Observatoire de Walter Laperle
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de Walter Laperle 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	18% 9% 4 7% 6% 3 5% 4 5% 7% 32%	74895
	Night	20% 11% 4 7% 6% 3 4 4 5% 6% 30%	75229
Transparency	Day	51% 6% 12% 20% 10% 1	74600
	Night	55% 6% 11% 18% 9% 1	75058
Seeing	Day	23% 14% 29% 22% 10% 3	13497
	Night	24% 13% 29% 25% 8% 1	22680

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	19% 19% 6% 10% 9% 4 5% 4 3 4 17%	1517
	11th to 20th	Day	18% 19% 6% 10% 8% 3 4 4 5% 4 19%	1520
	21st to end	Day	12% 15% 7% 10% 7% 5% 6% 6% 5% 7% 19%	1674
February	1st to 10th	Day	17% 15% 5% 11% 8% 4 5% 4 5% 5% 20%	1620
	11th to 20th	Day	17% 12% 5% 10% 7% 4 6% 4 5% 5% 25%	1645
	21st to end	Day	19% 16% 6% 8% 7% 3 4 4 5% 4 23%	1480
March	1st to 10th	Day	16% 14% 6% 9% 6% 4 5% 3 4 5% 28%	1797
	11th to 20th	Day	18% 12% 4 7% 6% 3 4 4 5% 6% 30%	1903
	21st to end	Day	21% 9% 4 6% 5% 2 4 4 4 5% 34%	2345
April	1st to 10th	Day	22% 11% 4 6% 6% 3 4 3 4 5% 32%	2160
	11th to 20th	Day	21% 7% 3 6% 7% 3 4 3 4 6% 37%	2134
	21st to end	Day	23% 10% 3 5% 5% 3 3 4 5% 6% 32%	2263
May	1st to 10th	Day	21% 7% 3 6% 6% 3 4 4 5% 6% 36%	2508
	11th to 20th	Day	16% 7% 3 6% 5% 3 4 5% 5% 6% 40%	2478
	21st to end	Day	16% 6% 3 5% 5% 3 5% 5% 5% 8% 37%	2725
June	1st to 10th	Day	23% 6% 3 6% 6% 3 5% 5% 5% 7% 31%	2520
	11th to 20th	Day	14% 6% 3 5% 6% 3 5% 5% 6% 9% 37%	2520
	21st to end	Day	21% 6% 3 6% 5% 3 6% 5% 6% 7% 33%	2520
July	1st to 10th	Day	12% 6% 3 5% 5% 3 5% 5% 6% 7% 43%	2520
	11th to 20th	Day	14% 5% 2 5% 6% 3 7% 6% 7% 9% 37%	2520
	21st to end	Day	13% 5% 2 5% 6% 3 6% 5% 7% 10% 38%	2744
August	1st to 10th	Day	13% 6% 3 6% 5% 4 6% 5% 5% 8% 39%	2506
	11th to 20th	Day	12% 4 2 5% 6% 3 5% 6% 6% 10% 40%	2245
	21st to end	Day	14% 6% 2 5% 5% 3 4 5% 5% 9% 40%	2376
September	1st to 10th	Day	14% 5% 2 5% 5% 3 5% 5% 5% 7% 44%	2160
	11th to 20th	Day	15% 5% 2 5% 4 3 3 4 5% 7% 48%	2110
	21st to end	Day	16% 7% 3 4 5% 3 5% 5% 5% 8% 39%	1980
October	1st to 10th	Day	22% 7% 2 6% 6% 3 5% 4 5% 7% 32%	1821
	11th to 20th	Day	19% 12% 3 8% 7% 4 5% 5% 5% 5% 28%	1799
	21st to end	Day	29% 11% 3 5% 5% 3 4 4 5% 5% 25%	1782
November	1st to 10th	Day	18% 9% 5% 7% 6% 4 4 5% 5% 7% 30%	1671
	11th to 20th	Day	18% 12% 4 8% 7% 4 4 5% 5% 7% 26%	1614
	21st to end	Day	21% 17% 5% 9% 6% 3 5% 5% 5% 6% 19%	1520
December	1st to 10th	Day	24% 15% 6% 10% 7% 4 5% 3 4 6% 15%	1520
	11th to 20th	Day	23% 17% 5% 8% 5% 3 5% 4 5% 6% 19%	1906
	21st to end	Day	27% 17% 5% 10% 6% 4 4 3 4 4 17%	2772
January	1st to 10th	Night	19% 19% 7% 11% 8% 4 5% 4 3 4 16%	2649
	11th to 20th	Night	18% 19% 6% 10% 7% 4 5% 4 4 4 18%	2660
	21st to end	Night	15% 15% 6% 10% 8% 5% 5% 4 4 5% 24%	2905
February	1st to 10th	Night	17% 15% 7% 14% 8% 4 5% 5% 4 5% 18%	2340
	11th to 20th	Night	16% 12% 6% 10% 7% 4 5% 4 4 5% 25%	2201
