Coral Lakes Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Coral Lakes Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	7% 5% 2 7% 6% 6% 7% 6% 8% 11% 34%	108757
	Night	7% 6% 2 7% 5% 5% 6% 5% 8% 10% 40%	109813
Transparency	Day	21% 46% 13% 14% 6% 1	112454
	Night	22% 44% 12% 14% 6% 1	114255
Seeing	Day	8% 4 16% 29% 26% 18%	16371
	Night	9% 3 13% 30% 32% 13%	26557

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	7% 6% 2 6% 4 3 4 4 6% 10% 48%	4056
	11th to 20th	Day	16% 7% 3 10% 6% 6% 6% 4 6% 9% 28%	4060
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 4 7% 7% 38%	4466
February	1st to 10th	Day	9% 6% 3 6% 5% 5% 5% 5% 7% 11% 40%	1995
	11th to 20th	Day	7% 4 1 5% 4 4 5% 4 7% 10% 48%	2075
	21st to end	Day	6% 6% 2 6% 4 6% 6% 5% 8% 9% 44%	1727
March	1st to 10th	Day	6% 5% 2 9% 6% 7% 9% 7% 9% 10% 29%	3435
	11th to 20th	Day	5% 3 1 4 4 5% 5% 5% 7% 12% 50%	3480
	21st to end	Day	9% 5% 2 5% 4 5% 5% 4 7% 9% 45%	3815
April	1st to 10th	Day	4 3 2 6% 5% 4 5% 6% 9% 12% 45%	2090
	11th to 20th	Day	7% 6% 2 5% 5% 5% 6% 6% 8% 9% 40%	2177
	21st to end	Day	6% 4 2 6% 6% 5% 6% 6% 7% 9% 43%	2280
May	1st to 10th	Day	2 2 1 3 3 4 4 5% 7% 11% 58%	3765
	11th to 20th	Day	7% 5% 2 7% 5% 6% 7% 5% 9% 11% 35%	3795
	21st to end	Day	5% 5% 2 5% 5% 5% 6% 6% 8% 11% 43%	4476
June	1st to 10th	Day	13% 8% 4 11% 8% 7% 8% 6% 8% 9% 18%	2470
	11th to 20th	Day	8% 7% 3 10% 8% 9% 9% 6% 9% 10% 20%	2470
	21st to end	Day	7% 6% 3 9% 8% 7% 9% 7% 10% 11% 23%	2470
July	1st to 10th	Day	5% 6% 2 8% 6% 6% 7% 6% 9% 13% 32%	4120
	11th to 20th	Day	6% 6% 2 7% 5% 7% 8% 6% 9% 14% 29%	4120
	21st to end	Day	4 4 2 6% 6% 6% 7% 7% 10% 15% 32%	4466
August	1st to 10th	Day	5% 5% 3 10% 9% 9% 9% 7% 10% 12% 21%	4015
	11th to 20th	Day	4 4 2 7% 6% 6% 8% 7% 10% 16% 30%	3730
	21st to end	Day	5% 4 2 7% 7% 7% 8% 7% 13% 18% 20%	3968
September	1st to 10th	Day	7% 6% 3 9% 8% 9% 8% 7% 11% 12% 20%	2090
	11th to 20th	Day	8% 6% 3 10% 8% 8% 10% 6% 10% 11% 20%	2090
	21st to end	Day	10% 8% 3 11% 7% 9% 9% 6% 8% 10% 19%	2090
October	1st to 10th	Day	9% 7% 3 9% 7% 7% 8% 8% 9% 11% 22%	1981
	11th to 20th	Day	8% 6% 3 9% 7% 7% 9% 6% 8% 9% 28%	1900
	21st to end	Day	10% 7% 2 8% 5% 6% 6% 5% 7% 10% 33%	2090
November	1st to 10th	Day	8% 5% 2 7% 5% 6% 7% 5% 8% 9% 38%	1957
	11th to 20th	Day	11% 7% 2 8% 7% 6% 6% 5% 8% 9% 32%	2000
	21st to end	Day	8% 6% 2 7% 6% 5% 7% 6% 7% 9% 38%	2000
December	1st to 10th	Day	6% 5% 2 5% 4 4 5% 4 6% 8% 52%	3199
	11th to 20th	Day	9% 5% 2 7% 5% 6% 7% 5% 8% 10% 35%	3373
	21st to end	Day	11% 7% 3 8% 6% 6% 6% 6% 9% 10% 29%	4466
January	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 5% 8% 9% 43%	5889
	11th to 20th	Night	15% 10% 3 9% 6% 5% 5% 4 7% 8% 29%	5900
	21st to end	Night	12% 7% 2 8% 6% 5% 4 4 6% 6% 40%	6490
February	1st to 10th	Night	7% 6% 2 6% 5% 4 4 4 6% 10% 46%	2400
