Mont Louis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mont Louis clear sky chart from 2005-11-06 to 2025-11-18.

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	15% 10% 4 9% 6% 5% 5% 4 5% 6% 30%	127325
	Night	15% 13% 5% 13% 8% 6% 5% 3 5% 5% 22%	123217
Transparency	Day	54% 9% 13% 17% 7%	130750
	Night	61% 7% 11% 14% 6% 1	127881
Seeing	Day	22% 17% 28% 23% 9% 2	17774
	Night	23% 17% 26% 23% 10% 1	25315

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% 23% 7% 15% 17% 7% 4 3 3 2 4	3157
	11th to 20th	Day	7% 17% 11% 23% 14% 5% 5% 3 5% 4 7%	3134
	21st to end	Day	3 7% 3 12% 17% 13% 15% 5% 5% 5% 15%	3882
February	1st to 10th	Day	8% 10% 5% 12% 10% 12% 7% 5% 6% 6% 19%	1800
	11th to 20th	Day	10% 16% 5% 11% 9% 6% 6% 5% 6% 5% 20%	1753
	21st to end	Day	7% 11% 4 10% 7% 9% 6% 5% 6% 8% 27%	1505
March	1st to 10th	Day	14% 14% 5% 12% 7% 8% 7% 4 5% 6% 19%	4722
	11th to 20th	Day	15% 15% 5% 10% 8% 7% 5% 5% 6% 6% 18%	4980
	21st to end	Day	18% 15% 5% 9% 8% 5% 5% 3 5% 6% 21%	5466
April	1st to 10th	Day	15% 12% 5% 11% 8% 5% 5% 3 5% 6% 26%	2419
	11th to 20th	Day	15% 12% 2 8% 5% 4 5% 4 6% 5% 34%	2572
	21st to end	Day	22% 12% 3 7% 5% 3 4 3 4 6% 33%	2600
May	1st to 10th	Day	19% 11% 4 8% 5% 4 4 4 5% 6% 31%	6179
	11th to 20th	Day	19% 8% 3 7% 4 5% 5% 4 6% 8% 31%	6409
	21st to end	Day	21% 8% 3 7% 4 4 4 3 5% 6% 36%	7451
June	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 4 6% 6% 33%	3000
	11th to 20th	Day	16% 6% 3 6% 5% 4 4 4 6% 8% 37%	3000
	21st to end	Day	18% 8% 3 7% 5% 4 4 4 6% 8% 34%	3000
July	1st to 10th	Day	15% 6% 2 7% 5% 4 5% 4 6% 7% 39%	6820
	11th to 20th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 8% 36%	6820
	21st to end	Day	12% 5% 2 5% 3 4 4 3 6% 7% 48%	7393
August	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 7% 9% 42%	4269
	11th to 20th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 7% 35%	4175
	21st to end	Day	12% 6% 2 6% 4 4 5% 3 6% 7% 46%	4675
September	1st to 10th	Day	18% 5% 2 6% 4 5% 4 3 6% 7% 40%	2429
	11th to 20th	Day	22% 7% 2 5% 4 3 4 2 4 5% 43%	2248
	21st to end	Day	17% 7% 2 7% 4 4 4 3 6% 7% 40%	2190
October	1st to 10th	Day	17% 6% 3 7% 4 4 5% 4 7% 7% 37%	2000
	11th to 20th	Day	20% 11% 3 8% 5% 4 5% 4 6% 7% 28%	2000
	21st to end	Day	24% 14% 3 8% 4 4 4 3 5% 6% 26%	2007
November	1st to 10th	Day	18% 16% 5% 10% 5% 6% 5% 4 6% 5% 21%	1645
	11th to 20th	Day	18% 17% 9% 11% 6% 5% 5% 3 4 6% 16%	1656
	21st to end	Day	19% 19% 8% 18% 5% 4 4 3 4 4 12%	1595
December	1st to 10th	Day	9% 19% 11% 28% 9% 5% 4 3 3 4 5%	2280
	11th to 20th	Day	16% 16% 7% 20% 8% 6% 5% 3 4 3 13%	2618
	21st to end	Day	11% 15% 6% 18% 11% 5% 5% 3 4 6% 15%	3476
January	1st to 10th	Night	17% 24% 7% 18% 14% 6% 3 2 3 2 4	6808
	11th to 20th	Night	9% 19% 10% 26% 13% 5% 4 3 3 2 7%	6771
	21st to end	Night	3 8% 5% 16% 16% 15% 13% 3 3 4 13%	7062
February	1st to 10th	Night	7% 12% 6% 13% 12% 11% 7% 4 6% 5% 17%	2600
