Prince George Centre Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prince George Centre Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 10% 4 10% 6% 6% 6% 5% 6% 8% 31%	130662
	Night	9% 13% 4 10% 6% 6% 6% 4 6% 7% 29%	127743
Transparency	Day	48% 10% 16% 19% 8%	130353
	Night	52% 9% 14% 18% 7% 1	127629
Seeing	Day	14% 13% 26% 26% 17% 4	17718
	Night	17% 11% 24% 29% 17% 2	25439

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	6% 16% 7% 17% 8% 7% 7% 4 6% 5% 17%	3013
	11th to 20th	Day	7% 14% 6% 11% 6% 5% 5% 4 6% 6% 30%	3159
	21st to end	Day	8% 18% 6% 12% 7% 6% 5% 3 6% 6% 24%	3498
February	1st to 10th	Day	13% 16% 4 11% 6% 6% 5% 4 5% 7% 24%	1627
	11th to 20th	Day	9% 16% 5% 10% 7% 5% 6% 4 6% 6% 26%	1755
	21st to end	Day	5% 17% 5% 11% 7% 6% 5% 4 5% 7% 29%	1485
March	1st to 10th	Day	4 13% 5% 13% 8% 7% 6% 4 6% 7% 26%	4486
	11th to 20th	Day	4 12% 4 9% 7% 7% 8% 7% 8% 9% 26%	5000
	21st to end	Day	7% 15% 4 9% 5% 5% 5% 4 6% 6% 34%	5695
April	1st to 10th	Day	5% 12% 6% 13% 8% 7% 7% 6% 6% 7% 24%	2400
	11th to 20th	Day	9% 14% 5% 10% 8% 6% 6% 5% 6% 6% 25%	2558
	21st to end	Day	11% 12% 4 9% 6% 6% 7% 4 6% 7% 29%	2780
May	1st to 10th	Day	7% 9% 4 10% 6% 6% 6% 5% 7% 9% 32%	6320
	11th to 20th	Day	8% 10% 5% 13% 8% 7% 7% 6% 7% 8% 20%	6551
	21st to end	Day	10% 9% 4 11% 8% 8% 8% 6% 7% 7% 22%	7716
June	1st to 10th	Day	9% 9% 3 9% 8% 6% 6% 5% 6% 8% 30%	3200
	11th to 20th	Day	13% 9% 4 9% 6% 6% 5% 6% 8% 8% 26%	3200
	21st to end	Day	10% 9% 4 8% 6% 6% 6% 5% 7% 9% 31%	3200
July	1st to 10th	Day	8% 6% 2 7% 5% 5% 5% 5% 7% 8% 43%	7240
	11th to 20th	Day	9% 6% 2 7% 4 5% 5% 4 6% 9% 44%	6782
	21st to end	Day	7% 4 2 7% 6% 6% 6% 4 7% 9% 40%	7375
August	1st to 10th	Day	4 4 2 4 4 3 4 4 5% 8% 58%	4645
	11th to 20th	Day	8% 5% 2 8% 4 5% 5% 5% 7% 10% 41%	4384
	21st to end	Day	9% 7% 3 9% 6% 5% 7% 5% 6% 8% 35%	4664
September	1st to 10th	Day	9% 6% 3 7% 6% 5% 5% 4 6% 9% 40%	2404
	11th to 20th	Day	9% 7% 3 7% 5% 6% 5% 5% 7% 9% 38%	2400
	21st to end	Day	14% 9% 3 10% 6% 5% 5% 4 6% 7% 28%	2107
October	1st to 10th	Day	14% 12% 4 9% 6% 6% 6% 5% 6% 7% 26%	3280
	11th to 20th	Day	14% 11% 3 8% 6% 6% 6% 5% 7% 7% 28%	3226
	21st to end	Day	10% 11% 3 9% 6% 5% 5% 4 5% 7% 33%	2853
November	1st to 10th	Day	16% 15% 4 10% 6% 5% 5% 4 5% 6% 24%	1568
	11th to 20th	Day	9% 16% 6% 14% 7% 6% 7% 4 6% 7% 18%	1600
	21st to end	Day	11% 20% 7% 9% 7% 6% 6% 4 5% 6% 18%	1277
December	1st to 10th	Day	14% 20% 6% 11% 5% 6% 5% 4 7% 7% 16%	1960
	11th to 20th	Day	9% 18% 6% 17% 10% 7% 6% 4 4 4 14%	2262
	21st to end	Day	5% 17% 6% 13% 7% 6% 6% 5% 6% 6% 22%	2992
January	1st to 10th	Night	7% 17% 6% 14% 8% 7% 7% 4 5% 7% 18%	6974
	11th to 20th	Night	10% 15% 4 11% 6% 6% 5% 4 5% 6% 28%	6739
	21st to end	Night	10% 20% 7% 11% 6% 5% 5% 4 5% 6% 23%	7458
February	1st to 10th	Night	12% 19% 5% 11% 7% 5% 5% 4 5% 5% 23%	2770
	11th to 20th	Night	9% 16% 6% 10% 6% 4 5% 4 5% 6% 27%	2532