	11th to 20th	Night	6% 4 2 5% 3 4 4 4 6% 7% 55%	2207
	21st to end	Night	5% 5% 2 5% 4 4 4 5% 7% 8% 52%	1728
March	1st to 10th	Night	6% 7% 3 9% 6% 6% 7% 5% 6% 8% 37%	3418
	11th to 20th	Night	4 2 1 4 3 4 5% 4 8% 11% 54%	3480
	21st to end	Night	7% 4 2 5% 3 4 5% 4 7% 8% 51%	3817
April	1st to 10th	Night	4 2 1 4 3 3 5% 4 7% 11% 55%	2090
	11th to 20th	Night	4 4 2 5% 5% 4 6% 5% 8% 10% 47%	1951
	21st to end	Night	5% 4 1 4 5% 5% 4 4 6% 10% 50%	1900
May	1st to 10th	Night	3 2 1 3 2 3 3 4 6% 9% 65%	3130
	11th to 20th	Night	5% 5% 2 5% 5% 5% 5% 5% 7% 10% 46%	3111
	21st to end	Night	5% 5% 2 5% 5% 4 5% 4 7% 9% 47%	3079
June	1st to 10th	Night	10% 8% 3 9% 6% 7% 7% 7% 8% 11% 24%	1710
	11th to 20th	Night	7% 6% 2 7% 6% 6% 6% 7% 10% 12% 30%	1710
	21st to end	Night	5% 6% 2 8% 6% 6% 7% 6% 10% 13% 31%	1710
July	1st to 10th	Night	5% 5% 3 7% 5% 4 6% 5% 8% 12% 41%	2840
	11th to 20th	Night	4 5% 2 7% 5% 5% 6% 7% 11% 13% 35%	2840
	21st to end	Night	2 3 2 7% 5% 6% 6% 5% 9% 14% 41%	3087
August	1st to 10th	Night	4 4 2 8% 6% 6% 7% 6% 10% 14% 33%	2879
	11th to 20th	Night	2 3 2 6% 4 6% 7% 6% 9% 14% 41%	3097
	21st to end	Night	4 4 2 7% 7% 7% 7% 7% 12% 16% 26%	3668
September	1st to 10th	Night	6% 6% 3 8% 7% 7% 8% 6% 10% 13% 26%	2090
	11th to 20th	Night	7% 7% 3 10% 5% 7% 8% 6% 8% 10% 29%	2090
	21st to end	Night	8% 7% 2 10% 8% 8% 8% 6% 9% 10% 23%	2090
October	1st to 10th	Night	9% 6% 3 10% 7% 6% 6% 6% 9% 10% 29%	2195
	11th to 20th	Night	8% 5% 3 8% 5% 5% 5% 5% 8% 10% 38%	2280
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 7% 9% 39%	2508
November	1st to 10th	Night	8% 5% 2 5% 5% 5% 5% 5% 7% 10% 42%	2341
	11th to 20th	Night	8% 7% 2 7% 6% 5% 6% 5% 7% 9% 37%	2400
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	2400
December	1st to 10th	Night	6% 4 2 5% 4 3 4 3 5% 8% 55%	3976
	11th to 20th	Night	12% 8% 3 8% 5% 5% 6% 5% 7% 9% 33%	4822
	21st to end	Night	9% 8% 4 10% 6% 7% 6% 5% 7% 9% 29%	6490

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	23% 27% 8% 23% 17% 2	4057
	11th to 20th	Day	41% 29% 11% 15% 4	4060
	21st to end	Day	32% 30% 4 23% 7% 3	4466
February	1st to 10th	Day	12% 63% 7% 11% 6% 1	1995
	11th to 20th	Day	10% 61% 3 14% 10% 3	2085
	21st to end	Day	12% 65% 5% 7% 8% 2	1727
March	1st to 10th	Day	31% 44% 12% 9% 3 1	3435
	11th to 20th	Day	12% 42% 15% 26% 4	3480
	21st to end	Day	21% 43% 6% 18% 10% 3	3815
April	1st to 10th	Day	7% 66% 8% 13% 6% 1	2090
	11th to 20th	Day	10% 66% 7% 8% 7% 2	2177
	21st to end	Day	10% 66% 4 11% 9% 1	2280
May	1st to 10th	Day	8% 42% 8% 24% 14% 3	3800
	11th to 20th	Day	19% 42% 19% 15% 4	3799
	21st to end	Day	11% 42% 11% 26% 9%	4489
June	1st to 10th	Day	18% 68% 8% 5% 1	2470
	11th to 20th	Day	18% 69% 7% 5%	2470
	21st to end	Day	11% 69% 16% 5%	2470
July	1st to 10th	Day	18% 41% 25% 15% 1	4055
	11th to 20th	Day	22% 41% 28% 8%	4051
	21st to end	Day	18% 43% 28% 11%	4324
