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 2025-02-21.

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% 11% 3 9% 5% 5% 5% 4 5% 7% 31%	108618
	Night	18% 13% 4 9% 5% 4 4 3 5% 5% 27%	109966
Transparency	Day	51% 6% 13% 21% 9% 1	112109
	Night	56% 6% 11% 18% 9% 1	114627
Seeing	Day	22% 15% 29% 22% 10% 3	14830
	Night	24% 13% 28% 25% 9% 1	24010

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	16% 23% 5% 13% 7% 5% 4 3 4 4 16%	3597
	11th to 20th	Day	12% 18% 8% 17% 6% 5% 5% 4 4 4 18%	3570
	21st to end	Day	13% 18% 7% 13% 6% 6% 5% 4 5% 6% 18%	3984
February	1st to 10th	Day	16% 16% 4 12% 7% 6% 5% 4 5% 5% 21%	1800
	11th to 20th	Day	16% 12% 4 12% 7% 6% 5% 4 5% 5% 24%	1833
	21st to end	Day	19% 17% 4 9% 6% 5% 4 3 6% 5% 24%	1570
March	1st to 10th	Day	21% 13% 4 9% 5% 5% 4 3 4 6% 26%	3116
	11th to 20th	Day	14% 15% 5% 12% 6% 5% 4 3 5% 6% 25%	3347
	21st to end	Day	16% 9% 3 10% 5% 3 4 3 4 5% 37%	4121
April	1st to 10th	Day	22% 12% 3 7% 5% 3 3 3 4 6% 32%	2280
	11th to 20th	Day	21% 9% 2 7% 6% 3 4 2 5% 6% 36%	2254
	21st to end	Day	22% 10% 3 7% 4 4 3 4 5% 6% 33%	2389
May	1st to 10th	Day	21% 9% 3 8% 5% 4 5% 4 5% 7% 30%	4414
	11th to 20th	Day	17% 10% 3 8% 5% 4 4 4 5% 7% 33%	4398
	21st to end	Day	15% 7% 2 6% 5% 4 6% 5% 6% 8% 37%	4837
June	1st to 10th	Day	22% 8% 3 7% 5% 4 5% 4 5% 7% 30%	2660
	11th to 20th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 8% 37%	2660
	21st to end	Day	21% 6% 2 7% 4 5% 5% 4 6% 7% 33%	2660
July	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 38%	4440
	11th to 20th	Day	13% 6% 2 6% 4 5% 6% 5% 7% 9% 37%	4440
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 7% 9% 42%	4814
August	1st to 10th	Day	16% 7% 3 7% 5% 4 6% 4 6% 9% 33%	4423
	11th to 20th	Day	16% 6% 3 6% 4 4 4 4 6% 8% 39%	3928
	21st to end	Day	17% 6% 2 5% 4 4 5% 4 6% 9% 38%	4180
September	1st to 10th	Day	14% 6% 2 5% 4 5% 5% 3 5% 7% 44%	2280
	11th to 20th	Day	14% 5% 2 6% 3 3 4 3 5% 6% 48%	2227
	21st to end	Day	17% 7% 2 5% 4 4 5% 3 5% 8% 39%	2090
October	1st to 10th	Day	21% 8% 2 7% 5% 4 4 3 5% 7% 33%	1922
	11th to 20th	Day	19% 12% 3 9% 5% 5% 4 4 4 5% 29%	1898
	21st to end	Day	27% 12% 3 6% 4 4 4 3 5% 5% 26%	1881
November	1st to 10th	Day	17% 11% 3 8% 5% 5% 4 4 5% 7% 30%	1761
	11th to 20th	Day	17% 12% 3 10% 6% 5% 4 4 5% 6% 27%	1698
	21st to end	Day	21% 18% 5% 10% 5% 5% 4 4 5% 6% 19%	1600
December	1st to 10th	Day	21% 19% 6% 13% 7% 6% 5% 3 4 5% 12%	2600
	11th to 20th	Day	18% 16% 5% 12% 5% 5% 5% 3 5% 5% 20%	2986
	21st to end	Day	22% 16% 5% 12% 5% 5% 4 4 7% 5% 16%	3960
January	1st to 10th	Night	15% 23% 9% 16% 6% 4 4 2 3 3 15%	6349
	11th to 20th	Night	11% 21% 8% 17% 7% 6% 5% 4 4 3 13%	6315
	21st to end	Night	15% 19% 7% 13% 6% 6% 5% 4 4 5% 18%	6968
February	1st to 10th	Night	16% 16% 6% 15% 8% 5% 4 4 4 5% 19%	2600
	11th to 20th	Night	15% 14% 6% 11% 7% 5% 5% 3 5% 4 24%	2453
