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 2024-01-22.

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	20% 13% 5% 8% 6% 3 5% 4 5% 6% 25%	74581
	Night	19% 14% 6% 9% 7% 3 4 4 4 5% 25%	75318
Transparency	Day	60% 4 10% 17% 8% 1	74286
	Night	62% 4 9% 16% 7% 1	75148
Seeing	Day	30% 17% 29% 17% 6% 1	13875
	Night	28% 16% 28% 21% 7% 1	22802

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	14% 21% 9% 15% 11% 5% 5% 4 3 3 11%	1517
	11th to 20th	Day	16% 18% 8% 14% 11% 3 4 4 4 3 16%	1520
	21st to end	Day	8% 15% 10% 13% 11% 5% 7% 5% 4 4 18%	1592
February	1st to 10th	Day	14% 19% 7% 14% 12% 5% 6% 4 3 3 14%	1491
	11th to 20th	Day	14% 18% 10% 15% 9% 4 4 4 3 4 15%	1748
	21st to end	Day	11% 20% 7% 11% 9% 5% 6% 4 4 4 19%	1480
March	1st to 10th	Day	14% 17% 8% 12% 8% 4 5% 3 4 4 23%	1797
	11th to 20th	Day	16% 15% 7% 8% 8% 4 5% 4 4 4 25%	1894
	21st to end	Day	18% 17% 6% 11% 7% 3 4 3 3 3 26%	2300
April	1st to 10th	Day	21% 18% 5% 10% 6% 2 3 3 4 4 23%	2160
	11th to 20th	Day	19% 15% 4 6% 5% 2 4 3 4 5% 33%	2204
	21st to end	Day	26% 15% 4 7% 5% 2 4 3 4 5% 26%	2340
May	1st to 10th	Day	24% 12% 4 6% 5% 3 4 3 4 5% 30%	2340
	11th to 20th	Day	22% 10% 2 6% 5% 3 5% 3 5% 6% 32%	2475
	21st to end	Day	22% 8% 3 7% 6% 3 5% 5% 6% 8% 27%	2725
June	1st to 10th	Day	30% 9% 3 6% 5% 3 4 4 5% 7% 24%	2556
	11th to 20th	Day	22% 7% 3 6% 5% 4 5% 5% 5% 9% 30%	2700
	21st to end	Day	25% 9% 3 7% 6% 3 5% 5% 5% 7% 24%	2547
July	1st to 10th	Day	19% 8% 3 6% 6% 4 5% 6% 5% 8% 32%	2520
	11th to 20th	Day	19% 7% 3 7% 5% 3 6% 6% 6% 9% 28%	2520
	21st to end	Day	22% 7% 3 6% 6% 4 6% 5% 6% 7% 28%	2744
August	1st to 10th	Day	22% 7% 3 6% 5% 4 5% 6% 6% 9% 26%	2456
	11th to 20th	Day	20% 8% 3 6% 4 4 6% 4 5% 9% 30%	2304
	21st to end	Day	22% 7% 2 4 6% 3 5% 6% 5% 7% 33%	2443
September	1st to 10th	Day	20% 8% 2 6% 5% 3 4 4 6% 7% 36%	2160
	11th to 20th	Day	21% 7% 2 5% 4 2 3 4 5% 6% 40%	1990
	21st to end	Day	21% 8% 3 5% 4 3 3 5% 6% 8% 33%	1980
October	1st to 10th	Day	23% 12% 3 5% 5% 2 5% 4 6% 7% 28%	1980
	11th to 20th	Day	21% 14% 5% 7% 6% 4 5% 5% 5% 5% 23%	1674
	21st to end	Day	26% 22% 4 6% 5% 3 3 3 4 4 20%	1782
November	1st to 10th	Day	20% 19% 5% 6% 6% 3 3 4 3 4 27%	1671
	11th to 20th	Day	17% 16% 9% 10% 7% 2 4 4 3 5% 20%	1710
	21st to end	Day	18% 19% 7% 11% 7% 3 5% 3 4 5% 18%	1539
December	1st to 10th	Day	19% 24% 10% 13% 6% 3 3 3 3 3 14%	1330
	11th to 20th	Day	19% 18% 8% 11% 7% 3 5% 3 3 3 19%	1668
	21st to end	Day	22% 21% 7% 14% 8% 3 4 3 2 3 14%	2724
January	1st to 10th	Night	13% 21% 10% 15% 11% 4 5% 4 3 3 11%	2649
	11th to 20th	Night	14% 18% 8% 14% 11% 5% 7% 4 4 3 12%	2660
	21st to end	Night	10% 15% 7% 14% 11% 6% 8% 5% 4 3 16%	2798
February	1st to 10th	Night	12% 17% 8% 13% 11% 5% 7% 3 4 4 16%	2466
	11th to 20th	Night	11% 18% 8% 15% 9% 5% 7% 3 3 4 17%	2098
