Observatoire Plourde
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Plourde clear sky chart from 2005-11-06 to 2025-10-04.

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	16% 13% 4 10% 6% 5% 5% 4 5% 6% 26%	126998
	Night	15% 15% 6% 12% 6% 6% 5% 4 5% 5% 22%	122517
Transparency	Day	58% 6% 10% 17% 8% 1	130433
	Night	63% 6% 9% 15% 7% 1	127171
Seeing	Day	24% 16% 32% 19% 7% 2	16427
	Night	24% 12% 31% 24% 8% 1	25268

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% 22% 7% 20% 9% 8% 6% 3 3 2 7%	3158
	11th to 20th	Day	6% 18% 6% 19% 11% 7% 6% 5% 9% 4 8%	3133
	21st to end	Day	7% 18% 8% 15% 7% 8% 7% 5% 5% 6% 14%	4005
February	1st to 10th	Day	7% 14% 6% 14% 9% 9% 7% 5% 7% 6% 15%	1800
	11th to 20th	Day	8% 19% 6% 14% 8% 6% 6% 5% 6% 6% 16%	1753
	21st to end	Day	9% 18% 6% 13% 7% 5% 6% 4 6% 6% 19%	1485
March	1st to 10th	Day	17% 16% 5% 11% 8% 5% 5% 4 6% 4 20%	4760
	11th to 20th	Day	16% 17% 5% 14% 6% 6% 3 2 3 4 23%	4980
	21st to end	Day	10% 17% 6% 13% 6% 6% 6% 3 4 5% 24%	5505
April	1st to 10th	Day	17% 17% 6% 12% 6% 4 3 3 5% 5% 23%	2400
	11th to 20th	Day	18% 15% 5% 10% 4 3 4 3 4 5% 29%	2550
	21st to end	Day	19% 13% 5% 8% 5% 4 4 3 4 5% 30%	2600
May	1st to 10th	Day	15% 10% 4 8% 5% 4 4 4 5% 5% 37%	6345
	11th to 20th	Day	14% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	6318
	21st to end	Day	19% 12% 3 7% 4 3 4 3 5% 6% 35%	7241
June	1st to 10th	Day	22% 9% 2 7% 4 5% 5% 4 6% 7% 29%	3000
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 7% 34%	3000
	21st to end	Day	20% 9% 2 7% 5% 4 5% 4 6% 7% 31%	3000
July	1st to 10th	Day	14% 7% 3 8% 6% 6% 7% 5% 8% 9% 26%	6819
	11th to 20th	Day	17% 7% 2 7% 5% 5% 6% 4 7% 9% 31%	6820
	21st to end	Day	17% 7% 3 7% 5% 4 5% 4 7% 9% 32%	7205
August	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 5% 7% 9% 31%	4407
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 6% 6% 34%	4174
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 5% 9% 37%	4612
September	1st to 10th	Day	21% 8% 3 8% 4 4 5% 3 5% 7% 31%	2400
	11th to 20th	Day	22% 11% 3 7% 5% 4 4 3 5% 7% 29%	2335
	21st to end	Day	23% 11% 2 9% 4 4 4 3 4 6% 31%	2130
October	1st to 10th	Day	26% 13% 3 7% 4 4 2 2 4 6% 30%	1930
	11th to 20th	Day	29% 21% 5% 8% 4 3 3 3 3 3 17%	1900
	21st to end	Day	28% 23% 6% 7% 4 4 4 3 3 3 14%	1920
November	1st to 10th	Day	22% 27% 6% 9% 5% 5% 4 2 3 5% 14%	1566
	11th to 20th	Day	22% 22% 7% 16% 6% 5% 3 2 3 3 11%	1600
	21st to end	Day	20% 27% 10% 13% 6% 5% 3 2 2 3 9%	1600
December	1st to 10th	Day	11% 25% 13% 19% 8% 5% 3 3 4 2 7%	2450
	11th to 20th	Day	9% 24% 7% 13% 9% 7% 4 3 4 5% 16%	2621
	21st to end	Day	18% 22% 8% 16% 8% 5% 4 3 5% 4 7%	3476
January	1st to 10th	Night	10% 19% 10% 22% 9% 8% 5% 3 4 3 9%	6808
	11th to 20th	Night	9% 17% 8% 19% 10% 7% 7% 6% 8% 4 7%	6772
	21st to end	Night	6% 17% 9% 16% 7% 6% 8% 6% 5% 5% 14%	6959