	21st to end	Night	16% 14% 6% 10% 7% 3 4 5% 5% 4 27%	1776
March	1st to 10th	Night	19% 15% 5% 7% 8% 4 5% 4 4 4 26%	2150
	11th to 20th	Night	18% 14% 4 7% 7% 3 5% 3 4 4 31%	2051
	21st to end	Night	20% 11% 4 6% 5% 3 4 3 4 4 35%	2010
April	1st to 10th	Night	24% 10% 4 7% 5% 3 4 3 4 5% 30%	1800
	11th to 20th	Night	23% 9% 3 6% 6% 3 4 3 4 6% 33%	1775
	21st to end	Night	22% 9% 4 6% 5% 3 4 3 5% 5% 33%	1697
May	1st to 10th	Night	25% 7% 3 6% 5% 2 4 4 4 6% 33%	1468
	11th to 20th	Night	20% 6% 3 6% 6% 3 5% 3 5% 5% 39%	1415
	21st to end	Night	16% 4 2 6% 5% 4 4 5% 5% 9% 40%	1550
June	1st to 10th	Night	20% 7% 3 6% 6% 3 5% 6% 7% 9% 28%	1440
	11th to 20th	Night	18% 6% 3 5% 5% 3 4 5% 6% 7% 40%	1440
	21st to end	Night	18% 7% 3 7% 5% 3 4 5% 6% 8% 35%	1440
July	1st to 10th	Night	11% 4 2 5% 5% 3 6% 5% 7% 9% 43%	1440
	11th to 20th	Night	12% 4 3 5% 5% 3 6% 6% 6% 10% 41%	1440
	21st to end	Night	11% 6% 3 5% 4 3 5% 6% 7% 10% 40%	1574
August	1st to 10th	Night	12% 5% 3 5% 4 3 4 5% 8% 9% 42%	1432
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 5% 7% 42%	1633
	21st to end	Night	14% 6% 2 5% 6% 3 4 5% 6% 7% 42%	1980
September	1st to 10th	Night	14% 6% 3 6% 5% 3 5% 5% 6% 7% 41%	1800
	11th to 20th	Night	16% 4 2 5% 4 3 3 4 5% 6% 47%	1849
	21st to end	Night	18% 5% 2 6% 5% 3 4 4 5% 7% 41%	1980
October	1st to 10th	Night	22% 8% 3 5% 5% 3 5% 5% 5% 6% 34%	2136
	11th to 20th	Night	21% 8% 3 6% 6% 3 6% 4 5% 5% 31%	2160
	21st to end	Night	25% 11% 3 8% 6% 3 4 4 4 6% 27%	2574
November	1st to 10th	Night	21% 11% 4 6% 5% 3 5% 5% 5% 7% 30%	2407
	11th to 20th	Night	19% 12% 4 6% 6% 3 4 4 5% 6% 30%	2565
	21st to end	Night	24% 13% 6% 8% 6% 3 4 5% 4 5% 22%	2660
December	1st to 10th	Night	22% 19% 7% 9% 7% 3 5% 4 4 5% 15%	2660
	11th to 20th	Night	25% 15% 6% 9% 5% 2 4 4 4 4 21%	3288
	21st to end	Night	30% 17% 5% 10% 6% 3 4 3 3 3 16%	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	71% 4 11% 11% 3	1518
	11th to 20th	Day	67% 4 9% 11% 8% 1	1520
	21st to end	Day	61% 6% 14% 12% 6% 1	1674
February	1st to 10th	Day	65% 6% 10% 15% 3 1	1620
	11th to 20th	Day	59% 5% 12% 16% 6% 1	1654
	21st to end	Day	63% 4 11% 13% 8% 1	1480
March	1st to 10th	Day	59% 4 12% 15% 8% 1	1800
	11th to 20th	Day	54% 5% 13% 15% 11% 2	1903
	21st to end	Day	51% 5% 8% 18% 14% 3	2345
April	1st to 10th	Day	54% 4 8% 21% 11% 2	2160
	11th to 20th	Day	49% 4 8% 22% 15% 2	2134
	21st to end	Day	53% 5% 11% 16% 13% 4	2263
May	1st to 10th	Day	49% 5% 9% 19% 15% 2	2508
	11th to 20th	Day	44% 5% 11% 22% 16% 2	2482
	21st to end	Day	44% 7% 13% 20% 13% 3	2725
June	1st to 10th	Day	51% 6% 12% 18% 12% 1	2520
	11th to 20th	Day	43% 7% 12% 24% 13% 2	2520
	21st to end	Day	48% 8% 13% 19% 11% 1	2520
July	1st to 10th	Day	37% 7% 13% 28% 14% 1	2450
	11th to 20th	Day	39% 8% 16% 28% 9%	2445
	21st to end	Day	39% 9% 18% 25% 9%	2591
August	1st to 10th	Day	42% 8% 17% 26% 8%	2492
