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 2026-09-29.

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	23% 9% 3 6% 4 4 4 3 5% 6% 33%	153745
	Night	28% 10% 3 6% 4 3 3 3 4 5% 31%	146406
Transparency	Day	34% 34% 12% 14% 5% 1	153453
	Night	37% 35% 10% 12% 5% 1	146280
Seeing	Day	32% 9% 21% 23% 12% 3	17837
	Night	36% 5% 14% 25% 17% 2	25896

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	44% 13% 3 8% 5% 4 4 3 3 4 9%	4125
	11th to 20th	Day	23% 9% 3 5% 3 4 3 3 5% 9% 32%	4119
	21st to end	Day	45% 7% 2 5% 3 3 3 2 3 4 24%	5202
February	1st to 10th	Day	34% 12% 4 8% 5% 5% 3 3 5% 5% 16%	1890
	11th to 20th	Day	32% 13% 3 7% 4 4 3 2 4 5% 22%	1845
	21st to end	Day	28% 15% 3 9% 6% 5% 4 2 4 4 20%	1577
March	1st to 10th	Day	29% 18% 4 9% 5% 4 4 3 4 5% 16%	4766
	11th to 20th	Day	28% 14% 4 7% 4 4 3 2 4 4 24%	5110
	21st to end	Day	34% 13% 4 9% 6% 4 4 3 4 5% 15%	5758
April	1st to 10th	Day	30% 13% 3 7% 6% 5% 5% 4 5% 5% 17%	2520
	11th to 20th	Day	24% 11% 3 8% 6% 5% 4 4 5% 6% 24%	2594
	21st to end	Day	26% 11% 4 6% 5% 4 5% 4 5% 5% 25%	2797
May	1st to 10th	Day	13% 8% 3 7% 4 4 4 4 5% 6% 42%	8195
	11th to 20th	Day	18% 9% 3 8% 4 5% 4 5% 7% 9% 28%	8160
	21st to end	Day	22% 7% 2 6% 4 4 5% 4 7% 9% 29%	9168
June	1st to 10th	Day	24% 9% 3 7% 5% 4 4 4 5% 7% 28%	3150
	11th to 20th	Day	23% 10% 3 8% 5% 5% 5% 3 5% 7% 24%	3150
	21st to end	Day	18% 9% 2 7% 5% 5% 5% 3 6% 7% 33%	3150
July	1st to 10th	Day	11% 4 2 5% 4 4 5% 3 4 6% 51%	8820
	11th to 20th	Day	6% 4 1 4 2 3 3 2 4 6% 64%	8820
	21st to end	Day	14% 4 1 4 3 3 3 4 5% 9% 51%	9075
August	1st to 10th	Day	7% 4 2 3 2 2 2 2 3 5% 67%	6402
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 49%	5923
	21st to end	Day	19% 7% 2 6% 4 4 5% 4 5% 7% 37%	6417
September	1st to 10th	Day	16% 7% 2 6% 4 3 4 3 6% 7% 43%	2520
	11th to 20th	Day	20% 8% 3 7% 5% 4 3 3 4 6% 38%	2520
	21st to end	Day	28% 7% 1 5% 3 4 4 4 5% 5% 33%	2188
October	1st to 10th	Day	27% 8% 3 8% 4 5% 4 3 5% 5% 28%	3280
	11th to 20th	Day	38% 7% 2 5% 4 4 4 3 5% 6% 21%	3261
	21st to end	Day	35% 13% 4 8% 5% 4 4 3 5% 5% 14%	3279
November	1st to 10th	Day	43% 10% 3 7% 5% 4 3 2 4 4 15%	1648
	11th to 20th	Day	43% 11% 3 6% 4 3 3 3 3 3 19%	1680
	21st to end	Day	43% 10% 3 6% 3 3 4 3 4 4 18%	1680
December	1st to 10th	Day	27% 11% 3 6% 5% 4 4 4 5% 10% 20%	2666
	11th to 20th	Day	47% 15% 4 7% 3 3 3 2 3 3 10%	2715
	21st to end	Day	47% 15% 4 8% 4 4 3 2 3 3 7%	3575
January	1st to 10th	Night	42% 14% 4 8% 4 4 3 2 3 3 13%	8770
	11th to 20th	Night	26% 7% 2 5% 3 3 3 3 5% 10% 33%	8779
	21st to end	Night	41% 8% 2 4 2 2 3 2 3 5% 28%	8965
February	1st to 10th	Night	34% 13% 3 7% 5% 4 4 3 4 4 20%	2730
	11th to 20th	Night	30% 13% 3 8% 4 3 3 3 4 4 25%	2662
	21st to end	Night	31% 13% 3 7% 4 3 3 3 5% 4 24%	2228
March	1st to 10th	Night	28% 17% 3 7% 4 4 4 3 4 4 22%	5344
