Lethbridge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lethbridge clear sky chart from 2005-11-06 to 2025-10-19.

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% 8% 3 7% 4 4 5% 4 6% 8% 45%	130803
	Night	6% 9% 3 8% 5% 5% 5% 4 6% 8% 42%	127634
Transparency	Day	36% 6% 14% 27% 14% 3	130504
	Night	39% 6% 15% 24% 12% 2	127511
Seeing	Day	13% 14% 29% 26% 15% 4	16678
	Night	12% 14% 34% 29% 11% 1	25760

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 15% 6% 12% 6% 5% 5% 4 6% 7% 30%	3179
	11th to 20th	Day	4 11% 4 10% 6% 6% 6% 5% 6% 6% 36%	3265
	21st to end	Day	2 7% 4 9% 5% 4 4 4 7% 8% 46%	4004
February	1st to 10th	Day	7% 12% 5% 10% 7% 5% 5% 4 7% 8% 29%	1685
	11th to 20th	Day	5% 12% 4 9% 6% 6% 5% 6% 7% 9% 31%	1755
	21st to end	Day	4 11% 5% 11% 7% 4 5% 3 6% 7% 36%	1580
March	1st to 10th	Day	4 10% 3 8% 4 4 4 4 7% 10% 42%	4577
	11th to 20th	Day	4 8% 4 8% 5% 5% 5% 4 6% 7% 43%	4999
	21st to end	Day	10% 16% 6% 10% 5% 4 5% 5% 7% 8% 25%	5486
April	1st to 10th	Day	7% 13% 4 8% 6% 6% 5% 4 5% 8% 33%	2445
	11th to 20th	Day	11% 12% 4 8% 5% 5% 4 4 5% 7% 35%	2570
	21st to end	Day	10% 10% 3 7% 5% 5% 6% 5% 6% 7% 37%	2600
May	1st to 10th	Day	15% 10% 3 5% 3 3 4 4 5% 6% 41%	5987
	11th to 20th	Day	11% 7% 2 7% 5% 6% 6% 5% 7% 9% 35%	6680
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	7405
June	1st to 10th	Day	12% 7% 2 6% 5% 4 5% 5% 7% 8% 40%	3000
	11th to 20th	Day	14% 7% 3 6% 5% 5% 6% 4 7% 8% 36%	3000
	21st to end	Day	8% 4 1 5% 4 3 5% 4 7% 9% 51%	3000
July	1st to 10th	Day	7% 4 1 4 3 3 3 4 6% 7% 58%	6780
	11th to 20th	Day	7% 3 2 4 3 3 4 3 6% 9% 56%	6780
	21st to end	Day	6% 3 1 3 2 2 3 2 5% 7% 64%	7181
August	1st to 10th	Day	8% 5% 2 4 3 4 5% 4 8% 9% 49%	4427
	11th to 20th	Day	5% 3 2 5% 3 3 4 4 6% 8% 57%	4158
	21st to end	Day	8% 3 1 4 3 3 3 4 5% 7% 61%	4664
September	1st to 10th	Day	10% 4 1 4 3 3 3 2 4 7% 58%	2465
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 8% 51%	2200
	21st to end	Day	10% 5% 2 6% 3 4 3 3 5% 8% 51%	2200
October	1st to 10th	Day	7% 5% 2 5% 4 3 5% 4 8% 9% 47%	3406
	11th to 20th	Day	4 6% 2 7% 4 5% 4 4 5% 8% 50%	3116
	21st to end	Day	7% 7% 2 5% 4 4 5% 5% 7% 7% 47%	3101
November	1st to 10th	Day	13% 13% 3 7% 4 4 4 3 7% 7% 35%	1630
	11th to 20th	Day	6% 9% 3 7% 5% 5% 5% 5% 6% 9% 39%	1600
	21st to end	Day	5% 11% 5% 9% 5% 5% 4 3 6% 10% 38%	1448
December	1st to 10th	Day	3 10% 4 9% 6% 6% 6% 4 5% 7% 40%	2290
	11th to 20th	Day	3 10% 4 8% 7% 6% 7% 6% 6% 9% 34%	2642
	21st to end	Day	4 10% 5% 9% 6% 5% 5% 4 6% 7% 40%	3498
January	1st to 10th	Night	4 15% 6% 12% 5% 5% 5% 5% 6% 8% 28%	6766
	11th to 20th	Night	3 13% 5% 10% 6% 6% 6% 5% 6% 7% 35%	6675
	21st to end	Night	3 8% 4 9% 6% 5% 6% 4 6% 7% 42%	6952
February	1st to 10th	Night	5% 13% 4 10% 7% 5% 5% 4 6% 7% 33%	2713
	11th to 20th	Night	6% 13% 4 10% 7% 6% 6% 4 6% 8% 30%	2532
	21st to end	Night	3 12% 4 10% 8% 5% 5% 3 6% 7% 36%	2047
