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 2026-06-16.

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	7% 5% 2 7% 5% 6% 6% 6% 8% 11% 36%	136730
	Night	7% 5% 2 7% 5% 5% 5% 5% 7% 10% 42%	135289
Transparency	Day	22% 39% 15% 16% 7% 1	140383
	Night	24% 37% 14% 16% 7% 2	139708
Seeing	Day	8% 4 16% 30% 26% 17%	18224
	Night	9% 3 14% 30% 31% 13%	28369

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	6% 5% 2 5% 3 3 4 4 6% 9% 53%	5258
	11th to 20th	Day	14% 7% 3 10% 6% 6% 6% 5% 7% 9% 28%	5280
	21st to end	Day	11% 7% 3 8% 5% 5% 5% 4 6% 7% 39%	5780
February	1st to 10th	Day	9% 5% 2 6% 5% 5% 4 4 7% 10% 42%	2095
	11th to 20th	Day	7% 4 1 5% 4 4 4 4 7% 10% 49%	2182
	21st to end	Day	6% 5% 2 6% 4 6% 6% 5% 8% 9% 44%	1903
March	1st to 10th	Day	5% 4 2 7% 6% 6% 7% 6% 8% 9% 40%	5045
	11th to 20th	Day	4 2 1 3 3 4 4 4 6% 10% 59%	5090
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 10% 42%	5586
April	1st to 10th	Day	4 3 2 5% 5% 4 5% 6% 9% 12% 45%	2310
	11th to 20th	Day	7% 6% 2 5% 5% 5% 5% 6% 8% 9% 42%	2409
	21st to end	Day	6% 4 2 6% 6% 5% 6% 5% 7% 10% 44%	2520
May	1st to 10th	Day	4 2 1 3 3 4 5% 4 7% 12% 54%	7021
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 9% 11% 41%	7089
	21st to end	Day	4 4 2 5% 5% 6% 8% 7% 10% 15% 36%	8391
June	1st to 10th	Day	13% 8% 4 11% 8% 8% 8% 6% 8% 9% 18%	2730
	11th to 20th	Day	8% 7% 3 10% 8% 8% 8% 6% 9% 11% 22%	2665
	21st to end	Day	7% 6% 3 9% 8% 7% 9% 7% 10% 11% 23%	2600
July	1st to 10th	Day	6% 6% 3 9% 7% 6% 7% 6% 9% 12% 29%	5900
	11th to 20th	Day	6% 5% 2 7% 6% 7% 7% 6% 9% 13% 31%	5900
	21st to end	Day	3 3 2 5% 5% 5% 7% 6% 10% 15% 39%	6424
August	1st to 10th	Day	5% 5% 3 10% 9% 8% 9% 7% 10% 12% 22%	4142
	11th to 20th	Day	4 4 2 7% 6% 6% 8% 7% 10% 16% 31%	3850
	21st to end	Day	5% 4 2 7% 7% 7% 8% 7% 13% 18% 21%	4093
September	1st to 10th	Day	8% 6% 3 9% 7% 9% 8% 7% 11% 12% 20%	2200
	11th to 20th	Day	8% 6% 3 9% 8% 8% 10% 6% 10% 11% 20%	2200
	21st to end	Day	10% 8% 3 11% 7% 9% 9% 6% 9% 10% 19%	2200
October	1st to 10th	Day	9% 7% 3 9% 7% 7% 8% 8% 9% 12% 22%	2085
	11th to 20th	Day	8% 6% 3 8% 7% 7% 9% 6% 8% 9% 29%	2000
	21st to end	Day	10% 7% 2 7% 5% 6% 6% 5% 7% 11% 34%	2200
November	1st to 10th	Day	8% 5% 2 7% 5% 6% 7% 6% 8% 9% 38%	2057
	11th to 20th	Day	11% 7% 2 8% 6% 6% 6% 4 8% 9% 34%	2100
	21st to end	Day	7% 6% 2 7% 6% 5% 7% 6% 7% 9% 38%	2100
December	1st to 10th	Day	6% 4 2 5% 4 4 5% 4 6% 8% 51%	3297
	11th to 20th	Day	9% 5% 2 7% 5% 6% 7% 5% 8% 10% 35%	3463
	21st to end	Day	11% 6% 3 8% 6% 6% 6% 6% 9% 10% 30%	4565
January	1st to 10th	Night	5% 4 2 6% 4 4 5% 4 7% 8% 52%	7635
	11th to 20th	Night	13% 9% 3 9% 6% 5% 5% 4 7% 9% 30%	7680
	21st to end	Night	12% 7% 3 8% 5% 5% 4 3 5% 6% 42%	8401
February	1st to 10th	Night	7% 5% 2 5% 5% 4 4 4 6% 10% 47%	2520
	11th to 20th	Night	7% 4 2 5% 3 4 4 4 6% 8% 55%	2320