	21st to end	Night	12% 17% 8% 11% 9% 4 6% 4 3 4 22%	1776
March	1st to 10th	Night	14% 18% 7% 11% 8% 3 4 4 4 4 24%	2150
	11th to 20th	Night	16% 16% 6% 10% 6% 4 4 4 4 4 26%	2065
	21st to end	Night	18% 16% 6% 10% 5% 3 3 3 3 4 28%	2052
April	1st to 10th	Night	22% 16% 5% 9% 5% 4 3 3 4 5% 26%	1800
	11th to 20th	Night	21% 13% 4 6% 4 2 4 4 5% 5% 32%	1705
	21st to end	Night	23% 14% 4 4 5% 2 4 4 4 5% 29%	1620
May	1st to 10th	Night	25% 11% 4 5% 4 3 4 5% 5% 6% 28%	1620
	11th to 20th	Night	20% 11% 4 6% 6% 3 5% 4 4 5% 33%	1433
	21st to end	Night	23% 7% 2 5% 6% 3 4 4 5% 8% 32%	1550
June	1st to 10th	Night	28% 9% 4 6% 4 3 3 4 6% 7% 27%	1396
	11th to 20th	Night	20% 7% 3 5% 5% 2 5% 4 5% 8% 35%	1260
	21st to end	Night	22% 8% 3 6% 6% 4 5% 5% 5% 7% 29%	1402
July	1st to 10th	Night	18% 7% 3 6% 5% 3 5% 5% 6% 9% 34%	1440
	11th to 20th	Night	16% 7% 2 5% 6% 4 6% 7% 7% 9% 32%	1440
	21st to end	Night	18% 7% 3 6% 5% 3 6% 5% 5% 7% 34%	1574
August	1st to 10th	Night	18% 6% 2 4 5% 4 4 5% 8% 8% 36%	1463
	11th to 20th	Night	16% 9% 3 6% 6% 3 4 6% 6% 7% 34%	1596
	21st to end	Night	19% 7% 2 6% 5% 3 4 3 5% 8% 38%	1911
September	1st to 10th	Night	19% 8% 3 6% 4 3 4 4 5% 6% 38%	1800
	11th to 20th	Night	20% 7% 3 4 4 2 4 4 5% 6% 42%	1970
	21st to end	Night	22% 7% 2 4 3 3 4 4 4 7% 41%	1980
October	1st to 10th	Night	20% 9% 4 6% 4 3 4 4 5% 7% 34%	1980
	11th to 20th	Night	23% 15% 3 6% 6% 3 4 5% 4 5% 26%	2281
	21st to end	Night	27% 16% 4 6% 5% 3 4 4 4 5% 23%	2574
November	1st to 10th	Night	21% 18% 5% 7% 6% 3 4 3 3 5% 24%	2407
	11th to 20th	Night	19% 15% 7% 11% 6% 3 4 3 4 5% 24%	2470
	21st to end	Night	17% 18% 8% 12% 8% 3 3 3 3 4 20%	2635
December	1st to 10th	Night	20% 20% 10% 14% 7% 4 4 3 4 3 12%	2850
	11th to 20th	Night	18% 19% 9% 15% 10% 4 4 2 3 3 13%	3531
	21st to end	Night	22% 20% 7% 10% 7% 3 4 3 3 3 19%	4916

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% 3 7% 8% 3	1518
	11th to 20th	Day	74% 3 6% 11% 5% 1	1520
	21st to end	Day	68% 4 7% 12% 7% 2	1592
February	1st to 10th	Day	77% 3 5% 8% 6% 1	1491
	11th to 20th	Day	76% 2 6% 10% 5% 1	1757
	21st to end	Day	70% 4 6% 13% 6% 2	1480
March	1st to 10th	Day	67% 3 7% 14% 6% 3	1800
	11th to 20th	Day	63% 3 9% 12% 11% 2	1894
	21st to end	Day	66% 2 6% 15% 9% 2	2300
April	1st to 10th	Day	66% 3 8% 13% 8% 2	2160
	11th to 20th	Day	55% 3 8% 21% 12% 1	2204
	21st to end	Day	62% 3 7% 15% 9% 3	2340
May	1st to 10th	Day	58% 3 6% 19% 12% 2	2340
	11th to 20th	Day	54% 4 12% 19% 10% 1	2479
	21st to end	Day	54% 5% 12% 18% 10% 1	2725
June	1st to 10th	Day	59% 5% 9% 20% 6% 1	2556
	11th to 20th	Day	51% 6% 11% 22% 9% 1	2700
	21st to end	Day	59% 6% 10% 18% 6%	2547
July	1st to 10th	Day	49% 6% 15% 22% 7%	2450
	11th to 20th	Day	52% 5% 14% 23% 6%	2445
	21st to end	Day	54% 7% 13% 20% 5%	2591
August	1st to 10th	Day	53% 6% 16% 22% 3	2442
