Cardston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cardston clear sky chart from 2005-11-06 to 2024-11-25.

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	7% 8% 3 7% 4 4 5% 4 6% 8% 45%	105642
	Night	5% 8% 3 7% 5% 4 5% 4 6% 8% 46%	102426
Transparency	Day	35% 6% 14% 26% 14% 4	105392
	Night	36% 6% 16% 26% 13% 3	102323
Seeing	Day	15% 13% 27% 26% 14% 5%	15413
	Night	12% 12% 31% 32% 12% 1	24594

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 9% 4 10% 6% 5% 5% 4 6% 9% 39%	2219
	11th to 20th	Day	2 9% 5% 11% 7% 7% 8% 6% 6% 7% 32%	2338
	21st to end	Day	2 8% 3 8% 5% 4 5% 4 7% 10% 45%	2794
February	1st to 10th	Day	3 11% 3 12% 7% 6% 5% 5% 8% 8% 31%	1596
	11th to 20th	Day	3 13% 4 9% 4 5% 6% 4 7% 8% 35%	1665
	21st to end	Day	3 10% 5% 11% 7% 5% 5% 4 5% 8% 37%	1535
March	1st to 10th	Day	1 7% 3 8% 5% 5% 4 5% 8% 9% 44%	3157
	11th to 20th	Day	3 10% 3 5% 3 3 4 4 7% 10% 49%	3489
	21st to end	Day	8% 17% 5% 11% 5% 4 4 4 5% 6% 30%	3825
April	1st to 10th	Day	7% 12% 4 8% 6% 5% 6% 5% 7% 9% 32%	2319
	11th to 20th	Day	9% 13% 4 8% 6% 4 5% 3 6% 7% 35%	2440
	21st to end	Day	11% 11% 3 9% 6% 4 5% 3 6% 6% 36%	2470
May	1st to 10th	Day	15% 16% 4 8% 4 3 4 4 5% 6% 31%	4110
	11th to 20th	Day	11% 8% 3 8% 5% 6% 6% 5% 7% 8% 33%	4652
	21st to end	Day	15% 8% 3 7% 5% 5% 6% 5% 7% 9% 32%	5161
June	1st to 10th	Day	12% 7% 2 6% 5% 6% 5% 5% 6% 8% 36%	2850
	11th to 20th	Day	15% 7% 2 7% 5% 6% 5% 5% 7% 8% 34%	2850
	21st to end	Day	7% 4 2 5% 4 4 5% 4 7% 9% 50%	2850
July	1st to 10th	Day	5% 4 1 4 4 4 5% 4 6% 8% 56%	4740
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 8% 64%	4740
	21st to end	Day	2 2 1 2 2 2 3 3 6% 7% 69%	4917
August	1st to 10th	Day	10% 5% 2 5% 3 4 5% 5% 6% 9% 46%	4295
	11th to 20th	Day	4 3 1 4 4 5% 5% 4 6% 8% 56%	4028
	21st to end	Day	7% 3 1 4 3 3 4 3 6% 7% 59%	4521
September	1st to 10th	Day	12% 4 1 4 4 3 3 3 5% 7% 55%	2323
	11th to 20th	Day	10% 6% 2 5% 3 4 5% 3 5% 7% 49%	2090
	21st to end	Day	11% 5% 2 4 4 3 3 3 5% 7% 52%	2090
October	1st to 10th	Day	7% 6% 2 5% 3 3 3 3 6% 11% 51%	3335
	11th to 20th	Day	4 7% 3 6% 4 3 5% 3 6% 10% 49%	3011
	21st to end	Day	6% 7% 3 7% 4 4 5% 4 7% 8% 47%	3101
November	1st to 10th	Day	8% 12% 3 9% 5% 3 4 3 6% 7% 40%	1671
	11th to 20th	Day	3 11% 3 7% 4 4 4 5% 6% 9% 45%	1600
	21st to end	Day	3 9% 5% 9% 7% 4 4 3 6% 9% 41%	1406
December	1st to 10th	Day	2 10% 5% 8% 6% 6% 5% 5% 7% 7% 39%	1330
	11th to 20th	Day	3 9% 4 9% 7% 5% 5% 5% 8% 8% 36%	1682
	21st to end	Day	3 9% 4 7% 4 4 5% 5% 6% 8% 46%	2442
January	1st to 10th	Night	2 8% 4 10% 6% 5% 5% 5% 7% 10% 38%	4726
	11th to 20th	Night	2 9% 4 11% 6% 7% 7% 5% 6% 8% 34%	4590
	21st to end	Night	3 8% 3 7% 5% 4 5% 5% 7% 9% 45%	4862
February	1st to 10th	Night	3 13% 5% 10% 6% 4 5% 4 6% 8% 36%	2584
	11th to 20th	Night	3 12% 4 10% 5% 6% 6% 5% 6% 8% 34%	2402
	21st to end	Night	2 10% 4 11% 7% 6% 5% 5% 6% 7% 37%	1918
