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 2024-03-18.

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	9% 8% 4 7% 6% 4 5% 6% 6% 8% 36%	78059
	Night	7% 9% 4 8% 7% 4 6% 6% 7% 9% 33%	81690
Transparency	Day	44% 8% 16% 22% 9% 1	77761
	Night	45% 8% 15% 21% 9% 1	81568
Seeing	Day	12% 6% 23% 31% 20% 6%	14468
	Night	12% 5% 25% 38% 17% 2	23809

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 11% 6% 12% 9% 5% 6% 7% 7% 8% 25%	2219
	11th to 20th	Day	4 9% 5% 9% 6% 5% 6% 10% 10% 7% 27%	2199
	21st to end	Day	5% 9% 4 10% 7% 4 5% 7% 9% 10% 30%	2707
February	1st to 10th	Day	8% 13% 4 10% 8% 6% 5% 5% 6% 9% 25%	1539
	11th to 20th	Day	6% 12% 7% 10% 9% 4 6% 6% 7% 8% 25%	1665
	21st to end	Day	4 11% 7% 12% 10% 6% 6% 7% 6% 7% 24%	1471
March	1st to 10th	Day	6% 12% 6% 13% 11% 6% 7% 5% 6% 6% 22%	1907
	11th to 20th	Day	7% 8% 5% 11% 8% 4 6% 7% 6% 8% 30%	2057
	21st to end	Day	10% 12% 7% 10% 7% 4 6% 5% 6% 8% 26%	2178
April	1st to 10th	Day	9% 10% 4 8% 7% 4 6% 5% 6% 9% 31%	2239
	11th to 20th	Day	12% 11% 4 7% 6% 4 5% 6% 7% 8% 29%	2310
	21st to end	Day	14% 9% 3 7% 7% 4 6% 4 5% 9% 32%	2340
May	1st to 10th	Day	13% 10% 4 6% 6% 4 5% 7% 6% 9% 30%	2501
	11th to 20th	Day	10% 6% 3 6% 6% 4 5% 6% 6% 8% 41%	2654
	21st to end	Day	18% 7% 3 7% 5% 4 5% 5% 5% 7% 34%	2917
June	1st to 10th	Day	16% 6% 2 6% 6% 3 5% 6% 8% 9% 33%	2700
	11th to 20th	Day	20% 7% 4 6% 6% 4 7% 5% 7% 8% 27%	2700
	21st to end	Day	9% 4 2 4 5% 3 6% 7% 7% 10% 43%	2700
July	1st to 10th	Day	8% 5% 2 5% 5% 3 5% 6% 8% 10% 42%	2700
	11th to 20th	Day	10% 5% 2 5% 5% 3 5% 6% 6% 10% 44%	2700
	21st to end	Day	6% 4 2 4 4 3 4 4 7% 9% 53%	2938
August	1st to 10th	Day	9% 5% 2 5% 5% 3 4 6% 6% 9% 47%	2523
	11th to 20th	Day	9% 4 2 4 5% 3 4 5% 6% 9% 50%	2304
	21st to end	Day	10% 4 2 4 4 3 4 6% 7% 9% 48%	2574
September	1st to 10th	Day	11% 5% 2 5% 5% 3 4 5% 6% 8% 46%	2260
	11th to 20th	Day	15% 7% 2 4 4 4 4 3 5% 7% 43%	1980
	21st to end	Day	8% 8% 2 5% 5% 3 4 4 5% 8% 47%	1980
October	1st to 10th	Day	12% 9% 4 6% 5% 3 4 4 5% 7% 40%	1866
	11th to 20th	Day	9% 6% 4 8% 6% 3 5% 4 6% 8% 40%	1702
	21st to end	Day	7% 11% 4 7% 7% 4 6% 6% 7% 7% 34%	1782
November	1st to 10th	Day	10% 16% 6% 10% 7% 3 5% 5% 6% 8% 24%	1493
	11th to 20th	Day	7% 11% 5% 9% 8% 4 6% 5% 6% 8% 31%	1470
	21st to end	Day	6% 11% 4 9% 9% 4 6% 4 6% 8% 31%	1330
December	1st to 10th	Day	8% 12% 6% 10% 9% 5% 8% 5% 5% 7% 25%	1330
	11th to 20th	Day	4 11% 7% 13% 9% 5% 7% 6% 8% 7% 23%	1682
	21st to end	Day	4 9% 4 8% 6% 3 4 5% 6% 8% 43%	2442
January	1st to 10th	Night	4 13% 6% 11% 9% 5% 7% 7% 8% 10% 21%	4726
	11th to 20th	Night	4 8% 5% 11% 10% 5% 6% 8% 7% 8% 28%	4699
	21st to end	Night	4 11% 5% 10% 8% 3 6% 7% 8% 9% 29%	4976
February	1st to 10th	Night	6% 12% 5% 11% 9% 4 6% 5% 6% 8% 28%	2641
	11th to 20th	Night	6% 11% 6% 11% 9% 5% 7% 6% 6% 8% 26%	2402
	21st to end	Night	4 9% 6% 10% 9% 4 5% 6% 6% 10% 31%	1979
