Alberta Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alberta Star Party clear sky chart from 2005-11-06 to 2026-10-14.

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 7% 8% 38%	154498
	Night	7% 9% 4 10% 6% 5% 6% 5% 7% 9% 33%	145995
Transparency	Day	41% 7% 16% 25% 10% 1	154195
	Night	44% 7% 15% 22% 9% 1	145881
Seeing	Day	12% 7% 24% 32% 20% 6%	18026
	Night	12% 6% 26% 39% 17% 2	26976

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	4 12% 6% 17% 8% 6% 6% 4 6% 8% 22%	4125
	11th to 20th	Day	2 8% 5% 11% 6% 7% 6% 6% 9% 9% 31%	4119
	21st to end	Day	4 10% 4 11% 6% 5% 5% 5% 8% 9% 35%	5043
February	1st to 10th	Day	7% 14% 4 12% 7% 7% 4 4 6% 9% 25%	1701
	11th to 20th	Day	6% 12% 6% 10% 8% 6% 6% 5% 7% 8% 26%	1845
	21st to end	Day	3 13% 5% 13% 9% 7% 6% 6% 6% 8% 24%	1621
March	1st to 10th	Day	3 10% 5% 12% 8% 6% 6% 4 8% 8% 28%	4650
	11th to 20th	Day	5% 11% 4 11% 6% 5% 5% 5% 6% 8% 34%	5109
	21st to end	Day	9% 18% 7% 15% 6% 5% 4 4 6% 6% 20%	5607
April	1st to 10th	Day	9% 11% 4 9% 6% 5% 6% 4 6% 8% 32%	2613
	11th to 20th	Day	11% 11% 4 8% 6% 6% 5% 5% 7% 8% 30%	2700
	21st to end	Day	13% 9% 3 9% 6% 6% 5% 3 6% 8% 32%	2730
May	1st to 10th	Day	12% 8% 3 5% 4 4 5% 5% 6% 9% 40%	8166
	11th to 20th	Day	13% 10% 3 8% 5% 5% 6% 5% 7% 8% 29%	8755
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 5% 6% 7% 39%	9649
June	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 5% 8% 9% 34%	3150
	11th to 20th	Day	18% 8% 3 7% 5% 6% 7% 4 7% 8% 28%	3150
	21st to end	Day	10% 4 2 5% 5% 5% 5% 5% 7% 9% 42%	3150
July	1st to 10th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 51%	8820
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 6% 9% 46%	8820
	21st to end	Day	6% 4 1 5% 4 4 4 3 6% 8% 55%	9569
August	1st to 10th	Day	6% 5% 2 5% 4 5% 5% 4 7% 9% 47%	6472
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 4 6% 10% 42%	5930
	21st to end	Day	8% 4 2 4 3 4 3 4 6% 9% 54%	6570
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 5% 7% 47%	2635
	11th to 20th	Day	14% 7% 2 5% 4 4 4 3 5% 7% 45%	2310
	21st to end	Day	8% 7% 2 6% 4 4 4 4 6% 8% 48%	2310
October	1st to 10th	Day	7% 7% 3 8% 4 5% 5% 4 6% 9% 42%	3485
	11th to 20th	Day	6% 5% 3 9% 5% 4 5% 4 7% 10% 42%	3100
	21st to end	Day	8% 9% 3 7% 5% 5% 6% 4 7% 7% 40%	3200
November	1st to 10th	Day	10% 17% 5% 10% 6% 5% 5% 4 6% 8% 25%	1653
	11th to 20th	Day	7% 13% 4 10% 7% 6% 5% 4 6% 7% 31%	1624
	21st to end	Day	6% 13% 4 11% 9% 6% 5% 4 6% 8% 28%	1470
December	1st to 10th	Day	10% 11% 5% 10% 7% 7% 7% 4 8% 8% 22%	2360
	11th to 20th	Day	7% 11% 5% 14% 10% 7% 6% 4 7% 7% 22%	2712
	21st to end	Day	3 7% 4 11% 5% 5% 5% 4 7% 10% 40%	3575
January	1st to 10th	Night	5% 13% 6% 15% 8% 7% 6% 5% 7% 9% 20%	8768
	11th to 20th	Night	3 8% 4 11% 6% 5% 6% 5% 7% 10% 35%	8779
	21st to end	Night	3 9% 4 11% 6% 6% 7% 5% 8% 9% 32%	9192
February	1st to 10th	Night	5% 12% 4 13% 8% 6% 6% 4 7% 8% 27%	2919
