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 2024-12-02.

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	17% 10% 4 9% 5% 5% 4 4 5% 7% 31%	101791
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	97836
Transparency	Day	51% 6% 13% 20% 9% 1	105250
	Night	52% 6% 13% 19% 9% 1	102466
Seeing	Day	24% 16% 29% 19% 8% 2	14540
	Night	23% 13% 29% 24% 9% 1	23598

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	15% 18% 6% 15% 7% 5% 4 3 4 4 19%	2517
	11th to 20th	Day	14% 15% 6% 17% 8% 4 3 3 4 5% 21%	2520
	21st to end	Day	12% 16% 7% 14% 7% 6% 6% 4 5% 5% 19%	2804
February	1st to 10th	Day	12% 17% 5% 14% 8% 5% 4 3 4 6% 22%	1710
	11th to 20th	Day	12% 18% 6% 12% 6% 4 5% 3 4 5% 26%	1691
	21st to end	Day	17% 16% 5% 10% 7% 5% 3 3 5% 5% 25%	1570
March	1st to 10th	Day	18% 16% 5% 10% 6% 4 4 3 4 5% 25%	3116
	11th to 20th	Day	11% 18% 6% 12% 7% 4 4 4 4 4 25%	3407
	21st to end	Day	13% 9% 3 7% 6% 4 4 3 6% 6% 38%	4070
April	1st to 10th	Day	21% 14% 3 8% 4 4 3 3 4 5% 31%	2280
	11th to 20th	Day	22% 10% 3 7% 4 4 4 3 5% 5% 33%	2254
	21st to end	Day	22% 10% 4 8% 6% 4 4 3 4 5% 30%	2430
May	1st to 10th	Day	20% 10% 3 8% 5% 4 5% 4 5% 6% 29%	4426
	11th to 20th	Day	19% 8% 3 7% 4 4 5% 4 7% 7% 32%	4398
	21st to end	Day	17% 7% 3 7% 5% 4 5% 4 6% 8% 35%	4837
June	1st to 10th	Day	22% 8% 3 8% 5% 5% 5% 3 6% 6% 28%	2660
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 5% 9% 36%	2660
	21st to end	Day	20% 7% 2 7% 5% 6% 5% 4 7% 7% 29%	2660
July	1st to 10th	Day	12% 7% 3 7% 6% 6% 6% 4 7% 10% 32%	4440
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 4 7% 9% 34%	4440
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 8% 44%	4814
August	1st to 10th	Day	14% 6% 3 8% 6% 6% 5% 4 7% 9% 32%	4423
	11th to 20th	Day	15% 7% 3 7% 5% 4 5% 4 6% 9% 36%	3992
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 9% 38%	4180
September	1st to 10th	Day	14% 6% 2 6% 5% 4 4 4 6% 7% 42%	2280
	11th to 20th	Day	14% 5% 2 6% 4 4 4 3 6% 7% 45%	2254
	21st to end	Day	20% 7% 2 6% 4 3 4 3 5% 7% 40%	2066
October	1st to 10th	Day	21% 9% 3 6% 5% 4 4 4 5% 6% 34%	1900
	11th to 20th	Day	24% 10% 4 9% 5% 4 5% 4 4 6% 26%	1900
	21st to end	Day	25% 12% 4 7% 4 5% 4 3 5% 5% 26%	1895
November	1st to 10th	Day	15% 11% 4 7% 6% 4 4 3 6% 6% 33%	1761
	11th to 20th	Day	19% 13% 4 10% 6% 5% 6% 3 6% 6% 24%	1630
	21st to end	Day	22% 15% 5% 10% 6% 4 4 3 5% 5% 21%	1600
December	1st to 10th	Day	22% 20% 5% 11% 5% 4 5% 3 4 5% 16%	1528
	11th to 20th	Day	19% 15% 6% 11% 6% 4 5% 3 3 5% 22%	1906
	21st to end	Day	25% 20% 6% 9% 5% 3 3 2 3 3 19%	2772
January	1st to 10th	Night	16% 17% 7% 10% 5% 5% 4 3 5% 6% 22%	4429
	11th to 20th	Night	12% 15% 7% 13% 8% 5% 4 3 5% 6% 23%	4440
	21st to end	Night	13% 17% 8% 13% 6% 6% 5% 3 4 5% 20%	4850
February	1st to 10th	Night	12% 15% 6% 12% 7% 5% 5% 4 5% 5% 24%	2470
	11th to 20th	Night	11% 15% 5% 11% 7% 5% 4 3 5% 5% 28%	2374
