Southwest Corner Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Southwest Corner Mountain clear sky chart from 2005-11-06 to 2024-11-28.

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	7% 7% 3 6% 4 4 4 4 6% 7% 48%	105530
	Night	8% 9% 3 7% 4 4 4 4 6% 7% 44%	102604
Transparency	Day	34% 7% 12% 22% 17% 8%	105299
	Night	38% 7% 14% 22% 14% 5%	102481
Seeing	Day	10% 8% 24% 32% 19% 6%	14327
	Night	12% 4 20% 41% 21% 2	23328

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	16% 21% 4 9% 6% 4 5% 4 6% 7% 20%	2539
	11th to 20th	Day	15% 14% 6% 10% 6% 4 4 3 4 6% 27%	2516
	21st to end	Day	25% 22% 7% 9% 4 3 3 2 4 5% 16%	2794
February	1st to 10th	Day	8% 13% 3 10% 6% 5% 6% 5% 7% 7% 30%	1621
	11th to 20th	Day	8% 15% 4 7% 4 4 4 3 7% 8% 37%	1665
	21st to end	Day	5% 13% 4 10% 5% 6% 6% 5% 6% 9% 32%	1570
March	1st to 10th	Day	7% 9% 3 7% 5% 5% 5% 4 7% 8% 39%	3138
	11th to 20th	Day	4 5% 1 4 3 4 5% 4 6% 8% 56%	3354
	21st to end	Day	9% 10% 3 8% 6% 5% 5% 5% 7% 9% 33%	4103
April	1st to 10th	Day	8% 8% 2 8% 6% 6% 6% 6% 7% 8% 35%	2280
	11th to 20th	Day	5% 7% 3 7% 6% 6% 5% 5% 8% 9% 39%	2252
	21st to end	Day	6% 7% 3 7% 6% 5% 6% 5% 6% 9% 40%	2538
May	1st to 10th	Day	7% 5% 2 5% 4 4 4 4 6% 8% 52%	4420
	11th to 20th	Day	5% 4 2 5% 5% 4 5% 4 6% 10% 49%	4377
	21st to end	Day	10% 9% 3 8% 7% 6% 6% 5% 7% 8% 32%	4813
June	1st to 10th	Day	9% 6% 2 6% 4 5% 6% 6% 8% 8% 39%	2660
	11th to 20th	Day	7% 5% 2 6% 4 5% 6% 5% 6% 8% 45%	2660
	21st to end	Day	5% 4 1 5% 4 4 5% 4 6% 8% 55%	2660
July	1st to 10th	Day	1 1 1 3 2 2 3 2 4 5% 76%	4420
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 4 78%	4420
	21st to end	Day	2 2 1 2 2 2 2 2 4 5% 76%	4790
August	1st to 10th	Day	2 2 1 2 2 3 3 3 5% 7% 71%	4401
	11th to 20th	Day	1 2 1 3 3 3 3 4 6% 8% 66%	4209
	21st to end	Day	3 4 1 5% 4 4 4 3 5% 6% 61%	4185
September	1st to 10th	Day	3 2 1 3 3 3 2 3 5% 8% 67%	2280
	11th to 20th	Day	4 3 2 5% 5% 5% 5% 4 6% 8% 53%	2266
	21st to end	Day	7% 4 2 4 4 3 4 4 6% 6% 57%	2068
October	1st to 10th	Day	4 3 2 4 3 4 4 3 6% 8% 60%	3180
	11th to 20th	Day	5% 4 2 7% 5% 5% 5% 5% 6% 9% 47%	3116
	21st to end	Day	8% 7% 2 6% 4 5% 5% 5% 7% 8% 44%	3101
November	1st to 10th	Day	12% 10% 3 6% 5% 5% 5% 4 7% 9% 33%	1719
	11th to 20th	Day	9% 14% 4 8% 5% 5% 5% 5% 5% 7% 34%	1600
