Prince Albert
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prince Albert 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	10% 8% 3 8% 5% 5% 5% 4 6% 8% 37%	130681
	Night	8% 9% 4 10% 6% 5% 5% 4 6% 8% 35%	127731
Transparency	Day	43% 7% 17% 24% 8%	130372
	Night	46% 7% 15% 23% 8% 1	127620
Seeing	Day	13% 9% 28% 29% 16% 4	17154
	Night	13% 6% 29% 36% 15% 2	26300

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% 15% 7% 15% 9% 7% 6% 3 6% 7% 20%	2831
	11th to 20th	Day	4 9% 4 13% 9% 10% 7% 5% 6% 7% 26%	3159
	21st to end	Day	6% 15% 6% 12% 7% 5% 5% 4 6% 8% 26%	3727
February	1st to 10th	Day	8% 9% 5% 11% 8% 7% 6% 7% 7% 8% 24%	1610
	11th to 20th	Day	6% 12% 5% 9% 6% 5% 6% 5% 6% 8% 32%	1755
	21st to end	Day	5% 11% 5% 13% 7% 7% 5% 5% 6% 7% 28%	1530
March	1st to 10th	Day	6% 12% 6% 12% 8% 8% 6% 4 6% 7% 24%	4500
	11th to 20th	Day	7% 12% 4 11% 6% 5% 5% 4 6% 8% 32%	4851
	21st to end	Day	9% 11% 3 8% 5% 5% 5% 4 5% 8% 36%	5832
April	1st to 10th	Day	6% 11% 3 7% 5% 5% 5% 4 7% 9% 38%	2400
	11th to 20th	Day	11% 12% 4 8% 5% 4 6% 4 7% 7% 32%	2498
	21st to end	Day	12% 9% 3 8% 6% 5% 5% 4 7% 8% 34%	2800
May	1st to 10th	Day	11% 8% 3 6% 5% 4 4 4 6% 8% 41%	6340
	11th to 20th	Day	12% 9% 3 9% 6% 6% 6% 5% 7% 7% 30%	6298
	21st to end	Day	15% 8% 3 6% 4 4 4 3 5% 7% 42%	7861
June	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 7% 8% 33%	3200
	11th to 20th	Day	22% 8% 3 7% 6% 5% 5% 4 7% 7% 27%	3200
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 5% 6% 8% 35%	3200
July	1st to 10th	Day	7% 4 2 5% 3 4 5% 4 7% 10% 50%	7240
	11th to 20th	Day	9% 4 2 5% 4 4 4 4 7% 10% 47%	6974
	21st to end	Day	7% 4 1 5% 4 4 5% 4 7% 10% 48%	7250
August	1st to 10th	Day	11% 4 2 5% 4 4 5% 4 7% 9% 44%	4552
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 3 6% 9% 44%	4483
	21st to end	Day	12% 6% 2 6% 3 4 4 3 6% 8% 46%	4582
September	1st to 10th	Day	10% 6% 2 7% 4 4 5% 4 5% 9% 45%	2400
	11th to 20th	Day	13% 8% 3 7% 6% 5% 5% 4 6% 8% 36%	2400
	21st to end	Day	9% 5% 2 5% 5% 4 4 4 5% 8% 49%	2135
October	1st to 10th	Day	10% 7% 3 6% 4 4 4 3 6% 8% 46%	3270
	11th to 20th	Day	7% 7% 2 7% 5% 6% 5% 5% 8% 12% 37%	3081
	21st to end	Day	8% 10% 4 9% 6% 6% 6% 5% 7% 7% 32%	3009
November	1st to 10th	Day	15% 17% 5% 9% 6% 6% 4 3 6% 6% 25%	1568
	11th to 20th	Day	12% 14% 5% 14% 7% 6% 5% 3 4 6% 24%	1519
	21st to end	Day	8% 16% 7% 12% 7% 5% 5% 3 5% 6% 25%	1400
December	1st to 10th	Day	9% 13% 4 11% 10% 7% 7% 5% 5% 7% 23%	1975
	11th to 20th	Day	5% 12% 7% 19% 9% 7% 6% 4 5% 8% 18%	2259
	21st to end	Day	4 8% 5% 15% 7% 6% 5% 3 5% 5% 37%	2992
January	1st to 10th	Night	4 10% 5% 19% 11% 7% 6% 4 7% 8% 20%	7111
	11th to 20th	Night	4 10% 4 14% 9% 9% 7% 6% 6% 7% 25%	6739
	21st to end	Night	5% 11% 4 13% 8% 6% 6% 4 6% 8% 29%	7238
February	1st to 10th	Night	6% 11% 5% 11% 8% 6% 5% 6% 7% 7% 27%	2800
	11th to 20th	Night	4 10% 4 12% 6% 7% 6% 4 7% 7% 33%	2540
