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 2024-11-05.

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 6% 4 4 4 4 6% 8% 45%	105542
	Night	7% 8% 3 7% 5% 5% 5% 4 6% 8% 43%	102085
Transparency	Day	36% 7% 14% 27% 13% 3	105288
	Night	38% 7% 15% 24% 13% 3	101986
Seeing	Day	13% 14% 29% 26% 15% 3	15406
	Night	12% 14% 34% 28% 11% 1	24539

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	3 12% 5% 9% 5% 6% 6% 5% 7% 10% 33%	2219
	11th to 20th	Day	4 10% 4 10% 8% 7% 7% 6% 6% 6% 32%	2263
	21st to end	Day	3 9% 3 9% 6% 5% 4 4 7% 9% 42%	2794
February	1st to 10th	Day	8% 12% 5% 10% 7% 5% 5% 4 7% 9% 29%	1600
	11th to 20th	Day	5% 12% 4 8% 6% 6% 6% 6% 6% 9% 31%	1665
	21st to end	Day	5% 12% 5% 12% 7% 4 5% 3 6% 7% 35%	1503
March	1st to 10th	Day	3 10% 3 9% 6% 5% 4 4 7% 9% 39%	3184
	11th to 20th	Day	5% 9% 4 5% 3 4 4 4 6% 8% 48%	3489
	21st to end	Day	11% 16% 5% 9% 5% 4 4 4 6% 7% 28%	3825
April	1st to 10th	Day	8% 14% 4 8% 6% 6% 5% 4 5% 8% 33%	2323
	11th to 20th	Day	11% 13% 4 8% 5% 5% 4 4 5% 7% 34%	2440
	21st to end	Day	11% 10% 3 7% 5% 5% 5% 5% 6% 7% 37%	2470
May	1st to 10th	Day	19% 13% 4 6% 4 4 4 4 5% 6% 31%	4189
	11th to 20th	Day	10% 7% 2 6% 6% 6% 6% 5% 8% 9% 36%	4653
	21st to end	Day	15% 7% 2 7% 6% 6% 5% 5% 7% 9% 32%	5161
June	1st to 10th	Day	12% 7% 2 6% 5% 4 5% 5% 7% 9% 38%	2850
	11th to 20th	Day	14% 7% 3 7% 5% 5% 6% 4 7% 7% 35%	2850
	21st to end	Day	7% 4 1 5% 4 4 5% 4 7% 9% 51%	2850
July	1st to 10th	Day	5% 4 1 4 4 3 3 4 6% 8% 57%	4740
	11th to 20th	Day	3 2 1 3 3 3 3 3 6% 9% 64%	4740
	21st to end	Day	2 1 1 3 2 2 3 3 6% 6% 71%	4999
August	1st to 10th	Day	8% 4 2 4 3 4 5% 5% 8% 9% 49%	4291
	11th to 20th	Day	5% 3 2 5% 3 3 4 4 6% 8% 57%	4028
	21st to end	Day	8% 3 1 4 3 3 3 4 6% 7% 60%	4521
September	1st to 10th	Day	11% 4 1 4 3 3 3 2 4 7% 58%	2342
	11th to 20th	Day	13% 5% 2 6% 3 3 3 3 5% 8% 50%	2090
	21st to end	Day	10% 5% 2 6% 3 4 3 3 5% 8% 51%	2090
October	1st to 10th	Day	7% 5% 2 5% 4 3 5% 4 8% 9% 47%	3302
	11th to 20th	Day	4 7% 2 7% 4 5% 4 4 5% 8% 50%	3038
	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 4 7% 7% 35%	1582
	11th to 20th	Day	6% 9% 3 7% 5% 5% 5% 5% 6% 9% 40%	1520
	21st to end	Day	5% 11% 5% 8% 4 4 4 3 6% 10% 39%	1376
December	1st to 10th	Day	4 13% 4 9% 5% 5% 6% 4 6% 7% 37%	1330
	11th to 20th	Day	4 11% 5% 10% 7% 5% 6% 5% 7% 9% 32%	1682
	21st to end	Day	4 9% 3 7% 5% 4 5% 4 6% 8% 45%	2442
January	1st to 10th	Night	4 11% 5% 11% 5% 5% 5% 5% 7% 9% 33%	4726
	11th to 20th	Night	3 11% 5% 10% 6% 6% 7% 5% 6% 7% 34%	4663
	21st to end	Night	4 9% 4 9% 5% 5% 5% 4 6% 7% 42%	4862
February	1st to 10th	Night	6% 13% 4 10% 7% 5% 5% 4 6% 7% 34%	2578
	11th to 20th	Night	6% 14% 4 9% 7% 6% 6% 5% 6% 8% 31%	2402
	21st to end	Night	4 12% 5% 10% 8% 5% 5% 3 6% 6% 35%	1948
March	1st to 10th	Night	3 8% 5% 11% 7% 6% 6% 4 6% 7% 37%	3689
