Trottier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Trottier Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	26% 10% 3 7% 4 4 4 3 4 5% 30%	117585
	Night	30% 10% 3 6% 4 3 3 3 4 5% 29%	120412
Transparency	Day	32% 42% 8% 11% 5% 1	117334
	Night	36% 41% 8% 10% 4 1	120309
Seeing	Day	32% 8% 20% 24% 13% 4	15980
	Night	37% 5% 14% 25% 17% 3	24301

By Month

Percentage Cloud Cover NW (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	42% 12% 3 8% 5% 4 4 4 4 4 11%	3179
	11th to 20th	Day	22% 11% 4 6% 4 4 4 3 4 7% 32%	3159
	21st to end	Day	48% 7% 2 5% 3 2 2 2 2 3 24%	4020
February	1st to 10th	Day	34% 12% 4 8% 5% 5% 3 3 5% 5% 17%	1800
	11th to 20th	Day	33% 13% 3 7% 4 4 3 2 4 5% 22%	1755
	21st to end	Day	28% 15% 3 9% 6% 5% 4 2 4 4 20%	1504
March	1st to 10th	Day	29% 18% 4 9% 5% 4 4 3 4 4 16%	4663
	11th to 20th	Day	28% 15% 4 8% 4 4 3 2 4 4 24%	5000
	21st to end	Day	34% 14% 3 8% 5% 4 4 2 4 5% 15%	5634
April	1st to 10th	Day	31% 13% 3 7% 6% 5% 5% 4 5% 5% 16%	2400
	11th to 20th	Day	24% 12% 3 8% 5% 5% 4 4 4 5% 25%	2469
	21st to end	Day	27% 11% 4 6% 5% 4 4 4 5% 5% 24%	2664
May	1st to 10th	Day	18% 10% 3 6% 4 3 4 4 5% 5% 38%	4560
	11th to 20th	Day	15% 8% 2 7% 4 5% 4 5% 7% 8% 34%	4517
	21st to end	Day	30% 10% 3 8% 5% 4 4 4 5% 6% 21%	4977
June	1st to 10th	Day	26% 9% 3 7% 5% 5% 4 4 5% 6% 26%	2850
	11th to 20th	Day	25% 11% 4 8% 6% 5% 5% 3 5% 7% 22%	2850
	21st to end	Day	18% 9% 2 7% 4 5% 5% 3 5% 7% 34%	2850
July	1st to 10th	Day	8% 3 2 4 3 3 3 3 3 5% 62%	4740
	11th to 20th	Day	7% 3 1 3 2 2 3 2 3 5% 68%	4740
	21st to end	Day	14% 5% 2 4 3 3 3 3 5% 7% 51%	4867
August	1st to 10th	Day	7% 3 2 3 2 2 2 2 4 5% 66%	4376
	11th to 20th	Day	15% 7% 3 8% 4 4 4 3 5% 6% 41%	4026
	21st to end	Day	20% 6% 2 6% 3 4 4 3 5% 7% 40%	4411
September	1st to 10th	Day	17% 7% 2 6% 4 3 4 3 5% 6% 44%	2280
	11th to 20th	Day	21% 8% 3 7% 5% 4 3 3 4 5% 37%	2280
	21st to end	Day	29% 7% 1 5% 3 4 4 4 5% 5% 34%	1997
October	1st to 10th	Day	27% 8% 3 8% 4 5% 4 3 5% 5% 28%	3180
	11th to 20th	Day	39% 7% 2 5% 4 4 4 3 5% 6% 20%	3161
	21st to end	Day	34% 13% 4 8% 5% 4 4 3 5% 5% 14%	3178
November	1st to 10th	Day	43% 10% 3 7% 5% 4 3 2 4 4 15%	1568
	11th to 20th	Day	43% 11% 3 6% 4 3 3 3 3 3 19%	1600
	21st to end	Day	42% 10% 3 6% 3 3 4 3 4 4 18%	1600
December	1st to 10th	Day	26% 11% 3 6% 5% 4 5% 4 5% 10% 21%	2587
	11th to 20th	Day	47% 15% 4 7% 4 3 3 2 3 3 10%	2645
	21st to end	Day	47% 15% 4 8% 4 4 3 2 3 3 8%	3498
January	1st to 10th	Night	41% 14% 4 8% 4 3 3 2 3 4 15%	6766
	11th to 20th	Night	24% 8% 3 6% 3 3 4 3 5% 9% 32%	6739
	21st to end	Night	43% 9% 2 4 2 2 2 2 3 4 27%	6936
February	1st to 10th	Night	34% 12% 3 7% 5% 4 4 3 4 4 20%	2600
	11th to 20th	Night	30% 13% 3 8% 4 3 3 3 4 4 24%	2532
	21st to end	Night	31% 13% 3 7% 3 3 3 3 5% 4 24%	2125
