Rothney Astrophysical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rothney Astrophysical Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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	8% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	130416
	Night	6% 9% 3 8% 5% 5% 5% 4 6% 8% 40%	126979
Transparency	Day	40% 7% 15% 24% 12% 2	130119
	Night	40% 6% 15% 24% 13% 2	126853
Seeing	Day	12% 10% 27% 31% 16% 4	16438
	Night	10% 7% 31% 36% 14% 2	25405

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	1 6% 6% 14% 6% 8% 7% 4 6% 7% 34%	3179
	11th to 20th	Day	1 7% 3 11% 6% 7% 6% 4 7% 7% 40%	3279
	21st to end	Day	2 6% 3 8% 5% 4 5% 4 5% 11% 48%	4004
February	1st to 10th	Day	2 9% 4 13% 8% 6% 7% 5% 7% 8% 32%	1600
	11th to 20th	Day	2 11% 4 10% 8% 7% 7% 5% 7% 8% 32%	1736
	21st to end	Day	1 10% 5% 12% 8% 6% 5% 4 7% 8% 33%	1625
March	1st to 10th	Day	2 8% 4 10% 6% 6% 5% 4 6% 10% 40%	4518
	11th to 20th	Day	3 10% 4 9% 5% 4 5% 4 6% 7% 42%	4926
	21st to end	Day	5% 19% 7% 14% 6% 5% 5% 4 6% 7% 22%	5606
April	1st to 10th	Day	8% 14% 4 10% 6% 5% 5% 4 7% 8% 30%	2440
	11th to 20th	Day	9% 13% 4 9% 6% 4 5% 4 6% 8% 31%	2570
	21st to end	Day	10% 12% 3 8% 6% 4 5% 5% 6% 8% 32%	2600
May	1st to 10th	Day	13% 13% 3 6% 4 3 4 4 5% 6% 39%	6157
	11th to 20th	Day	11% 11% 4 10% 6% 6% 6% 5% 7% 8% 26%	6734
	21st to end	Day	12% 7% 3 7% 6% 6% 5% 4 6% 8% 36%	7405
June	1st to 10th	Day	16% 7% 3 8% 6% 6% 5% 5% 7% 7% 30%	3000
	11th to 20th	Day	19% 9% 3 8% 5% 4 5% 4 7% 7% 27%	3000
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 6% 7% 10% 40%	3000
July	1st to 10th	Day	8% 5% 2 5% 4 4 5% 4 7% 8% 48%	6780
	11th to 20th	Day	11% 4 1 5% 3 4 5% 4 7% 9% 47%	6780
	21st to end	Day	10% 5% 2 5% 3 4 4 4 7% 9% 49%	7134
August	1st to 10th	Day	10% 5% 2 6% 4 4 5% 5% 7% 10% 42%	4541
	11th to 20th	Day	7% 5% 2 6% 4 4 6% 5% 7% 10% 44%	4206
	21st to end	Day	9% 4 2 4 3 4 4 4 6% 7% 54%	4664
September	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 5% 7% 50%	2386
	11th to 20th	Day	12% 7% 2 5% 4 4 4 3 6% 6% 45%	2090
	21st to end	Day	8% 8% 2 6% 3 4 5% 3 6% 8% 46%	2090
October	1st to 10th	Day	6% 7% 3 6% 4 4 4 4 6% 9% 46%	3370
	11th to 20th	Day	5% 6% 3 7% 4 4 5% 5% 7% 11% 42%	2888
	21st to end	Day	6% 7% 2 6% 5% 5% 4 4 6% 6% 48%	3101
November	1st to 10th	Day	6% 14% 5% 8% 6% 5% 4 4 7% 7% 35%	1659
	11th to 20th	Day	5% 11% 3 8% 5% 4 4 5% 7% 7% 43%	1518
	21st to end	Day	3 11% 5% 10% 5% 4 5% 5% 5% 10% 38%	1400
December	1st to 10th	Day	2 7% 5% 11% 6% 6% 6% 5% 7% 8% 37%	2290
	11th to 20th	Day	1 7% 5% 11% 6% 7% 7% 4 6% 10% 36%	2642
	21st to end	Day	3 10% 4 9% 5% 5% 4 4 5% 8% 42%	3498
January	1st to 10th	Night	2 11% 6% 12% 8% 8% 7% 5% 6% 7% 29%	6766
	11th to 20th	Night	2 6% 4 12% 7% 6% 7% 6% 7% 8% 36%	6648
	21st to end	Night	2 8% 4 9% 5% 5% 5% 4 6% 8% 44%	6952
February	1st to 10th	Night	3 10% 4 10% 7% 7% 6% 5% 7% 9% 34%	2800
