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 2024-09-29.

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%	101611
	Night	8% 10% 4 10% 6% 5% 5% 4 6% 8% 34%	96701
Transparency	Day	43% 8% 16% 23% 9% 1	101353
	Night	45% 8% 15% 22% 9% 2	96606
Seeing	Day	12% 7% 25% 32% 19% 6%	15316
	Night	11% 5% 28% 37% 16% 2	24303

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	5% 12% 5% 13% 7% 5% 6% 4 6% 8% 30%	2219
	11th to 20th	Day	4 10% 4 11% 6% 5% 6% 6% 9% 9% 29%	2199
	21st to end	Day	5% 9% 3 11% 6% 5% 5% 4 6% 9% 36%	2734
February	1st to 10th	Day	8% 14% 3 11% 7% 6% 6% 4 6% 7% 28%	1521
	11th to 20th	Day	6% 14% 5% 12% 6% 6% 6% 5% 6% 8% 25%	1665
	21st to end	Day	4 13% 6% 12% 9% 7% 6% 4 6% 8% 25%	1485
March	1st to 10th	Day	4 14% 5% 13% 9% 7% 7% 4 6% 6% 25%	3138
	11th to 20th	Day	7% 9% 2 8% 5% 5% 4 4 6% 9% 41%	3484
	21st to end	Day	8% 21% 7% 14% 6% 4 4 4 4 5% 23%	3841
April	1st to 10th	Day	9% 13% 4 9% 6% 5% 5% 5% 7% 8% 31%	2364
	11th to 20th	Day	12% 12% 4 8% 6% 5% 5% 4 6% 8% 29%	2440
	21st to end	Day	12% 10% 4 10% 5% 5% 5% 5% 6% 8% 32%	2470
May	1st to 10th	Day	20% 12% 4 7% 5% 4 5% 4 5% 6% 28%	4425
	11th to 20th	Day	11% 8% 3 9% 5% 5% 6% 5% 6% 8% 33%	4694
	21st to end	Day	16% 8% 4 9% 6% 6% 6% 5% 6% 8% 27%	5161
June	1st to 10th	Day	16% 6% 2 6% 5% 5% 6% 5% 8% 10% 32%	2850
	11th to 20th	Day	18% 9% 3 8% 6% 5% 5% 5% 6% 8% 27%	2850
	21st to end	Day	9% 4 2 6% 5% 5% 6% 6% 8% 10% 40%	2850
July	1st to 10th	Day	7% 4 2 5% 4 4 4 4 7% 9% 51%	4740
	11th to 20th	Day	8% 4 1 5% 4 4 5% 4 7% 9% 50%	4740
	21st to end	Day	5% 4 1 4 4 4 4 4 6% 9% 55%	5138
August	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 5% 7% 8% 42%	4401
	11th to 20th	Day	7% 4 2 6% 4 5% 6% 5% 8% 11% 42%	4057
	21st to end	Day	11% 4 2 4 3 4 3 4 6% 8% 50%	4521
September	1st to 10th	Day	11% 5% 2 6% 4 3 4 3 6% 7% 47%	2380
	11th to 20th	Day	16% 7% 2 6% 3 3 4 3 5% 7% 43%	2090
	21st to end	Day	8% 8% 3 6% 4 4 5% 4 6% 9% 45%	2069
October	1st to 10th	Day	13% 11% 3 7% 4 3 4 3 5% 7% 40%	1877
	11th to 20th	Day	9% 8% 4 8% 5% 5% 5% 4 6% 7% 38%	1684
	21st to end	Day	7% 12% 3 8% 5% 6% 5% 5% 6% 7% 35%	1782
November	1st to 10th	Day	10% 17% 5% 10% 6% 4 4 4 6% 8% 25%	1505
	11th to 20th	Day	8% 13% 4 12% 5% 5% 4 4 5% 7% 33%	1453
	21st to end	Day	6% 12% 4 11% 6% 6% 4 4 5% 9% 32%	1330
December	1st to 10th	Day	7% 13% 5% 12% 7% 9% 5% 5% 5% 6% 27%	1330
	11th to 20th	Day	4 11% 6% 13% 8% 6% 5% 4 6% 8% 27%	1682
	21st to end	Day	4 9% 3 9% 6% 5% 5% 4 5% 8% 43%	2442
January	1st to 10th	Night	5% 13% 5% 12% 8% 6% 6% 5% 7% 8% 25%	4726
	11th to 20th	Night	4 9% 4 14% 8% 5% 6% 5% 7% 7% 31%	4699
	21st to end	Night	4 11% 5% 11% 6% 5% 5% 4 6% 8% 34%	4951
February	1st to 10th	Night	6% 13% 5% 13% 7% 6% 6% 4 6% 8% 28%	2658
	11th to 20th	Night	5% 14% 6% 12% 7% 6% 5% 5% 7% 7% 27%	2402
	21st to end	Night	4 10% 4 11% 7% 5% 5% 5% 8% 10% 30%	1965
