Mont St-Alex
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mont St-Alex clear sky chart from 2005-11-06 to 2026-05-17.

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% 4 8% 5% 5% 5% 4 5% 7% 29%	131904
	Night	18% 13% 4 10% 5% 5% 4 3 5% 5% 27%	130929
Transparency	Day	55% 3 12% 21% 8% 1	135360
	Night	59% 3 10% 19% 9% 1	135560
Seeing	Day	25% 10% 27% 23% 12% 3	16494
	Night	26% 8% 27% 28% 10% 1	25531

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	17% 21% 8% 14% 6% 6% 4 3 4 4 16%	4661
	11th to 20th	Day	12% 18% 7% 18% 8% 6% 5% 3 4 5% 15%	4650
	21st to end	Day	11% 14% 6% 11% 9% 7% 6% 4 6% 6% 20%	5124
February	1st to 10th	Day	17% 14% 5% 11% 7% 6% 5% 4 4 5% 22%	1818
	11th to 20th	Day	15% 17% 6% 11% 6% 4 4 4 5% 5% 24%	1991
	21st to end	Day	16% 16% 4 7% 7% 5% 5% 4 5% 6% 24%	1730
March	1st to 10th	Day	19% 13% 5% 11% 6% 5% 6% 3 5% 5% 23%	4576
	11th to 20th	Day	14% 14% 4 8% 5% 5% 4 3 5% 7% 29%	4772
	21st to end	Day	19% 13% 4 9% 4 4 5% 3 4 5% 30%	6102
April	1st to 10th	Day	22% 12% 3 7% 6% 5% 4 4 4 5% 29%	2520
	11th to 20th	Day	21% 10% 3 7% 4 4 4 2 4 6% 34%	2494
	21st to end	Day	22% 10% 3 6% 5% 4 4 3 4 6% 32%	2714
May	1st to 10th	Day	28% 11% 3 9% 5% 4 4 3 5% 5% 22%	6286
	11th to 20th	Day	20% 7% 3 8% 5% 6% 5% 4 6% 7% 30%	6402
	21st to end	Day	21% 9% 3 7% 4 4 5% 4 6% 7% 30%	6949
June	1st to 10th	Day	23% 7% 2 7% 5% 4 5% 4 6% 7% 30%	2800
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 6% 8% 38%	2800
	21st to end	Day	21% 8% 3 7% 4 5% 4 4 6% 6% 32%	2800
July	1st to 10th	Day	13% 7% 3 8% 5% 6% 6% 5% 8% 9% 31%	6360
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 8% 9% 39%	6360
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 9% 43%	6926
August	1st to 10th	Day	15% 6% 3 7% 6% 5% 6% 5% 7% 9% 31%	4563
	11th to 20th	Day	14% 7% 3 6% 5% 4 5% 4 6% 8% 38%	4104
	21st to end	Day	15% 6% 2 6% 4 4 4 3 6% 8% 40%	4312
September	1st to 10th	Day	13% 6% 3 5% 4 4 4 4 5% 8% 44%	2400
	11th to 20th	Day	15% 5% 2 5% 3 4 4 2 5% 7% 48%	2290
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 9% 39%	2200
October	1st to 10th	Day	21% 8% 2 6% 5% 5% 5% 4 6% 6% 33%	2106
	11th to 20th	Day	20% 9% 3 7% 5% 5% 4 4 6% 6% 30%	1931
	21st to end	Day	26% 11% 3 8% 4 5% 4 3 4 5% 27%	1980
November	1st to 10th	Day	19% 14% 2 6% 5% 5% 5% 4 5% 5% 30%	1851
	11th to 20th	Day	18% 14% 4 9% 6% 5% 4 4 6% 6% 26%	1858
	21st to end	Day	22% 16% 5% 9% 5% 4 4 4 5% 5% 20%	1680
December	1st to 10th	Day	18% 21% 8% 12% 7% 4 4 3 4 4 15%	2680
	11th to 20th	Day	21% 14% 6% 10% 5% 4 4 4 5% 5% 23%	3066
	21st to end	Day	22% 16% 5% 10% 6% 5% 5% 5% 6% 4 15%	4048
January	1st to 10th	Night	15% 24% 8% 12% 5% 4 3 2 3 4 18%	8234
	11th to 20th	Night	12% 20% 8% 17% 8% 6% 5% 3 4 4 12%	8235
	21st to end	Night	12% 15% 5% 14% 9% 6% 5% 3 4 5% 22%	9057
February	1st to 10th	Night	17% 16% 6% 13% 6% 5% 4 3 5% 5% 20%	2798