	11th to 20th	Night	30% 14% 3 6% 4 3 3 3 4 5% 25%	5109
	21st to end	Night	40% 13% 4 7% 4 3 3 2 3 3 17%	5459
April	1st to 10th	Night	30% 12% 4 7% 5% 4 3 3 5% 5% 22%	2100
	11th to 20th	Night	21% 12% 3 8% 5% 4 4 4 5% 6% 28%	1974
	21st to end	Night	24% 10% 4 6% 5% 4 4 3 5% 6% 29%	1822
May	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 4 4 50%	4724
	11th to 20th	Night	15% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	4700
	21st to end	Night	18% 9% 3 7% 5% 4 5% 4 6% 8% 32%	4998
June	1st to 10th	Night	24% 7% 3 6% 5% 5% 5% 4 5% 5% 31%	1470
	11th to 20th	Night	17% 10% 3 8% 5% 4 5% 3 6% 8% 31%	1470
	21st to end	Night	16% 7% 2 6% 5% 4 4 3 5% 8% 40%	1470
July	1st to 10th	Night	10% 4 1 5% 3 3 4 3 5% 7% 55%	4140
	11th to 20th	Night	6% 3 1 2 2 2 2 2 3 5% 72%	4140
	21st to end	Night	11% 4 2 4 3 3 3 3 6% 7% 55%	4990
August	1st to 10th	Night	4 2 1 2 2 2 2 2 4 6% 73%	3747
	11th to 20th	Night	8% 5% 2 5% 3 4 4 3 6% 7% 52%	4142
	21st to end	Night	14% 7% 3 7% 4 4 4 3 5% 7% 42%	4738
September	1st to 10th	Night	10% 4 2 6% 3 3 4 3 5% 6% 53%	2100
	11th to 20th	Night	21% 6% 2 5% 4 4 3 3 4 5% 43%	2100
	21st to end	Night	25% 8% 3 7% 3 3 4 2 4 6% 36%	2385
October	1st to 10th	Night	24% 9% 3 7% 5% 6% 5% 4 5% 5% 29%	3920
	11th to 20th	Night	34% 9% 3 6% 3 3 3 3 4 5% 27%	3897
	21st to end	Night	35% 12% 4 7% 3 4 3 2 4 4 21%	4521
November	1st to 10th	Night	42% 11% 2 6% 4 4 4 2 3 4 17%	2870
	11th to 20th	Night	41% 10% 3 6% 4 3 3 2 4 3 21%	2940
	21st to end	Night	39% 12% 3 7% 4 3 4 3 3 3 20%	2940
December	1st to 10th	Night	30% 10% 3 6% 4 4 4 4 6% 8% 21%	4729
	11th to 20th	Night	47% 14% 3 7% 4 4 3 2 3 3 11%	5710
	21st to end	Night	47% 16% 4 7% 3 3 3 2 3 3 11%	7623

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% 23% 7% 6% 1	4125
	11th to 20th	Day	37% 24% 12% 18% 8% 2	4140
	21st to end	Day	54% 22% 6% 13% 5%	5203
February	1st to 10th	Day	28% 60% 4 4 2 2	1890
	11th to 20th	Day	28% 59% 4 7% 2 1	1854
	21st to end	Day	27% 59% 5% 5% 2 1	1577
March	1st to 10th	Day	52% 28% 8% 9% 2	4766
	11th to 20th	Day	45% 30% 7% 16% 1	5110
	21st to end	Day	53% 30% 11% 5% 2	5758
April	1st to 10th	Day	26% 61% 6% 6% 1	2520
	11th to 20th	Day	25% 60% 5% 7% 2 1	2594
	21st to end	Day	23% 60% 6% 8% 3	2797
May	1st to 10th	Day	30% 24% 14% 22% 9%	8195
	11th to 20th	Day	39% 27% 18% 14% 2	8163
	21st to end	Day	37% 25% 20% 15% 3	9169
June	1st to 10th	Day	20% 60% 8% 9% 3	3150
	11th to 20th	Day	20% 60% 7% 10% 4	3150
	21st to end	Day	20% 59% 9% 9% 3	3150
July	1st to 10th	Day	26% 23% 15% 18% 13% 4	8746
	11th to 20th	Day	15% 22% 14% 30% 17% 2	8739
	21st to end	Day	25% 23% 24% 20% 7%	8879
August	1st to 10th	Day	12% 27% 10% 31% 17% 3	6385
	11th to 20th	Day	24% 31% 26% 16% 3	5962
	21st to end	Day	34% 31% 15% 14% 7%	6416