	21st to end	Night	4 9% 4 11% 7% 5% 6% 5% 7% 7% 35%	2100
March	1st to 10th	Night	7% 11% 5% 12% 7% 7% 6% 5% 8% 9% 24%	5374
	11th to 20th	Night	7% 10% 4 9% 5% 7% 7% 4 7% 9% 30%	5142
	21st to end	Night	7% 10% 3 9% 5% 5% 4 5% 6% 9% 36%	5126
April	1st to 10th	Night	5% 10% 2 7% 5% 5% 6% 5% 6% 9% 40%	2000
	11th to 20th	Night	12% 10% 4 8% 3 4 4 4 6% 8% 35%	1851
	21st to end	Night	13% 8% 3 7% 5% 6% 5% 4 6% 7% 36%	1600
May	1st to 10th	Night	13% 8% 3 8% 5% 5% 4 3 5% 6% 40%	3640
	11th to 20th	Night	10% 9% 3 9% 6% 6% 7% 5% 6% 9% 31%	3618
	21st to end	Night	14% 8% 3 7% 4 4 3 3 4 6% 43%	3013
June	1st to 10th	Night	13% 6% 2 5% 4 4 5% 5% 7% 8% 41%	1200
	11th to 20th	Night	20% 7% 3 7% 4 4 4 4 8% 8% 33%	1200
	21st to end	Night	13% 8% 1 5% 4 4 4 4 6% 8% 42%	1200
July	1st to 10th	Night	10% 4 2 4 4 4 5% 4 7% 9% 46%	2720
	11th to 20th	Night	9% 3 1 4 3 3 4 4 7% 9% 52%	2982
	21st to end	Night	6% 3 2 5% 4 4 5% 3 6% 8% 55%	3597
August	1st to 10th	Night	9% 3 1 4 4 4 5% 4 6% 9% 51%	2608
	11th to 20th	Night	9% 4 2 5% 4 4 5% 4 7% 9% 47%	2576
	21st to end	Night	10% 4 2 5% 3 3 4 3 5% 8% 52%	3324
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 6% 50%	2000
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 6% 7% 45%	2000
	21st to end	Night	8% 4 2 4 4 4 5% 3 6% 9% 50%	2261
October	1st to 10th	Night	10% 7% 2 5% 4 3 4 3 6% 7% 48%	3920
	11th to 20th	Night	7% 7% 2 7% 5% 6% 5% 4 6% 6% 46%	3992
	21st to end	Night	10% 11% 4 9% 5% 4 4 3 5% 8% 37%	4547
November	1st to 10th	Night	15% 16% 5% 9% 6% 4 5% 4 5% 7% 24%	2730
	11th to 20th	Night	9% 13% 6% 15% 7% 5% 5% 4 6% 6% 23%	2880
	21st to end	Night	8% 14% 5% 11% 7% 6% 6% 4 4 7% 28%	3000
December	1st to 10th	Night	7% 15% 7% 15% 9% 7% 6% 4 5% 5% 21%	5205
	11th to 20th	Night	5% 11% 5% 14% 8% 7% 7% 4 7% 9% 23%	5933
	21st to end	Night	4 8% 4 16% 7% 5% 4 4 5% 5% 37%	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	61% 5% 9% 19% 6%	2831
	11th to 20th	Day	55% 7% 13% 18% 7%	3180
	21st to end	Day	55% 7% 19% 14% 4	3727
February	1st to 10th	Day	52% 8% 17% 14% 10%	1610
	11th to 20th	Day	46% 7% 14% 23% 10%	1764
	21st to end	Day	52% 7% 14% 18% 8%	1530
March	1st to 10th	Day	56% 8% 16% 15% 6%	4500
	11th to 20th	Day	47% 9% 15% 21% 8%	4852
	21st to end	Day	43% 6% 11% 24% 15%	5832
April	1st to 10th	Day	40% 8% 19% 22% 11% 1	2400
	11th to 20th	Day	49% 6% 14% 20% 8% 2	2498
	21st to end	Day	46% 8% 15% 23% 8% 1	2800
May	1st to 10th	Day	40% 6% 12% 25% 15% 1	6340
	11th to 20th	Day	47% 9% 11% 25% 7% 1	6302
	21st to end	Day	40% 7% 12% 24% 16% 1	7861
June	1st to 10th	Day	47% 8% 15% 20% 9% 1	3200
	11th to 20th	Day	54% 9% 13% 17% 7% 1	3200
	21st to end	Day	44% 9% 18% 23% 6%	3200
July	1st to 10th	Day	27% 8% 29% 32% 5%	7161
	11th to 20th	Day	32% 7% 17% 41% 3	6888
	21st to end	Day	30% 8% 25% 31% 6%	7045
August	1st to 10th	Day	35% 9% 25% 27% 4	4536