	11th to 20th	Night	5% 11% 5% 13% 7% 6% 6% 5% 7% 8% 26%	2662
	21st to end	Night	3 10% 5% 11% 7% 6% 6% 5% 6% 10% 31%	2181
March	1st to 10th	Night	3 8% 4 13% 9% 7% 6% 6% 8% 10% 27%	5464
	11th to 20th	Night	5% 8% 3 11% 7% 6% 7% 6% 9% 9% 30%	5110
	21st to end	Night	7% 17% 7% 16% 7% 6% 6% 4 6% 7% 17%	5610
April	1st to 10th	Night	11% 10% 4 9% 6% 5% 6% 4 7% 8% 31%	2006
	11th to 20th	Night	8% 11% 5% 11% 7% 6% 6% 4 7% 7% 29%	1871
	21st to end	Night	10% 10% 4 10% 5% 4 6% 4 7% 8% 32%	1890
May	1st to 10th	Night	13% 10% 3 8% 4 4 5% 4 6% 7% 36%	4757
	11th to 20th	Night	11% 9% 3 9% 6% 6% 6% 5% 8% 9% 27%	4105
	21st to end	Night	12% 8% 4 8% 6% 5% 6% 5% 6% 8% 33%	4522
June	1st to 10th	Night	15% 9% 4 8% 6% 5% 6% 4 7% 8% 29%	1470
	11th to 20th	Night	15% 8% 3 9% 8% 6% 6% 5% 7% 6% 26%	1470
	21st to end	Night	10% 5% 3 7% 5% 5% 5% 5% 6% 8% 40%	1470
July	1st to 10th	Night	5% 3 2 5% 4 4 5% 4 7% 10% 49%	4140
	11th to 20th	Night	7% 4 2 6% 4 5% 5% 4 7% 10% 46%	4140
	21st to end	Night	8% 4 1 6% 4 4 4 3 6% 10% 50%	4499
August	1st to 10th	Night	10% 7% 3 7% 4 4 4 4 5% 8% 44%	3662
	11th to 20th	Night	9% 6% 3 8% 5% 6% 6% 5% 8% 10% 35%	4136
	21st to end	Night	7% 4 2 5% 5% 4 5% 4 7% 9% 49%	4578
September	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 8% 47%	1982
	11th to 20th	Night	13% 6% 2 6% 4 4 3 3 5% 8% 46%	2310
	21st to end	Night	7% 7% 2 6% 3 4 4 4 5% 9% 48%	2310
October	1st to 10th	Night	8% 6% 3 7% 4 4 5% 4 7% 8% 44%	3927
	11th to 20th	Night	6% 8% 3 7% 4 4 5% 4 7% 8% 43%	4114
	21st to end	Night	6% 7% 3 8% 5% 6% 6% 4 7% 9% 39%	4600
November	1st to 10th	Night	10% 13% 5% 9% 5% 5% 5% 4 6% 8% 31%	2845
	11th to 20th	Night	5% 9% 4 11% 7% 6% 7% 5% 7% 9% 30%	2993
	21st to end	Night	6% 11% 4 10% 7% 6% 6% 5% 7% 9% 29%	3150
December	1st to 10th	Night	7% 14% 5% 12% 7% 6% 6% 5% 8% 9% 21%	5040
	11th to 20th	Night	6% 12% 5% 14% 9% 7% 7% 5% 7% 8% 20%	5700
	21st to end	Night	3 7% 3 10% 7% 6% 6% 5% 7% 11% 35%	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	57% 6% 8% 19% 9% 1	4125
	11th to 20th	Day	45% 6% 15% 14% 17% 2	4140
	21st to end	Day	42% 7% 18% 27% 6%	5044
February	1st to 10th	Day	56% 5% 13% 15% 10% 1	1701
	11th to 20th	Day	53% 8% 12% 17% 9% 1	1854
	21st to end	Day	55% 6% 11% 18% 9%	1621
March	1st to 10th	Day	49% 7% 17% 18% 9%	4650
	11th to 20th	Day	45% 7% 14% 21% 12%	5110
	21st to end	Day	60% 8% 10% 16% 6%	5607
April	1st to 10th	Day	47% 8% 18% 18% 8% 1	2613
	11th to 20th	Day	49% 8% 14% 20% 8% 1	2700
	21st to end	Day	48% 8% 13% 23% 7% 1	2730
May	1st to 10th	Day	39% 6% 11% 29% 12% 2	8166
	11th to 20th	Day	48% 8% 12% 23% 9% 1	8759
	21st to end	Day	40% 8% 16% 26% 9% 1	9650
June	1st to 10th	Day	44% 9% 15% 21% 10% 1	3150
	11th to 20th	Day	51% 9% 15% 19% 6% 1	3150
	21st to end	Day	34% 11% 20% 27% 7% 1	3150
July	1st to 10th	Day	28% 8% 18% 32% 13% 1	8746
	11th to 20th	Day	33% 8% 18% 27% 11% 3	8739
	21st to end	Day	25% 7% 17% 31% 16% 3	9362
August	1st to 10th	Day	31% 8% 18% 30% 11% 2	6454
