Three Hills
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Three Hills clear sky chart from 2005-11-06 to 2026-07-01.

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	9% 9% 3 8% 5% 5% 5% 4 6% 8% 37%	142185
	Night	7% 9% 4 10% 6% 5% 5% 4 6% 8% 34%	138867
Transparency	Day	42% 6% 16% 25% 10% 1	141883
	Night	45% 6% 15% 22% 9% 1	138753
Seeing	Day	12% 7% 25% 32% 19% 6%	17562
	Night	11% 6% 28% 37% 16% 2	26628

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 12% 6% 16% 8% 6% 5% 3 5% 7% 27%	4125
	11th to 20th	Day	3 8% 4 11% 7% 6% 6% 5% 7% 9% 35%	4119
	21st to end	Day	4 10% 4 11% 5% 4 4 3 6% 9% 40%	5084
February	1st to 10th	Day	7% 13% 4 12% 7% 6% 6% 4 6% 8% 28%	1682
	11th to 20th	Day	6% 14% 4 12% 6% 6% 6% 5% 7% 8% 25%	1845
	21st to end	Day	4 13% 5% 11% 9% 7% 7% 5% 6% 9% 26%	1637
March	1st to 10th	Day	3 11% 4 11% 8% 6% 6% 4 7% 8% 32%	4618
	11th to 20th	Day	5% 11% 4 10% 6% 5% 4 4 6% 8% 37%	5104
	21st to end	Day	9% 20% 7% 14% 6% 5% 4 4 5% 6% 21%	5623
April	1st to 10th	Day	9% 13% 3 8% 6% 5% 5% 5% 7% 8% 31%	2613
	11th to 20th	Day	11% 12% 4 8% 6% 5% 5% 4 6% 8% 29%	2700
	21st to end	Day	12% 10% 4 10% 5% 5% 4 5% 6% 8% 33%	2730
May	1st to 10th	Day	13% 8% 3 5% 4 4 5% 4 6% 8% 40%	8182
	11th to 20th	Day	13% 10% 3 9% 5% 6% 5% 5% 7% 8% 28%	8755
	21st to end	Day	12% 6% 3 7% 5% 5% 6% 5% 6% 8% 37%	9649
June	1st to 10th	Day	16% 6% 2 6% 5% 5% 6% 5% 7% 9% 33%	3150
	11th to 20th	Day	17% 9% 3 8% 7% 5% 5% 5% 6% 8% 27%	3150
	21st to end	Day	10% 4 2 6% 5% 5% 6% 5% 7% 10% 40%	3150
July	1st to 10th	Day	7% 5% 2 5% 4 4 4 4 6% 9% 52%	6781
	11th to 20th	Day	11% 4 2 5% 4 5% 5% 4 7% 9% 44%	6780
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 8% 48%	7376
August	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 5% 7% 9% 42%	4541
	11th to 20th	Day	7% 4 2 6% 4 5% 6% 5% 8% 11% 42%	4188
	21st to end	Day	11% 4 2 4 3 3 3 4 6% 8% 51%	4664
September	1st to 10th	Day	10% 5% 2 6% 4 3 4 4 6% 7% 48%	2505
	11th to 20th	Day	15% 7% 2 5% 3 3 4 3 5% 7% 45%	2200
	21st to end	Day	8% 8% 3 6% 4 4 5% 4 6% 9% 46%	2200
October	1st to 10th	Day	7% 7% 2 7% 5% 5% 5% 4 6% 9% 43%	3407
	11th to 20th	Day	6% 7% 3 8% 5% 5% 5% 4 7% 9% 42%	3038
	21st to end	Day	8% 9% 3 7% 5% 5% 5% 4 5% 6% 42%	3200
November	1st to 10th	Day	10% 17% 5% 10% 6% 4 4 4 6% 7% 26%	1667
	11th to 20th	Day	7% 13% 4 12% 5% 5% 4 4 5% 7% 33%	1605
	21st to end	Day	7% 12% 4 12% 7% 7% 4 4 5% 8% 30%	1470
December	1st to 10th	Day	9% 13% 5% 10% 6% 7% 5% 5% 6% 7% 27%	2360
	11th to 20th	Day	6% 11% 5% 13% 9% 7% 5% 4 5% 7% 26%	2712
	21st to end	Day	3 8% 4 10% 6% 5% 5% 4 5% 8% 42%	3575
January	1st to 10th	Night	5% 13% 6% 15% 8% 7% 5% 4 6% 7% 24%	8768
	11th to 20th	Night	3 8% 4 11% 6% 5% 6% 4 6% 8% 37%	8779
	21st to end	Night	3 9% 4 11% 6% 6% 6% 5% 6% 8% 37%	9139
February	1st to 10th	Night	5% 12% 4 13% 7% 6% 6% 4 6% 8% 28%	2937
	11th to 20th	Night	5% 13% 6% 12% 7% 6% 5% 5% 8% 7% 27%	2662
