Southern Alberta Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Southern Alberta Star Party clear sky chart from 2005-11-06 to 2026-09-14.

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	7% 7% 3 8% 5% 5% 5% 4 6% 8% 43%	158872
	Night	5% 8% 3 8% 5% 5% 5% 4 7% 8% 41%	149554
Transparency	Day	36% 8% 15% 26% 13% 3	153507
	Night	37% 8% 16% 24% 13% 3	145931
Seeing	Day	13% 14% 31% 27% 13% 3	18095
	Night	11% 13% 35% 29% 11% 1	27217

By Month

Percentage Cloud Cover SW (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	2 10% 5% 12% 7% 7% 5% 4 7% 9% 30%	4124
	11th to 20th	Day	1 5% 5% 14% 6% 4 7% 7% 7% 9% 35%	4140
	21st to end	Day	1 5% 3 11% 7% 7% 6% 4 5% 6% 45%	5008
February	1st to 10th	Day	4 12% 5% 13% 7% 6% 5% 5% 6% 8% 30%	1890
	11th to 20th	Day	3 12% 5% 10% 7% 6% 6% 6% 9% 7% 29%	1845
	21st to end	Day	3 11% 4 10% 7% 7% 5% 5% 6% 8% 32%	1557
March	1st to 10th	Day	3 6% 3 10% 6% 5% 4 4 6% 9% 43%	4847
	11th to 20th	Day	4 7% 3 9% 6% 6% 6% 5% 8% 11% 37%	5110
	21st to end	Day	5% 15% 6% 13% 7% 6% 7% 5% 8% 7% 22%	5607
April	1st to 10th	Day	6% 12% 4 9% 7% 6% 5% 4 7% 7% 34%	2529
	11th to 20th	Day	9% 14% 5% 11% 5% 5% 5% 4 5% 7% 30%	2697
	21st to end	Day	11% 12% 4 9% 4 5% 4 3 6% 8% 34%	2730
May	1st to 10th	Day	10% 10% 4 7% 5% 4 5% 4 6% 7% 38%	8137
	11th to 20th	Day	10% 11% 4 10% 6% 6% 6% 5% 7% 8% 28%	8246
	21st to end	Day	11% 7% 2 7% 5% 5% 5% 5% 6% 8% 39%	9648
June	1st to 10th	Day	14% 9% 2 8% 5% 4 5% 4 7% 8% 34%	3150
	11th to 20th	Day	17% 8% 3 7% 5% 6% 5% 4 6% 8% 31%	3150
	21st to end	Day	10% 4 2 6% 5% 5% 5% 5% 7% 9% 43%	3150
July	1st to 10th	Day	6% 4 2 5% 3 3 4 4 6% 8% 55%	8819
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 4 7% 8% 49%	8820
	21st to end	Day	5% 4 1 3 2 3 3 3 5% 7% 64%	9575
August	1st to 10th	Day	7% 5% 2 5% 4 5% 5% 4 6% 8% 50%	7737
	11th to 20th	Day	6% 4 2 5% 4 5% 6% 5% 7% 10% 46%	7565
	21st to end	Day	7% 4 2 4 3 3 3 3 5% 7% 60%	8374
September	1st to 10th	Day	12% 6% 2 5% 3 4 3 3 5% 7% 51%	2554
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 3 5% 8% 45%	2313
	21st to end	Day	10% 5% 2 4 3 3 4 4 5% 7% 52%	2188
October	1st to 10th	Day	6% 6% 2 6% 4 4 4 4 6% 9% 49%	3270
	11th to 20th	Day	3 6% 3 5% 4 4 4 4 6% 8% 52%	3250
	21st to end	Day	4 8% 3 8% 4 4 4 3 6% 7% 49%	3190
November	1st to 10th	Day	7% 15% 5% 10% 7% 5% 4 4 5% 7% 32%	1648
	11th to 20th	Day	4 9% 4 10% 7% 5% 4 4 7% 8% 38%	1680
	21st to end	Day	4 11% 4 11% 5% 7% 6% 4 7% 8% 33%	1674
December	1st to 10th	Day	2 12% 5% 14% 8% 6% 5% 3 6% 7% 32%	2360
	11th to 20th	Day	3 10% 4 10% 6% 7% 7% 5% 8% 9% 32%	2715
	21st to end	Day	2 7% 4 9% 7% 6% 5% 4 6% 7% 42%	3575
January	1st to 10th	Night	2 7% 4 10% 7% 7% 6% 5% 8% 10% 34%	8768
	11th to 20th	Night	1 7% 6% 15% 7% 5% 5% 5% 7% 9% 35%	8820
	21st to end	Night	1 5% 4 12% 8% 6% 6% 5% 8% 9% 37%	9208
February	1st to 10th	Night	4 12% 5% 11% 6% 6% 7% 6% 6% 9% 29%	2730
	11th to 20th	Night	2 10% 4 10% 7% 7% 6% 5% 7% 8% 32%	2662
