Drummondville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Drummondville clear sky chart from 2005-11-06 to 2024-01-31.

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% 10% 4 7% 6% 3 5% 4 5% 6% 31%	74915
	Night	19% 11% 4 7% 6% 3 4 4 4 6% 30%	75212
Transparency	Day	53% 6% 12% 19% 9% 1	74620
	Night	55% 6% 10% 18% 9% 1	75041
Seeing	Day	24% 13% 28% 22% 10% 3	13540
	Night	25% 10% 28% 27% 9% 1	22588

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	21% 19% 6% 10% 7% 3 4 4 4 4 18%	1517
	11th to 20th	Day	20% 16% 7% 11% 7% 3 4 4 3 5% 21%	1520
	21st to end	Day	12% 14% 6% 10% 8% 3 6% 4 5% 6% 26%	1664
February	1st to 10th	Day	15% 19% 6% 10% 6% 4 5% 4 4 4 23%	1510
	11th to 20th	Day	16% 15% 6% 10% 7% 4 6% 4 5% 5% 23%	1712
	21st to end	Day	16% 16% 6% 9% 6% 3 4 5% 4 6% 25%	1480
March	1st to 10th	Day	16% 13% 4 11% 7% 4 4 5% 4 5% 27%	1797
	11th to 20th	Day	18% 12% 5% 7% 6% 3 5% 4 4 6% 31%	1844
	21st to end	Day	18% 12% 4 8% 5% 3 5% 4 5% 6% 31%	2371
April	1st to 10th	Day	23% 11% 5% 7% 7% 4 5% 3 3 5% 28%	2160
	11th to 20th	Day	22% 9% 2 5% 5% 3 4 4 5% 6% 36%	2158
	21st to end	Day	25% 10% 3 7% 5% 2 4 4 5% 5% 30%	2340
May	1st to 10th	Day	23% 8% 3 5% 6% 3 4 3 5% 5% 34%	2425
	11th to 20th	Day	18% 6% 3 6% 6% 3 4 4 5% 7% 37%	2478
	21st to end	Day	19% 7% 3 6% 6% 4 5% 5% 6% 6% 33%	2725
June	1st to 10th	Day	24% 7% 3 5% 5% 3 5% 5% 6% 8% 30%	2520
	11th to 20th	Day	18% 6% 3 5% 5% 3 5% 5% 6% 8% 36%	2520
	21st to end	Day	21% 8% 4 7% 5% 4 5% 5% 5% 6% 30%	2520
July	1st to 10th	Day	15% 5% 3 5% 6% 3 6% 6% 6% 9% 36%	2520
	11th to 20th	Day	13% 5% 3 6% 6% 4 6% 7% 6% 9% 36%	2520
	21st to end	Day	15% 7% 3 6% 5% 4 6% 5% 7% 9% 34%	2744
August	1st to 10th	Day	16% 7% 3 6% 6% 4 5% 6% 6% 9% 33%	2506
	11th to 20th	Day	13% 6% 2 6% 6% 3 5% 6% 6% 9% 38%	2320
	21st to end	Day	15% 6% 2 5% 6% 3 4 4 6% 8% 40%	2376
September	1st to 10th	Day	14% 6% 3 5% 5% 3 5% 6% 6% 8% 40%	2160
	11th to 20th	Day	19% 5% 2 3 3 2 4 4 5% 8% 45%	2039
	21st to end	Day	19% 6% 3 4 4 3 4 5% 5% 7% 40%	1980
October	1st to 10th	Day	21% 7% 3 6% 6% 3 4 5% 6% 8% 32%	1924
	11th to 20th	Day	22% 10% 4 7% 6% 5% 5% 4 5% 7% 28%	1706
	21st to end	Day	27% 12% 4 7% 5% 3 3 4 5% 6% 25%	1782
November	1st to 10th	Day	20% 11% 4 6% 7% 4 3 3 5% 6% 32%	1671
	11th to 20th	Day	18% 14% 6% 9% 6% 3 4 4 4 6% 26%	1688
	21st to end	Day	23% 14% 5% 7% 6% 3 5% 4 5% 5% 23%	1520
December	1st to 10th	Day	26% 17% 6% 9% 6% 3 4 4 4 3 18%	1520
	11th to 20th	Day	23% 14% 7% 7% 5% 3 3 4 3 5% 26%	1906
	21st to end	Day	26% 17% 5% 11% 6% 2 3 3 3 4 20%	2772
January	1st to 10th	Night	20% 18% 7% 10% 5% 3 5% 4 4 4 18%	2649
	11th to 20th	Night	17% 16% 7% 12% 8% 3 5% 4 4 4 20%	2660
	21st to end	Night	13% 15% 6% 11% 8% 4 4 4 4 5% 26%	2919
February	1st to 10th	Night	15% 16% 6% 11% 9% 4 5% 4 5% 5% 21%	2448
	11th to 20th	Night	15% 14% 6% 11% 7% 3 5% 4 4 5% 25%	2133
