McCauley Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McCauley Lake Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	15% 12% 4 9% 5% 5% 4 4 5% 7% 30%	150505
	Night	14% 13% 5% 11% 6% 5% 4 3 5% 6% 29%	140836
Transparency	Day	52% 6% 11% 20% 10% 1	154048
	Night	54% 7% 11% 18% 9% 1	145381
Seeing	Day	23% 18% 32% 18% 7% 2	17081
	Night	21% 16% 32% 22% 8% 1	25889

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	10% 16% 6% 15% 9% 8% 6% 3 5% 5% 16%	4661
	11th to 20th	Day	9% 21% 8% 17% 8% 6% 5% 3 4 4 12%	4650
	21st to end	Day	9% 16% 9% 17% 9% 7% 5% 3 3 4 18%	5720
February	1st to 10th	Day	10% 17% 6% 15% 10% 5% 4 3 4 4 20%	1890
	11th to 20th	Day	10% 15% 6% 14% 8% 6% 5% 3 4 5% 23%	1843
	21st to end	Day	14% 17% 5% 11% 7% 5% 5% 4 5% 5% 20%	1619
March	1st to 10th	Day	16% 17% 5% 12% 5% 4 4 3 4 6% 24%	4896
	11th to 20th	Day	11% 14% 5% 12% 6% 5% 5% 4 5% 7% 25%	5090
	21st to end	Day	15% 15% 5% 10% 5% 4 3 3 3 5% 33%	5673
April	1st to 10th	Day	19% 15% 5% 9% 4 4 4 3 4 5% 28%	2520
	11th to 20th	Day	22% 13% 3 7% 3 4 4 3 4 4 33%	2531
	21st to end	Day	21% 12% 4 8% 4 4 3 3 4 4 33%	2730
May	1st to 10th	Day	17% 11% 4 9% 5% 5% 4 4 5% 7% 29%	8054
	11th to 20th	Day	17% 11% 3 7% 5% 4 4 3 5% 6% 34%	8221
	21st to end	Day	19% 10% 3 7% 4 4 4 3 5% 5% 37%	9060
June	1st to 10th	Day	20% 9% 3 7% 6% 5% 4 4 5% 7% 30%	2940
	11th to 20th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 9% 35%	2976
	21st to end	Day	17% 7% 3 7% 5% 5% 6% 5% 5% 8% 32%	3150
July	1st to 10th	Day	14% 6% 2 6% 4 5% 5% 4 6% 10% 36%	8543
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 7% 9% 46%	8280
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 7% 9% 41%	8993
August	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 7% 32%	6024
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 7% 9% 36%	5951
	21st to end	Day	13% 7% 2 6% 5% 6% 6% 5% 7% 11% 33%	6266
September	1st to 10th	Day	16% 7% 3 6% 4 5% 5% 4 6% 8% 38%	2520
	11th to 20th	Day	13% 7% 2 6% 4 4 4 3 5% 8% 44%	2503
	21st to end	Day	19% 9% 3 7% 5% 5% 4 3 5% 6% 34%	2181
October	1st to 10th	Day	18% 11% 3 7% 4 5% 3 4 5% 6% 34%	2000
	11th to 20th	Day	22% 15% 3 9% 6% 5% 3 3 4 5% 25%	2000
	21st to end	Day	26% 17% 5% 9% 5% 4 4 2 3 5% 20%	2150
November	1st to 10th	Day	16% 16% 5% 9% 4 4 4 3 6% 6% 28%	1716
	11th to 20th	Day	15% 18% 6% 11% 7% 5% 4 3 4 5% 22%	1680
	21st to end	Day	19% 20% 8% 13% 6% 5% 4 3 3 4 14%	1680
December	1st to 10th	Day	14% 24% 11% 14% 7% 4 4 3 4 4 11%	2680
	11th to 20th	Day	15% 21% 7% 14% 8% 4 3 2 3 3 19%	3066
	21st to end	Day	18% 21% 8% 13% 7% 3 3 2 3 4 17%	4048
January	1st to 10th	Night	11% 17% 9% 18% 9% 6% 5% 3 4 4 14%	8234
	11th to 20th	Night	8% 17% 8% 19% 10% 8% 6% 4 4 4 13%	8235
	21st to end	Night	8% 14% 8% 17% 8% 8% 7% 4 5% 5% 17%	8447
February	1st to 10th	Night	11% 17% 6% 13% 8% 6% 4 4 5% 6% 19%	2730