	21st to end	Night	6% 4 1 4 3 4 4 5% 7% 8% 53%	1903
March	1st to 10th	Night	4 5% 2 7% 5% 5% 5% 4 6% 8% 49%	5028
	11th to 20th	Night	5% 2 2 4 3 4 4 3 6% 9% 59%	5090
	21st to end	Night	6% 4 2 6% 4 5% 6% 5% 7% 8% 47%	5588
April	1st to 10th	Night	4 3 1 4 3 3 5% 4 7% 11% 55%	2310
	11th to 20th	Night	4 3 2 5% 4 4 5% 5% 8% 10% 49%	2159
	21st to end	Night	5% 4 1 4 5% 5% 4 4 7% 10% 51%	2100
May	1st to 10th	Night	4 3 1 4 3 4 4 4 7% 10% 56%	5833
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 5% 7% 10% 46%	5797
	21st to end	Night	4 4 2 6% 5% 5% 6% 6% 10% 13% 39%	5761
June	1st to 10th	Night	9% 8% 3 9% 7% 7% 7% 7% 8% 10% 24%	1890
	11th to 20th	Night	6% 6% 2 7% 6% 6% 6% 7% 10% 13% 31%	1845
	21st to end	Night	5% 6% 2 8% 6% 6% 7% 6% 10% 13% 32%	1800
July	1st to 10th	Night	5% 6% 3 9% 6% 5% 6% 5% 8% 12% 36%	4058
	11th to 20th	Night	3 5% 2 7% 6% 5% 6% 6% 10% 13% 37%	4057
	21st to end	Night	2 3 1 5% 4 5% 5% 4 8% 13% 49%	4427
August	1st to 10th	Night	3 4 2 8% 6% 6% 7% 6% 10% 14% 33%	2971
	11th to 20th	Night	2 3 2 6% 4 5% 7% 6% 9% 14% 42%	3197
	21st to end	Night	4 4 2 8% 7% 7% 8% 7% 12% 16% 26%	3784
September	1st to 10th	Night	6% 6% 3 8% 7% 7% 8% 6% 10% 13% 26%	2200
	11th to 20th	Night	7% 7% 3 11% 5% 6% 8% 6% 8% 10% 29%	2200
	21st to end	Night	8% 7% 2 10% 8% 7% 8% 6% 9% 10% 23%	2200
October	1st to 10th	Night	9% 6% 3 9% 7% 6% 6% 6% 9% 10% 29%	2311
	11th to 20th	Night	8% 5% 3 8% 5% 5% 5% 5% 8% 10% 38%	2400
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	2640
November	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 41%	2461
	11th to 20th	Night	8% 6% 2 7% 6% 5% 6% 5% 7% 9% 38%	2520
	21st to end	Night	8% 6% 2 7% 6% 5% 5% 4 6% 9% 41%	2520
December	1st to 10th	Night	6% 4 2 5% 4 3 4 3 6% 8% 54%	4098
	11th to 20th	Night	12% 8% 3 7% 5% 5% 6% 5% 7% 9% 34%	4952
	21st to end	Night	9% 8% 4 10% 6% 6% 6% 5% 7% 9% 30%	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	21% 22% 8% 31% 15% 4	5259
	11th to 20th	Day	42% 25% 11% 16% 5% 2	5280
	21st to end	Day	34% 24% 6% 24% 9% 2	5780
February	1st to 10th	Day	12% 60% 7% 11% 7% 3	2095
	11th to 20th	Day	10% 58% 4 15% 10% 3	2192
	21st to end	Day	14% 60% 6% 9% 9% 3	1903
March	1st to 10th	Day	28% 31% 11% 16% 14% 1	5045
	11th to 20th	Day	11% 29% 12% 24% 15% 10%	5090
	21st to end	Day	26% 31% 8% 22% 10% 3	5586
April	1st to 10th	Day	9% 60% 8% 13% 8% 2	2310
	11th to 20th	Day	11% 60% 7% 8% 10% 4	2409
	21st to end	Day	10% 60% 4 14% 11% 1	2520
May	1st to 10th	Day	15% 25% 16% 31% 11% 2	7053
	11th to 20th	Day	23% 25% 22% 24% 5% 1	7093
	21st to end	Day	20% 29% 25% 21% 6%	8405
June	1st to 10th	Day	22% 62% 8% 6% 1	2730
	11th to 20th	Day	19% 64% 10% 7% 1	2665
	21st to end	Day	12% 66% 17% 5%	2600
July	1st to 10th	Day	29% 33% 27% 11% 1	5835
	11th to 20th	Day	26% 34% 29% 11%	5831
	21st to end	Day	18% 32% 29% 21% 1	6240
August	1st to 10th	Day	30% 44% 19% 7% 1	4129
