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 2026-10-14.

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% 8% 3 8% 6% 6% 6% 5% 6% 8% 38%	153979
	Night	3 7% 3 9% 6% 7% 7% 5% 7% 9% 37%	146514
Transparency	Day	34% 7% 19% 23% 14% 4	153692
	Night	34% 8% 20% 23% 12% 3	146384
Seeing	Day	18% 20% 29% 20% 10% 2	17868
	Night	14% 20% 32% 24% 9% 1	26917

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 6% 6% 16% 10% 10% 9% 7% 8% 9% 20%	4125
	11th to 20th	Day	1 5% 3 12% 9% 7% 6% 6% 7% 8% 36%	4518
	21st to end	Day	1 7% 4 11% 8% 9% 6% 5% 6% 8% 34%	5186
February	1st to 10th	Day	2 11% 4 14% 10% 8% 8% 6% 8% 9% 17%	1738
	11th to 20th	Day	2 12% 5% 14% 9% 9% 9% 5% 6% 7% 22%	1845
	21st to end	Day	2 10% 5% 13% 12% 10% 9% 5% 7% 7% 21%	1725
March	1st to 10th	Day	1 7% 5% 12% 8% 9% 8% 5% 6% 8% 32%	4618
	11th to 20th	Day	1 8% 5% 13% 10% 10% 7% 5% 6% 7% 28%	5081
	21st to end	Day	3 18% 8% 16% 9% 9% 6% 4 5% 5% 15%	5644
April	1st to 10th	Day	4 13% 6% 14% 10% 9% 6% 6% 5% 7% 20%	2554
	11th to 20th	Day	5% 15% 7% 13% 7% 7% 6% 5% 6% 7% 23%	2700
	21st to end	Day	7% 16% 5% 11% 7% 7% 6% 4 6% 7% 24%	2730
May	1st to 10th	Day	7% 17% 4 7% 5% 4 4 3 4 5% 39%	7626
	11th to 20th	Day	7% 12% 3 8% 7% 7% 8% 6% 8% 8% 25%	8631
	21st to end	Day	11% 9% 3 7% 6% 7% 6% 5% 8% 8% 30%	9649
June	1st to 10th	Day	13% 10% 3 9% 7% 8% 7% 5% 5% 7% 26%	3150
	11th to 20th	Day	14% 10% 3 9% 7% 8% 7% 6% 6% 7% 24%	3150
	21st to end	Day	8% 6% 2 7% 6% 6% 7% 5% 8% 7% 39%	3150
July	1st to 10th	Day	4 3 1 4 4 4 5% 4 7% 8% 56%	8820
	11th to 20th	Day	5% 3 1 4 4 4 4 4 6% 9% 57%	8820
	21st to end	Day	5% 2 1 3 2 3 3 4 5% 6% 66%	9000
August	1st to 10th	Day	5% 5% 2 5% 4 4 4 4 6% 7% 54%	6353
	11th to 20th	Day	9% 4 1 4 4 4 5% 4 6% 8% 49%	5925
	21st to end	Day	5% 3 1 4 3 4 4 4 6% 9% 58%	6570
September	1st to 10th	Day	10% 8% 2 5% 4 4 5% 4 6% 8% 46%	2552
	11th to 20th	Day	9% 7% 2 7% 5% 5% 6% 4 6% 9% 41%	2310
	21st to end	Day	8% 7% 2 5% 4 6% 5% 3 5% 9% 45%	2310
October	1st to 10th	Day	6% 9% 2 6% 4 4 4 3 5% 6% 50%	3584
	11th to 20th	Day	2 6% 3 8% 6% 6% 6% 5% 8% 11% 39%	3097
	21st to end	Day	4 7% 4 10% 5% 7% 7% 6% 7% 9% 34%	3200
November	1st to 10th	Day	4 11% 4 10% 9% 10% 8% 5% 7% 10% 21%	1796
	11th to 20th	Day	1 9% 4 8% 7% 8% 8% 7% 8% 12% 29%	1680
	21st to end	Day	2 10% 3 13% 8% 8% 7% 6% 6% 10% 26%	1495
December	1st to 10th	Day	1 6% 3 6% 5% 5% 5% 5% 7% 10% 46%	2360
	11th to 20th	Day	1 8% 4 11% 9% 9% 8% 5% 8% 10% 28%	2712
	21st to end	Day	1 8% 5% 11% 7% 9% 9% 6% 8% 9% 28%	3575
January	1st to 10th	Night	1 6% 5% 14% 9% 10% 10% 7% 8% 9% 21%	8768
	11th to 20th	Night	1 5% 3 11% 8% 7% 8% 6% 7% 9% 36%	8426
	21st to end	Night	1 7% 4 10% 8% 11% 8% 6% 7% 8% 31%	8994
February	1st to 10th	Night	2 12% 6% 14% 9% 7% 9% 7% 7% 7% 19%	2882
	11th to 20th	Night	1 10% 5% 12% 10% 9% 9% 7% 8% 7% 22%	2662
	21st to end	Night	2 8% 4 13% 10% 9% 11% 6% 8% 8% 21%	2078
