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 2025-05-11.

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% 8% 3 8% 5% 5% 5% 4 6% 8% 41%	117246
	Night	5% 8% 3 9% 6% 5% 5% 5% 7% 8% 40%	120468
Transparency	Day	38% 8% 15% 25% 12% 2	116968
	Night	38% 8% 16% 24% 12% 2	120338
Seeing	Day	12% 13% 30% 28% 14% 3	16168
	Night	11% 13% 35% 29% 11% 1	25605

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	3 12% 6% 14% 7% 7% 5% 4 6% 7% 28%	3179
	11th to 20th	Day	1 6% 6% 14% 5% 4 7% 7% 7% 9% 34%	3180
	21st to end	Day	1 5% 3 12% 6% 6% 5% 4 5% 7% 46%	3866
February	1st to 10th	Day	4 13% 5% 14% 7% 6% 5% 5% 5% 8% 29%	1800
	11th to 20th	Day	3 12% 5% 11% 8% 6% 6% 6% 9% 7% 28%	1755
	21st to end	Day	3 12% 5% 11% 7% 7% 5% 5% 7% 8% 32%	1485
March	1st to 10th	Day	3 6% 3 10% 6% 5% 4 4 6% 9% 43%	4742
	11th to 20th	Day	4 7% 3 9% 6% 6% 6% 5% 8% 11% 37%	5000
	21st to end	Day	5% 16% 6% 12% 7% 6% 7% 5% 8% 7% 22%	5486
April	1st to 10th	Day	6% 12% 4 9% 7% 6% 5% 4 7% 7% 33%	2409
	11th to 20th	Day	9% 14% 5% 11% 5% 5% 5% 4 5% 6% 30%	2567
	21st to end	Day	12% 12% 4 8% 4 5% 5% 4 6% 8% 33%	2600
May	1st to 10th	Day	15% 15% 6% 9% 5% 5% 4 3 4 5% 30%	4524
	11th to 20th	Day	8% 9% 2 8% 6% 6% 6% 6% 7% 7% 34%	4419
	21st to end	Day	15% 9% 3 8% 6% 5% 5% 5% 6% 7% 31%	5161
June	1st to 10th	Day	14% 8% 2 7% 5% 4 5% 4 7% 8% 34%	2850
	11th to 20th	Day	18% 8% 2 7% 5% 6% 5% 4 6% 7% 31%	2850
	21st to end	Day	9% 4 2 6% 5% 5% 4 5% 6% 9% 45%	2850
July	1st to 10th	Day	6% 5% 2 6% 5% 4 4 4 7% 9% 49%	4740
	11th to 20th	Day	5% 3 2 4 3 4 4 4 6% 7% 58%	4740
	21st to end	Day	3 2 1 3 3 3 4 3 5% 7% 68%	5138
August	1st to 10th	Day	6% 5% 2 5% 5% 5% 5% 5% 6% 9% 49%	4197
	11th to 20th	Day	5% 3 1 4 3 4 5% 5% 7% 9% 52%	4028
	21st to end	Day	6% 4 2 3 3 3 3 4 6% 8% 58%	4521
September	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 7% 51%	2311
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 3 5% 8% 45%	2162
	21st to end	Day	10% 5% 2 4 3 3 4 4 5% 7% 51%	2079
October	1st to 10th	Day	6% 6% 2 6% 4 4 4 4 6% 10% 48%	3170
	11th to 20th	Day	3 6% 2 5% 4 4 4 4 6% 8% 53%	3150
	21st to end	Day	4 8% 3 8% 4 4 4 3 6% 7% 48%	3091
November	1st to 10th	Day	8% 15% 5% 10% 7% 5% 4 4 5% 7% 32%	1568
	11th to 20th	Day	4 9% 4 10% 7% 5% 4 4 7% 8% 38%	1600
	21st to end	Day	5% 11% 4 11% 5% 7% 6% 4 6% 8% 33%	1595
December	1st to 10th	Day	2 12% 5% 14% 8% 6% 5% 3 6% 7% 32%	2290
	11th to 20th	Day	3 10% 4 10% 6% 7% 7% 5% 8% 8% 32%	2645
	21st to end	Day	2 7% 4 9% 7% 6% 5% 4 6% 7% 42%	3498
January	1st to 10th	Night	2 9% 4 12% 8% 7% 7% 5% 8% 9% 30%	6766
	11th to 20th	Night	1 8% 6% 15% 7% 6% 5% 5% 6% 8% 32%	6780
	21st to end	Night	1 6% 4 13% 7% 5% 5% 5% 7% 9% 38%	7123
February	1st to 10th	Night	4 13% 5% 11% 6% 6% 7% 6% 6% 8% 29%	2600
	11th to 20th	Night	2 11% 4 10% 7% 7% 6% 5% 7% 8% 31%	2532
	21st to end	Night	3 11% 4 10% 7% 6% 5% 4 7% 9% 33%	2145