	21st to end	Night	13% 15% 5% 10% 5% 5% 5% 4 5% 5% 29%	1884
March	1st to 10th	Night	19% 14% 4 9% 4 4 4 3 4 5% 31%	3724
	11th to 20th	Night	15% 19% 4 8% 5% 4 4 4 5% 5% 27%	3548
	21st to end	Night	14% 10% 3 9% 6% 4 4 3 5% 5% 38%	3548
April	1st to 10th	Night	21% 11% 3 6% 5% 4 4 3 4 6% 32%	1900
	11th to 20th	Night	20% 9% 3 7% 4 3 4 3 5% 6% 35%	1875
	21st to end	Night	19% 10% 3 8% 4 4 4 3 5% 5% 34%	1765
May	1st to 10th	Night	19% 11% 3 9% 5% 5% 5% 3 6% 7% 29%	2514
	11th to 20th	Night	19% 7% 3 6% 4 4 4 4 5% 7% 37%	2495
	21st to end	Night	16% 7% 2 8% 5% 4 5% 4 6% 6% 36%	2738
June	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 4 7% 9% 32%	1520
	11th to 20th	Night	15% 7% 3 6% 5% 3 4 3 5% 8% 40%	1520
	21st to end	Night	19% 7% 2 7% 4 5% 4 4 6% 6% 37%	1520
July	1st to 10th	Night	12% 7% 2 7% 6% 5% 6% 4 6% 7% 36%	2520
	11th to 20th	Night	15% 6% 3 6% 4 4 4 4 6% 9% 40%	2520
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 6% 8% 47%	2739
August	1st to 10th	Night	11% 7% 3 8% 5% 5% 5% 4 6% 8% 39%	2506
	11th to 20th	Night	15% 6% 3 6% 3 4 4 3 6% 6% 42%	2801
	21st to end	Night	14% 6% 2 7% 4 4 5% 3 5% 7% 42%	3476
September	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 6% 7% 46%	1900
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 7% 49%	1925
	21st to end	Night	17% 6% 2 5% 4 4 3 3 6% 6% 44%	2109
October	1st to 10th	Night	18% 8% 3 7% 4 5% 4 4 5% 6% 37%	2280
	11th to 20th	Night	20% 10% 3 7% 5% 4 5% 3 5% 6% 32%	2280
	21st to end	Night	22% 10% 3 9% 5% 4 5% 3 4 6% 29%	2703
November	1st to 10th	Night	18% 11% 4 8% 4 4 4 4 5% 7% 31%	2537
	11th to 20th	Night	18% 12% 4 9% 6% 5% 5% 4 5% 6% 27%	2768
	21st to end	Night	19% 16% 5% 10% 6% 5% 5% 3 4 6% 22%	2800
December	1st to 10th	Night	19% 19% 5% 10% 6% 5% 4 3 4 5% 19%	2686
	11th to 20th	Night	22% 15% 4 9% 6% 4 5% 3 4 4 25%	3288
	21st to end	Night	25% 22% 4 9% 4 3 3 2 3 3 19%	4884

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	69% 6% 9% 11% 5% 1	2518
	11th to 20th	Day	65% 6% 9% 13% 6% 1	2520
	21st to end	Day	67% 7% 10% 10% 6%	2804
February	1st to 10th	Day	63% 4 8% 16% 7% 2	1710
	11th to 20th	Day	60% 5% 10% 14% 9% 1	1700
	21st to end	Day	62% 4 9% 14% 10% 1	1570
March	1st to 10th	Day	60% 7% 7% 15% 9% 1	3119
	11th to 20th	Day	59% 6% 12% 12% 9% 2	3407
	21st to end	Day	44% 5% 10% 20% 18% 2	4070
April	1st to 10th	Day	55% 3 9% 18% 11% 3	2280
	11th to 20th	Day	52% 5% 8% 19% 14% 2	2254
	21st to end	Day	55% 5% 8% 15% 12% 4	2430
May	1st to 10th	Day	52% 8% 9% 20% 9% 2	4440
	11th to 20th	Day	47% 8% 17% 18% 9% 1	4402
	21st to end	Day	45% 7% 12% 20% 15% 1	4837
June	1st to 10th	Day	54% 7% 11% 17% 11% 1	2660
	11th to 20th	Day	44% 7% 13% 22% 11% 3	2660
	21st to end	Day	51% 8% 13% 17% 9% 1	2660
July	1st to 10th	Day	45% 6% 16% 24% 9%	4370
	11th to 20th	Day	45% 6% 16% 26% 7%	4365
	21st to end	Day	38% 6% 15% 31% 10%	4661
