Terrebonne
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Terrebonne clear sky chart from 2005-11-06 to 2026-03-18.

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% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	131216
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	130266
Transparency	Day	51% 4 12% 21% 10% 1	134661
	Night	54% 4 11% 20% 10% 1	134908
Seeing	Day	20% 10% 28% 25% 13% 4	16180
	Night	22% 7% 29% 31% 10% 1	25270

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% 17% 6% 12% 5% 4 4 3 4 5% 26%	4661
	11th to 20th	Day	11% 16% 6% 13% 7% 5% 4 3 4 6% 24%	4650
	21st to end	Day	10% 13% 5% 11% 8% 7% 5% 4 5% 6% 26%	5124
February	1st to 10th	Day	14% 15% 4 8% 7% 5% 4 3 5% 6% 29%	1832
	11th to 20th	Day	13% 12% 5% 10% 6% 5% 5% 4 6% 6% 28%	1953
	21st to end	Day	16% 16% 4 10% 5% 4 4 3 5% 5% 27%	1730
March	1st to 10th	Day	19% 12% 4 10% 6% 6% 5% 3 4 5% 26%	4576
	11th to 20th	Day	12% 14% 4 9% 5% 4 4 3 5% 7% 32%	4760
	21st to end	Day	19% 9% 4 10% 5% 4 4 3 4 4 34%	5970
April	1st to 10th	Day	21% 12% 3 7% 5% 4 4 3 3 5% 32%	2400
	11th to 20th	Day	21% 8% 3 7% 5% 4 3 3 4 5% 37%	2374
	21st to end	Day	21% 9% 3 8% 4 4 4 3 4 5% 32%	2570
May	1st to 10th	Day	26% 9% 3 9% 5% 5% 4 4 5% 6% 25%	6254
	11th to 20th	Day	21% 8% 3 8% 5% 5% 5% 3 6% 7% 31%	6318
	21st to end	Day	22% 7% 3 6% 5% 4 5% 4 6% 7% 31%	6949
June	1st to 10th	Day	21% 8% 2 7% 6% 5% 5% 4 5% 7% 31%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 4 6% 4 6% 8% 38%	2800
	21st to end	Day	19% 8% 3 8% 6% 5% 5% 4 5% 7% 31%	2800
July	1st to 10th	Day	14% 5% 2 8% 6% 6% 6% 5% 6% 8% 34%	6360
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	6360
	21st to end	Day	10% 5% 2 5% 3 4 4 4 6% 9% 47%	6926
August	1st to 10th	Day	14% 5% 2 8% 5% 5% 6% 5% 6% 9% 35%	4563
	11th to 20th	Day	14% 6% 2 8% 5% 4 5% 4 6% 8% 38%	4133
	21st to end	Day	16% 5% 2 6% 4 4 4 4 6% 8% 40%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 4 6% 3 6% 9% 43%	2400
	11th to 20th	Day	14% 4 2 4 4 3 4 3 4 7% 51%	2311
	21st to end	Day	16% 7% 2 6% 4 5% 5% 2 5% 7% 42%	2200
October	1st to 10th	Day	19% 7% 2 7% 4 4 4 3 5% 7% 38%	2070
	11th to 20th	Day	19% 7% 3 9% 5% 6% 4 4 6% 6% 31%	1950
	21st to end	Day	23% 9% 3 7% 4 4 5% 3 5% 6% 32%	1980
November	1st to 10th	Day	16% 11% 3 7% 3 4 4 4 6% 6% 34%	1851
	11th to 20th	Day	18% 11% 3 8% 5% 4 4 3 5% 7% 32%	1805
	21st to end	Day	23% 12% 4 7% 5% 5% 5% 4 4 6% 26%	1680
December	1st to 10th	Day	20% 17% 6% 11% 5% 5% 4 3 4 4 20%	2680
	11th to 20th	Day	19% 13% 4 9% 5% 4 4 4 5% 4 29%	3066
	21st to end	Day	23% 13% 3 10% 5% 4 5% 3 5% 5% 23%	4048
January	1st to 10th	Night	13% 16% 7% 15% 6% 4 4 3 3 4 23%	8234
	11th to 20th	Night	11% 16% 6% 16% 8% 5% 5% 3 4 5% 20%	8235
	21st to end	Night	11% 12% 5% 13% 9% 6% 5% 4 5% 5% 27%	9057
February	1st to 10th	Night	13% 14% 5% 12% 6% 5% 4 4 5% 5% 25%	2783