February	1st to 10th	Night	7% 15% 5% 11% 10% 9% 9% 6% 7% 6% 14%	2600
	11th to 20th	Night	8% 18% 7% 14% 8% 7% 6% 5% 5% 6% 16%	2533
	21st to end	Night	9% 18% 4 13% 8% 6% 7% 5% 7% 6% 18%	2145
March	1st to 10th	Night	15% 17% 6% 11% 6% 5% 5% 5% 4 4 21%	5079
	11th to 20th	Night	16% 17% 6% 15% 9% 6% 5% 3 4 4 16%	4979
	21st to end	Night	12% 14% 5% 13% 8% 9% 5% 3 4 5% 23%	5427
April	1st to 10th	Night	17% 17% 5% 11% 4 4 3 3 4 5% 28%	2000
	11th to 20th	Night	19% 11% 4 10% 5% 5% 4 3 5% 5% 29%	1817
	21st to end	Night	20% 12% 5% 8% 5% 3 4 3 4 6% 31%	1800
May	1st to 10th	Night	16% 11% 4 8% 4 5% 4 4 5% 6% 33%	3594
	11th to 20th	Night	14% 10% 4 6% 4 4 4 3 6% 5% 39%	3575
	21st to end	Night	20% 10% 3 7% 5% 4 4 4 5% 7% 33%	3630
June	1st to 10th	Night	20% 10% 3 7% 5% 4 5% 3 5% 7% 32%	1400
	11th to 20th	Night	15% 9% 3 7% 4 4 3 4 5% 6% 39%	1400
	21st to end	Night	18% 6% 2 6% 5% 5% 5% 3 6% 7% 37%	1400
July	1st to 10th	Night	17% 9% 3 7% 4 4 4 3 5% 7% 37%	3160
	11th to 20th	Night	20% 9% 3 6% 5% 4 3 4 5% 6% 35%	3160
	21st to end	Night	13% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	3656
August	1st to 10th	Night	9% 7% 3 8% 6% 5% 5% 5% 6% 7% 40%	2740
	11th to 20th	Night	14% 9% 3 8% 4 4 4 3 6% 6% 39%	2875
	21st to end	Night	17% 8% 3 7% 4 4 4 3 4 5% 43%	3252
September	1st to 10th	Night	20% 9% 2 8% 5% 4 4 3 5% 6% 34%	2000
	11th to 20th	Night	18% 10% 4 6% 4 4 4 3 5% 7% 36%	2061
	21st to end	Night	22% 10% 3 6% 4 3 3 2 4 6% 38%	2270
October	1st to 10th	Night	22% 12% 3 7% 4 4 4 3 5% 6% 30%	2317
	11th to 20th	Night	27% 20% 5% 8% 5% 3 3 2 4 5% 16%	2280
	21st to end	Night	22% 23% 6% 10% 5% 4 5% 2 4 5% 15%	2674
November	1st to 10th	Night	22% 22% 6% 11% 4 4 4 3 4 4 17%	2732
	11th to 20th	Night	21% 21% 7% 13% 7% 5% 3 3 3 4 14%	2800
	21st to end	Night	20% 23% 8% 14% 8% 5% 4 2 3 3 10%	2800
December	1st to 10th	Night	12% 25% 9% 17% 9% 7% 5% 3 3 3 6%	4734
	11th to 20th	Night	14% 19% 7% 13% 9% 9% 7% 4 4 4 9%	5586
	21st to end	Night	19% 21% 9% 15% 7% 5% 4 3 4 3 11%	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% 3 6% 6% 2	3159
	11th to 20th	Day	74% 5% 4 11% 6%	3160
	21st to end	Day	71% 4 6% 15% 5%	4005
February	1st to 10th	Day	67% 5% 7% 14% 7%	1800
	11th to 20th	Day	67% 5% 8% 14% 6%	1762
	21st to end	Day	64% 6% 7% 16% 6% 1	1485
March	1st to 10th	Day	66% 4 5% 13% 10% 1	4763
	11th to 20th	Day	67% 4 6% 14% 8% 1	4980
	21st to end	Day	64% 4 7% 16% 9% 1	5505
April	1st to 10th	Day	63% 5% 7% 13% 10% 1	2400
	11th to 20th	Day	58% 5% 7% 17% 12% 1	2550
	21st to end	Day	56% 5% 8% 15% 13% 2	2600
May	1st to 10th	Day	47% 6% 7% 18% 20% 2	6356
	11th to 20th	Day	47% 8% 13% 20% 9% 3	6322
	21st to end	Day	49% 6% 7% 19% 16% 3	7241
June	1st to 10th	Day	52% 6% 10% 21% 10% 1	3000
	11th to 20th	Day	48% 6% 9% 22% 13% 2	3000
	21st to end	Day	51% 7% 10% 22% 10% 1	3000
July	1st to 10th	Day	49% 9% 14% 21% 7%	6746
