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 2024-02-05.

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	10% 8% 4 7% 6% 4 5% 5% 6% 8% 36%	77657
	Night	8% 9% 4 8% 7% 4 6% 5% 6% 8% 34%	81153
Transparency	Day	44% 8% 16% 22% 9% 1	77360
	Night	45% 8% 15% 21% 10% 2	81031
Seeing	Day	12% 7% 25% 32% 19% 5%	14306
	Night	12% 5% 28% 37% 16% 2	23606

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% 11% 5% 11% 8% 3 6% 5% 6% 8% 30%	2219
	11th to 20th	Day	5% 10% 4 10% 7% 4 6% 8% 8% 9% 29%	2199
	21st to end	Day	5% 9% 4 10% 8% 3 5% 5% 6% 10% 36%	2734
February	1st to 10th	Day	8% 12% 5% 10% 7% 4 5% 6% 6% 8% 28%	1473
	11th to 20th	Day	7% 13% 6% 10% 8% 4 6% 6% 6% 8% 24%	1575
	21st to end	Day	5% 11% 8% 10% 11% 5% 7% 5% 6% 9% 24%	1400
March	1st to 10th	Day	6% 13% 5% 12% 11% 5% 7% 5% 6% 6% 25%	1800
	11th to 20th	Day	8% 9% 3 10% 8% 5% 5% 6% 6% 7% 32%	1974
	21st to end	Day	9% 14% 7% 9% 7% 4 5% 6% 5% 7% 27%	2182
April	1st to 10th	Day	9% 11% 5% 8% 7% 4 5% 6% 7% 9% 31%	2239
	11th to 20th	Day	12% 12% 4 8% 6% 4 6% 5% 6% 8% 29%	2310
	21st to end	Day	13% 9% 4 9% 5% 4 5% 6% 5% 8% 33%	2340
May	1st to 10th	Day	15% 10% 4 6% 6% 4 5% 6% 6% 8% 30%	2506
	11th to 20th	Day	10% 6% 3 6% 6% 3 6% 6% 6% 8% 40%	2654
	21st to end	Day	19% 6% 3 6% 5% 3 5% 5% 5% 8% 34%	2917
June	1st to 10th	Day	16% 5% 2 5% 6% 4 6% 6% 7% 10% 32%	2700
	11th to 20th	Day	19% 8% 4 6% 7% 4 6% 6% 6% 8% 26%	2700
	21st to end	Day	9% 4 2 5% 6% 4 7% 7% 7% 11% 40%	2700
July	1st to 10th	Day	9% 5% 2 5% 6% 4 5% 6% 8% 9% 42%	2700
	11th to 20th	Day	11% 5% 2 5% 5% 3 5% 5% 7% 9% 44%	2700
	21st to end	Day	6% 4 2 4 5% 3 4 5% 6% 10% 52%	2938
August	1st to 10th	Day	11% 5% 2 5% 4 3 5% 6% 7% 9% 45%	2507
	11th to 20th	Day	10% 4 2 4 4 3 4 5% 7% 9% 49%	2316
	21st to end	Day	10% 4 2 4 4 3 4 6% 7% 8% 48%	2574
September	1st to 10th	Day	11% 5% 3 5% 5% 3 4 4 6% 8% 45%	2255
	11th to 20th	Day	16% 7% 2 5% 4 2 4 4 5% 7% 43%	1980
	21st to end	Day	9% 8% 3 5% 5% 3 5% 4 5% 9% 45%	1980
October	1st to 10th	Day	13% 10% 3 6% 5% 2 4 4 5% 7% 40%	1877
	11th to 20th	Day	9% 7% 4 7% 6% 4 5% 6% 6% 7% 38%	1684
	21st to end	Day	8% 12% 3 7% 7% 4 5% 6% 6% 8% 35%	1782
November	1st to 10th	Day	10% 16% 6% 9% 7% 3 4 5% 6% 8% 25%	1505
	11th to 20th	Day	8% 12% 5% 11% 7% 4 5% 5% 5% 7% 34%	1453
	21st to end	Day	7% 10% 5% 10% 8% 4 5% 5% 5% 9% 32%	1330