	21st to end	Night	4 10% 4 11% 8% 5% 6% 5% 8% 10% 30%	2165
March	1st to 10th	Night	3 8% 3 13% 8% 6% 6% 5% 7% 9% 31%	5496
	11th to 20th	Night	5% 9% 4 11% 6% 5% 6% 6% 9% 9% 30%	5115
	21st to end	Night	8% 19% 6% 15% 6% 6% 5% 4 6% 6% 19%	5578
April	1st to 10th	Night	10% 10% 4 9% 6% 6% 5% 4 6% 8% 31%	2006
	11th to 20th	Night	8% 11% 4 10% 7% 7% 5% 4 7% 7% 29%	1871
	21st to end	Night	10% 12% 3 9% 5% 5% 6% 4 7% 7% 32%	1890
May	1st to 10th	Night	13% 9% 3 8% 5% 4 5% 4 5% 7% 37%	4738
	11th to 20th	Night	12% 9% 3 9% 7% 6% 6% 5% 7% 9% 28%	4105
	21st to end	Night	12% 7% 3 9% 6% 6% 5% 5% 6% 8% 34%	4522
June	1st to 10th	Night	15% 8% 3 9% 7% 6% 4 4 7% 8% 29%	1470
	11th to 20th	Night	16% 8% 3 9% 8% 6% 7% 5% 6% 7% 24%	1470
	21st to end	Night	12% 5% 3 7% 5% 5% 5% 4 7% 8% 39%	1470
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 4 7% 9% 44%	3180
	11th to 20th	Night	9% 4 2 7% 5% 3 6% 5% 6% 10% 42%	3180
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 10% 45%	3467
August	1st to 10th	Night	10% 6% 2 7% 4 3 5% 4 6% 9% 44%	2606
	11th to 20th	Night	6% 4 2 7% 5% 5% 5% 4 8% 11% 41%	2868
	21st to end	Night	8% 5% 2 5% 4 4 4 4 7% 10% 48%	3245
September	1st to 10th	Night	10% 5% 2 5% 3 4 4 4 6% 8% 49%	1891
	11th to 20th	Night	13% 6% 2 5% 4 4 4 4 5% 8% 44%	2200
	21st to end	Night	9% 6% 2 6% 4 4 4 4 7% 8% 48%	2200
October	1st to 10th	Night	8% 6% 2 5% 4 5% 5% 5% 6% 8% 46%	3787
	11th to 20th	Night	7% 8% 3 7% 4 5% 5% 5% 5% 9% 43%	4105
	21st to end	Night	7% 8% 3 8% 5% 5% 6% 4 6% 7% 40%	4600
November	1st to 10th	Night	10% 16% 4 8% 6% 4 5% 4 6% 8% 30%	2830
	11th to 20th	Night	5% 10% 5% 11% 7% 6% 6% 4 7% 8% 32%	3014
	21st to end	Night	5% 11% 4 10% 7% 6% 5% 5% 7% 8% 31%	3150
December	1st to 10th	Night	7% 13% 5% 12% 7% 6% 6% 4 6% 8% 26%	5040
	11th to 20th	Night	6% 13% 5% 13% 8% 6% 7% 4 7% 8% 22%	5700
	21st to end	Night	3 8% 3 9% 6% 6% 5% 4 6% 8% 42%	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	56% 5% 10% 19% 10% 1	4125
	11th to 20th	Day	44% 5% 15% 16% 17% 3	4140
	21st to end	Day	40% 5% 16% 29% 9%	5085
February	1st to 10th	Day	54% 6% 13% 15% 10% 1	1682
	11th to 20th	Day	53% 7% 13% 17% 9% 1	1854
	21st to end	Day	54% 7% 11% 19% 10%	1637
March	1st to 10th	Day	48% 5% 16% 20% 11%	4618
	11th to 20th	Day	44% 7% 15% 21% 13%	5105
	21st to end	Day	63% 6% 10% 15% 6%	5623
April	1st to 10th	Day	47% 8% 17% 17% 9% 1	2613
	11th to 20th	Day	49% 7% 14% 21% 8% 1	2700
	21st to end	Day	49% 7% 13% 22% 7% 1	2730
May	1st to 10th	Day	41% 5% 11% 29% 13% 2	8182
	11th to 20th	Day	48% 7% 13% 24% 8%	8759
	21st to end	Day	42% 6% 17% 26% 8%	9650
June	1st to 10th	Day	44% 8% 16% 21% 10% 1	3150
	11th to 20th	Day	52% 9% 13% 19% 5% 1	3150
	21st to end	Day	36% 9% 20% 27% 7% 1	3150
July	1st to 10th	Day	28% 6% 19% 32% 14% 1	6707
	11th to 20th	Day	34% 7% 20% 30% 9%	6699
	21st to end	Day	32% 6% 17% 27% 14% 3	7169
August	1st to 10th	Day	37% 9% 17% 30% 7% 1	4524
