Calgary
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Calgary clear sky chart from 2005-11-06 to 2026-03-03.

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% 38%	134865
	Night	6% 9% 4 9% 5% 5% 5% 4 6% 8% 38%	134995
Transparency	Day	41% 7% 16% 24% 11% 2	134566
	Night	42% 7% 15% 23% 11% 2	134873
Seeing	Day	13% 9% 29% 30% 15% 4	16954
	Night	11% 7% 33% 35% 13% 2	26220

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	2 8% 5% 14% 8% 7% 6% 4 8% 8% 31%	4125
	11th to 20th	Day	2 8% 3 9% 6% 5% 5% 5% 7% 7% 44%	4221
	21st to end	Day	3 5% 2 9% 5% 5% 5% 4 7% 10% 45%	5186
February	1st to 10th	Day	4 10% 5% 14% 7% 6% 6% 5% 7% 8% 29%	1680
	11th to 20th	Day	4 9% 4 10% 8% 6% 7% 5% 8% 8% 29%	1829
	21st to end	Day	4 11% 5% 10% 6% 5% 6% 5% 7% 9% 32%	1688
March	1st to 10th	Day	2 9% 4 11% 5% 5% 6% 4 6% 12% 37%	4539
	11th to 20th	Day	4 10% 4 9% 4 4 5% 4 6% 9% 40%	4944
	21st to end	Day	8% 21% 7% 13% 6% 5% 5% 4 5% 6% 19%	5573
April	1st to 10th	Day	7% 15% 4 10% 7% 5% 6% 5% 6% 9% 27%	2450
	11th to 20th	Day	10% 14% 5% 9% 6% 5% 5% 4 6% 7% 29%	2570
	21st to end	Day	11% 13% 4 8% 6% 6% 6% 4 6% 8% 29%	2600
May	1st to 10th	Day	13% 12% 3 6% 4 4 4 4 6% 6% 37%	6189
	11th to 20th	Day	12% 11% 4 10% 6% 6% 7% 5% 8% 8% 23%	6734
	21st to end	Day	11% 8% 3 8% 7% 5% 6% 4 6% 7% 35%	7405
June	1st to 10th	Day	16% 7% 3 8% 6% 6% 7% 5% 6% 8% 29%	3000
	11th to 20th	Day	19% 9% 3 8% 7% 6% 6% 4 7% 7% 26%	3000
	21st to end	Day	9% 5% 2 6% 5% 6% 6% 5% 8% 9% 40%	3000
July	1st to 10th	Day	7% 4 2 6% 4 5% 6% 5% 7% 10% 46%	6780
	11th to 20th	Day	11% 4 1 5% 3 4 5% 5% 7% 10% 45%	6780
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 9% 48%	7197
August	1st to 10th	Day	9% 6% 2 6% 4 4 6% 5% 8% 8% 42%	4541
	11th to 20th	Day	6% 5% 2 5% 4 4 5% 5% 8% 11% 44%	4202
	21st to end	Day	9% 4 1 4 3 3 4 3 6% 8% 53%	4664
September	1st to 10th	Day	10% 6% 2 6% 4 3 3 4 5% 8% 49%	2479
	11th to 20th	Day	13% 8% 2 5% 4 3 4 3 5% 8% 44%	2200
	21st to end	Day	7% 8% 3 7% 5% 4 5% 4 6% 8% 44%	2200
October	1st to 10th	Day	7% 7% 2 7% 4 4 5% 4 7% 9% 44%	3454
	11th to 20th	Day	5% 7% 3 7% 4 4 5% 5% 7% 10% 43%	3000
	21st to end	Day	7% 7% 3 7% 5% 5% 5% 4 6% 7% 46%	3200
November	1st to 10th	Day	8% 14% 5% 9% 7% 3 5% 4 5% 7% 33%	1723
	11th to 20th	Day	6% 11% 3 8% 5% 6% 5% 4 6% 8% 39%	1595
	21st to end	Day	4 13% 4 10% 6% 4 5% 4 6% 10% 34%	1470
December	1st to 10th	Day	3 11% 5% 12% 7% 5% 6% 5% 6% 8% 32%	2360
	11th to 20th	Day	3 9% 5% 12% 7% 8% 8% 5% 7% 7% 29%	2712
	21st to end	Day	3 10% 4 9% 5% 5% 5% 4 6% 8% 42%	3575
January	1st to 10th	Night	3 11% 5% 12% 8% 7% 6% 4 7% 8% 27%	8768
	11th to 20th	Night	2 7% 4 12% 5% 5% 6% 5% 7% 8% 39%	8722
	21st to end	Night	2 8% 4 10% 6% 6% 6% 4 6% 8% 40%	8994
February	1st to 10th	Night	4 10% 4 11% 7% 6% 6% 5% 7% 7% 32%	2940
	11th to 20th	Night	4 10% 5% 10% 6% 6% 7% 6% 8% 8% 30%	2678
	21st to end	Night	2 10% 5% 13% 7% 6% 6% 5% 6% 7% 33%	2114