	11th to 20th	Night	15% 12% 5% 10% 5% 5% 4 4 5% 5% 31%	2551
	21st to end	Night	15% 14% 5% 8% 6% 5% 4 3 5% 5% 31%	2076
March	1st to 10th	Night	22% 13% 3 8% 5% 4 4 3 4 5% 30%	5484
	11th to 20th	Night	14% 14% 4 8% 5% 4 5% 4 5% 7% 31%	5336
	21st to end	Night	16% 11% 3 9% 5% 5% 5% 3 4 5% 33%	4962
April	1st to 10th	Night	24% 9% 3 7% 4 3 4 3 5% 6% 31%	2000
	11th to 20th	Night	22% 9% 3 7% 4 4 4 3 4 5% 35%	1975
	21st to end	Night	22% 9% 2 6% 4 3 3 3 5% 6% 36%	1830
May	1st to 10th	Night	26% 9% 3 9% 5% 4 5% 4 4 6% 25%	3715
	11th to 20th	Night	23% 9% 3 8% 4 3 5% 3 5% 7% 31%	3575
	21st to end	Night	19% 10% 3 8% 5% 5% 4 4 5% 7% 31%	3926
June	1st to 10th	Night	21% 8% 2 6% 5% 4 4 4 5% 8% 33%	1600
	11th to 20th	Night	16% 7% 3 6% 4 4 5% 4 7% 8% 39%	1600
	21st to end	Night	19% 7% 2 6% 6% 4 5% 4 6% 7% 34%	1600
July	1st to 10th	Night	13% 5% 3 8% 5% 6% 6% 5% 7% 9% 35%	3600
	11th to 20th	Night	14% 5% 2 6% 4 5% 5% 4 6% 7% 42%	3600
	21st to end	Night	10% 5% 2 7% 5% 5% 4 4 6% 8% 45%	3926
August	1st to 10th	Night	14% 6% 3 8% 5% 4 5% 4 5% 7% 38%	2586
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 4 6% 7% 39%	2864
	21st to end	Night	17% 5% 3 7% 5% 5% 4 4 5% 7% 39%	3586
September	1st to 10th	Night	12% 4 2 7% 5% 4 4 4 5% 7% 46%	2000
	11th to 20th	Night	13% 6% 2 5% 3 2 3 3 5% 7% 51%	2087
	21st to end	Night	16% 6% 2 7% 4 4 4 3 5% 6% 45%	2200
October	1st to 10th	Night	18% 6% 2 6% 3 4 5% 3 6% 6% 42%	2329
	11th to 20th	Night	19% 8% 2 7% 5% 4 3 4 4 6% 37%	2450
	21st to end	Night	22% 8% 3 8% 4 4 4 3 5% 6% 32%	2860
November	1st to 10th	Night	20% 9% 4 7% 4 4 4 3 6% 6% 33%	2667
	11th to 20th	Night	19% 10% 3 8% 5% 4 4 3 5% 5% 34%	2814
	21st to end	Night	22% 13% 4 8% 6% 4 4 3 3 5% 29%	2940
December	1st to 10th	Night	17% 16% 6% 11% 7% 6% 5% 4 5% 5% 20%	4720
	11th to 20th	Night	23% 13% 4 9% 5% 4 3 3 5% 5% 26%	5348
	21st to end	Night	24% 15% 5% 9% 4 4 3 3 5% 4 24%	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	63% 3 11% 18% 4	4662
	11th to 20th	Day	63% 4 8% 15% 10% 1	4680
	21st to end	Day	59% 4 11% 14% 11% 1	5124
February	1st to 10th	Day	57% 4 10% 20% 7% 2	1832
	11th to 20th	Day	57% 6% 11% 16% 10% 1	1962
	21st to end	Day	59% 3 9% 16% 11% 2	1730
March	1st to 10th	Day	61% 4 7% 14% 11% 4	4579
	11th to 20th	Day	52% 5% 17% 14% 11% 2	4760
	21st to end	Day	53% 3 7% 17% 18% 2	5970
April	1st to 10th	Day	55% 4 7% 19% 13% 2	2400
	11th to 20th	Day	50% 4 7% 22% 16% 2	2374
	21st to end	Day	54% 5% 8% 17% 13% 3	2570
May	1st to 10th	Day	61% 4 8% 19% 7% 1	6265
	11th to 20th	Day	53% 5% 14% 18% 8% 2	6322
	21st to end	Day	51% 5% 12% 19% 12% 1	6949
June	1st to 10th	Day	51% 6% 10% 19% 12% 2	2800
	11th to 20th	Day	43% 5% 11% 24% 13% 3	2800
	21st to end	Day	51% 5% 12% 19% 10% 1	2800
July	1st to 10th	Day	47% 4 14% 26% 8%	6290
	11th to 20th	Day	40% 4 15% 31% 11%	6285
	21st to end	Day	34% 4 15% 37% 9%	6728