March	1st to 10th	Night	3 9% 4 16% 10% 7% 7% 5% 6% 6% 26%	3736
	11th to 20th	Night	7% 8% 3 8% 6% 5% 6% 5% 8% 8% 37%	3495
	21st to end	Night	7% 20% 7% 16% 6% 6% 5% 5% 6% 6% 17%	3809
April	1st to 10th	Night	9% 11% 4 10% 6% 6% 6% 4 6% 8% 30%	1815
	11th to 20th	Night	9% 12% 4 10% 7% 6% 5% 4 7% 8% 28%	1691
	21st to end	Night	10% 12% 3 9% 5% 5% 6% 4 7% 7% 32%	1710
May	1st to 10th	Night	21% 12% 3 8% 5% 4 4 4 5% 5% 28%	2538
	11th to 20th	Night	10% 7% 2 8% 7% 6% 6% 5% 7% 9% 33%	2199
	21st to end	Night	18% 10% 3 11% 7% 6% 5% 4 5% 6% 23%	2414
June	1st to 10th	Night	14% 9% 3 8% 7% 6% 4 5% 7% 8% 30%	1330
	11th to 20th	Night	17% 9% 3 8% 8% 6% 7% 5% 6% 7% 25%	1330
	21st to end	Night	11% 5% 3 7% 5% 5% 4 4 6% 8% 41%	1330
July	1st to 10th	Night	8% 5% 2 7% 6% 5% 5% 4 7% 9% 44%	2220
	11th to 20th	Night	6% 3 2 6% 5% 3 6% 5% 7% 11% 46%	2220
	21st to end	Night	5% 5% 2 6% 4 4 5% 4 6% 10% 51%	2414
August	1st to 10th	Night	10% 6% 2 7% 4 3 5% 4 6% 9% 44%	2526
	11th to 20th	Night	6% 4 2 7% 5% 5% 5% 4 8% 11% 41%	2779
	21st to end	Night	8% 5% 2 5% 4 4 4 4 7% 10% 47%	3146
September	1st to 10th	Night	10% 5% 2 6% 3 4 4 4 6% 7% 48%	1796
	11th to 20th	Night	14% 7% 2 5% 4 4 4 4 5% 8% 43%	2090
	21st to end	Night	9% 6% 2 6% 4 4 4 4 7% 8% 47%	2079
October	1st to 10th	Night	12% 10% 3 5% 5% 4 4 5% 5% 7% 41%	2083
	11th to 20th	Night	9% 8% 3 9% 6% 5% 5% 4 5% 9% 38%	2272
	21st to end	Night	7% 9% 4 10% 6% 4 5% 4 7% 8% 35%	2574
November	1st to 10th	Night	10% 16% 4 7% 6% 4 4 3 6% 8% 31%	2554
	11th to 20th	Night	5% 10% 4 11% 6% 6% 6% 4 7% 8% 32%	2726
	21st to end	Night	5% 11% 4 10% 7% 6% 5% 4 7% 9% 33%	2850
December	1st to 10th	Night	7% 12% 4 12% 7% 6% 5% 3 5% 8% 31%	2850
	11th to 20th	Night	3 10% 5% 12% 8% 7% 7% 4 7% 8% 29%	3510
	21st to end	Night	4 8% 3 8% 5% 5% 5% 4 6% 8% 44%	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	49% 8% 14% 20% 9% 1	2219
	11th to 20th	Day	47% 7% 14% 13% 16% 2	2220
	21st to end	Day	42% 8% 16% 26% 7%	2734
February	1st to 10th	Day	54% 6% 14% 16% 9% 1	1521
	11th to 20th	Day	54% 8% 14% 15% 8% 1	1674
	21st to end	Day	57% 7% 10% 17% 10%	1485
March	1st to 10th	Day	57% 6% 15% 13% 9%	3138
	11th to 20th	Day	38% 8% 19% 22% 12%	3485
	21st to end	Day	62% 7% 11% 15% 5%	3841
April	1st to 10th	Day	48% 8% 18% 17% 9% 1	2364
	11th to 20th	Day	50% 7% 14% 21% 8% 1	2440
	21st to end	Day	49% 8% 14% 21% 7% 1	2470
May	1st to 10th	Day	54% 8% 13% 21% 5%	4425
	11th to 20th	Day	42% 10% 14% 25% 9%	4698
	21st to end	Day	50% 9% 17% 18% 6% 1	5162
June	1st to 10th	Day	44% 9% 17% 20% 8% 1	2850
	11th to 20th	Day	52% 9% 14% 19% 5% 1	2850
	21st to end	Day	34% 10% 20% 27% 7% 1	2850
July	1st to 10th	Day	28% 7% 19% 28% 16% 1	4666
	11th to 20th	Day	28% 8% 18% 36% 10%	4659
	21st to end	Day	24% 6% 17% 29% 20% 4	4976
August	1st to 10th	Day	36% 9% 17% 30% 7% 1	4384
	11th to 20th	Day	32% 8% 24% 28% 6% 1	4096