September	1st to 10th	Day	22% 57% 7% 10% 4 1	2520
	11th to 20th	Day	23% 56% 6% 9% 6%	2520
	21st to end	Day	23% 56% 7% 9% 4	2188
October	1st to 10th	Day	39% 38% 11% 10% 2	3280
	11th to 20th	Day	41% 37% 8% 9% 5%	3261
	21st to end	Day	45% 40% 6% 8% 2	3279
November	1st to 10th	Day	33% 55% 5% 6% 1	1648
	11th to 20th	Day	31% 55% 5% 5% 2 2	1680
	21st to end	Day	29% 55% 5% 6% 4 1	1680
December	1st to 10th	Day	38% 39% 8% 13% 2	2666
	11th to 20th	Day	59% 31% 4 4 1	2718
	21st to end	Day	64% 26% 5% 4 1	3575
January	1st to 10th	Night	62% 22% 6% 8% 2	8770
	11th to 20th	Night	35% 26% 12% 17% 8% 1	8820
	21st to end	Night	49% 24% 9% 16% 3	8965
February	1st to 10th	Night	28% 59% 3 4 3 2	2730
	11th to 20th	Night	29% 58% 4 5% 4 1	2664
	21st to end	Night	24% 60% 6% 7% 2 2	2228
March	1st to 10th	Night	47% 31% 6% 10% 6%	5344
	11th to 20th	Night	42% 30% 10% 16% 3	5110
	21st to end	Night	55% 29% 10% 6% 1	5459
April	1st to 10th	Night	26% 59% 5% 8% 2	2100
	11th to 20th	Night	23% 60% 6% 7% 3 1	1974
	21st to end	Night	21% 59% 8% 7% 5%	1822
May	1st to 10th	Night	28% 23% 13% 26% 10%	4721
	11th to 20th	Night	37% 28% 17% 15% 3	4700
	21st to end	Night	39% 25% 20% 14% 2	5002
June	1st to 10th	Night	20% 59% 8% 8% 3 1	1470
	11th to 20th	Night	18% 59% 7% 12% 4	1470
	21st to end	Night	18% 59% 10% 9% 4	1470
July	1st to 10th	Night	22% 23% 17% 24% 12% 3	4105
	11th to 20th	Night	12% 21% 15% 32% 19% 2	4098
	21st to end	Night	24% 22% 21% 23% 9% 1	4881
August	1st to 10th	Night	7% 26% 15% 29% 20% 3	3739
	11th to 20th	Night	23% 30% 28% 16% 2 1	4169
	21st to end	Night	31% 32% 17% 14% 6% 1	4738
September	1st to 10th	Night	18% 56% 10% 10% 6% 1	2100
	11th to 20th	Night	22% 56% 7% 8% 6% 1	2100
	21st to end	Night	24% 55% 7% 10% 4	2385
October	1st to 10th	Night	37% 40% 13% 8% 2	3920
	11th to 20th	Night	41% 37% 6% 11% 5%	3897
	21st to end	Night	42% 39% 7% 9% 3	4521
November	1st to 10th	Night	34% 54% 5% 5% 2	2870
	11th to 20th	Night	31% 54% 4 6% 3 2	2940
	21st to end	Night	32% 54% 4 5% 5%	2940
December	1st to 10th	Night	39% 42% 9% 6% 3	4729
	11th to 20th	Night	57% 32% 5% 5% 1	5706
	21st to end	Night	60% 27% 5% 6% 2	7623

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	49% 7% 13% 18% 7% 6%	220
	11th to 20th	Day	45% 9% 14% 16% 15% 2	220
	21st to end	Day	50% 5% 15% 15% 14% 2	261
February	1st to 10th	Day	43% 8% 21% 18% 9% 1	430
	11th to 20th	Day	39% 8% 21% 21% 9% 1	425
	21st to end	Day	39% 12% 22% 19% 7% 1	366
March	1st to 10th	Day	38% 11% 21% 19% 9% 2	540
	11th to 20th	Day	44% 9% 14% 17% 14% 2	540
	21st to end	Day	43% 12% 22% 15% 6% 2	594
April	1st to 10th	Day	37% 13% 19% 19% 10% 1	540
	11th to 20th	Day	32% 12% 23% 21% 9% 3	532
	21st to end	Day	36% 12% 23% 18% 8% 3	540
May	1st to 10th	Day	25% 10% 26% 25% 12% 2	543
	11th to 20th	Day	29% 10% 21% 23% 13% 4	568