March	1st to 10th	Night	1 7% 4 11% 6% 7% 6% 5% 6% 7% 40%	3717
	11th to 20th	Night	3 6% 2 6% 4 4 5% 4 7% 8% 51%	3490
	21st to end	Night	7% 20% 6% 10% 4 4 4 4 5% 7% 29%	3828
April	1st to 10th	Night	7% 11% 2 6% 4 4 4 5% 7% 7% 42%	1859
	11th to 20th	Night	10% 11% 4 8% 5% 4 5% 4 7% 8% 36%	1691
	21st to end	Night	8% 11% 2 6% 5% 5% 6% 4 6% 6% 42%	1710
May	1st to 10th	Night	14% 16% 3 9% 5% 4 4 3 5% 5% 32%	2860
	11th to 20th	Night	10% 7% 2 5% 4 4 5% 4 6% 8% 45%	2240
	21st to end	Night	16% 6% 2 7% 5% 5% 5% 5% 7% 8% 34%	2414
June	1st to 10th	Night	11% 7% 2 5% 5% 3 4 4 6% 8% 45%	1330
	11th to 20th	Night	15% 6% 3 6% 5% 4 4 4 7% 7% 39%	1330
	21st to end	Night	5% 4 1 4 3 4 5% 4 7% 7% 56%	1330
July	1st to 10th	Night	3 3 1 3 3 3 4 4 6% 9% 62%	2220
	11th to 20th	Night	3 2 3 2 3 3 3 6% 5% 70%	2220
	21st to end	Night	3 2 1 3 2 3 3 2 4 5% 71%	2632
August	1st to 10th	Night	10% 4 2 4 3 3 3 3 4 8% 57%	2624
	11th to 20th	Night	5% 3 1 6% 4 5% 5% 4 7% 9% 49%	2811
	21st to end	Night	5% 3 1 4 3 3 3 3 4 8% 62%	3146
September	1st to 10th	Night	12% 4 1 4 2 2 3 3 6% 7% 56%	1852
	11th to 20th	Night	10% 7% 2 4 3 4 4 4 6% 6% 52%	2090
	21st to end	Night	9% 5% 2 4 4 3 4 3 5% 7% 54%	2090
October	1st to 10th	Night	7% 5% 1 3 2 3 4 3 7% 8% 57%	3640
	11th to 20th	Night	4 5% 2 5% 4 4 4 3 6% 9% 54%	3915
	21st to end	Night	4 6% 4 8% 4 3 4 4 5% 8% 51%	4457
November	1st to 10th	Night	7% 12% 4 6% 6% 4 4 4 6% 7% 41%	2607
	11th to 20th	Night	2 9% 3 6% 5% 3 5% 4 7% 9% 47%	2800
	21st to end	Night	3 10% 4 8% 6% 5% 3 4 6% 9% 42%	2867
December	1st to 10th	Night	2 9% 4 9% 6% 5% 5% 5% 6% 8% 42%	2850
	11th to 20th	Night	2 8% 4 10% 6% 5% 6% 4 7% 9% 39%	3510
	21st to end	Night	2 8% 3 7% 5% 4 5% 4 6% 9% 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	39% 8% 17% 24% 10% 1	2219
	11th to 20th	Day	49% 8% 13% 14% 10% 6%	2360
	21st to end	Day	32% 9% 23% 30% 7% 1	2794
February	1st to 10th	Day	47% 8% 16% 19% 7% 2	1596
	11th to 20th	Day	46% 6% 19% 20% 9% 1	1674
	21st to end	Day	45% 7% 15% 21% 11% 2	1535
March	1st to 10th	Day	33% 6% 17% 26% 17% 1	3157
	11th to 20th	Day	30% 7% 22% 31% 9% 1	3490
	21st to end	Day	52% 7% 14% 20% 5%	3825
April	1st to 10th	Day	46% 8% 15% 22% 8%	2319
	11th to 20th	Day	46% 8% 12% 22% 10% 2	2440
	21st to end	Day	46% 6% 14% 23% 10% 1	2470
May	1st to 10th	Day	54% 5% 9% 25% 6%	4110
	11th to 20th	Day	42% 9% 13% 26% 10%	4656
	21st to end	Day	44% 9% 16% 21% 10%	5162
June	1st to 10th	Day	42% 8% 15% 22% 11% 1	2850
	11th to 20th	Day	46% 6% 12% 21% 13% 2	2850
	21st to end	Day	29% 6% 17% 30% 14% 3	2850
July	1st to 10th	Day	24% 5% 11% 29% 22% 8%	4666
	11th to 20th	Day	18% 5% 8% 31% 28% 10%	4659
	21st to end	Day	13% 4 7% 27% 27% 22%	4762
August	1st to 10th	Day	32% 5% 14% 23% 21% 5%	4278
	11th to 20th	Day	25% 5% 14% 37% 17% 3	4067
	21st to end	Day	23% 4 9% 40% 21% 4	4520