December	1st to 10th	Day	7% 12% 6% 10% 8% 7% 6% 6% 5% 7% 27%	1330
	11th to 20th	Day	4 11% 6% 12% 9% 4 6% 5% 6% 8% 28%	1682
	21st to end	Day	4 9% 4 8% 7% 3 5% 5% 5% 8% 43%	2442
January	1st to 10th	Night	5% 13% 6% 10% 9% 5% 6% 6% 7% 9% 25%	4726
	11th to 20th	Night	4 9% 5% 12% 9% 4 6% 7% 7% 7% 31%	4699
	21st to end	Night	4 11% 5% 10% 7% 4 6% 5% 6% 8% 34%	4951
February	1st to 10th	Night	6% 11% 5% 11% 8% 4 6% 6% 6% 8% 29%	2580
	11th to 20th	Night	5% 13% 7% 10% 9% 5% 6% 6% 7% 7% 25%	2272
	21st to end	Night	5% 9% 5% 10% 9% 4 5% 6% 8% 10% 30%	1852
March	1st to 10th	Night	6% 10% 5% 9% 11% 5% 6% 6% 6% 8% 29%	2160
	11th to 20th	Night	7% 9% 5% 10% 8% 4 6% 5% 8% 8% 31%	1985
	21st to end	Night	9% 14% 6% 8% 8% 4 6% 6% 7% 8% 24%	2171
April	1st to 10th	Night	9% 9% 4 10% 6% 4 7% 5% 6% 8% 31%	1720
	11th to 20th	Night	10% 11% 4 9% 9% 5% 5% 5% 6% 8% 29%	1601
	21st to end	Night	11% 10% 4 8% 6% 4 5% 5% 6% 8% 33%	1620
May	1st to 10th	Night	13% 11% 3 7% 6% 3 5% 6% 7% 8% 31%	1452
	11th to 20th	Night	10% 7% 2 6% 8% 4 5% 6% 5% 6% 41%	1239
	21st to end	Night	19% 7% 3 7% 6% 3 4 4 5% 8% 35%	1358
June	1st to 10th	Night	15% 8% 4 6% 7% 4 5% 5% 7% 9% 30%	1260
	11th to 20th	Night	18% 8% 3 7% 8% 5% 7% 6% 6% 7% 25%	1260
	21st to end	Night	11% 5% 3 6% 6% 4 5% 6% 6% 8% 40%	1260
July	1st to 10th	Night	9% 5% 2 5% 5% 3 6% 5% 7% 10% 43%	1260
	11th to 20th	Night	10% 4 2 6% 7% 3 7% 6% 7% 9% 38%	1260
	21st to end	Night	6% 4 2 5% 5% 3 4 6% 6% 10% 51%	1379
August	1st to 10th	Night	9% 4 2 6% 6% 3 5% 6% 7% 9% 43%	1429
	11th to 20th	Night	9% 5% 3 6% 5% 3 4 4 7% 10% 44%	1583
	21st to end	Night	9% 4 2 5% 4 3 5% 5% 6% 9% 48%	1782
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 5% 6% 8% 47%	1701
	11th to 20th	Night	13% 6% 3 5% 5% 3 4 4 5% 8% 44%	1980
	21st to end	Night	9% 6% 3 5% 5% 3 4 4 7% 8% 47%	1980
October	1st to 10th	Night	12% 9% 3 5% 5% 3 5% 5% 5% 7% 41%	2083
	11th to 20th	Night	9% 7% 3 8% 6% 3 4 6% 5% 9% 39%	2272
	21st to end	Night	7% 9% 4 9% 8% 4 5% 5% 6% 9% 35%	2574
November	1st to 10th	Night	10% 15% 4 7% 6% 3 5% 4 6% 8% 31%	2554
	11th to 20th	Night	5% 10% 5% 10% 7% 4 7% 6% 7% 8% 32%	2726
	21st to end	Night	5% 10% 4 9% 8% 4 5% 6% 7% 9% 33%	2850
December	1st to 10th	Night	7% 11% 5% 10% 8% 5% 6% 4 5% 8% 31%	2850
	11th to 20th	Night	3 9% 5% 10% 10% 5% 8% 5% 7% 8% 30%	3510