	21st to end	Night	3 11% 4 10% 7% 6% 5% 4 7% 9% 34%	2249
March	1st to 10th	Night	3 7% 3 9% 7% 6% 7% 5% 7% 10% 38%	5261
	11th to 20th	Night	5% 6% 3 8% 5% 5% 6% 5% 7% 10% 42%	5110
	21st to end	Night	6% 15% 6% 13% 6% 5% 6% 5% 9% 8% 20%	5610
April	1st to 10th	Night	7% 11% 4 9% 6% 6% 6% 5% 6% 8% 33%	2088
	11th to 20th	Night	9% 10% 4 8% 6% 6% 5% 5% 7% 8% 32%	1871
	21st to end	Night	8% 10% 4 10% 5% 5% 4 4 7% 7% 36%	1890
May	1st to 10th	Night	12% 10% 3 5% 4 4 4 3 5% 6% 45%	4808
	11th to 20th	Night	8% 11% 4 9% 6% 5% 6% 5% 7% 9% 29%	4641
	21st to end	Night	10% 7% 2 6% 5% 5% 5% 5% 6% 9% 40%	4525
June	1st to 10th	Night	14% 8% 2 6% 4 4 4 5% 6% 9% 39%	1470
	11th to 20th	Night	12% 6% 3 7% 5% 4 5% 4 6% 8% 39%	1470
	21st to end	Night	8% 4 2 5% 3 3 4 4 7% 10% 50%	1470
July	1st to 10th	Night	6% 4 1 4 3 3 3 3 6% 8% 60%	4140
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 50%	4140
	21st to end	Night	4 4 1 4 2 3 4 4 5% 6% 64%	4500
August	1st to 10th	Night	9% 4 2 5% 4 4 4 3 6% 8% 53%	5068
	11th to 20th	Night	7% 5% 2 5% 4 5% 5% 5% 8% 10% 46%	5291
	21st to end	Night	9% 4 1 4 2 3 3 3 4 7% 60%	5842
September	1st to 10th	Night	11% 4 2 4 4 3 4 3 4 8% 53%	2065
	11th to 20th	Night	10% 8% 2 6% 3 4 4 3 5% 7% 48%	2150
	21st to end	Night	7% 6% 2 5% 3 3 3 3 6% 8% 52%	2211
October	1st to 10th	Night	6% 5% 2 5% 4 4 5% 4 6% 7% 50%	3920
	11th to 20th	Night	5% 7% 3 6% 4 4 4 4 6% 9% 48%	3898
	21st to end	Night	4 7% 3 8% 4 4 5% 4 5% 7% 49%	4564
November	1st to 10th	Night	8% 12% 5% 9% 5% 5% 5% 4 6% 8% 34%	2870
	11th to 20th	Night	3 10% 4 9% 7% 7% 6% 5% 6% 7% 36%	2940
	21st to end	Night	4 10% 4 10% 5% 5% 6% 5% 7% 8% 36%	2945
December	1st to 10th	Night	2 11% 5% 12% 7% 5% 6% 5% 6% 8% 32%	5040
	11th to 20th	Night	2 9% 4 12% 7% 7% 6% 5% 8% 9% 31%	5696
	21st to end	Night	3 7% 3 7% 6% 6% 5% 4 6% 7% 45%	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	45% 9% 18% 22% 5% 1	4125
	11th to 20th	Day	42% 8% 17% 18% 12% 4	4140
	21st to end	Day	37% 7% 23% 24% 8% 2	5008
February	1st to 10th	Day	49% 8% 17% 14% 10% 1	1890
	11th to 20th	Day	44% 11% 16% 18% 8% 3	1854
	21st to end	Day	47% 9% 15% 19% 9% 1	1557
March	1st to 10th	Day	35% 8% 18% 24% 14% 1	4847
	11th to 20th	Day	37% 9% 18% 24% 12%	5110
	21st to end	Day	52% 13% 17% 12% 5%	5607
April	1st to 10th	Day	45% 6% 16% 24% 8%	2529
	11th to 20th	Day	51% 6% 13% 23% 6% 1	2700
	21st to end	Day	46% 8% 13% 25% 8% 1	2730
May	1st to 10th	Day	40% 7% 12% 26% 11% 3	8137
	11th to 20th	Day	47% 9% 11% 25% 8%	8250
	21st to end	Day	37% 9% 14% 29% 11%	9650
June	1st to 10th	Day	44% 7% 16% 21% 10% 1	3150
	11th to 20th	Day	48% 7% 12% 21% 10% 1	3150
	21st to end	Day	34% 8% 18% 27% 12% 1	3150
July	1st to 10th	Day	24% 7% 16% 28% 21% 5%	8746
	11th to 20th	Day	29% 8% 15% 25% 16% 7%	8739
	21st to end	Day	19% 6% 11% 27% 24% 14%	9362
August	1st to 10th	Day	24% 7% 13% 32% 20% 5%	6127
	11th to 20th	Day	29% 10% 19% 31% 11% 1	5964