March	1st to 10th	Night	3 8% 4 12% 6% 7% 6% 5% 7% 10% 34%	5400
	11th to 20th	Night	4 9% 4 10% 6% 5% 6% 5% 7% 8% 38%	5054
	21st to end	Night	9% 19% 7% 14% 6% 5% 4 4 6% 7% 22%	5388
April	1st to 10th	Night	8% 14% 3 10% 7% 5% 6% 4 5% 7% 30%	1949
	11th to 20th	Night	10% 12% 4 11% 7% 4 5% 5% 6% 7% 30%	1781
	21st to end	Night	10% 11% 3 10% 5% 4 6% 4 6% 7% 32%	1800
May	1st to 10th	Night	14% 11% 3 8% 5% 4 4 4 5% 6% 37%	3764
	11th to 20th	Night	13% 10% 4 8% 5% 4 5% 5% 7% 10% 29%	3159
	21st to end	Night	10% 8% 3 9% 5% 6% 5% 4 6% 6% 37%	3470
June	1st to 10th	Night	14% 9% 3 8% 5% 4 4 5% 6% 7% 34%	1400
	11th to 20th	Night	17% 8% 3 7% 4 5% 6% 4 7% 9% 30%	1400
	21st to end	Night	8% 4 2 6% 5% 6% 6% 4 9% 9% 41%	1400
July	1st to 10th	Night	7% 5% 2 7% 5% 4 5% 3 6% 7% 47%	3180
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 7% 9% 45%	3180
	21st to end	Night	11% 4 2 5% 3 3 3 3 6% 9% 52%	3643
August	1st to 10th	Night	9% 6% 3 7% 4 4 4 5% 7% 10% 42%	2609
	11th to 20th	Night	6% 4 2 6% 4 5% 5% 5% 7% 11% 45%	2855
	21st to end	Night	8% 4 2 4 3 3 4 4 6% 9% 54%	3245
September	1st to 10th	Night	10% 6% 2 5% 3 3 3 4 6% 8% 49%	1918
	11th to 20th	Night	13% 7% 3 4 3 3 4 4 5% 8% 48%	2200
	21st to end	Night	10% 7% 2 5% 3 4 4 4 6% 9% 47%	2200
October	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 6% 8% 48%	3728
	11th to 20th	Night	5% 7% 2 6% 4 4 5% 4 6% 8% 48%	4147
	21st to end	Night	5% 8% 4 7% 5% 5% 5% 4 6% 7% 45%	4600
November	1st to 10th	Night	7% 13% 4 8% 4 5% 5% 5% 6% 7% 35%	2772
	11th to 20th	Night	3 11% 4 8% 6% 5% 6% 4 7% 8% 38%	3024
	21st to end	Night	5% 10% 4 9% 5% 6% 5% 4 6% 9% 37%	3150
December	1st to 10th	Night	3 11% 4 10% 7% 6% 6% 6% 7% 8% 33%	5040
	11th to 20th	Night	3 10% 5% 14% 8% 7% 5% 4 6% 9% 29%	5700
	21st to end	Night	4 10% 4 9% 5% 5% 6% 4 6% 7% 40%	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	48% 6% 12% 20% 12% 2	4125
	11th to 20th	Day	37% 5% 15% 21% 18% 3	4242
	21st to end	Day	34% 7% 15% 30% 13% 1	5187
February	1st to 10th	Day	50% 7% 15% 15% 11% 2	1680
	11th to 20th	Day	47% 8% 13% 20% 10% 1	1838
	21st to end	Day	46% 7% 14% 20% 12% 2	1688
March	1st to 10th	Day	40% 5% 16% 22% 16% 1	4539
	11th to 20th	Day	39% 7% 15% 25% 14% 1	4945
	21st to end	Day	61% 7% 13% 13% 6% 1	5573
April	1st to 10th	Day	51% 8% 15% 18% 8%	2450
	11th to 20th	Day	51% 7% 12% 19% 9% 1	2570
	21st to end	Day	50% 8% 15% 19% 8% 1	2600
May	1st to 10th	Day	45% 5% 10% 22% 14% 3	6189
	11th to 20th	Day	51% 9% 14% 19% 7%	6738
	21st to end	Day	44% 8% 15% 23% 9% 1	7406
June	1st to 10th	Day	48% 9% 15% 17% 9% 1	3000
	11th to 20th	Day	52% 9% 14% 17% 6% 2	3000
	21st to end	Day	35% 10% 19% 24% 10% 2	3000
July	1st to 10th	Day	30% 8% 18% 27% 14% 3	6706
	11th to 20th	Day	32% 8% 21% 29% 9% 1	6699
	21st to end	Day	31% 7% 17% 25% 16% 5%	6993
August	1st to 10th	Day	34% 8% 17% 34% 6% 1	4524
	11th to 20th	Day	30% 8% 27% 28% 6% 1	4241