March	1st to 10th	Night	4 9% 5% 10% 11% 5% 6% 5% 7% 7% 29%	2273
	11th to 20th	Night	6% 10% 4 9% 8% 3 8% 6% 8% 8% 31%	2068
	21st to end	Night	9% 13% 6% 9% 8% 4 7% 7% 6% 8% 24%	2178
April	1st to 10th	Night	10% 8% 4 8% 8% 4 6% 6% 7% 8% 30%	1720
	11th to 20th	Night	9% 11% 5% 10% 8% 4 6% 5% 6% 7% 30%	1601
	21st to end	Night	11% 9% 4 9% 6% 3 6% 5% 7% 8% 32%	1620
May	1st to 10th	Night	12% 10% 3 7% 6% 4 6% 7% 7% 7% 31%	1457
	11th to 20th	Night	9% 7% 3 6% 8% 3 5% 6% 5% 8% 40%	1239
	21st to end	Night	18% 8% 3 6% 5% 4 4 4 7% 8% 32%	1358
June	1st to 10th	Night	15% 8% 4 7% 6% 3 6% 5% 7% 8% 31%	1260
	11th to 20th	Night	16% 8% 3 8% 9% 6% 6% 6% 7% 6% 26%	1260
	21st to end	Night	9% 4 3 7% 7% 4 5% 6% 6% 8% 42%	1260
July	1st to 10th	Night	9% 4 3 5% 6% 3 5% 6% 6% 11% 42%	1260
	11th to 20th	Night	9% 3 2 5% 7% 3 6% 7% 8% 10% 39%	1260
	21st to end	Night	6% 3 2 4 5% 2 5% 5% 6% 10% 52%	1379
August	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 7% 7% 9% 43%	1414
	11th to 20th	Night	8% 5% 2 5% 5% 4 5% 5% 6% 10% 45%	1597
	21st to end	Night	8% 5% 2 4 5% 3 4 6% 6% 8% 49%	1782
September	1st to 10th	Night	11% 5% 2 4 4 3 4 5% 6% 8% 47%	1698
	11th to 20th	Night	13% 6% 2 5% 6% 3 3 5% 5% 8% 44%	1980
	21st to end	Night	7% 7% 2 6% 4 3 4 5% 6% 10% 47%	1980
October	1st to 10th	Night	12% 8% 3 6% 5% 3 4 5% 6% 8% 40%	2093
	11th to 20th	Night	9% 8% 3 7% 6% 3 4 5% 7% 7% 39%	2258
	21st to end	Night	7% 9% 3 8% 7% 4 6% 6% 6% 9% 35%	2574
November	1st to 10th	Night	11% 12% 6% 8% 6% 4 5% 6% 6% 8% 31%	2567
	11th to 20th	Night	5% 8% 5% 9% 9% 5% 7% 7% 7% 9% 30%	2707
	21st to end	Night	6% 10% 4 9% 8% 4 6% 7% 6% 9% 31%	2850
December	1st to 10th	Night	7% 12% 5% 10% 7% 5% 5% 5% 6% 9% 28%	2850
	11th to 20th	Night	3 10% 5% 11% 9% 5% 7% 6% 6% 9% 28%	3510
	21st to end	Night	4 7% 3 8% 7% 4 5% 5% 6% 10% 41%	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	51% 9% 11% 20% 8% 1	2219
	11th to 20th	Day	48% 7% 15% 12% 16% 2	2220
	21st to end	Day	44% 8% 17% 25% 6%	2707
February	1st to 10th	Day	56% 5% 14% 15% 9% 1	1539
	11th to 20th	Day	55% 8% 13% 16% 8% 1	1674
	21st to end	Day	57% 6% 11% 17% 9%	1471
March	1st to 10th	Day	60% 8% 14% 13% 5%	1907
	11th to 20th	Day	49% 7% 14% 21% 9% 1	2058
	21st to end	Day	54% 9% 16% 15% 6% 1	2178
April	1st to 10th	Day	48% 8% 18% 18% 8% 1	2239
	11th to 20th	Day	50% 8% 14% 20% 8% 1	2310
	21st to end	Day	49% 8% 13% 21% 7% 1	2340
May	1st to 10th	Day	49% 9% 14% 20% 8%	2501
	11th to 20th	Day	39% 8% 14% 26% 11% 1	2658
	21st to end	Day	47% 8% 15% 21% 8% 1	2918
June	1st to 10th	Day	45% 9% 17% 21% 8%	2700
	11th to 20th	Day	52% 9% 15% 18% 6% 1	2700
	21st to end	Day	34% 11% 20% 27% 6% 1	2700
July	1st to 10th	Day	35% 9% 20% 26% 8% 1	2626
	11th to 20th	Day	34% 11% 20% 27% 8%	2619
	21st to end	Day	28% 9% 21% 30% 12% 1	2775
August	1st to 10th	Day	34% 8% 21% 27% 8% 1	2505
	11th to 20th	Day	31% 7% 24% 27% 10% 1	2304
