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 2025-06-10.

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% 9% 3 8% 5% 5% 5% 4 7% 8% 37%	123427
	Night	7% 9% 4 10% 6% 6% 6% 5% 7% 9% 32%	123504
Transparency	Day	42% 8% 15% 24% 10% 1	123168
	Night	45% 8% 15% 22% 9% 1	123406
Seeing	Day	12% 7% 23% 32% 20% 6%	16159
	Night	12% 6% 25% 38% 17% 2	25334

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% 17% 9% 6% 6% 4 6% 8% 23%	3179
	11th to 20th	Day	3 10% 5% 11% 6% 7% 6% 7% 10% 9% 25%	3159
	21st to end	Day	4 9% 4 10% 6% 5% 5% 5% 8% 9% 33%	3889
February	1st to 10th	Day	7% 14% 5% 12% 7% 8% 4 4 6% 8% 24%	1620
	11th to 20th	Day	6% 13% 6% 11% 8% 6% 6% 6% 7% 8% 25%	1755
	21st to end	Day	3 12% 5% 13% 9% 8% 6% 5% 6% 8% 24%	1546
March	1st to 10th	Day	3 10% 5% 12% 8% 6% 7% 4 8% 8% 28%	4550
	11th to 20th	Day	5% 11% 4 11% 6% 4 5% 4 6% 8% 34%	4999
	21st to end	Day	10% 18% 7% 15% 6% 5% 4 4 6% 6% 20%	5486
April	1st to 10th	Day	8% 12% 4 9% 6% 5% 5% 4 6% 8% 33%	2489
	11th to 20th	Day	12% 11% 4 8% 5% 6% 5% 5% 7% 8% 30%	2570
	21st to end	Day	14% 9% 3 9% 6% 6% 5% 3 6% 8% 32%	2600
May	1st to 10th	Day	14% 9% 3 5% 4 4 5% 4 6% 7% 38%	6300
	11th to 20th	Day	13% 11% 4 8% 5% 5% 5% 5% 6% 8% 29%	6734
	21st to end	Day	11% 6% 3 8% 5% 5% 5% 4 6% 7% 40%	7405
June	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 5% 8% 9% 35%	2988
	11th to 20th	Day	18% 8% 3 7% 5% 6% 6% 4 7% 8% 28%	2850
	21st to end	Day	8% 4 2 5% 5% 5% 5% 5% 7% 10% 43%	2850
July	1st to 10th	Day	7% 4 2 5% 4 4 5% 4 7% 9% 50%	4740
	11th to 20th	Day	7% 4 1 5% 3 4 5% 4 6% 10% 50%	4740
	21st to end	Day	4 4 1 5% 4 4 4 3 6% 9% 57%	5138
August	1st to 10th	Day	7% 6% 2 6% 5% 5% 5% 4 7% 8% 43%	4417
	11th to 20th	Day	7% 4 2 5% 5% 5% 5% 5% 7% 11% 44%	4033
	21st to end	Day	10% 4 2 5% 3 3 3 4 6% 8% 51%	4521
September	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 5% 7% 47%	2384
	11th to 20th	Day	15% 7% 2 5% 4 4 4 2 6% 7% 43%	2090
	21st to end	Day	8% 8% 2 6% 4 4 4 4 6% 8% 47%	2090
October	1st to 10th	Day	7% 7% 3 8% 4 5% 5% 4 6% 9% 41%	3278
	11th to 20th	Day	6% 5% 3 9% 5% 4 5% 4 7% 11% 42%	2976
	21st to end	Day	8% 9% 3 8% 5% 5% 5% 5% 7% 6% 40%	3101
November	1st to 10th	Day	10% 17% 5% 10% 6% 5% 5% 3 5% 8% 25%	1573
	11th to 20th	Day	7% 13% 4 10% 7% 6% 5% 4 6% 7% 31%	1547
	21st to end	Day	6% 13% 4 11% 9% 6% 5% 3 6% 8% 29%	1400
December	1st to 10th	Day	10% 11% 5% 10% 7% 7% 6% 4 8% 8% 22%	2290
	11th to 20th	Day	7% 11% 5% 14% 10% 7% 6% 4 7% 7% 22%	2642
	21st to end	Day	3 7% 4 11% 5% 5% 5% 4 7% 10% 40%	3498
January	1st to 10th	Night	4 13% 6% 15% 8% 7% 6% 5% 7% 9% 20%	6766
	11th to 20th	Night	3 9% 5% 12% 7% 6% 6% 6% 6% 9% 30%	6739
	21st to end	Night	3 10% 5% 10% 6% 5% 6% 5% 7% 8% 34%	7108
February	1st to 10th	Night	5% 12% 4 13% 7% 6% 6% 4 6% 8% 27%	2780
	11th to 20th	Night	5% 12% 5% 13% 7% 6% 6% 5% 7% 8% 25%	2532
	21st to end	Night	3 10% 5% 11% 7% 5% 6% 5% 6% 10% 31%	2080
