The Observatory on the Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Observatory on the Lake 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	16% 10% 3 8% 5% 4 4 3 5% 7% 33%	138160
	Night	17% 11% 4 8% 5% 5% 4 4 5% 6% 32%	133841
Transparency	Day	50% 6% 12% 21% 10% 1	141679
	Night	51% 6% 11% 20% 11% 1	138411
Seeing	Day	23% 16% 31% 19% 8% 2	16501
	Night	23% 13% 31% 24% 9% 1	25715

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	14% 14% 5% 11% 6% 5% 5% 4 5% 6% 25%	4661
	11th to 20th	Day	12% 21% 8% 16% 7% 5% 4 3 4 4 17%	4650
	21st to end	Day	11% 16% 8% 15% 7% 6% 5% 3 4 5% 20%	5600
February	1st to 10th	Day	17% 14% 4 12% 7% 5% 4 3 6% 5% 23%	1890
	11th to 20th	Day	13% 16% 5% 11% 7% 5% 4 3 5% 5% 25%	1843
	21st to end	Day	19% 16% 5% 9% 6% 6% 5% 3 4 5% 23%	1714
March	1st to 10th	Day	20% 12% 4 10% 5% 4 3 2 4 5% 31%	4709
	11th to 20th	Day	11% 14% 4 9% 6% 5% 5% 4 5% 6% 31%	5090
	21st to end	Day	20% 10% 3 8% 4 4 3 3 4 5% 37%	5813
April	1st to 10th	Day	21% 14% 3 8% 4 4 3 3 3 5% 31%	2520
	11th to 20th	Day	21% 11% 2 5% 4 4 4 2 4 6% 36%	2494
	21st to end	Day	21% 10% 3 8% 4 4 4 3 4 6% 33%	2629
May	1st to 10th	Day	23% 10% 3 8% 5% 4 5% 3 5% 6% 28%	8194
	11th to 20th	Day	16% 9% 3 8% 5% 4 4 3 5% 7% 35%	8221
	21st to end	Day	20% 8% 2 6% 4 4 4 4 6% 8% 34%	9060
June	1st to 10th	Day	18% 8% 3 6% 5% 5% 5% 4 6% 7% 34%	2940
	11th to 20th	Day	14% 6% 3 6% 5% 4 5% 4 6% 9% 38%	2870
	21st to end	Day	17% 7% 2 6% 5% 5% 4 4 6% 8% 36%	2800
July	1st to 10th	Day	11% 6% 2 7% 5% 4 5% 4 7% 10% 38%	6360
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 44%	6360
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 45%	6926
August	1st to 10th	Day	16% 7% 2 7% 4 3 4 4 6% 8% 38%	4335
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 7% 9% 36%	4147
	21st to end	Day	14% 5% 3 5% 4 4 4 4 6% 10% 41%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 7% 7% 44%	2400
	11th to 20th	Day	11% 6% 2 6% 4 3 4 3 5% 7% 50%	2400
	21st to end	Day	17% 7% 2 6% 5% 5% 4 4 6% 6% 39%	2117
October	1st to 10th	Day	19% 7% 3 6% 4 4 4 4 4 7% 38%	2000
	11th to 20th	Day	20% 8% 2 7% 5% 4 5% 3 5% 8% 32%	2000
	21st to end	Day	25% 10% 3 7% 4 5% 4 4 5% 6% 27%	2100
November	1st to 10th	Day	16% 9% 3 8% 5% 4 5% 4 5% 7% 33%	1851
	11th to 20th	Day	15% 12% 4 10% 6% 5% 3 3 5% 7% 31%	1680
	21st to end	Day	21% 14% 4 8% 5% 4 4 4 6% 5% 24%	1680
December	1st to 10th	Day	20% 15% 4 11% 6% 5% 5% 4 4 6% 20%	2680
	11th to 20th	Day	18% 19% 6% 11% 5% 3 4 3 4 4 23%	3066
	21st to end	Day	22% 15% 5% 10% 5% 4 4 2 3 5% 25%	4048
January	1st to 10th	Night	14% 14% 6% 13% 6% 6% 4 4 4 6% 23%	8234
	11th to 20th	Night	13% 17% 7% 15% 7% 6% 5% 4 5% 4 16%	8235
	21st to end	Night	14% 14% 6% 11% 6% 6% 5% 4 5% 5% 24%	8565
February	1st to 10th	Night	16% 15% 3 11% 6% 6% 5% 3 5% 5% 25%	2730
	11th to 20th	Night	15% 13% 5% 10% 6% 5% 4 3 5% 6% 29%	2663