	11th to 20th	Night	15% 14% 5% 13% 8% 5% 4 3 4 5% 23%	2515
	21st to end	Night	17% 15% 5% 10% 6% 4 4 3 4 5% 28%	2076
March	1st to 10th	Night	22% 14% 4 8% 6% 4 4 3 4 4 28%	5484
	11th to 20th	Night	17% 15% 4 8% 5% 4 4 3 5% 7% 28%	5404
	21st to end	Night	17% 15% 5% 10% 6% 5% 4 3 4 4 28%	5072
April	1st to 10th	Night	23% 12% 4 7% 6% 4 4 3 5% 5% 28%	2100
	11th to 20th	Night	23% 12% 4 7% 5% 3 4 3 3 5% 31%	2075
	21st to end	Night	21% 10% 4 7% 4 3 4 3 5% 5% 35%	1906
May	1st to 10th	Night	26% 11% 4 8% 5% 5% 4 4 4 6% 24%	3918
	11th to 20th	Night	23% 9% 3 9% 4 4 4 3 4 7% 30%	3623
	21st to end	Night	19% 9% 3 8% 5% 5% 4 4 5% 6% 32%	3926
June	1st to 10th	Night	25% 7% 2 6% 5% 4 5% 4 5% 7% 31%	1600
	11th to 20th	Night	17% 6% 3 6% 5% 4 4 4 7% 8% 37%	1600
	21st to end	Night	19% 9% 3 6% 5% 5% 4 4 5% 8% 34%	1600
July	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 5% 6% 9% 33%	3600
	11th to 20th	Night	15% 6% 2 7% 5% 4 5% 4 6% 7% 39%	3600
	21st to end	Night	11% 5% 2 8% 6% 5% 5% 4 6% 8% 40%	3926
August	1st to 10th	Night	18% 8% 3 8% 4 5% 4 3 5% 7% 35%	2586
	11th to 20th	Night	15% 7% 3 7% 5% 4 5% 4 6% 6% 36%	2895
	21st to end	Night	17% 6% 3 6% 4 4 4 4 5% 7% 41%	3586
September	1st to 10th	Night	14% 7% 2 6% 5% 4 4 4 5% 7% 43%	2000
	11th to 20th	Night	14% 5% 2 6% 3 3 3 3 5% 6% 48%	2110
	21st to end	Night	18% 7% 3 6% 4 4 4 3 5% 6% 41%	2200
October	1st to 10th	Night	20% 6% 3 7% 5% 4 4 3 6% 6% 37%	2290
	11th to 20th	Night	21% 10% 3 8% 4 4 3 3 5% 6% 33%	2467
	21st to end	Night	25% 11% 3 8% 4 4 4 3 4 5% 27%	2860
November	1st to 10th	Night	21% 12% 4 7% 4 4 5% 4 5% 6% 30%	2667
	11th to 20th	Night	20% 12% 3 10% 4 4 4 4 4 5% 29%	2761
	21st to end	Night	22% 15% 4 10% 5% 5% 4 3 4 5% 24%	2940
December	1st to 10th	Night	18% 24% 7% 13% 6% 4 4 3 3 4 13%	4720
	11th to 20th	Night	23% 16% 5% 12% 5% 4 4 2 4 3 22%	5348
	21st to end	Night	23% 17% 5% 10% 5% 4 4 4 7% 4 18%	7150

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	75% 2 10% 13% 2	4662
	11th to 20th	Day	75% 2 8% 10% 5%	4680
	21st to end	Day	65% 3 10% 12% 9%	5124
February	1st to 10th	Day	66% 3 9% 16% 5% 1	1818
	11th to 20th	Day	64% 2 11% 18% 5% 1	2000
	21st to end	Day	62% 2 12% 16% 8% 1	1730
March	1st to 10th	Day	65% 3 7% 15% 8% 2	4579
	11th to 20th	Day	55% 3 15% 17% 10% 1	4772
	21st to end	Day	59% 2 6% 19% 14% 1	6102
April	1st to 10th	Day	59% 3 8% 20% 10% 1	2520
	11th to 20th	Day	53% 2 8% 22% 13% 1	2494
	21st to end	Day	55% 3 10% 19% 11% 2	2714
May	1st to 10th	Day	65% 2 8% 18% 7% 1	6297
	11th to 20th	Day	54% 3 15% 18% 9% 1	6406
	21st to end	Day	53% 3 11% 21% 10% 1	6949
June	1st to 10th	Day	53% 3 11% 21% 10% 1	2800
	11th to 20th	Day	44% 5% 12% 25% 13% 2	2800
	21st to end	Day	52% 4 12% 22% 9% 1	2800
July	1st to 10th	Day	47% 5% 15% 25% 8%	6290
	11th to 20th	Day	40% 6% 14% 31% 10%	6285
	21st to end	Day	37% 5% 15% 36% 8%	6728