March	1st to 10th	Night	3 7% 3 9% 7% 6% 7% 5% 7% 9% 38%	5146
	11th to 20th	Night	5% 6% 3 8% 5% 5% 6% 5% 7% 10% 42%	5000
	21st to end	Night	7% 15% 6% 13% 6% 5% 6% 5% 9% 8% 20%	5489
April	1st to 10th	Night	7% 11% 4 9% 6% 6% 5% 5% 6% 8% 33%	1989
	11th to 20th	Night	10% 10% 5% 8% 6% 5% 4 5% 6% 8% 32%	1781
	21st to end	Night	8% 10% 3 9% 6% 5% 4 4 7% 7% 36%	1800
May	1st to 10th	Night	17% 15% 4 6% 4 4 4 3 4 5% 33%	2664
	11th to 20th	Night	7% 7% 2 7% 5% 5% 7% 5% 9% 9% 37%	2489
	21st to end	Night	13% 9% 3 8% 6% 6% 5% 5% 6% 8% 30%	2414
June	1st to 10th	Night	14% 8% 2 7% 4 4 4 5% 6% 8% 38%	1330
	11th to 20th	Night	12% 7% 3 6% 5% 4 5% 4 6% 8% 39%	1330
	21st to end	Night	7% 3 2 5% 3 4 5% 4 6% 10% 51%	1330
July	1st to 10th	Night	6% 5% 1 4 4 3 4 4 6% 8% 56%	2220
	11th to 20th	Night	4 3 1 4 3 4 3 4 5% 7% 61%	2220
	21st to end	Night	2 2 1 2 2 2 4 4 6% 6% 70%	2413
August	1st to 10th	Night	9% 4 1 4 3 3 4 4 6% 7% 55%	2739
	11th to 20th	Night	5% 3 2 5% 4 5% 5% 5% 8% 9% 50%	2811
	21st to end	Night	10% 4 2 4 3 3 3 3 4 7% 56%	3146
September	1st to 10th	Night	11% 4 2 4 4 3 4 3 4 8% 53%	1868
	11th to 20th	Night	10% 8% 2 6% 3 4 4 3 5% 7% 47%	2014
	21st to end	Night	7% 7% 2 5% 4 4 4 3 7% 8% 51%	2100
October	1st to 10th	Night	6% 6% 2 6% 4 4 5% 4 6% 7% 50%	3800
	11th to 20th	Night	5% 6% 3 6% 4 4 4 4 6% 9% 48%	3778
	21st to end	Night	4 7% 4 8% 4 4 5% 4 5% 7% 48%	4422
November	1st to 10th	Night	8% 12% 5% 8% 5% 5% 5% 4 5% 8% 34%	2730
	11th to 20th	Night	3 10% 4 9% 7% 7% 6% 5% 6% 7% 36%	2800
	21st to end	Night	4 10% 4 10% 6% 5% 6% 5% 7% 8% 36%	2805
December	1st to 10th	Night	2 10% 5% 12% 7% 5% 6% 5% 6% 8% 33%	4890
	11th to 20th	Night	2 9% 4 12% 7% 7% 6% 5% 8% 9% 31%	5546
	21st to end	Night	3 7% 3 7% 6% 6% 5% 4 6% 7% 46%	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	51% 9% 14% 22% 3 1	3179
	11th to 20th	Day	43% 8% 14% 18% 13% 4	3180
	21st to end	Day	36% 7% 23% 27% 7%	3866
February	1st to 10th	Day	51% 8% 17% 13% 10% 1	1800
	11th to 20th	Day	45% 11% 16% 17% 7% 3	1764
	21st to end	Day	47% 9% 15% 18% 10% 1	1485
March	1st to 10th	Day	35% 8% 18% 24% 14% 1	4742
	11th to 20th	Day	37% 9% 18% 24% 12%	5000
	21st to end	Day	52% 13% 17% 12% 5%	5486
April	1st to 10th	Day	45% 6% 16% 24% 8%	2409
	11th to 20th	Day	51% 6% 13% 22% 6% 1	2570
	21st to end	Day	46% 8% 14% 24% 7% 1	2600
May	1st to 10th	Day	53% 8% 12% 21% 6%	4524
	11th to 20th	Day	39% 10% 15% 27% 8%	4423
	21st to end	Day	46% 9% 15% 22% 8%	5162
June	1st to 10th	Day	44% 8% 17% 21% 10% 1	2850
	11th to 20th	Day	49% 7% 12% 20% 11% 1	2850
	21st to end	Day	33% 8% 18% 27% 12% 1	2850
July	1st to 10th	Day	29% 8% 15% 25% 18% 5%	4666
	11th to 20th	Day	22% 7% 13% 32% 19% 7%	4659
	21st to end	Day	15% 5% 9% 29% 30% 12%	4976
August	1st to 10th	Day	28% 8% 12% 31% 19% 3	4182
	11th to 20th	Day	24% 9% 15% 37% 14% 1	4067
	21st to end	Day	22% 5% 13% 40% 19% 2	4520