	21st to end	Night	17% 13% 4 9% 6% 5% 4 4 5% 5% 29%	2090
March	1st to 10th	Night	20% 11% 4 8% 5% 6% 4 3 4 4 30%	5348
	11th to 20th	Night	15% 12% 4 7% 5% 4 4 4 6% 8% 32%	5089
	21st to end	Night	20% 10% 3 7% 4 3 3 3 5% 7% 35%	5359
April	1st to 10th	Night	21% 10% 2 7% 4 4 3 3 5% 6% 33%	2100
	11th to 20th	Night	20% 9% 3 7% 4 4 3 2 5% 5% 37%	2075
	21st to end	Night	21% 9% 3 7% 5% 5% 4 3 5% 6% 34%	2009
May	1st to 10th	Night	24% 11% 3 7% 4 5% 4 4 5% 6% 26%	4702
	11th to 20th	Night	15% 9% 3 7% 4 4 5% 3 5% 7% 37%	4639
	21st to end	Night	18% 7% 2 5% 4 4 5% 4 6% 8% 36%	5114
June	1st to 10th	Night	15% 7% 3 7% 5% 4 5% 4 7% 9% 34%	1680
	11th to 20th	Night	12% 5% 2 7% 5% 5% 4 4 6% 10% 42%	1640
	21st to end	Night	16% 5% 3 7% 5% 5% 5% 4 6% 8% 37%	1600
July	1st to 10th	Night	12% 5% 3 8% 4 6% 5% 5% 6% 9% 38%	3600
	11th to 20th	Night	13% 6% 3 6% 4 3 4 4 6% 8% 45%	3600
	21st to end	Night	9% 6% 2 5% 4 4 4 4 6% 8% 49%	3926
August	1st to 10th	Night	11% 7% 2 6% 5% 4 4 4 6% 8% 44%	2810
	11th to 20th	Night	14% 6% 2 6% 4 3 3 4 5% 8% 44%	2875
	21st to end	Night	9% 4 2 7% 5% 5% 5% 5% 7% 10% 41%	3578
September	1st to 10th	Night	11% 6% 2 8% 4 3 4 3 5% 7% 48%	2000
	11th to 20th	Night	11% 5% 1 5% 3 3 3 3 6% 7% 52%	2000
	21st to end	Night	17% 5% 3 6% 4 4 4 3 5% 6% 42%	2278
October	1st to 10th	Night	17% 8% 2 6% 4 5% 4 4 5% 6% 40%	2400
	11th to 20th	Night	19% 8% 3 7% 6% 4 5% 3 5% 6% 33%	2400
	21st to end	Night	21% 9% 3 9% 4 4 4 4 5% 6% 30%	2740
November	1st to 10th	Night	18% 9% 3 7% 3 3 4 3 5% 6% 37%	2667
	11th to 20th	Night	18% 10% 3 6% 4 4 5% 4 5% 7% 34%	2937
	21st to end	Night	20% 12% 3 8% 5% 5% 6% 4 5% 5% 26%	2940
December	1st to 10th	Night	19% 16% 4 9% 6% 5% 5% 4 5% 7% 20%	4720
	11th to 20th	Night	22% 15% 4 8% 4 3 3 3 4 5% 27%	5348
	21st to end	Night	27% 15% 4 9% 4 4 4 3 4 5% 22%	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	59% 6% 11% 17% 6% 1	4662
	11th to 20th	Day	71% 5% 8% 8% 7% 1	4680
	21st to end	Day	66% 5% 8% 11% 9% 1	5600
February	1st to 10th	Day	62% 5% 9% 17% 6% 1	1890
	11th to 20th	Day	61% 5% 10% 15% 9% 1	1852
	21st to end	Day	64% 5% 9% 12% 9% 1	1714
March	1st to 10th	Day	56% 4 10% 14% 14% 2	4712
	11th to 20th	Day	52% 5% 19% 12% 11% 2	5090
	21st to end	Day	50% 4 6% 20% 16% 3	5813
April	1st to 10th	Day	56% 4 8% 17% 13% 3	2520
	11th to 20th	Day	50% 4 8% 20% 15% 2	2494
	21st to end	Day	54% 5% 8% 17% 13% 3	2629
May	1st to 10th	Day	55% 6% 10% 18% 10% 2	8206
	11th to 20th	Day	47% 7% 12% 21% 12% 1	8225
	21st to end	Day	47% 6% 12% 24% 10% 1	9061
June	1st to 10th	Day	48% 7% 11% 19% 13% 1	2940
	11th to 20th	Day	42% 6% 12% 26% 13% 2	2870
	21st to end	Day	45% 8% 14% 21% 12% 1	2800
July	1st to 10th	Day	40% 6% 15% 27% 12%	6290
	11th to 20th	Day	35% 6% 16% 29% 14%	6285
	21st to end	Day	34% 6% 15% 35% 10%	6728
August	1st to 10th	Day	44% 6% 15% 25% 10% 1	4321