	11th to 20th	Day	32% 8% 25% 28% 6% 1	4227
	21st to end	Day	30% 6% 22% 30% 11% 1	4663
September	1st to 10th	Day	34% 7% 16% 28% 11% 4	2505
	11th to 20th	Day	38% 6% 15% 30% 10% 1	2200
	21st to end	Day	35% 7% 17% 27% 11% 3	2200
October	1st to 10th	Day	38% 6% 20% 28% 7% 1	3407
	11th to 20th	Day	37% 6% 19% 26% 11% 2	3038
	21st to end	Day	41% 6% 10% 34% 10% 1	3200
November	1st to 10th	Day	55% 7% 13% 16% 7% 2	1667
	11th to 20th	Day	49% 7% 14% 21% 8% 1	1605
	21st to end	Day	51% 7% 16% 18% 7% 1	1470
December	1st to 10th	Day	54% 6% 17% 17% 5% 1	2360
	11th to 20th	Day	56% 6% 16% 18% 3 1	2714
	21st to end	Day	38% 5% 12% 26% 15% 3	3575
January	1st to 10th	Night	58% 5% 11% 15% 8% 1	8769
	11th to 20th	Night	44% 4 16% 20% 13% 3	8820
	21st to end	Night	44% 6% 18% 24% 8%	9139
February	1st to 10th	Night	52% 6% 12% 19% 11% 1	2937
	11th to 20th	Night	52% 6% 12% 17% 11% 1	2663
	21st to end	Night	46% 7% 15% 20% 11% 1	2165
March	1st to 10th	Night	47% 6% 18% 19% 9%	5496
	11th to 20th	Night	44% 7% 19% 22% 7% 1	5115
	21st to end	Night	63% 7% 10% 13% 6% 1	5578
April	1st to 10th	Night	48% 8% 16% 16% 12%	2006
	11th to 20th	Night	51% 6% 14% 21% 7% 1	1871
	21st to end	Night	48% 7% 12% 23% 8% 2	1890
May	1st to 10th	Night	46% 5% 9% 26% 13% 2	4735
	11th to 20th	Night	48% 7% 13% 23% 8%	4105
	21st to end	Night	46% 7% 15% 24% 6% 1	4526
June	1st to 10th	Night	49% 9% 13% 18% 10% 1	1470
	11th to 20th	Night	56% 8% 12% 17% 6% 2	1470
	21st to end	Night	41% 8% 17% 26% 7% 1	1470
July	1st to 10th	Night	34% 9% 19% 24% 14% 1	3145
	11th to 20th	Night	35% 7% 18% 31% 5% 3	3138
	21st to end	Night	32% 7% 19% 26% 13% 2	3369
August	1st to 10th	Night	35% 8% 17% 31% 9%	2599
	11th to 20th	Night	34% 8% 21% 31% 5% 1	2895
	21st to end	Night	31% 6% 21% 31% 9% 2	3245
September	1st to 10th	Night	32% 7% 16% 26% 14% 6%	1891
	11th to 20th	Night	37% 7% 14% 27% 11% 4	2200
	21st to end	Night	33% 5% 17% 28% 15% 2	2200
October	1st to 10th	Night	35% 5% 19% 27% 12% 1	3787
	11th to 20th	Night	38% 5% 17% 28% 11% 2	4105
	21st to end	Night	41% 6% 14% 30% 9% 1	4600
November	1st to 10th	Night	50% 6% 11% 22% 9% 1	2830
	11th to 20th	Night	47% 7% 16% 18% 10% 1	3014
	21st to end	Night	48% 9% 16% 18% 7% 2	3150
December	1st to 10th	Night	55% 6% 17% 16% 5% 1	5040
	11th to 20th	Night	57% 8% 17% 14% 5%	5697
	21st to end	Night	39% 5% 12% 25% 16% 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	7% 6% 25% 35% 20% 7%	220
	11th to 20th	Day	9% 7% 35% 30% 18% 1	220
	21st to end	Day	5% 11% 28% 32% 19% 6%	242
February	1st to 10th	Day	9% 12% 22% 37% 14% 6%	222
	11th to 20th	Day	10% 6% 26% 35% 20% 3	330
	21st to end	Day	7% 7% 26% 36% 19% 5%	353
March	1st to 10th	Day	8% 6% 30% 33% 18% 5%	540
	11th to 20th	Day	7% 4 28% 36% 18% 6%	540
	21st to end	Day	12% 7% 29% 31% 15% 5%	594
April	1st to 10th	Day	11% 13% 27% 29% 16% 5%	540
	11th to 20th	Day	14% 11% 31% 27% 14% 4	532
	21st to end	Day	17% 9% 25% 32% 12% 4	540