August	1st to 10th	Day	30% 45% 18% 6% 1	4002
	11th to 20th	Day	17% 44% 23% 16% 1	3766
	21st to end	Day	21% 46% 29% 4	3968
September	1st to 10th	Day	17% 64% 12% 6%	2090
	11th to 20th	Day	18% 63% 12% 6% 1	2090
	21st to end	Day	19% 65% 8% 7% 2	2090
October	1st to 10th	Day	31% 43% 13% 11% 2	3293
	11th to 20th	Day	29% 39% 12% 10% 9% 1	3140
	21st to end	Day	20% 38% 8% 22% 10% 2	3428
November	1st to 10th	Day	16% 57% 8% 12% 5% 1	1957
	11th to 20th	Day	20% 58% 5% 9% 5% 3	2000
	21st to end	Day	17% 57% 5% 10% 8% 3	2000
December	1st to 10th	Day	16% 37% 13% 20% 12% 2	3199
	11th to 20th	Day	26% 35% 10% 14% 12% 4	3370
	21st to end	Day	37% 31% 13% 12% 7%	4466
January	1st to 10th	Night	27% 28% 10% 21% 11% 3	5889
	11th to 20th	Night	43% 28% 13% 11% 4	5900
	21st to end	Night	34% 29% 4 22% 8% 3	6490
February	1st to 10th	Night	10% 62% 8% 13% 6%	2400
	11th to 20th	Night	9% 60% 3 11% 12% 4	2208
	21st to end	Night	11% 65% 5% 8% 9% 3	1728
March	1st to 10th	Night	33% 43% 10% 10% 3 1	3418
	11th to 20th	Night	12% 41% 20% 20% 5% 1	3480
	21st to end	Night	17% 42% 9% 15% 15% 2	3817
April	1st to 10th	Night	6% 65% 7% 15% 5% 1	2090
	11th to 20th	Night	10% 66% 7% 9% 8% 1	1951
	21st to end	Night	7% 65% 8% 12% 6% 2	1900
May	1st to 10th	Night	9% 41% 13% 26% 8% 3	3160
	11th to 20th	Night	16% 41% 17% 21% 5%	3112
	21st to end	Night	13% 43% 11% 27% 7%	3087
June	1st to 10th	Night	15% 67% 11% 6% 1	1710
	11th to 20th	Night	17% 69% 8% 6%	1710
	21st to end	Night	11% 66% 17% 6%	1710
July	1st to 10th	Night	18% 41% 26% 15%	2795
	11th to 20th	Night	20% 40% 33% 7%	2786
	21st to end	Night	18% 42% 28% 12%	2988
August	1st to 10th	Night	27% 43% 19% 10% 1	2868
	11th to 20th	Night	16% 41% 22% 19% 1	3127
	21st to end	Night	23% 46% 26% 4	3668
September	1st to 10th	Night	18% 65% 12% 5%	2090
	11th to 20th	Night	17% 64% 14% 5% 1	2090
	21st to end	Night	17% 65% 8% 7% 3	2090
October	1st to 10th	Night	35% 42% 12% 9% 2	3663
	11th to 20th	Night	31% 39% 9% 12% 9%	3779
	21st to end	Night	21% 37% 8% 21% 10% 3	4120
November	1st to 10th	Night	16% 57% 8% 12% 6% 1	2341
	11th to 20th	Night	19% 58% 6% 10% 5% 3	2400
	21st to end	Night	18% 57% 6% 9% 8% 3	2400
December	1st to 10th	Night	17% 37% 11% 22% 13% 1	3976
	11th to 20th	Night	30% 34% 9% 12% 10% 5%	4824
	21st to end	Night	40% 31% 12% 11% 6% 1	6490

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	10% 5% 37% 36% 11% 1	300
	11th to 20th	Day	14% 7% 33% 31% 12% 3	300
	21st to end	Day	13% 6% 42% 33% 6%	330
February	1st to 10th	Day	9% 6% 25% 40% 17% 3	300
	11th to 20th	Day	8% 6% 24% 39% 18% 4	428
	21st to end	Day	5% 5% 22% 41% 24% 3	382
March	1st to 10th	Day	4 6% 34% 37% 17% 3	460
	11th to 20th	Day	7% 5% 27% 36% 23% 3	460
	21st to end	Day	6% 4 28% 38% 20% 5%	506
April	1st to 10th	Day	4 5% 32% 36% 19% 5%	460
	11th to 20th	Day	9% 6% 25% 36% 19% 6%	452