	11th to 20th	Night	2 11% 6% 10% 7% 5% 5% 5% 7% 9% 33%	2549
	21st to end	Night	2 8% 4 13% 8% 5% 5% 5% 5% 7% 37%	2004
March	1st to 10th	Night	2 7% 3 12% 7% 6% 6% 4 7% 11% 36%	5376
	11th to 20th	Night	3 8% 3 9% 6% 5% 6% 4 7% 9% 40%	5073
	21st to end	Night	8% 19% 6% 12% 6% 5% 4 4 6% 7% 25%	5368
April	1st to 10th	Night	6% 15% 4 9% 5% 5% 5% 4 6% 8% 33%	1959
	11th to 20th	Night	10% 12% 6% 10% 5% 4 4 4 5% 6% 34%	1781
	21st to end	Night	9% 11% 4 9% 4 5% 4 4 7% 7% 36%	1800
May	1st to 10th	Night	12% 12% 3 7% 4 4 4 3 5% 7% 39%	3807
	11th to 20th	Night	13% 10% 4 9% 5% 4 5% 4 6% 9% 30%	3159
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 4 6% 7% 41%	3470
June	1st to 10th	Night	15% 8% 2 6% 5% 5% 4 4 7% 8% 36%	1400
	11th to 20th	Night	18% 8% 3 6% 5% 4 5% 4 7% 9% 32%	1400
	21st to end	Night	8% 5% 2 3 5% 4 4 5% 7% 10% 46%	1400
July	1st to 10th	Night	8% 6% 2 6% 5% 5% 4 4 5% 6% 50%	3180
	11th to 20th	Night	9% 5% 2 5% 3 4 4 4 6% 8% 50%	3180
	21st to end	Night	12% 5% 2 5% 3 3 3 3 5% 8% 52%	3693
August	1st to 10th	Night	12% 5% 2 5% 3 4 4 4 7% 10% 44%	2609
	11th to 20th	Night	7% 4 1 5% 5% 4 5% 5% 7% 10% 47%	2839
	21st to end	Night	9% 3 1 4 3 3 4 3 6% 8% 56%	3245
September	1st to 10th	Night	9% 6% 2 5% 4 4 4 3 5% 8% 51%	1857
	11th to 20th	Night	13% 7% 2 4 3 3 4 3 5% 7% 49%	2090
	21st to end	Night	9% 8% 2 6% 3 3 3 3 7% 8% 48%	2090
October	1st to 10th	Night	6% 6% 2 5% 4 3 4 4 6% 7% 52%	3605
	11th to 20th	Night	5% 7% 2 5% 4 3 4 4 6% 9% 51%	4024
	21st to end	Night	5% 8% 3 7% 4 4 5% 4 5% 7% 47%	4457
November	1st to 10th	Night	5% 13% 5% 7% 5% 4 5% 4 6% 7% 38%	2620
	11th to 20th	Night	3 9% 4 8% 6% 5% 5% 4 6% 8% 42%	2880
	21st to end	Night	3 10% 4 10% 6% 5% 5% 4 6% 8% 40%	3000
December	1st to 10th	Night	2 8% 4 8% 6% 6% 6% 5% 8% 10% 39%	4890
	11th to 20th	Night	1 9% 4 12% 7% 6% 6% 5% 8% 10% 33%	5550
	21st to end	Night	3 10% 4 10% 5% 5% 5% 4 6% 8% 40%	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	46% 6% 11% 22% 12% 3	3179
	11th to 20th	Day	41% 5% 12% 20% 20% 3	3300
	21st to end	Day	29% 6% 16% 32% 15% 2	4004
February	1st to 10th	Day	47% 8% 15% 18% 9% 3	1600
	11th to 20th	Day	46% 7% 15% 22% 9% 2	1745
	21st to end	Day	46% 7% 14% 18% 14% 1	1625
March	1st to 10th	Day	38% 6% 16% 21% 18% 1	4518
	11th to 20th	Day	38% 6% 15% 25% 16% 1	4927
	21st to end	Day	58% 8% 13% 14% 6% 1	5606
April	1st to 10th	Day	49% 9% 15% 19% 8% 1	2440
	11th to 20th	Day	48% 8% 13% 20% 10% 1	2570
	21st to end	Day	47% 8% 15% 20% 9% 1	2600
May	1st to 10th	Day	44% 6% 9% 23% 16% 2	6157
	11th to 20th	Day	50% 9% 13% 21% 7%	6738
	21st to end	Day	42% 8% 17% 22% 11% 1	7406
June	1st to 10th	Day	48% 9% 15% 17% 10% 1	3000
	11th to 20th	Day	53% 8% 14% 16% 8% 2	3000
	21st to end	Day	35% 11% 19% 23% 11% 2	3000
July	1st to 10th	Day	32% 6% 16% 27% 15% 4	6706
	11th to 20th	Day	32% 6% 20% 29% 11% 2	6699
	21st to end	Day	31% 5% 15% 26% 18% 6%	6933
August	1st to 10th	Day	35% 7% 15% 35% 6% 1	4524