March	1st to 10th	Night	3 9% 4 13% 9% 7% 6% 6% 8% 10% 27%	5344
	11th to 20th	Night	5% 8% 3 10% 7% 6% 7% 6% 9% 8% 30%	5000
	21st to end	Night	7% 17% 7% 16% 7% 6% 6% 4 6% 7% 17%	5489
April	1st to 10th	Night	10% 9% 4 9% 6% 5% 6% 4 7% 8% 31%	1910
	11th to 20th	Night	8% 11% 5% 11% 7% 6% 6% 4 7% 7% 29%	1781
	21st to end	Night	11% 10% 4 10% 5% 5% 6% 4 7% 8% 32%	1800
May	1st to 10th	Night	15% 11% 3 8% 4 4 5% 4 6% 6% 34%	3657
	11th to 20th	Night	11% 10% 3 10% 7% 5% 6% 4 8% 8% 28%	3159
	21st to end	Night	12% 9% 4 9% 6% 6% 5% 4 6% 6% 33%	3470
June	1st to 10th	Night	14% 9% 4 8% 6% 5% 6% 4 7% 7% 30%	1396
	11th to 20th	Night	16% 9% 3 9% 8% 6% 6% 5% 7% 6% 26%	1330
	21st to end	Night	9% 5% 3 7% 5% 5% 5% 5% 6% 8% 42%	1330
July	1st to 10th	Night	8% 4 3 7% 6% 5% 6% 4 7% 9% 43%	2220
	11th to 20th	Night	6% 3 1 5% 5% 5% 5% 5% 9% 13% 43%	2220
	21st to end	Night	5% 4 1 6% 4 4 4 4 7% 10% 51%	2414
August	1st to 10th	Night	9% 6% 2 7% 4 5% 5% 4 5% 9% 44%	2497
	11th to 20th	Night	6% 5% 2 7% 4 5% 6% 5% 8% 11% 42%	2806
	21st to end	Night	8% 5% 2 5% 4 3 4 4 7% 10% 47%	3146
September	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 8% 48%	1793
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 8% 44%	2090
	21st to end	Night	7% 7% 2 6% 3 4 4 4 6% 10% 47%	2090
October	1st to 10th	Night	8% 6% 3 6% 4 4 4 4 7% 8% 45%	3694
	11th to 20th	Night	6% 8% 3 7% 4 4 5% 4 7% 8% 43%	3952
	21st to end	Night	6% 7% 3 7% 5% 6% 6% 4 7% 8% 39%	4457
November	1st to 10th	Night	10% 13% 5% 9% 5% 5% 4 5% 6% 8% 31%	2706
	11th to 20th	Night	5% 9% 4 11% 8% 6% 7% 5% 7% 8% 29%	2850
	21st to end	Night	6% 11% 5% 10% 8% 6% 6% 5% 6% 9% 29%	3000
December	1st to 10th	Night	7% 13% 5% 11% 7% 6% 6% 5% 8% 9% 22%	4890
	11th to 20th	Night	6% 12% 5% 14% 9% 7% 7% 5% 7% 8% 20%	5550
	21st to end	Night	3 7% 3 10% 7% 6% 6% 5% 7% 11% 35%	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	56% 7% 9% 19% 8% 1	3179
	11th to 20th	Day	48% 6% 14% 12% 18% 2	3180
	21st to end	Day	42% 7% 17% 26% 7%	3889
February	1st to 10th	Day	57% 5% 14% 14% 9% 1	1620
	11th to 20th	Day	55% 8% 13% 16% 8% 1	1764
	21st to end	Day	55% 6% 11% 18% 9%	1546
March	1st to 10th	Day	49% 7% 17% 18% 9%	4550
	11th to 20th	Day	45% 7% 14% 21% 12%	5000
	21st to end	Day	60% 8% 10% 16% 6%	5486
April	1st to 10th	Day	47% 8% 17% 19% 8% 1	2489
	11th to 20th	Day	48% 8% 13% 21% 8% 1	2570
	21st to end	Day	48% 8% 13% 22% 7% 1	2600
May	1st to 10th	Day	44% 7% 11% 23% 13% 3	6300
	11th to 20th	Day	49% 8% 13% 22% 7%	6738
	21st to end	Day	40% 8% 17% 25% 10% 1	7406
June	1st to 10th	Day	43% 9% 16% 21% 10% 1	2988
	11th to 20th	Day	51% 9% 15% 19% 6% 1	2850
	21st to end	Day	33% 11% 20% 27% 7% 1	2850
July	1st to 10th	Day	29% 8% 17% 28% 16% 2	4666
	11th to 20th	Day	27% 9% 17% 35% 12%	4659
	21st to end	Day	24% 7% 15% 30% 20% 4	4976
August	1st to 10th	Day	35% 9% 17% 30% 8% 1	4399
	11th to 20th	Day	31% 9% 26% 28% 7% 1	4072
