Observatoire Monty-Guilmette
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Monty-Guilmette 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	18% 13% 5% 11% 6% 5% 5% 3 5% 6% 24%	131199
	Night	17% 15% 6% 12% 7% 5% 5% 3 4 5% 23%	131626
Transparency	Day	61% 5% 10% 17% 7% 1	134680
	Night	64% 4 9% 15% 7% 1	136233
Seeing	Day	30% 18% 29% 16% 6% 1	16854
	Night	27% 16% 28% 21% 7% 1	25756

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	11% 16% 9% 20% 11% 8% 8% 3 3 3 9%	4661
	11th to 20th	Day	7% 16% 8% 27% 17% 7% 4 2 2 2 9%	4650
	21st to end	Day	6% 15% 7% 16% 9% 14% 8% 4 5% 3 12%	5124
February	1st to 10th	Day	13% 18% 6% 16% 11% 6% 5% 3 3 3 16%	1740
	11th to 20th	Day	12% 19% 8% 18% 8% 6% 4 3 3 3 16%	2048
	21st to end	Day	10% 20% 6% 13% 8% 7% 5% 3 5% 4 19%	1730
March	1st to 10th	Day	17% 17% 6% 15% 7% 4 6% 2 3 3 19%	4576
	11th to 20th	Day	14% 18% 5% 12% 6% 5% 4 3 4 5% 24%	4873
	21st to end	Day	20% 17% 6% 12% 5% 4 3 3 3 4 24%	5910
April	1st to 10th	Day	21% 20% 4 11% 5% 3 3 2 3 4 24%	2520
	11th to 20th	Day	19% 17% 4 7% 4 3 4 2 4 5% 31%	2576
	21st to end	Day	24% 15% 4 8% 5% 3 3 3 5% 5% 27%	2730
May	1st to 10th	Day	28% 13% 4 9% 5% 6% 5% 3 4 5% 19%	6016
	11th to 20th	Day	21% 9% 3 9% 5% 5% 5% 4 6% 6% 27%	6399
	21st to end	Day	21% 10% 3 7% 5% 5% 5% 4 6% 7% 27%	6949
June	1st to 10th	Day	29% 9% 2 7% 4 4 4 4 6% 7% 24%	2840
	11th to 20th	Day	21% 8% 2 6% 4 5% 5% 4 6% 8% 30%	3000
	21st to end	Day	25% 10% 3 7% 5% 4 5% 4 5% 7% 24%	2829
July	1st to 10th	Day	18% 9% 3 9% 6% 6% 7% 5% 7% 7% 23%	6360
	11th to 20th	Day	17% 7% 2 7% 5% 5% 6% 5% 7% 8% 31%	6360
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 38%	6926
August	1st to 10th	Day	20% 8% 3 7% 5% 6% 6% 5% 6% 8% 25%	4469
	11th to 20th	Day	21% 9% 3 7% 5% 6% 6% 4 6% 8% 24%	4175
	21st to end	Day	21% 8% 2 5% 4 4 5% 4 6% 7% 33%	4424
September	1st to 10th	Day	19% 8% 2 6% 4 4 4 3 5% 7% 38%	2400
	11th to 20th	Day	19% 8% 2 5% 3 3 4 3 5% 6% 41%	2210
	21st to end	Day	20% 9% 2 5% 4 4 4 4 6% 8% 33%	2200
October	1st to 10th	Day	21% 12% 3 6% 4 4 4 3 6% 7% 31%	2200
	11th to 20th	Day	20% 16% 4 8% 5% 5% 4 4 5% 5% 24%	1860
	21st to end	Day	24% 22% 5% 7% 4 4 3 3 4 4 21%	1980
November	1st to 10th	Day	19% 22% 6% 7% 5% 3 4 3 3 4 26%	1851
	11th to 20th	Day	16% 19% 8% 13% 5% 3 4 3 3 4 21%	1890
	21st to end	Day	17% 22% 8% 13% 5% 4 4 2 4 4 16%	1699
December	1st to 10th	Day	14% 23% 9% 18% 11% 5% 3 2 3 3 10%	2350
	11th to 20th	Day	16% 18% 7% 14% 7% 5% 4 3 3 3 19%	2688
	21st to end	Day	17% 19% 6% 15% 6% 4 5% 3 3 4 17%	3986
January	1st to 10th	Night	9% 16% 9% 23% 11% 8% 7% 3 3 3 9%	8234
	11th to 20th	Night	8% 18% 9% 25% 14% 7% 4 2 3 2 7%	8235
	21st to end	Night	7% 13% 7% 17% 10% 13% 8% 5% 5% 3 13%	9057
February	1st to 10th	Night	11% 17% 7% 16% 9% 6% 6% 3 4 4 18%	2876