May	1st to 10th	Day	15% 12% 25% 27% 17% 5%	649
	11th to 20th	Day	13% 8% 22% 33% 20% 4	775
	21st to end	Day	15% 6% 22% 29% 22% 7%	844
June	1st to 10th	Day	16% 9% 25% 34% 13% 4	780
	11th to 20th	Day	17% 9% 26% 30% 15% 4	780
	21st to end	Day	11% 5% 23% 30% 24% 7%	780
July	1st to 10th	Day	9% 4 19% 35% 25% 8%	722
	11th to 20th	Day	11% 4 19% 35% 24% 7%	718
	21st to end	Day	8% 3 19% 33% 26% 10%	794
August	1st to 10th	Day	10% 5% 24% 31% 21% 10%	527
	11th to 20th	Day	7% 5% 21% 33% 28% 6%	517
	21st to end	Day	9% 8% 20% 29% 24% 9%	583
September	1st to 10th	Day	13% 6% 26% 30% 20% 6%	530
	11th to 20th	Day	17% 5% 21% 29% 22% 5%	530
	21st to end	Day	12% 12% 30% 28% 14% 4	530
October	1st to 10th	Day	17% 6% 22% 33% 16% 6%	415
	11th to 20th	Day	10% 10% 35% 27% 15% 4	330
	21st to end	Day	13% 15% 32% 27% 12% 1	363
November	1st to 10th	Day	19% 6% 27% 33% 12% 3	330
	11th to 20th	Day	11% 7% 32% 33% 13% 2	290
	21st to end	Day	9% 5% 30% 35% 16% 6%	220
December	1st to 10th	Day	10% 4 30% 34% 19% 4	220
	11th to 20th	Day	8% 8% 33% 33% 14% 3	220
	21st to end	Day	10% 2 21% 38% 23% 6%	242
January	1st to 10th	Night	11% 6% 32% 35% 14% 2	1050
	11th to 20th	Night	12% 5% 33% 36% 13% 1	1050
	21st to end	Night	11% 7% 30% 38% 12% 2	978
February	1st to 10th	Night	10% 7% 27% 38% 17% 2	840
	11th to 20th	Night	10% 5% 32% 37% 15% 1	824
	21st to end	Night	7% 4 25% 40% 21% 4	692
March	1st to 10th	Night	8% 4 29% 40% 16% 3	840
	11th to 20th	Night	9% 5% 30% 40% 14% 3	840
	21st to end	Night	14% 6% 27% 35% 16% 2	924
April	1st to 10th	Night	14% 7% 28% 33% 16% 2	746
	11th to 20th	Night	15% 7% 32% 35% 10% 1	623
	21st to end	Night	14% 8% 28% 35% 13%	630
May	1st to 10th	Night	16% 8% 26% 37% 12% 1	539
	11th to 20th	Night	11% 7% 29% 37% 16% 1	414
	21st to end	Night	17% 7% 24% 31% 19% 2	455
June	1st to 10th	Night	17% 7% 28% 33% 13% 2	420
	11th to 20th	Night	16% 5% 31% 31% 16%	420
	21st to end	Night	14% 3 21% 37% 23% 2	420
July	1st to 10th	Night	10% 2 16% 47% 23% 1	390
	11th to 20th	Night	12% 4 19% 44% 21% 2	388
	21st to end	Night	6% 4 20% 40% 28% 2	438
August	1st to 10th	Night	11% 3 18% 39% 27% 3	398
	11th to 20th	Night	7% 5% 19% 43% 24% 2	573
	21st to end	Night	8% 5% 20% 45% 21%	660
September	1st to 10th	Night	12% 3 23% 41% 19% 2	656
	11th to 20th	Night	14% 5% 20% 40% 21% 1	800
	21st to end	Night	10% 4 30% 38% 15% 2	800
October	1st to 10th	Night	15% 5% 26% 36% 17% 1	800
	11th to 20th	Night	11% 6% 31% 37% 14% 2	800
	21st to end	Night	11% 8% 32% 36% 12% 1	880
November	1st to 10th	Night	15% 9% 30% 36% 9% 1	985
	11th to 20th	Night	8% 7% 38% 33% 11% 2	1050
	21st to end	Night	9% 4 33% 39% 12% 2	1050
December	1st to 10th	Night	12% 7% 31% 34% 15% 2	1050
	11th to 20th	Night	9% 5% 35% 37% 12% 2	1050
	21st to end	Night	10% 4 29% 38% 15% 2	1155

Page generated at 2026-06-28 00:10:43. Version ac07

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