August	1st to 10th	Day	49% 5% 17% 22% 7%	4409
	11th to 20th	Day	43% 6% 16% 28% 7%	4032
	21st to end	Day	43% 4 18% 22% 12% 1	4180
September	1st to 10th	Day	40% 7% 14% 27% 13%	2280
	11th to 20th	Day	37% 6% 14% 30% 11% 1	2254
	21st to end	Day	43% 6% 15% 26% 10%	2066
October	1st to 10th	Day	50% 4 11% 27% 7%	3160
	11th to 20th	Day	49% 7% 14% 23% 7%	3140
	21st to end	Day	50% 5% 20% 18% 6%	3095
November	1st to 10th	Day	49% 6% 16% 21% 6% 1	1761
	11th to 20th	Day	60% 7% 11% 12% 9% 1	1630
	21st to end	Day	65% 6% 12% 11% 5% 1	1600
December	1st to 10th	Day	70% 6% 10% 11% 3	1528
	11th to 20th	Day	64% 7% 12% 13% 5%	1906
	21st to end	Day	70% 5% 7% 13% 5%	2772
January	1st to 10th	Night	63% 7% 16% 11% 3	4429
	11th to 20th	Night	60% 9% 13% 13% 5% 1	4440
	21st to end	Night	66% 7% 10% 9% 7% 1	4850
February	1st to 10th	Night	60% 5% 10% 17% 7% 1	2470
	11th to 20th	Night	59% 4 10% 18% 8% 1	2376
	21st to end	Night	56% 5% 10% 17% 11% 1	1884
March	1st to 10th	Night	57% 5% 10% 16% 10% 3	3734
	11th to 20th	Night	57% 6% 12% 15% 9% 2	3549
	21st to end	Night	47% 5% 8% 21% 15% 3	3548
April	1st to 10th	Night	53% 5% 9% 17% 13% 2	1900
	11th to 20th	Night	49% 6% 8% 22% 15% 1	1875
	21st to end	Night	51% 5% 10% 17% 14% 3	1765
May	1st to 10th	Night	52% 7% 11% 18% 10% 2	2520
	11th to 20th	Night	45% 8% 14% 18% 13% 2	2496
	21st to end	Night	47% 7% 10% 20% 15% 2	2739
June	1st to 10th	Night	46% 8% 13% 22% 10%	1520
	11th to 20th	Night	42% 7% 12% 23% 14% 2	1520
	21st to end	Night	48% 8% 12% 19% 13% 1	1520
July	1st to 10th	Night	46% 5% 18% 20% 11% 1	2480
	11th to 20th	Night	41% 5% 17% 30% 7%	2472
	21st to end	Night	36% 6% 18% 27% 14%	2651
August	1st to 10th	Night	43% 6% 16% 27% 8%	2496
	11th to 20th	Night	42% 5% 17% 29% 7%	2827
	21st to end	Night	42% 6% 16% 23% 12% 2	3476
September	1st to 10th	Night	37% 7% 14% 28% 13% 1	1900
	11th to 20th	Night	35% 6% 15% 34% 10% 1	1925
	21st to end	Night	40% 6% 13% 29% 11%	2109
October	1st to 10th	Night	43% 4 14% 27% 11%	3800
	11th to 20th	Night	45% 5% 15% 26% 9%	3779
	21st to end	Night	49% 5% 19% 17% 10%	4453
November	1st to 10th	Night	53% 6% 14% 19% 7%	2537
	11th to 20th	Night	55% 7% 12% 15% 10% 1	2768
	21st to end	Night	63% 6% 12% 15% 4	2800
December	1st to 10th	Night	68% 6% 11% 12% 4	2686
	11th to 20th	Night	63% 5% 9% 15% 7% 1	3288
	21st to end	Night	70% 5% 7% 14% 5%	4884

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	26% 22% 28% 19% 6%	180
	11th to 20th	Day	33% 14% 33% 13% 6%	180
	21st to end	Day	22% 16% 34% 22% 5%	207
February	1st to 10th	Day	31% 26% 30% 10% 3	270
	11th to 20th	Day	29% 29% 31% 10%	298
	21st to end	Day	29% 23% 31% 16% 1	382
March	1st to 10th	Day	27% 21% 31% 15% 6%	460
	11th to 20th	Day	26% 22% 33% 14% 4 1	460
	21st to end	Day	31% 22% 25% 15% 6% 1	506
April	1st to 10th	Day	30% 19% 29% 14% 6% 1	460
	11th to 20th	Day	35% 18% 23% 13% 9% 2	452
