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 2024-01-31.

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	17% 9% 4 7% 6% 3 5% 5% 5% 7% 34%	74870
	Night	18% 10% 4 7% 6% 3 5% 4 5% 6% 33%	75252
Transparency	Day	50% 6% 12% 20% 11% 1	74575
	Night	51% 6% 12% 19% 11% 1	75081
Seeing	Day	23% 16% 30% 20% 9% 3	13438
	Night	23% 13% 31% 24% 9% 1	22768

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	18% 16% 5% 7% 9% 3 6% 4 4 5% 24%	1517
	11th to 20th	Day	19% 16% 7% 10% 7% 3 4 4 3 4 24%	1520
	21st to end	Day	14% 14% 6% 11% 8% 4 6% 5% 4 5% 22%	1815
February	1st to 10th	Day	18% 13% 5% 11% 9% 4 6% 4 5% 5% 21%	1620
	11th to 20th	Day	14% 16% 5% 10% 7% 3 5% 4 5% 5% 26%	1573
	21st to end	Day	20% 14% 6% 7% 7% 5% 5% 4 4 5% 24%	1466
March	1st to 10th	Day	17% 11% 4 10% 9% 4 5% 3 4 4 29%	1843
	11th to 20th	Day	16% 14% 4 8% 7% 4 5% 3 4 4 30%	1980
	21st to end	Day	21% 10% 4 6% 4 3 3 3 4 5% 36%	2261
April	1st to 10th	Day	21% 12% 4 6% 4 3 4 4 3 6% 33%	2160
	11th to 20th	Day	21% 9% 2 5% 4 3 4 3 4 6% 39%	2134
	21st to end	Day	21% 10% 3 7% 6% 3 4 4 5% 5% 32%	2253
May	1st to 10th	Day	18% 8% 3 6% 6% 3 6% 5% 5% 6% 33%	2505
	11th to 20th	Day	16% 6% 3 6% 5% 3 4 5% 5% 8% 38%	2478
	21st to end	Day	14% 6% 3 5% 6% 3 5% 5% 7% 8% 40%	2725
June	1st to 10th	Day	19% 8% 3 5% 5% 3 5% 5% 6% 7% 33%	2520
	11th to 20th	Day	15% 5% 3 5% 5% 3 4 5% 5% 9% 39%	2520
	21st to end	Day	16% 7% 3 5% 5% 3 5% 5% 6% 8% 36%	2520
July	1st to 10th	Day	11% 5% 3 5% 4 3 5% 6% 7% 8% 43%	2520
	11th to 20th	Day	12% 5% 3 5% 5% 3 5% 6% 6% 11% 39%	2520
	21st to end	Day	11% 5% 2 5% 5% 4 7% 6% 7% 9% 38%	2744
August	1st to 10th	Day	11% 6% 2 6% 6% 3 5% 6% 7% 9% 40%	2383
	11th to 20th	Day	11% 5% 3 6% 6% 3 6% 5% 7% 9% 39%	2291
	21st to end	Day	13% 6% 3 5% 5% 3 5% 5% 6% 8% 41%	2376
September	1st to 10th	Day	13% 6% 2 5% 5% 3 4 5% 7% 8% 43%	2160
	11th to 20th	Day	12% 6% 3 5% 4 2 4 4 5% 8% 48%	2160
	21st to end	Day	17% 7% 2 5% 5% 3 4 5% 6% 7% 40%	1907
October	1st to 10th	Day	21% 7% 3 5% 5% 3 4 4 4 7% 36%	1800
	11th to 20th	Day	22% 8% 3 7% 6% 3 4 5% 5% 8% 30%	1800
	21st to end	Day	27% 10% 3 6% 5% 3 4 4 5% 6% 26%	1890
November	1st to 10th	Day	16% 8% 4 6% 6% 3 5% 6% 5% 8% 34%	1671
	11th to 20th	Day	17% 10% 4 8% 7% 4 3 3 5% 7% 31%	1520
	21st to end	Day	21% 12% 5% 7% 6% 3 5% 5% 6% 6% 25%	1520
December	1st to 10th	Day	22% 14% 5% 9% 6% 3 5% 4 4 6% 21%	1520
	11th to 20th	Day	21% 14% 5% 9% 7% 3 5% 4 4 5% 23%	1906
	21st to end	Day	27% 16% 6% 8% 6% 3 4 3 3 4 21%	2772
January	1st to 10th	Night	20% 14% 5% 9% 6% 4 5% 4 4 5% 22%	2649
	11th to 20th	Night	19% 15% 6% 9% 7% 4 5% 4 4 5% 22%	2660
	21st to end	Night	15% 13% 6% 10% 7% 4 5% 4 5% 5% 27%	2766
February	1st to 10th	Night	18% 14% 4 10% 7% 4 6% 4 5% 5% 23%	2340
	11th to 20th	Night	16% 12% 6% 9% 7% 3 4 4 5% 6% 30%	2273
	21st to end	Night	19% 11% 4 7% 7% 3 4 5% 5% 5% 30%	1789