	11th to 20th	Night	13% 17% 5% 12% 9% 8% 6% 5% 5% 5% 16%	2533
	21st to end	Night	9% 14% 4 10% 9% 8% 5% 4 5% 7% 26%	2122
March	1st to 10th	Night	13% 15% 4 12% 9% 9% 5% 3 4 5% 20%	5117
	11th to 20th	Night	18% 19% 4 11% 8% 7% 5% 3 4 5% 15%	4979
	21st to end	Night	21% 18% 4 9% 9% 4 4 3 4 5% 19%	5466
April	1st to 10th	Night	19% 13% 4 11% 6% 5% 5% 4 5% 5% 21%	1976
	11th to 20th	Night	18% 12% 3 8% 3 4 5% 4 5% 7% 31%	1778
	21st to end	Night	22% 11% 2 8% 5% 3 4 4 4 4 33%	1800
May	1st to 10th	Night	21% 11% 4 9% 5% 4 4 3 4 5% 30%	3760
	11th to 20th	Night	20% 7% 4 8% 5% 4 5% 3 5% 5% 34%	3533
	21st to end	Night	18% 10% 3 7% 5% 5% 5% 4 6% 8% 28%	3446
June	1st to 10th	Night	20% 10% 3 7% 5% 5% 4 4 5% 7% 30%	1400
	11th to 20th	Night	17% 7% 4 8% 4 5% 4 3 6% 7% 35%	1400
	21st to end	Night	18% 9% 3 7% 6% 5% 5% 5% 6% 7% 30%	1400
July	1st to 10th	Night	16% 6% 2 7% 5% 6% 5% 4 6% 7% 35%	3160
	11th to 20th	Night	10% 6% 2 7% 6% 6% 6% 5% 9% 10% 32%	3160
	21st to end	Night	12% 7% 3 8% 4 4 5% 4 6% 7% 40%	3446
August	1st to 10th	Night	12% 7% 3 8% 6% 6% 6% 5% 7% 9% 29%	2868
	11th to 20th	Night	12% 6% 3 8% 7% 5% 6% 4 7% 8% 36%	2872
	21st to end	Night	18% 6% 2 5% 4 4 4 3 5% 6% 44%	3223
September	1st to 10th	Night	18% 6% 3 6% 4 4 4 4 5% 7% 38%	1970
	11th to 20th	Night	21% 7% 2 5% 3 4 4 3 5% 5% 40%	2150
	21st to end	Night	18% 6% 2 6% 4 3 4 3 5% 7% 42%	2210
October	1st to 10th	Night	20% 8% 2 6% 4 3 4 3 5% 7% 38%	2400
	11th to 20th	Night	26% 10% 3 7% 5% 5% 4 3 4 6% 27%	2400
	21st to end	Night	21% 13% 3 7% 5% 4 4 4 5% 8% 25%	2827
November	1st to 10th	Night	19% 16% 5% 9% 6% 5% 5% 4 4 6% 19%	2873
	11th to 20th	Night	20% 19% 7% 10% 6% 4 4 3 4 5% 17%	2899
	21st to end	Night	20% 18% 8% 17% 5% 4 5% 4 4 5% 10%	2805
December	1st to 10th	Night	12% 20% 12% 28% 6% 4 3 3 3 3 6%	4910
	11th to 20th	Night	14% 15% 7% 21% 10% 6% 4 3 3 3 13%	5591
	21st to end	Night	13% 16% 7% 17% 11% 5% 5% 3 4 6% 14%	7502

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	83% 5% 9% 2	3158
	11th to 20th	Day	77% 5% 10% 6% 2	3160
	21st to end	Day	57% 11% 20% 8% 4	3882
February	1st to 10th	Day	62% 7% 12% 13% 5% 1	1800
	11th to 20th	Day	63% 8% 12% 12% 5%	1761
	21st to end	Day	54% 9% 12% 20% 6%	1505
March	1st to 10th	Day	63% 7% 11% 13% 5%	4726
	11th to 20th	Day	64% 10% 9% 12% 5%	4980
	21st to end	Day	64% 8% 8% 13% 7% 1	5466
April	1st to 10th	Day	61% 7% 9% 12% 10% 1	2419
	11th to 20th	Day	53% 8% 9% 18% 12% 1	2572
	21st to end	Day	55% 8% 10% 15% 11% 1	2600
May	1st to 10th	Day	55% 10% 9% 16% 9% 1	6191
	11th to 20th	Day	50% 10% 15% 17% 7%	6413
	21st to end	Day	50% 10% 11% 20% 9% 1	7451
June	1st to 10th	Day	50% 10% 11% 18% 10% 1	3000
	11th to 20th	Day	47% 8% 14% 18% 12% 1	3000
	21st to end	Day	49% 12% 11% 18% 10% 1	3000
July	1st to 10th	Day	43% 9% 13% 25% 10%	6745
	11th to 20th	Day	45% 10% 19% 21% 6%	6739
