Waterton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Waterton 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	5% 9% 3 9% 7% 7% 6% 5% 6% 8% 35%	123223
	Night	3 8% 3 9% 7% 8% 7% 5% 7% 8% 35%	123719
Transparency	Day	35% 8% 19% 22% 12% 3	122977
	Night	35% 9% 21% 21% 12% 3	123608
Seeing	Day	17% 21% 29% 21% 10% 2	16014
	Night	13% 21% 32% 24% 9% 1	25271

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	1 7% 7% 16% 10% 9% 8% 6% 7% 9% 21%	3179
	11th to 20th	Day	1 5% 3 14% 12% 9% 7% 7% 7% 7% 27%	3460
	21st to end	Day	1 6% 4 9% 9% 10% 7% 5% 6% 9% 34%	4004
February	1st to 10th	Day	2 12% 5% 15% 11% 9% 8% 6% 8% 9% 17%	1655
	11th to 20th	Day	2 12% 5% 14% 9% 9% 9% 5% 6% 7% 21%	1755
	21st to end	Day	2 10% 5% 13% 12% 10% 9% 5% 7% 7% 20%	1645
March	1st to 10th	Day	1 7% 5% 12% 8% 9% 8% 5% 6% 8% 32%	4518
	11th to 20th	Day	1 8% 5% 13% 10% 10% 7% 5% 6% 7% 28%	4972
	21st to end	Day	3 18% 8% 16% 9% 9% 7% 4 5% 5% 15%	5522
April	1st to 10th	Day	4 14% 6% 14% 10% 9% 7% 6% 6% 7% 19%	2432
	11th to 20th	Day	5% 16% 6% 13% 7% 7% 6% 5% 6% 7% 23%	2570
	21st to end	Day	7% 16% 5% 10% 7% 7% 6% 4 6% 7% 24%	2600
May	1st to 10th	Day	8% 19% 4 7% 5% 4 3 3 4 4 39%	5880
	11th to 20th	Day	9% 13% 3 9% 7% 7% 8% 5% 7% 7% 25%	6636
	21st to end	Day	10% 9% 3 8% 7% 8% 6% 5% 7% 7% 28%	7405
June	1st to 10th	Day	13% 10% 4 9% 8% 7% 7% 5% 5% 7% 26%	2988
	11th to 20th	Day	15% 11% 3 9% 7% 9% 7% 6% 6% 7% 21%	2850
	21st to end	Day	7% 5% 2 7% 6% 6% 7% 5% 8% 7% 41%	2850
July	1st to 10th	Day	4 4 2 5% 6% 5% 6% 5% 7% 7% 50%	4740
	11th to 20th	Day	2 3 1 3 4 4 4 4 6% 8% 61%	4740
	21st to end	Day	2 2 1 3 2 3 4 4 6% 7% 66%	4833
August	1st to 10th	Day	7% 7% 2 7% 5% 5% 5% 5% 7% 8% 44%	4341
	11th to 20th	Day	5% 4 2 5% 5% 5% 5% 3 6% 8% 53%	4028
	21st to end	Day	7% 3 1 4 3 4 4 4 6% 8% 56%	4521
September	1st to 10th	Day	9% 8% 2 5% 4 4 5% 4 5% 8% 47%	2310
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 9% 41%	2090
	21st to end	Day	9% 8% 2 5% 4 6% 5% 3 6% 10% 42%	2090
October	1st to 10th	Day	6% 9% 3 6% 4 4 4 3 5% 6% 50%	3372
	11th to 20th	Day	2 5% 2 8% 6% 6% 6% 6% 8% 11% 39%	2973
	21st to end	Day	4 8% 4 10% 6% 7% 7% 5% 7% 9% 33%	3101
November	1st to 10th	Day	4 11% 4 10% 9% 10% 8% 5% 7% 10% 21%	1709
	11th to 20th	Day	1 9% 4 8% 7% 9% 8% 7% 7% 12% 28%	1600
	21st to end	Day	2 10% 3 14% 8% 8% 7% 6% 6% 10% 26%	1424
December	1st to 10th	Day	1 5% 3 6% 5% 5% 5% 5% 7% 10% 46%	2290
	11th to 20th	Day	1 7% 4 11% 9% 9% 8% 5% 8% 10% 28%	2642
	21st to end	Day	1 8% 5% 11% 8% 9% 9% 6% 8% 9% 27%	3498
January	1st to 10th	Night	1 7% 5% 14% 9% 11% 11% 7% 8% 8% 19%	6766
	11th to 20th	Night	1 6% 3 13% 9% 8% 9% 7% 7% 9% 29%	6470
	21st to end	Night	1 7% 3 10% 8% 13% 7% 5% 7% 8% 30%	6952
February	1st to 10th	Night	3 12% 6% 14% 9% 7% 9% 7% 7% 8% 19%	2745
	11th to 20th	Night	1 10% 5% 12% 10% 9% 9% 7% 8% 7% 21%	2532
	21st to end	Night	2 8% 4 14% 10% 9% 11% 6% 8% 8% 20%	1982