	11th to 20th	Night	5% 7% 3 6% 4 4 5% 4 6% 9% 47%	3490
	21st to end	Night	13% 18% 6% 10% 5% 4 4 4 5% 7% 24%	3828
April	1st to 10th	Night	8% 14% 3 8% 5% 4 4 4 6% 7% 38%	1855
	11th to 20th	Night	10% 12% 4 8% 6% 4 4 4 6% 7% 35%	1691
	21st to end	Night	11% 9% 3 8% 5% 6% 5% 5% 6% 5% 37%	1710
May	1st to 10th	Night	18% 12% 3 8% 6% 5% 4 3 6% 5% 30%	2779
	11th to 20th	Night	11% 6% 3 7% 6% 4 6% 4 6% 9% 39%	2238
	21st to end	Night	16% 7% 3 8% 5% 6% 5% 5% 7% 8% 30%	2414
June	1st to 10th	Night	13% 6% 3 7% 4 3 5% 4 7% 8% 41%	1330
	11th to 20th	Night	17% 5% 2 6% 6% 4 5% 4 6% 9% 36%	1330
	21st to end	Night	6% 4 2 4 4 4 4 4 6% 9% 53%	1330
July	1st to 10th	Night	4 3 1 5% 4 5% 5% 4 7% 9% 53%	2220
	11th to 20th	Night	4 3 1 4 3 4 5% 3 6% 8% 59%	2220
	21st to end	Night	2 2 1 4 3 3 3 4 5% 6% 66%	2538
August	1st to 10th	Night	8% 3 2 5% 4 4 5% 4 5% 9% 52%	2642
	11th to 20th	Night	5% 4 2 6% 4 5% 5% 5% 7% 10% 47%	2811
	21st to end	Night	7% 3 1 4 3 3 4 3 5% 7% 60%	3146
September	1st to 10th	Night	10% 4 2 5% 3 2 3 4 6% 6% 55%	1834
	11th to 20th	Night	12% 5% 1 4 3 4 3 4 5% 7% 52%	2090
	21st to end	Night	10% 5% 2 5% 3 4 4 3 6% 7% 51%	2090
October	1st to 10th	Night	7% 4 2 5% 3 3 3 3 6% 8% 55%	3672
	11th to 20th	Night	6% 7% 2 6% 4 4 4 4 5% 9% 51%	3888
	21st to end	Night	6% 5% 2 6% 5% 5% 6% 4 6% 7% 48%	4457
November	1st to 10th	Night	10% 12% 3 6% 4 4 4 5% 6% 7% 38%	2578
	11th to 20th	Night	5% 8% 3 7% 5% 5% 4 5% 6% 7% 43%	2660
	21st to end	Night	4 11% 4 8% 5% 4 3 4 7% 10% 41%	2802
December	1st to 10th	Night	5% 12% 4 9% 7% 5% 5% 5% 6% 7% 36%	2850
	11th to 20th	Night	3 8% 4 10% 7% 5% 5% 5% 6% 10% 37%	3510
	21st to end	Night	4 9% 3 7% 5% 5% 4 4 6% 8% 46%	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% 9% 15% 24% 8% 1	2219
	11th to 20th	Day	50% 7% 13% 12% 14% 4	2284
	21st to end	Day	37% 8% 19% 30% 5% 1	2794
February	1st to 10th	Day	50% 8% 15% 19% 7% 2	1600
	11th to 20th	Day	46% 9% 17% 20% 8% 1	1674
	21st to end	Day	48% 7% 12% 21% 10% 2	1503
March	1st to 10th	Day	38% 7% 16% 26% 12%	3184
	11th to 20th	Day	32% 7% 24% 26% 9% 1	3490
	21st to end	Day	53% 8% 15% 19% 5%	3825
April	1st to 10th	Day	49% 6% 14% 22% 7% 1	2323
	11th to 20th	Day	48% 6% 12% 22% 10% 2	2440
	21st to end	Day	45% 7% 13% 23% 11% 1	2470
May	1st to 10th	Day	53% 7% 10% 25% 6%	4189
	11th to 20th	Day	39% 8% 16% 26% 11% 1	4657
	21st to end	Day	44% 8% 16% 21% 9% 1	5162
June	1st to 10th	Day	41% 8% 17% 22% 10% 2	2850
	11th to 20th	Day	44% 8% 12% 21% 12% 2	2850
	21st to end	Day	27% 8% 16% 32% 13% 3	2850
July	1st to 10th	Day	24% 6% 13% 29% 21% 7%	4666
	11th to 20th	Day	17% 6% 14% 35% 21% 7%	4659
	21st to end	Day	13% 4 8% 28% 29% 18%	4841
August	1st to 10th	Day	28% 7% 14% 30% 18% 3	4274
	11th to 20th	Day	25% 5% 15% 38% 13% 3	4067
	21st to end	Day	23% 5% 10% 39% 20% 3	4520
September	1st to 10th	Day	28% 5% 12% 27% 18% 10%	2342