August	1st to 10th	Day	46% 4 18% 23% 8%	4549
	11th to 20th	Day	43% 4 19% 27% 7%	4175
	21st to end	Day	43% 4 16% 22% 15% 1	4312
September	1st to 10th	Day	37% 6% 14% 31% 12% 1	2400
	11th to 20th	Day	34% 4 13% 33% 15% 1	2311
	21st to end	Day	43% 5% 14% 26% 12% 1	2200
October	1st to 10th	Day	44% 4 14% 28% 10%	3369
	11th to 20th	Day	44% 4 17% 27% 7%	3153
	21st to end	Day	44% 3 20% 24% 9% 1	3180
November	1st to 10th	Day	49% 4 13% 24% 8% 1	1851
	11th to 20th	Day	52% 5% 11% 19% 11% 1	1805
	21st to end	Day	59% 6% 14% 15% 6% 1	1680
December	1st to 10th	Day	69% 3 8% 16% 3	2680
	11th to 20th	Day	59% 4 9% 18% 10%	3066
	21st to end	Day	65% 3 12% 14% 7%	4048
January	1st to 10th	Night	66% 3 11% 14% 5% 1	8234
	11th to 20th	Night	67% 4 9% 14% 6%	8280
	21st to end	Night	60% 3 8% 15% 13%	9057
February	1st to 10th	Night	61% 4 9% 17% 8% 1	2783
	11th to 20th	Night	54% 4 11% 21% 10% 1	2553
	21st to end	Night	55% 5% 8% 18% 13% 1	2076
March	1st to 10th	Night	58% 3 8% 14% 12% 4	5494
	11th to 20th	Night	53% 4 13% 15% 13% 2	5337
	21st to end	Night	54% 3 5% 22% 13% 2	4962
April	1st to 10th	Night	54% 4 7% 20% 14% 2	2000
	11th to 20th	Night	52% 4 7% 21% 14% 2	1975
	21st to end	Night	50% 4 8% 20% 14% 4	1830
May	1st to 10th	Night	61% 4 9% 17% 7% 2	3721
	11th to 20th	Night	55% 4 10% 19% 10% 2	3576
	21st to end	Night	54% 3 10% 18% 14% 1	3927
June	1st to 10th	Night	50% 6% 10% 20% 14% 1	1600
	11th to 20th	Night	43% 5% 13% 22% 14% 3	1600
	21st to end	Night	48% 7% 12% 19% 14% 1	1600
July	1st to 10th	Night	46% 3 18% 23% 10%	3560
	11th to 20th	Night	41% 3 18% 28% 9%	3552
	21st to end	Night	37% 4 17% 30% 13%	3809
August	1st to 10th	Night	47% 4 13% 27% 9%	2576
	11th to 20th	Night	44% 4 19% 25% 8%	2888
	21st to end	Night	45% 5% 13% 22% 13% 1	3586
September	1st to 10th	Night	37% 6% 13% 28% 15% 1	2000
	11th to 20th	Night	35% 5% 14% 34% 13% 1	2087
	21st to end	Night	41% 5% 11% 27% 16%	2200
October	1st to 10th	Night	44% 4 13% 27% 11% 1	3810
	11th to 20th	Night	44% 3 12% 32% 9%	3986
	21st to end	Night	47% 3 16% 20% 12% 2	4610
November	1st to 10th	Night	52% 5% 12% 23% 8% 1	2667
	11th to 20th	Night	52% 4 12% 20% 10% 1	2814
	21st to end	Night	60% 4 11% 18% 7%	2940
December	1st to 10th	Night	67% 3 12% 13% 4	4720
	11th to 20th	Night	61% 4 8% 15% 11%	5348
	21st to end	Night	64% 3 8% 15% 10%	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	23% 14% 26% 28% 9%	220
	11th to 20th	Day	25% 14% 34% 21% 5% 1	220
	21st to end	Day	17% 14% 34% 27% 7% 1	242
February	1st to 10th	Day	27% 15% 34% 20% 3	300
	11th to 20th	Day	20% 15% 35% 25% 5%	440
	21st to end	Day	25% 13% 33% 25% 4	446
March	1st to 10th	Day	23% 14% 33% 22% 8%	540
	11th to 20th	Day	23% 13% 34% 24% 5% 1	528
	21st to end	Day	26% 12% 32% 20% 9% 2	550
April	1st to 10th	Day	26% 17% 26% 20% 8% 2	500
	11th to 20th	Day	27% 17% 23% 21% 9% 2	492
	21st to end	Day	27% 14% 29% 20% 7% 3	500
