Night	21% 10% 15% 31% 22% 2	720
October	1st to 10th	Night	20% 9% 16% 26% 23% 5%	720
	11th to 20th	Night	20% 15% 23% 26% 14% 1	720
	21st to end	Night	20% 18% 22% 28% 11% 2	792
November	1st to 10th	Night	18% 15% 22% 30% 14% 2	740
	11th to 20th	Night	23% 20% 21% 27% 9% 1	760
	21st to end	Night	22% 19% 23% 26% 9% 1	760
December	1st to 10th	Night	30% 20% 26% 19% 4	760
	11th to 20th	Night	23% 26% 23% 24% 4	760
	21st to end	Night	27% 22% 27% 22% 3	836

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