	11th to 20th	Day	16% 43% 23% 17% 1	3886
	21st to end	Day	22% 45% 29% 4	4093
September	1st to 10th	Day	19% 63% 12% 6%	2200
	11th to 20th	Day	19% 61% 13% 6% 1	2200
	21st to end	Day	20% 63% 9% 7% 2	2200
October	1st to 10th	Day	31% 42% 13% 11% 2	3397
	11th to 20th	Day	29% 38% 11% 10% 10% 1	3240
	21st to end	Day	20% 37% 8% 22% 10% 3	3538
November	1st to 10th	Day	17% 55% 9% 13% 5% 1	2057
	11th to 20th	Day	20% 56% 5% 10% 7% 3	2100
	21st to end	Day	17% 55% 6% 11% 8% 3	2100
December	1st to 10th	Day	16% 36% 13% 21% 12% 2	3297
	11th to 20th	Day	26% 34% 10% 14% 12% 3	3460
	21st to end	Day	37% 30% 13% 13% 8%	4565
January	1st to 10th	Night	23% 23% 10% 27% 12% 5%	7636
	11th to 20th	Night	42% 24% 15% 13% 5% 2	7680
	21st to end	Night	34% 24% 6% 25% 8% 3	8401
February	1st to 10th	Night	10% 59% 8% 14% 7% 3	2520
	11th to 20th	Night	10% 57% 4 12% 13% 4	2321
	21st to end	Night	13% 60% 5% 9% 11% 3	1903
March	1st to 10th	Night	26% 30% 10% 17% 16% 2	5028
	11th to 20th	Night	14% 29% 15% 20% 13% 8%	5090
	21st to end	Night	24% 30% 10% 20% 15% 2	5588
April	1st to 10th	Night	8% 59% 8% 15% 8% 1	2310
	11th to 20th	Night	10% 59% 7% 9% 11% 3	2159
	21st to end	Night	8% 60% 8% 14% 8% 1	2100
May	1st to 10th	Night	17% 25% 22% 26% 8% 1	5863
	11th to 20th	Night	23% 25% 20% 24% 7%	5798
	21st to end	Night	22% 28% 25% 21% 4	5771
June	1st to 10th	Night	19% 61% 11% 7% 1	1890
	11th to 20th	Night	18% 65% 10% 7% 1	1845
	21st to end	Night	12% 64% 18% 6%	1800
July	1st to 10th	Night	29% 31% 29% 11%	4015
	11th to 20th	Night	25% 31% 35% 8%	4006
	21st to end	Night	15% 31% 32% 20% 1	4297
August	1st to 10th	Night	27% 42% 19% 10% 1	2960
	11th to 20th	Night	16% 40% 23% 20% 1	3227
	21st to end	Night	24% 45% 26% 4 1	3784
September	1st to 10th	Night	20% 63% 12% 5%	2200
	11th to 20th	Night	18% 61% 13% 6% 1	2200
	21st to end	Night	18% 63% 9% 7% 3	2200
October	1st to 10th	Night	35% 41% 12% 9% 2	3779
	11th to 20th	Night	31% 38% 9% 12% 10% 1	3899
	21st to end	Night	22% 36% 8% 21% 10% 3	4252
November	1st to 10th	Night	17% 54% 9% 12% 6% 2	2461
	11th to 20th	Night	19% 55% 6% 11% 6% 3	2520
	21st to end	Night	18% 54% 6% 11% 8% 3	2520
December	1st to 10th	Night	17% 36% 12% 22% 12% 1	4098
	11th to 20th	Night	30% 34% 9% 12% 10% 5%	4954
	21st to end	Night	39% 30% 12% 11% 7% 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	9% 5% 35% 36% 13% 2	330
	11th to 20th	Day	14% 8% 34% 29% 12% 3	330
	21st to end	Day	13% 7% 42% 31% 6%	363
February	1st to 10th	Day	8% 7% 28% 38% 15% 3	330
	11th to 20th	Day	8% 6% 26% 39% 18% 4	465
	21st to end	Day	6% 6% 24% 40% 22% 3	446
March	1st to 10th	Day	4 7% 33% 38% 16% 2	540
	11th to 20th	Day	8% 7% 27% 35% 20% 2	540
	21st to end	Day	5% 5% 28% 37% 20% 5%	594
April	1st to 10th	Day	4 5% 32% 37% 18% 4	540
	11th to 20th	Day	8% 5% 25% 38% 18% 5%	532