March	1st to 10th	Night	15% 11% 6% 6% 6% 4 6% 4 5% 5% 32%	2101
	11th to 20th	Night	19% 12% 5% 6% 6% 3 4 3 4 6% 33%	1979
	21st to end	Night	18% 9% 4 7% 5% 2 3 3 4 6% 38%	2093
April	1st to 10th	Night	21% 9% 4 6% 4 3 4 5% 5% 6% 35%	1800
	11th to 20th	Night	19% 7% 4 6% 5% 3 4 4 4 6% 38%	1775
	21st to end	Night	22% 8% 3 7% 6% 4 3 4 6% 6% 32%	1723
May	1st to 10th	Night	22% 8% 3 4 5% 3 4 5% 6% 7% 36%	1454
	11th to 20th	Night	13% 6% 3 4 4 3 6% 5% 5% 7% 43%	1415
	21st to end	Night	11% 5% 2 5% 5% 3 6% 5% 6% 9% 44%	1550
June	1st to 10th	Night	16% 6% 2 6% 6% 3 6% 5% 7% 9% 34%	1440
	11th to 20th	Night	13% 4 2 5% 5% 3 4 5% 6% 9% 43%	1440
	21st to end	Night	15% 5% 3 6% 6% 4 5% 5% 6% 8% 38%	1440
July	1st to 10th	Night	10% 4 2 5% 5% 3 6% 5% 5% 8% 47%	1440
	11th to 20th	Night	11% 6% 3 5% 4 3 4 5% 7% 9% 44%	1440
	21st to end	Night	10% 5% 2 4 4 3 5% 5% 6% 9% 47%	1574
August	1st to 10th	Night	9% 5% 2 4 5% 3 4 5% 6% 9% 46%	1551
	11th to 20th	Night	10% 6% 2 6% 5% 2 3 5% 7% 9% 44%	1598
	21st to end	Night	9% 5% 2 5% 6% 3 4 5% 6% 8% 45%	1972
September	1st to 10th	Night	12% 6% 2 6% 5% 2 3 4 5% 7% 47%	1800
	11th to 20th	Night	13% 5% 2 4 3 2 3 4 6% 7% 51%	1800
	21st to end	Night	17% 5% 2 5% 5% 3 4 4 5% 6% 43%	2048
October	1st to 10th	Night	18% 8% 3 6% 5% 4 4 6% 5% 6% 36%	2160
	11th to 20th	Night	21% 8% 3 6% 7% 3 5% 4 5% 6% 32%	2160
	21st to end	Night	23% 9% 3 8% 6% 3 4 5% 5% 5% 28%	2466
November	1st to 10th	Night	19% 8% 3 6% 4 2 4 4 5% 6% 38%	2407
	11th to 20th	Night	19% 9% 3 6% 5% 3 5% 5% 5% 7% 34%	2657
	21st to end	Night	21% 11% 3 7% 5% 3 6% 5% 5% 6% 27%	2660
December	1st to 10th	Night	21% 15% 5% 8% 7% 3 4 4 5% 6% 22%	2660
	11th to 20th	Night	21% 13% 5% 8% 6% 3 5% 4 4 5% 28%	3288
	21st to end	Night	30% 15% 5% 7% 6% 3 4 4 4 5% 18%	4884

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	61% 6% 11% 15% 5% 1	1518
	11th to 20th	Day	65% 5% 9% 11% 9% 1	1520
	21st to end	Day	63% 7% 9% 11% 8% 2	1815
February	1st to 10th	Day	65% 5% 9% 15% 5% 1	1620
	11th to 20th	Day	61% 5% 10% 14% 9% 1	1582
	21st to end	Day	64% 5% 9% 12% 9% 1	1466
March	1st to 10th	Day	59% 4 12% 15% 9% 2	1846
	11th to 20th	Day	57% 4 10% 14% 11% 2	1980
	21st to end	Day	51% 4 7% 18% 13% 6%	2261
April	1st to 10th	Day	55% 4 8% 17% 13% 3	2160
	11th to 20th	Day	48% 4 8% 22% 16% 2	2134
	21st to end	Day	54% 5% 8% 16% 13% 4	2253
May	1st to 10th	Day	50% 6% 9% 17% 15% 3	2505
	11th to 20th	Day	43% 6% 10% 23% 16% 2	2482
	21st to end	Day	41% 8% 15% 21% 12% 3	2725
June	1st to 10th	Day	48% 8% 12% 18% 13% 1	2520
	11th to 20th	Day	41% 6% 12% 25% 14% 2	2520
	21st to end	Day	44% 8% 14% 21% 12% 1	2520
July	1st to 10th	Day	36% 8% 13% 27% 16% 1	2450
	11th to 20th	Day	38% 8% 17% 27% 10%	2445
	21st to end	Day	40% 8% 17% 24% 11%	2591
August	1st to 10th	Day	39% 9% 16% 26% 10% 1	2369
