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 2025-01-22.

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	19% 13% 4 10% 6% 5% 5% 3 5% 6% 25%	104654
	Night	18% 15% 5% 11% 6% 5% 4 3 4 5% 24%	104715
Transparency	Day	60% 5% 10% 17% 7% 1	108153
	Night	63% 4 10% 16% 7% 1	109306
Seeing	Day	30% 17% 29% 17% 6% 1	15155
	Night	27% 16% 28% 21% 7% 1	24068

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	12% 20% 9% 17% 11% 7% 5% 3 3 3 9%	2597
	11th to 20th	Day	10% 15% 6% 25% 15% 6% 3 2 3 2 14%	2600
	21st to end	Day	10% 19% 10% 17% 7% 7% 4 3 4 4 16%	2780
February	1st to 10th	Day	14% 19% 6% 16% 10% 6% 4 3 3 3 16%	1574
	11th to 20th	Day	13% 20% 9% 17% 8% 6% 4 3 3 4 15%	1848
	21st to end	Day	11% 21% 6% 12% 8% 7% 5% 3 5% 4 19%	1570
March	1st to 10th	Day	20% 17% 5% 12% 6% 4 4 3 3 4 22%	3116
	11th to 20th	Day	15% 24% 7% 14% 5% 4 3 2 3 3 19%	3338
	21st to end	Day	17% 17% 5% 12% 5% 4 3 2 3 4 28%	4034
April	1st to 10th	Day	21% 19% 4 11% 5% 3 3 2 4 4 24%	2280
	11th to 20th	Day	18% 16% 3 7% 4 3 4 2 5% 5% 32%	2328
	21st to end	Day	24% 15% 4 8% 4 3 3 3 4 5% 27%	2470
May	1st to 10th	Day	24% 11% 4 9% 5% 6% 5% 3 5% 5% 25%	4106
	11th to 20th	Day	21% 9% 3 9% 5% 5% 5% 4 6% 7% 26%	4395
	21st to end	Day	17% 8% 3 8% 5% 5% 5% 4 7% 8% 29%	4837
June	1st to 10th	Day	29% 9% 2 6% 4 4 4 4 6% 7% 24%	2698
	11th to 20th	Day	21% 8% 2 6% 4 5% 5% 4 6% 8% 30%	2850
	21st to end	Day	26% 10% 3 7% 5% 4 5% 4 5% 7% 24%	2688
July	1st to 10th	Day	16% 8% 3 8% 5% 6% 6% 5% 7% 8% 29%	4440
	11th to 20th	Day	18% 8% 3 8% 5% 5% 6% 5% 7% 8% 28%	4440
	21st to end	Day	18% 7% 3 6% 5% 5% 5% 4 5% 6% 37%	4814
August	1st to 10th	Day	21% 8% 3 8% 5% 6% 6% 5% 6% 8% 24%	4332
	11th to 20th	Day	22% 9% 3 8% 5% 6% 6% 4 6% 8% 23%	4045
	21st to end	Day	22% 8% 2 5% 4 4 5% 4 6% 7% 33%	4289
September	1st to 10th	Day	19% 8% 2 6% 4 4 4 3 6% 7% 37%	2280
	11th to 20th	Day	20% 8% 2 6% 3 3 4 3 5% 6% 40%	2100
	21st to end	Day	21% 9% 2 6% 4 4 4 4 6% 8% 32%	2090
October	1st to 10th	Day	22% 13% 3 6% 4 4 5% 3 6% 7% 28%	2090
	11th to 20th	Day	20% 16% 4 8% 5% 5% 4 4 5% 5% 23%	1767
	21st to end	Day	24% 22% 4 7% 4 4 3 3 4 4 21%	1881
November	1st to 10th	Day	19% 20% 5% 7% 5% 3 4 3 3 4 27%	1761
	11th to 20th	Day	17% 19% 8% 12% 6% 3 4 3 3 5% 21%	1800
	21st to end	Day	17% 21% 7% 13% 5% 5% 4 2 4 5% 17%	1619
December	1st to 10th	Day	14% 23% 9% 18% 11% 5% 3 2 2 2 10%	2280
	11th to 20th	Day	16% 19% 8% 14% 6% 5% 4 3 3 3 19%	2618
	21st to end	Day	17% 20% 6% 15% 6% 4 5% 3 3 4 17%	3899
January	1st to 10th	Night	12% 19% 9% 21% 11% 5% 5% 2 2 3 10%	4569
	11th to 20th	Night	10% 18% 7% 20% 14% 8% 5% 3 3 2 10%	4580
	21st to end	Night	11% 19% 9% 17% 8% 8% 5% 3 3 3 14%	4899
February	1st to 10th	Night	12% 17% 7% 16% 9% 6% 5% 3 4 4 17%	2603
	11th to 20th	Night	11% 20% 7% 18% 8% 7% 6% 2 3 4 16%	2218