March	1st to 10th	Night	9% 4 14% 9% 9% 7% 6% 6% 7% 29%	5376
	11th to 20th	Night	1 5% 3 9% 7% 9% 8% 5% 8% 11% 34%	5027
	21st to end	Night	3 15% 7% 14% 9% 11% 7% 5% 6% 8% 15%	5451
April	1st to 10th	Night	4 11% 4 8% 7% 7% 7% 5% 8% 10% 27%	1967
	11th to 20th	Night	4 13% 4 8% 6% 7% 7% 5% 8% 9% 30%	1781
	21st to end	Night	6% 12% 3 9% 8% 7% 7% 3 5% 7% 33%	1800
May	1st to 10th	Night	8% 15% 4 7% 5% 4 4 4 5% 6% 40%	4083
	11th to 20th	Night	8% 8% 3 10% 6% 7% 7% 5% 6% 8% 32%	3256
	21st to end	Night	9% 9% 2 7% 8% 7% 6% 4 6% 6% 37%	3470
June	1st to 10th	Night	11% 8% 3 7% 8% 8% 5% 4 6% 6% 34%	1396
	11th to 20th	Night	15% 7% 3 6% 6% 7% 7% 4 7% 8% 31%	1330
	21st to end	Night	5% 4 2 6% 4 4 5% 6% 6% 7% 52%	1330
July	1st to 10th	Night	3 3 1 4 4 4 4 4 6% 7% 60%	2220
	11th to 20th	Night	2 2 1 4 3 3 3 3 4 6% 69%	2220
	21st to end	Night	3 2 2 2 3 4 3 4 5% 71%	2717
August	1st to 10th	Night	9% 7% 2 5% 2 3 4 3 4 8% 53%	2589
	11th to 20th	Night	4 3 1 4 4 4 6% 5% 8% 9% 52%	2811
	21st to end	Night	3 4 1 3 3 3 4 4 6% 9% 61%	3146
September	1st to 10th	Night	9% 5% 2 6% 4 3 4 4 5% 7% 52%	1866
	11th to 20th	Night	6% 7% 2 7% 5% 5% 5% 4 7% 9% 45%	2090
	21st to end	Night	7% 8% 2 5% 3 5% 4 4 7% 9% 46%	2090
October	1st to 10th	Night	5% 6% 2 5% 4 4 4 4 6% 8% 52%	3604
	11th to 20th	Night	2 5% 2 5% 4 6% 5% 4 7% 11% 50%	3952
	21st to end	Night	2 6% 3 7% 5% 8% 7% 5% 7% 9% 40%	4457
November	1st to 10th	Night	4 11% 5% 10% 8% 10% 7% 6% 7% 9% 24%	2570
	11th to 20th	Night	1 6% 3 10% 8% 10% 9% 9% 7% 10% 26%	2800
	21st to end	Night	2 10% 3 10% 7% 9% 8% 6% 9% 10% 26%	2975
December	1st to 10th	Night	1 6% 4 9% 6% 7% 7% 6% 8% 11% 36%	4890
	11th to 20th	Night	1 9% 4 12% 9% 9% 9% 7% 9% 9% 23%	5550
	21st to end	Night	1 9% 6% 12% 8% 9% 9% 6% 8% 9% 24%	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	44% 9% 27% 14% 5% 1	3179
	11th to 20th	Day	46% 10% 18% 12% 10% 4	3484
	21st to end	Day	30% 13% 23% 22% 11%	4004
February	1st to 10th	Day	48% 15% 19% 11% 5% 2	1655
	11th to 20th	Day	45% 11% 25% 13% 5% 1	1764
	21st to end	Day	44% 13% 21% 15% 6% 1	1645
March	1st to 10th	Day	29% 8% 25% 20% 15% 3	4518
	11th to 20th	Day	30% 10% 25% 23% 11% 1	4973
	21st to end	Day	53% 9% 21% 12% 6%	5522
April	1st to 10th	Day	46% 9% 26% 16% 3	2432
	11th to 20th	Day	49% 9% 20% 17% 5% 1	2570
	21st to end	Day	47% 9% 20% 17% 6% 1	2600
May	1st to 10th	Day	44% 5% 12% 21% 15% 3	5880
	11th to 20th	Day	44% 7% 18% 23% 7% 1	6640
	21st to end	Day	37% 7% 25% 20% 11%	7406
June	1st to 10th	Day	46% 10% 19% 19% 7%	2988
	11th to 20th	Day	49% 9% 17% 17% 7% 1	2850
	21st to end	Day	31% 8% 22% 25% 12% 2	2850
July	1st to 10th	Day	26% 6% 11% 31% 18% 8%	4666
	11th to 20th	Day	18% 6% 10% 23% 28% 15%	4659
	21st to end	Day	13% 3 8% 27% 27% 20%	4680
August	1st to 10th	Day	35% 5% 14% 21% 19% 6%	4324
	11th to 20th	Day	26% 4 17% 32% 19% 2	4067
	21st to end	Day	22% 4 12% 41% 19% 2	4520