	11th to 20th	Day	45% 8% 18% 21% 8% 1	6739
	21st to end	Day	46% 7% 15% 26% 6%	6999
August	1st to 10th	Day	46% 9% 17% 24% 5%	4393
	11th to 20th	Day	49% 7% 14% 23% 6% 1	4213
	21st to end	Day	44% 6% 14% 28% 8%	4612
September	1st to 10th	Day	52% 7% 14% 19% 7%	2400
	11th to 20th	Day	54% 7% 15% 20% 4	2335
	21st to end	Day	55% 6% 9% 22% 7%	2130
October	1st to 10th	Day	62% 4 11% 16% 5%	3190
	11th to 20th	Day	66% 5% 10% 14% 4	3140
	21st to end	Day	73% 4 11% 7% 5%	3135
November	1st to 10th	Day	75% 5% 9% 6% 4	1566
	11th to 20th	Day	81% 3 6% 9% 2	1600
	21st to end	Day	84% 4 6% 5% 1	1600
December	1st to 10th	Day	83% 4 5% 6% 2	2450
	11th to 20th	Day	73% 3 6% 14% 3	2621
	21st to end	Day	80% 3 5% 9% 3	3476
January	1st to 10th	Night	81% 4 7% 6% 2	6808
	11th to 20th	Night	75% 6% 5% 8% 6%	6820
	21st to end	Night	70% 5% 4 15% 5%	6959
February	1st to 10th	Night	65% 7% 9% 11% 8%	2600
	11th to 20th	Night	68% 5% 8% 13% 6%	2535
	21st to end	Night	64% 6% 7% 14% 7% 1	2145
March	1st to 10th	Night	66% 4 6% 11% 10% 3	5089
	11th to 20th	Night	72% 5% 7% 9% 6% 1	4980
	21st to end	Night	62% 4 6% 15% 11% 1	5427
April	1st to 10th	Night	60% 4 6% 16% 12% 2	2000
	11th to 20th	Night	56% 5% 7% 17% 13% 1	1817
	21st to end	Night	53% 4 7% 17% 15% 3	1800
May	1st to 10th	Night	48% 6% 10% 16% 18% 1	3600
	11th to 20th	Night	44% 7% 10% 23% 13% 2	3576
	21st to end	Night	50% 6% 10% 16% 13% 5%	3631
June	1st to 10th	Night	51% 6% 9% 21% 12% 1	1400
	11th to 20th	Night	45% 5% 10% 21% 18% 1	1400
	21st to end	Night	46% 6% 12% 23% 11% 2	1400
July	1st to 10th	Night	47% 6% 11% 29% 8%	3125
	11th to 20th	Night	48% 6% 13% 23% 9% 1	3118
	21st to end	Night	42% 9% 14% 30% 5%	3548
August	1st to 10th	Night	40% 8% 16% 30% 6%	2730
	11th to 20th	Night	43% 7% 17% 27% 5%	2902
	21st to end	Night	44% 6% 13% 26% 10% 1	3252
September	1st to 10th	Night	51% 7% 13% 21% 7%	2000
	11th to 20th	Night	47% 7% 16% 24% 5%	2061
	21st to end	Night	49% 6% 13% 25% 7%	2270
October	1st to 10th	Night	56% 6% 11% 21% 6%	3837
	11th to 20th	Night	64% 5% 9% 20% 2	3779
	21st to end	Night	67% 6% 12% 11% 5%	4409
November	1st to 10th	Night	72% 6% 10% 10% 2	2732
	11th to 20th	Night	76% 5% 8% 9% 3	2800
	21st to end	Night	81% 5% 6% 7% 1	2800
December	1st to 10th	Night	83% 5% 5% 5% 2	4734
	11th to 20th	Night	78% 5% 7% 8% 2	5585
	21st to end	Night	77% 4 6% 9% 4	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	24% 19% 38% 16% 3 1	200
	11th to 20th	Day	15% 17% 38% 23% 6% 1	200
	21st to end	Day	16% 16% 38% 25% 4 1	273
February	1st to 10th	Day	15% 25% 38% 18% 2 1	300
	11th to 20th	Day	18% 22% 37% 19% 3 1	300
	21st to end	Day	24% 23% 40% 12% 1	249
March	1st to 10th	Day	20% 18% 42% 15% 4 1	470
	11th to 20th	Day	25% 17% 38% 16% 4	500
	21st to end	Day	20% 24% 35% 15% 5% 1	550
April	1st to 10th	Day	23% 22% 29% 17% 7% 1	500
	11th to 20th	Day	28% 17% 30% 16% 6% 2	492