	21st to end	Night	8% 14% 5% 11% 7% 5% 5% 3 6% 6% 30%	2145
March	1st to 10th	Night	4 14% 5% 12% 7% 6% 7% 5% 7% 7% 27%	5401
	11th to 20th	Night	4 12% 4 10% 7% 7% 6% 5% 8% 9% 28%	4999
	21st to end	Night	8% 11% 4 8% 5% 5% 5% 4 6% 7% 37%	5280
April	1st to 10th	Night	8% 12% 4 10% 7% 6% 6% 5% 7% 8% 27%	2000
	11th to 20th	Night	7% 13% 4 9% 7% 7% 6% 5% 8% 7% 27%	1790
	21st to end	Night	8% 12% 3 9% 7% 5% 6% 5% 6% 6% 32%	1620
May	1st to 10th	Night	10% 10% 4 10% 7% 5% 6% 5% 7% 8% 29%	3640
	11th to 20th	Night	12% 11% 4 9% 6% 6% 6% 5% 8% 10% 22%	3329
	21st to end	Night	11% 12% 4 11% 8% 7% 5% 5% 5% 6% 27%	3156
June	1st to 10th	Night	13% 7% 3 8% 7% 7% 6% 5% 7% 8% 29%	1200
	11th to 20th	Night	13% 10% 3 9% 7% 6% 6% 5% 6% 7% 27%	1200
	21st to end	Night	9% 7% 4 9% 8% 6% 8% 5% 7% 9% 27%	1200
July	1st to 10th	Night	5% 6% 2 7% 5% 6% 5% 4 6% 8% 46%	2720
	11th to 20th	Night	9% 6% 2 6% 4 3 4 4 6% 7% 49%	3175
	21st to end	Night	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 41%	3468
August	1st to 10th	Night	4 3 1 3 4 3 3 3 4 7% 65%	2503
	11th to 20th	Night	6% 5% 2 5% 4 4 5% 4 6% 10% 48%	2665
	21st to end	Night	11% 8% 4 9% 5% 5% 5% 5% 6% 8% 35%	3245
September	1st to 10th	Night	10% 7% 2 7% 5% 4 5% 4 6% 9% 40%	1996
	11th to 20th	Night	9% 8% 2 6% 5% 4 6% 4 6% 7% 43%	2000
	21st to end	Night	14% 11% 3 9% 6% 5% 5% 4 6% 7% 30%	2290
October	1st to 10th	Night	13% 14% 5% 10% 6% 5% 5% 4 6% 7% 25%	3920
	11th to 20th	Night	9% 8% 3 9% 7% 6% 7% 5% 8% 8% 29%	3888
	21st to end	Night	11% 11% 4 8% 6% 6% 6% 4 6% 7% 30%	4697
November	1st to 10th	Night	15% 14% 5% 8% 6% 5% 6% 4 6% 6% 23%	2730
	11th to 20th	Night	10% 15% 4 12% 7% 6% 5% 4 6% 7% 23%	2800
	21st to end	Night	9% 20% 5% 11% 6% 5% 6% 4 6% 7% 19%	3099
December	1st to 10th	Night	12% 19% 5% 10% 6% 6% 5% 4 5% 5% 23%	5220
	11th to 20th	Night	10% 20% 6% 14% 8% 7% 5% 4 6% 5% 15%	5930
	21st to end	Night	4 16% 6% 14% 8% 7% 6% 5% 7% 6% 21%	7964

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	62% 11% 13% 10% 4	3013
	11th to 20th	Day	53% 8% 14% 13% 9% 3	3180
	21st to end	Day	59% 10% 14% 13% 4	3498
February	1st to 10th	Day	59% 8% 12% 10% 11% 1	1627
	11th to 20th	Day	56% 8% 17% 12% 5%	1764
	21st to end	Day	53% 9% 14% 15% 8% 2	1485
March	1st to 10th	Day	52% 10% 11% 17% 10%	4486
	11th to 20th	Day	46% 12% 16% 17% 9% 1	5000
	21st to end	Day	47% 9% 13% 20% 11% 1	5695
April	1st to 10th	Day	55% 11% 12% 17% 5% 1	2400
	11th to 20th	Day	55% 8% 11% 19% 7%	2558
	21st to end	Day	52% 8% 14% 18% 7%	2780
May	1st to 10th	Day	44% 9% 16% 21% 9% 1	6320
	11th to 20th	Day	53% 12% 13% 15% 6%	6554
	21st to end	Day	52% 12% 14% 16% 4 1	7717
June	1st to 10th	Day	47% 12% 13% 18% 9%	3200
	11th to 20th	Day	51% 11% 15% 20% 4	3200
	21st to end	Day	47% 12% 17% 19% 6%	3200
July	1st to 10th	Day	37% 8% 13% 22% 19% 1	7161
	11th to 20th	Day	36% 8% 19% 28% 9%	6700
	21st to end	Day	36% 8% 22% 24% 9%	7168