	21st to end	Night	12% 18% 6% 13% 8% 6% 4 3 3 4 23%	1884
March	1st to 10th	Night	20% 19% 4 9% 5% 3 5% 2 4 4 25%	3724
	11th to 20th	Night	13% 24% 8% 13% 5% 4 3 2 3 3 21%	3621
	21st to end	Night	19% 18% 5% 11% 6% 3 3 2 3 4 26%	3594
April	1st to 10th	Night	21% 17% 4 9% 3 4 2 3 4 4 26%	1900
	11th to 20th	Night	21% 14% 4 7% 4 3 4 3 5% 5% 31%	1800
	21st to end	Night	23% 16% 3 5% 4 3 5% 3 4 5% 29%	1710
May	1st to 10th	Night	26% 10% 3 6% 4 4 5% 5% 6% 7% 25%	2834
	11th to 20th	Night	20% 10% 4 7% 5% 4 4 3 5% 6% 32%	2527
	21st to end	Night	19% 7% 2 5% 4 4 5% 4 6% 9% 34%	2738
June	1st to 10th	Night	28% 10% 3 6% 4 4 3 4 6% 7% 26%	1473
	11th to 20th	Night	20% 8% 2 6% 4 4 4 4 6% 8% 34%	1330
	21st to end	Night	22% 9% 3 7% 6% 6% 4 4 5% 7% 28%	1480
July	1st to 10th	Night	17% 9% 3 8% 5% 5% 5% 3 6% 8% 29%	2520
	11th to 20th	Night	22% 10% 2 6% 4 4 5% 4 6% 6% 31%	2520
	21st to end	Night	17% 8% 2 7% 5% 4 5% 3 5% 6% 38%	2739
August	1st to 10th	Night	25% 9% 3 7% 4 4 4 3 6% 6% 28%	2565
	11th to 20th	Night	18% 10% 3 7% 6% 5% 5% 5% 7% 6% 28%	2782
	21st to end	Night	20% 8% 2 6% 4 4 4 3 5% 7% 38%	3365
September	1st to 10th	Night	19% 8% 3 7% 3 4 4 3 5% 6% 39%	1900
	11th to 20th	Night	19% 7% 2 5% 4 3 4 3 5% 5% 44%	2080
	21st to end	Night	21% 7% 2 5% 3 4 4 3 4 7% 41%	2090
October	1st to 10th	Night	20% 10% 3 7% 4 4 4 3 5% 7% 34%	2090
	11th to 20th	Night	23% 16% 3 7% 5% 4 4 3 4 5% 27%	2408
	21st to end	Night	25% 17% 4 7% 4 4 4 3 4 5% 24%	2717
November	1st to 10th	Night	21% 19% 5% 8% 5% 4 4 2 4 5% 24%	2537
	11th to 20th	Night	18% 17% 6% 12% 5% 4 3 2 4 5% 25%	2600
	21st to end	Night	17% 20% 7% 14% 6% 4 3 3 3 4 19%	2775
December	1st to 10th	Night	14% 21% 12% 23% 8% 4 3 2 3 2 8%	4910
	11th to 20th	Night	16% 20% 7% 17% 8% 5% 3 3 3 3 15%	5591
	21st to end	Night	17% 20% 6% 11% 5% 4 4 2 3 4 22%	7042

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	79% 4 8% 8% 2	2598
	11th to 20th	Day	77% 3 4 12% 3 1	2600
	21st to end	Day	72% 5% 7% 8% 6% 2	2780
February	1st to 10th	Day	75% 3 5% 10% 7% 1	1574
	11th to 20th	Day	76% 2 6% 10% 5% 1	1857
	21st to end	Day	69% 4 7% 13% 6% 2	1570
March	1st to 10th	Day	66% 4 6% 12% 9% 2	3119
	11th to 20th	Day	71% 4 7% 10% 7% 1	3338
	21st to end	Day	62% 3 5% 15% 14% 2	4034
April	1st to 10th	Day	65% 3 7% 14% 8% 2	2280
	11th to 20th	Day	55% 3 7% 21% 12% 1	2328
	21st to end	Day	61% 4 7% 16% 10% 2	2470
May	1st to 10th	Day	60% 6% 9% 17% 7% 1	4120
	11th to 20th	Day	55% 7% 17% 15% 6% 1	4399
	21st to end	Day	49% 7% 12% 22% 10% 1	4837
June	1st to 10th	Day	59% 5% 9% 21% 6% 1	2698
	11th to 20th	Day	50% 6% 12% 22% 9% 1	2850
	21st to end	Day	59% 6% 10% 19% 6%	2688
July	1st to 10th	Day	50% 6% 15% 22% 7%	4370
	11th to 20th	Day	51% 6% 15% 25% 3	4365
	21st to end	Day	47% 6% 12% 28% 6%	4661
August	1st to 10th	Day	56% 6% 15% 20% 4	4318