March	1st to 10th	Night	28% 17% 3 7% 4 4 4 3 4 4 21%	5227
	11th to 20th	Night	29% 14% 3 6% 4 3 3 3 4 5% 26%	4999
	21st to end	Night	41% 13% 4 7% 4 3 3 2 3 3 17%	5341
April	1st to 10th	Night	30% 12% 4 8% 5% 4 3 3 4 5% 22%	2000
	11th to 20th	Night	22% 13% 3 8% 5% 4 4 4 4 6% 29%	1879
	21st to end	Night	25% 10% 4 7% 5% 4 4 3 5% 6% 27%	1735
May	1st to 10th	Night	18% 8% 2 5% 4 3 4 3 4 4 46%	2620
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 6% 8% 44%	2596
	21st to end	Night	26% 11% 4 7% 5% 4 4 3 4 5% 27%	2709
June	1st to 10th	Night	25% 7% 3 6% 5% 5% 5% 4 5% 5% 31%	1330
	11th to 20th	Night	19% 11% 4 8% 5% 4 5% 3 7% 8% 28%	1330
	21st to end	Night	16% 7% 2 7% 5% 4 4 4 5% 7% 40%	1330
July	1st to 10th	Night	6% 3 1 4 3 3 3 3 4 5% 66%	2220
	11th to 20th	Night	6% 2 1 2 2 2 2 2 3 5% 74%	2220
	21st to end	Night	10% 4 2 4 3 3 3 3 6% 6% 55%	2682
August	1st to 10th	Night	5% 3 1 2 1 2 2 2 4 6% 72%	2552
	11th to 20th	Night	11% 6% 3 6% 3 4 3 3 5% 6% 49%	2812
	21st to end	Night	15% 7% 3 6% 4 3 4 3 5% 7% 42%	3260
September	1st to 10th	Night	10% 4 2 6% 4 3 4 3 4 6% 54%	1900
	11th to 20th	Night	22% 6% 2 5% 4 4 3 3 4 5% 42%	1900
	21st to end	Night	24% 8% 3 6% 3 3 3 2 4 5% 37%	2179
October	1st to 10th	Night	24% 10% 3 7% 5% 6% 5% 4 5% 5% 28%	3800
	11th to 20th	Night	35% 9% 3 6% 3 3 3 3 4 5% 27%	3777
	21st to end	Night	36% 12% 4 7% 3 4 3 3 4 4 21%	4380
November	1st to 10th	Night	43% 11% 2 6% 4 4 4 2 3 4 17%	2730
	11th to 20th	Night	41% 10% 3 6% 4 3 3 2 3 3 22%	2800
	21st to end	Night	38% 11% 2 7% 4 3 4 3 3 4 21%	2800
December	1st to 10th	Night	29% 10% 3 6% 4 4 4 4 6% 9% 22%	4588
	11th to 20th	Night	47% 14% 3 7% 4 4 3 2 3 3 11%	5560
	21st to end	Night	47% 16% 4 7% 3 3 3 2 2 3 11%	7458

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	58% 27% 7% 7% 1	3179
	11th to 20th	Day	36% 28% 13% 14% 6% 2	3180
	21st to end	Day	50% 27% 5% 14% 4	4020
February	1st to 10th	Day	26% 62% 3 4 2 3	1800
	11th to 20th	Day	27% 61% 4 6% 1 1	1764
	21st to end	Day	25% 62% 5% 5% 2 1	1504
March	1st to 10th	Day	52% 29% 8% 9% 2	4663
	11th to 20th	Day	44% 31% 7% 16% 1	5000
	21st to end	Day	53% 30% 10% 5% 2	5634
April	1st to 10th	Day	26% 64% 5% 6% 1	2400
	11th to 20th	Day	23% 62% 5% 7% 2 1	2469
	21st to end	Day	22% 63% 5% 7% 3	2664
May	1st to 10th	Day	26% 38% 10% 17% 8%	4560
	11th to 20th	Day	28% 40% 16% 14% 2	4520
	21st to end	Day	41% 39% 12% 7% 1	4978
June	1st to 10th	Day	19% 65% 6% 7% 2	2850
	11th to 20th	Day	19% 66% 6% 7% 3	2850
	21st to end	Day	17% 64% 8% 8% 3	2850
July	1st to 10th	Day	13% 36% 5% 18% 20% 8%	4666
	11th to 20th	Day	8% 35% 7% 30% 18% 1	4659
	21st to end	Day	22% 38% 16% 16% 6% 1	4713
August	1st to 10th	Day	9% 37% 8% 33% 13% 1	4359
	11th to 20th	Day	27% 42% 17% 10% 4	4065