	21st to end	Day	31% 20% 31% 11% 6%	460
May	1st to 10th	Day	27% 18% 31% 16% 6% 2	463
	11th to 20th	Day	22% 18% 29% 21% 8% 2	487
	21st to end	Day	24% 16% 32% 18% 9% 2	529
June	1st to 10th	Day	26% 11% 32% 22% 8%	490
	11th to 20th	Day	18% 18% 32% 21% 9% 2	490
	21st to end	Day	24% 13% 29% 22% 11% 1	490
July	1st to 10th	Day	15% 13% 30% 28% 9% 4	485
	11th to 20th	Day	15% 11% 33% 27% 11% 2	484
	21st to end	Day	13% 9% 33% 31% 11% 2	532
August	1st to 10th	Day	15% 8% 29% 35% 11% 2	486
	11th to 20th	Day	14% 9% 26% 28% 15% 8%	479
	21st to end	Day	18% 12% 29% 22% 12% 7%	539
September	1st to 10th	Day	16% 11% 32% 22% 13% 6%	490
	11th to 20th	Day	13% 11% 24% 24% 19% 9%	490
	21st to end	Day	21% 15% 26% 19% 14% 5%	490
October	1st to 10th	Day	26% 17% 22% 18% 14% 3	490
	11th to 20th	Day	29% 22% 26% 14% 8% 1	490
	21st to end	Day	32% 16% 30% 17% 5%	436
November	1st to 10th	Day	32% 15% 29% 18% 5% 1	395
	11th to 20th	Day	28% 20% 34% 11% 6% 1	230
	21st to end	Day	33% 21% 29% 15% 3 1	200
December	1st to 10th	Day	32% 13% 35% 18% 2	182
	11th to 20th	Day	28% 22% 37% 11% 2 1	180
	21st to end	Day	32% 25% 26% 13% 3 1	198
January	1st to 10th	Night	28% 17% 36% 16% 4	760
	11th to 20th	Night	29% 16% 29% 20% 5%	760
	21st to end	Night	20% 15% 36% 22% 6% 1	836
February	1st to 10th	Night	19% 18% 35% 22% 5%	760
	11th to 20th	Night	20% 20% 38% 18% 4	743
	21st to end	Night	23% 17% 32% 24% 3 1	628
March	1st to 10th	Night	24% 17% 33% 21% 5%	760
	11th to 20th	Night	25% 18% 31% 20% 5% 1	760
	21st to end	Night	24% 16% 31% 22% 7%	836
April	1st to 10th	Night	28% 16% 29% 23% 4	760
	11th to 20th	Night	26% 16% 30% 20% 6%	751
	21st to end	Night	26% 15% 30% 22% 7%	625
May	1st to 10th	Night	25% 13% 25% 28% 8% 2	380
	11th to 20th	Night	22% 11% 29% 29% 10%	374
	21st to end	Night	16% 10% 37% 24% 11% 1	411
June	1st to 10th	Night	17% 9% 31% 34% 8%	380
	11th to 20th	Night	19% 8% 28% 32% 12% 1	380
	21st to end	Night	21% 6% 26% 31% 15% 1	380
July	1st to 10th	Night	9% 6% 33% 36% 15%	370
	11th to 20th	Night	15% 4 21% 42% 17% 1	368
	21st to end	Night	11% 5% 27% 39% 16% 2	416
August	1st to 10th	Night	13% 5% 20% 40% 20% 1	378
	11th to 20th	Night	14% 4 20% 34% 25% 3	566
	21st to end	Night	16% 8% 24% 32% 18% 2	836
September	1st to 10th	Night	15% 6% 30% 31% 16% 1	760
	11th to 20th	Night	17% 8% 21% 29% 23% 2	760
	21st to end	Night	21% 9% 24% 27% 17% 2	760
October	1st to 10th	Night	22% 11% 27% 24% 15% 1	760
	11th to 20th	Night	26% 14% 28% 23% 8% 1	760
	21st to end	Night	27% 14% 29% 25% 5%	836
November	1st to 10th	Night	27% 14% 27% 23% 8% 1	780
	11th to 20th	Night	25% 17% 31% 16% 11%	800
	21st to end	Night	30% 19% 27% 19% 6%	800
December	1st to 10th	Night	30% 16% 31% 18% 5%	768
	11th to 20th	Night	29% 17% 31% 17% 4 1	760
	21st to end	Night	35% 15% 31% 16% 4	836

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