March	1st to 10th	Night	3 8% 4 10% 6% 5% 5% 4 6% 11% 40%	5316
	11th to 20th	Night	3 6% 3 9% 6% 5% 6% 5% 7% 9% 41%	5000
	21st to end	Night	12% 17% 6% 10% 5% 4 4 4 5% 8% 26%	5489
April	1st to 10th	Night	8% 14% 3 8% 4 4 4 4 6% 7% 38%	1952
	11th to 20th	Night	10% 11% 3 8% 6% 4 4 4 6% 7% 36%	1781
	21st to end	Night	11% 9% 3 8% 6% 6% 5% 5% 6% 6% 37%	1800
May	1st to 10th	Night	15% 10% 3 7% 5% 4 4 4 6% 6% 38%	3991
	11th to 20th	Night	12% 6% 3 8% 6% 5% 6% 4 6% 9% 34%	3212
	21st to end	Night	11% 6% 2 6% 4 5% 4 4 6% 7% 44%	3470
June	1st to 10th	Night	13% 6% 2 7% 4 3 5% 4 7% 7% 42%	1400
	11th to 20th	Night	16% 5% 2 6% 6% 4 5% 4 7% 9% 37%	1400
	21st to end	Night	7% 4 2 4 4 4 4 4 6% 9% 53%	1400
July	1st to 10th	Night	6% 2 1 4 4 4 4 4 7% 8% 55%	3180
	11th to 20th	Night	6% 3 1 4 3 4 5% 4 6% 9% 55%	3180
	21st to end	Night	5% 4 2 5% 3 4 4 3 5% 6% 59%	3647
August	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 5% 9% 51%	2726
	11th to 20th	Night	5% 4 2 6% 4 5% 5% 5% 7% 10% 47%	2901
	21st to end	Night	7% 3 1 4 3 3 4 3 5% 7% 60%	3245
September	1st to 10th	Night	9% 4 2 5% 3 2 3 4 6% 6% 55%	1931
	11th to 20th	Night	11% 4 2 4 3 4 3 4 5% 7% 52%	2200
	21st to end	Night	9% 5% 2 5% 3 4 4 3 6% 7% 52%	2200
October	1st to 10th	Night	7% 4 2 5% 3 3 4 3 6% 8% 55%	3788
	11th to 20th	Night	6% 6% 2 6% 4 4 4 4 5% 9% 51%	3986
	21st to end	Night	6% 5% 2 6% 5% 5% 6% 4 6% 7% 48%	4457
November	1st to 10th	Night	10% 13% 3 6% 4 4 4 5% 6% 7% 38%	2649
	11th to 20th	Night	5% 8% 3 7% 5% 5% 5% 5% 6% 7% 43%	2800
	21st to end	Night	4 11% 4 8% 5% 4 3 4 7% 10% 40%	2950
December	1st to 10th	Night	3 10% 3 7% 6% 5% 7% 6% 8% 9% 36%	4890
	11th to 20th	Night	3 10% 4 10% 6% 6% 6% 5% 6% 9% 34%	5550
	21st to end	Night	4 10% 4 8% 5% 5% 5% 4 6% 8% 41%	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% 7% 11% 23% 7% 1	3179
	11th to 20th	Day	48% 6% 13% 16% 14% 2	3286
	21st to end	Day	34% 6% 15% 33% 11% 1	4004
February	1st to 10th	Day	50% 7% 14% 18% 8% 2	1685
	11th to 20th	Day	46% 8% 17% 20% 8% 1	1764
	21st to end	Day	46% 7% 13% 21% 11% 2	1580
March	1st to 10th	Day	36% 6% 14% 28% 17%	4577
	11th to 20th	Day	37% 6% 18% 25% 13% 1	5000
	21st to end	Day	54% 8% 17% 15% 6%	5486
April	1st to 10th	Day	48% 6% 13% 23% 8% 1	2445
	11th to 20th	Day	48% 6% 12% 22% 10% 2	2570
	21st to end	Day	44% 7% 13% 24% 11% 1	2600
May	1st to 10th	Day	43% 5% 9% 24% 15% 5%	5987
	11th to 20th	Day	42% 7% 15% 26% 10% 1	6684
	21st to end	Day	36% 7% 15% 27% 15% 1	7406
June	1st to 10th	Day	40% 8% 16% 23% 11% 2	3000
	11th to 20th	Day	44% 8% 12% 21% 13% 2	3000
	21st to end	Day	28% 8% 15% 33% 13% 3	3000
July	1st to 10th	Day	25% 5% 14% 26% 24% 6%	6706
	11th to 20th	Day	25% 6% 17% 31% 17% 5%	6699
	21st to end	Day	20% 4 14% 28% 21% 13%	6978
August	1st to 10th	Day	28% 7% 14% 30% 18% 3	4410
	11th to 20th	Day	25% 5% 16% 38% 14% 3	4197