	11th to 20th	Night	10% 16% 6% 14% 7% 6% 4 3 5% 5% 24%	2663
	21st to end	Night	11% 17% 5% 13% 6% 6% 5% 4 5% 6% 22%	2186
March	1st to 10th	Night	15% 14% 5% 12% 6% 5% 4 4 3 5% 27%	5160
	11th to 20th	Night	9% 13% 5% 10% 6% 5% 6% 5% 7% 8% 25%	5089
	21st to end	Night	15% 13% 4 10% 6% 4 4 3 5% 6% 29%	5500
April	1st to 10th	Night	19% 15% 4 9% 4 3 3 3 5% 5% 29%	2100
	11th to 20th	Night	18% 11% 4 7% 5% 4 3 3 4 6% 36%	2055
	21st to end	Night	17% 11% 5% 8% 5% 5% 4 3 5% 6% 31%	1890
May	1st to 10th	Night	18% 10% 3 6% 4 5% 4 4 5% 8% 34%	4843
	11th to 20th	Night	16% 9% 3 6% 3 4 3 3 5% 7% 41%	4639
	21st to end	Night	18% 8% 2 6% 4 4 4 3 5% 7% 38%	5114
June	1st to 10th	Night	15% 10% 3 6% 4 4 5% 4 5% 8% 35%	1680
	11th to 20th	Night	14% 7% 2 6% 4 4 3 3 6% 8% 43%	1633
	21st to end	Night	15% 8% 3 7% 5% 4 4 5% 6% 6% 37%	1470
July	1st to 10th	Night	12% 7% 3 7% 5% 4 5% 3 6% 8% 40%	4395
	11th to 20th	Night	11% 7% 2 5% 4 4 4 4 5% 6% 48%	4680
	21st to end	Night	10% 7% 2 6% 4 4 4 3 6% 7% 47%	5084
August	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 7% 9% 41%	4109
	11th to 20th	Night	15% 7% 2 6% 3 4 4 4 5% 7% 43%	4095
	21st to end	Night	15% 7% 2 5% 4 4 4 4 6% 8% 41%	4812
September	1st to 10th	Night	12% 6% 1 5% 4 4 5% 4 5% 8% 46%	2100
	11th to 20th	Night	13% 8% 2 6% 3 3 3 3 4 6% 49%	2114
	21st to end	Night	16% 9% 3 7% 3 4 4 3 5% 6% 41%	2395
October	1st to 10th	Night	18% 10% 3 6% 4 3 4 3 5% 6% 39%	2400
	11th to 20th	Night	19% 14% 4 8% 5% 5% 4 3 4 6% 28%	2400
	21st to end	Night	22% 16% 4 9% 5% 4 4 3 4 5% 23%	2688
November	1st to 10th	Night	16% 15% 5% 9% 5% 4 4 4 4 5% 28%	2798
	11th to 20th	Night	15% 15% 5% 10% 7% 6% 4 3 5% 6% 24%	2940
	21st to end	Night	18% 20% 7% 12% 7% 5% 4 3 4 5% 15%	2940
December	1st to 10th	Night	15% 22% 7% 14% 8% 5% 5% 4 4 5% 11%	4720
	11th to 20th	Night	15% 20% 8% 13% 6% 4 3 3 4 4 19%	5348
	21st to end	Night	20% 20% 7% 15% 7% 4 3 3 3 4 14%	7150

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	68% 7% 9% 11% 5% 1	4662
	11th to 20th	Day	73% 5% 6% 7% 8% 1	4680
	21st to end	Day	71% 4 5% 11% 8% 1	5720
February	1st to 10th	Day	66% 6% 7% 13% 7% 1	1890
	11th to 20th	Day	63% 5% 9% 13% 9% 1	1852
	21st to end	Day	64% 5% 8% 13% 10% 1	1619
March	1st to 10th	Day	61% 5% 10% 9% 14% 2	4899
	11th to 20th	Day	56% 7% 13% 15% 7% 2	5090
	21st to end	Day	54% 4 7% 18% 14% 3	5673
April	1st to 10th	Day	59% 3 8% 18% 10% 2	2520
	11th to 20th	Day	54% 5% 7% 21% 12% 1	2531
	21st to end	Day	54% 5% 7% 18% 13% 3	2730
May	1st to 10th	Day	51% 7% 10% 20% 9% 4	8064
	11th to 20th	Day	48% 7% 10% 22% 13% 1	8225
	21st to end	Day	49% 6% 9% 21% 14% 2	9061
June	1st to 10th	Day	51% 7% 10% 19% 12% 1	2940
	11th to 20th	Day	44% 7% 11% 23% 13% 2	2976
	21st to end	Day	48% 7% 12% 21% 10% 1	3150
July	1st to 10th	Day	40% 8% 16% 25% 11%	8474
	11th to 20th	Day	33% 6% 14% 35% 12%	8205
	21st to end	Day	37% 7% 15% 31% 10%	8795