	21st to end	Night	16% 15% 5% 7% 7% 3 4 5% 4 4 30%	1776
March	1st to 10th	Night	17% 15% 5% 8% 7% 4 4 3 4 5% 28%	2150
	11th to 20th	Night	21% 12% 5% 8% 5% 3 5% 4 4 5% 29%	2115
	21st to end	Night	18% 13% 5% 5% 5% 3 4 4 4 5% 34%	1980
April	1st to 10th	Night	25% 10% 5% 8% 5% 2 4 5% 5% 5% 27%	1800
	11th to 20th	Night	23% 8% 3 5% 5% 3 4 5% 4 7% 34%	1750
	21st to end	Night	23% 10% 3 6% 5% 2 4 4 5% 6% 31%	1620
May	1st to 10th	Night	24% 9% 3 7% 5% 2 4 3 5% 6% 31%	1551
	11th to 20th	Night	20% 7% 3 7% 5% 3 4 4 4 8% 35%	1415
	21st to end	Night	20% 6% 3 5% 5% 3 4 5% 6% 7% 36%	1550
June	1st to 10th	Night	23% 7% 3 5% 5% 3 5% 4 6% 9% 31%	1440
	11th to 20th	Night	19% 6% 2 5% 4 3 6% 5% 5% 9% 37%	1440
	21st to end	Night	18% 8% 3 6% 6% 3 5% 5% 7% 8% 33%	1440
July	1st to 10th	Night	13% 6% 3 6% 5% 4 5% 5% 6% 9% 38%	1440
	11th to 20th	Night	11% 6% 2 7% 6% 4 6% 7% 7% 9% 37%	1440
	21st to end	Night	13% 4 3 6% 5% 3 6% 5% 8% 8% 37%	1574
August	1st to 10th	Night	16% 4 2 5% 5% 2 5% 5% 6% 9% 39%	1432
	11th to 20th	Night	15% 6% 2 6% 6% 3 5% 6% 6% 8% 37%	1561
	21st to end	Night	15% 6% 3 6% 5% 3 4 4 5% 8% 40%	1979
September	1st to 10th	Night	15% 7% 3 6% 6% 3 4 4 5% 6% 43%	1800
	11th to 20th	Night	18% 5% 1 5% 4 2 3 4 4 7% 47%	1920
	21st to end	Night	18% 6% 3 6% 5% 3 4 4 4 7% 41%	1980
October	1st to 10th	Night	19% 6% 3 6% 5% 3 4 4 5% 6% 39%	2034
	11th to 20th	Night	22% 10% 3 6% 5% 4 5% 4 5% 6% 30%	2254
	21st to end	Night	26% 11% 3 6% 6% 3 4 4 4 5% 28%	2574
November	1st to 10th	Night	20% 12% 4 6% 7% 3 4 4 5% 7% 29%	2407
	11th to 20th	Night	20% 12% 5% 7% 6% 4 4 4 4 6% 29%	2489
	21st to end	Night	22% 15% 5% 7% 7% 3 4 4 4 4 26%	2660
December	1st to 10th	Night	25% 17% 7% 9% 7% 3 4 3 3 5% 18%	2660
	11th to 20th	Night	21% 16% 7% 9% 7% 3 3 3 3 5% 23%	3288
	21st to end	Night	26% 19% 6% 8% 5% 3 3 3 3 4 22%	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	70% 6% 8% 12% 3	1518
	11th to 20th	Day	67% 5% 8% 13% 6%	1520
	21st to end	Day	58% 6% 10% 18% 7% 1	1664
February	1st to 10th	Day	65% 5% 8% 17% 5% 1	1510
	11th to 20th	Day	64% 5% 11% 12% 8% 1	1721
	21st to end	Day	61% 5% 8% 16% 8% 1	1480
March	1st to 10th	Day	59% 6% 10% 15% 8% 2	1800
	11th to 20th	Day	55% 5% 13% 13% 13% 2	1844
	21st to end	Day	54% 5% 9% 20% 10% 2	2371
April	1st to 10th	Day	60% 4 9% 16% 9% 2	2160
	11th to 20th	Day	49% 6% 9% 20% 14% 2	2158
	21st to end	Day	56% 6% 8% 16% 11% 3	2340
May	1st to 10th	Day	51% 4 10% 19% 13% 2	2425
	11th to 20th	Day	46% 6% 11% 21% 13% 2	2482
	21st to end	Day	50% 7% 11% 18% 13% 1	2725
June	1st to 10th	Day	51% 7% 12% 19% 9% 2	2520
	11th to 20th	Day	44% 7% 13% 22% 12% 1	2520
	21st to end	Day	54% 7% 10% 20% 9%	2520
July	1st to 10th	Day	43% 8% 15% 25% 9%	2450
	11th to 20th	Day	42% 8% 16% 26% 8%	2445
	21st to end	Day	46% 9% 16% 22% 7%	2591
August	1st to 10th	Day	47% 8% 18% 22% 5%	2492