	21st to end	Day	6% 3 21% 40% 24% 6%	540
May	1st to 10th	Day	3 2 19% 41% 23% 11%	543
	11th to 20th	Day	10% 4 17% 33% 24% 12%	567
	21st to end	Day	9% 2 9% 27% 33% 20%	617
June	1st to 10th	Day	12% 3 8% 26% 30% 22%	570
	11th to 20th	Day	10% 3 4 21% 31% 31%	550
	21st to end	Day	4 3 2 16% 33% 42%	530
July	1st to 10th	Day	4 3 2 16% 31% 44%	525
	11th to 20th	Day	5% 2 1 15% 39% 39%	524
	21st to end	Day	4 3 2 18% 34% 41%	576
August	1st to 10th	Day	7% 4 3 16% 31% 39%	526
	11th to 20th	Day	4 2 1 14% 34% 44%	519
	21st to end	Day	9% 2 4 20% 34% 31%	583
September	1st to 10th	Day	8% 5% 2 14% 37% 34%	530
	11th to 20th	Day	6% 2 4 20% 31% 37%	530
	21st to end	Day	8% 5% 8% 20% 32% 26%	530
October	1st to 10th	Day	11% 3 8% 36% 32% 10%	530
	11th to 20th	Day	9% 4 15% 31% 31% 10%	530
	21st to end	Day	8% 5% 18% 37% 25% 8%	583
November	1st to 10th	Day	10% 6% 17% 36% 23% 8%	535
	11th to 20th	Day	11% 3 21% 36% 24% 4	540
	21st to end	Day	6% 4 24% 40% 23% 3	540
December	1st to 10th	Day	10% 4 20% 41% 21% 4	503
	11th to 20th	Day	8% 4 26% 42% 17% 4	330
	21st to end	Day	8% 10% 25% 33% 20% 4	363
January	1st to 10th	Night	9% 5% 22% 37% 25% 2	840
	11th to 20th	Night	11% 7% 27% 35% 19% 2	840
	21st to end	Night	10% 7% 28% 40% 14% 1	924
February	1st to 10th	Night	10% 6% 20% 39% 24% 2	840
	11th to 20th	Night	9% 4 25% 39% 21% 2	823
	21st to end	Night	6% 3 22% 45% 23% 1	692
March	1st to 10th	Night	5% 4 30% 40% 19% 2	840
	11th to 20th	Night	8% 4 22% 44% 21% 2	840
	21st to end	Night	7% 4 22% 44% 21% 2	924
April	1st to 10th	Night	5% 3 19% 45% 25% 3	840
	11th to 20th	Night	5% 4 19% 45% 25% 2	831
	21st to end	Night	6% 2 18% 44% 28% 2	840
May	1st to 10th	Night	5% 2 12% 43% 34% 3	840
	11th to 20th	Night	10% 3 12% 39% 31% 5%	822
	21st to end	Night	9% 2 4 28% 44% 13%	683
June	1st to 10th	Night	12% 3 3 25% 41% 17%	630
	11th to 20th	Night	7% 2 2 13% 44% 31%	615
	21st to end	Night	6% 1 2 11% 41% 40%	600
July	1st to 10th	Night	7% 1 1 5% 42% 43%	585
	11th to 20th	Night	6% 1 1 7% 40% 44%	582
	21st to end	Night	3 1 1 7% 44% 44%	657
August	1st to 10th	Night	7% 2 1 8% 45% 37%	627
	11th to 20th	Night	5% 1 1 9% 47% 37%	790
	21st to end	Night	10% 2 2 14% 41% 31%	880
September	1st to 10th	Night	9% 3 3 11% 39% 35%	800
	11th to 20th	Night	10% 3 3 13% 41% 32%	800
	21st to end	Night	12% 2 5% 17% 35% 29%	800
October	1st to 10th	Night	11% 4 4 31% 39% 11%	800
	11th to 20th	Night	12% 2 7% 30% 38% 11%	800
	21st to end	Night	14% 2 14% 34% 29% 7%	880
November	1st to 10th	Night	10% 4 18% 33% 30% 5%	820
	11th to 20th	Night	10% 3 18% 32% 32% 5%	840
	21st to end	Night	10% 4 17% 40% 26% 3	840
December	1st to 10th	Night	9% 5% 16% 36% 31% 4	840
	11th to 20th	Night	12% 5% 18% 36% 26% 3	840
	21st to end	Night	11% 5% 20% 33% 27% 5%	924

Page generated at 2026-06-13 02:31:44. Version ac07

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