	21st to end	Day	6% 3 21% 38% 25% 7%	460
May	1st to 10th	Day	3 2 16% 41% 25% 13%	463
	11th to 20th	Day	11% 3 15% 32% 25% 13%	487
	21st to end	Day	9% 2 9% 24% 33% 22%	529
June	1st to 10th	Day	11% 3 7% 25% 30% 23%	490
	11th to 20th	Day	11% 3 4 21% 29% 32%	490
	21st to end	Day	4 3 2 14% 33% 45%	490
July	1st to 10th	Day	3 3 2 15% 31% 46%	485
	11th to 20th	Day	5% 2 1 13% 39% 41%	484
	21st to end	Day	5% 3 1 16% 33% 42%	532
August	1st to 10th	Day	7% 4 3 16% 31% 39%	486
	11th to 20th	Day	5% 3 1 13% 34% 46%	479
	21st to end	Day	9% 2 4 21% 32% 31%	539
September	1st to 10th	Day	8% 4 2 15% 38% 34%	490
	11th to 20th	Day	6% 2 3 19% 31% 39%	490
	21st to end	Day	9% 5% 7% 20% 31% 27%	490
October	1st to 10th	Day	12% 3 8% 35% 32% 11%	490
	11th to 20th	Day	9% 4 16% 29% 32% 11%	490
	21st to end	Day	9% 5% 17% 36% 25% 8%	539
November	1st to 10th	Day	9% 6% 18% 35% 23% 9%	495
	11th to 20th	Day	12% 3 21% 36% 24% 4	500
	21st to end	Day	6% 4 24% 40% 23% 3	500
December	1st to 10th	Day	10% 5% 20% 41% 20% 4	465
	11th to 20th	Day	9% 2 26% 42% 17% 4	300
	21st to end	Day	8% 11% 24% 34% 20% 3	330
January	1st to 10th	Night	10% 5% 23% 37% 24% 2	800
	11th to 20th	Night	11% 6% 26% 36% 20% 2	800
	21st to end	Night	10% 6% 27% 40% 15% 1	880
February	1st to 10th	Night	10% 6% 18% 40% 25% 2	800
	11th to 20th	Night	8% 4 25% 39% 22% 2	783
	21st to end	Night	6% 3 21% 45% 24% 1	629
March	1st to 10th	Night	5% 4 30% 39% 20% 2	760
	11th to 20th	Night	7% 2 21% 46% 22% 2	760
	21st to end	Night	7% 3 22% 45% 21% 2	836
April	1st to 10th	Night	5% 2 19% 45% 27% 3	760
	11th to 20th	Night	5% 4 18% 45% 25% 2	751
	21st to end	Night	7% 2 18% 44% 27% 2	760
May	1st to 10th	Night	5% 2 12% 42% 37% 4	760
	11th to 20th	Night	11% 3 10% 39% 31% 5%	743
	21st to end	Night	9% 2 4 27% 44% 14%	617
June	1st to 10th	Night	12% 2 3 25% 41% 18%	570
	11th to 20th	Night	8% 2 2 14% 43% 31%	570
	21st to end	Night	6% 1 1 11% 40% 41%	570
July	1st to 10th	Night	7% 1 1 5% 42% 44%	555
	11th to 20th	Night	7% 1 1 7% 40% 45%	552
	21st to end	Night	3 1 7% 43% 45%	624
August	1st to 10th	Night	8% 2 2 8% 45% 36%	595
	11th to 20th	Night	5% 1 1 9% 46% 38%	750
	21st to end	Night	10% 2 2 14% 40% 31%	836
September	1st to 10th	Night	8% 3 3 11% 39% 36%	760
	11th to 20th	Night	9% 3 2 11% 41% 33%	760
	21st to end	Night	13% 2 5% 16% 36% 29%	760
October	1st to 10th	Night	12% 4 4 29% 40% 11%	760
	11th to 20th	Night	12% 3 7% 30% 38% 11%	760
	21st to end	Night	15% 2 13% 34% 29% 8%	836
November	1st to 10th	Night	10% 4 18% 32% 31% 5%	780
	11th to 20th	Night	10% 2 19% 32% 33% 5%	800
	21st to end	Night	10% 4 17% 41% 26% 3	800
December	1st to 10th	Night	9% 5% 16% 36% 31% 5%	800
	11th to 20th	Night	12% 4 18% 36% 27% 4	800
	21st to end	Night	11% 4 20% 33% 27% 4	880

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