	21st to end	Night	4 8% 3 7% 6% 4 5% 5% 5% 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	53% 7% 14% 16% 9% 1	1473
	11th to 20th	Day	55% 8% 14% 15% 7% 1	1584
	21st to end	Day	57% 7% 10% 17% 10%	1400
March	1st to 10th	Day	58% 8% 14% 13% 6%	1800
	11th to 20th	Day	49% 6% 14% 21% 9% 1	1975
	21st to end	Day	55% 8% 15% 15% 6% 1	2182
April	1st to 10th	Day	48% 8% 18% 17% 9% 1	2239
	11th to 20th	Day	51% 7% 14% 20% 8% 1	2310
	21st to end	Day	49% 8% 14% 21% 7% 1	2340
May	1st to 10th	Day	48% 9% 14% 21% 8%	2506
	11th to 20th	Day	38% 9% 15% 26% 11%	2658
	21st to end	Day	47% 7% 15% 22% 8%	2918
June	1st to 10th	Day	45% 9% 18% 20% 8%	2700
	11th to 20th	Day	53% 9% 14% 18% 5% 1	2700
	21st to end	Day	35% 10% 20% 27% 6% 1	2700
July	1st to 10th	Day	34% 10% 22% 25% 8% 1	2626
	11th to 20th	Day	35% 11% 20% 28% 7%	2619
	21st to end	Day	27% 8% 22% 31% 11% 1	2775
August	1st to 10th	Day	33% 10% 21% 27% 8% 1	2490
	11th to 20th	Day	30% 8% 24% 27% 9% 1	2316
	21st to end	Day	31% 9% 19% 28% 12% 1	2574
September	1st to 10th	Day	36% 7% 17% 27% 10% 4	2255
	11th to 20th	Day	39% 7% 14% 29% 10% 2	1980
	21st to end	Day	36% 7% 17% 26% 11% 3	1980
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	2580
	11th to 20th	Night	55% 7% 12% 15% 9% 1	2273
	21st to end	Night	46% 8% 15% 19% 12% 1	1852
March	1st to 10th	Night	52% 7% 14% 20% 6% 1	2160
	11th to 20th	Night	46% 7% 17% 21% 9% 1	1985
	21st to end	Night	54% 9% 14% 16% 6% 1	2171
April	1st to 10th	Night	48% 9% 16% 15% 12% 1	1720
	11th to 20th	Night	51% 6% 14% 21% 7% 1	1601
	21st to end	Night	48% 7% 13% 22% 7% 2	1620
May	1st to 10th	Night	49% 7% 13% 20% 10% 1	1452
	11th to 20th	Night	41% 7% 12% 23% 16%	1239
	21st to end	Night	49% 7% 14% 22% 7% 1	1358
June	1st to 10th	Night	48% 9% 14% 19% 9%	1260
	11th to 20th	Night	56% 8% 12% 16% 6% 2	1260
	21st to end	Night	40% 9% 17% 25% 8% 1	1260
July	1st to 10th	Night	35% 12% 21% 24% 8% 1	1225
	11th to 20th	Night	37% 11% 21% 21% 8% 1	1218
	21st to end	Night	26% 11% 21% 29% 12% 1	1302
August	1st to 10th	Night	33% 11% 23% 24% 8% 1	1422
	11th to 20th	Night	33% 8% 18% 30% 9% 1	1583
	21st to end	Night	30% 8% 18% 29% 12% 3	1782
September	1st to 10th	Night	33% 7% 17% 25% 13% 5%	1701
	11th to 20th	Night	38% 7% 14% 26% 11% 4	1980
	21st to end	Night	34% 6% 17% 27% 14% 3	1980
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	12% 11% 21% 38% 13% 5%	169