March	1st to 10th	Night	9% 4 14% 9% 9% 7% 6% 6% 7% 29%	5496
	11th to 20th	Night	1 5% 3 9% 7% 8% 8% 5% 8% 11% 34%	5138
	21st to end	Night	3 15% 7% 14% 8% 11% 7% 5% 6% 8% 15%	5571
April	1st to 10th	Night	4 11% 4 8% 7% 7% 7% 5% 8% 10% 29%	2065
	11th to 20th	Night	4 12% 4 9% 6% 7% 6% 5% 8% 9% 30%	1871
	21st to end	Night	6% 12% 3 9% 8% 7% 6% 3 5% 7% 32%	1890
May	1st to 10th	Night	8% 13% 3 7% 4 4 4 4 5% 7% 42%	5305
	11th to 20th	Night	6% 9% 3 9% 6% 7% 7% 5% 6% 8% 34%	4228
	21st to end	Night	10% 8% 2 7% 7% 7% 6% 4 6% 7% 35%	4522
June	1st to 10th	Night	11% 9% 3 8% 7% 8% 4 4 5% 6% 34%	1470
	11th to 20th	Night	14% 7% 3 6% 6% 7% 6% 4 7% 8% 33%	1470
	21st to end	Night	5% 5% 2 6% 5% 4 5% 6% 6% 7% 49%	1470
July	1st to 10th	Night	4 2 1 4 3 3 4 3 5% 7% 64%	4140
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 7% 65%	4140
	21st to end	Night	6% 5% 1 3 2 3 3 2 3 6% 67%	5052
August	1st to 10th	Night	6% 5% 2 5% 2 3 3 3 3 8% 60%	3799
	11th to 20th	Night	9% 5% 1 4 3 4 5% 4 7% 10% 49%	4141
	21st to end	Night	2 3 1 3 2 3 4 3 6% 9% 64%	4578
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 8% 52%	2064
	11th to 20th	Night	6% 7% 2 6% 5% 4 5% 4 7% 9% 45%	2310
	21st to end	Night	6% 8% 2 5% 3 5% 4 4 7% 8% 48%	2310
October	1st to 10th	Night	5% 6% 2 5% 3 4 4 4 6% 8% 52%	3831
	11th to 20th	Night	2 5% 2 5% 4 5% 5% 4 7% 11% 50%	4114
	21st to end	Night	2 6% 3 7% 5% 8% 7% 5% 7% 9% 41%	4600
November	1st to 10th	Night	4 11% 4 10% 8% 10% 7% 6% 7% 9% 24%	2702
	11th to 20th	Night	1 7% 3 10% 8% 10% 9% 8% 7% 10% 26%	2940
	21st to end	Night	2 10% 4 10% 7% 8% 7% 6% 9% 10% 26%	3124
December	1st to 10th	Night	6% 4 9% 6% 7% 7% 6% 8% 11% 35%	5040
	11th to 20th	Night	1 9% 4 12% 9% 9% 9% 7% 9% 9% 23%	5700
	21st to end	Night	1 9% 6% 12% 9% 9% 9% 6% 8% 9% 24%	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	38% 7% 27% 17% 10% 1	4125
	11th to 20th	Day	40% 8% 20% 16% 10% 5%	4542
	21st to end	Day	35% 10% 21% 21% 12%	5187
February	1st to 10th	Day	47% 14% 19% 11% 6% 2	1738
	11th to 20th	Day	45% 11% 24% 13% 7% 1	1854
	21st to end	Day	42% 13% 21% 16% 6% 1	1725
March	1st to 10th	Day	29% 8% 25% 20% 15% 3	4618
	11th to 20th	Day	30% 10% 25% 23% 11% 1	5082
	21st to end	Day	52% 8% 21% 12% 6%	5644
April	1st to 10th	Day	45% 9% 26% 16% 3	2554
	11th to 20th	Day	48% 8% 20% 18% 4 1	2700
	21st to end	Day	48% 9% 20% 18% 6% 1	2730
May	1st to 10th	Day	42% 4 14% 23% 14% 2	7626
	11th to 20th	Day	41% 6% 16% 24% 12% 1	8635
	21st to end	Day	37% 7% 25% 23% 9%	9650
June	1st to 10th	Day	46% 9% 18% 19% 8%	3150
	11th to 20th	Day	47% 9% 17% 18% 9% 2	3150
	21st to end	Day	33% 8% 21% 26% 12% 2	3150
July	1st to 10th	Day	23% 4 10% 30% 24% 9%	8746
	11th to 20th	Day	23% 5% 12% 24% 22% 14%	8739
	21st to end	Day	19% 2 10% 24% 26% 19%	8805
August	1st to 10th	Day	27% 4 16% 21% 22% 10%	6336
	11th to 20th	Day	30% 4 20% 31% 14% 2	5964
	21st to end	Day	20% 3 12% 43% 19% 2	6569