	21st to end	Day	30% 7% 23% 30% 9% 1	4520
September	1st to 10th	Day	34% 9% 16% 26% 12% 4	2384
	11th to 20th	Day	41% 5% 15% 29% 9% 1	2090
	21st to end	Day	34% 8% 17% 27% 10% 4	2090
October	1st to 10th	Day	39% 6% 19% 28% 7% 1	3278
	11th to 20th	Day	35% 7% 19% 27% 10% 1	2976
	21st to end	Day	40% 8% 11% 32% 8% 1	3101
November	1st to 10th	Day	57% 6% 13% 17% 6% 1	1573
	11th to 20th	Day	50% 6% 14% 20% 8% 1	1547
	21st to end	Day	52% 6% 16% 18% 7% 1	1400
December	1st to 10th	Day	56% 6% 18% 14% 4 1	2290
	11th to 20th	Day	57% 9% 15% 16% 3 1	2644
	21st to end	Day	38% 5% 12% 28% 14% 2	3498
January	1st to 10th	Night	56% 8% 12% 17% 6% 1	6766
	11th to 20th	Night	48% 6% 13% 16% 16% 2	6780
	21st to end	Night	43% 8% 17% 25% 7%	7108
February	1st to 10th	Night	53% 6% 11% 19% 10%	2780
	11th to 20th	Night	53% 7% 12% 16% 11% 1	2533
	21st to end	Night	47% 6% 14% 21% 12% 1	2080
March	1st to 10th	Night	47% 8% 20% 18% 6%	5344
	11th to 20th	Night	45% 8% 17% 22% 7% 1	5000
	21st to end	Night	63% 8% 10% 13% 5% 1	5489
April	1st to 10th	Night	49% 7% 15% 17% 12% 1	1910
	11th to 20th	Night	50% 7% 12% 21% 8% 1	1781
	21st to end	Night	46% 8% 13% 23% 8% 2	1800
May	1st to 10th	Night	49% 5% 8% 22% 14% 2	3654
	11th to 20th	Night	47% 9% 14% 23% 7%	3159
	21st to end	Night	48% 10% 13% 21% 7% 1	3470
June	1st to 10th	Night	48% 9% 13% 19% 10%	1396
	11th to 20th	Night	53% 10% 14% 15% 7% 1	1330
	21st to end	Night	38% 10% 18% 24% 9% 1	1330
July	1st to 10th	Night	35% 9% 19% 21% 15% 1	2185
	11th to 20th	Night	28% 10% 17% 35% 7% 3	2178
	21st to end	Night	26% 7% 16% 29% 18% 4	2337
August	1st to 10th	Night	36% 9% 15% 30% 10% 1	2490
	11th to 20th	Night	32% 10% 21% 30% 6% 1	2833
	21st to end	Night	29% 7% 22% 32% 8% 2	3146
September	1st to 10th	Night	33% 8% 16% 23% 13% 6%	1793
	11th to 20th	Night	39% 6% 14% 26% 11% 4	2090
	21st to end	Night	32% 7% 16% 28% 15% 2	2090
October	1st to 10th	Night	36% 6% 20% 26% 11% 1	3694
	11th to 20th	Night	36% 7% 16% 31% 9% 1	3952
	21st to end	Night	39% 7% 15% 31% 6% 1	4457
November	1st to 10th	Night	50% 6% 13% 21% 8% 1	2706
	11th to 20th	Night	49% 9% 14% 18% 9% 1	2850
	21st to end	Night	49% 9% 16% 18% 6% 2	3000
December	1st to 10th	Night	55% 8% 16% 15% 5% 1	4890
	11th to 20th	Night	58% 9% 16% 13% 4	5547
	21st to end	Night	40% 6% 12% 24% 15% 2	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% 4 23% 33% 23% 11%	200
	11th to 20th	Day	9% 5% 27% 34% 25% 2	200
	21st to end	Day	12% 9% 21% 32% 20% 6%	220
February	1st to 10th	Day	11% 8% 26% 30% 19% 7%	210
	11th to 20th	Day	10% 6% 25% 36% 19% 5%	300
	21st to end	Day	7% 4 22% 41% 20% 5%	310
March	1st to 10th	Day	10% 3 30% 31% 21% 5%	500
	11th to 20th	Day	9% 4 24% 35% 21% 6%	500
	21st to end	Day	13% 5% 26% 31% 18% 5%	550
April	1st to 10th	Day	11% 12% 26% 33% 12% 6%	500
	11th to 20th	Day	13% 10% 32% 25% 15% 4	492
	21st to end	Day	18% 9% 25% 28% 15% 4	500