	11th to 20th	Day	56% 7% 17% 16% 5%	4084
	21st to end	Day	49% 6% 13% 18% 12% 2	4289
September	1st to 10th	Day	46% 5% 14% 25% 10% 1	2280
	11th to 20th	Day	45% 4 10% 27% 13%	2100
	21st to end	Day	49% 4 13% 23% 10% 1	2090
October	1st to 10th	Day	54% 4 14% 21% 7%	3480
	11th to 20th	Day	58% 4 10% 22% 6%	2918
	21st to end	Day	57% 3 14% 18% 7%	3081
November	1st to 10th	Day	62% 4 10% 17% 7% 1	1761
	11th to 20th	Day	67% 3 8% 13% 7% 1	1800
	21st to end	Day	71% 5% 10% 12% 3	1619
December	1st to 10th	Day	82% 3 6% 6% 3	2280
	11th to 20th	Day	70% 4 6% 13% 8%	2618
	21st to end	Day	72% 3 5% 10% 8% 2	3899
January	1st to 10th	Night	82% 3 7% 6% 2	4569
	11th to 20th	Night	82% 3 5% 7% 2	4580
	21st to end	Night	76% 3 7% 8% 5% 1	4899
February	1st to 10th	Night	70% 4 7% 12% 6% 1	2603
	11th to 20th	Night	74% 3 6% 10% 6% 1	2220
	21st to end	Night	67% 2 6% 15% 9% 1	1884
March	1st to 10th	Night	63% 4 8% 16% 6% 2	3734
	11th to 20th	Night	69% 4 7% 13% 6% 1	3622
	21st to end	Night	65% 2 5% 16% 11% 2	3594
April	1st to 10th	Night	62% 3 6% 16% 11% 1	1900
	11th to 20th	Night	54% 5% 9% 20% 11% 2	1800
	21st to end	Night	57% 5% 9% 17% 10% 2	1710
May	1st to 10th	Night	55% 7% 13% 17% 7% 1	2840
	11th to 20th	Night	52% 6% 15% 20% 6% 1	2528
	21st to end	Night	46% 7% 15% 17% 14%	2739
June	1st to 10th	Night	57% 5% 11% 20% 7%	1473
	11th to 20th	Night	45% 7% 13% 23% 11% 2	1330
	21st to end	Night	56% 6% 10% 19% 8%	1480
July	1st to 10th	Night	52% 5% 16% 20% 7%	2480
	11th to 20th	Night	52% 6% 15% 22% 5%	2472
	21st to end	Night	46% 6% 14% 27% 6%	2651
August	1st to 10th	Night	55% 5% 15% 22% 3	2555
	11th to 20th	Night	54% 8% 16% 16% 5%	2809
	21st to end	Night	46% 5% 13% 22% 13%	3365
September	1st to 10th	Night	47% 4 13% 26% 10%	1900
	11th to 20th	Night	42% 5% 13% 29% 11% 1	2080
	21st to end	Night	45% 4 12% 25% 11% 1	2090
October	1st to 10th	Night	51% 4 11% 23% 11%	3480
	11th to 20th	Night	57% 4 9% 22% 7%	3996
	21st to end	Night	56% 3 14% 16% 8% 2	4467
November	1st to 10th	Night	64% 3 11% 15% 7% 1	2537
	11th to 20th	Night	64% 3 9% 14% 9% 1	2600
	21st to end	Night	70% 4 10% 12% 4	2775
December	1st to 10th	Night	84% 3 5% 6% 2	4910
	11th to 20th	Night	76% 3 5% 10% 6%	5592
	21st to end	Night	68% 3 6% 13% 9% 1	7042

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% 22% 36% 10%	200
	11th to 20th	Day	32% 24% 28% 15% 1 1	200
	21st to end	Day	23% 27% 35% 13% 3 1	200
February	1st to 10th	Day	38% 24% 28% 7% 2	226
	11th to 20th	Day	32% 27% 32% 7% 2	456
	21st to end	Day	32% 24% 30% 13% 1	382
March	1st to 10th	Day	33% 23% 30% 11% 4	460
	11th to 20th	Day	34% 26% 24% 12% 4	460
	21st to end	Day	34% 22% 24% 14% 5% 1	506
April	1st to 10th	Day	40% 16% 26% 13% 4 1	460
	11th to 20th	Day	36% 20% 26% 12% 5% 1	452
	21st to end	Day	38% 19% 28% 11% 5% 1	460
May	1st to 10th	Day	38% 15% 28% 14% 4 1	463