	21st to end	Day	28% 17% 30% 18% 7% 1	500
May	1st to 10th	Day	23% 15% 33% 20% 7% 1	503
	11th to 20th	Day	24% 17% 32% 18% 7% 2	527
	21st to end	Day	25% 16% 32% 19% 8%	696
June	1st to 10th	Day	26% 15% 32% 19% 7%	730
	11th to 20th	Day	24% 18% 30% 20% 6% 2	730
	21st to end	Day	23% 11% 34% 22% 8% 2	730
July	1st to 10th	Day	18% 16% 33% 22% 9% 3	721
	11th to 20th	Day	18% 13% 34% 24% 10% 1	718
	21st to end	Day	20% 10% 31% 27% 9% 3	699
August	1st to 10th	Day	23% 10% 29% 25% 10% 3	526
	11th to 20th	Day	19% 10% 27% 26% 13% 6%	519
	21st to end	Day	25% 12% 27% 20% 12% 4	583
September	1st to 10th	Day	26% 14% 27% 21% 9% 2	530
	11th to 20th	Day	25% 10% 31% 22% 10% 2	530
	21st to end	Day	29% 12% 30% 17% 8% 3	530
October	1st to 10th	Day	31% 13% 29% 17% 8% 1	502
	11th to 20th	Day	38% 20% 26% 12% 3 1	490
	21st to end	Day	36% 20% 29% 12% 2 1	409
November	1st to 10th	Day	39% 17% 34% 11% 1	200
	11th to 20th	Day	32% 22% 32% 12% 3 1	200
	21st to end	Day	38% 23% 27% 9% 5%	200
December	1st to 10th	Day	30% 12% 34% 15% 8% 2	200
	11th to 20th	Day	24% 21% 38% 11% 7% 1	200
	21st to end	Day	29% 17% 30% 16% 5% 2	220
January	1st to 10th	Night	22% 13% 36% 21% 6% 1	1000
	11th to 20th	Night	24% 11% 37% 23% 5%	1000
	21st to end	Night	16% 11% 32% 28% 11% 1	974
February	1st to 10th	Night	15% 13% 39% 25% 9% 1	800
	11th to 20th	Night	19% 13% 36% 24% 7%	783
	21st to end	Night	18% 15% 38% 22% 6% 1	660
March	1st to 10th	Night	17% 17% 34% 25% 6% 1	800
	11th to 20th	Night	24% 15% 32% 22% 6%	800
	21st to end	Night	19% 15% 37% 22% 6% 1	880
April	1st to 10th	Night	27% 14% 32% 21% 5% 1	800
	11th to 20th	Night	25% 14% 29% 23% 7% 1	633
	21st to end	Night	24% 11% 32% 25% 8% 1	600
May	1st to 10th	Night	18% 10% 31% 31% 10%	400
	11th to 20th	Night	26% 11% 26% 30% 6% 1	394
	21st to end	Night	22% 11% 33% 28% 5%	433
June	1st to 10th	Night	24% 11% 32% 25% 8% 1	400
	11th to 20th	Night	17% 15% 31% 29% 8% 1	400
	21st to end	Night	20% 6% 28% 35% 10% 1	400
July	1st to 10th	Night	17% 7% 31% 33% 11% 1	390
	11th to 20th	Night	18% 7% 28% 36% 11%	388
	21st to end	Night	15% 5% 27% 38% 13% 1	438
August	1st to 10th	Night	16% 7% 28% 29% 18% 2	487
	11th to 20th	Night	19% 6% 23% 33% 18% 1	594
	21st to end	Night	24% 8% 24% 28% 15% 1	676
September	1st to 10th	Night	25% 11% 28% 23% 12% 1	800
	11th to 20th	Night	24% 10% 27% 26% 12% 2	800
	21st to end	Night	27% 9% 23% 28% 11% 2	800
October	1st to 10th	Night	28% 11% 27% 25% 8% 1	773
	11th to 20th	Night	37% 13% 26% 18% 5% 1	760
	21st to end	Night	34% 12% 29% 19% 5% 1	836
November	1st to 10th	Night	32% 16% 31% 16% 4 1	780
	11th to 20th	Night	30% 17% 28% 20% 5%	800
	21st to end	Night	33% 14% 34% 15% 4 1	800
December	1st to 10th	Night	27% 15% 30% 21% 7%	889
	11th to 20th	Night	26% 12% 35% 19% 6% 1	1000
	21st to end	Night	28% 13% 29% 21% 7% 2	1100

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