	11th to 20th	Night	10% 19% 7% 18% 8% 6% 5% 2 3 3 17%	2458
	21st to end	Night	11% 18% 6% 13% 8% 7% 4 3 3 4 23%	2076
March	1st to 10th	Night	18% 17% 6% 15% 5% 4 6% 2 3 4 19%	5484
	11th to 20th	Night	13% 19% 6% 12% 6% 5% 4 3 5% 5% 23%	5306
	21st to end	Night	18% 18% 6% 12% 6% 4 3 2 3 4 24%	5260
April	1st to 10th	Night	22% 17% 5% 9% 4 4 2 3 4 4 26%	2100
	11th to 20th	Night	21% 15% 4 7% 4 3 4 3 5% 5% 29%	1992
	21st to end	Night	22% 15% 3 6% 4 3 5% 3 5% 5% 29%	1890
May	1st to 10th	Night	27% 12% 4 8% 5% 4 4 4 5% 6% 21%	4144
	11th to 20th	Night	24% 10% 3 7% 5% 4 4 3 5% 6% 29%	3670
	21st to end	Night	22% 9% 3 6% 4 4 4 4 5% 7% 31%	3926
June	1st to 10th	Night	28% 10% 3 6% 4 4 3 4 6% 6% 27%	1550
	11th to 20th	Night	19% 8% 3 6% 4 5% 4 4 6% 8% 34%	1400
	21st to end	Night	22% 9% 3 6% 6% 6% 5% 4 5% 7% 28%	1558
July	1st to 10th	Night	20% 10% 4 9% 5% 4 5% 3 5% 8% 27%	3600
	11th to 20th	Night	21% 9% 2 6% 4 4 4 4 5% 6% 36%	3600
	21st to end	Night	14% 8% 3 7% 5% 5% 6% 4 6% 7% 36%	3926
August	1st to 10th	Night	25% 9% 3 6% 4 4 4 3 5% 6% 30%	2647
	11th to 20th	Night	18% 9% 3 7% 6% 5% 5% 5% 7% 6% 29%	2872
	21st to end	Night	20% 8% 2 6% 4 4 4 3 5% 7% 38%	3471
September	1st to 10th	Night	18% 8% 2 7% 4 4 4 3 5% 6% 39%	2000
	11th to 20th	Night	18% 6% 2 5% 4 3 4 3 5% 5% 45%	2190
	21st to end	Night	21% 7% 2 5% 3 4 4 3 4 6% 41%	2200
October	1st to 10th	Night	19% 9% 3 6% 3 4 4 3 5% 7% 36%	2200
	11th to 20th	Night	22% 16% 3 7% 5% 4 4 3 4 5% 27%	2535
	21st to end	Night	25% 17% 4 7% 4 4 4 3 4 5% 24%	2860
November	1st to 10th	Night	21% 19% 5% 8% 5% 4 4 2 3 5% 23%	2667
	11th to 20th	Night	18% 17% 7% 13% 5% 4 3 2 4 5% 23%	2730
	21st to end	Night	17% 20% 8% 15% 6% 4 3 3 3 4 19%	2914
December	1st to 10th	Night	13% 21% 11% 23% 9% 4 3 2 3 2 9%	5060
	11th to 20th	Night	16% 20% 7% 17% 9% 5% 3 3 3 3 15%	5741
	21st to end	Night	17% 20% 6% 11% 6% 4 4 2 3 4 22%	7197

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	81% 3 6% 7% 2	4662
	11th to 20th	Day	83% 3 3 8% 3 1	4680
	21st to end	Day	75% 4 6% 7% 8% 1	5124
February	1st to 10th	Day	74% 3 4 10% 7% 1	1740
	11th to 20th	Day	75% 2 6% 11% 5% 1	2057
	21st to end	Day	69% 4 6% 13% 6% 2	1730
March	1st to 10th	Day	71% 3 5% 10% 8% 3	4579
	11th to 20th	Day	62% 4 12% 12% 8% 1	4873
	21st to end	Day	66% 3 4 13% 12% 1	5910
April	1st to 10th	Day	65% 3 7% 14% 8% 2	2520
	11th to 20th	Day	57% 3 7% 20% 12% 1	2576
	21st to end	Day	61% 3 7% 17% 10% 2	2730
May	1st to 10th	Day	67% 6% 7% 15% 5% 1	6027
	11th to 20th	Day	56% 7% 14% 15% 6% 2	6403
	21st to end	Day	54% 6% 11% 19% 9% 1	6949
June	1st to 10th	Day	58% 5% 9% 20% 6% 1	2840
	11th to 20th	Day	50% 6% 12% 22% 9% 1	3000
	21st to end	Day	58% 6% 10% 19% 7%	2829
July	1st to 10th	Day	56% 6% 15% 17% 6%	6290
	11th to 20th	Day	48% 6% 14% 27% 5%	6285
	21st to end	Day	44% 6% 14% 29% 7%	6728