September	1st to 10th	Day	32% 4 14% 24% 19% 7%	2311
	11th to 20th	Day	37% 6% 13% 28% 13% 3	2162
	21st to end	Day	30% 6% 14% 31% 16% 2	2079
October	1st to 10th	Day	31% 6% 14% 32% 15% 3	3170
	11th to 20th	Day	27% 5% 19% 32% 14% 2	3150
	21st to end	Day	34% 6% 11% 35% 13%	3091
November	1st to 10th	Day	51% 7% 14% 18% 9% 1	1568
	11th to 20th	Day	40% 9% 20% 23% 8% 1	1600
	21st to end	Day	47% 9% 16% 19% 8% 1	1595
December	1st to 10th	Day	50% 7% 21% 17% 4 1	2290
	11th to 20th	Day	44% 10% 21% 22% 3	2644
	21st to end	Day	38% 7% 13% 24% 13% 5%	3498
January	1st to 10th	Night	44% 11% 17% 20% 6% 1	6766
	11th to 20th	Night	47% 6% 15% 13% 13% 5%	6780
	21st to end	Night	38% 10% 19% 25% 8% 1	7123
February	1st to 10th	Night	47% 8% 17% 14% 12% 2	2600
	11th to 20th	Night	45% 9% 15% 17% 12% 2	2533
	21st to end	Night	45% 7% 15% 22% 9% 3	2145
March	1st to 10th	Night	37% 8% 22% 17% 13% 2	5146
	11th to 20th	Night	34% 7% 18% 27% 14% 1	5000
	21st to end	Night	52% 14% 16% 12% 5% 1	5489
April	1st to 10th	Night	45% 7% 15% 22% 10% 1	1989
	11th to 20th	Night	44% 7% 14% 21% 12% 1	1781
	21st to end	Night	44% 8% 13% 26% 8% 1	1800
May	1st to 10th	Night	48% 9% 10% 23% 10% 1	2664
	11th to 20th	Night	32% 12% 15% 30% 11%	2490
	21st to end	Night	43% 12% 18% 20% 7%	2414
June	1st to 10th	Night	41% 8% 17% 23% 10% 1	1330
	11th to 20th	Night	39% 7% 13% 24% 15% 2	1330
	21st to end	Night	25% 7% 18% 34% 14% 2	1330
July	1st to 10th	Night	24% 7% 12% 33% 22% 2	2185
	11th to 20th	Night	20% 6% 11% 33% 23% 7%	2178
	21st to end	Night	12% 5% 9% 31% 31% 13%	2337
August	1st to 10th	Night	26% 6% 11% 34% 20% 4	2732
	11th to 20th	Night	25% 8% 15% 34% 15% 3	2838
	21st to end	Night	27% 5% 11% 35% 20% 3	3146
September	1st to 10th	Night	30% 5% 12% 27% 17% 9%	1868
	11th to 20th	Night	36% 7% 11% 28% 14% 5%	2014
	21st to end	Night	30% 6% 14% 30% 18% 2	2100
October	1st to 10th	Night	30% 6% 13% 35% 15% 1	3800
	11th to 20th	Night	30% 6% 16% 29% 17% 2	3778
	21st to end	Night	33% 6% 11% 36% 13%	4422
November	1st to 10th	Night	46% 8% 14% 21% 9% 2	2730
	11th to 20th	Night	41% 10% 19% 19% 11%	2800
	21st to end	Night	41% 9% 23% 17% 9% 1	2805
December	1st to 10th	Night	45% 11% 20% 17% 6% 1	4890
	11th to 20th	Night	45% 9% 24% 17% 4	5547
	21st to end	Night	35% 6% 13% 26% 15% 6%	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	7% 20% 36% 22% 12% 4	200
	11th to 20th	Day	9% 21% 32% 25% 12% 3	200
	21st to end	Day	8% 19% 44% 21% 7% 1	233
February	1st to 10th	Day	13% 23% 34% 22% 7%	300
	11th to 20th	Day	11% 12% 42% 24% 9% 2	300
	21st to end	Day	6% 19% 39% 26% 8% 2	249
March	1st to 10th	Day	9% 17% 36% 26% 10% 3	471
	11th to 20th	Day	9% 15% 40% 25% 8% 3	500
	21st to end	Day	12% 12% 37% 24% 12% 3	550
April	1st to 10th	Day	11% 18% 35% 27% 7% 1	500
	11th to 20th	Day	14% 18% 36% 23% 8% 1	492
	21st to end	Day	17% 13% 30% 28% 11% 2	500
May	1st to 10th	Day	15% 10% 25% 29% 17% 5%	670
