L'Observatoire des Lacs
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the L'Observatoire des Lacs clear sky chart from 2005-11-06 to 2025-01-07.

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% 8% 3 6% 4 4 4 4 5% 7% 37%	104409
	Night	18% 8% 3 7% 4 4 4 4 5% 7% 36%	104634
Transparency	Day	47% 3 11% 23% 14% 2	107934
	Night	48% 3 11% 23% 14% 2	109199
Seeing	Day	25% 13% 31% 20% 9% 2	15475
	Night	24% 11% 31% 26% 8% 1	24206

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	15% 8% 2 7% 4 4 5% 5% 5% 6% 37%	2565
	11th to 20th	Day	17% 7% 3 9% 4 4 4 3 5% 7% 38%	2520
	21st to end	Day	15% 11% 4 8% 5% 3 4 4 5% 6% 34%	2772
February	1st to 10th	Day	16% 11% 3 6% 5% 4 5% 4 6% 6% 35%	1670
	11th to 20th	Day	12% 11% 3 8% 6% 5% 5% 4 5% 6% 35%	1824
	21st to end	Day	15% 11% 3 6% 5% 5% 5% 3 6% 5% 34%	1570
March	1st to 10th	Day	25% 12% 3 7% 5% 4 4 3 5% 4 29%	3116
	11th to 20th	Day	17% 9% 2 7% 5% 5% 5% 4 5% 6% 36%	3433
	21st to end	Day	25% 9% 3 6% 3 3 3 3 6% 6% 33%	3898
April	1st to 10th	Day	20% 10% 4 7% 5% 4 4 3 4 5% 35%	2280
	11th to 20th	Day	18% 6% 2 6% 4 4 4 3 6% 7% 39%	2370
	21st to end	Day	21% 8% 3 8% 5% 4 4 3 4 5% 34%	2470
May	1st to 10th	Day	21% 8% 3 7% 4 4 5% 5% 6% 7% 30%	4106
	11th to 20th	Day	18% 7% 3 8% 6% 5% 5% 4 6% 7% 31%	4166
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 5% 6% 9% 37%	4837
June	1st to 10th	Day	22% 8% 2 6% 4 4 6% 4 6% 7% 31%	2769
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 4 5% 8% 38%	2850
	21st to end	Day	24% 6% 2 6% 5% 4 5% 4 5% 7% 32%	2754
July	1st to 10th	Day	14% 6% 2 7% 5% 5% 5% 5% 6% 9% 35%	4440
	11th to 20th	Day	12% 5% 3 6% 4 5% 6% 5% 7% 9% 37%	4440
	21st to end	Day	18% 7% 3 7% 4 4 4 3 6% 7% 36%	4814
August	1st to 10th	Day	15% 6% 2 8% 5% 5% 6% 4 6% 8% 34%	4142
	11th to 20th	Day	16% 9% 4 8% 4 4 4 4 6% 7% 34%	4045
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 45%	4386
September	1st to 10th	Day	11% 5% 2 5% 4 4 5% 3 5% 7% 48%	2231
	11th to 20th	Day	12% 5% 2 4 3 4 4 3 5% 6% 52%	2090
	21st to end	Day	17% 5% 2 5% 3 4 4 3 6% 7% 44%	2090
October	1st to 10th	Day	18% 7% 2 7% 4 4 5% 3 5% 6% 39%	2085
	11th to 20th	Day	19% 6% 2 6% 4 5% 4 2 5% 7% 41%	1824
	21st to end	Day	23% 8% 3 7% 4 4 2 3 4 5% 36%	1881
November	1st to 10th	Day	17% 6% 2 5% 4 4 5% 4 4 6% 42%	1761
	11th to 20th	Day	20% 6% 2 5% 4 4 4 4 7% 6% 41%	1800
	21st to end	Day	22% 7% 2 5% 4 3 3 2 4 6% 42%	1690
December	1st to 10th	Day	17% 11% 2 6% 4 4 4 4 6% 6% 35%	2293
	11th to 20th	Day	20% 8% 2 5% 3 3 3 3 5% 6% 43%	2618
	21st to end	Day	21% 11% 3 6% 5% 4 4 3 5% 5% 33%	3809
January	1st to 10th	Night	15% 9% 3 8% 5% 4 4 4 5% 6% 38%	4514
	11th to 20th	Night	13% 10% 4 11% 6% 5% 4 4 6% 6% 31%	4440
	21st to end	Night	17% 11% 4 9% 5% 4 4 4 5% 6% 31%	4884
February	1st to 10th	Night	15% 10% 3 9% 5% 5% 5% 4 5% 6% 33%	2508
	11th to 20th	Night	12% 11% 3 7% 5% 4 5% 4 4 6% 39%	2240
