Plouffe Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Plouffe Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 11% 4 9% 6% 5% 5% 4 5% 7% 30%	131161
	Night	15% 12% 4 10% 5% 5% 4 3 5% 6% 30%	129979
Transparency	Day	53% 5% 12% 19% 9% 1	134602
	Night	55% 5% 11% 19% 10% 1	134625
Seeing	Day	24% 17% 30% 19% 8% 2	16025
	Night	23% 13% 30% 24% 9% 1	25199

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% 7% 16% 8% 7% 5% 3 4 5% 18%	4661
	11th to 20th	Day	10% 21% 9% 17% 8% 4 3 2 3 5% 18%	4650
	21st to end	Day	7% 13% 7% 16% 9% 8% 6% 4 5% 5% 21%	5184
February	1st to 10th	Day	11% 16% 5% 14% 9% 5% 4 3 4 6% 23%	1890
	11th to 20th	Day	11% 17% 5% 12% 6% 4 4 3 4 6% 26%	1875
	21st to end	Day	16% 16% 5% 11% 7% 5% 4 3 5% 5% 24%	1730
March	1st to 10th	Day	17% 16% 5% 12% 7% 4 3 3 4 5% 23%	4496
	11th to 20th	Day	10% 14% 5% 11% 7% 4 5% 4 5% 7% 28%	4879
	21st to end	Day	17% 14% 3 7% 5% 4 4 3 5% 5% 34%	5832
April	1st to 10th	Day	21% 14% 3 8% 4 4 3 3 4 5% 31%	2400
	11th to 20th	Day	22% 10% 3 7% 4 4 4 3 4 5% 34%	2374
	21st to end	Day	22% 10% 4 8% 6% 4 4 3 4 5% 30%	2558
May	1st to 10th	Day	22% 11% 4 9% 5% 5% 5% 4 5% 6% 24%	6346
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	6318
	21st to end	Day	22% 9% 3 7% 5% 4 4 3 5% 7% 30%	6949
June	1st to 10th	Day	22% 8% 3 8% 5% 5% 4 3 6% 6% 29%	2800
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 4 5% 10% 36%	2800
	21st to end	Day	20% 7% 2 7% 5% 6% 5% 4 7% 8% 29%	2800
July	1st to 10th	Day	14% 7% 3 8% 6% 6% 6% 4 7% 9% 31%	6360
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 7% 8% 38%	6360
	21st to end	Day	9% 5% 2 6% 4 4 4 4 7% 9% 45%	6926
August	1st to 10th	Day	13% 6% 3 8% 5% 6% 5% 4 6% 9% 33%	4563
	11th to 20th	Day	15% 7% 3 7% 5% 4 5% 4 6% 9% 36%	4120
	21st to end	Day	14% 6% 3 6% 5% 5% 5% 4 6% 9% 38%	4312
September	1st to 10th	Day	14% 6% 2 6% 5% 4 4 4 6% 7% 42%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 6% 7% 47%	2372
	21st to end	Day	20% 7% 1 6% 4 3 4 3 5% 7% 40%	2175
October	1st to 10th	Day	21% 8% 3 6% 5% 4 4 4 5% 6% 36%	2000
	11th to 20th	Day	23% 10% 4 8% 5% 4 5% 4 4 6% 27%	2000
	21st to end	Day	25% 11% 4 8% 4 5% 4 3 5% 5% 26%	1995
November	1st to 10th	Day	15% 12% 4 8% 6% 4 4 3 6% 6% 32%	1851
	11th to 20th	Day	18% 14% 5% 10% 6% 5% 5% 3 5% 6% 23%	1711
	21st to end	Day	22% 16% 5% 10% 6% 4 4 3 5% 5% 20%	1680
December	1st to 10th	Day	19% 21% 7% 12% 6% 4 4 4 5% 5% 13%	2680
	11th to 20th	Day	16% 16% 8% 13% 6% 4 5% 3 3 4 22%	3066
	21st to end	Day	20% 18% 5% 9% 6% 4 4 4 4 4 22%	4048
January	1st to 10th	Night	12% 18% 8% 14% 6% 4 4 3 5% 6% 20%	8234
	11th to 20th	Night	10% 19% 8% 17% 8% 5% 4 3 4 4 17%	8235
	21st to end	Night	8% 14% 7% 14% 7% 7% 6% 4 5% 6% 21%	8981
February	1st to 10th	Night	11% 14% 5% 13% 7% 5% 5% 4 5% 5% 25%	2730
	11th to 20th	Night	11% 15% 5% 12% 8% 5% 4 3 4 5% 28%	2630