	11th to 20th	Day	33% 7% 14% 27% 16% 4	2090
	21st to end	Day	32% 6% 12% 33% 14% 3	2090
October	1st to 10th	Day	31% 7% 12% 35% 14% 1	3302
	11th to 20th	Day	32% 5% 18% 28% 16% 2	3038
	21st to end	Day	33% 6% 11% 34% 12% 3	3101
November	1st to 10th	Day	47% 7% 13% 23% 9% 1	1582
	11th to 20th	Day	39% 8% 19% 24% 9% 1	1520
	21st to end	Day	41% 6% 20% 21% 10% 2	1376
December	1st to 10th	Day	45% 7% 17% 19% 9% 3	1330
	11th to 20th	Day	46% 9% 18% 23% 4 1	1684
	21st to end	Day	35% 6% 16% 23% 19% 1	2442
January	1st to 10th	Night	45% 8% 17% 22% 7% 1	4726
	11th to 20th	Night	48% 7% 14% 15% 14% 2	4704
	21st to end	Night	39% 7% 20% 26% 8%	4862
February	1st to 10th	Night	48% 8% 14% 17% 11% 2	2578
	11th to 20th	Night	48% 7% 15% 19% 9% 1	2403
	21st to end	Night	47% 7% 15% 21% 8% 2	1948
March	1st to 10th	Night	43% 7% 16% 23% 10%	3689
	11th to 20th	Night	32% 9% 24% 25% 10%	3490
	21st to end	Night	58% 8% 14% 16% 4 1	3828
April	1st to 10th	Night	45% 6% 15% 24% 9% 1	1855
	11th to 20th	Night	47% 7% 12% 23% 10% 1	1691
	21st to end	Night	45% 8% 12% 24% 9% 1	1710
May	1st to 10th	Night	54% 7% 11% 21% 7%	2779
	11th to 20th	Night	37% 10% 13% 29% 11% 1	2238
	21st to end	Night	47% 10% 16% 19% 7%	2414
June	1st to 10th	Night	39% 9% 16% 25% 11% 1	1330
	11th to 20th	Night	43% 7% 14% 23% 12% 2	1330
	21st to end	Night	26% 7% 17% 31% 15% 3	1330
July	1st to 10th	Night	25% 7% 16% 26% 21% 5%	2185
	11th to 20th	Night	22% 5% 15% 29% 20% 9%	2178
	21st to end	Night	17% 5% 9% 26% 25% 18%	2458
August	1st to 10th	Night	29% 7% 12% 28% 17% 7%	2634
	11th to 20th	Night	29% 8% 16% 29% 16% 3	2838
	21st to end	Night	24% 4 12% 36% 20% 4	3146
September	1st to 10th	Night	28% 6% 13% 27% 18% 8%	1834
	11th to 20th	Night	31% 7% 13% 27% 17% 6%	2090
	21st to end	Night	32% 5% 14% 30% 17% 3	2090
October	1st to 10th	Night	27% 6% 11% 33% 21% 3	3672
	11th to 20th	Night	31% 5% 16% 28% 17% 3	3888
	21st to end	Night	34% 6% 12% 34% 13% 1	4457
November	1st to 10th	Night	44% 8% 15% 22% 10% 1	2578
	11th to 20th	Night	36% 7% 21% 21% 14% 1	2660
	21st to end	Night	38% 6% 22% 21% 11% 2	2802
December	1st to 10th	Night	45% 8% 16% 20% 10% 2	2850
	11th to 20th	Night	41% 8% 23% 22% 5% 1	3507
	21st to end	Night	36% 6% 16% 24% 14% 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	11% 19% 34% 19% 8% 8%	180
	11th to 20th	Day	16% 16% 29% 27% 12% 1	180
	21st to end	Day	12% 21% 41% 18% 5% 3	198
February	1st to 10th	Day	19% 21% 29% 21% 8% 2	218
	11th to 20th	Day	14% 19% 35% 23% 8% 1	270
	21st to end	Day	15% 19% 33% 24% 8% 1	315
March	1st to 10th	Day	11% 22% 35% 20% 10% 2	460
	11th to 20th	Day	11% 20% 28% 27% 9% 5%	460
	21st to end	Day	12% 21% 36% 21% 8% 2	506
April	1st to 10th	Day	17% 21% 32% 22% 7%	460
	11th to 20th	Day	17% 18% 30% 23% 11% 1	452
	21st to end	Day	17% 20% 29% 22% 11% 2	460