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 5% 6% 38%	1884
March	1st to 10th	Night	29% 11% 3 5% 4 4 4 3 4 5% 30%	3724
	11th to 20th	Night	16% 10% 4 7% 5% 5% 5% 4 6% 6% 32%	3525
	21st to end	Night	26% 10% 3 7% 4 4 4 3 5% 6% 29%	3732
April	1st to 10th	Night	21% 12% 3 6% 4 4 3 3 3 5% 36%	1900
	11th to 20th	Night	19% 7% 2 7% 4 5% 5% 3 5% 6% 37%	1758
	21st to end	Night	25% 8% 2 7% 4 4 3 3 5% 6% 34%	1710
May	1st to 10th	Night	23% 9% 2 5% 4 4 4 4 6% 8% 31%	2834
	11th to 20th	Night	18% 8% 3 8% 5% 5% 5% 5% 6% 8% 29%	2755
	21st to end	Night	16% 6% 3 7% 5% 3 5% 4 6% 7% 39%	2738
June	1st to 10th	Night	23% 8% 2 6% 5% 4 4 3 6% 9% 31%	1402
	11th to 20th	Night	17% 6% 2 6% 4 4 3 5% 6% 8% 38%	1330
	21st to end	Night	24% 8% 2 5% 4 4 5% 5% 6% 7% 31%	1417
July	1st to 10th	Night	12% 6% 2 6% 5% 6% 6% 5% 7% 10% 35%	2520
	11th to 20th	Night	15% 8% 2 7% 4 5% 5% 3 5% 8% 38%	2520
	21st to end	Night	15% 6% 2 5% 3 4 5% 5% 6% 8% 41%	2739
August	1st to 10th	Night	20% 8% 2 8% 5% 5% 5% 4 5% 7% 30%	2754
	11th to 20th	Night	15% 9% 3 8% 5% 4 5% 4 6% 7% 34%	2782
	21st to end	Night	11% 6% 2 7% 4 4 5% 3 5% 8% 45%	3268
September	1st to 10th	Night	14% 5% 2 5% 3 4 4 3 5% 7% 49%	1948
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 4 6% 52%	2090
	21st to end	Night	16% 6% 2 5% 3 4 4 3 4 7% 44%	2090
October	1st to 10th	Night	18% 6% 2 7% 4 5% 4 3 5% 6% 39%	2095
	11th to 20th	Night	17% 6% 2 5% 4 4 5% 4 5% 6% 41%	2354
	21st to end	Night	25% 8% 2 5% 4 4 4 3 5% 6% 35%	2717
November	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 4 6% 42%	2537
	11th to 20th	Night	17% 6% 2 6% 4 3 4 4 4 8% 41%	2600
	21st to end	Night	18% 8% 2 7% 4 4 4 3 5% 7% 39%	2705
December	1st to 10th	Night	17% 11% 3 7% 4 4 5% 4 6% 7% 33%	4895
	11th to 20th	Night	23% 8% 2 5% 4 4 4 3 4 5% 38%	5591
	21st to end	Night	21% 10% 3 6% 3 4 3 3 5% 5% 36%	7134

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	47% 2 12% 27% 11% 1	2566
	11th to 20th	Day	46% 3 11% 26% 12% 3	2520
	21st to end	Day	51% 2 14% 19% 13% 1	2772
February	1st to 10th	Day	50% 3 10% 21% 14% 3	1670
	11th to 20th	Day	50% 3 10% 20% 14% 3	1833
	21st to end	Day	50% 3 10% 20% 12% 6%	1570
March	1st to 10th	Day	60% 3 8% 12% 14% 3	3119
	11th to 20th	Day	50% 3 9% 21% 15% 3	3433
	21st to end	Day	51% 3 5% 17% 19% 5%	3898
April	1st to 10th	Day	53% 3 6% 18% 15% 5%	2280
	11th to 20th	Day	45% 3 8% 25% 16% 2	2370
	21st to end	Day	53% 2 7% 20% 14% 4	2470
May	1st to 10th	Day	55% 3 11% 20% 9% 2	4120
	11th to 20th	Day	53% 3 13% 19% 9% 2	4170
	21st to end	Day	43% 4 13% 23% 16% 1	4837
June	1st to 10th	Day	52% 3 12% 24% 9%	2769
	11th to 20th	Day	45% 3 11% 22% 16% 3	2850
	21st to end	Day	53% 4 11% 20% 11% 1	2754
July	1st to 10th	Day	45% 3 13% 26% 13%	4370
	11th to 20th	Day	42% 4 14% 28% 12% 1	4365
	21st to end	Day	48% 3 11% 21% 16% 1	4661
August	1st to 10th	Day	49% 3 13% 27% 8%	4128
