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 2024-11-02.

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%	101821
	Night	6% 8% 3 8% 5% 5% 5% 5% 7% 8% 40%	97303
Transparency	Day	37% 8% 15% 25% 12% 2	101543
	Night	37% 9% 16% 24% 12% 2	97173
Seeing	Day	12% 12% 30% 28% 14% 3	15579
	Night	10% 12% 35% 30% 12% 1	24798

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 11% 6% 11% 5% 6% 5% 5% 8% 9% 32%	2219
	11th to 20th	Day	2 7% 3 13% 7% 5% 8% 9% 6% 8% 32%	2220
	21st to end	Day	2 7% 3 8% 6% 6% 5% 5% 6% 7% 45%	2684
February	1st to 10th	Day	4 12% 5% 14% 7% 6% 5% 5% 5% 8% 29%	1710
	11th to 20th	Day	3 13% 5% 11% 8% 6% 6% 6% 9% 7% 27%	1665
	21st to end	Day	3 12% 5% 11% 7% 7% 5% 5% 6% 7% 31%	1413
March	1st to 10th	Day	2 7% 3 11% 7% 5% 5% 5% 7% 9% 40%	3298
	11th to 20th	Day	6% 7% 3 7% 5% 5% 5% 4 7% 11% 39%	3490
	21st to end	Day	6% 16% 5% 13% 6% 5% 6% 5% 7% 6% 23%	3825
April	1st to 10th	Day	6% 12% 4 9% 7% 6% 5% 4 7% 7% 32%	2288
	11th to 20th	Day	9% 14% 5% 11% 5% 5% 5% 4 5% 6% 29%	2437
	21st to end	Day	12% 12% 4 8% 4 5% 4 4 6% 8% 33%	2470
May	1st to 10th	Day	16% 15% 6% 9% 5% 5% 4 3 4 5% 29%	4385
	11th to 20th	Day	8% 9% 2 8% 6% 6% 6% 6% 7% 7% 34%	4417
	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	9% 9% 3 8% 4 4 3 4 6% 9% 41%	1900
	11th to 20th	Day	5% 9% 3 7% 5% 4 5% 4 7% 7% 44%	1900
	21st to end	Day	6% 12% 4 8% 5% 4 5% 3 5% 8% 40%	1884
November	1st to 10th	Day	8% 15% 5% 9% 7% 4 4 4 5% 6% 32%	1496
	11th to 20th	Day	4 9% 4 10% 7% 5% 4 4 6% 8% 38%	1520
	21st to end	Day	4 10% 4 11% 4 7% 6% 5% 6% 8% 34%	1516
December	1st to 10th	Day	4 13% 5% 11% 8% 7% 6% 3 6% 6% 32%	1330
	11th to 20th	Day	3 9% 4 9% 6% 7% 7% 5% 8% 9% 34%	1685
	21st to end	Day	2 8% 3 8% 6% 5% 4 4 5% 8% 48%	2442
January	1st to 10th	Night	2 8% 3 8% 7% 7% 7% 5% 8% 10% 34%	4726
	11th to 20th	Night	2 8% 5% 12% 7% 6% 6% 6% 8% 9% 30%	4740
	21st to end	Night	1 7% 4 9% 6% 5% 5% 6% 8% 10% 39%	4989
February	1st to 10th	Night	5% 12% 5% 11% 6% 6% 6% 6% 6% 8% 29%	2470
	11th to 20th	Night	2 11% 4 11% 7% 7% 6% 5% 7% 8% 30%	2402
	21st to end	Night	3 11% 4 11% 7% 6% 6% 4 7% 8% 33%	2041
March	1st to 10th	Night	3 7% 4 10% 8% 7% 8% 6% 7% 8% 33%	3571
	11th to 20th	Night	6% 7% 2 6% 5% 4 5% 4 7% 10% 44%	3490
	21st to end	Night	8% 15% 6% 14% 7% 5% 5% 5% 10% 7% 19%	3828
April	1st to 10th	Night	7% 11% 4 9% 7% 6% 5% 5% 6% 8% 32%	1890
	11th to 20th	Night	10% 10% 5% 8% 6% 5% 4 5% 6% 8% 32%	1691
	21st to end	Night	8% 10% 3 9% 6% 5% 4 5% 8% 7% 35%	1710
May	1st to 10th	Night	17% 15% 4 6% 4 4 4 3 4 5% 32%	2583
	11th to 20th	Night	7% 7% 2 7% 5% 5% 7% 5% 9% 9% 38%	2485
	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	10% 8% 3 6% 4 4 5% 4 6% 7% 44%	2280
	11th to 20th	Night	5% 8% 3 7% 6% 4 4 4 6% 9% 43%	2280
	21st to end	Night	6% 10% 4 9% 5% 5% 5% 4 6% 7% 38%	2693
November	1st to 10th	Night	8% 12% 5% 8% 5% 5% 6% 4 5% 8% 34%	2605
	11th to 20th	Night	3 10% 4 8% 6% 7% 6% 5% 6% 8% 37%	2660
	21st to end	Night	4 10% 4 9% 5% 5% 5% 5% 7% 8% 37%	2664
December	1st to 10th	Night	2 12% 4 10% 8% 6% 5% 5% 6% 8% 34%	2850
	11th to 20th	Night	2 8% 4 10% 7% 6% 6% 5% 8% 10% 35%	3506
	21st to end	Night	3 7% 2 6% 5% 5% 5% 4 6% 7% 49%	5214