	11th to 20th	Day	40% 8% 15% 25% 11% 1	2291
	21st to end	Day	39% 7% 13% 28% 13% 1	2376
September	1st to 10th	Day	38% 7% 16% 27% 12% 1	2160
	11th to 20th	Day	35% 5% 13% 30% 16% 1	2160
	21st to end	Day	43% 7% 13% 28% 9%	1907
October	1st to 10th	Day	48% 5% 13% 22% 12% 1	1800
	11th to 20th	Day	52% 7% 16% 19% 6% 1	1800
	21st to end	Day	59% 6% 9% 18% 8%	1890
November	1st to 10th	Day	48% 8% 13% 21% 9% 1	1671
	11th to 20th	Day	54% 6% 11% 18% 10% 1	1520
	21st to end	Day	60% 7% 13% 11% 9%	1520
December	1st to 10th	Day	66% 5% 12% 12% 5% 1	1520
	11th to 20th	Day	63% 6% 15% 12% 4	1906
	21st to end	Day	69% 4 8% 14% 5%	2772
January	1st to 10th	Night	64% 6% 10% 13% 7%	2649
	11th to 20th	Night	65% 6% 9% 12% 7% 1	2660
	21st to end	Night	60% 5% 9% 15% 10% 1	2766
February	1st to 10th	Night	63% 6% 10% 15% 6% 1	2340
	11th to 20th	Night	57% 4 10% 19% 10% 1	2275
	21st to end	Night	56% 6% 11% 17% 10% 1	1789
March	1st to 10th	Night	54% 4 11% 18% 11% 1	2111
	11th to 20th	Night	53% 5% 10% 20% 12% 1	1980
	21st to end	Night	48% 3 8% 19% 18% 3	2093
April	1st to 10th	Night	50% 5% 8% 20% 15% 2	1800
	11th to 20th	Night	48% 5% 9% 19% 18% 2	1775
	21st to end	Night	52% 5% 9% 15% 16% 3	1723
May	1st to 10th	Night	48% 4 10% 17% 17% 5%	1454
	11th to 20th	Night	39% 6% 11% 25% 16% 3	1416
	21st to end	Night	36% 7% 16% 20% 19% 3	1551
June	1st to 10th	Night	45% 8% 12% 21% 12% 1	1440
	11th to 20th	Night	37% 6% 13% 21% 20% 2	1440
	21st to end	Night	42% 10% 12% 22% 13% 1	1440
July	1st to 10th	Night	32% 8% 11% 24% 22% 2	1400
	11th to 20th	Night	35% 7% 13% 31% 14%	1392
	21st to end	Night	32% 8% 15% 30% 13% 1	1486
August	1st to 10th	Night	34% 9% 14% 29% 13% 1	1541
	11th to 20th	Night	36% 8% 14% 27% 14% 1	1598
	21st to end	Night	35% 8% 13% 29% 14%	1972
September	1st to 10th	Night	37% 6% 14% 29% 14% 1	1800
	11th to 20th	Night	32% 5% 15% 33% 14% 1	1800
	21st to end	Night	40% 6% 10% 31% 12% 1	2048
October	1st to 10th	Night	47% 7% 10% 20% 14% 1	2160
	11th to 20th	Night	51% 6% 14% 20% 8% 1	2160
	21st to end	Night	56% 7% 14% 13% 10% 1	2466
November	1st to 10th	Night	46% 6% 14% 22% 11% 1	2407
	11th to 20th	Night	49% 7% 13% 18% 11% 2	2657
	21st to end	Night	57% 7% 13% 16% 6%	2660
December	1st to 10th	Night	62% 6% 12% 14% 5%	2660
	11th to 20th	Night	59% 5% 12% 15% 7% 1	3288
	21st to end	Night	69% 5% 11% 12% 3	4884

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	27% 20% 33% 14% 5% 1	180
	11th to 20th	Day	33% 18% 32% 14% 2 1	180
	21st to end	Day	28% 19% 31% 18% 4	267
February	1st to 10th	Day	37% 28% 29% 6% 1	240
	11th to 20th	Day	31% 28% 34% 7%	240
	21st to end	Day	35% 20% 31% 11% 2	332
March	1st to 10th	Day	23% 22% 33% 17% 5%	420
	11th to 20th	Day	29% 21% 30% 16% 4	420
	21st to end	Day	33% 21% 27% 12% 6% 1	462
April	1st to 10th	Day	29% 20% 31% 14% 5%	420
	11th to 20th	Day	30% 23% 22% 13% 9% 3	412