	11th to 20th	Day	42% 7% 14% 27% 9% 1	4186
	21st to end	Day	38% 5% 16% 30% 11%	4312
September	1st to 10th	Day	37% 6% 15% 29% 12% 1	2400
	11th to 20th	Day	34% 5% 13% 31% 16% 1	2400
	21st to end	Day	43% 7% 13% 27% 10%	2117
October	1st to 10th	Day	40% 5% 15% 30% 10%	3260
	11th to 20th	Day	44% 5% 15% 27% 8% 1	3240
	21st to end	Day	49% 7% 13% 23% 8%	3377
November	1st to 10th	Day	48% 8% 13% 22% 9% 1	1851
	11th to 20th	Day	54% 6% 11% 18% 11% 1	1680
	21st to end	Day	60% 7% 13% 12% 8%	1680
December	1st to 10th	Day	66% 4 16% 10% 4	2680
	11th to 20th	Day	65% 4 11% 13% 7%	3066
	21st to end	Day	64% 4 10% 16% 6%	4048
January	1st to 10th	Night	62% 6% 13% 13% 6%	8234
	11th to 20th	Night	69% 7% 8% 11% 6%	8280
	21st to end	Night	61% 5% 9% 13% 12% 1	8565
February	1st to 10th	Night	60% 6% 10% 17% 6% 1	2730
	11th to 20th	Night	57% 4 10% 19% 9% 1	2665
	21st to end	Night	56% 6% 11% 18% 9% 1	2090
March	1st to 10th	Night	57% 4 7% 17% 11% 4	5358
	11th to 20th	Night	49% 5% 17% 17% 9% 3	5090
	21st to end	Night	49% 3 9% 21% 16% 2	5359
April	1st to 10th	Night	51% 5% 8% 20% 14% 2	2100
	11th to 20th	Night	49% 4 9% 19% 17% 1	2075
	21st to end	Night	51% 4 9% 17% 16% 3	2009
May	1st to 10th	Night	57% 5% 9% 17% 11% 2	4708
	11th to 20th	Night	45% 7% 12% 22% 13% 1	4640
	21st to end	Night	44% 5% 12% 21% 16% 1	5115
June	1st to 10th	Night	45% 7% 11% 20% 14% 1	1680
	11th to 20th	Night	38% 6% 14% 21% 19% 3	1640
	21st to end	Night	44% 9% 12% 22% 13% 1	1600
July	1st to 10th	Night	41% 5% 16% 20% 17% 1	3560
	11th to 20th	Night	36% 6% 12% 32% 13% 1	3552
	21st to end	Night	32% 7% 13% 33% 13% 2	3809
August	1st to 10th	Night	38% 7% 14% 27% 13% 1	2800
	11th to 20th	Night	38% 6% 14% 32% 11% 1	2902
	21st to end	Night	35% 7% 16% 30% 11% 1	3578
September	1st to 10th	Night	36% 6% 13% 30% 14% 1	2000
	11th to 20th	Night	30% 5% 15% 34% 15% 1	2000
	21st to end	Night	42% 6% 10% 28% 13% 1	2278
October	1st to 10th	Night	37% 6% 11% 27% 18% 1	3920
	11th to 20th	Night	42% 5% 15% 27% 10%	3899
	21st to end	Night	48% 6% 14% 24% 8%	4413
November	1st to 10th	Night	46% 5% 14% 23% 11% 1	2667
	11th to 20th	Night	50% 6% 13% 19% 11% 2	2937
	21st to end	Night	58% 7% 12% 16% 6%	2940
December	1st to 10th	Night	63% 5% 14% 14% 4	4720
	11th to 20th	Night	60% 6% 10% 15% 9% 1	5348
	21st to end	Night	66% 4 9% 11% 10%	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	26% 19% 38% 13% 4	220
	11th to 20th	Day	35% 19% 30% 13% 2 1	220
	21st to end	Day	27% 22% 31% 16% 4	330
February	1st to 10th	Day	33% 25% 29% 11% 2	330
	11th to 20th	Day	31% 27% 35% 8%	330
	21st to end	Day	33% 20% 34% 11% 2	430
March	1st to 10th	Day	27% 21% 32% 16% 4	540
	11th to 20th	Day	32% 21% 30% 15% 3	540
	21st to end	Day	34% 20% 28% 12% 5% 1	594
April	1st to 10th	Day	32% 22% 30% 12% 4	540
	11th to 20th	Day	32% 23% 23% 12% 7% 2	532
	21st to end	Day	28% 23% 31% 11% 6% 1	540