	21st to end	Day	25% 11% 25% 25% 10% 4	675
June	1st to 10th	Day	30% 9% 25% 25% 9% 1	780
	11th to 20th	Day	30% 13% 26% 24% 7% 2	780
	21st to end	Day	26% 8% 23% 27% 13% 3	780
July	1st to 10th	Day	19% 8% 23% 33% 13% 2	771
	11th to 20th	Day	14% 9% 29% 32% 13% 3	768
	21st to end	Day	14% 5% 22% 37% 17% 5%	676
August	1st to 10th	Day	17% 7% 22% 29% 18% 8%	567
	11th to 20th	Day	14% 6% 20% 33% 20% 7%	557
	21st to end	Day	20% 8% 22% 29% 16% 5%	627
September	1st to 10th	Day	20% 9% 25% 26% 15% 4	570
	11th to 20th	Day	27% 7% 19% 27% 17% 3	570
	21st to end	Day	34% 7% 20% 22% 13% 5%	563
October	1st to 10th	Day	35% 6% 19% 19% 18% 4	530
	11th to 20th	Day	42% 8% 15% 16% 14% 5%	530
	21st to end	Day	48% 9% 15% 16% 8% 4	432
November	1st to 10th	Day	44% 12% 18% 15% 10% 1	220
	11th to 20th	Day	52% 5% 11% 12% 11% 8%	220
	21st to end	Day	50% 9% 14% 14% 6% 8%	220
December	1st to 10th	Day	48% 8% 12% 9% 16% 7%	220
	11th to 20th	Day	55% 7% 9% 16% 10% 3	220
	21st to end	Day	49% 7% 14% 21% 7% 2	242
January	1st to 10th	Night	54% 7% 12% 18% 8% 1	950
	11th to 20th	Night	43% 6% 16% 24% 11% 1	950
	21st to end	Night	46% 5% 15% 23% 11% 1	931
February	1st to 10th	Night	41% 7% 17% 26% 8% 1	840
	11th to 20th	Night	37% 8% 18% 26% 11%	824
	21st to end	Night	40% 8% 18% 23% 11% 1	692
March	1st to 10th	Night	41% 7% 16% 21% 14% 1	840
	11th to 20th	Night	47% 6% 14% 20% 12% 1	840
	21st to end	Night	44% 5% 15% 22% 12% 2	924
April	1st to 10th	Night	39% 5% 13% 29% 12% 2	840
	11th to 20th	Night	32% 5% 19% 29% 13% 2	726
	21st to end	Night	35% 4 16% 30% 13% 2	562
May	1st to 10th	Night	20% 4 16% 36% 23% 1	420
	11th to 20th	Night	25% 4 10% 31% 26% 4	414
	21st to end	Night	24% 3 12% 36% 23% 1	455
June	1st to 10th	Night	28% 4 14% 33% 21% 1	420
	11th to 20th	Night	24% 4 20% 32% 18% 2	420
	21st to end	Night	20% 3 15% 32% 26% 3	420
July	1st to 10th	Night	12% 3 11% 34% 38% 2	410
	11th to 20th	Night	12% 2 10% 33% 39% 5%	408
	21st to end	Night	8% 2 7% 32% 43% 7%	460
August	1st to 10th	Night	14% 2 4 30% 41% 10%	430
	11th to 20th	Night	12% 1 4 28% 45% 10%	624
	21st to end	Night	17% 3 11% 31% 34% 4	761
September	1st to 10th	Night	15% 2 13% 35% 31% 4	840
	11th to 20th	Night	25% 2 10% 24% 31% 6%	840
	21st to end	Night	33% 3 15% 27% 20% 2	832
October	1st to 10th	Night	31% 4 15% 26% 22% 4	800
	11th to 20th	Night	42% 3 14% 24% 14% 3	800
	21st to end	Night	45% 3 17% 24% 9% 3	880
November	1st to 10th	Night	51% 5% 16% 21% 6%	820
	11th to 20th	Night	51% 4 16% 18% 10% 1	840
	21st to end	Night	47% 6% 16% 18% 10% 3	840
December	1st to 10th	Night	39% 5% 16% 26% 12% 2	848
	11th to 20th	Night	55% 9% 13% 18% 5% 1	950
	21st to end	Night	52% 8% 17% 17% 5%	1045

Page generated at 2026-09-26 01:18:38. Version ac07

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