August	1st to 10th	Day	24% 7% 18% 37% 14%	4627
	11th to 20th	Day	36% 9% 21% 28% 6%	4426
	21st to end	Day	45% 9% 23% 19% 4	4663
September	1st to 10th	Day	39% 10% 24% 21% 6%	2404
	11th to 20th	Day	41% 11% 19% 24% 6%	2400
	21st to end	Day	52% 12% 15% 16% 4 1	2107
October	1st to 10th	Day	55% 9% 19% 12% 6%	3280
	11th to 20th	Day	52% 10% 17% 19% 2	3226
	21st to end	Day	47% 10% 22% 15% 5%	2853
November	1st to 10th	Day	59% 8% 14% 13% 5% 1	1568
	11th to 20th	Day	62% 11% 12% 11% 4	1600
	21st to end	Day	66% 11% 10% 11% 2	1277
December	1st to 10th	Day	66% 10% 14% 9% 2	1960
	11th to 20th	Day	72% 8% 10% 6% 4	2264
	21st to end	Day	57% 10% 14% 14% 4	2992
January	1st to 10th	Night	60% 10% 12% 13% 5%	6974
	11th to 20th	Night	55% 7% 13% 11% 11% 4	6780
	21st to end	Night	62% 9% 15% 11% 2	7458
February	1st to 10th	Night	62% 8% 11% 10% 9% 1	2770
	11th to 20th	Night	56% 9% 14% 15% 6%	2534
	21st to end	Night	52% 7% 14% 17% 8% 1	2145
March	1st to 10th	Night	52% 9% 11% 16% 12% 1	5401
	11th to 20th	Night	47% 9% 11% 22% 11%	5000
	21st to end	Night	42% 9% 12% 23% 14%	5280
April	1st to 10th	Night	50% 11% 13% 19% 8%	2000
	11th to 20th	Night	51% 9% 12% 19% 9%	1790
	21st to end	Night	49% 9% 11% 21% 9% 1	1620
May	1st to 10th	Night	47% 9% 14% 21% 9% 1	3637
	11th to 20th	Night	49% 13% 14% 20% 4	3329
	21st to end	Night	56% 10% 9% 21% 4 1	3156
June	1st to 10th	Night	48% 12% 11% 19% 9% 1	1200
	11th to 20th	Night	52% 11% 12% 20% 6%	1200
	21st to end	Night	48% 12% 17% 18% 5%	1200
July	1st to 10th	Night	34% 8% 14% 25% 19% 1	2690
	11th to 20th	Night	34% 7% 17% 34% 8%	3134
	21st to end	Night	34% 9% 25% 21% 10%	3370
August	1st to 10th	Night	20% 6% 15% 41% 17% 1	2496
	11th to 20th	Night	31% 7% 19% 35% 8%	2689
	21st to end	Night	46% 9% 22% 18% 5%	3245
September	1st to 10th	Night	38% 10% 19% 25% 7% 1	1996
	11th to 20th	Night	37% 10% 16% 28% 8%	2000
	21st to end	Night	51% 9% 14% 17% 8%	2290
October	1st to 10th	Night	56% 9% 15% 15% 5%	3920
	11th to 20th	Night	48% 9% 17% 21% 5%	3888
	21st to end	Night	51% 9% 17% 18% 4	4697
November	1st to 10th	Night	59% 11% 12% 13% 5%	2730
	11th to 20th	Night	59% 11% 12% 13% 5%	2800
	21st to end	Night	62% 11% 12% 11% 4	3099
December	1st to 10th	Night	61% 8% 15% 13% 3	5220
	11th to 20th	Night	69% 9% 10% 8% 3	5927
	21st to end	Night	58% 10% 14% 13% 4	7964

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	14% 15% 30% 26% 13% 3	200
	11th to 20th	Day	14% 21% 29% 25% 11% 2	200
	21st to end	Day	21% 20% 27% 18% 13% 2	220
February	1st to 10th	Day	22% 20% 24% 21% 10% 3	258
	11th to 20th	Day	14% 14% 33% 27% 10% 2	395
	21st to end	Day	16% 23% 29% 22% 8% 3	331
March	1st to 10th	Day	14% 17% 29% 24% 13% 3	483
	11th to 20th	Day	14% 18% 33% 23% 11% 2	500
	21st to end	Day	14% 22% 25% 20% 13% 5%	550
April	1st to 10th	Day	12% 18% 30% 26% 11% 2	500
	11th to 20th	Day	15% 18% 30% 20% 14% 4	492
	21st to end	Day	12% 17% 31% 24% 14% 3	500