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	42% 11% 19% 23% 5% 1	2219
	11th to 20th	Day	45% 9% 15% 18% 13% 1	2220
	21st to end	Day	34% 9% 25% 25% 6%	2684
February	1st to 10th	Day	50% 9% 17% 14% 9% 1	1710
	11th to 20th	Day	46% 11% 16% 18% 7% 2	1674
	21st to end	Day	49% 9% 15% 16% 10% 1	1413
March	1st to 10th	Day	36% 9% 18% 26% 10%	3298
	11th to 20th	Day	35% 9% 22% 23% 9% 1	3490
	21st to end	Day	51% 14% 17% 12% 5% 1	3825
April	1st to 10th	Day	46% 6% 17% 23% 8%	2288
	11th to 20th	Day	52% 6% 13% 22% 6% 1	2440
	21st to end	Day	47% 8% 14% 24% 7% 1	2470
May	1st to 10th	Day	54% 8% 12% 21% 5%	4385
	11th to 20th	Day	39% 9% 15% 27% 8%	4421
	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	39% 7% 15% 25% 12% 1	1900
	11th to 20th	Day	37% 6% 18% 25% 12% 2	1900
	21st to end	Day	42% 7% 15% 25% 11%	1884
November	1st to 10th	Day	51% 7% 14% 18% 9% 1	1496
	11th to 20th	Day	40% 9% 20% 22% 8% 1	1520
	21st to end	Day	45% 9% 16% 20% 9% 1	1516
December	1st to 10th	Day	50% 8% 18% 16% 7% 2	1330
	11th to 20th	Day	41% 11% 20% 23% 5%	1684
	21st to end	Day	34% 7% 15% 24% 14% 7%	2442
January	1st to 10th	Night	37% 13% 22% 22% 5% 1	4726
	11th to 20th	Night	46% 7% 14% 16% 13% 3	4740
	21st to end	Night	34% 13% 19% 27% 7% 1	4989
February	1st to 10th	Night	47% 8% 17% 15% 11% 2	2470
	11th to 20th	Night	46% 10% 15% 16% 11% 1	2403
	21st to end	Night	45% 8% 14% 22% 9% 3	2041
March	1st to 10th	Night	43% 10% 20% 19% 8% 1	3571
	11th to 20th	Night	33% 8% 22% 26% 11% 1	3490
	21st to end	Night	54% 16% 15% 11% 3 1	3828
April	1st to 10th	Night	47% 7% 16% 21% 9%	1890
	11th to 20th	Night	45% 7% 14% 21% 12% 1	1691
	21st to end	Night	44% 8% 13% 26% 8% 1	1710
May	1st to 10th	Night	49% 9% 10% 22% 9%	2583
	11th to 20th	Night	32% 12% 15% 30% 11%	2486
	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	36% 8% 15% 26% 14% 2	2280
	11th to 20th	Night	35% 6% 17% 26% 14% 2	2280
	21st to end	Night	41% 9% 14% 26% 10%	2693
November	1st to 10th	Night	46% 8% 15% 21% 9% 2	2605
	11th to 20th	Night	41% 9% 20% 19% 11%	2660
	21st to end	Night	40% 9% 24% 17% 9% 1	2664
December	1st to 10th	Night	44% 11% 16% 18% 9% 1	2850
	11th to 20th	Night	39% 10% 25% 21% 5% 1	3507
	21st to end	Night	31% 7% 15% 25% 15% 8%	5214