	11th to 20th	Day	50% 3 14% 25% 8%	4084
	21st to end	Day	36% 2 13% 28% 19% 2	4386
September	1st to 10th	Day	35% 3 12% 29% 19% 1	2231
	11th to 20th	Day	33% 2 9% 31% 23% 2	2090
	21st to end	Day	39% 3 11% 30% 17% 1	2090
October	1st to 10th	Day	54% 2 8% 23% 13% 1	3462
	11th to 20th	Day	37% 2 9% 31% 20% 1	3014
	21st to end	Day	39% 1 12% 24% 20% 3	3081
November	1st to 10th	Day	43% 3 12% 23% 16% 2	1761
	11th to 20th	Day	42% 3 9% 23% 19% 4	1800
	21st to end	Day	46% 2 14% 24% 12% 1	1690
December	1st to 10th	Day	49% 2 13% 23% 12% 1	2293
	11th to 20th	Day	43% 3 10% 20% 17% 7%	2618
	21st to end	Day	55% 1 9% 18% 14% 3	3809
January	1st to 10th	Night	47% 2 12% 27% 10% 2	4514
	11th to 20th	Night	50% 4 13% 18% 12% 3	4440
	21st to end	Night	54% 2 9% 21% 13% 2	4884
February	1st to 10th	Night	51% 3 11% 19% 13% 3	2508
	11th to 20th	Night	47% 3 9% 22% 17% 3	2242
	21st to end	Night	48% 3 8% 21% 15% 4	1884
March	1st to 10th	Night	59% 2 7% 17% 10% 4	3734
	11th to 20th	Night	52% 4 10% 21% 10% 3	3526
	21st to end	Night	57% 2 5% 16% 17% 3	3732
April	1st to 10th	Night	53% 2 6% 19% 16% 4	1900
	11th to 20th	Night	48% 3 7% 23% 18% 1	1758
	21st to end	Night	52% 3 11% 17% 15% 3	1710
May	1st to 10th	Night	51% 3 16% 20% 9% 1	2840
	11th to 20th	Night	53% 3 13% 21% 8% 2	2756
	21st to end	Night	45% 3 16% 22% 13% 1	2739
June	1st to 10th	Night	52% 3 11% 24% 10% 1	1402
	11th to 20th	Night	44% 3 16% 23% 13% 2	1330
	21st to end	Night	51% 5% 13% 20% 9% 2	1417
July	1st to 10th	Night	44% 3 17% 23% 12%	2480
	11th to 20th	Night	46% 2 14% 28% 10%	2472
	21st to end	Night	41% 4 15% 27% 14% 1	2651
August	1st to 10th	Night	55% 4 13% 21% 7%	2744
	11th to 20th	Night	48% 4 16% 21% 11%	2809
	21st to end	Night	39% 3 13% 27% 16% 2	3268
September	1st to 10th	Night	36% 3 13% 30% 17%	1948
	11th to 20th	Night	34% 3 10% 33% 18% 2	2090
	21st to end	Night	40% 3 9% 31% 14% 2	2090
October	1st to 10th	Night	48% 2 12% 24% 14% 1	3498
	11th to 20th	Night	36% 2 10% 30% 21% 1	3903
	21st to end	Night	40% 2 11% 28% 16% 2	4467
November	1st to 10th	Night	44% 3 10% 27% 15% 1	2537
	11th to 20th	Night	43% 4 9% 25% 17% 3	2600
	21st to end	Night	46% 3 13% 22% 14% 2	2705
December	1st to 10th	Night	50% 1 8% 30% 10% 1	4895
	11th to 20th	Night	50% 2 9% 18% 19% 2	5592
	21st to end	Night	52% 2 10% 19% 17% 1	7134

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	31% 17% 31% 16% 6%	192
	11th to 20th	Day	33% 15% 29% 17% 4 1	180
	21st to end	Day	22% 16% 36% 23% 2 1	198
February	1st to 10th	Day	30% 17% 31% 19% 3	330
	11th to 20th	Day	27% 20% 36% 15% 2	444
	21st to end	Day	25% 18% 33% 19% 4	382
March	1st to 10th	Day	30% 16% 35% 16% 4	460
	11th to 20th	Day	28% 21% 33% 13% 4	460
	21st to end	Day	30% 19% 29% 13% 7% 2	506
April	1st to 10th	Day	34% 16% 31% 13% 5% 1	460
	11th to 20th	Day	31% 18% 30% 14% 4 3	452
	21st to end	Day	31% 17% 29% 16% 5% 3	460