	11th to 20th	Day	14% 7% 26% 30% 19% 4	676
	21st to end	Day	21% 9% 20% 25% 18% 7%	734
June	1st to 10th	Day	18% 11% 24% 28% 17% 3	680
	11th to 20th	Day	19% 15% 31% 22% 11% 1	680
	21st to end	Day	13% 8% 25% 30% 20% 4	680
July	1st to 10th	Day	5% 6% 25% 41% 20% 4	671
	11th to 20th	Day	7% 7% 29% 36% 17% 3	668
	21st to end	Day	6% 7% 24% 35% 22% 6%	739
August	1st to 10th	Day	9% 7% 22% 40% 18% 5%	553
	11th to 20th	Day	7% 7% 26% 35% 19% 5%	477
	21st to end	Day	7% 8% 29% 32% 19% 4	539
September	1st to 10th	Day	14% 9% 32% 29% 15% 1	490
	11th to 20th	Day	15% 10% 28% 31% 12% 3	490
	21st to end	Day	15% 15% 35% 24% 9% 3	479
October	1st to 10th	Day	16% 13% 30% 23% 15% 4	300
	11th to 20th	Day	7% 21% 31% 28% 12% 1	300
	21st to end	Day	16% 19% 36% 21% 7% 1	330
November	1st to 10th	Day	19% 21% 33% 18% 8% 1	300
	11th to 20th	Day	14% 23% 42% 16% 4 1	300
	21st to end	Day	9% 20% 38% 21% 9% 4	297
December	1st to 10th	Day	10% 29% 31% 23% 7% 2	200
	11th to 20th	Day	10% 29% 35% 20% 5% 2	200
	21st to end	Day	10% 16% 32% 29% 9% 5%	220
January	1st to 10th	Night	9% 18% 41% 24% 7% 1	1000
	11th to 20th	Night	8% 18% 40% 26% 8% 1	1000
	21st to end	Night	8% 20% 41% 21% 8% 2	983
February	1st to 10th	Night	10% 16% 33% 24% 14% 2	800
	11th to 20th	Night	6% 16% 44% 24% 9% 1	784
	21st to end	Night	10% 11% 35% 28% 13% 3	660
March	1st to 10th	Night	9% 12% 37% 31% 10% 1	800
	11th to 20th	Night	9% 12% 39% 28% 9% 2	800
	21st to end	Night	13% 12% 35% 26% 11% 2	880
April	1st to 10th	Night	10% 12% 38% 27% 10% 1	620
	11th to 20th	Night	16% 9% 36% 29% 10% 1	593
	21st to end	Night	13% 11% 30% 35% 11%	600
May	1st to 10th	Night	15% 8% 25% 32% 19% 2	600
	11th to 20th	Night	11% 6% 27% 35% 18% 3	545
	21st to end	Night	21% 9% 22% 30% 17% 3	411
June	1st to 10th	Night	17% 10% 26% 30% 16% 2	380
	11th to 20th	Night	15% 11% 33% 31% 11%	380
	21st to end	Night	10% 6% 26% 38% 18% 3	380
July	1st to 10th	Night	7% 6% 22% 45% 18% 1	370
	11th to 20th	Night	8% 5% 33% 39% 14% 1	368
	21st to end	Night	5% 5% 22% 42% 24% 2	416
August	1st to 10th	Night	8% 5% 21% 44% 19% 2	566
	11th to 20th	Night	9% 4 20% 42% 24% 1	564
	21st to end	Night	12% 5% 25% 37% 20% 1	627
September	1st to 10th	Night	13% 4 29% 38% 13% 2	570
	11th to 20th	Night	13% 8% 26% 36% 16% 1	684
	21st to end	Night	11% 9% 32% 33% 12% 3	760
October	1st to 10th	Night	15% 10% 30% 32% 13% 1	760
	11th to 20th	Night	11% 12% 34% 30% 11% 2	760
	21st to end	Night	11% 19% 36% 26% 7%	869
November	1st to 10th	Night	15% 15% 41% 24% 6%	975
	11th to 20th	Night	12% 21% 41% 21% 5% 1	1000
	21st to end	Night	9% 15% 44% 26% 5% 1	1000
December	1st to 10th	Night	9% 16% 39% 25% 9% 1	1000
	11th to 20th	Night	6% 18% 42% 25% 9% 1	1000
	21st to end	Night	9% 16% 40% 25% 9% 2	1100

Page generated at 2025-05-08 07:33:55. Version ac07

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