May	1st to 10th	Day	15% 11% 22% 27% 19% 6%	618
	11th to 20th	Day	13% 9% 21% 34% 19% 4	725
	21st to end	Day	17% 5% 21% 29% 21% 7%	789
June	1st to 10th	Day	15% 8% 24% 33% 17% 3	726
	11th to 20th	Day	18% 8% 23% 31% 15% 4	680
	21st to end	Day	10% 4 23% 31% 25% 8%	680
July	1st to 10th	Day	10% 5% 17% 34% 26% 8%	671
	11th to 20th	Day	11% 5% 18% 33% 26% 7%	668
	21st to end	Day	6% 4 19% 31% 29% 11%	739
August	1st to 10th	Day	10% 4 21% 34% 21% 10%	503
	11th to 20th	Day	9% 4 19% 31% 28% 8%	477
	21st to end	Day	11% 9% 20% 27% 23% 10%	539
September	1st to 10th	Day	14% 6% 26% 27% 21% 6%	490
	11th to 20th	Day	19% 7% 20% 28% 21% 5%	490
	21st to end	Day	11% 13% 25% 32% 15% 5%	490
October	1st to 10th	Day	17% 7% 22% 28% 19% 7%	369
	11th to 20th	Day	10% 11% 29% 30% 14% 6%	300
	21st to end	Day	14% 11% 30% 30% 13% 2	330
November	1st to 10th	Day	20% 7% 22% 34% 14% 3	300
	11th to 20th	Day	14% 4 29% 35% 15% 3	273
	21st to end	Day	10% 4 27% 33% 17% 10%	200
December	1st to 10th	Day	10% 5% 26% 33% 20% 8%	200
	11th to 20th	Day	8% 4 34% 36% 18% 3	200
	21st to end	Day	9% 2 18% 35% 26% 9%	220
January	1st to 10th	Night	10% 6% 27% 39% 16% 2	1000
	11th to 20th	Night	12% 5% 29% 39% 14% 2	1000
	21st to end	Night	12% 7% 24% 41% 14% 2	945
February	1st to 10th	Night	11% 7% 23% 37% 19% 3	800
	11th to 20th	Night	11% 5% 28% 38% 17% 1	784
	21st to end	Night	7% 3 21% 43% 21% 5%	660
March	1st to 10th	Night	8% 5% 25% 41% 18% 4	800
	11th to 20th	Night	9% 5% 24% 42% 18% 3	800
	21st to end	Night	15% 5% 24% 37% 17% 2	880
April	1st to 10th	Night	13% 6% 27% 33% 17% 2	700
	11th to 20th	Night	12% 9% 31% 36% 11% 1	593
	21st to end	Night	15% 8% 25% 38% 14% 1	600
May	1st to 10th	Night	17% 9% 24% 35% 14% 1	533
	11th to 20th	Night	12% 6% 26% 37% 17% 2	394
	21st to end	Night	21% 5% 23% 31% 19% 2	433
June	1st to 10th	Night	19% 7% 25% 36% 12% 1	399
	11th to 20th	Night	17% 5% 27% 32% 18% 1	380
	21st to end	Night	11% 4 23% 36% 23% 2	380
July	1st to 10th	Night	10% 2 18% 44% 24% 2	370
	11th to 20th	Night	11% 2 20% 45% 21% 2	368
	21st to end	Night	6% 2 22% 40% 27% 3	416
August	1st to 10th	Night	12% 3 19% 38% 26% 2	378
	11th to 20th	Night	8% 5% 16% 45% 25% 1	559
	21st to end	Night	9% 5% 19% 42% 25%	627
September	1st to 10th	Night	13% 3 20% 41% 20% 2	608
	11th to 20th	Night	16% 4 18% 40% 21% 1	760
	21st to end	Night	11% 5% 27% 40% 16% 2	760
October	1st to 10th	Night	13% 5% 24% 37% 18% 2	760
	11th to 20th	Night	13% 5% 27% 38% 14% 2	760
	21st to end	Night	11% 7% 29% 39% 13% 1	836
November	1st to 10th	Night	16% 7% 29% 36% 11% 1	951
	11th to 20th	Night	9% 7% 36% 34% 12% 2	1000
	21st to end	Night	10% 5% 28% 41% 15% 1	1000
December	1st to 10th	Night	13% 7% 25% 36% 16% 3	1000
	11th to 20th	Night	10% 5% 31% 39% 13% 2	1000
	21st to end	Night	11% 4 25% 39% 17% 3	1100

Page generated at 2025-06-07 08:28:19. Version ac07

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