May	1st to 10th	Day	25% 17% 30% 21% 6% 2	543
	11th to 20th	Day	20% 15% 36% 20% 8% 2	567
	21st to end	Day	19% 16% 32% 23% 9% 2	617
June	1st to 10th	Day	23% 13% 34% 22% 7% 2	570
	11th to 20th	Day	17% 16% 35% 20% 10% 2	550
	21st to end	Day	20% 14% 33% 21% 9% 2	530
July	1st to 10th	Day	13% 10% 31% 29% 15% 3	525
	11th to 20th	Day	15% 8% 32% 27% 14% 5%	524
	21st to end	Day	13% 7% 37% 28% 11% 4	576
August	1st to 10th	Day	13% 8% 31% 28% 16% 4	526
	11th to 20th	Day	13% 6% 29% 29% 16% 6%	519
	21st to end	Day	14% 10% 33% 25% 11% 7%	583
September	1st to 10th	Day	14% 11% 30% 26% 12% 7%	530
	11th to 20th	Day	10% 10% 23% 26% 19% 12%	530
	21st to end	Day	17% 12% 29% 25% 13% 3	530
October	1st to 10th	Day	26% 15% 25% 18% 12% 4	530
	11th to 20th	Day	23% 22% 31% 15% 6% 2	530
	21st to end	Day	30% 19% 26% 21% 3	528
November	1st to 10th	Day	28% 19% 28% 17% 7%	425
	11th to 20th	Day	26% 17% 38% 10% 7% 2	220
	21st to end	Day	29% 23% 34% 10% 4	220
December	1st to 10th	Day	34% 20% 31% 11% 4	220
	11th to 20th	Day	32% 23% 34% 9% 2	220
	21st to end	Day	36% 19% 32% 11% 3	242
January	1st to 10th	Night	29% 18% 32% 17% 4	840
	11th to 20th	Night	30% 20% 28% 17% 5%	840
	21st to end	Night	22% 17% 35% 21% 5%	924
February	1st to 10th	Night	26% 18% 33% 21% 2	840
	11th to 20th	Night	24% 17% 39% 17% 3	823
	21st to end	Night	26% 20% 33% 18% 3	692
March	1st to 10th	Night	25% 18% 34% 20% 3	840
	11th to 20th	Night	28% 17% 30% 20% 4 1	840
	21st to end	Night	25% 13% 37% 18% 6%	924
April	1st to 10th	Night	26% 15% 35% 21% 3	840
	11th to 20th	Night	24% 18% 33% 17% 7% 1	831
	21st to end	Night	26% 12% 30% 23% 9%	749
May	1st to 10th	Night	26% 9% 27% 25% 11% 1	436
	11th to 20th	Night	15% 10% 36% 31% 8%	414
	21st to end	Night	14% 8% 37% 29% 11% 1	455
June	1st to 10th	Night	16% 8% 33% 32% 11%	420
	11th to 20th	Night	15% 9% 31% 33% 12% 1	410
	21st to end	Night	17% 5% 27% 39% 13%	400
July	1st to 10th	Night	10% 5% 28% 38% 19%	390
	11th to 20th	Night	13% 4 28% 38% 16% 1	388
	21st to end	Night	13% 3 27% 41% 15% 1	438
August	1st to 10th	Night	12% 3 25% 36% 23% 1	531
	11th to 20th	Night	13% 4 24% 33% 23% 3	594
	21st to end	Night	10% 6% 29% 36% 17% 2	872
September	1st to 10th	Night	15% 7% 28% 32% 17% 3	800
	11th to 20th	Night	13% 7% 21% 28% 26% 6%	800
	21st to end	Night	19% 9% 25% 27% 18% 3	800
October	1st to 10th	Night	21% 12% 28% 20% 17% 2	800
	11th to 20th	Night	24% 15% 29% 22% 8% 1	800
	21st to end	Night	26% 16% 29% 24% 5%	880
November	1st to 10th	Night	26% 14% 30% 21% 8%	820
	11th to 20th	Night	25% 17% 32% 18% 7% 1	840
	21st to end	Night	30% 18% 30% 16% 7%	840
December	1st to 10th	Night	30% 18% 29% 19% 3	840
	11th to 20th	Night	28% 20% 33% 16% 3 1	840
	21st to end	Night	35% 16% 30% 17% 2	924

Page generated at 2026-06-13 02:31:41. Version ac07

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