May	1st to 10th	Day	24% 10% 31% 24% 10% 2	503
	11th to 20th	Day	20% 8% 30% 26% 13% 4	527
	21st to end	Day	20% 11% 29% 26% 13% 2	573
June	1st to 10th	Day	22% 11% 26% 27% 12% 2	530
	11th to 20th	Day	16% 10% 26% 29% 15% 3	530
	21st to end	Day	20% 9% 28% 25% 14% 3	530
July	1st to 10th	Day	13% 6% 25% 30% 20% 5%	525
	11th to 20th	Day	14% 4 28% 32% 18% 4	524
	21st to end	Day	12% 5% 24% 34% 19% 5%	576
August	1st to 10th	Day	13% 4 22% 32% 21% 7%	526
	11th to 20th	Day	12% 4 19% 30% 23% 11%	519
	21st to end	Day	15% 6% 25% 24% 21% 10%	583
September	1st to 10th	Day	12% 6% 27% 27% 19% 9%	530
	11th to 20th	Day	12% 7% 18% 27% 21% 14%	530
	21st to end	Day	18% 8% 25% 24% 19% 7%	530
October	1st to 10th	Day	19% 9% 27% 24% 15% 6%	530
	11th to 20th	Day	18% 12% 32% 24% 11% 3	502
	21st to end	Day	30% 10% 25% 25% 10%	462
November	1st to 10th	Day	26% 13% 28% 22% 11% 1	425
	11th to 20th	Day	24% 13% 31% 22% 9% 1	345
	21st to end	Day	32% 13% 27% 19% 7% 1	220
December	1st to 10th	Day	28% 10% 29% 22% 10% 2	220
	11th to 20th	Day	25% 12% 39% 19% 4 1	220
	21st to end	Day	29% 14% 31% 18% 8% 1	242
January	1st to 10th	Night	26% 9% 32% 27% 6%	840
	11th to 20th	Night	24% 11% 32% 26% 7%	840
	21st to end	Night	19% 8% 35% 30% 8% 1	924
February	1st to 10th	Night	20% 7% 36% 30% 7% 1	840
	11th to 20th	Night	21% 10% 36% 27% 5%	823
	21st to end	Night	23% 8% 30% 34% 4	692
March	1st to 10th	Night	24% 9% 34% 28% 6%	840
	11th to 20th	Night	26% 10% 32% 25% 7%	829
	21st to end	Night	22% 9% 30% 31% 7%	880
April	1st to 10th	Night	27% 10% 27% 29% 6%	800
	11th to 20th	Night	25% 11% 31% 27% 6% 1	791
	21st to end	Night	27% 7% 33% 24% 8%	630
May	1st to 10th	Night	24% 7% 32% 29% 7% 1	456
	11th to 20th	Night	23% 5% 30% 34% 8% 1	394
	21st to end	Night	19% 5% 35% 32% 9%	433
June	1st to 10th	Night	24% 5% 24% 36% 11%	400
	11th to 20th	Night	18% 5% 27% 38% 12% 1	400
	21st to end	Night	22% 4 26% 34% 15% 1	400
July	1st to 10th	Night	11% 5% 26% 41% 17% 1	390
	11th to 20th	Night	11% 3 24% 45% 16% 1	388
	21st to end	Night	13% 3 20% 47% 17% 1	438
August	1st to 10th	Night	14% 4 12% 47% 22% 1	398
	11th to 20th	Night	15% 2 20% 35% 25% 2	580
	21st to end	Night	17% 2 22% 36% 22% 2	880
September	1st to 10th	Night	14% 3 23% 40% 18% 1	800
	11th to 20th	Night	15% 5% 17% 37% 23% 3	800
	21st to end	Night	18% 4 25% 34% 17% 2	800
October	1st to 10th	Night	20% 6% 27% 33% 14% 1	800
	11th to 20th	Night	24% 7% 27% 31% 11% 1	800
	21st to end	Night	26% 8% 30% 30% 7%	880
November	1st to 10th	Night	24% 10% 28% 29% 8%	820
	11th to 20th	Night	23% 9% 32% 26% 10% 1	840
	21st to end	Night	28% 10% 30% 22% 8%	840
December	1st to 10th	Night	28% 9% 31% 25% 7%	840
	11th to 20th	Night	27% 11% 33% 23% 6% 1	840
	21st to end	Night	32% 11% 29% 21% 7%	924

Page generated at 2026-03-15 02:23:50. Version ac07

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