September	1st to 10th	Day	32% 4 16% 26% 16% 7%	2552
	11th to 20th	Day	32% 7% 17% 27% 15% 3	2310
	21st to end	Day	29% 7% 22% 26% 13% 3	2310
October	1st to 10th	Day	30% 5% 17% 31% 16% 1	3584
	11th to 20th	Day	27% 6% 24% 23% 18% 1	3097
	21st to end	Day	32% 7% 20% 27% 12% 2	3200
November	1st to 10th	Day	40% 14% 26% 14% 5% 1	1796
	11th to 20th	Day	32% 16% 29% 15% 7% 1	1680
	21st to end	Day	41% 13% 25% 12% 8% 1	1495
December	1st to 10th	Day	26% 12% 29% 23% 8% 2	2360
	11th to 20th	Day	36% 13% 26% 18% 6% 1	2714
	21st to end	Day	36% 10% 19% 20% 13% 2	3575
January	1st to 10th	Night	38% 9% 23% 20% 9% 1	8769
	11th to 20th	Night	37% 7% 21% 20% 9% 5%	8464
	21st to end	Night	35% 13% 22% 20% 10%	8994
February	1st to 10th	Night	48% 12% 17% 12% 9% 2	2882
	11th to 20th	Night	42% 14% 23% 13% 7% 1	2663
	21st to end	Night	42% 12% 23% 17% 5% 2	2078
March	1st to 10th	Night	34% 10% 23% 19% 10% 4	5496
	11th to 20th	Night	25% 10% 27% 24% 14%	5138
	21st to end	Night	50% 8% 24% 12% 5%	5571
April	1st to 10th	Night	37% 9% 25% 22% 6%	2065
	11th to 20th	Night	40% 10% 21% 23% 7%	1871
	21st to end	Night	44% 8% 17% 22% 8% 1	1890
May	1st to 10th	Night	40% 5% 15% 26% 12% 2	5302
	11th to 20th	Night	38% 5% 18% 26% 13% 1	4228
	21st to end	Night	37% 6% 21% 27% 9%	4526
June	1st to 10th	Night	40% 8% 21% 22% 9%	1470
	11th to 20th	Night	40% 7% 18% 20% 12% 1	1470
	21st to end	Night	26% 7% 23% 28% 13% 2	1470
July	1st to 10th	Night	18% 3 15% 31% 23% 12%	4105
	11th to 20th	Night	19% 3 11% 29% 24% 14%	4098
	21st to end	Night	23% 2 9% 22% 24% 19%	4942
August	1st to 10th	Night	25% 3 12% 22% 23% 16%	3791
	11th to 20th	Night	30% 4 21% 32% 11% 2	4168
	21st to end	Night	17% 3 11% 43% 23% 3	4578
September	1st to 10th	Night	28% 5% 17% 28% 16% 6%	2064
	11th to 20th	Night	31% 6% 18% 28% 14% 4	2310
	21st to end	Night	27% 6% 22% 25% 16% 3	2310
October	1st to 10th	Night	23% 6% 15% 33% 21% 3	3831
	11th to 20th	Night	22% 6% 26% 26% 18% 2	4114
	21st to end	Night	28% 8% 18% 33% 13% 1	4600
November	1st to 10th	Night	42% 13% 24% 14% 7% 2	2702
	11th to 20th	Night	33% 16% 29% 16% 7%	2940
	21st to end	Night	37% 15% 26% 13% 7% 2	3124
December	1st to 10th	Night	30% 11% 28% 20% 10% 1	5040
	11th to 20th	Night	42% 12% 26% 14% 5% 1	5697
	21st to end	Night	40% 9% 21% 17% 11% 3	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	19% 29% 29% 14% 6% 4	220
	11th to 20th	Day	16% 30% 29% 21% 3 2	220
	21st to end	Day	24% 29% 30% 12% 4 2	242
February	1st to 10th	Day	21% 24% 35% 14% 5% 1	250
	11th to 20th	Day	12% 27% 38% 16% 7%	330
	21st to end	Day	23% 29% 30% 13% 4	441
March	1st to 10th	Day	19% 28% 33% 15% 4 2	540
	11th to 20th	Day	20% 24% 29% 16% 9% 2	540
	21st to end	Day	20% 27% 32% 15% 6% 2	594
April	1st to 10th	Day	20% 25% 34% 14% 6%	540
	11th to 20th	Day	17% 31% 27% 17% 7% 1	532
	21st to end	Day	20% 21% 29% 19% 8% 3	540