	11th to 20th	Day	37% 8% 22% 27% 6%	4525
	21st to end	Day	36% 7% 22% 31% 4	4581
September	1st to 10th	Day	36% 8% 18% 27% 10% 1	2400
	11th to 20th	Day	44% 9% 18% 24% 5%	2400
	21st to end	Day	34% 7% 19% 28% 11% 1	2135
October	1st to 10th	Day	37% 5% 20% 28% 10%	3270
	11th to 20th	Day	39% 6% 23% 26% 5% 1	3081
	21st to end	Day	50% 7% 19% 22% 3	3009
November	1st to 10th	Day	59% 7% 13% 16% 4	1568
	11th to 20th	Day	62% 8% 14% 13% 3	1519
	21st to end	Day	59% 5% 16% 15% 4	1400
December	1st to 10th	Day	59% 10% 12% 15% 5%	1975
	11th to 20th	Day	62% 6% 15% 12% 4	2260
	21st to end	Day	49% 6% 10% 18% 15% 1	2992
January	1st to 10th	Night	59% 7% 10% 19% 5%	7111
	11th to 20th	Night	56% 7% 13% 16% 8%	6780
	21st to end	Night	52% 10% 20% 15% 4	7238
February	1st to 10th	Night	52% 8% 14% 17% 9%	2800
	11th to 20th	Night	47% 7% 14% 23% 9%	2541
	21st to end	Night	44% 7% 15% 23% 11%	2100
March	1st to 10th	Night	52% 9% 17% 17% 5%	5374
	11th to 20th	Night	46% 9% 15% 20% 9%	5142
	21st to end	Night	42% 6% 10% 23% 19%	5126
April	1st to 10th	Night	38% 8% 16% 23% 14% 1	2000
	11th to 20th	Night	45% 6% 13% 20% 14% 2	1851
	21st to end	Night	45% 7% 14% 23% 11% 1	1600
May	1st to 10th	Night	43% 6% 10% 22% 17% 2	3640
	11th to 20th	Night	46% 8% 13% 21% 10% 1	3618
	21st to end	Night	40% 7% 9% 23% 19% 1	3013
June	1st to 10th	Night	37% 8% 17% 26% 11% 2	1200
	11th to 20th	Night	47% 9% 18% 16% 9%	1200
	21st to end	Night	38% 9% 18% 25% 9% 1	1200
July	1st to 10th	Night	32% 8% 24% 32% 4	2690
	11th to 20th	Night	27% 6% 17% 43% 7%	2945
	21st to end	Night	26% 7% 24% 37% 7%	3497
August	1st to 10th	Night	29% 6% 25% 33% 7%	2600
	11th to 20th	Night	32% 8% 21% 33% 7%	2600
	21st to end	Night	30% 6% 22% 33% 9%	3324
September	1st to 10th	Night	32% 7% 17% 30% 11% 3	2000
	11th to 20th	Night	37% 7% 17% 30% 8%	2000
	21st to end	Night	29% 7% 16% 31% 15% 1	2261
October	1st to 10th	Night	35% 5% 21% 31% 8%	3920
	11th to 20th	Night	38% 5% 15% 30% 11%	3992
	21st to end	Night	46% 6% 21% 23% 5%	4547
November	1st to 10th	Night	59% 8% 12% 15% 5% 1	2730
	11th to 20th	Night	60% 8% 15% 14% 4	2880
	21st to end	Night	55% 8% 16% 17% 3	3000
December	1st to 10th	Night	64% 7% 10% 15% 4	5205
	11th to 20th	Night	56% 8% 15% 17% 4	5931
	21st to end	Night	48% 6% 11% 22% 12% 1	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	13% 7% 32% 26% 17% 6%	200
	11th to 20th	Day	11% 7% 32% 36% 14% 2	200
	21st to end	Day	12% 9% 33% 31% 13% 1	270
February	1st to 10th	Day	11% 6% 30% 37% 15% 2	300
	11th to 20th	Day	7% 4 27% 37% 22% 2	300
	21st to end	Day	7% 5% 33% 33% 16% 6%	249
March	1st to 10th	Day	10% 8% 29% 37% 14% 2	300
	11th to 20th	Day	10% 7% 37% 30% 12% 4	430
	21st to end	Day	12% 10% 31% 29% 16% 2	550
April	1st to 10th	Day	10% 10% 36% 29% 11% 4	500
	11th to 20th	Day	16% 12% 32% 27% 10% 4	535
	21st to end	Day	12% 10% 33% 28% 15% 2	700