	21st to end	Day	23% 5% 12% 41% 18% 1	6569
September	1st to 10th	Day	32% 4 14% 26% 18% 6%	2554
	11th to 20th	Day	37% 6% 13% 28% 13% 3	2313
	21st to end	Day	30% 6% 14% 31% 17% 2	2188
October	1st to 10th	Day	30% 5% 14% 33% 15% 3	3270
	11th to 20th	Day	28% 6% 19% 32% 14% 2	3250
	21st to end	Day	34% 6% 11% 35% 14%	3190
November	1st to 10th	Day	50% 7% 14% 18% 9% 1	1648
	11th to 20th	Day	39% 9% 20% 23% 8% 1	1680
	21st to end	Day	46% 9% 16% 19% 8% 1	1674
December	1st to 10th	Day	50% 7% 21% 17% 4 1	2360
	11th to 20th	Day	44% 10% 21% 22% 3	2714
	21st to end	Day	38% 7% 13% 24% 13% 5%	3575
January	1st to 10th	Night	39% 11% 18% 23% 9% 1	8769
	11th to 20th	Night	44% 6% 19% 16% 12% 4	8820
	21st to end	Night	39% 9% 19% 22% 8% 2	9208
February	1st to 10th	Night	46% 8% 17% 15% 11% 2	2730
	11th to 20th	Night	44% 9% 15% 17% 13% 2	2663
	21st to end	Night	44% 7% 15% 23% 9% 2	2249
March	1st to 10th	Night	37% 8% 22% 18% 13% 2	5261
	11th to 20th	Night	34% 7% 18% 27% 14% 1	5110
	21st to end	Night	52% 14% 16% 12% 5% 1	5610
April	1st to 10th	Night	45% 7% 16% 22% 9%	2088
	11th to 20th	Night	44% 7% 14% 22% 12% 1	1871
	21st to end	Night	44% 8% 13% 26% 9% 1	1890
May	1st to 10th	Night	37% 7% 9% 29% 16% 2	4808
	11th to 20th	Night	43% 10% 12% 24% 10% 1	4642
	21st to end	Night	33% 10% 17% 30% 9%	4526
June	1st to 10th	Night	40% 7% 16% 25% 11% 1	1470
	11th to 20th	Night	39% 8% 12% 25% 15% 1	1470
	21st to end	Night	26% 7% 17% 35% 13% 2	1470
July	1st to 10th	Night	21% 5% 14% 32% 22% 5%	4105
	11th to 20th	Night	29% 6% 12% 25% 20% 8%	4098
	21st to end	Night	19% 5% 10% 28% 23% 15%	4401
August	1st to 10th	Night	23% 5% 11% 33% 22% 7%	4003
	11th to 20th	Night	32% 9% 18% 29% 11% 2	4168
	21st to end	Night	26% 5% 11% 37% 20% 2	4578
September	1st to 10th	Night	30% 5% 12% 28% 16% 8%	2065
	11th to 20th	Night	34% 7% 12% 29% 14% 4	2150
	21st to end	Night	29% 6% 14% 31% 18% 2	2211
October	1st to 10th	Night	29% 6% 13% 35% 15% 1	3920
	11th to 20th	Night	30% 6% 16% 29% 17% 2	3898
	21st to end	Night	32% 6% 11% 36% 13%	4564
November	1st to 10th	Night	46% 8% 14% 22% 9% 2	2870
	11th to 20th	Night	41% 9% 20% 19% 11%	2940
	21st to end	Night	41% 9% 23% 17% 9% 1	2945
December	1st to 10th	Night	46% 10% 20% 17% 6% 1	5040
	11th to 20th	Night	45% 9% 24% 17% 4	5697
	21st to end	Night	35% 6% 13% 26% 15% 6%	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	9% 21% 36% 20% 11% 4	220
	11th to 20th	Day	12% 22% 30% 22% 11% 2	220
	21st to end	Day	9% 20% 45% 20% 6% 1	256
February	1st to 10th	Day	13% 24% 35% 20% 7%	330
	11th to 20th	Day	12% 13% 42% 24% 8% 2	330
	21st to end	Day	10% 19% 38% 25% 8% 1	273
March	1st to 10th	Day	11% 17% 36% 25% 9% 2	510
	11th to 20th	Day	12% 16% 38% 24% 8% 3	540
	21st to end	Day	14% 13% 37% 22% 11% 3	594
April	1st to 10th	Day	12% 18% 36% 26% 7% 1	540
	11th to 20th	Day	13% 18% 38% 23% 8% 1	532
	21st to end	Day	17% 13% 30% 28% 10% 2	540