	21st to end	Night	12% 15% 6% 11% 6% 5% 5% 4 5% 5% 28%	2076
March	1st to 10th	Night	20% 16% 4 10% 5% 4 4 3 4 5% 27%	5389
	11th to 20th	Night	11% 14% 3 7% 5% 5% 6% 4 7% 8% 30%	5076
	21st to end	Night	17% 11% 3 10% 6% 5% 4 4 5% 5% 31%	5086
April	1st to 10th	Night	21% 11% 3 7% 5% 5% 4 3 4 6% 32%	2000
	11th to 20th	Night	20% 9% 3 8% 4 3 4 3 4 6% 35%	1975
	21st to end	Night	20% 10% 3 8% 4 4 4 3 5% 6% 35%	1858
May	1st to 10th	Night	20% 11% 4 9% 5% 6% 5% 4 6% 7% 24%	3594
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 36%	3575
	21st to end	Night	20% 10% 3 9% 5% 4 5% 4 5% 6% 30%	3926
June	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 4 7% 9% 32%	1600
	11th to 20th	Night	15% 7% 3 6% 5% 3 4 3 5% 8% 40%	1600
	21st to end	Night	19% 7% 2 7% 4 5% 4 4 6% 6% 37%	1600
July	1st to 10th	Night	14% 7% 2 7% 6% 5% 7% 5% 7% 8% 34%	3600
	11th to 20th	Night	13% 6% 3 6% 4 4 4 4 6% 8% 42%	3600
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 8% 49%	3926
August	1st to 10th	Night	11% 6% 3 8% 4 5% 5% 4 6% 8% 39%	2586
	11th to 20th	Night	15% 6% 3 6% 3 4 4 3 6% 6% 43%	2892
	21st to end	Night	15% 6% 2 7% 4 4 5% 3 5% 7% 41%	3586
September	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 6% 7% 47%	2000
	11th to 20th	Night	14% 5% 1 5% 4 3 4 3 5% 7% 50%	2027
	21st to end	Night	17% 6% 2 5% 4 4 3 3 5% 6% 44%	2220
October	1st to 10th	Night	18% 8% 3 6% 4 4 3 4 5% 6% 39%	2400
	11th to 20th	Night	19% 10% 3 7% 5% 4 5% 3 5% 6% 33%	2400
	21st to end	Night	22% 10% 3 8% 5% 4 5% 3 4 6% 30%	2845
November	1st to 10th	Night	19% 11% 4 8% 4 4 4 4 5% 6% 31%	2667
	11th to 20th	Night	17% 12% 4 10% 6% 5% 5% 4 5% 6% 27%	2907
	21st to end	Night	18% 16% 5% 10% 6% 5% 5% 3 4 6% 21%	2940
December	1st to 10th	Night	17% 17% 5% 12% 6% 6% 5% 4 4 6% 18%	4720
	11th to 20th	Night	23% 13% 4 10% 6% 4 4 3 4 4 25%	5348
	21st to end	Night	21% 21% 4 9% 4 4 3 2 4 5% 22%	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	68% 4 9% 15% 4	4662
	11th to 20th	Day	70% 4 6% 10% 9% 1	4680
	21st to end	Day	66% 4 9% 10% 10% 1	5184
February	1st to 10th	Day	62% 4 8% 17% 7% 2	1890
	11th to 20th	Day	60% 5% 10% 15% 9% 1	1884
	21st to end	Day	63% 4 9% 13% 10% 1	1730
March	1st to 10th	Day	63% 5% 6% 11% 12% 3	4499
	11th to 20th	Day	54% 5% 17% 13% 10% 1	4879
	21st to end	Day	52% 3 8% 16% 18% 2	5832
April	1st to 10th	Day	55% 3 9% 18% 12% 3	2400
	11th to 20th	Day	52% 5% 7% 19% 15% 2	2374
	21st to end	Day	55% 5% 9% 15% 13% 4	2558
May	1st to 10th	Day	59% 6% 8% 18% 8% 1	6357
	11th to 20th	Day	51% 6% 15% 17% 10% 1	6322
	21st to end	Day	53% 6% 11% 17% 13% 1	6949
June	1st to 10th	Day	53% 6% 11% 17% 11% 1	2800
	11th to 20th	Day	44% 6% 13% 23% 11% 2	2800
	21st to end	Day	51% 8% 13% 18% 9% 1	2800
July	1st to 10th	Day	48% 5% 15% 25% 7%	6290
	11th to 20th	Day	42% 5% 14% 30% 9%	6285
	21st to end	Day	35% 5% 16% 35% 9%	6728
August	1st to 10th	Day	47% 5% 17% 23% 7%	4549