August	1st to 10th	Day	49% 7% 12% 20% 10% 1	6010
	11th to 20th	Day	41% 8% 18% 27% 5%	5990
	21st to end	Day	42% 8% 17% 26% 7%	6266
September	1st to 10th	Day	42% 7% 14% 26% 10% 1	2520
	11th to 20th	Day	38% 6% 13% 29% 14%	2503
	21st to end	Day	49% 7% 10% 23% 11%	2181
October	1st to 10th	Day	49% 7% 10% 25% 8%	3260
	11th to 20th	Day	55% 5% 9% 19% 10% 1	3240
	21st to end	Day	63% 6% 11% 16% 4 1	3452
November	1st to 10th	Day	55% 7% 13% 19% 6% 1	1716
	11th to 20th	Day	63% 7% 10% 12% 7% 1	1680
	21st to end	Day	73% 6% 8% 9% 4 1	1680
December	1st to 10th	Day	76% 6% 7% 8% 2	2680
	11th to 20th	Day	70% 5% 5% 11% 8% 1	3066
	21st to end	Day	72% 4 5% 13% 6%	4048
January	1st to 10th	Night	72% 7% 8% 9% 5%	8234
	11th to 20th	Night	70% 8% 6% 7% 7% 1	8280
	21st to end	Night	66% 6% 6% 11% 10% 1	8447
February	1st to 10th	Night	64% 5% 11% 13% 7% 1	2730
	11th to 20th	Night	61% 5% 9% 16% 8%	2665
	21st to end	Night	60% 6% 9% 17% 7% 1	2186
March	1st to 10th	Night	58% 5% 9% 13% 10% 4	5170
	11th to 20th	Night	52% 9% 16% 15% 7% 1	5090
	21st to end	Night	53% 6% 9% 18% 12% 2	5500
April	1st to 10th	Night	57% 4 8% 18% 11% 2	2100
	11th to 20th	Night	50% 5% 8% 23% 13% 1	2055
	21st to end	Night	54% 4 8% 17% 15% 2	1890
May	1st to 10th	Night	46% 8% 9% 24% 10% 3	4849
	11th to 20th	Night	42% 8% 11% 23% 14% 2	4640
	21st to end	Night	43% 8% 10% 24% 13% 1	5115
June	1st to 10th	Night	45% 7% 11% 21% 14% 1	1680
	11th to 20th	Night	38% 7% 13% 26% 15% 1	1633
	21st to end	Night	45% 8% 14% 20% 12%	1470
July	1st to 10th	Night	40% 9% 17% 22% 12%	4355
	11th to 20th	Night	34% 6% 15% 32% 13%	4632
	21st to end	Night	37% 7% 11% 32% 13%	4967
August	1st to 10th	Night	38% 8% 15% 23% 14% 1	4099
	11th to 20th	Night	37% 8% 20% 28% 6%	4122
	21st to end	Night	39% 8% 16% 28% 7% 1	4812
September	1st to 10th	Night	35% 7% 16% 29% 12% 1	2100
	11th to 20th	Night	36% 5% 13% 32% 13%	2114
	21st to end	Night	44% 6% 12% 25% 11% 1	2395
October	1st to 10th	Night	39% 5% 13% 28% 14%	3920
	11th to 20th	Night	51% 5% 13% 19% 11% 1	3899
	21st to end	Night	55% 6% 18% 16% 5%	4336
November	1st to 10th	Night	56% 6% 13% 17% 7%	2798
	11th to 20th	Night	60% 7% 11% 15% 6% 1	2940
	21st to end	Night	70% 6% 9% 11% 3	2940
December	1st to 10th	Night	74% 6% 10% 7% 2	4720
	11th to 20th	Night	68% 5% 6% 11% 9%	5348
	21st to end	Night	75% 4 5% 10% 6%	7150

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	20% 24% 40% 12% 3 1	220
	11th to 20th	Day	29% 16% 36% 15% 2 2	220
	21st to end	Day	24% 22% 33% 17% 4	352
February	1st to 10th	Day	28% 27% 32% 9% 2 1	330
	11th to 20th	Day	27% 30% 35% 6% 1	330
	21st to end	Day	34% 21% 35% 9% 1	335
March	1st to 10th	Day	26% 23% 36% 13% 2	540
	11th to 20th	Day	28% 21% 33% 14% 3 1	540
	21st to end	Day	28% 26% 29% 13% 4	594
April	1st to 10th	Day	29% 22% 31% 12% 4 1	540
	11th to 20th	Day	36% 22% 20% 14% 6% 2	532
	21st to end	Day	30% 18% 33% 12% 5% 1	540