	11th to 20th	Day	33% 7% 26% 26% 7% 1	4245
	21st to end	Day	28% 5% 17% 34% 13% 1	4663
September	1st to 10th	Day	32% 6% 17% 25% 17% 4	2386
	11th to 20th	Day	37% 7% 16% 25% 13% 2	2090
	21st to end	Day	35% 7% 16% 27% 13% 3	2090
October	1st to 10th	Day	35% 4 17% 35% 8% 1	3370
	11th to 20th	Day	34% 6% 20% 25% 12% 3	2888
	21st to end	Day	35% 6% 12% 32% 14% 2	3101
November	1st to 10th	Day	47% 7% 14% 20% 11% 1	1659
	11th to 20th	Day	38% 7% 17% 25% 11% 2	1518
	21st to end	Day	41% 7% 17% 23% 9% 4	1400
December	1st to 10th	Day	44% 5% 14% 26% 9% 2	2290
	11th to 20th	Day	42% 6% 22% 21% 9% 1	2644
	21st to end	Day	39% 5% 9% 24% 19% 4	3498
January	1st to 10th	Night	51% 7% 12% 22% 6% 2	6766
	11th to 20th	Night	44% 6% 12% 20% 13% 5%	6689
	21st to end	Night	36% 6% 14% 26% 17% 1	6952
February	1st to 10th	Night	44% 7% 14% 20% 13% 3	2800
	11th to 20th	Night	45% 7% 15% 19% 12% 2	2550
	21st to end	Night	45% 5% 12% 23% 13% 3	2004
March	1st to 10th	Night	40% 6% 15% 23% 13% 2	5376
	11th to 20th	Night	38% 7% 15% 24% 15% 1	5073
	21st to end	Night	56% 8% 13% 14% 9% 1	5368
April	1st to 10th	Night	47% 7% 14% 20% 11% 1	1959
	11th to 20th	Night	49% 6% 13% 21% 9% 1	1781
	21st to end	Night	45% 7% 16% 21% 9% 2	1800
May	1st to 10th	Night	45% 5% 10% 22% 14% 3	3804
	11th to 20th	Night	47% 8% 13% 26% 6% 1	3159
	21st to end	Night	38% 8% 16% 28% 9% 1	3470
June	1st to 10th	Night	44% 7% 16% 23% 10% 1	1400
	11th to 20th	Night	47% 7% 15% 19% 10% 2	1400
	21st to end	Night	31% 10% 20% 26% 12% 1	1400
July	1st to 10th	Night	35% 5% 15% 24% 18% 3	3145
	11th to 20th	Night	31% 5% 22% 31% 8% 3	3138
	21st to end	Night	32% 4 16% 26% 17% 6%	3590
August	1st to 10th	Night	35% 6% 17% 29% 13% 1	2601
	11th to 20th	Night	31% 6% 27% 26% 10% 1	2865
	21st to end	Night	26% 5% 19% 34% 15% 2	3245
September	1st to 10th	Night	31% 5% 19% 25% 16% 4	1857
	11th to 20th	Night	35% 6% 15% 28% 13% 4	2090
	21st to end	Night	32% 5% 18% 26% 14% 4	2090
October	1st to 10th	Night	30% 5% 15% 26% 21% 2	3605
	11th to 20th	Night	30% 4 20% 25% 18% 2	4024
	21st to end	Night	36% 5% 12% 32% 15% 1	4457
November	1st to 10th	Night	44% 7% 14% 21% 12% 2	2620
	11th to 20th	Night	38% 7% 16% 22% 15% 2	2880
	21st to end	Night	39% 6% 17% 22% 11% 4	3000
December	1st to 10th	Night	39% 6% 22% 22% 9% 2	4890
	11th to 20th	Night	45% 8% 20% 17% 9%	5547
	21st to end	Night	40% 5% 10% 24% 16% 4	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	4 6% 25% 39% 17% 11%	200
	11th to 20th	Day	7% 6% 30% 38% 16% 3	200
	21st to end	Day	6% 7% 22% 47% 14% 3	220
February	1st to 10th	Day	7% 12% 36% 32% 10% 5%	200
	11th to 20th	Day	6% 9% 29% 40% 14% 2	291
	21st to end	Day	4 13% 32% 34% 15% 2	389
March	1st to 10th	Day	6% 13% 39% 27% 12% 3	500
	11th to 20th	Day	7% 12% 32% 33% 13% 3	500
	21st to end	Day	11% 14% 33% 27% 11% 3	550
April	1st to 10th	Day	12% 15% 34% 28% 9% 2	500
	11th to 20th	Day	14% 16% 26% 29% 13% 2	492