May	1st to 10th	Day	15% 15% 29% 26% 11% 3	503
	11th to 20th	Day	12% 10% 27% 30% 17% 4	644
	21st to end	Day	13% 11% 25% 26% 21% 4	919
June	1st to 10th	Day	14% 12% 28% 28% 17% 2	930
	11th to 20th	Day	16% 9% 25% 28% 18% 4	930
	21st to end	Day	12% 6% 25% 28% 24% 4	930
July	1st to 10th	Day	13% 7% 20% 31% 25% 3	916
	11th to 20th	Day	12% 9% 23% 28% 24% 5%	719
	21st to end	Day	8% 6% 21% 34% 25% 5%	794
August	1st to 10th	Day	7% 7% 22% 31% 27% 6%	593
	11th to 20th	Day	9% 9% 26% 29% 21% 7%	517
	21st to end	Day	10% 11% 26% 28% 19% 6%	583
September	1st to 10th	Day	13% 9% 24% 27% 21% 6%	530
	11th to 20th	Day	12% 12% 25% 26% 20% 5%	530
	21st to end	Day	20% 17% 32% 17% 10% 2	530
October	1st to 10th	Day	24% 12% 25% 24% 13% 3	530
	11th to 20th	Day	22% 15% 29% 22% 12% 1	519
	21st to end	Day	20% 17% 27% 24% 10% 2	252
November	1st to 10th	Day	31% 17% 24% 20% 7% 3	200
	11th to 20th	Day	18% 20% 29% 22% 7% 5%	200
	21st to end	Day	22% 16% 31% 19% 10% 3	200
December	1st to 10th	Day	19% 9% 28% 27% 16% 3	200
	11th to 20th	Day	14% 12% 27% 30% 16% 2	200
	21st to end	Day	22% 11% 25% 25% 15% 1	220
January	1st to 10th	Night	21% 15% 29% 26% 8% 2	991
	11th to 20th	Night	21% 14% 28% 25% 11%	900
	21st to end	Night	19% 16% 30% 22% 11% 1	990
February	1st to 10th	Night	23% 16% 22% 25% 13% 2	815
	11th to 20th	Night	17% 11% 26% 32% 13% 1	784
	21st to end	Night	16% 13% 25% 30% 13% 2	660
March	1st to 10th	Night	15% 13% 26% 29% 14% 2	800
	11th to 20th	Night	14% 14% 29% 29% 14% 1	800
	21st to end	Night	12% 12% 21% 29% 22% 4	880
April	1st to 10th	Night	14% 11% 21% 33% 19% 3	800
	11th to 20th	Night	13% 8% 27% 31% 17% 4	602
	21st to end	Night	11% 10% 27% 33% 17% 1	420
May	1st to 10th	Night	14% 10% 21% 37% 16% 3	400
	11th to 20th	Night	14% 3 23% 34% 23% 3	394
	21st to end	Night	13% 9% 21% 31% 24% 2	433
June	1st to 10th	Night	15% 6% 26% 34% 17% 3	400
	11th to 20th	Night	18% 4 19% 33% 23% 3	400
	21st to end	Night	12% 5% 19% 37% 27% 1	400
July	1st to 10th	Night	10% 4 17% 39% 28% 1	390
	11th to 20th	Night	12% 3 17% 35% 29% 3	388
	21st to end	Night	12% 3 15% 35% 29% 6%	438
August	1st to 10th	Night	8% 3 14% 31% 38% 7%	398
	11th to 20th	Night	8% 3 15% 38% 33% 3	445
	21st to end	Night	16% 4 18% 32% 26% 4	660
September	1st to 10th	Night	13% 5% 17% 34% 29% 2	796
	11th to 20th	Night	12% 6% 16% 31% 28% 6%	800
	21st to end	Night	20% 12% 25% 25% 16% 2	800
October	1st to 10th	Night	20% 9% 22% 31% 15% 3	800
	11th to 20th	Night	20% 10% 26% 29% 15% 1	792
	21st to end	Night	21% 14% 29% 25% 11% 1	836
November	1st to 10th	Night	24% 16% 23% 27% 8% 2	780
	11th to 20th	Night	19% 17% 29% 25% 9% 1	800
	21st to end	Night	20% 16% 29% 25% 9% 2	1037
December	1st to 10th	Night	17% 13% 25% 31% 13% 1	1100
	11th to 20th	Night	22% 12% 27% 27% 11% 1	1100
	21st to end	Night	18% 13% 30% 27% 11% 1	1210

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