	21st to end	Night	16% 15% 4 11% 6% 4 4 4 5% 4 27%	1885
March	1st to 10th	Night	21% 12% 3 7% 6% 5% 4 3 4 4 31%	3724
	11th to 20th	Night	18% 20% 4 9% 5% 4 3 2 4 5% 25%	3607
	21st to end	Night	15% 13% 4 8% 5% 4 4 3 4 5% 36%	3510
April	1st to 10th	Night	22% 12% 4 8% 4 4 4 3 5% 4 30%	1900
	11th to 20th	Night	23% 11% 2 7% 5% 4 3 3 5% 6% 32%	1875
	21st to end	Night	21% 10% 3 7% 4 4 3 3 5% 5% 34%	1791
May	1st to 10th	Night	24% 10% 3 7% 5% 4 5% 4 5% 7% 27%	2556
	11th to 20th	Night	23% 8% 3 6% 4 4 5% 3 5% 5% 34%	2495
	21st to end	Night	16% 6% 2 7% 4 5% 4 4 5% 9% 38%	2738
June	1st to 10th	Night	19% 7% 3 7% 5% 5% 5% 5% 7% 9% 28%	1520
	11th to 20th	Night	18% 6% 3 5% 4 4 4 4 6% 8% 40%	1520
	21st to end	Night	18% 8% 3 8% 5% 4 4 4 6% 7% 34%	1520
July	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 3 6% 7% 34%	2520
	11th to 20th	Night	15% 5% 3 6% 4 4 4 4 5% 8% 41%	2520
	21st to end	Night	12% 8% 3 7% 4 4 4 4 7% 8% 39%	2739
August	1st to 10th	Night	17% 8% 4 8% 4 3 3 3 6% 7% 38%	2506
	11th to 20th	Night	18% 7% 3 7% 4 5% 4 3 6% 7% 37%	2864
	21st to end	Night	15% 6% 2 6% 5% 5% 4 4 6% 7% 40%	3476
September	1st to 10th	Night	14% 7% 3 6% 4 4 5% 3 6% 7% 42%	1900
	11th to 20th	Night	15% 5% 2 5% 4 4 3 3 5% 6% 48%	1952
	21st to end	Night	18% 5% 2 6% 5% 4 5% 3 5% 7% 41%	2090
October	1st to 10th	Night	21% 9% 2 6% 4 4 4 4 5% 6% 34%	2255
	11th to 20th	Night	21% 9% 3 8% 5% 5% 5% 3 5% 5% 32%	2280
	21st to end	Night	24% 11% 3 9% 5% 4 4 3 4 6% 28%	2717
November	1st to 10th	Night	20% 12% 3 7% 4 4 4 4 5% 6% 30%	2537
	11th to 20th	Night	19% 12% 4 7% 5% 4 4 3 5% 6% 31%	2700
	21st to end	Night	23% 14% 5% 9% 5% 4 4 3 4 5% 22%	2800
December	1st to 10th	Night	19% 24% 7% 11% 6% 4 4 3 4 4 13%	4580
	11th to 20th	Night	24% 15% 5% 11% 5% 4 4 3 4 5% 20%	5208
	21st to end	Night	25% 17% 5% 11% 4 4 3 3 5% 5% 17%	6996

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	70% 6% 11% 11% 2	3598
	11th to 20th	Day	66% 6% 11% 10% 6%	3600
	21st to end	Day	63% 6% 13% 10% 7% 1	3984
February	1st to 10th	Day	63% 6% 10% 16% 4 1	1800
	11th to 20th	Day	60% 5% 12% 16% 6% 1	1842
	21st to end	Day	61% 4 11% 13% 8% 1	1570
March	1st to 10th	Day	58% 6% 10% 18% 6% 1	3119
	11th to 20th	Day	58% 6% 13% 14% 8% 1	3347
	21st to end	Day	46% 5% 10% 20% 17% 2	4121
April	1st to 10th	Day	54% 5% 8% 21% 11% 2	2280
	11th to 20th	Day	50% 4 8% 21% 15% 2	2254
	21st to end	Day	53% 4 10% 16% 13% 3	2389
May	1st to 10th	Day	52% 7% 11% 19% 10% 1	4428
	11th to 20th	Day	49% 8% 14% 19% 9% 1	4402
	21st to end	Day	41% 8% 13% 24% 12% 2	4837
June	1st to 10th	Day	51% 6% 12% 18% 12% 1	2660
	11th to 20th	Day	43% 7% 12% 24% 13% 2	2660
	21st to end	Day	49% 8% 13% 19% 10% 1	2660
July	1st to 10th	Day	41% 7% 13% 28% 10% 1	4370
	11th to 20th	Day	39% 8% 16% 30% 7%	4365
	21st to end	Day	36% 8% 14% 32% 9%	4661
August	1st to 10th	Day	46% 7% 15% 23% 9%	4409