	21st to end	Day	28% 6% 19% 33% 12% 1	4663
September	1st to 10th	Day	32% 6% 16% 27% 14% 5%	2479
	11th to 20th	Day	38% 6% 15% 27% 13% 2	2200
	21st to end	Day	36% 7% 15% 28% 12% 3	2200
October	1st to 10th	Day	36% 6% 18% 33% 6% 1	3454
	11th to 20th	Day	35% 6% 20% 23% 13% 3	3000
	21st to end	Day	37% 5% 12% 33% 12% 1	3200
November	1st to 10th	Day	50% 6% 12% 20% 11% 2	1723
	11th to 20th	Day	43% 6% 16% 24% 9% 2	1595
	21st to end	Day	45% 6% 16% 23% 9% 2	1470
December	1st to 10th	Day	49% 7% 13% 22% 7% 2	2360
	11th to 20th	Day	50% 8% 16% 18% 7% 1	2714
	21st to end	Day	39% 5% 11% 23% 17% 4	3575
January	1st to 10th	Night	51% 7% 12% 18% 9% 2	8769
	11th to 20th	Night	40% 6% 16% 20% 12% 5%	8763
	21st to end	Night	40% 6% 16% 26% 11% 1	8994
February	1st to 10th	Night	48% 7% 12% 19% 12% 3	2940
	11th to 20th	Night	48% 7% 13% 18% 12% 2	2679
	21st to end	Night	46% 6% 12% 21% 12% 2	2114
March	1st to 10th	Night	42% 7% 18% 20% 11% 2	5400
	11th to 20th	Night	40% 8% 15% 22% 14% 1	5054
	21st to end	Night	61% 6% 11% 12% 8% 1	5388
April	1st to 10th	Night	51% 6% 13% 18% 11% 1	1949
	11th to 20th	Night	50% 8% 12% 19% 9% 1	1781
	21st to end	Night	47% 8% 15% 20% 8% 2	1800
May	1st to 10th	Night	46% 6% 10% 20% 16% 1	3761
	11th to 20th	Night	45% 10% 15% 24% 5%	3159
	21st to end	Night	42% 9% 16% 25% 8% 1	3470
June	1st to 10th	Night	46% 10% 14% 19% 11% 1	1400
	11th to 20th	Night	48% 9% 16% 18% 7% 2	1400
	21st to end	Night	34% 10% 22% 25% 8% 1	1400
July	1st to 10th	Night	34% 7% 17% 25% 16% 2	3145
	11th to 20th	Night	34% 8% 21% 28% 7% 3	3138
	21st to end	Night	30% 6% 20% 26% 15% 4	3542
August	1st to 10th	Night	35% 8% 15% 30% 12% 1	2601
	11th to 20th	Night	31% 9% 26% 26% 8% 1	2882
	21st to end	Night	26% 7% 21% 32% 12% 2	3245
September	1st to 10th	Night	31% 8% 16% 26% 15% 5%	1918
	11th to 20th	Night	34% 6% 16% 27% 13% 4	2200
	21st to end	Night	33% 6% 18% 26% 14% 3	2200
October	1st to 10th	Night	33% 6% 17% 25% 18% 1	3728
	11th to 20th	Night	33% 6% 18% 25% 16% 2	4147
	21st to end	Night	37% 6% 12% 32% 13% 1	4600
November	1st to 10th	Night	46% 8% 12% 20% 12% 2	2772
	11th to 20th	Night	41% 8% 17% 20% 13% 1	3024
	21st to end	Night	42% 7% 17% 22% 10% 3	3150
December	1st to 10th	Night	48% 7% 16% 20% 7% 2	5040
	11th to 20th	Night	50% 7% 18% 17% 8%	5697
	21st to end	Night	40% 6% 11% 25% 15% 4	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	6% 8% 33% 33% 11% 9%	220
	11th to 20th	Day	8% 12% 40% 22% 16% 2	220
	21st to end	Day	7% 10% 32% 35% 12% 3	242
February	1st to 10th	Day	10% 10% 36% 27% 13% 5%	220
	11th to 20th	Day	8% 7% 36% 38% 9% 2	322
	21st to end	Day	9% 11% 34% 32% 13% 2	404
March	1st to 10th	Day	9% 10% 38% 28% 11% 3	509
	11th to 20th	Day	9% 12% 32% 32% 12% 3	500
	21st to end	Day	14% 17% 32% 22% 10% 4	550
April	1st to 10th	Day	13% 15% 37% 24% 9% 2	500
	11th to 20th	Day	17% 12% 30% 28% 11% 3	492
	21st to end	Day	18% 15% 27% 26% 11% 2	500