May	1st to 10th	Day	25% 21% 29% 17% 6% 2	543
	11th to 20th	Day	22% 19% 31% 18% 8% 2	567
	21st to end	Day	21% 21% 29% 18% 9% 2	617
June	1st to 10th	Day	22% 14% 38% 19% 6% 1	570
	11th to 20th	Day	19% 17% 35% 19% 9% 1	606
	21st to end	Day	19% 15% 36% 21% 7% 1	780
July	1st to 10th	Day	14% 10% 38% 28% 8% 2	660
	11th to 20th	Day	13% 15% 35% 25% 10% 2	564
	21st to end	Day	12% 14% 34% 28% 10% 3	620
August	1st to 10th	Day	15% 11% 30% 30% 11% 4	566
	11th to 20th	Day	15% 10% 28% 31% 12% 4	559
	21st to end	Day	15% 13% 34% 24% 11% 4	627
September	1st to 10th	Day	19% 12% 32% 21% 12% 4	570
	11th to 20th	Day	15% 10% 26% 24% 16% 9%	570
	21st to end	Day	20% 16% 26% 21% 13% 4	562
October	1st to 10th	Day	23% 18% 25% 20% 10% 3	530
	11th to 20th	Day	29% 25% 27% 14% 4 1	530
	21st to end	Day	34% 19% 28% 15% 3	555
November	1st to 10th	Day	32% 22% 32% 11% 3	290
	11th to 20th	Day	30% 22% 33% 11% 4	220
	21st to end	Day	31% 23% 38% 5% 3	220
December	1st to 10th	Day	28% 23% 33% 15% 1	220
	11th to 20th	Day	34% 26% 28% 10% 1 1	220
	21st to end	Day	32% 23% 26% 15% 3 1	242
January	1st to 10th	Night	23% 22% 36% 16% 3 1	840
	11th to 20th	Night	25% 20% 32% 18% 5% 1	840
	21st to end	Night	17% 18% 37% 22% 6% 1	924
February	1st to 10th	Night	20% 22% 38% 17% 3	840
	11th to 20th	Night	19% 21% 40% 17% 4	823
	21st to end	Night	22% 23% 33% 20% 2	692
March	1st to 10th	Night	21% 22% 36% 18% 3	840
	11th to 20th	Night	21% 21% 36% 16% 5% 1	840
	21st to end	Night	20% 19% 34% 20% 6%	924
April	1st to 10th	Night	27% 17% 33% 18% 4	840
	11th to 20th	Night	25% 20% 28% 19% 9%	811
	21st to end	Night	22% 15% 30% 22% 10%	630
May	1st to 10th	Night	22% 11% 27% 30% 9% 1	486
	11th to 20th	Night	21% 12% 33% 26% 7%	414
	21st to end	Night	16% 12% 34% 27% 10%	455
June	1st to 10th	Night	18% 10% 35% 30% 6% 1	420
	11th to 20th	Night	18% 11% 30% 30% 10%	420
	21st to end	Night	19% 10% 28% 33% 10%	420
July	1st to 10th	Night	10% 7% 34% 36% 13%	410
	11th to 20th	Night	15% 8% 30% 33% 14%	408
	21st to end	Night	10% 8% 37% 33% 12% 1	460
August	1st to 10th	Night	12% 5% 26% 37% 18% 2	614
	11th to 20th	Night	11% 5% 25% 37% 20% 3	624
	21st to end	Night	15% 8% 29% 31% 14% 2	817
September	1st to 10th	Night	15% 10% 30% 29% 15% 1	840
	11th to 20th	Night	15% 9% 26% 27% 18% 4	840
	21st to end	Night	20% 11% 24% 26% 17% 2	833
October	1st to 10th	Night	24% 13% 28% 23% 11% 2	800
	11th to 20th	Night	26% 18% 32% 17% 7% 1	800
	21st to end	Night	30% 17% 29% 20% 4	880
November	1st to 10th	Night	27% 18% 31% 19% 5%	820
	11th to 20th	Night	24% 23% 31% 15% 7%	840
	21st to end	Night	31% 19% 29% 17% 3	840
December	1st to 10th	Night	28% 21% 34% 12% 5%	840
	11th to 20th	Night	25% 23% 33% 16% 3 1	840
	21st to end	Night	33% 19% 29% 15% 3 1	924

Page generated at 2026-09-26 04:18:44. Version ac07

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