	11th to 20th	Day	38% 8% 18% 26% 10%	2245
	21st to end	Day	39% 8% 14% 26% 12% 1	2376
September	1st to 10th	Day	39% 6% 14% 28% 13%	2160
	11th to 20th	Day	36% 5% 14% 31% 13% 1	2110
	21st to end	Day	41% 8% 14% 27% 9% 1	1980
October	1st to 10th	Day	49% 6% 12% 21% 11%	1821
	11th to 20th	Day	56% 6% 15% 17% 7%	1799
	21st to end	Day	60% 5% 11% 16% 7% 1	1782
November	1st to 10th	Day	51% 7% 13% 21% 7% 1	1671
	11th to 20th	Day	56% 6% 12% 15% 9% 1	1614
	21st to end	Day	65% 7% 13% 12% 4	1520
December	1st to 10th	Day	72% 6% 11% 10% 2	1520
	11th to 20th	Day	65% 7% 13% 12% 3	1906
	21st to end	Day	71% 5% 9% 12% 3	2772
January	1st to 10th	Night	71% 4 10% 10% 4	2649
	11th to 20th	Night	69% 5% 10% 10% 6%	2660
	21st to end	Night	62% 5% 11% 14% 8% 1	2905
February	1st to 10th	Night	69% 5% 9% 12% 5% 1	2340
	11th to 20th	Night	60% 4 11% 17% 7%	2203
	21st to end	Night	59% 5% 9% 16% 10% 1	1776
March	1st to 10th	Night	61% 5% 9% 15% 9% 1	2160
	11th to 20th	Night	56% 5% 10% 17% 10% 3	2052
	21st to end	Night	53% 4 7% 20% 13% 4	2010
April	1st to 10th	Night	56% 4 9% 18% 11% 2	1800
	11th to 20th	Night	53% 4 10% 19% 13% 1	1775
	21st to end	Night	53% 5% 8% 17% 14% 4	1697
May	1st to 10th	Night	52% 4 8% 15% 16% 4	1468
	11th to 20th	Night	47% 4 9% 19% 18% 3	1416
	21st to end	Night	41% 7% 13% 21% 15% 2	1551
June	1st to 10th	Night	49% 8% 13% 18% 11% 1	1440
	11th to 20th	Night	42% 7% 12% 21% 16% 2	1440
	21st to end	Night	46% 7% 13% 23% 11%	1440
July	1st to 10th	Night	35% 8% 15% 24% 18% 1	1400
	11th to 20th	Night	36% 7% 14% 31% 12%	1392
	21st to end	Night	36% 10% 17% 24% 13% 1	1486
August	1st to 10th	Night	35% 8% 20% 25% 11%	1422
	11th to 20th	Night	39% 7% 15% 25% 12% 1	1633
	21st to end	Night	39% 7% 16% 26% 11% 1	1980
September	1st to 10th	Night	42% 6% 11% 26% 14% 1	1800
	11th to 20th	Night	37% 5% 13% 33% 11% 1	1849
	21st to end	Night	42% 6% 11% 26% 13% 1	1980
October	1st to 10th	Night	49% 6% 11% 23% 10% 1	2136
	11th to 20th	Night	53% 7% 13% 20% 8%	2160
	21st to end	Night	59% 6% 13% 14% 7% 1	2574
November	1st to 10th	Night	54% 6% 12% 19% 8% 1	2407
	11th to 20th	Night	55% 6% 12% 18% 8% 2	2565
	21st to end	Night	64% 6% 12% 14% 4	2660
December	1st to 10th	Night	71% 5% 9% 12% 2	2660
	11th to 20th	Night	67% 6% 8% 14% 5%	3288
	21st to end	Night	72% 5% 9% 11% 2	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	29% 15% 29% 23% 4	180
	11th to 20th	Day	32% 21% 29% 13% 4 1	180
	21st to end	Day	19% 18% 36% 22% 4	201
February	1st to 10th	Day	33% 26% 30% 10% 1	240
	11th to 20th	Day	25% 20% 40% 13% 2	312
	21st to end	Day	31% 20% 31% 17% 2	346
March	1st to 10th	Day	25% 18% 33% 17% 6%	420
	11th to 20th	Day	24% 20% 33% 17% 5% 1	420
	21st to end	Day	28% 19% 26% 16% 10% 1	462
April	1st to 10th	Day	30% 15% 30% 19% 5% 1	420
	11th to 20th	Day	27% 19% 26% 17% 9% 1	412