	21st to end	Day	31% 6% 22% 30% 10% 1	4520
September	1st to 10th	Day	35% 7% 17% 27% 11% 4	2380
	11th to 20th	Day	39% 6% 15% 29% 9% 1	2090
	21st to end	Day	36% 7% 17% 26% 11% 4	2069
October	1st to 10th	Day	43% 6% 16% 24% 9% 1	1877
	11th to 20th	Day	42% 8% 16% 23% 10% 1	1684
	21st to end	Day	46% 8% 13% 21% 11% 1	1782
November	1st to 10th	Day	56% 7% 13% 16% 7% 2	1505
	11th to 20th	Day	50% 7% 13% 22% 8% 1	1453
	21st to end	Day	48% 7% 17% 19% 8% 1	1330
December	1st to 10th	Day	56% 6% 12% 15% 8% 2	1330
	11th to 20th	Day	52% 8% 15% 20% 3 1	1684
	21st to end	Day	38% 6% 12% 24% 16% 4	2442
January	1st to 10th	Night	53% 8% 13% 19% 6% 2	4726
	11th to 20th	Night	51% 6% 12% 16% 14% 3	4740
	21st to end	Night	45% 9% 15% 23% 8% 1	4951
February	1st to 10th	Night	52% 6% 12% 19% 10% 1	2658
	11th to 20th	Night	53% 7% 12% 15% 10% 1	2403
	21st to end	Night	46% 8% 15% 20% 11% 1	1965
March	1st to 10th	Night	54% 7% 13% 19% 6% 1	3736
	11th to 20th	Night	39% 8% 24% 19% 8% 1	3495
	21st to end	Night	64% 9% 12% 11% 4 1	3809
April	1st to 10th	Night	48% 9% 16% 15% 12% 1	1815
	11th to 20th	Night	51% 6% 14% 21% 7% 1	1691
	21st to end	Night	48% 7% 13% 22% 7% 2	1710
May	1st to 10th	Night	55% 6% 10% 22% 6% 1	2538
	11th to 20th	Night	41% 10% 13% 26% 10%	2199
	21st to end	Night	58% 10% 12% 15% 4 1	2414
June	1st to 10th	Night	49% 9% 14% 19% 9% 1	1330
	11th to 20th	Night	56% 8% 12% 16% 6% 2	1330
	21st to end	Night	39% 8% 17% 26% 8% 1	1330
July	1st to 10th	Night	35% 9% 17% 23% 15% 1	2185
	11th to 20th	Night	28% 9% 17% 36% 7% 4	2178
	21st to end	Night	27% 7% 15% 30% 18% 3	2337
August	1st to 10th	Night	35% 8% 16% 31% 9%	2519
	11th to 20th	Night	34% 8% 21% 31% 5% 1	2806
	21st to end	Night	31% 7% 22% 30% 8% 2	3146
September	1st to 10th	Night	33% 7% 16% 24% 14% 6%	1796
	11th to 20th	Night	39% 7% 14% 26% 10% 4	2090
	21st to end	Night	34% 5% 17% 27% 15% 2	2079
October	1st to 10th	Night	42% 6% 15% 24% 11% 1	2083
	11th to 20th	Night	42% 7% 17% 22% 10% 2	2272
	21st to end	Night	45% 9% 15% 23% 8% 1	2574
November	1st to 10th	Night	50% 7% 11% 22% 9% 1	2554
	11th to 20th	Night	47% 8% 15% 18% 10% 1	2726
	21st to end	Night	46% 9% 18% 19% 7% 2	2850
December	1st to 10th	Night	52% 7% 12% 20% 9% 2	2850
	11th to 20th	Night	49% 10% 17% 18% 6%	3507
	21st to end	Night	37% 6% 13% 24% 15% 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	7% 5% 24% 35% 21% 8%	180
	11th to 20th	Day	9% 4 34% 30% 22% 2	180
	21st to end	Day	6% 11% 25% 30% 21% 7%	198
February	1st to 10th	Day	10% 12% 21% 38% 13% 6%	181
	11th to 20th	Day	11% 7% 26% 35% 18% 4	270
	21st to end	Day	8% 6% 25% 36% 20% 6%	297
March	1st to 10th	Day	10% 6% 30% 32% 17% 5%	460
	11th to 20th	Day	8% 4 26% 37% 19% 6%	460
	21st to end	Day	12% 6% 28% 32% 17% 6%	506
April	1st to 10th	Day	10% 13% 29% 30% 14% 4	460
	11th to 20th	Day	13% 10% 30% 27% 15% 4	452