	11th to 20th	Day	42% 7% 17% 28% 7%	3969
	21st to end	Day	41% 7% 17% 23% 12%	4180
September	1st to 10th	Day	39% 5% 14% 29% 13%	2280
	11th to 20th	Day	36% 5% 14% 31% 14% 1	2227
	21st to end	Day	41% 8% 14% 27% 10% 1	2090
October	1st to 10th	Day	47% 5% 12% 28% 8%	3195
	11th to 20th	Day	46% 6% 16% 25% 7%	3126
	21st to end	Day	48% 4 19% 23% 6%	3081
November	1st to 10th	Day	51% 7% 13% 21% 7% 1	1761
	11th to 20th	Day	55% 6% 12% 16% 10% 1	1698
	21st to end	Day	65% 7% 13% 12% 3	1600
December	1st to 10th	Day	73% 6% 10% 10% 1	2600
	11th to 20th	Day	64% 6% 10% 13% 6%	2986
	21st to end	Day	68% 6% 9% 14% 3	3960
January	1st to 10th	Night	72% 5% 10% 9% 3	6349
	11th to 20th	Night	73% 6% 9% 9% 4	6360
	21st to end	Night	67% 5% 10% 9% 9% 1	6968
February	1st to 10th	Night	67% 6% 9% 13% 5% 1	2600
	11th to 20th	Night	61% 4 11% 16% 7%	2455
	21st to end	Night	58% 5% 9% 17% 10% 1	1885
March	1st to 10th	Night	54% 6% 9% 21% 8% 2	3734
	11th to 20th	Night	61% 6% 10% 14% 7% 2	3608
	21st to end	Night	50% 5% 7% 22% 14% 2	3510
April	1st to 10th	Night	56% 4 9% 18% 11% 2	1900
	11th to 20th	Night	53% 5% 10% 19% 12% 1	1875
	21st to end	Night	52% 5% 8% 17% 14% 4	1791
May	1st to 10th	Night	53% 7% 11% 17% 9% 2	2562
	11th to 20th	Night	49% 7% 12% 20% 11% 2	2496
	21st to end	Night	41% 8% 11% 21% 17% 1	2739
June	1st to 10th	Night	49% 9% 13% 17% 11% 1	1520
	11th to 20th	Night	42% 7% 13% 21% 16% 2	1520
	21st to end	Night	48% 7% 13% 22% 11%	1520
July	1st to 10th	Night	46% 6% 13% 21% 13%	2480
	11th to 20th	Night	39% 6% 17% 30% 8%	2472
	21st to end	Night	40% 8% 14% 24% 13%	2651
August	1st to 10th	Night	44% 7% 15% 26% 8%	2496
	11th to 20th	Night	45% 7% 17% 24% 7%	2889
	21st to end	Night	40% 7% 16% 23% 13% 1	3476
September	1st to 10th	Night	41% 6% 11% 27% 14% 1	1900
	11th to 20th	Night	36% 5% 13% 33% 12% 1	1952
	21st to end	Night	42% 6% 12% 26% 14% 1	2090
October	1st to 10th	Night	45% 6% 13% 28% 8%	3762
	11th to 20th	Night	46% 6% 13% 26% 9%	3779
	21st to end	Night	49% 5% 16% 20% 9% 1	4467
November	1st to 10th	Night	54% 6% 12% 19% 8% 1	2537
	11th to 20th	Night	54% 5% 12% 18% 9% 2	2700
	21st to end	Night	64% 6% 11% 14% 4	2800
December	1st to 10th	Night	73% 5% 10% 9% 2	4580
	11th to 20th	Night	66% 5% 8% 13% 8%	5208
	21st to end	Night	67% 5% 8% 12% 8%	6996

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	30% 15% 31% 21% 4	200
	11th to 20th	Day	31% 20% 32% 15% 4 1	200
	21st to end	Day	18% 20% 35% 22% 4	226
February	1st to 10th	Day	30% 23% 30% 13% 3	300
	11th to 20th	Day	24% 23% 39% 12% 1	380
	21st to end	Day	30% 20% 31% 17% 2	382
March	1st to 10th	Day	26% 17% 32% 17% 7%	460
	11th to 20th	Day	24% 21% 34% 16% 4 1	460
	21st to end	Day	27% 20% 27% 15% 9% 1	506
April	1st to 10th	Day	30% 15% 30% 20% 4 1	460
	11th to 20th	Day	28% 20% 27% 16% 9% 1	452
	21st to end	Day	28% 16% 30% 16% 8% 2	460