	11th to 20th	Day	41% 9% 18% 24% 9%	2320
	21st to end	Day	41% 6% 16% 24% 12%	2376
September	1st to 10th	Day	41% 8% 15% 26% 10% 1	2160
	11th to 20th	Day	39% 5% 14% 31% 12%	2039
	21st to end	Day	43% 6% 15% 26% 10%	1980
October	1st to 10th	Day	50% 6% 13% 23% 7%	1924
	11th to 20th	Day	56% 6% 15% 18% 4	1706
	21st to end	Day	60% 6% 11% 15% 7% 1	1782
November	1st to 10th	Day	54% 5% 14% 19% 8%	1671
	11th to 20th	Day	59% 6% 11% 15% 9%	1688
	21st to end	Day	62% 7% 14% 14% 3	1520
December	1st to 10th	Day	71% 5% 9% 12% 4	1520
	11th to 20th	Day	63% 4 9% 18% 6%	1906
	21st to end	Day	70% 5% 9% 10% 6%	2772
January	1st to 10th	Night	69% 5% 9% 12% 4	2649
	11th to 20th	Night	68% 5% 8% 12% 6% 1	2660
	21st to end	Night	61% 5% 10% 15% 9% 1	2919
February	1st to 10th	Night	66% 4 9% 14% 7% 1	2448
	11th to 20th	Night	62% 4 9% 15% 9% 1	2135
	21st to end	Night	58% 5% 8% 15% 12% 1	1776
March	1st to 10th	Night	59% 4 9% 16% 9% 1	2160
	11th to 20th	Night	58% 5% 8% 13% 12% 3	2116
	21st to end	Night	53% 5% 7% 20% 12% 3	1980
April	1st to 10th	Night	58% 5% 9% 16% 12% 1	1800
	11th to 20th	Night	50% 5% 8% 20% 14% 2	1750
	21st to end	Night	53% 6% 9% 17% 13% 2	1620
May	1st to 10th	Night	54% 4 8% 16% 15% 3	1551
	11th to 20th	Night	49% 6% 11% 16% 15% 3	1416
	21st to end	Night	45% 7% 11% 19% 15% 3	1551
June	1st to 10th	Night	50% 6% 12% 21% 10% 1	1440
	11th to 20th	Night	45% 7% 10% 23% 12% 3	1440
	21st to end	Night	49% 8% 12% 20% 12% 1	1440
July	1st to 10th	Night	41% 8% 13% 23% 14% 1	1400
	11th to 20th	Night	41% 8% 14% 25% 12%	1392
	21st to end	Night	41% 9% 17% 24% 9%	1486
August	1st to 10th	Night	40% 8% 18% 26% 8%	1422
	11th to 20th	Night	42% 8% 17% 24% 8%	1561
	21st to end	Night	42% 8% 13% 26% 11% 1	1979
September	1st to 10th	Night	41% 7% 14% 25% 13% 1	1800
	11th to 20th	Night	38% 5% 14% 31% 10% 1	1920
	21st to end	Night	44% 6% 10% 26% 13% 1	1980
October	1st to 10th	Night	47% 6% 10% 25% 12%	2034
	11th to 20th	Night	55% 7% 11% 22% 6%	2254
	21st to end	Night	59% 6% 10% 15% 9%	2574
November	1st to 10th	Night	55% 6% 12% 19% 7% 1	2407
	11th to 20th	Night	57% 5% 12% 16% 8% 1	2489
	21st to end	Night	62% 5% 11% 16% 6%	2660
December	1st to 10th	Night	70% 5% 10% 11% 3	2660
	11th to 20th	Night	65% 6% 8% 15% 5%	3288
	21st to end	Night	68% 4 8% 15% 4	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	31% 13% 33% 19% 3 1	180
	11th to 20th	Day	31% 17% 31% 17% 4	180
	21st to end	Day	21% 17% 30% 21% 10% 1	196
February	1st to 10th	Day	35% 22% 34% 7% 1 1	192
	11th to 20th	Day	27% 18% 34% 16% 4	380
	21st to end	Day	24% 19% 34% 20% 3	346
March	1st to 10th	Day	26% 17% 29% 23% 5%	420
	11th to 20th	Day	26% 17% 30% 20% 6% 1	420
	21st to end	Day	29% 14% 28% 20% 8% 2	462
April	1st to 10th	Day	29% 14% 28% 19% 8% 2	420
	11th to 20th	Day	30% 17% 24% 17% 9% 2	412
	21st to end	Day	28% 15% 34% 14% 8% 2	420