	21st to end	Day	30% 9% 19% 28% 12% 1	2574
September	1st to 10th	Day	35% 9% 16% 26% 11% 4	2260
	11th to 20th	Day	41% 6% 14% 29% 9% 1	1980
	21st to end	Day	34% 8% 17% 27% 10% 3	1980
October	1st to 10th	Day	44% 6% 16% 24% 9% 1	1866
	11th to 20th	Day	41% 8% 17% 24% 10% 1	1702
	21st to end	Day	45% 9% 13% 21% 10% 1	1782
November	1st to 10th	Day	57% 6% 13% 17% 6% 1	1493
	11th to 20th	Day	51% 6% 14% 21% 8% 1	1470
	21st to end	Day	50% 7% 16% 19% 8% 1	1330
December	1st to 10th	Day	58% 6% 12% 16% 6% 2	1330
	11th to 20th	Day	54% 10% 14% 19% 3 1	1684
	21st to end	Day	37% 6% 12% 26% 16% 3	2442
January	1st to 10th	Night	52% 10% 12% 19% 5% 1	4726
	11th to 20th	Night	50% 6% 12% 15% 14% 2	4740
	21st to end	Night	45% 9% 15% 23% 8% 1	4976
February	1st to 10th	Night	52% 6% 12% 19% 10%	2641
	11th to 20th	Night	53% 7% 13% 16% 10% 1	2403
	21st to end	Night	47% 6% 14% 21% 11% 1	1979
March	1st to 10th	Night	51% 9% 15% 20% 6% 1	2273
	11th to 20th	Night	46% 7% 14% 22% 8% 1	2068
	21st to end	Night	55% 8% 14% 16% 6% 1	2178
April	1st to 10th	Night	49% 7% 16% 16% 11% 1	1720
	11th to 20th	Night	51% 7% 12% 21% 7% 1	1601
	21st to end	Night	46% 8% 14% 23% 8% 2	1620
May	1st to 10th	Night	49% 7% 12% 21% 11% 1	1457
	11th to 20th	Night	40% 8% 12% 24% 16%	1239
	21st to end	Night	49% 8% 11% 22% 8% 1	1358
June	1st to 10th	Night	48% 9% 13% 20% 10%	1260
	11th to 20th	Night	54% 10% 13% 15% 7% 1	1260
	21st to end	Night	38% 11% 18% 23% 8% 1	1260
July	1st to 10th	Night	35% 11% 23% 23% 8% 1	1225
	11th to 20th	Night	36% 12% 21% 22% 8% 1	1218
	21st to end	Night	27% 8% 23% 30% 13% 1	1302
August	1st to 10th	Night	34% 10% 22% 25% 8% 1	1407
	11th to 20th	Night	33% 9% 18% 29% 9% 1	1597
	21st to end	Night	30% 8% 17% 30% 12% 3	1782
September	1st to 10th	Night	34% 8% 17% 23% 13% 6%	1698
	11th to 20th	Night	38% 6% 14% 26% 12% 4	1980
	21st to end	Night	32% 7% 16% 28% 15% 2	1980
October	1st to 10th	Night	41% 7% 15% 25% 11% 1	2093
	11th to 20th	Night	42% 7% 16% 23% 10% 2	2258
	21st to end	Night	45% 8% 16% 23% 7% 1	2574
November	1st to 10th	Night	50% 6% 13% 21% 8% 1	2567
	11th to 20th	Night	48% 8% 15% 19% 10% 1	2707
	21st to end	Night	47% 10% 17% 19% 6% 2	2850
December	1st to 10th	Night	53% 7% 11% 20% 8% 1	2850
	11th to 20th	Night	49% 10% 18% 17% 6%	3507
	21st to end	Night	38% 6% 14% 24% 15% 4	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	7% 4 22% 32% 22% 12%	180
	11th to 20th	Day	9% 4 26% 33% 26% 2	180
	21st to end	Day	12% 8% 22% 30% 21% 7%	198
February	1st to 10th	Day	12% 8% 25% 31% 18% 6%	189
	11th to 20th	Day	11% 6% 25% 34% 19% 5%	270
	21st to end	Day	8% 5% 21% 41% 20% 6%	283
March	1st to 10th	Day	11% 4 31% 31% 19% 5%	460
	11th to 20th	Day	8% 4 24% 35% 22% 6%	449
	21st to end	Day	11% 5% 29% 30% 18% 6%	462
April	1st to 10th	Day	10% 13% 27% 34% 12% 5%	420
	11th to 20th	Day	14% 8% 30% 25% 18% 4	412
	21st to end	Day	19% 9% 25% 29% 15% 3	420