	21st to end	Day	14% 17% 29% 23% 13% 4	500
May	1st to 10th	Day	18% 12% 26% 27% 12% 5%	521
	11th to 20th	Day	18% 9% 21% 27% 20% 5%	725
	21st to end	Day	18% 7% 22% 29% 20% 4	789
June	1st to 10th	Day	16% 11% 28% 31% 12% 3	730
	11th to 20th	Day	21% 9% 27% 27% 13% 3	730
	21st to end	Day	10% 6% 25% 30% 23% 5%	730
July	1st to 10th	Day	12% 5% 23% 33% 22% 5%	721
	11th to 20th	Day	13% 5% 24% 32% 23% 4	718
	21st to end	Day	6% 6% 27% 35% 20% 7%	692
August	1st to 10th	Day	12% 4 25% 31% 21% 7%	527
	11th to 20th	Day	9% 5% 26% 34% 18% 8%	517
	21st to end	Day	9% 10% 25% 30% 16% 10%	583
September	1st to 10th	Day	12% 9% 27% 28% 17% 6%	503
	11th to 20th	Day	17% 8% 24% 32% 16% 4	490
	21st to end	Day	13% 15% 28% 30% 11% 3	490
October	1st to 10th	Day	12% 10% 29% 28% 17% 3	422
	11th to 20th	Day	13% 10% 27% 31% 14% 5%	300
	21st to end	Day	12% 14% 34% 25% 11% 4	330
November	1st to 10th	Day	14% 12% 30% 30% 11% 2	300
	11th to 20th	Day	12% 13% 29% 34% 9% 3	258
	21st to end	Day	8% 9% 32% 30% 13% 10%	200
December	1st to 10th	Day	7% 6% 29% 43% 12% 5%	200
	11th to 20th	Day	3 10% 32% 40% 11% 5%	200
	21st to end	Day	5% 9% 23% 35% 22% 6%	220
January	1st to 10th	Night	7% 11% 37% 34% 10% 2	1000
	11th to 20th	Night	6% 7% 40% 37% 8% 1	961
	21st to end	Night	6% 12% 37% 36% 9% 1	880
February	1st to 10th	Night	6% 11% 32% 33% 13% 5%	800
	11th to 20th	Night	7% 8% 31% 39% 13% 1	784
	21st to end	Night	5% 6% 35% 33% 18% 3	660
March	1st to 10th	Night	6% 6% 38% 35% 12% 4	800
	11th to 20th	Night	7% 7% 36% 38% 11% 2	800
	21st to end	Night	10% 8% 30% 37% 12% 3	880
April	1st to 10th	Night	13% 7% 33% 32% 14% 2	759
	11th to 20th	Night	15% 8% 33% 34% 10%	593
	21st to end	Night	15% 8% 31% 35% 11% 1	600
May	1st to 10th	Night	16% 8% 24% 36% 15% 1	496
	11th to 20th	Night	14% 6% 25% 34% 20% 1	394
	21st to end	Night	18% 5% 21% 36% 18% 1	433
June	1st to 10th	Night	17% 7% 24% 37% 14% 1	400
	11th to 20th	Night	19% 5% 29% 33% 15% 1	400
	21st to end	Night	10% 2 20% 40% 27% 2	400
July	1st to 10th	Night	9% 4 15% 45% 26% 1	390
	11th to 20th	Night	11% 2 18% 48% 19% 2	388
	21st to end	Night	7% 3 17% 44% 27% 2	438
August	1st to 10th	Night	13% 3 12% 41% 30% 3	398
	11th to 20th	Night	10% 2 15% 45% 25% 3	556
	21st to end	Night	9% 4 21% 43% 21% 2	660
September	1st to 10th	Night	12% 4 23% 36% 21% 4	694
	11th to 20th	Night	18% 4 16% 39% 19% 4	760
	21st to end	Night	13% 5% 29% 37% 11% 4	760
October	1st to 10th	Night	14% 3 27% 39% 14% 3	760
	11th to 20th	Night	11% 6% 32% 34% 14% 3	760
	21st to end	Night	9% 8% 34% 36% 11% 2	836
November	1st to 10th	Night	12% 9% 37% 32% 9% 1	865
	11th to 20th	Night	9% 12% 41% 32% 6% 1	1000
	21st to end	Night	6% 8% 43% 32% 9% 2	1000
December	1st to 10th	Night	7% 8% 37% 34% 12% 2	1000
	11th to 20th	Night	4 8% 39% 38% 10% 1	1000
	21st to end	Night	8% 8% 36% 34% 12% 2	1100

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