August	1st to 10th	Day	54% 6% 15% 20% 4	4455
	11th to 20th	Day	55% 7% 16% 17% 5%	4214
	21st to end	Day	49% 6% 13% 19% 12% 2	4424
September	1st to 10th	Day	46% 5% 13% 27% 9% 1	2400
	11th to 20th	Day	44% 4 11% 28% 13%	2210
	21st to end	Day	49% 5% 12% 23% 11% 1	2200
October	1st to 10th	Day	52% 4 14% 22% 8%	3590
	11th to 20th	Day	57% 4 10% 22% 6%	3011
	21st to end	Day	58% 3 14% 17% 8%	3180
November	1st to 10th	Day	63% 4 9% 16% 7% 1	1851
	11th to 20th	Day	68% 3 8% 12% 7% 1	1890
	21st to end	Day	72% 4 9% 12% 2	1699
December	1st to 10th	Day	81% 3 6% 6% 3	2350
	11th to 20th	Day	70% 4 6% 12% 8%	2688
	21st to end	Day	72% 3 5% 10% 8% 2	3986
January	1st to 10th	Night	82% 3 6% 6% 2	8234
	11th to 20th	Night	85% 2 4 6% 3	8280
	21st to end	Night	73% 3 6% 8% 8% 1	9057
February	1st to 10th	Night	70% 3 7% 13% 6% 1	2876
	11th to 20th	Night	74% 3 6% 10% 6% 1	2460
	21st to end	Night	67% 2 6% 15% 9% 1	2076
March	1st to 10th	Night	70% 3 7% 12% 6% 2	5494
	11th to 20th	Night	64% 4 12% 12% 7% 1	5307
	21st to end	Night	65% 2 5% 14% 12% 1	5260
April	1st to 10th	Night	62% 3 6% 16% 11% 1	2100
	11th to 20th	Night	56% 4 8% 19% 11% 1	1992
	21st to end	Night	57% 5% 9% 17% 10% 2	1890
May	1st to 10th	Night	62% 6% 11% 14% 7% 1	4150
	11th to 20th	Night	55% 6% 12% 21% 5% 1	3671
	21st to end	Night	51% 6% 14% 16% 13%	3927
June	1st to 10th	Night	57% 5% 11% 20% 7%	1550
	11th to 20th	Night	46% 7% 12% 22% 11% 2	1400
	21st to end	Night	55% 6% 11% 19% 9%	1558
July	1st to 10th	Night	56% 4 16% 19% 5%	3560
	11th to 20th	Night	49% 5% 15% 25% 6%	3552
	21st to end	Night	46% 6% 15% 27% 6%	3809
August	1st to 10th	Night	54% 5% 15% 23% 4	2637
	11th to 20th	Night	53% 8% 17% 17% 6%	2899
	21st to end	Night	46% 5% 13% 23% 13%	3471
September	1st to 10th	Night	46% 4 13% 27% 10%	2000
	11th to 20th	Night	41% 5% 13% 30% 11% 1	2190
	21st to end	Night	45% 5% 12% 25% 12% 1	2200
October	1st to 10th	Night	50% 4 11% 24% 11%	3590
	11th to 20th	Night	57% 4 9% 23% 7%	4123
	21st to end	Night	57% 3 13% 16% 8% 2	4610
November	1st to 10th	Night	65% 3 10% 15% 6% 1	2667
	11th to 20th	Night	65% 3 9% 13% 8% 1	2730
	21st to end	Night	71% 4 10% 12% 4	2914
December	1st to 10th	Night	84% 3 5% 6% 2	5060
	11th to 20th	Night	77% 3 5% 9% 6%	5742
	21st to end	Night	68% 3 6% 13% 9% 1	7197

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	33% 23% 35% 9%	220
	11th to 20th	Day	32% 25% 28% 14% 1	220
	21st to end	Day	26% 27% 33% 11% 2	242
February	1st to 10th	Day	34% 25% 28% 10% 2 1	272
	11th to 20th	Day	31% 26% 33% 8% 1	536
	21st to end	Day	31% 24% 32% 12% 1	446
March	1st to 10th	Day	33% 25% 29% 10% 3	540
	11th to 20th	Day	34% 28% 24% 11% 4	540
	21st to end	Day	37% 21% 24% 12% 5% 1	594
April	1st to 10th	Day	40% 17% 26% 12% 4 1	540
	11th to 20th	Day	38% 20% 27% 11% 4 1	532
	21st to end	Day	37% 18% 28% 11% 5% 1	540