	21st to end	Day	35% 9% 19% 27% 9%	7181
August	1st to 10th	Day	39% 10% 19% 27% 4	4255
	11th to 20th	Day	46% 13% 18% 17% 6%	4214
	21st to end	Day	37% 10% 18% 28% 7%	4675
September	1st to 10th	Day	43% 10% 17% 21% 9%	2429
	11th to 20th	Day	44% 8% 15% 25% 7%	2248
	21st to end	Day	45% 9% 13% 24% 8% 1	2190
October	1st to 10th	Day	45% 11% 16% 21% 7% 1	3260
	11th to 20th	Day	56% 8% 13% 19% 4	3240
	21st to end	Day	56% 9% 14% 16% 5% 1	3220
November	1st to 10th	Day	65% 9% 13% 8% 4	1645
	11th to 20th	Day	70% 8% 10% 9% 3	1656
	21st to end	Day	75% 8% 9% 5% 3	1595
December	1st to 10th	Day	81% 5% 8% 5%	2280
	11th to 20th	Day	73% 6% 8% 9% 4	2618
	21st to end	Day	65% 9% 12% 10% 4	3476
January	1st to 10th	Night	86% 5% 5% 3 1	6808
	11th to 20th	Night	78% 6% 10% 4 2	6820
	21st to end	Night	66% 9% 12% 7% 5% 2	7062
February	1st to 10th	Night	63% 6% 13% 10% 7% 2	2600
	11th to 20th	Night	68% 7% 9% 11% 4	2536
	21st to end	Night	57% 7% 8% 19% 7%	2122
March	1st to 10th	Night	65% 8% 8% 12% 7%	5126
	11th to 20th	Night	69% 6% 9% 11% 5% 1	4980
	21st to end	Night	66% 7% 9% 12% 5% 1	5466
April	1st to 10th	Night	64% 7% 10% 9% 9% 1	1976
	11th to 20th	Night	52% 6% 9% 18% 13% 1	1778
	21st to end	Night	56% 6% 8% 15% 13% 2	1800
May	1st to 10th	Night	55% 7% 12% 15% 9% 2	3765
	11th to 20th	Night	50% 6% 10% 23% 11%	3534
	21st to end	Night	49% 9% 13% 20% 9%	3447
June	1st to 10th	Night	52% 7% 11% 18% 11%	1400
	11th to 20th	Night	49% 7% 13% 18% 13% 1	1400
	21st to end	Night	50% 10% 12% 18% 10% 1	1400
July	1st to 10th	Night	45% 8% 13% 24% 10%	3125
	11th to 20th	Night	41% 11% 19% 26% 4	3118
	21st to end	Night	40% 9% 20% 21% 10%	3343
August	1st to 10th	Night	46% 10% 19% 21% 4	2858
	11th to 20th	Night	43% 10% 22% 20% 5%	2899
	21st to end	Night	39% 9% 14% 28% 9% 1	3223
September	1st to 10th	Night	45% 7% 14% 25% 8% 1	1970
	11th to 20th	Night	45% 7% 15% 23% 9%	2150
	21st to end	Night	43% 6% 13% 25% 11% 1	2210
October	1st to 10th	Night	46% 7% 13% 26% 7% 1	3920
	11th to 20th	Night	56% 6% 11% 19% 7%	3899
	21st to end	Night	56% 10% 12% 17% 4	4565
November	1st to 10th	Night	64% 8% 14% 10% 4	2873
	11th to 20th	Night	71% 7% 9% 10% 3	2899
	21st to end	Night	76% 8% 8% 5% 2	2805
December	1st to 10th	Night	81% 6% 7% 5% 1	4910
	11th to 20th	Night	71% 8% 9% 9% 2	5592
	21st to end	Night	71% 7% 9% 11% 2	7502

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% 26% 25% 17% 7% 1	200
	11th to 20th	Day	25% 22% 24% 20% 10% 1	200
	21st to end	Day	17% 24% 34% 17% 7% 2	234
February	1st to 10th	Day	16% 25% 33% 17% 8% 2	400
	11th to 20th	Day	18% 29% 31% 16% 6% 1	396
	21st to end	Day	14% 23% 36% 20% 5% 2	341
March	1st to 10th	Day	19% 23% 32% 19% 5% 2	500
	11th to 20th	Day	21% 22% 29% 19% 7% 2	500
	21st to end	Day	27% 23% 27% 17% 4 2	550
April	1st to 10th	Day	25% 25% 29% 16% 4 1	500
	11th to 20th	Day	23% 21% 30% 18% 6% 2	492