	11th to 20th	Day	43% 6% 16% 28% 7% 1	4160
	21st to end	Day	43% 4 18% 23% 11% 1	4312
September	1st to 10th	Day	39% 7% 14% 28% 12%	2400
	11th to 20th	Day	36% 6% 14% 32% 12% 1	2372
	21st to end	Day	43% 5% 15% 26% 10%	2175
October	1st to 10th	Day	49% 4 11% 28% 8%	3260
	11th to 20th	Day	49% 7% 14% 23% 7% 1	3240
	21st to end	Day	51% 5% 20% 18% 6%	3195
November	1st to 10th	Day	51% 6% 15% 20% 7% 1	1851
	11th to 20th	Day	61% 6% 11% 12% 9% 1	1711
	21st to end	Day	67% 6% 11% 10% 5% 1	1680
December	1st to 10th	Day	72% 4 10% 11% 2	2680
	11th to 20th	Day	66% 5% 8% 14% 6%	3066
	21st to end	Day	66% 4 8% 16% 6%	4048
January	1st to 10th	Night	66% 4 11% 13% 5% 1	8234
	11th to 20th	Night	70% 5% 8% 11% 5%	8280
	21st to end	Night	64% 4 7% 12% 12%	8981
February	1st to 10th	Night	60% 5% 10% 18% 7% 1	2730
	11th to 20th	Night	59% 4 9% 19% 8% 1	2632
	21st to end	Night	57% 5% 9% 17% 11% 1	2076
March	1st to 10th	Night	61% 3 7% 13% 12% 4	5399
	11th to 20th	Night	50% 6% 15% 16% 9% 3	5077
	21st to end	Night	54% 4 7% 21% 12% 2	5086
April	1st to 10th	Night	54% 4 9% 18% 13% 2	2000
	11th to 20th	Night	49% 6% 8% 21% 15% 1	1975
	21st to end	Night	51% 5% 10% 18% 14% 3	1858
May	1st to 10th	Night	57% 6% 9% 18% 8% 2	3600
	11th to 20th	Night	47% 7% 15% 18% 12% 2	3576
	21st to end	Night	55% 5% 10% 17% 12% 1	3927
June	1st to 10th	Night	46% 8% 13% 22% 11%	1600
	11th to 20th	Night	42% 6% 12% 23% 14% 2	1600
	21st to end	Night	48% 7% 12% 19% 13% 1	1600
July	1st to 10th	Night	47% 4 19% 21% 8%	3560
	11th to 20th	Night	40% 4 17% 31% 8%	3552
	21st to end	Night	33% 5% 16% 32% 14%	3809
August	1st to 10th	Night	43% 6% 16% 27% 8%	2576
	11th to 20th	Night	41% 5% 17% 29% 8%	2918
	21st to end	Night	42% 5% 16% 23% 12% 2	3586
September	1st to 10th	Night	36% 7% 14% 29% 13% 1	2000
	11th to 20th	Night	34% 5% 15% 35% 11% 1	2027
	21st to end	Night	40% 5% 13% 29% 12%	2220
October	1st to 10th	Night	42% 4 14% 27% 11% 1	3920
	11th to 20th	Night	45% 5% 14% 26% 9%	3899
	21st to end	Night	48% 5% 18% 18% 10%	4595
November	1st to 10th	Night	53% 6% 14% 20% 7%	2667
	11th to 20th	Night	56% 6% 13% 15% 9% 1	2907
	21st to end	Night	64% 6% 12% 14% 4	2940
December	1st to 10th	Night	68% 4 13% 12% 3	4720
	11th to 20th	Night	64% 3 6% 15% 11% 1	5348
	21st to end	Night	66% 4 5% 14% 11%	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	24% 20% 31% 19% 5%	220
	11th to 20th	Day	34% 17% 33% 11% 5%	220
	21st to end	Day	20% 19% 33% 23% 5%	253
February	1st to 10th	Day	29% 25% 32% 11% 2	330
	11th to 20th	Day	28% 28% 33% 10% 1	362
	21st to end	Day	27% 22% 33% 16% 1	446
March	1st to 10th	Day	29% 21% 31% 14% 5%	508
	11th to 20th	Day	27% 23% 32% 15% 3 1	500
	21st to end	Day	32% 22% 25% 14% 5% 1	550
April	1st to 10th	Day	32% 21% 27% 14% 6% 1	500
	11th to 20th	Day	35% 19% 22% 13% 9% 2	492
	21st to end	Day	32% 21% 31% 11% 6%	500