	21st to end	Day	28% 15% 30% 17% 8% 2	420
May	1st to 10th	Day	24% 16% 32% 17% 9% 2	423
	11th to 20th	Day	20% 13% 34% 21% 13% 1	447
	21st to end	Day	18% 15% 31% 24% 9% 2	485
June	1st to 10th	Day	25% 13% 28% 25% 6% 2	450
	11th to 20th	Day	15% 13% 30% 28% 11% 3	450
	21st to end	Day	20% 10% 33% 20% 13% 4	450
July	1st to 10th	Day	14% 8% 31% 27% 15% 5%	445
	11th to 20th	Day	15% 8% 28% 33% 13% 3	444
	21st to end	Day	15% 9% 31% 30% 13% 2	488
August	1st to 10th	Day	15% 8% 26% 31% 16% 4	446
	11th to 20th	Day	12% 8% 25% 34% 15% 6%	439
	21st to end	Day	15% 11% 29% 22% 15% 7%	495
September	1st to 10th	Day	15% 6% 31% 26% 15% 6%	450
	11th to 20th	Day	16% 10% 22% 25% 17% 10%	450
	21st to end	Day	17% 11% 28% 24% 16% 4	450
October	1st to 10th	Day	24% 14% 24% 20% 13% 4	450
	11th to 20th	Day	25% 19% 27% 19% 9% 2	449
	21st to end	Day	36% 14% 24% 20% 7%	396
November	1st to 10th	Day	30% 17% 25% 18% 10%	365
	11th to 20th	Day	30% 22% 24% 17% 6% 1	274
	21st to end	Day	29% 17% 29% 19% 5% 1	180
December	1st to 10th	Day	28% 19% 27% 21% 4	180
	11th to 20th	Day	30% 22% 33% 12% 3 1	180
	21st to end	Day	35% 21% 25% 16% 3 1	198
January	1st to 10th	Night	29% 16% 33% 19% 3	760
	11th to 20th	Night	30% 17% 29% 19% 4 1	760
	21st to end	Night	23% 14% 34% 24% 4	835
February	1st to 10th	Night	25% 20% 30% 22% 3	720
	11th to 20th	Night	23% 17% 35% 20% 4	703
	21st to end	Night	24% 15% 35% 23% 3	592
March	1st to 10th	Night	26% 16% 30% 23% 5%	720
	11th to 20th	Night	26% 18% 31% 22% 4	720
	21st to end	Night	27% 11% 28% 27% 7% 1	792
April	1st to 10th	Night	28% 15% 29% 24% 4	720
	11th to 20th	Night	27% 14% 29% 23% 6%	711
	21st to end	Night	28% 12% 27% 25% 8%	617
May	1st to 10th	Night	26% 11% 27% 27% 8% 1	388
	11th to 20th	Night	23% 6% 33% 28% 9%	354
	21st to end	Night	16% 9% 34% 28% 12% 1	389
June	1st to 10th	Night	22% 8% 28% 34% 7% 1	360
	11th to 20th	Night	19% 10% 27% 31% 12% 1	360
	21st to end	Night	17% 8% 30% 29% 15% 1	360
July	1st to 10th	Night	12% 5% 31% 36% 15%	350
	11th to 20th	Night	12% 5% 28% 40% 14% 1	348
	21st to end	Night	13% 5% 31% 38% 12% 1	394
August	1st to 10th	Night	13% 3 22% 41% 19% 1	358
	11th to 20th	Night	15% 5% 21% 35% 21% 3	569
	21st to end	Night	14% 7% 29% 31% 18% 2	792
September	1st to 10th	Night	16% 7% 28% 34% 14% 2	720
	11th to 20th	Night	18% 7% 21% 33% 17% 3	720
	21st to end	Night	19% 10% 24% 27% 18% 2	720
October	1st to 10th	Night	25% 11% 25% 24% 14% 1	720
	11th to 20th	Night	26% 13% 26% 25% 9%	720
	21st to end	Night	30% 15% 29% 22% 4	792
November	1st to 10th	Night	28% 16% 25% 23% 7%	740
	11th to 20th	Night	26% 18% 28% 18% 9%	760
	21st to end	Night	30% 17% 24% 21% 6%	760
December	1st to 10th	Night	31% 18% 29% 19% 4	760
	11th to 20th	Night	32% 18% 29% 17% 4 1	760
	21st to end	Night	36% 16% 29% 15% 4	836

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