	21st to end	Day	26% 17% 27% 21% 6% 2	500
May	1st to 10th	Day	26% 14% 25% 22% 9% 3	628
	11th to 20th	Day	22% 16% 24% 23% 13% 2	724
	21st to end	Day	28% 16% 25% 22% 7% 1	790
June	1st to 10th	Day	21% 11% 30% 25% 11% 2	730
	11th to 20th	Day	22% 11% 26% 26% 11% 3	730
	21st to end	Day	21% 9% 28% 29% 11% 1	730
July	1st to 10th	Day	20% 8% 31% 26% 14% 2	720
	11th to 20th	Day	15% 11% 25% 35% 12% 1	718
	21st to end	Day	18% 9% 25% 31% 15% 3	795
August	1st to 10th	Day	14% 9% 24% 36% 15% 2	547
	11th to 20th	Day	15% 9% 25% 31% 18% 3	519
	21st to end	Day	18% 9% 26% 31% 13% 3	583
September	1st to 10th	Day	23% 11% 29% 22% 12% 3	530
	11th to 20th	Day	26% 13% 29% 21% 9% 2	530
	21st to end	Day	22% 18% 27% 23% 10% 1	524
October	1st to 10th	Day	19% 19% 32% 20% 9% 1	420
	11th to 20th	Day	30% 18% 27% 15% 8% 1	420
	21st to end	Day	28% 24% 23% 17% 6% 2	462
November	1st to 10th	Day	29% 27% 24% 16% 3	425
	11th to 20th	Day	27% 27% 25% 15% 6% 1	421
	21st to end	Day	30% 23% 28% 15% 4 1	395
December	1st to 10th	Day	20% 20% 33% 21% 5% 1	200
	11th to 20th	Day	24% 31% 31% 11% 5%	200
	21st to end	Day	14% 32% 29% 23% 2	220
January	1st to 10th	Night	23% 24% 29% 18% 6% 1	900
	11th to 20th	Night	19% 23% 31% 20% 6% 1	900
	21st to end	Night	14% 25% 30% 21% 9%	900
February	1st to 10th	Night	14% 26% 32% 19% 8% 1	800
	11th to 20th	Night	24% 20% 31% 18% 6%	783
	21st to end	Night	15% 18% 35% 22% 8% 2	660
March	1st to 10th	Night	18% 20% 31% 23% 8% 1	800
	11th to 20th	Night	21% 22% 28% 19% 9% 1	800
	21st to end	Night	26% 19% 25% 21% 8% 1	880
April	1st to 10th	Night	26% 17% 26% 22% 7% 1	645
	11th to 20th	Night	22% 19% 23% 23% 10% 2	594
	21st to end	Night	28% 12% 26% 21% 11% 2	600
May	1st to 10th	Night	29% 10% 20% 26% 14% 2	600
	11th to 20th	Night	24% 9% 22% 25% 17% 2	568
	21st to end	Night	22% 10% 30% 29% 7% 1	433
June	1st to 10th	Night	24% 5% 22% 34% 15% 1	400
	11th to 20th	Night	21% 7% 22% 32% 17% 1	400
	21st to end	Night	23% 6% 27% 28% 15% 2	400
July	1st to 10th	Night	17% 7% 26% 31% 18% 1	390
	11th to 20th	Night	14% 6% 23% 41% 16%	388
	21st to end	Night	17% 7% 20% 35% 20% 2	438
August	1st to 10th	Night	15% 7% 17% 38% 21% 2	588
	11th to 20th	Night	15% 5% 22% 35% 21% 2	592
	21st to end	Night	17% 7% 24% 29% 20% 3	660
September	1st to 10th	Night	21% 12% 24% 27% 14% 2	600
	11th to 20th	Night	24% 11% 23% 30% 11% 2	750
	21st to end	Night	23% 13% 27% 28% 9% 1	800
October	1st to 10th	Night	25% 17% 23% 25% 9% 1	800
	11th to 20th	Night	33% 13% 22% 19% 11% 1	800
	21st to end	Night	29% 17% 26% 18% 9% 1	890
November	1st to 10th	Night	30% 22% 30% 15% 3	930
	11th to 20th	Night	29% 23% 25% 16% 6% 1	936
	21st to end	Night	27% 23% 28% 15% 6%	900
December	1st to 10th	Night	23% 22% 29% 19% 5% 1	900
	11th to 20th	Night	25% 30% 28% 14% 4	900
	21st to end	Night	25% 23% 26% 19% 5% 2	990

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