	21st to end	Day	23% 5% 10% 39% 20% 3	4663
September	1st to 10th	Day	28% 5% 13% 28% 18% 9%	2465
	11th to 20th	Day	32% 6% 13% 28% 16% 4	2200
	21st to end	Day	32% 5% 12% 33% 15% 3	2200
October	1st to 10th	Day	31% 7% 12% 35% 14% 1	3406
	11th to 20th	Day	32% 5% 18% 28% 17% 1	3116
	21st to end	Day	33% 6% 11% 34% 12% 3	3101
November	1st to 10th	Day	47% 7% 13% 23% 9% 1	1630
	11th to 20th	Day	40% 8% 19% 24% 9% 1	1600
	21st to end	Day	42% 5% 21% 21% 10% 2	1448
December	1st to 10th	Day	43% 6% 17% 26% 6% 2	2290
	11th to 20th	Day	45% 7% 22% 22% 4 1	2644
	21st to end	Day	41% 5% 13% 23% 17% 1	3498
January	1st to 10th	Night	52% 7% 14% 21% 5% 1	6766
	11th to 20th	Night	48% 5% 15% 17% 13% 2	6716
	21st to end	Night	39% 6% 16% 27% 11% 1	6952
February	1st to 10th	Night	48% 8% 14% 16% 12% 2	2713
	11th to 20th	Night	49% 7% 15% 19% 9% 1	2533
	21st to end	Night	47% 6% 15% 21% 8% 2	2047
March	1st to 10th	Night	37% 6% 19% 23% 13% 2	5316
	11th to 20th	Night	36% 8% 18% 27% 10%	5000
	21st to end	Night	55% 7% 16% 16% 6%	5489
April	1st to 10th	Night	44% 6% 15% 25% 10% 1	1952
	11th to 20th	Night	46% 7% 12% 23% 11% 1	1781
	21st to end	Night	45% 8% 12% 24% 10% 1	1800
May	1st to 10th	Night	45% 6% 9% 24% 12% 4	3988
	11th to 20th	Night	42% 9% 12% 27% 9% 1	3212
	21st to end	Night	37% 8% 16% 26% 13% 1	3470
June	1st to 10th	Night	38% 9% 15% 26% 12% 1	1400
	11th to 20th	Night	42% 7% 14% 23% 12% 2	1400
	21st to end	Night	27% 7% 16% 32% 15% 3	1400
July	1st to 10th	Night	24% 6% 16% 23% 22% 8%	3145
	11th to 20th	Night	25% 6% 18% 28% 17% 6%	3138
	21st to end	Night	25% 6% 15% 25% 17% 12%	3546
August	1st to 10th	Night	29% 7% 13% 27% 17% 6%	2718
	11th to 20th	Night	29% 8% 17% 29% 16% 3	2928
	21st to end	Night	23% 4 12% 36% 21% 5%	3245
September	1st to 10th	Night	28% 5% 13% 28% 18% 8%	1931
	11th to 20th	Night	31% 6% 14% 27% 17% 5%	2200
	21st to end	Night	31% 5% 14% 30% 17% 3	2200
October	1st to 10th	Night	27% 5% 11% 34% 21% 2	3788
	11th to 20th	Night	32% 5% 16% 28% 17% 3	3986
	21st to end	Night	34% 6% 12% 34% 13% 1	4457
November	1st to 10th	Night	44% 8% 15% 22% 10% 1	2649
	11th to 20th	Night	37% 7% 20% 22% 14% 1	2800
	21st to end	Night	40% 6% 22% 20% 10% 2	2950
December	1st to 10th	Night	42% 9% 20% 20% 9% 1	4890
	11th to 20th	Night	45% 7% 22% 19% 5% 1	5547
	21st to end	Night	40% 5% 14% 25% 13% 3	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	13% 18% 36% 18% 9% 8%	200
	11th to 20th	Day	16% 17% 30% 27% 11% 1	200
	21st to end	Day	13% 21% 42% 18% 5% 2	220
February	1st to 10th	Day	18% 20% 30% 22% 9% 2	243
	11th to 20th	Day	12% 17% 36% 26% 8% 1	300
	21st to end	Day	17% 20% 32% 23% 7% 1	344
March	1st to 10th	Day	12% 21% 34% 20% 10% 3	500
	11th to 20th	Day	11% 20% 29% 26% 9% 5%	500
	21st to end	Day	13% 21% 34% 21% 8% 2	550
April	1st to 10th	Day	17% 20% 31% 23% 9% 1	500
	11th to 20th	Day	17% 19% 30% 23% 10% 1	492