September	1st to 10th	Day	30% 3 11% 26% 20% 9%	2323
	11th to 20th	Day	35% 6% 12% 26% 17% 5%	2090
	21st to end	Day	32% 5% 13% 32% 13% 4	2090
October	1st to 10th	Day	27% 4 12% 37% 17% 2	3335
	11th to 20th	Day	31% 3 18% 28% 19% 1	3011
	21st to end	Day	35% 5% 13% 30% 14% 3	3101
November	1st to 10th	Day	44% 8% 13% 24% 10% 1	1671
	11th to 20th	Day	36% 8% 19% 25% 11% 1	1600
	21st to end	Day	40% 6% 21% 19% 11% 3	1406
December	1st to 10th	Day	42% 8% 18% 20% 7% 4	1330
	11th to 20th	Day	43% 7% 22% 22% 5% 1	1684
	21st to end	Day	34% 8% 12% 25% 17% 4	2442
January	1st to 10th	Night	40% 9% 18% 24% 8% 2	4726
	11th to 20th	Night	47% 7% 14% 16% 11% 5%	4630
	21st to end	Night	34% 10% 20% 27% 8% 1	4862
February	1st to 10th	Night	45% 8% 15% 18% 11% 2	2584
	11th to 20th	Night	47% 6% 15% 20% 10% 1	2403
	21st to end	Night	46% 5% 14% 23% 10% 2	1918
March	1st to 10th	Night	40% 8% 16% 22% 14% 1	3717
	11th to 20th	Night	28% 10% 24% 27% 11%	3490
	21st to end	Night	56% 6% 13% 19% 6% 1	3828
April	1st to 10th	Night	39% 6% 18% 28% 8%	1859
	11th to 20th	Night	44% 7% 14% 25% 10% 1	1691
	21st to end	Night	42% 6% 16% 27% 8% 1	1710
May	1st to 10th	Night	52% 6% 12% 23% 7%	2860
	11th to 20th	Night	35% 8% 13% 29% 16%	2240
	21st to end	Night	44% 8% 17% 21% 10%	2414
June	1st to 10th	Night	38% 7% 17% 28% 9% 1	1330
	11th to 20th	Night	42% 5% 16% 22% 13% 2	1330
	21st to end	Night	25% 6% 19% 29% 18% 2	1330
July	1st to 10th	Night	20% 4 13% 31% 23% 9%	2185
	11th to 20th	Night	16% 3 12% 32% 24% 14%	2178
	21st to end	Night	16% 4 7% 27% 27% 20%	2549
August	1st to 10th	Night	28% 4 12% 26% 21% 9%	2616
	11th to 20th	Night	29% 4 17% 30% 17% 3	2838
	21st to end	Night	22% 4 10% 37% 22% 5%	3146
September	1st to 10th	Night	28% 4 12% 29% 18% 9%	1852
	11th to 20th	Night	31% 5% 15% 27% 17% 5%	2090
	21st to end	Night	30% 4 16% 30% 16% 3	2090
October	1st to 10th	Night	25% 4 11% 34% 22% 3	3640
	11th to 20th	Night	29% 3 17% 30% 18% 3	3915
	21st to end	Night	33% 5% 13% 34% 14% 1	4457
November	1st to 10th	Night	42% 7% 18% 21% 11% 2	2607
	11th to 20th	Night	34% 7% 22% 22% 14% 1	2800
	21st to end	Night	39% 6% 22% 20% 11% 3	2867
December	1st to 10th	Night	40% 7% 18% 22% 10% 4	2850
	11th to 20th	Night	38% 10% 23% 22% 5% 1	3507
	21st to end	Night	33% 7% 16% 24% 15% 5%	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	12% 17% 30% 22% 10% 8%	180
	11th to 20th	Day	13% 12% 27% 33% 11% 4	180
	21st to end	Day	17% 18% 35% 19% 7% 5%	198
February	1st to 10th	Day	21% 15% 25% 28% 9% 3	216
	11th to 20th	Day	16% 12% 37% 25% 9% 2	270
	21st to end	Day	17% 21% 30% 24% 7% 1	347
March	1st to 10th	Day	13% 20% 33% 23% 7% 3	460
	11th to 20th	Day	14% 17% 26% 24% 13% 5%	460
	21st to end	Day	14% 18% 35% 23% 8% 2	506
April	1st to 10th	Day	18% 22% 30% 22% 7%	460
	11th to 20th	Day	19% 17% 29% 22% 8% 3	452
	21st to end	Day	23% 17% 25% 21% 11% 3	460