	11th to 20th	Day	36% 8% 25% 26% 5%	5969
	21st to end	Day	26% 6% 20% 36% 11% 1	6569
September	1st to 10th	Day	35% 8% 15% 27% 11% 4	2635
	11th to 20th	Day	39% 6% 14% 30% 10% 1	2310
	21st to end	Day	33% 8% 17% 28% 11% 4	2310
October	1st to 10th	Day	38% 6% 20% 28% 8% 1	3485
	11th to 20th	Day	35% 7% 19% 27% 11% 1	3100
	21st to end	Day	40% 8% 11% 32% 9% 1	3200
November	1st to 10th	Day	57% 6% 12% 18% 6% 1	1653
	11th to 20th	Day	50% 6% 15% 20% 8% 1	1624
	21st to end	Day	52% 6% 16% 18% 7% 1	1470
December	1st to 10th	Day	57% 6% 18% 14% 4 1	2360
	11th to 20th	Day	57% 8% 15% 16% 3 1	2714
	21st to end	Day	39% 5% 12% 28% 14% 2	3575
January	1st to 10th	Night	58% 7% 11% 15% 8% 1	8769
	11th to 20th	Night	42% 5% 17% 19% 14% 2	8820
	21st to end	Night	43% 7% 19% 22% 7%	9192
February	1st to 10th	Night	52% 6% 12% 19% 11%	2919
	11th to 20th	Night	52% 7% 12% 17% 11% 1	2663
	21st to end	Night	47% 6% 13% 21% 11% 1	2181
March	1st to 10th	Night	47% 8% 20% 18% 6%	5464
	11th to 20th	Night	45% 8% 17% 22% 7% 1	5110
	21st to end	Night	63% 8% 10% 13% 5% 1	5610
April	1st to 10th	Night	50% 7% 15% 17% 11% 1	2006
	11th to 20th	Night	50% 7% 12% 21% 8% 1	1871
	21st to end	Night	46% 8% 13% 23% 9% 2	1890
May	1st to 10th	Night	46% 6% 9% 25% 13% 1	4754
	11th to 20th	Night	46% 9% 13% 23% 8%	4105
	21st to end	Night	46% 10% 14% 23% 7% 1	4526
June	1st to 10th	Night	49% 9% 13% 19% 10%	1470
	11th to 20th	Night	54% 10% 13% 15% 7% 1	1470
	21st to end	Night	39% 10% 19% 24% 8% 1	1470
July	1st to 10th	Night	28% 8% 23% 27% 13% 1	4105
	11th to 20th	Night	32% 7% 18% 31% 9% 3	4098
	21st to end	Night	29% 7% 17% 30% 14% 2	4401
August	1st to 10th	Night	38% 7% 15% 30% 10% 1	3655
	11th to 20th	Night	39% 11% 21% 25% 4 1	4163
	21st to end	Night	29% 6% 19% 36% 8% 1	4578
September	1st to 10th	Night	34% 8% 15% 25% 13% 6%	1982
	11th to 20th	Night	36% 6% 15% 28% 12% 3	2310
	21st to end	Night	32% 7% 16% 29% 15% 2	2310
October	1st to 10th	Night	36% 6% 20% 27% 11% 1	3927
	11th to 20th	Night	36% 6% 16% 31% 9% 1	4114
	21st to end	Night	39% 7% 15% 31% 6% 1	4600
November	1st to 10th	Night	50% 6% 13% 21% 8% 1	2845
	11th to 20th	Night	48% 9% 15% 19% 9% 1	2993
	21st to end	Night	49% 9% 16% 18% 6% 1	3150
December	1st to 10th	Night	56% 8% 16% 15% 5% 1	5040
	11th to 20th	Night	58% 9% 16% 13% 4	5697
	21st to end	Night	40% 6% 12% 24% 15% 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	8% 4 22% 35% 21% 10%	220
	11th to 20th	Day	9% 6% 27% 35% 22% 1	220
	21st to end	Day	11% 9% 23% 32% 19% 6%	242
February	1st to 10th	Day	10% 8% 26% 30% 19% 6%	231
	11th to 20th	Day	9% 6% 23% 37% 19% 5%	330
	21st to end	Day	7% 5% 23% 40% 20% 5%	337
March	1st to 10th	Day	9% 4 30% 31% 20% 5%	540
	11th to 20th	Day	8% 4 27% 34% 20% 6%	540
	21st to end	Day	13% 6% 28% 30% 17% 5%	594
April	1st to 10th	Day	12% 12% 25% 33% 12% 6%	540
	11th to 20th	Day	14% 10% 32% 26% 15% 4	532
	21st to end	Day	18% 10% 24% 28% 15% 4	540