May	1st to 10th	Day	18% 21% 27% 21% 10% 3	543
	11th to 20th	Day	25% 15% 25% 18% 13% 4	727
	21st to end	Day	24% 12% 22% 21% 16% 4	844
June	1st to 10th	Day	20% 22% 24% 22% 11% 2	780
	11th to 20th	Day	23% 20% 26% 18% 10% 2	780
	21st to end	Day	14% 14% 28% 26% 14% 4	780
July	1st to 10th	Day	7% 14% 30% 28% 16% 4	771
	11th to 20th	Day	10% 11% 32% 30% 14% 3	768
	21st to end	Day	7% 13% 33% 29% 13% 4	652
August	1st to 10th	Day	12% 12% 26% 29% 16% 5%	567
	11th to 20th	Day	9% 9% 33% 29% 16% 3	557
	21st to end	Day	14% 14% 32% 27% 10% 3	627
September	1st to 10th	Day	18% 19% 25% 22% 12% 4	570
	11th to 20th	Day	14% 16% 31% 25% 11% 3	570
	21st to end	Day	23% 24% 27% 19% 7% 1	570
October	1st to 10th	Day	18% 21% 31% 17% 12% 1	496
	11th to 20th	Day	21% 21% 29% 18% 9% 3	339
	21st to end	Day	25% 26% 28% 12% 7% 1	363
November	1st to 10th	Day	24% 28% 29% 15% 4	330
	11th to 20th	Day	27% 30% 29% 12% 2	330
	21st to end	Day	22% 37% 19% 12% 7% 3	233
December	1st to 10th	Day	22% 34% 26% 13% 5% 1	220
	11th to 20th	Day	28% 25% 28% 17% 2	220
	21st to end	Day	19% 26% 28% 16% 8% 2	242
January	1st to 10th	Night	17% 31% 32% 15% 4 1	1050
	11th to 20th	Night	14% 28% 36% 16% 4 1	928
	21st to end	Night	13% 32% 36% 14% 4 2	924
February	1st to 10th	Night	14% 21% 35% 21% 7% 1	840
	11th to 20th	Night	10% 24% 38% 21% 6%	824
	21st to end	Night	14% 25% 31% 22% 8%	692
March	1st to 10th	Night	10% 21% 37% 24% 6% 1	840
	11th to 20th	Night	13% 24% 32% 22% 7% 1	840
	21st to end	Night	12% 21% 30% 25% 10% 2	924
April	1st to 10th	Night	12% 21% 32% 24% 10% 1	805
	11th to 20th	Night	12% 19% 38% 22% 8% 1	623
	21st to end	Night	13% 16% 33% 27% 10% 1	630
May	1st to 10th	Night	15% 15% 27% 29% 12% 1	626
	11th to 20th	Night	14% 9% 24% 34% 16% 1	414
	21st to end	Night	24% 9% 22% 25% 18% 2	455
June	1st to 10th	Night	19% 13% 27% 28% 13% 1	420
	11th to 20th	Night	18% 15% 30% 26% 10% 1	420
	21st to end	Night	8% 7% 32% 34% 18% 1	420
July	1st to 10th	Night	7% 9% 22% 40% 21% 1	410
	11th to 20th	Night	8% 5% 31% 40% 14% 1	408
	21st to end	Night	9% 6% 23% 41% 18% 2	460
August	1st to 10th	Night	9% 3 22% 41% 23% 2	456
	11th to 20th	Night	7% 5% 25% 38% 23% 3	624
	21st to end	Night	10% 11% 25% 38% 14% 2	693
September	1st to 10th	Night	15% 8% 25% 30% 18% 4	767
	11th to 20th	Night	12% 10% 24% 33% 16% 5%	840
	21st to end	Night	15% 15% 32% 23% 12% 3	840
October	1st to 10th	Night	16% 18% 27% 27% 12% 1	840
	11th to 20th	Night	13% 20% 29% 28% 9% 1	812
	21st to end	Night	18% 25% 34% 17% 5% 1	880
November	1st to 10th	Night	18% 29% 34% 16% 4	857
	11th to 20th	Night	17% 32% 34% 14% 2 1	1050
	21st to end	Night	15% 28% 35% 17% 5% 1	1050
December	1st to 10th	Night	15% 26% 36% 18% 5% 1	1050
	11th to 20th	Night	13% 29% 38% 16% 4	1050
	21st to end	Night	15% 26% 37% 17% 5%	1155

Page generated at 2026-10-11 03:06:28. Version ac07

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