May	1st to 10th	Day	17% 11% 27% 28% 14% 3	540
	11th to 20th	Day	17% 7% 26% 28% 19% 4	725
	21st to end	Day	18% 6% 22% 30% 20% 5%	789
June	1st to 10th	Day	16% 10% 28% 29% 14% 3	730
	11th to 20th	Day	21% 10% 27% 28% 12% 2	730
	21st to end	Day	11% 6% 25% 32% 20% 5%	730
July	1st to 10th	Day	10% 5% 26% 32% 22% 5%	721
	11th to 20th	Day	12% 4 27% 33% 20% 4	718
	21st to end	Day	6% 5% 27% 36% 20% 6%	716
August	1st to 10th	Day	11% 6% 25% 31% 21% 6%	527
	11th to 20th	Day	9% 5% 23% 36% 21% 5%	517
	21st to end	Day	11% 7% 22% 30% 20% 9%	583
September	1st to 10th	Day	10% 8% 28% 29% 19% 6%	530
	11th to 20th	Day	18% 6% 23% 30% 17% 6%	530
	21st to end	Day	12% 15% 30% 30% 11% 3	530
October	1st to 10th	Day	15% 7% 30% 30% 14% 4	449
	11th to 20th	Day	11% 12% 35% 27% 13% 3	330
	21st to end	Day	16% 14% 35% 23% 9% 3	363
November	1st to 10th	Day	17% 12% 31% 30% 8% 2	330
	11th to 20th	Day	13% 12% 37% 26% 10% 2	285
	21st to end	Day	8% 10% 36% 25% 15% 6%	220
December	1st to 10th	Day	7% 5% 41% 31% 12% 3	220
	11th to 20th	Day	8% 9% 40% 27% 14% 2	220
	21st to end	Day	8% 5% 31% 37% 16% 3	242
January	1st to 10th	Night	9% 10% 37% 33% 10% 2	1050
	11th to 20th	Night	8% 9% 40% 34% 8% 1	1032
	21st to end	Night	9% 11% 38% 32% 8% 1	924
February	1st to 10th	Night	8% 9% 33% 34% 13% 3	840
	11th to 20th	Night	9% 7% 35% 37% 10% 1	824
	21st to end	Night	6% 8% 35% 31% 17% 3	692
March	1st to 10th	Night	8% 6% 37% 34% 13% 3	808
	11th to 20th	Night	8% 7% 36% 36% 11% 2	800
	21st to end	Night	13% 6% 32% 34% 12% 3	880
April	1st to 10th	Night	13% 8% 31% 32% 15% 1	749
	11th to 20th	Night	17% 6% 37% 31% 10%	593
	21st to end	Night	15% 9% 31% 34% 11%	600
May	1st to 10th	Night	17% 8% 28% 31% 16%	498
	11th to 20th	Night	14% 8% 24% 37% 16%	394
	21st to end	Night	19% 5% 23% 34% 18% 1	433
June	1st to 10th	Night	18% 9% 28% 31% 14% 1	400
	11th to 20th	Night	20% 4 28% 34% 15% 1	400
	21st to end	Night	10% 4 20% 47% 19% 2	400
July	1st to 10th	Night	8% 3 20% 44% 25% 1	390
	11th to 20th	Night	11% 2 23% 43% 18% 2	388
	21st to end	Night	7% 3 20% 46% 22% 2	438
August	1st to 10th	Night	12% 2 13% 41% 29% 3	398
	11th to 20th	Night	10% 2 19% 46% 23% 2	560
	21st to end	Night	10% 4 21% 42% 23%	660
September	1st to 10th	Night	13% 3 25% 36% 21% 2	707
	11th to 20th	Night	16% 4 20% 38% 20% 3	800
	21st to end	Night	12% 6% 32% 34% 14% 2	800
October	1st to 10th	Night	14% 5% 29% 34% 16% 2	800
	11th to 20th	Night	12% 5% 34% 33% 14% 2	800
	21st to end	Night	11% 8% 35% 35% 10% 1	880
November	1st to 10th	Night	14% 8% 40% 28% 9% 1	927
	11th to 20th	Night	8% 11% 43% 29% 8% 1	1050
	21st to end	Night	8% 8% 41% 33% 9% 2	1050
December	1st to 10th	Night	10% 8% 38% 30% 12% 2	1050
	11th to 20th	Night	6% 8% 40% 37% 8% 1	1050
	21st to end	Night	8% 6% 38% 35% 10% 2	1155

Page generated at 2026-02-27 23:40:21. Version ac07

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