September	1st to 10th	Day	31% 4 15% 25% 16% 8%	2310
	11th to 20th	Day	31% 7% 18% 26% 14% 3	2090
	21st to end	Day	30% 8% 22% 26% 11% 3	2090
October	1st to 10th	Day	31% 6% 16% 30% 16% 1	3372
	11th to 20th	Day	27% 7% 25% 24% 17% 1	2973
	21st to end	Day	32% 7% 21% 27% 11% 2	3101
November	1st to 10th	Day	40% 15% 26% 13% 5% 1	1709
	11th to 20th	Day	32% 17% 29% 14% 8% 1	1600
	21st to end	Day	41% 14% 24% 12% 8% 1	1424
December	1st to 10th	Day	26% 12% 29% 23% 8% 2	2290
	11th to 20th	Day	36% 13% 26% 18% 6% 1	2644
	21st to end	Day	36% 11% 19% 19% 13% 2	3498
January	1st to 10th	Night	44% 11% 25% 15% 4 1	6766
	11th to 20th	Night	43% 9% 18% 17% 8% 5%	6508
	21st to end	Night	32% 15% 22% 20% 10%	6952
February	1st to 10th	Night	48% 13% 17% 12% 8% 2	2745
	11th to 20th	Night	43% 14% 24% 12% 6% 1	2533
	21st to end	Night	44% 12% 22% 15% 5% 1	1982
March	1st to 10th	Night	34% 10% 23% 19% 9% 4	5376
	11th to 20th	Night	25% 10% 27% 24% 14%	5027
	21st to end	Night	51% 8% 24% 11% 5%	5451
April	1st to 10th	Night	38% 10% 26% 21% 6%	1967
	11th to 20th	Night	40% 10% 21% 22% 6% 1	1781
	21st to end	Night	43% 9% 18% 22% 8% 1	1800
May	1st to 10th	Night	43% 6% 12% 21% 15% 2	4080
	11th to 20th	Night	38% 6% 20% 25% 9% 1	3256
	21st to end	Night	35% 7% 21% 25% 12%	3470
June	1st to 10th	Night	39% 8% 22% 22% 9%	1396
	11th to 20th	Night	42% 7% 19% 19% 11% 1	1330
	21st to end	Night	24% 8% 24% 27% 15% 3	1330
July	1st to 10th	Night	19% 4 15% 30% 22% 10%	2185
	11th to 20th	Night	15% 4 10% 26% 27% 17%	2178
	21st to end	Night	14% 3 8% 26% 29% 19%	2631
August	1st to 10th	Night	29% 3 16% 23% 20% 9%	2581
	11th to 20th	Night	25% 4 19% 34% 15% 2	2838
	21st to end	Night	19% 4 11% 40% 23% 4	3146
September	1st to 10th	Night	28% 6% 17% 26% 17% 7%	1866
	11th to 20th	Night	30% 6% 18% 27% 14% 4	2090
	21st to end	Night	28% 7% 24% 24% 15% 3	2090
October	1st to 10th	Night	23% 6% 14% 32% 21% 3	3604
	11th to 20th	Night	21% 6% 27% 27% 17% 2	3952
	21st to end	Night	28% 8% 18% 32% 13% 1	4457
November	1st to 10th	Night	42% 13% 24% 13% 7% 2	2570
	11th to 20th	Night	33% 16% 28% 15% 7%	2800
	21st to end	Night	38% 15% 26% 12% 7% 2	2975
December	1st to 10th	Night	29% 11% 28% 20% 10% 1	4890
	11th to 20th	Night	42% 12% 27% 14% 5% 1	5547
	21st to end	Night	40% 9% 21% 16% 11% 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	15% 30% 31% 15% 7% 4	200
	11th to 20th	Day	15% 28% 30% 23% 3 2	200
	21st to end	Day	22% 30% 30% 11% 4 2	220
February	1st to 10th	Day	19% 22% 37% 15% 6% 1	227
	11th to 20th	Day	12% 27% 38% 17% 7%	300
	21st to end	Day	22% 29% 31% 14% 4	409
March	1st to 10th	Day	17% 29% 32% 16% 5% 2	500
	11th to 20th	Day	16% 24% 30% 18% 9% 2	500
	21st to end	Day	18% 27% 32% 16% 6% 2	550
April	1st to 10th	Day	20% 26% 36% 13% 6%	500
	11th to 20th	Day	17% 32% 26% 17% 7% 1	492
	21st to end	Day	21% 23% 28% 19% 7% 3	500