May	1st to 10th	Day	27% 19% 32% 15% 5% 2	503
	11th to 20th	Day	22% 18% 30% 20% 8% 2	527
	21st to end	Day	26% 15% 31% 18% 9% 2	573
June	1st to 10th	Day	27% 12% 32% 21% 8%	530
	11th to 20th	Day	18% 19% 33% 20% 9% 2	530
	21st to end	Day	24% 13% 31% 20% 10% 1	530
July	1st to 10th	Day	15% 13% 31% 28% 9% 4	525
	11th to 20th	Day	15% 12% 33% 27% 10% 2	524
	21st to end	Day	13% 11% 34% 30% 11% 2	576
August	1st to 10th	Day	14% 8% 28% 37% 11% 2	526
	11th to 20th	Day	14% 9% 27% 28% 15% 8%	519
	21st to end	Day	18% 12% 30% 21% 13% 7%	583
September	1st to 10th	Day	16% 11% 32% 22% 12% 6%	530
	11th to 20th	Day	12% 11% 24% 25% 20% 9%	530
	21st to end	Day	21% 15% 26% 20% 13% 5%	530
October	1st to 10th	Day	25% 16% 23% 18% 14% 3	530
	11th to 20th	Day	28% 22% 26% 15% 7% 1	530
	21st to end	Day	33% 15% 30% 17% 5%	470
November	1st to 10th	Day	32% 16% 28% 18% 5% 1	425
	11th to 20th	Day	28% 19% 34% 12% 6% 1	251
	21st to end	Day	33% 22% 28% 14% 3	220
December	1st to 10th	Day	30% 14% 37% 16% 3	220
	11th to 20th	Day	28% 21% 38% 11% 1 1	220
	21st to end	Day	30% 23% 29% 14% 4 1	242
January	1st to 10th	Night	26% 16% 37% 17% 4	840
	11th to 20th	Night	29% 17% 29% 20% 5%	840
	21st to end	Night	20% 15% 36% 23% 6% 1	924
February	1st to 10th	Night	18% 19% 35% 23% 6%	840
	11th to 20th	Night	20% 19% 38% 18% 4	823
	21st to end	Night	23% 17% 32% 24% 3 1	692
March	1st to 10th	Night	25% 17% 33% 20% 5%	809
	11th to 20th	Night	25% 18% 32% 21% 5% 1	800
	21st to end	Night	25% 16% 30% 22% 6%	880
April	1st to 10th	Night	28% 16% 29% 23% 4	800
	11th to 20th	Night	26% 16% 30% 21% 6%	791
	21st to end	Night	26% 15% 30% 22% 7%	658
May	1st to 10th	Night	25% 13% 26% 27% 7% 2	400
	11th to 20th	Night	21% 12% 29% 29% 9%	394
	21st to end	Night	17% 10% 36% 25% 11% 1	433
June	1st to 10th	Night	17% 9% 33% 33% 8%	400
	11th to 20th	Night	19% 8% 29% 32% 12% 1	400
	21st to end	Night	21% 7% 27% 31% 15% 1	400
July	1st to 10th	Night	9% 7% 32% 37% 15%	390
	11th to 20th	Night	15% 4 21% 43% 17% 1	388
	21st to end	Night	10% 6% 27% 39% 16% 2	438
August	1st to 10th	Night	13% 5% 20% 40% 21% 1	398
	11th to 20th	Night	13% 4 20% 35% 25% 3	597
	21st to end	Night	16% 7% 25% 31% 18% 2	880
September	1st to 10th	Night	15% 6% 30% 31% 17% 1	800
	11th to 20th	Night	16% 8% 22% 29% 23% 2	800
	21st to end	Night	20% 9% 25% 27% 18% 2	800
October	1st to 10th	Night	21% 11% 27% 25% 15% 1	800
	11th to 20th	Night	25% 14% 29% 24% 8% 1	800
	21st to end	Night	27% 13% 30% 25% 5%	880
November	1st to 10th	Night	27% 14% 28% 23% 7% 1	820
	11th to 20th	Night	25% 17% 31% 16% 10%	840
	21st to end	Night	29% 19% 26% 20% 6%	840
December	1st to 10th	Night	30% 16% 31% 19% 4	840
	11th to 20th	Night	30% 18% 31% 17% 4 1	840
	21st to end	Night	34% 16% 29% 17% 4	924

Page generated at 2026-02-28 01:40:23. Version ac07

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