May	1st to 10th	Day	39% 17% 28% 13% 3 1	543
	11th to 20th	Day	30% 16% 28% 21% 4 1	551
	21st to end	Day	25% 20% 31% 18% 5% 1	573
June	1st to 10th	Day	35% 14% 31% 15% 4 1	570
	11th to 20th	Day	26% 14% 28% 23% 9%	730
	21st to end	Day	28% 16% 32% 17% 7% 1	559
July	1st to 10th	Day	21% 13% 36% 23% 7%	525
	11th to 20th	Day	22% 11% 39% 20% 7% 1	524
	21st to end	Day	23% 12% 34% 25% 6% 1	576
August	1st to 10th	Day	21% 11% 32% 25% 10% 1	526
	11th to 20th	Day	22% 9% 28% 26% 12% 3	519
	21st to end	Day	26% 13% 26% 23% 9% 4	583
September	1st to 10th	Day	23% 12% 30% 21% 10% 3	530
	11th to 20th	Day	21% 9% 26% 22% 16% 5%	530
	21st to end	Day	23% 12% 30% 21% 9% 4	530
October	1st to 10th	Day	27% 14% 26% 19% 10% 3	530
	11th to 20th	Day	31% 16% 30% 14% 10% 1	442
	21st to end	Day	38% 19% 26% 15% 2	462
November	1st to 10th	Day	37% 18% 30% 11% 4	425
	11th to 20th	Day	29% 23% 30% 11% 6% 1	430
	21st to end	Day	35% 22% 28% 12% 2 1	292
December	1st to 10th	Day	32% 23% 31% 10% 3	220
	11th to 20th	Day	37% 27% 27% 6% 2	220
	21st to end	Day	36% 23% 27% 10% 3	242
January	1st to 10th	Night	27% 25% 35% 10% 2	840
	11th to 20th	Night	27% 25% 30% 15% 3	840
	21st to end	Night	20% 26% 33% 17% 3	924
February	1st to 10th	Night	22% 24% 36% 14% 5%	840
	11th to 20th	Night	25% 25% 34% 14% 2	823
	21st to end	Night	26% 20% 36% 17% 2	692
March	1st to 10th	Night	28% 20% 34% 16% 2	840
	11th to 20th	Night	29% 22% 30% 15% 4	840
	21st to end	Night	30% 18% 27% 20% 4 1	924
April	1st to 10th	Night	33% 19% 27% 16% 4	840
	11th to 20th	Night	32% 15% 28% 19% 5% 1	748
	21st to end	Night	32% 16% 28% 18% 5%	630
May	1st to 10th	Night	30% 14% 30% 20% 5%	630
	11th to 20th	Night	25% 10% 32% 26% 7%	427
	21st to end	Night	26% 8% 31% 28% 6%	433
June	1st to 10th	Night	33% 8% 26% 25% 8% 1	400
	11th to 20th	Night	24% 8% 22% 34% 13% 1	400
	21st to end	Night	25% 7% 28% 30% 10%	400
July	1st to 10th	Night	20% 6% 29% 33% 11% 1	390
	11th to 20th	Night	18% 5% 29% 36% 11% 1	388
	21st to end	Night	20% 6% 23% 39% 12%	438
August	1st to 10th	Night	20% 4 19% 38% 18% 1	433
	11th to 20th	Night	19% 4 24% 32% 20% 1	592
	21st to end	Night	22% 6% 25% 30% 16% 2	804
September	1st to 10th	Night	23% 6% 25% 32% 12% 2	800
	11th to 20th	Night	22% 7% 24% 26% 19% 2	800
	21st to end	Night	24% 9% 27% 25% 14% 2	800
October	1st to 10th	Night	25% 9% 27% 25% 13% 1	800
	11th to 20th	Night	32% 12% 25% 23% 7% 2	800
	21st to end	Night	36% 15% 27% 18% 4	880
November	1st to 10th	Night	37% 17% 26% 16% 5%	820
	11th to 20th	Night	30% 19% 29% 15% 7% 1	840
	21st to end	Night	30% 23% 25% 17% 4 1	865
December	1st to 10th	Night	31% 22% 28% 14% 4	950
	11th to 20th	Night	32% 24% 25% 14% 4	950
	21st to end	Night	35% 22% 27% 13% 3	935

Page generated at 2026-05-14 01:58:55. Version ac07

Copyright © 2026 Attilla Danko.