August	1st to 10th	Day	49% 5% 17% 22% 7%	4549
	11th to 20th	Day	44% 5% 19% 25% 6%	4144
	21st to end	Day	41% 4 18% 22% 13%	4312
September	1st to 10th	Day	40% 4 14% 32% 11% 1	2400
	11th to 20th	Day	37% 3 12% 35% 13%	2290
	21st to end	Day	42% 5% 15% 27% 11% 1	2200
October	1st to 10th	Day	44% 4 13% 31% 8%	3431
	11th to 20th	Day	48% 4 16% 26% 5%	3121
	21st to end	Day	51% 3 18% 22% 6% 1	3180
November	1st to 10th	Day	57% 3 12% 21% 7% 1	1851
	11th to 20th	Day	60% 3 12% 16% 9%	1858
	21st to end	Day	66% 4 14% 14% 3	1680
December	1st to 10th	Day	74% 4 9% 11% 2	2680
	11th to 20th	Day	64% 3 10% 17% 6%	3066
	21st to end	Day	70% 3 10% 12% 5%	4048
January	1st to 10th	Night	72% 1 11% 12% 3	8234
	11th to 20th	Night	76% 2 7% 10% 4	8280
	21st to end	Night	66% 2 8% 13% 11%	9057
February	1st to 10th	Night	67% 2 8% 16% 6%	2798
	11th to 20th	Night	64% 3 9% 18% 6%	2517
	21st to end	Night	61% 2 9% 16% 10% 2	2076
March	1st to 10th	Night	61% 2 7% 16% 11% 3	5494
	11th to 20th	Night	58% 3 12% 17% 10% 1	5405
	21st to end	Night	61% 2 5% 22% 9% 2	5072
April	1st to 10th	Night	60% 2 7% 20% 10% 1	2100
	11th to 20th	Night	57% 3 6% 21% 13% 1	2075
	21st to end	Night	52% 3 8% 20% 14% 3	1906
May	1st to 10th	Night	62% 3 9% 17% 6% 2	3924
	11th to 20th	Night	56% 3 10% 21% 9% 1	3624
	21st to end	Night	54% 4 8% 20% 14% 1	3927
June	1st to 10th	Night	53% 3 10% 22% 11% 1	1600
	11th to 20th	Night	45% 5% 12% 22% 14% 3	1600
	21st to end	Night	49% 5% 12% 22% 12% 1	1600
July	1st to 10th	Night	47% 6% 16% 22% 9%	3560
	11th to 20th	Night	43% 5% 14% 28% 10%	3552
	21st to end	Night	41% 5% 15% 27% 12%	3809
August	1st to 10th	Night	49% 4 13% 25% 8%	2576
	11th to 20th	Night	47% 5% 17% 22% 8% 1	2921
	21st to end	Night	43% 5% 12% 24% 14% 1	3586
September	1st to 10th	Night	41% 4 12% 29% 14% 1	2000
	11th to 20th	Night	37% 3 14% 32% 13% 1	2110
	21st to end	Night	44% 4 11% 26% 15% 1	2200
October	1st to 10th	Night	47% 3 12% 26% 10%	3745
	11th to 20th	Night	49% 2 11% 29% 9%	4016
	21st to end	Night	53% 3 12% 20% 11% 1	4610
November	1st to 10th	Night	56% 3 12% 22% 7% 1	2667
	11th to 20th	Night	58% 3 10% 19% 9% 2	2761
	21st to end	Night	65% 2 12% 16% 5%	2940
December	1st to 10th	Night	77% 3 9% 9% 2	4720
	11th to 20th	Night	70% 2 6% 15% 7%	5348
	21st to end	Night	67% 2 9% 14% 8%	7150

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% 11% 26% 22% 9% 1	220
	11th to 20th	Day	28% 17% 34% 15% 5% 1	220
	21st to end	Day	22% 13% 37% 18% 9% 1	242
February	1st to 10th	Day	29% 20% 32% 15% 4	293
	11th to 20th	Day	26% 15% 34% 19% 5%	479
	21st to end	Day	27% 14% 32% 23% 4	446
March	1st to 10th	Day	29% 12% 28% 25% 6%	540
	11th to 20th	Day	30% 10% 34% 19% 6% 2	540
	21st to end	Day	31% 16% 24% 20% 8% 1	594
April	1st to 10th	Day	33% 13% 26% 19% 9% 1	540
	11th to 20th	Day	34% 13% 26% 16% 7% 4	532
	21st to end	Day	30% 12% 29% 16% 11% 2	540