May	1st to 10th	Day	19% 21% 26% 21% 11% 3	503
	11th to 20th	Day	25% 14% 25% 18% 14% 4	679
	21st to end	Day	25% 12% 21% 22% 16% 4	789
June	1st to 10th	Day	18% 22% 24% 23% 11% 2	726
	11th to 20th	Day	24% 21% 24% 18% 11% 2	680
	21st to end	Day	12% 14% 26% 28% 16% 4	680
July	1st to 10th	Day	7% 13% 28% 30% 18% 4	671
	11th to 20th	Day	8% 11% 32% 32% 14% 3	668
	21st to end	Day	6% 12% 33% 30% 14% 4	561
August	1st to 10th	Day	13% 12% 27% 28% 16% 5%	487
	11th to 20th	Day	8% 8% 34% 31% 16% 4	477
	21st to end	Day	14% 15% 31% 27% 11% 3	539
September	1st to 10th	Day	16% 19% 26% 23% 12% 3	490
	11th to 20th	Day	13% 18% 32% 23% 12% 2	490
	21st to end	Day	24% 23% 27% 18% 7% 1	490
October	1st to 10th	Day	18% 22% 30% 17% 12% 1	424
	11th to 20th	Day	20% 22% 29% 18% 9% 3	300
	21st to end	Day	25% 25% 29% 13% 7% 2	330
November	1st to 10th	Day	21% 30% 31% 15% 4	300
	11th to 20th	Day	26% 32% 29% 12% 2	300
	21st to end	Day	20% 37% 20% 13% 8% 3	212
December	1st to 10th	Day	19% 35% 28% 13% 5% 1	200
	11th to 20th	Day	24% 26% 30% 19% 2 1	200
	21st to end	Day	17% 27% 27% 18% 9% 2	220
January	1st to 10th	Night	16% 31% 33% 16% 4 1	1000
	11th to 20th	Night	13% 28% 36% 17% 5% 1	884
	21st to end	Night	12% 31% 36% 14% 4 2	880
February	1st to 10th	Night	13% 21% 36% 22% 8% 1	800
	11th to 20th	Night	11% 24% 37% 22% 6%	784
	21st to end	Night	12% 25% 32% 22% 8%	660
March	1st to 10th	Night	9% 22% 37% 25% 7% 1	800
	11th to 20th	Night	11% 24% 33% 23% 8% 2	800
	21st to end	Night	13% 20% 29% 26% 10% 2	880
April	1st to 10th	Night	12% 22% 32% 24% 9% 1	767
	11th to 20th	Night	12% 20% 37% 22% 8% 1	593
	21st to end	Night	13% 17% 33% 26% 10% 1	600
May	1st to 10th	Night	16% 15% 25% 29% 13% 2	596
	11th to 20th	Night	14% 9% 24% 35% 17% 2	394
	21st to end	Night	25% 9% 21% 25% 18% 2	433
June	1st to 10th	Night	17% 14% 26% 29% 14% 1	399
	11th to 20th	Night	19% 16% 29% 25% 10% 1	380
	21st to end	Night	7% 7% 32% 33% 19% 1	380
July	1st to 10th	Night	7% 9% 20% 40% 22% 1	370
	11th to 20th	Night	8% 5% 30% 41% 15% 1	368
	21st to end	Night	7% 6% 22% 43% 20% 2	416
August	1st to 10th	Night	9% 3 21% 41% 24% 2	412
	11th to 20th	Night	6% 5% 24% 37% 24% 3	564
	21st to end	Night	11% 11% 26% 37% 14% 2	627
September	1st to 10th	Night	14% 9% 26% 29% 17% 5%	693
	11th to 20th	Night	11% 11% 24% 31% 17% 5%	760
	21st to end	Night	15% 16% 31% 23% 12% 3	760
October	1st to 10th	Night	17% 17% 25% 28% 12% 1	760
	11th to 20th	Night	12% 20% 28% 29% 9% 1	760
	21st to end	Night	17% 25% 34% 18% 5% 1	836
November	1st to 10th	Night	16% 29% 34% 16% 4	815
	11th to 20th	Night	17% 32% 35% 14% 2 1	1000
	21st to end	Night	14% 29% 34% 17% 5% 1	1000
December	1st to 10th	Night	13% 26% 37% 18% 5% 1	1000
	11th to 20th	Night	11% 29% 39% 17% 4	1000
	21st to end	Night	15% 25% 37% 18% 5%	1100

Page generated at 2025-06-07 08:28:12. Version ac07

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