May	1st to 10th	Day	15% 11% 26% 27% 16% 4	718
	11th to 20th	Day	16% 10% 26% 28% 17% 3	775
	21st to end	Day	19% 9% 23% 25% 16% 7%	844
June	1st to 10th	Day	19% 13% 25% 26% 15% 2	780
	11th to 20th	Day	19% 16% 33% 21% 10% 1	780
	21st to end	Day	14% 8% 27% 28% 18% 4	780
July	1st to 10th	Day	5% 8% 26% 39% 19% 3	771
	11th to 20th	Day	8% 9% 31% 33% 16% 2	768
	21st to end	Day	6% 9% 26% 34% 20% 5%	849
August	1st to 10th	Day	10% 8% 24% 39% 16% 4	639
	11th to 20th	Day	8% 8% 29% 33% 18% 5%	557
	21st to end	Day	7% 10% 29% 32% 18% 4	627
September	1st to 10th	Day	15% 10% 32% 28% 14% 1	570
	11th to 20th	Day	15% 10% 28% 32% 12% 3	543
	21st to end	Day	14% 15% 36% 24% 8% 3	518
October	1st to 10th	Day	16% 14% 30% 22% 15% 4	330
	11th to 20th	Day	9% 22% 30% 26% 11% 1	330
	21st to end	Day	16% 20% 37% 20% 6% 1	363
November	1st to 10th	Day	21% 22% 32% 17% 7% 1	330
	11th to 20th	Day	14% 24% 41% 16% 4 1	330
	21st to end	Day	10% 20% 37% 20% 10% 3	326
December	1st to 10th	Day	12% 29% 30% 21% 6% 2	220
	11th to 20th	Day	14% 28% 33% 19% 5% 1	220
	21st to end	Day	10% 19% 32% 26% 8% 4	242
January	1st to 10th	Night	9% 20% 41% 23% 7% 1	1050
	11th to 20th	Night	10% 18% 39% 24% 8% 1	1050
	21st to end	Night	9% 19% 41% 22% 8% 1	1032
February	1st to 10th	Night	10% 16% 34% 25% 13% 2	840
	11th to 20th	Night	7% 16% 44% 24% 8% 1	824
	21st to end	Night	10% 12% 34% 27% 12% 3	692
March	1st to 10th	Night	10% 12% 37% 30% 10% 1	840
	11th to 20th	Night	11% 13% 39% 28% 8% 2	840
	21st to end	Night	14% 13% 36% 25% 11% 2	924
April	1st to 10th	Night	11% 13% 38% 27% 10% 1	651
	11th to 20th	Night	15% 10% 37% 28% 9% 1	623
	21st to end	Night	14% 11% 29% 35% 11%	630
May	1st to 10th	Night	15% 8% 25% 33% 18% 2	630
	11th to 20th	Night	12% 8% 28% 33% 17% 2	601
	21st to end	Night	20% 9% 23% 30% 17% 2	455
June	1st to 10th	Night	18% 11% 25% 29% 15% 2	420
	11th to 20th	Night	15% 12% 33% 30% 10%	420
	21st to end	Night	11% 6% 27% 37% 17% 2	420
July	1st to 10th	Night	7% 7% 25% 43% 17% 1	410
	11th to 20th	Night	8% 6% 34% 38% 13% 1	408
	21st to end	Night	5% 6% 24% 42% 22% 2	460
August	1st to 10th	Night	8% 6% 21% 45% 18% 2	626
	11th to 20th	Night	9% 4 21% 42% 22% 1	624
	21st to end	Night	12% 6% 26% 36% 19% 1	693
September	1st to 10th	Night	13% 5% 30% 37% 13% 2	630
	11th to 20th	Night	13% 8% 27% 36% 16% 1	729
	21st to end	Night	11% 9% 33% 33% 11% 3	800
October	1st to 10th	Night	14% 9% 31% 32% 13% 1	800
	11th to 20th	Night	11% 14% 34% 29% 11% 2	800
	21st to end	Night	12% 19% 36% 25% 7%	915
November	1st to 10th	Night	15% 16% 40% 23% 6%	1025
	11th to 20th	Night	12% 20% 41% 21% 5% 1	1050
	21st to end	Night	9% 16% 44% 25% 5% 1	1050
December	1st to 10th	Night	10% 16% 39% 24% 9% 1	1050
	11th to 20th	Night	8% 19% 40% 24% 8% 1	1050
	21st to end	Night	9% 16% 41% 24% 8% 2	1155

Page generated at 2026-09-11 02:11:52. Version ac07

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