May	1st to 10th	Day	19% 12% 28% 22% 14% 5%	463
	11th to 20th	Day	23% 9% 20% 25% 17% 5%	670
	21st to end	Day	25% 7% 19% 25% 18% 7%	734
June	1st to 10th	Day	16% 16% 24% 25% 15% 4	680
	11th to 20th	Day	23% 14% 25% 23% 12% 4	680
	21st to end	Day	9% 9% 28% 29% 20% 6%	680
July	1st to 10th	Day	8% 7% 22% 31% 25% 7%	671
	11th to 20th	Day	8% 8% 23% 37% 19% 5%	668
	21st to end	Day	5% 7% 26% 36% 17% 8%	606
August	1st to 10th	Day	11% 7% 23% 31% 20% 8%	487
	11th to 20th	Day	6% 7% 21% 31% 26% 8%	477
	21st to end	Day	12% 8% 24% 33% 18% 6%	539
September	1st to 10th	Day	14% 12% 24% 26% 18% 5%	490
	11th to 20th	Day	14% 10% 29% 26% 15% 5%	490
	21st to end	Day	20% 17% 28% 19% 13% 4	490
October	1st to 10th	Day	19% 11% 22% 29% 14% 5%	400
	11th to 20th	Day	15% 14% 29% 22% 13% 7%	300
	21st to end	Day	18% 16% 33% 21% 9% 4	330
November	1st to 10th	Day	24% 18% 28% 23% 6% 1	300
	11th to 20th	Day	18% 19% 36% 21% 5%	300
	21st to end	Day	11% 22% 34% 18% 10% 6%	211
December	1st to 10th	Day	12% 23% 31% 23% 7% 4	180
	11th to 20th	Day	17% 18% 31% 23% 8% 2	180
	21st to end	Day	13% 11% 32% 25% 12% 8%	198
January	1st to 10th	Night	13% 21% 33% 24% 8% 1	950
	11th to 20th	Night	13% 14% 38% 27% 7% 1	879
	21st to end	Night	9% 19% 35% 28% 7% 2	836
February	1st to 10th	Night	13% 14% 30% 29% 10% 3	760
	11th to 20th	Night	14% 14% 34% 28% 10%	744
	21st to end	Night	10% 13% 34% 28% 14% 1	628
March	1st to 10th	Night	8% 13% 37% 30% 10% 3	760
	11th to 20th	Night	10% 13% 32% 34% 9% 2	760
	21st to end	Night	12% 12% 30% 32% 12% 1	836
April	1st to 10th	Night	14% 14% 32% 29% 10% 1	705
	11th to 20th	Night	15% 12% 35% 30% 8%	563
	21st to end	Night	17% 9% 32% 30% 11%	570
May	1st to 10th	Night	17% 11% 25% 29% 17%	570
	11th to 20th	Night	14% 6% 26% 37% 16% 1	397
	21st to end	Night	22% 8% 20% 30% 19% 1	411
June	1st to 10th	Night	17% 7% 28% 34% 12% 2	380
	11th to 20th	Night	20% 9% 30% 30% 9% 1	380
	21st to end	Night	8% 5% 30% 39% 17% 1	380
July	1st to 10th	Night	6% 4 21% 47% 22% 1	370
	11th to 20th	Night	7% 4 27% 45% 16% 1	368
	21st to end	Night	6% 4 19% 53% 17% 1	416
August	1st to 10th	Night	8% 2 18% 48% 24% 1	434
	11th to 20th	Night	8% 2 22% 43% 24% 2	564
	21st to end	Night	9% 5% 23% 45% 16% 2	627
September	1st to 10th	Night	15% 3 23% 39% 17% 3	660
	11th to 20th	Night	15% 4 22% 37% 19% 3	760
	21st to end	Night	13% 10% 28% 35% 12% 3	760
October	1st to 10th	Night	16% 7% 24% 35% 15% 3	760
	11th to 20th	Night	11% 14% 29% 32% 13% 1	760
	21st to end	Night	14% 17% 33% 28% 8% 1	836
November	1st to 10th	Night	17% 13% 37% 28% 5%	852
	11th to 20th	Night	13% 21% 37% 23% 5% 1	1000
	21st to end	Night	11% 18% 37% 24% 8% 1	973
December	1st to 10th	Night	10% 17% 32% 28% 10% 2	950
	11th to 20th	Night	9% 17% 36% 29% 8%	950
	21st to end	Night	12% 16% 34% 26% 9% 2	1045

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