	11th to 20th	Day	12% 7% 27% 35% 17% 2	240
	21st to end	Day	8% 5% 24% 39% 19% 6%	266
March	1st to 10th	Day	9% 6% 31% 31% 17% 5%	420
	11th to 20th	Day	8% 4 26% 37% 18% 7%	420
	21st to end	Day	12% 4 28% 32% 17% 7%	462
April	1st to 10th	Day	9% 12% 28% 31% 15% 5%	420
	11th to 20th	Day	14% 9% 30% 26% 16% 4	412
	21st to end	Day	17% 9% 26% 33% 12% 2	420
May	1st to 10th	Day	15% 11% 23% 28% 18% 5%	513
	11th to 20th	Day	13% 6% 21% 35% 21% 4	625
	21st to end	Day	18% 5% 22% 28% 21% 7%	679
June	1st to 10th	Day	15% 6% 26% 36% 14% 3	630
	11th to 20th	Day	18% 8% 26% 29% 15% 3	630
	21st to end	Day	9% 4 23% 31% 25% 8%	630
July	1st to 10th	Day	10% 4 19% 35% 25% 8%	621
	11th to 20th	Day	11% 5% 20% 35% 22% 6%	618
	21st to end	Day	6% 3 20% 35% 27% 10%	684
August	1st to 10th	Day	10% 5% 23% 32% 19% 10%	447
	11th to 20th	Day	8% 4 20% 32% 29% 5%	437
	21st to end	Day	10% 8% 21% 30% 23% 9%	495
September	1st to 10th	Day	15% 7% 28% 29% 18% 4	450
	11th to 20th	Day	18% 6% 23% 29% 20% 4	450
	21st to end	Day	13% 12% 27% 28% 15% 5%	450
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% 38% 17% 2	738
	11th to 20th	Night	12% 5% 33% 36% 13% 1	704
	21st to end	Night	7% 3 22% 41% 22% 4	592
March	1st to 10th	Night	9% 5% 28% 39% 15% 4	720
	11th to 20th	Night	9% 5% 28% 40% 14% 3	720
	21st to end	Night	14% 5% 26% 35% 18% 2	792
April	1st to 10th	Night	13% 6% 28% 34% 17% 3	640
	11th to 20th	Night	16% 7% 29% 37% 11% 1	533
	21st to end	Night	14% 8% 29% 36% 12% 1	540
May	1st to 10th	Night	16% 7% 27% 36% 13% 1	463
	11th to 20th	Night	11% 5% 29% 37% 16% 1	354
	21st to end	Night	19% 6% 24% 32% 17% 2	389
June	1st to 10th	Night	17% 7% 26% 34% 14% 2	360
	11th to 20th	Night	17% 4 31% 31% 17%	360
	21st to end	Night	13% 3 22% 38% 23% 2	360
July	1st to 10th	Night	11% 3 16% 46% 23% 1	350
	11th to 20th	Night	11% 4 20% 42% 21% 2	348
	21st to end	Night	5% 4 21% 40% 29% 2	394
August	1st to 10th	Night	11% 3 17% 40% 27% 3	358
	11th to 20th	Night	8% 5% 18% 43% 24% 1	515
	21st to end	Night	8% 5% 20% 44% 23%	594
September	1st to 10th	Night	13% 3 24% 41% 18% 2	590
	11th to 20th	Night	14% 5% 22% 40% 19% 1	720
	21st to end	Night	11% 4 29% 38% 17% 2	720
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

Page generated at 2024-02-02 03:20:33. Version ac07

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