	21st to end	Day	28% 22% 30% 12% 6% 2	420
May	1st to 10th	Day	25% 15% 30% 21% 8% 2	423
	11th to 20th	Day	20% 14% 36% 19% 9% 2	447
	21st to end	Day	17% 14% 32% 24% 10% 2	485
June	1st to 10th	Day	23% 12% 34% 21% 7% 2	450
	11th to 20th	Day	17% 14% 32% 22% 11% 3	450
	21st to end	Day	19% 13% 33% 22% 11% 2	450
July	1st to 10th	Day	14% 9% 27% 31% 16% 4	445
	11th to 20th	Day	15% 8% 31% 27% 14% 5%	444
	21st to end	Day	13% 8% 37% 29% 10% 3	488
August	1st to 10th	Day	13% 9% 31% 28% 16% 4	446
	11th to 20th	Day	11% 6% 30% 30% 17% 6%	439
	21st to end	Day	14% 11% 32% 26% 10% 8%	495
September	1st to 10th	Day	15% 9% 32% 26% 12% 7%	450
	11th to 20th	Day	11% 11% 26% 25% 17% 10%	450
	21st to end	Day	16% 14% 29% 24% 14% 3	450
October	1st to 10th	Day	28% 16% 24% 16% 12% 4	450
	11th to 20th	Day	24% 23% 30% 15% 6% 1	450
	21st to end	Day	32% 18% 25% 21% 4	450
November	1st to 10th	Day	25% 18% 28% 19% 8% 1	365
	11th to 20th	Day	28% 17% 34% 12% 6% 2	180
	21st to end	Day	23% 25% 34% 12% 4 1	180
December	1st to 10th	Day	32% 22% 30% 12% 4	180
	11th to 20th	Day	28% 24% 36% 9% 3	180
	21st to end	Day	34% 21% 32% 11% 2	198
January	1st to 10th	Night	30% 16% 30% 18% 4	760
	11th to 20th	Night	29% 20% 28% 18% 5%	760
	21st to end	Night	22% 16% 35% 22% 4	835
February	1st to 10th	Night	27% 19% 33% 19% 2	720
	11th to 20th	Night	23% 17% 40% 17% 3	703
	21st to end	Night	26% 20% 33% 18% 3	592
March	1st to 10th	Night	23% 18% 36% 21% 3	720
	11th to 20th	Night	28% 15% 30% 21% 5% 1	720
	21st to end	Night	24% 13% 35% 20% 7% 1	792
April	1st to 10th	Night	25% 14% 37% 21% 3	720
	11th to 20th	Night	22% 17% 34% 18% 8% 1	711
	21st to end	Night	26% 11% 30% 23% 10%	643
May	1st to 10th	Night	26% 8% 27% 26% 12% 1	374
	11th to 20th	Night	14% 9% 36% 32% 8%	354
	21st to end	Night	13% 8% 36% 30% 13% 1	389
June	1st to 10th	Night	17% 7% 33% 31% 11% 1	360
	11th to 20th	Night	15% 8% 31% 32% 13% 1	360
	21st to end	Night	18% 4 25% 40% 14%	360
July	1st to 10th	Night	10% 5% 28% 37% 20%	350
	11th to 20th	Night	12% 3 28% 38% 17% 1	348
	21st to end	Night	12% 3 28% 40% 15% 1	394
August	1st to 10th	Night	12% 3 25% 35% 24% 1	477
	11th to 20th	Night	13% 4 23% 34% 23% 4	534
	21st to end	Night	10% 6% 30% 34% 17% 2	784
September	1st to 10th	Night	15% 7% 28% 32% 15% 3	720
	11th to 20th	Night	15% 7% 22% 28% 23% 5%	720
	21st to end	Night	18% 9% 25% 26% 18% 3	720
October	1st to 10th	Night	22% 13% 28% 19% 17% 2	720
	11th to 20th	Night	25% 15% 29% 22% 8% 1	720
	21st to end	Night	27% 15% 28% 24% 5%	792
November	1st to 10th	Night	25% 13% 30% 22% 9%	740
	11th to 20th	Night	25% 18% 31% 18% 8% 1	760
	21st to end	Night	29% 16% 30% 17% 7%	760
December	1st to 10th	Night	31% 18% 29% 19% 3	760
	11th to 20th	Night	27% 20% 33% 17% 3 1	760
	21st to end	Night	35% 16% 31% 17% 2	836

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