	21st to end	Day	29% 40% 11% 11% 8%	4410
September	1st to 10th	Day	20% 61% 5% 9% 4 1	2280
	11th to 20th	Day	22% 60% 5% 6% 6%	2280
	21st to end	Day	21% 60% 6% 8% 4	1997
October	1st to 10th	Day	38% 39% 11% 10% 2	3180
	11th to 20th	Day	41% 38% 8% 8% 4	3161
	21st to end	Day	44% 41% 6% 8% 2	3178
November	1st to 10th	Day	31% 57% 5% 5% 1	1568
	11th to 20th	Day	29% 57% 5% 6% 2 2	1600
	21st to end	Day	27% 57% 4 6% 4 1	1600
December	1st to 10th	Day	36% 40% 8% 13% 2	2587
	11th to 20th	Day	58% 31% 4 4 1	2648
	21st to end	Day	64% 27% 5% 4 1	3498
January	1st to 10th	Night	57% 26% 6% 9% 2	6766
	11th to 20th	Night	32% 31% 14% 16% 6% 2	6780
	21st to end	Night	47% 29% 9% 13% 2	6936
February	1st to 10th	Night	26% 62% 3 4 3 3	2600
	11th to 20th	Night	27% 61% 4 4 3 1	2534
	21st to end	Night	21% 62% 5% 7% 2 2	2125
March	1st to 10th	Night	47% 31% 6% 10% 6%	5227
	11th to 20th	Night	41% 30% 10% 16% 3	5000
	21st to end	Night	55% 29% 10% 5% 1	5341
April	1st to 10th	Night	25% 62% 4 7% 1	2000
	11th to 20th	Night	21% 63% 5% 7% 3 1	1879
	21st to end	Night	21% 61% 7% 6% 4	1735
May	1st to 10th	Night	25% 38% 9% 19% 8% 1	2620
	11th to 20th	Night	21% 41% 19% 15% 5%	2596
	21st to end	Night	40% 38% 12% 8% 2	2709
June	1st to 10th	Night	18% 65% 7% 8% 2 1	1330
	11th to 20th	Night	17% 65% 8% 8% 3	1330
	21st to end	Night	15% 65% 9% 7% 4	1330
July	1st to 10th	Night	9% 36% 5% 24% 20% 6%	2185
	11th to 20th	Night	6% 35% 6% 33% 19% 1	2178
	21st to end	Night	22% 37% 11% 19% 11% 1	2597
August	1st to 10th	Night	6% 36% 9% 30% 18% 1	2544
	11th to 20th	Night	26% 40% 19% 11% 3 1	2839
	21st to end	Night	27% 40% 13% 12% 7%	3260
September	1st to 10th	Night	16% 61% 8% 8% 6% 1	1900
	11th to 20th	Night	21% 60% 6% 6% 5% 1	1900
	21st to end	Night	20% 60% 7% 9% 4	2179
October	1st to 10th	Night	37% 41% 12% 8% 2	3800
	11th to 20th	Night	41% 38% 6% 11% 4	3777
	21st to end	Night	42% 40% 7% 9% 3	4380
November	1st to 10th	Night	32% 56% 5% 5% 2	2730
	11th to 20th	Night	29% 57% 4 6% 3 2	2800
	21st to end	Night	29% 57% 4 4 6%	2800
December	1st to 10th	Night	38% 43% 10% 6% 3	4588
	11th to 20th	Night	56% 33% 5% 5% 1	5556
	21st to end	Night	60% 28% 5% 6% 2	7458

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	48% 6% 13% 19% 8% 7%	200
	11th to 20th	Day	47% 10% 14% 14% 14% 2	200
	21st to end	Day	51% 5% 13% 14% 14% 3	238
February	1st to 10th	Day	43% 8% 21% 18% 9% 1	400
	11th to 20th	Day	41% 7% 21% 21% 9% 2	395
	21st to end	Day	38% 11% 23% 19% 8% 1	341
March	1st to 10th	Day	38% 11% 21% 20% 10% 2	500
	11th to 20th	Day	41% 9% 15% 19% 15% 2	500
	21st to end	Day	43% 11% 22% 15% 7% 2	550
April	1st to 10th	Day	39% 13% 18% 19% 10% 1	500
	11th to 20th	Day	32% 12% 22% 20% 10% 3	492
	21st to end	Day	35% 11% 24% 19% 8% 3	500
May	1st to 10th	Day	26% 10% 24% 27% 12% 2	503