May	1st to 10th	Day	28% 14% 31% 16% 9% 1	423
	11th to 20th	Day	19% 12% 32% 26% 9% 2	447
	21st to end	Day	21% 16% 31% 19% 11% 2	485
June	1st to 10th	Day	23% 14% 30% 23% 8% 2	450
	11th to 20th	Day	19% 11% 31% 23% 13% 2	450
	21st to end	Day	24% 9% 29% 22% 12% 3	450
July	1st to 10th	Day	16% 8% 26% 29% 17% 4	445
	11th to 20th	Day	15% 8% 30% 34% 10% 4	444
	21st to end	Day	18% 8% 27% 32% 14% 2	488
August	1st to 10th	Day	19% 8% 24% 27% 18% 5%	446
	11th to 20th	Day	17% 5% 23% 31% 20% 5%	439
	21st to end	Day	18% 11% 25% 22% 14% 10%	495
September	1st to 10th	Day	16% 9% 28% 27% 13% 7%	450
	11th to 20th	Day	20% 9% 23% 22% 18% 8%	450
	21st to end	Day	18% 10% 26% 24% 17% 5%	450
October	1st to 10th	Day	22% 10% 27% 25% 12% 5%	450
	11th to 20th	Day	25% 16% 27% 17% 13% 1	403
	21st to end	Day	36% 12% 25% 21% 7%	396
November	1st to 10th	Day	30% 12% 30% 21% 6% 1	365
	11th to 20th	Day	28% 19% 26% 18% 7% 2	348
	21st to end	Day	34% 19% 28% 12% 7% 1	180
December	1st to 10th	Day	35% 15% 22% 19% 6% 3	180
	11th to 20th	Day	29% 22% 28% 17% 3 1	180
	21st to end	Day	36% 21% 22% 15% 6% 1	198
January	1st to 10th	Night	30% 14% 30% 21% 6%	760
	11th to 20th	Night	28% 12% 33% 20% 7%	760
	21st to end	Night	20% 11% 34% 28% 6%	835
February	1st to 10th	Night	25% 11% 35% 23% 6%	720
	11th to 20th	Night	24% 14% 35% 21% 5% 1	703
	21st to end	Night	25% 12% 32% 26% 5%	592
March	1st to 10th	Night	25% 12% 34% 26% 4	720
	11th to 20th	Night	28% 12% 30% 23% 6% 1	720
	21st to end	Night	23% 11% 30% 27% 8% 1	792
April	1st to 10th	Night	31% 13% 24% 25% 7%	720
	11th to 20th	Night	26% 13% 28% 25% 7%	686
	21st to end	Night	29% 11% 30% 21% 8%	540
May	1st to 10th	Night	27% 10% 29% 26% 8%	471
	11th to 20th	Night	24% 7% 31% 31% 7% 1	354
	21st to end	Night	23% 5% 34% 30% 8%	389
June	1st to 10th	Night	26% 6% 29% 31% 8%	360
	11th to 20th	Night	21% 5% 27% 33% 14%	360
	21st to end	Night	18% 7% 30% 34% 11% 1	360
July	1st to 10th	Night	13% 5% 28% 39% 14% 1	350
	11th to 20th	Night	10% 4 25% 45% 15% 1	348
	21st to end	Night	15% 5% 23% 45% 12% 1	394
August	1st to 10th	Night	17% 3 20% 39% 20% 1	358
	11th to 20th	Night	17% 4 22% 34% 21% 2	497
	21st to end	Night	17% 5% 23% 33% 18% 3	791
September	1st to 10th	Night	18% 4 24% 37% 14% 2	720
	11th to 20th	Night	20% 6% 22% 31% 20% 2	720
	21st to end	Night	21% 6% 27% 28% 16% 2	720
October	1st to 10th	Night	21% 9% 26% 31% 12% 1	720
	11th to 20th	Night	28% 9% 25% 27% 10% 1	720
	21st to end	Night	32% 11% 30% 23% 5%	792
November	1st to 10th	Night	28% 12% 29% 23% 8%	740
	11th to 20th	Night	26% 15% 29% 19% 10% 1	760
	21st to end	Night	32% 14% 27% 21% 6% 1	760
December	1st to 10th	Night	34% 13% 26% 21% 6%	760
	11th to 20th	Night	28% 15% 29% 21% 6% 1	760
	21st to end	Night	37% 13% 25% 20% 5%	836

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