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	5% 19% 37% 22% 12% 4	180
	11th to 20th	Day	9% 18% 30% 27% 13% 3	180
	21st to end	Day	8% 17% 43% 24% 8% 1	209
February	1st to 10th	Day	13% 22% 33% 23% 8%	270
	11th to 20th	Day	11% 13% 41% 24% 9% 2	270
	21st to end	Day	5% 17% 39% 28% 9% 2	225
March	1st to 10th	Day	8% 17% 36% 27% 9% 2	432
	11th to 20th	Day	8% 13% 39% 27% 9% 4	460
	21st to end	Day	12% 12% 36% 25% 12% 3	506
April	1st to 10th	Day	12% 18% 34% 28% 8% 1	460
	11th to 20th	Day	13% 17% 36% 25% 9% 1	452
	21st to end	Day	18% 13% 28% 28% 12% 2	460
May	1st to 10th	Day	15% 9% 24% 29% 18% 5%	621
	11th to 20th	Day	14% 7% 26% 30% 19% 4	675
	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% 32% 18% 8% 1	273
	11th to 20th	Day	12% 22% 44% 17% 5% 1	270
	21st to end	Day	8% 20% 39% 20% 9% 4	268
December	1st to 10th	Day	8% 27% 32% 24% 6% 2	180
	11th to 20th	Day	9% 29% 34% 21% 6% 2	180
	21st to end	Day	11% 13% 31% 30% 10% 5%	198
January	1st to 10th	Night	9% 18% 42% 23% 7% 1	950
	11th to 20th	Night	7% 17% 40% 26% 9% 1	950
	21st to end	Night	7% 19% 41% 22% 9% 2	934
February	1st to 10th	Night	10% 15% 33% 25% 14% 3	760
	11th to 20th	Night	6% 16% 44% 24% 9% 1	744
	21st to end	Night	9% 10% 35% 29% 13% 4	628
March	1st to 10th	Night	8% 12% 38% 30% 10% 1	760
	11th to 20th	Night	9% 12% 39% 28% 9% 2	760
	21st to end	Night	13% 12% 35% 26% 12% 2	836
April	1st to 10th	Night	11% 12% 38% 27% 10% 1	589
	11th to 20th	Night	15% 8% 36% 29% 10% 1	563
	21st to end	Night	14% 11% 30% 34% 11%	570
May	1st to 10th	Night	15% 8% 24% 32% 19% 2	570
	11th to 20th	Night	11% 6% 27% 35% 18% 3	543
	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% 14% 41% 23% 6%	931
	11th to 20th	Night	11% 19% 41% 22% 5% 1	950
	21st to end	Night	9% 15% 45% 26% 5% 1	950
December	1st to 10th	Night	9% 14% 40% 26% 10% 1	950
	11th to 20th	Night	6% 18% 41% 26% 9% 1	950
	21st to end	Night	9% 15% 40% 26% 9% 2	1045

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