May	1st to 10th	Day	29% 12% 28% 17% 11% 2	543
	11th to 20th	Day	20% 10% 28% 28% 11% 4	551
	21st to end	Day	21% 7% 32% 26% 12% 2	573
June	1st to 10th	Day	25% 8% 26% 29% 10% 2	530
	11th to 20th	Day	19% 9% 26% 28% 15% 2	530
	21st to end	Day	22% 8% 32% 22% 12% 3	530
July	1st to 10th	Day	14% 5% 25% 34% 17% 5%	525
	11th to 20th	Day	18% 4 28% 30% 16% 4	524
	21st to end	Day	19% 5% 22% 33% 18% 3	576
August	1st to 10th	Day	16% 4 22% 32% 21% 5%	526
	11th to 20th	Day	18% 4 21% 32% 19% 7%	519
	21st to end	Day	18% 7% 24% 25% 18% 8%	583
September	1st to 10th	Day	15% 7% 25% 26% 18% 8%	530
	11th to 20th	Day	15% 6% 20% 25% 23% 12%	530
	21st to end	Day	18% 7% 26% 24% 19% 7%	530
October	1st to 10th	Day	21% 8% 29% 20% 16% 5%	530
	11th to 20th	Day	27% 12% 26% 20% 12% 2	491
	21st to end	Day	36% 12% 22% 23% 8%	462
November	1st to 10th	Day	33% 14% 26% 20% 7%	425
	11th to 20th	Day	30% 14% 27% 18% 10% 2	398
	21st to end	Day	33% 17% 25% 19% 5%	220
December	1st to 10th	Day	36% 12% 23% 20% 7% 2	220
	11th to 20th	Day	28% 17% 36% 14% 5%	220
	21st to end	Day	33% 15% 26% 19% 5% 2	242
January	1st to 10th	Night	30% 11% 30% 23% 5% 1	840
	11th to 20th	Night	28% 11% 31% 22% 7%	840
	21st to end	Night	24% 9% 30% 29% 7% 1	924
February	1st to 10th	Night	26% 9% 32% 28% 6%	840
	11th to 20th	Night	23% 12% 33% 27% 4	823
	21st to end	Night	26% 10% 31% 28% 5%	692
March	1st to 10th	Night	27% 11% 28% 28% 5%	840
	11th to 20th	Night	30% 11% 27% 24% 7%	840
	21st to end	Night	29% 11% 27% 24% 7% 1	924
April	1st to 10th	Night	31% 10% 27% 26% 6%	840
	11th to 20th	Night	30% 11% 27% 23% 8%	831
	21st to end	Night	26% 11% 28% 26% 7%	646
May	1st to 10th	Night	26% 6% 32% 25% 10% 1	534
	11th to 20th	Night	24% 6% 29% 31% 10% 1	406
	21st to end	Night	20% 5% 29% 37% 9%	433
June	1st to 10th	Night	29% 5% 25% 31% 11% 1	400
	11th to 20th	Night	21% 4 27% 34% 15% 1	400
	21st to end	Night	22% 4 24% 37% 13% 1	400
July	1st to 10th	Night	14% 3 25% 43% 14% 1	390
	11th to 20th	Night	12% 2 19% 50% 16% 1	388
	21st to end	Night	15% 3 20% 48% 14%	438
August	1st to 10th	Night	15% 2 17% 41% 23% 2	398
	11th to 20th	Night	17% 3 17% 39% 23% 2	600
	21st to end	Night	19% 4 17% 38% 21% 2	880
September	1st to 10th	Night	17% 6% 20% 40% 16% 2	800
	11th to 20th	Night	17% 5% 17% 33% 24% 4	800
	21st to end	Night	22% 5% 26% 28% 17% 2	800
October	1st to 10th	Night	24% 4 26% 30% 16% 1	800
	11th to 20th	Night	29% 6% 27% 29% 9% 1	800
	21st to end	Night	32% 6% 29% 25% 7%	880
November	1st to 10th	Night	29% 11% 28% 24% 7% 1	820
	11th to 20th	Night	28% 11% 26% 24% 9% 1	840
	21st to end	Night	33% 10% 30% 20% 6%	840
December	1st to 10th	Night	34% 11% 28% 21% 5%	840
	11th to 20th	Night	33% 11% 29% 20% 6% 1	840
	21st to end	Night	35% 10% 29% 21% 5%	924

Page generated at 2026-05-14 01:58:58. Version ac07

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