May	1st to 10th	Day	24% 15% 32% 19% 9% 2	463
	11th to 20th	Day	20% 12% 32% 21% 14% 1	487
	21st to end	Day	18% 16% 29% 24% 10% 2	529
June	1st to 10th	Day	24% 12% 28% 26% 7% 2	490
	11th to 20th	Day	14% 13% 31% 28% 11% 3	490
	21st to end	Day	22% 11% 33% 19% 12% 3	490
July	1st to 10th	Day	14% 7% 32% 27% 15% 5%	485
	11th to 20th	Day	15% 8% 29% 32% 13% 3	484
	21st to end	Day	14% 9% 30% 31% 14% 2	532
August	1st to 10th	Day	15% 8% 26% 31% 16% 3	486
	11th to 20th	Day	13% 8% 25% 33% 15% 6%	479
	21st to end	Day	16% 10% 28% 23% 16% 7%	539
September	1st to 10th	Day	15% 7% 31% 26% 15% 6%	490
	11th to 20th	Day	14% 9% 21% 24% 19% 12%	490
	21st to end	Day	17% 10% 28% 23% 17% 4	490
October	1st to 10th	Day	23% 14% 25% 21% 13% 4	490
	11th to 20th	Day	25% 19% 26% 19% 9% 2	488
	21st to end	Day	34% 14% 25% 20% 7%	429
November	1st to 10th	Day	30% 17% 25% 17% 10%	395
	11th to 20th	Day	28% 23% 25% 17% 5% 1	298
	21st to end	Day	32% 18% 27% 18% 5% 1	200
December	1st to 10th	Day	28% 19% 28% 21% 5%	200
	11th to 20th	Day	30% 22% 34% 12% 3 1	200
	21st to end	Day	32% 20% 25% 17% 4 1	220
January	1st to 10th	Night	29% 16% 34% 19% 3	800
	11th to 20th	Night	29% 18% 29% 19% 5% 1	800
	21st to end	Night	22% 14% 35% 24% 5%	880
February	1st to 10th	Night	24% 19% 30% 24% 3	800
	11th to 20th	Night	23% 19% 35% 19% 4	783
	21st to end	Night	25% 15% 34% 23% 3	629
March	1st to 10th	Night	26% 16% 30% 23% 5%	760
	11th to 20th	Night	26% 18% 31% 21% 3	760
	21st to end	Night	27% 11% 28% 26% 7%	836
April	1st to 10th	Night	28% 15% 30% 24% 4	760
	11th to 20th	Night	28% 14% 29% 22% 6%	751
	21st to end	Night	28% 12% 27% 25% 8%	651
May	1st to 10th	Night	26% 11% 27% 27% 8% 1	409
	11th to 20th	Night	24% 6% 31% 28% 10%	374
	21st to end	Night	16% 9% 35% 29% 11%	411
June	1st to 10th	Night	21% 9% 28% 34% 8% 1	380
	11th to 20th	Night	18% 10% 26% 33% 12% 1	380
	21st to end	Night	18% 8% 30% 29% 14% 1	380
July	1st to 10th	Night	14% 5% 31% 36% 15%	370
	11th to 20th	Night	13% 5% 28% 40% 14% 1	368
	21st to end	Night	13% 5% 31% 38% 12% 1	416
August	1st to 10th	Night	15% 3 21% 42% 18% 1	378
	11th to 20th	Night	15% 5% 21% 35% 22% 3	602
	21st to end	Night	14% 7% 28% 32% 17% 2	836
September	1st to 10th	Night	15% 7% 28% 34% 14% 1	760
	11th to 20th	Night	17% 7% 20% 32% 20% 4	760
	21st to end	Night	19% 9% 23% 28% 18% 2	760
October	1st to 10th	Night	25% 10% 25% 25% 14% 1	760
	11th to 20th	Night	26% 13% 26% 25% 9%	760
	21st to end	Night	29% 15% 29% 22% 5%	836
November	1st to 10th	Night	28% 17% 25% 23% 7%	780
	11th to 20th	Night	26% 17% 29% 18% 9%	800
	21st to end	Night	30% 18% 25% 21% 6%	800
December	1st to 10th	Night	31% 18% 29% 19% 4	800
	11th to 20th	Night	32% 18% 28% 17% 4 1	800
	21st to end	Night	36% 16% 29% 16% 4	880

Page generated at 2025-02-18 03:26:31. Version ac07

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