	21st to end	Day	16% 19% 28% 22% 13% 2	500
May	1st to 10th	Day	14% 14% 28% 25% 16% 4	545
	11th to 20th	Day	17% 7% 27% 26% 19% 4	725
	21st to end	Day	19% 8% 22% 28% 18% 5%	789
June	1st to 10th	Day	15% 13% 28% 25% 17% 2	730
	11th to 20th	Day	16% 16% 29% 24% 14% 2	730
	21st to end	Day	9% 8% 26% 33% 20% 4	730
July	1st to 10th	Day	8% 6% 23% 33% 25% 5%	721
	11th to 20th	Day	8% 6% 28% 34% 20% 3	718
	21st to end	Day	4 8% 26% 35% 22% 6%	713
August	1st to 10th	Day	9% 6% 24% 33% 23% 5%	527
	11th to 20th	Day	5% 6% 26% 31% 25% 7%	517
	21st to end	Day	9% 11% 22% 29% 22% 6%	583
September	1st to 10th	Day	11% 12% 27% 24% 19% 7%	530
	11th to 20th	Day	13% 11% 28% 26% 18% 5%	530
	21st to end	Day	15% 18% 30% 23% 11% 4	530
October	1st to 10th	Day	15% 12% 25% 26% 18% 4	414
	11th to 20th	Day	12% 20% 26% 23% 14% 5%	324
	21st to end	Day	18% 25% 29% 19% 6% 2	330
November	1st to 10th	Day	24% 20% 29% 18% 7% 1	300
	11th to 20th	Day	17% 19% 43% 16% 4 1	300
	21st to end	Day	11% 26% 33% 14% 13% 3	223
December	1st to 10th	Day	16% 26% 30% 22% 7% 1	200
	11th to 20th	Day	12% 25% 38% 16% 9% 1	200
	21st to end	Day	15% 20% 30% 20% 11% 3	220
January	1st to 10th	Night	10% 23% 36% 22% 8% 1	1000
	11th to 20th	Night	12% 18% 39% 24% 6% 1	975
	21st to end	Night	11% 20% 39% 23% 6% 1	880
February	1st to 10th	Night	13% 15% 35% 25% 10% 2	800
	11th to 20th	Night	13% 15% 34% 29% 8% 1	784
	21st to end	Night	12% 16% 34% 25% 12% 1	660
March	1st to 10th	Night	10% 15% 35% 28% 11% 1	800
	11th to 20th	Night	10% 15% 38% 27% 8% 2	800
	21st to end	Night	14% 14% 32% 28% 10% 2	880
April	1st to 10th	Night	14% 15% 36% 25% 9% 1	718
	11th to 20th	Night	15% 11% 39% 26% 8%	593
	21st to end	Night	15% 12% 32% 32% 10%	600
May	1st to 10th	Night	13% 10% 27% 33% 16% 1	600
	11th to 20th	Night	14% 7% 28% 34% 16% 1	421
	21st to end	Night	19% 7% 25% 29% 18% 2	433
June	1st to 10th	Night	15% 10% 32% 30% 13% 1	400
	11th to 20th	Night	18% 9% 35% 27% 11%	400
	21st to end	Night	8% 6% 29% 39% 16% 2	400
July	1st to 10th	Night	6% 6% 21% 45% 21%	390
	11th to 20th	Night	8% 4 26% 45% 16% 1	388
	21st to end	Night	4 4 25% 44% 21% 1	438
August	1st to 10th	Night	6% 6% 21% 44% 21% 2	463
	11th to 20th	Night	7% 3 27% 40% 21% 2	594
	21st to end	Night	7% 6% 27% 42% 17% 1	660
September	1st to 10th	Night	12% 7% 25% 37% 19% 1	661
	11th to 20th	Night	13% 5% 23% 37% 20% 1	800
	21st to end	Night	12% 11% 33% 32% 10% 2	800
October	1st to 10th	Night	15% 8% 29% 32% 15% 1	800
	11th to 20th	Night	13% 13% 33% 30% 10% 1	792
	21st to end	Night	13% 17% 39% 23% 8%	836
November	1st to 10th	Night	18% 17% 38% 22% 5%	894
	11th to 20th	Night	13% 21% 43% 19% 4 1	1000
	21st to end	Night	11% 19% 38% 25% 6% 1	1000
December	1st to 10th	Night	11% 18% 38% 23% 8% 2	1000
	11th to 20th	Night	8% 21% 37% 26% 8%	1000
	21st to end	Night	13% 17% 39% 21% 9% 1	1100

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