	11th to 20th	Day	50% 6% 16% 20% 8%	2304
	21st to end	Day	49% 6% 13% 21% 11%	2443
September	1st to 10th	Day	48% 5% 13% 24% 9% 1	2160
	11th to 20th	Day	46% 4 10% 27% 13% 1	1990
	21st to end	Day	49% 4 12% 23% 10% 1	1980
October	1st to 10th	Day	55% 4 13% 19% 9%	1980
	11th to 20th	Day	62% 5% 13% 16% 5%	1674
	21st to end	Day	69% 3 8% 13% 6% 1	1782
November	1st to 10th	Day	62% 4 10% 16% 7% 1	1671
	11th to 20th	Day	67% 4 8% 13% 6% 1	1710
	21st to end	Day	69% 5% 10% 13% 3	1539
December	1st to 10th	Day	78% 4 5% 8% 5%	1330
	11th to 20th	Day	71% 3 4 13% 7% 1	1668
	21st to end	Day	77% 4 4 9% 5%	2724
January	1st to 10th	Night	80% 2 6% 8% 3	2649
	11th to 20th	Night	78% 3 6% 9% 4 1	2660
	21st to end	Night	72% 3 7% 11% 5% 1	2798
February	1st to 10th	Night	72% 3 7% 12% 5% 1	2466
	11th to 20th	Night	74% 3 6% 10% 6% 1	2100
	21st to end	Night	67% 2 6% 14% 9% 1	1776
March	1st to 10th	Night	65% 3 7% 16% 7% 1	2160
	11th to 20th	Night	62% 5% 8% 13% 10% 2	2066
	21st to end	Night	61% 2 7% 18% 9% 3	2052
April	1st to 10th	Night	63% 3 6% 15% 11% 1	1800
	11th to 20th	Night	53% 4 9% 20% 11% 2	1705
	21st to end	Night	57% 5% 10% 17% 9% 2	1620
May	1st to 10th	Night	56% 3 8% 18% 13% 1	1620
	11th to 20th	Night	53% 3 12% 20% 10% 2	1434
	21st to end	Night	50% 6% 13% 20% 11% 1	1551
June	1st to 10th	Night	57% 4 11% 20% 7%	1396
	11th to 20th	Night	46% 7% 12% 23% 11% 2	1260
	21st to end	Night	54% 7% 11% 20% 8%	1402
July	1st to 10th	Night	46% 6% 18% 21% 9%	1400
	11th to 20th	Night	45% 8% 16% 23% 8%	1392
	21st to end	Night	49% 7% 14% 22% 8%	1486
August	1st to 10th	Night	43% 7% 21% 24% 5%	1453
	11th to 20th	Night	47% 8% 16% 22% 8%	1596
	21st to end	Night	44% 5% 14% 26% 10%	1911
September	1st to 10th	Night	47% 4 13% 26% 10%	1800
	11th to 20th	Night	43% 5% 13% 29% 10% 1	1970
	21st to end	Night	45% 5% 12% 25% 12% 1	1980
October	1st to 10th	Night	51% 4 10% 23% 11%	1980
	11th to 20th	Night	60% 5% 12% 18% 5%	2281
	21st to end	Night	65% 4 9% 14% 7% 1	2574
November	1st to 10th	Night	64% 3 11% 15% 7% 1	2407
	11th to 20th	Night	64% 3 9% 13% 8% 1	2470
	21st to end	Night	69% 4 10% 12% 4	2635
December	1st to 10th	Night	79% 3 6% 8% 3	2850
	11th to 20th	Night	79% 3 5% 9% 6%	3532
	21st to end	Night	72% 3 6% 13% 5%	4916

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	32% 22% 36% 11%	180
	11th to 20th	Day	34% 23% 26% 15% 1 1	180
	21st to end	Day	23% 28% 34% 13% 2 1	178
February	1st to 10th	Day	39% 25% 27% 6% 2	203
	11th to 20th	Day	32% 27% 33% 6% 2	416
	21st to end	Day	33% 24% 29% 13% 1	346
March	1st to 10th	Day	31% 24% 31% 10% 4	420
	11th to 20th	Day	32% 26% 24% 13% 4	420
	21st to end	Day	34% 20% 24% 15% 6% 1	462
April	1st to 10th	Day	40% 15% 27% 13% 5% 1	420
	11th to 20th	Day	35% 20% 27% 12% 5% 1	412
	21st to end	Day	39% 17% 28% 11% 5% 1	420
May	1st to 10th	Day	38% 16% 28% 13% 4 1	423