May	1st to 10th	Day	32% 15% 27% 21% 3 2	463
	11th to 20th	Day	24% 16% 32% 19% 7% 3	487
	21st to end	Day	22% 15% 35% 20% 7% 1	529
June	1st to 10th	Day	23% 11% 30% 23% 10% 2	599
	11th to 20th	Day	22% 11% 28% 22% 13% 3	680
	21st to end	Day	25% 10% 29% 24% 10% 2	584
July	1st to 10th	Day	20% 8% 33% 27% 10% 3	485
	11th to 20th	Day	15% 5% 38% 24% 13% 5%	484
	21st to end	Day	16% 5% 36% 27% 12% 3	532
August	1st to 10th	Day	14% 6% 30% 30% 15% 3	486
	11th to 20th	Day	18% 7% 28% 26% 15% 5%	479
	21st to end	Day	15% 8% 31% 26% 14% 5%	539
September	1st to 10th	Day	16% 9% 31% 24% 17% 3	490
	11th to 20th	Day	19% 10% 26% 22% 14% 9%	490
	21st to end	Day	20% 9% 30% 23% 13% 5%	490
October	1st to 10th	Day	23% 12% 25% 23% 12% 5%	487
	11th to 20th	Day	27% 11% 31% 17% 11% 3	390
	21st to end	Day	33% 9% 31% 19% 7%	429
November	1st to 10th	Day	27% 12% 30% 21% 9% 2	395
	11th to 20th	Day	29% 14% 31% 20% 5% 1	400
	21st to end	Day	32% 17% 25% 19% 7% 1	400
December	1st to 10th	Day	30% 13% 35% 17% 6%	213
	11th to 20th	Day	37% 15% 37% 8% 4	200
	21st to end	Day	32% 13% 30% 16% 7% 1	220
January	1st to 10th	Night	24% 14% 35% 24% 3	785
	11th to 20th	Night	27% 16% 30% 23% 4	760
	21st to end	Night	19% 17% 37% 22% 4	836
February	1st to 10th	Night	23% 15% 33% 25% 4	760
	11th to 20th	Night	22% 19% 36% 20% 3	743
	21st to end	Night	22% 12% 37% 24% 4	628
March	1st to 10th	Night	28% 13% 37% 20% 2	760
	11th to 20th	Night	24% 15% 33% 21% 7%	760
	21st to end	Night	27% 15% 31% 22% 5% 1	836
April	1st to 10th	Night	30% 14% 31% 21% 3	760
	11th to 20th	Night	24% 14% 34% 22% 5%	634
	21st to end	Night	31% 11% 33% 21% 5%	570
May	1st to 10th	Night	27% 10% 29% 28% 6%	570
	11th to 20th	Night	24% 10% 35% 25% 5%	530
	21st to end	Night	21% 7% 31% 35% 5% 1	411
June	1st to 10th	Night	26% 4 32% 30% 7% 1	380
	11th to 20th	Night	19% 6% 27% 37% 10%	380
	21st to end	Night	27% 3 29% 31% 9%	380
July	1st to 10th	Night	19% 2 31% 36% 13% 1	370
	11th to 20th	Night	20% 1 26% 40% 12% 1	368
	21st to end	Night	17% 3 26% 37% 17% 1	416
August	1st to 10th	Night	17% 3 26% 40% 14% 1	522
	11th to 20th	Night	16% 4 26% 35% 17% 1	562
	21st to end	Night	17% 7% 22% 37% 16% 2	706
September	1st to 10th	Night	17% 5% 33% 29% 15% 1	760
	11th to 20th	Night	18% 7% 26% 27% 18% 4	760
	21st to end	Night	21% 8% 29% 28% 12% 2	760
October	1st to 10th	Night	22% 8% 26% 29% 14% 1	760
	11th to 20th	Night	22% 11% 29% 26% 11% 1	760
	21st to end	Night	31% 12% 29% 22% 6%	836
November	1st to 10th	Night	24% 10% 32% 26% 7% 1	780
	11th to 20th	Night	22% 13% 34% 22% 8% 1	800
	21st to end	Night	26% 16% 29% 24% 5%	853
December	1st to 10th	Night	28% 12% 33% 20% 6%	900
	11th to 20th	Night	26% 16% 32% 21% 5%	900
	21st to end	Night	29% 13% 32% 23% 3	910

Page generated at 2025-01-04 03:24:03. Version ac07

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