May	1st to 10th	Day	15% 10% 21% 27% 20% 6%	526
	11th to 20th	Day	12% 8% 20% 35% 20% 4	625
	21st to end	Day	18% 5% 20% 28% 22% 7%	679
June	1st to 10th	Day	15% 6% 24% 34% 18% 3	630
	11th to 20th	Day	20% 8% 23% 31% 15% 3	630
	21st to end	Day	10% 4 21% 30% 26% 8%	630
July	1st to 10th	Day	10% 5% 18% 34% 26% 9%	621
	11th to 20th	Day	12% 5% 18% 33% 24% 6%	618
	21st to end	Day	6% 4 18% 32% 29% 10%	684
August	1st to 10th	Day	11% 4 21% 35% 20% 10%	463
	11th to 20th	Day	10% 4 20% 30% 29% 7%	437
	21st to end	Day	9% 9% 19% 29% 23% 10%	495
September	1st to 10th	Day	15% 6% 28% 27% 20% 5%	450
	11th to 20th	Day	18% 7% 21% 29% 21% 5%	450
	21st to end	Day	12% 12% 24% 32% 16% 5%	450
October	1st to 10th	Day	18% 7% 20% 28% 20% 7%	336
	11th to 20th	Day	10% 11% 28% 30% 14% 6%	270
	21st to end	Day	14% 12% 31% 30% 12% 1	297
November	1st to 10th	Day	20% 7% 23% 32% 14% 3	270
	11th to 20th	Day	15% 4 28% 35% 15% 3	246
	21st to end	Day	8% 4 28% 32% 17% 11%	180
December	1st to 10th	Day	8% 5% 25% 33% 21% 9%	180
	11th to 20th	Day	7% 3 36% 36% 17% 2	180
	21st to end	Day	10% 2 19% 36% 25% 8%	198
January	1st to 10th	Night	10% 6% 27% 38% 16% 2	950
	11th to 20th	Night	12% 5% 27% 39% 15% 2	950
	21st to end	Night	12% 7% 24% 40% 14% 2	898
February	1st to 10th	Night	11% 7% 23% 37% 19% 3	760
	11th to 20th	Night	12% 5% 28% 36% 18% 1	744
	21st to end	Night	8% 3 20% 42% 21% 6%	628
March	1st to 10th	Night	8% 5% 25% 40% 18% 4	760
	11th to 20th	Night	9% 5% 24% 41% 18% 3	752
	21st to end	Night	15% 5% 24% 36% 18% 2	792
April	1st to 10th	Night	12% 6% 27% 33% 18% 3	630
	11th to 20th	Night	13% 8% 30% 36% 12% 1	533
	21st to end	Night	16% 8% 25% 37% 14% 1	540
May	1st to 10th	Night	16% 10% 23% 35% 15% 1	481
	11th to 20th	Night	11% 6% 27% 37% 18% 2	354
	21st to end	Night	22% 4 22% 31% 19% 2	389
June	1st to 10th	Night	19% 7% 23% 37% 13% 1	360
	11th to 20th	Night	18% 5% 27% 32% 18% 1	360
	21st to end	Night	10% 4 22% 37% 24% 3	360
July	1st to 10th	Night	11% 2 18% 44% 24% 2	350
	11th to 20th	Night	11% 2 20% 45% 20% 2	348
	21st to end	Night	7% 3 21% 40% 27% 3	394
August	1st to 10th	Night	11% 3 19% 38% 26% 2	358
	11th to 20th	Night	8% 5% 16% 46% 25% 1	529
	21st to end	Night	9% 5% 18% 42% 25% 1	594
September	1st to 10th	Night	13% 3 20% 41% 19% 2	576
	11th to 20th	Night	15% 5% 18% 41% 21% 1	720
	21st to end	Night	11% 5% 26% 40% 16% 2	720
October	1st to 10th	Night	14% 5% 23% 37% 18% 2	720
	11th to 20th	Night	13% 5% 28% 38% 15% 2	720
	21st to end	Night	11% 7% 30% 38% 12% 1	792
November	1st to 10th	Night	16% 7% 29% 35% 12% 1	902
	11th to 20th	Night	9% 7% 36% 34% 12% 2	950
	21st to end	Night	10% 5% 28% 41% 14% 1	950
December	1st to 10th	Night	12% 8% 25% 36% 16% 3	950
	11th to 20th	Night	9% 4 31% 39% 13% 2	950
	21st to end	Night	11% 5% 26% 38% 17% 3	1045

Page generated at 2024-03-15 12:01:40. Version ac07

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