	11th to 20th	Day	30% 16% 30% 20% 4 1	447
	21st to end	Day	24% 20% 30% 19% 5% 1	485
June	1st to 10th	Day	37% 14% 30% 14% 5% 1	486
	11th to 20th	Day	25% 13% 28% 23% 10%	630
	21st to end	Day	26% 15% 32% 18% 8% 1	477
July	1st to 10th	Day	22% 11% 36% 24% 7%	445
	11th to 20th	Day	22% 10% 38% 20% 8% 1	444
	21st to end	Day	25% 11% 33% 25% 6% 1	488
August	1st to 10th	Day	23% 11% 31% 25% 10% 1	446
	11th to 20th	Day	23% 8% 28% 27% 11% 3	439
	21st to end	Day	28% 13% 23% 22% 10% 4	495
September	1st to 10th	Day	24% 11% 31% 21% 10% 4	450
	11th to 20th	Day	24% 10% 27% 20% 13% 5%	450
	21st to end	Day	23% 12% 31% 20% 9% 5%	450
October	1st to 10th	Day	29% 14% 25% 19% 10% 3	450
	11th to 20th	Day	30% 16% 30% 13% 10% 1	378
	21st to end	Day	40% 18% 25% 15% 2	396
November	1st to 10th	Day	36% 16% 30% 13% 4	365
	11th to 20th	Day	29% 23% 29% 11% 6% 1	370
	21st to end	Day	34% 20% 29% 14% 2 1	246
December	1st to 10th	Day	36% 23% 26% 11% 4	180
	11th to 20th	Day	38% 27% 26% 7% 3	180
	21st to end	Day	39% 24% 25% 10% 2 1	198
January	1st to 10th	Night	28% 25% 34% 11% 2	760
	11th to 20th	Night	27% 26% 30% 15% 3	760
	21st to end	Night	19% 25% 34% 18% 4 1	800
February	1st to 10th	Night	23% 25% 36% 13% 3	720
	11th to 20th	Night	24% 26% 34% 14% 3	703
	21st to end	Night	27% 21% 33% 17% 2	592
March	1st to 10th	Night	26% 20% 34% 17% 2	720
	11th to 20th	Night	28% 22% 30% 15% 5%	720
	21st to end	Night	28% 17% 27% 22% 5% 1	792
April	1st to 10th	Night	32% 20% 26% 17% 5%	720
	11th to 20th	Night	30% 14% 29% 21% 6% 1	641
	21st to end	Night	32% 16% 28% 18% 5%	540
May	1st to 10th	Night	31% 13% 31% 18% 6% 1	540
	11th to 20th	Night	25% 11% 32% 26% 6% 1	372
	21st to end	Night	25% 8% 32% 28% 6% 1	389
June	1st to 10th	Night	33% 9% 26% 24% 8% 1	360
	11th to 20th	Night	24% 6% 23% 34% 13% 1	360
	21st to end	Night	25% 7% 27% 31% 10%	360
July	1st to 10th	Night	20% 6% 30% 31% 12% 1	350
	11th to 20th	Night	17% 4 30% 36% 11% 1	348
	21st to end	Night	21% 5% 23% 38% 13% 1	394
August	1st to 10th	Night	19% 5% 20% 38% 18% 1	389
	11th to 20th	Night	20% 4 25% 33% 19% 1	532
	21st to end	Night	22% 6% 25% 29% 16% 2	723
September	1st to 10th	Night	23% 6% 26% 33% 11% 2	720
	11th to 20th	Night	24% 7% 25% 26% 16% 2	720
	21st to end	Night	25% 9% 28% 24% 13% 2	720
October	1st to 10th	Night	26% 10% 27% 25% 12% 1	720
	11th to 20th	Night	32% 12% 25% 22% 7% 2	720
	21st to end	Night	37% 15% 26% 18% 4	792
November	1st to 10th	Night	35% 16% 27% 16% 6%	740
	11th to 20th	Night	30% 18% 29% 15% 8% 1	760
	21st to end	Night	30% 22% 26% 17% 4 1	780
December	1st to 10th	Night	32% 22% 28% 14% 4	850
	11th to 20th	Night	32% 23% 26% 14% 5%	850
	21st to end	Night	36% 21% 27% 13% 2	845

Page generated at 2024-01-19 09:28:35. Version ac07

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