	11th to 20th	Day	30% 8% 20% 24% 13% 4	528
	21st to end	Day	25% 9% 25% 26% 11% 4	602
June	1st to 10th	Day	31% 8% 24% 26% 10% 1	680
	11th to 20th	Day	31% 11% 25% 24% 7% 2	680
	21st to end	Day	25% 7% 23% 28% 15% 3	680
July	1st to 10th	Day	19% 6% 22% 36% 15% 3	671
	11th to 20th	Day	13% 7% 27% 35% 14% 4	668
	21st to end	Day	13% 5% 19% 39% 18% 6%	582
August	1st to 10th	Day	17% 5% 18% 31% 20% 8%	487
	11th to 20th	Day	14% 4 18% 35% 22% 6%	477
	21st to end	Day	20% 8% 23% 28% 15% 6%	539
September	1st to 10th	Day	21% 7% 26% 27% 15% 4	490
	11th to 20th	Day	30% 7% 17% 27% 17% 3	490
	21st to end	Day	33% 6% 19% 22% 15% 5%	490
October	1st to 10th	Day	35% 4 19% 19% 18% 4	490
	11th to 20th	Day	43% 6% 15% 16% 15% 5%	490
	21st to end	Day	47% 7% 16% 17% 9% 4	397
November	1st to 10th	Day	43% 13% 17% 15% 12% 1	200
	11th to 20th	Day	51% 6% 11% 12% 13% 9%	200
	21st to end	Day	49% 9% 14% 14% 7% 9%	200
December	1st to 10th	Day	47% 7% 13% 9% 18% 8%	200
	11th to 20th	Day	54% 6% 9% 17% 11% 4	200
	21st to end	Day	49% 7% 15% 22% 6% 2	220
January	1st to 10th	Night	54% 6% 12% 18% 8% 1	900
	11th to 20th	Night	44% 6% 15% 24% 11% 1	900
	21st to end	Night	46% 5% 14% 22% 11% 1	887
February	1st to 10th	Night	41% 6% 17% 27% 9% 1	800
	11th to 20th	Night	38% 7% 18% 25% 12%	784
	21st to end	Night	39% 7% 18% 23% 11% 1	660
March	1st to 10th	Night	40% 7% 16% 21% 14% 1	800
	11th to 20th	Night	46% 7% 14% 21% 13% 1	800
	21st to end	Night	45% 5% 13% 22% 12% 2	880
April	1st to 10th	Night	40% 5% 13% 28% 12% 2	800
	11th to 20th	Night	32% 5% 18% 29% 13% 2	691
	21st to end	Night	36% 4 15% 30% 14% 2	535
May	1st to 10th	Night	21% 4 16% 36% 23% 2	400
	11th to 20th	Night	25% 4 10% 30% 27% 4	394
	21st to end	Night	25% 3 12% 35% 24% 1	422
June	1st to 10th	Night	29% 3 13% 31% 23% 1	380
	11th to 20th	Night	26% 5% 18% 30% 19% 2	380
	21st to end	Night	21% 3 14% 31% 27% 4	380
July	1st to 10th	Night	13% 2 11% 34% 38% 2	370
	11th to 20th	Night	11% 2 7% 32% 42% 6%	368
	21st to end	Night	8% 2 7% 31% 44% 7%	416
August	1st to 10th	Night	14% 2 3 28% 43% 11%	388
	11th to 20th	Night	11% 1 4 28% 45% 11%	564
	21st to end	Night	18% 3 11% 30% 35% 4	689
September	1st to 10th	Night	15% 2 12% 36% 31% 4	760
	11th to 20th	Night	27% 2 10% 22% 31% 7%	760
	21st to end	Night	33% 3 14% 27% 21% 3	760
October	1st to 10th	Night	31% 3 15% 25% 22% 4	760
	11th to 20th	Night	42% 2 13% 24% 15% 4	760
	21st to end	Night	45% 2 17% 24% 9% 3	836
November	1st to 10th	Night	51% 5% 16% 21% 7%	780
	11th to 20th	Night	51% 4 15% 18% 11% 1	800
	21st to end	Night	46% 5% 16% 19% 11% 3	800
December	1st to 10th	Night	39% 5% 16% 27% 13% 2	807
	11th to 20th	Night	53% 8% 14% 19% 5% 1	900
	21st to end	Night	53% 7% 17% 17% 6%	990

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