May	1st to 10th	Day	14% 9% 26% 32% 16% 3	703
	11th to 20th	Day	12% 12% 27% 26% 18% 4	725
	21st to end	Day	16% 9% 23% 27% 20% 4	789
June	1st to 10th	Day	18% 11% 28% 26% 13% 3	730
	11th to 20th	Day	23% 11% 25% 22% 14% 4	730
	21st to end	Day	17% 7% 21% 29% 19% 6%	730
July	1st to 10th	Day	9% 6% 25% 28% 25% 7%	721
	11th to 20th	Day	9% 9% 25% 30% 19% 8%	718
	21st to end	Day	8% 7% 23% 30% 25% 8%	794
August	1st to 10th	Day	13% 6% 21% 30% 22% 7%	726
	11th to 20th	Day	10% 9% 22% 32% 20% 8%	715
	21st to end	Day	15% 8% 25% 31% 16% 4	613
September	1st to 10th	Day	13% 12% 29% 27% 16% 2	530
	11th to 20th	Day	17% 8% 28% 32% 13% 2	530
	21st to end	Day	11% 14% 28% 29% 16% 2	355
October	1st to 10th	Day	15% 9% 32% 28% 15% 2	330
	11th to 20th	Day	15% 10% 35% 26% 10% 4	324
	21st to end	Day	15% 12% 32% 26% 13% 1	330
November	1st to 10th	Day	20% 8% 32% 28% 9% 3	300
	11th to 20th	Day	15% 12% 41% 24% 7%	300
	21st to end	Day	15% 7% 38% 27% 10% 3	300
December	1st to 10th	Day	11% 8% 28% 31% 17% 5%	207
	11th to 20th	Day	13% 6% 34% 39% 10% 1	200
	21st to end	Day	9% 3 31% 29% 19% 9%	220
January	1st to 10th	Night	12% 5% 30% 38% 15% 1	1000
	11th to 20th	Night	13% 6% 32% 38% 10% 1	1000
	21st to end	Night	13% 7% 35% 35% 8% 1	1100
February	1st to 10th	Night	11% 4 31% 38% 15% 2	1000
	11th to 20th	Night	8% 5% 28% 37% 19% 3	792
	21st to end	Night	7% 5% 23% 40% 22% 2	660
March	1st to 10th	Night	13% 5% 27% 36% 18% 2	800
	11th to 20th	Night	12% 7% 31% 37% 13% 1	800
	21st to end	Night	12% 4 27% 38% 16% 3	724
April	1st to 10th	Night	9% 6% 30% 39% 15% 2	600
	11th to 20th	Night	17% 7% 31% 30% 14% 1	593
	21st to end	Night	16% 5% 29% 39% 11% 1	600
May	1st to 10th	Night	14% 7% 31% 35% 13% 1	600
	11th to 20th	Night	11% 9% 28% 36% 15% 1	591
	21st to end	Night	19% 7% 19% 37% 16% 2	452
June	1st to 10th	Night	17% 5% 29% 34% 15% 1	400
	11th to 20th	Night	20% 6% 22% 32% 19% 2	400
	21st to end	Night	17% 3 22% 40% 17% 2	400
July	1st to 10th	Night	9% 4 21% 42% 24% 1	390
	11th to 20th	Night	15% 4 21% 35% 22% 3	388
	21st to end	Night	6% 4 21% 38% 29% 3	493
August	1st to 10th	Night	7% 4 21% 37% 28% 3	597
	11th to 20th	Night	11% 4 21% 37% 24% 3	594
	21st to end	Night	12% 5% 26% 37% 20% 1	660
September	1st to 10th	Night	12% 5% 26% 36% 20% 2	600
	11th to 20th	Night	17% 5% 21% 41% 15% 1	600
	21st to end	Night	9% 6% 33% 39% 12%	687
October	1st to 10th	Night	17% 5% 30% 36% 10% 2	800
	11th to 20th	Night	14% 6% 30% 36% 13% 1	859
	21st to end	Night	18% 8% 31% 31% 11% 1	1045
November	1st to 10th	Night	22% 6% 31% 32% 8% 2	975
	11th to 20th	Night	14% 7% 33% 32% 11% 2	1000
	21st to end	Night	12% 8% 35% 34% 10% 2	1000
December	1st to 10th	Night	10% 5% 31% 36% 16% 2	1000
	11th to 20th	Night	13% 7% 32% 33% 13% 2	1000
	21st to end	Night	11% 4 26% 36% 18% 4	1100

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