May	1st to 10th	Day	15% 14% 29% 23% 16% 4	503
	11th to 20th	Day	17% 7% 26% 27% 20% 4	675
	21st to end	Day	20% 8% 22% 28% 17% 5%	734
June	1st to 10th	Day	16% 13% 28% 24% 17% 2	680
	11th to 20th	Day	16% 17% 29% 23% 13% 2	680
	21st to end	Day	8% 8% 27% 32% 21% 4	680
July	1st to 10th	Day	7% 6% 23% 34% 25% 5%	671
	11th to 20th	Day	7% 7% 29% 34% 21% 3	668
	21st to end	Day	3 8% 27% 35% 21% 5%	662
August	1st to 10th	Day	9% 6% 25% 33% 22% 5%	487
	11th to 20th	Day	5% 6% 25% 30% 26% 8%	477
	21st to end	Day	10% 12% 24% 28% 21% 6%	539
September	1st to 10th	Day	11% 13% 28% 24% 18% 6%	490
	11th to 20th	Day	13% 12% 30% 26% 15% 4	490
	21st to end	Day	16% 19% 29% 22% 10% 4	490
October	1st to 10th	Day	16% 11% 26% 26% 18% 3	380
	11th to 20th	Day	11% 21% 27% 23% 13% 5%	300
	21st to end	Day	18% 25% 29% 19% 6% 2	330
November	1st to 10th	Day	25% 20% 29% 18% 7%	282
	11th to 20th	Day	17% 17% 44% 17% 4	270
	21st to end	Day	11% 27% 33% 13% 13% 2	201
December	1st to 10th	Day	16% 26% 31% 21% 6% 1	180
	11th to 20th	Day	12% 26% 39% 14% 8% 1	180
	21st to end	Day	15% 19% 31% 21% 12% 3	198
January	1st to 10th	Night	10% 24% 36% 23% 7% 1	950
	11th to 20th	Night	12% 18% 39% 24% 6% 1	927
	21st to end	Night	11% 20% 38% 23% 7% 1	836
February	1st to 10th	Night	14% 15% 34% 25% 10% 2	760
	11th to 20th	Night	14% 16% 34% 28% 8% 1	744
	21st to end	Night	12% 14% 35% 25% 13% 1	628
March	1st to 10th	Night	9% 15% 36% 28% 10% 1	760
	11th to 20th	Night	11% 15% 37% 27% 9% 2	760
	21st to end	Night	14% 14% 32% 28% 10% 2	836
April	1st to 10th	Night	15% 16% 36% 24% 9%	682
	11th to 20th	Night	15% 11% 39% 26% 9%	563
	21st to end	Night	15% 12% 32% 31% 9%	570
May	1st to 10th	Night	14% 10% 27% 32% 16% 1	570
	11th to 20th	Night	14% 6% 28% 35% 16% 1	400
	21st to end	Night	20% 7% 26% 29% 17% 2	411
June	1st to 10th	Night	16% 10% 32% 29% 13% 1	380
	11th to 20th	Night	19% 9% 35% 26% 11%	380
	21st to end	Night	8% 6% 30% 38% 16% 2	380
July	1st to 10th	Night	6% 6% 21% 45% 21%	370
	11th to 20th	Night	7% 5% 27% 44% 16% 1	368
	21st to end	Night	3 4 26% 44% 21% 1	416
August	1st to 10th	Night	6% 5% 21% 44% 21% 1	439
	11th to 20th	Night	7% 4 27% 40% 21% 2	564
	21st to end	Night	8% 6% 27% 41% 17%	627
September	1st to 10th	Night	12% 7% 26% 37% 18% 1	628
	11th to 20th	Night	14% 5% 24% 37% 18% 1	760
	21st to end	Night	12% 11% 33% 32% 10% 2	760
October	1st to 10th	Night	16% 8% 28% 32% 15% 1	760
	11th to 20th	Night	13% 13% 34% 29% 11% 1	760
	21st to end	Night	13% 17% 39% 23% 8%	836
November	1st to 10th	Night	18% 18% 37% 22% 5%	869
	11th to 20th	Night	13% 21% 43% 18% 4 1	950
	21st to end	Night	11% 20% 39% 24% 6% 1	950
December	1st to 10th	Night	11% 18% 39% 22% 9% 2	950
	11th to 20th	Night	8% 22% 37% 26% 8%	950
	21st to end	Night	13% 18% 38% 22% 9% 1	1045

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