	11th to 20th	Day	29% 15% 29% 22% 4 1	487
	21st to end	Day	24% 20% 31% 19% 5% 1	529
June	1st to 10th	Day	36% 14% 30% 15% 5% 1	528
	11th to 20th	Day	25% 13% 29% 23% 10%	680
	21st to end	Day	28% 15% 31% 17% 7% 1	518
July	1st to 10th	Day	21% 12% 36% 24% 7%	485
	11th to 20th	Day	23% 11% 39% 19% 7% 1	484
	21st to end	Day	23% 11% 33% 26% 6% 1	532
August	1st to 10th	Day	23% 12% 31% 24% 9% 1	486
	11th to 20th	Day	23% 9% 28% 26% 10% 3	479
	21st to end	Day	27% 12% 25% 23% 10% 4	539
September	1st to 10th	Day	24% 12% 30% 21% 10% 3	490
	11th to 20th	Day	22% 10% 26% 22% 15% 5%	490
	21st to end	Day	23% 12% 30% 21% 10% 5%	490
October	1st to 10th	Day	28% 14% 26% 19% 10% 3	490
	11th to 20th	Day	31% 16% 29% 13% 10% 1	410
	21st to end	Day	38% 19% 25% 15% 2	429
November	1st to 10th	Day	37% 17% 30% 12% 4	395
	11th to 20th	Day	29% 23% 30% 11% 6% 1	400
	21st to end	Day	35% 21% 29% 13% 2 1	269
December	1st to 10th	Day	33% 23% 29% 12% 4	200
	11th to 20th	Day	38% 27% 27% 7% 3	200
	21st to end	Day	36% 23% 26% 11% 4	220
January	1st to 10th	Night	27% 25% 35% 10% 2	800
	11th to 20th	Night	27% 25% 31% 15% 3	800
	21st to end	Night	20% 24% 34% 18% 4	841
February	1st to 10th	Night	22% 24% 37% 14% 4	760
	11th to 20th	Night	24% 26% 33% 13% 3	743
	21st to end	Night	26% 20% 35% 17% 2	628
March	1st to 10th	Night	27% 19% 34% 18% 2	760
	11th to 20th	Night	28% 21% 30% 16% 5%	760
	21st to end	Night	29% 17% 27% 21% 5% 1	836
April	1st to 10th	Night	32% 19% 27% 17% 5%	760
	11th to 20th	Night	30% 15% 29% 20% 5% 1	676
	21st to end	Night	33% 16% 28% 18% 5%	570
May	1st to 10th	Night	31% 13% 31% 19% 6% 1	570
	11th to 20th	Night	25% 11% 31% 26% 7% 1	393
	21st to end	Night	25% 9% 32% 27% 6%	411
June	1st to 10th	Night	33% 8% 26% 24% 8% 1	380
	11th to 20th	Night	24% 6% 22% 34% 13% 1	380
	21st to end	Night	26% 7% 27% 30% 10%	380
July	1st to 10th	Night	20% 6% 29% 32% 11% 1	370
	11th to 20th	Night	18% 5% 29% 36% 11% 1	368
	21st to end	Night	20% 5% 23% 38% 13%	416
August	1st to 10th	Night	21% 4 19% 37% 18% 1	411
	11th to 20th	Night	20% 4 24% 32% 19% 1	562
	21st to end	Night	22% 6% 24% 29% 16% 2	764
September	1st to 10th	Night	23% 6% 26% 32% 11% 2	760
	11th to 20th	Night	23% 7% 24% 25% 19% 2	760
	21st to end	Night	24% 9% 27% 24% 14% 2	760
October	1st to 10th	Night	26% 9% 27% 25% 12% 1	760
	11th to 20th	Night	33% 12% 24% 22% 7% 2	760
	21st to end	Night	36% 16% 26% 18% 4	836
November	1st to 10th	Night	35% 17% 26% 16% 5%	780
	11th to 20th	Night	30% 18% 29% 15% 7% 1	800
	21st to end	Night	31% 22% 26% 17% 4 1	823
December	1st to 10th	Night	32% 22% 28% 14% 4	900
	11th to 20th	Night	32% 23% 25% 14% 5%	900
	21st to end	Night	35% 21% 27% 13% 3	890

Page generated at 2025-01-19 03:06:48. Version ac07

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