	21st to end	Day	17% 10% 26% 33% 12% 3	460
May	1st to 10th	Day	16% 11% 23% 27% 18% 5%	559
	11th to 20th	Day	12% 7% 22% 35% 20% 4	675
	21st to end	Day	17% 5% 22% 28% 22% 7%	734
June	1st to 10th	Day	15% 8% 26% 35% 13% 4	680
	11th to 20th	Day	18% 8% 26% 30% 14% 4	680
	21st to end	Day	9% 5% 24% 31% 24% 8%	680
July	1st to 10th	Day	10% 4 18% 35% 26% 8%	671
	11th to 20th	Day	10% 4 19% 36% 24% 7%	668
	21st to end	Day	7% 3 20% 33% 27% 10%	739
August	1st to 10th	Day	10% 5% 24% 31% 20% 10%	487
	11th to 20th	Day	8% 4 19% 33% 30% 6%	477
	21st to end	Day	10% 9% 22% 29% 23% 8%	539
September	1st to 10th	Day	14% 6% 27% 29% 18% 6%	490
	11th to 20th	Day	19% 6% 23% 28% 20% 4	490
	21st to end	Day	12% 12% 28% 29% 14% 4	483
October	1st to 10th	Day	19% 7% 22% 29% 17% 6%	347
	11th to 20th	Day	11% 10% 31% 28% 15% 4	270
	21st to end	Day	14% 13% 33% 26% 12% 1	297
November	1st to 10th	Day	20% 7% 25% 34% 11% 3	270
	11th to 20th	Day	13% 7% 32% 32% 14% 2	238
	21st to end	Day	8% 5% 30% 33% 16% 8%	180
December	1st to 10th	Day	8% 3 30% 33% 21% 4	180
	11th to 20th	Day	6% 7% 32% 37% 14% 3	180
	21st to end	Day	11% 3 20% 38% 24% 5%	198
January	1st to 10th	Night	11% 6% 31% 35% 14% 2	950
	11th to 20th	Night	12% 5% 31% 37% 14% 1	950
	21st to end	Night	11% 7% 29% 38% 13% 2	886
February	1st to 10th	Night	10% 7% 26% 37% 18% 2	760
	11th to 20th	Night	11% 5% 32% 36% 15% 1	744
	21st to end	Night	7% 4 23% 39% 22% 4	628
March	1st to 10th	Night	9% 5% 28% 39% 15% 4	760
	11th to 20th	Night	9% 5% 29% 40% 14% 3	760
	21st to end	Night	14% 5% 26% 35% 17% 2	836
April	1st to 10th	Night	13% 7% 29% 33% 16% 2	675
	11th to 20th	Night	15% 7% 30% 36% 10% 1	563
	21st to end	Night	14% 9% 29% 35% 12% 1	570
May	1st to 10th	Night	17% 7% 26% 36% 12% 1	488
	11th to 20th	Night	10% 6% 29% 37% 16% 1	374
	21st to end	Night	18% 7% 24% 31% 18% 1	411
June	1st to 10th	Night	16% 7% 28% 33% 14% 2	380
	11th to 20th	Night	16% 4 32% 30% 17%	380
	21st to end	Night	13% 3 22% 38% 22% 2	380
July	1st to 10th	Night	11% 2 16% 47% 23% 1	370
	11th to 20th	Night	11% 4 19% 43% 22% 2	368
	21st to end	Night	5% 4 21% 40% 29% 2	416
August	1st to 10th	Night	11% 3 17% 39% 26% 2	378
	11th to 20th	Night	8% 5% 18% 43% 25% 1	544
	21st to end	Night	8% 5% 21% 44% 22%	627
September	1st to 10th	Night	13% 3 23% 41% 18% 2	623
	11th to 20th	Night	14% 5% 21% 39% 20% 1	760
	21st to end	Night	10% 4 29% 39% 16% 2	756
October	1st to 10th	Night	15% 5% 25% 35% 18% 2	720
	11th to 20th	Night	11% 5% 30% 37% 15% 2	720
	21st to end	Night	11% 8% 32% 36% 13% 1	792
November	1st to 10th	Night	15% 9% 29% 36% 9% 1	889
	11th to 20th	Night	9% 7% 39% 33% 10% 2	950
	21st to end	Night	9% 4 33% 39% 12% 2	950
December	1st to 10th	Night	10% 7% 30% 35% 16% 2	950
	11th to 20th	Night	9% 5% 34% 38% 12% 2	950
	21st to end	Night	11% 4 29% 38% 16% 2	1045

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