May	1st to 10th	Day	15% 10% 23% 27% 20% 6%	664
	11th to 20th	Day	13% 9% 21% 34% 19% 4	775
	21st to end	Day	16% 5% 21% 30% 21% 7%	844
June	1st to 10th	Day	16% 8% 24% 33% 16% 3	780
	11th to 20th	Day	17% 9% 23% 32% 15% 4	780
	21st to end	Day	12% 4 22% 31% 25% 7%	780
July	1st to 10th	Day	9% 5% 17% 35% 26% 8%	771
	11th to 20th	Day	11% 5% 20% 33% 24% 7%	768
	21st to end	Day	7% 3 19% 32% 28% 10%	849
August	1st to 10th	Day	10% 5% 22% 33% 21% 10%	585
	11th to 20th	Day	10% 4 20% 32% 26% 7%	557
	21st to end	Day	9% 8% 19% 30% 23% 10%	627
September	1st to 10th	Day	15% 5% 25% 28% 21% 6%	570
	11th to 20th	Day	17% 6% 19% 29% 22% 7%	570
	21st to end	Day	11% 12% 26% 33% 15% 4	570
October	1st to 10th	Day	14% 6% 24% 31% 19% 7%	436
	11th to 20th	Day	10% 12% 29% 29% 14% 6%	339
	21st to end	Day	13% 12% 31% 30% 12% 2	363
November	1st to 10th	Day	20% 7% 24% 33% 13% 3	330
	11th to 20th	Day	14% 4 29% 34% 16% 3	300
	21st to end	Day	10% 4 25% 35% 17% 9%	220
December	1st to 10th	Day	10% 4 27% 33% 19% 7%	220
	11th to 20th	Day	9% 5% 35% 33% 16% 2	220
	21st to end	Day	9% 2 20% 35% 25% 8%	242
January	1st to 10th	Night	10% 6% 27% 39% 16% 2	1050
	11th to 20th	Night	11% 6% 29% 38% 13% 2	1050
	21st to end	Night	12% 7% 25% 41% 13% 2	992
February	1st to 10th	Night	11% 7% 23% 38% 19% 3	840
	11th to 20th	Night	11% 5% 27% 39% 17% 1	824
	21st to end	Night	8% 3 21% 43% 20% 5%	692
March	1st to 10th	Night	8% 5% 26% 41% 18% 4	840
	11th to 20th	Night	9% 5% 26% 41% 17% 3	840
	21st to end	Night	15% 5% 25% 36% 17% 2	924
April	1st to 10th	Night	14% 6% 27% 33% 17% 2	735
	11th to 20th	Night	12% 9% 30% 36% 12% 1	623
	21st to end	Night	16% 8% 24% 38% 14% 1	630
May	1st to 10th	Night	17% 9% 24% 35% 14% 1	560
	11th to 20th	Night	12% 6% 26% 37% 17% 2	414
	21st to end	Night	20% 5% 23% 31% 20% 2	455
June	1st to 10th	Night	20% 7% 25% 35% 12% 1	420
	11th to 20th	Night	16% 5% 29% 32% 17%	420
	21st to end	Night	13% 4 23% 36% 22% 2	420
July	1st to 10th	Night	9% 3 18% 47% 22% 1	410
	11th to 20th	Night	11% 2 20% 46% 19% 1	408
	21st to end	Night	7% 2 21% 42% 26% 2	460
August	1st to 10th	Night	12% 4 18% 38% 26% 2	418
	11th to 20th	Night	8% 5% 16% 45% 24% 1	619
	21st to end	Night	9% 5% 19% 43% 24% 1	693
September	1st to 10th	Night	13% 4 20% 42% 20% 2	672
	11th to 20th	Night	14% 4 18% 41% 23% 1	840
	21st to end	Night	10% 5% 27% 41% 15% 2	840
October	1st to 10th	Night	13% 6% 24% 38% 17% 2	840
	11th to 20th	Night	13% 6% 28% 38% 14% 2	812
	21st to end	Night	12% 7% 31% 38% 12% 1	880
November	1st to 10th	Night	16% 7% 29% 36% 11% 1	1000
	11th to 20th	Night	9% 7% 35% 36% 12% 2	1050
	21st to end	Night	10% 5% 28% 42% 14% 1	1050
December	1st to 10th	Night	13% 7% 26% 36% 15% 2	1050
	11th to 20th	Night	10% 5% 32% 38% 13% 2	1050
	21st to end	Night	11% 5% 26% 39% 17% 3	1155

Page generated at 2026-10-11 03:06:26. Version ac07

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