	21st to end	Day	9% 15% 6% 10% 6% 5% 5% 4 6% 7% 28%	1576
December	1st to 10th	Day	13% 15% 4 10% 5% 4 5% 3 6% 6% 29%	1520
	11th to 20th	Day	16% 18% 7% 12% 5% 3 4 4 5% 5% 22%	1925
	21st to end	Day	13% 21% 6% 12% 6% 5% 5% 4 4 5% 19%	2794
January	1st to 10th	Night	12% 22% 6% 11% 5% 5% 5% 4 5% 6% 19%	4406
	11th to 20th	Night	16% 15% 4 10% 4 3 4 3 6% 9% 26%	4382
	21st to end	Night	23% 21% 8% 10% 5% 4 4 3 4 4 13%	4862
February	1st to 10th	Night	11% 13% 4 9% 6% 5% 5% 4 7% 8% 30%	2556
	11th to 20th	Night	8% 11% 3 8% 4 4 5% 4 7% 8% 36%	2401
	21st to end	Night	7% 12% 4 8% 5% 5% 5% 4 7% 8% 33%	1884
March	1st to 10th	Night	6% 10% 3 6% 5% 5% 6% 4 7% 10% 37%	3736
	11th to 20th	Night	5% 6% 2 4 3 4 4 3 5% 6% 58%	3623
	21st to end	Night	9% 10% 3 8% 6% 6% 6% 5% 7% 10% 30%	3550
April	1st to 10th	Night	6% 7% 3 7% 6% 5% 6% 6% 7% 9% 39%	1900
	11th to 20th	Night	5% 8% 3 6% 4 5% 5% 4 6% 7% 47%	1879
	21st to end	Night	6% 6% 3 5% 4 4 5% 4 7% 8% 48%	1639
May	1st to 10th	Night	5% 5% 3 5% 3 4 5% 4 6% 8% 53%	2540
	11th to 20th	Night	6% 4 1 5% 3 3 3 4 6% 8% 56%	2516
	21st to end	Night	6% 4 2 6% 4 4 5% 4 8% 9% 47%	2762
June	1st to 10th	Night	8% 4 2 6% 4 4 5% 4 8% 8% 47%	1520
	11th to 20th	Night	5% 5% 2 6% 4 4 4 4 7% 8% 50%	1520
	21st to end	Night	3 3 2 3 3 3 5% 4 5% 6% 62%	1520
July	1st to 10th	Night	1 1 1 2 2 2 2 2 4 4 80%	2540
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 83%	2540
	21st to end	Night	1 1 1 2 2 2 2 2 4 6% 78%	2762
August	1st to 10th	Night	2 2 1 4 3 4 4 2 4 6% 68%	2529
	11th to 20th	Night	1 2 1 2 2 3 3 4 6% 9% 67%	2619
	21st to end	Night	4 3 1 4 4 4 4 3 4 7% 64%	3493
September	1st to 10th	Night	2 2 1 3 3 3 4 3 5% 7% 67%	1900
	11th to 20th	Night	4 4 1 4 3 3 4 3 5% 7% 60%	1914
	21st to end	Night	6% 4 1 3 2 3 4 3 6% 9% 58%	2109
October	1st to 10th	Night	5% 4 1 3 3 3 5% 4 7% 7% 58%	3800
	11th to 20th	Night	4 6% 2 6% 4 4 5% 5% 6% 9% 50%	3807
	21st to end	Night	10% 6% 2 7% 5% 5% 4 4 6% 8% 44%	4457
November	1st to 10th	Night	15% 9% 3 5% 4 4 4 5% 6% 8% 36%	2578
	11th to 20th	Night	11% 11% 3 7% 5% 5% 6% 5% 6% 8% 33%	2800
	21st to end	Night	11% 18% 5% 10% 4 4 4 4 6% 7% 27%	2758
December	1st to 10th	Night	12% 15% 5% 8% 5% 4 5% 3 6% 8% 29%	2660
	11th to 20th	Night	13% 18% 7% 14% 6% 6% 5% 4 5% 6% 17%	3280
	21st to end	Night	16% 23% 4 10% 5% 5% 4 3 5% 5% 19%	4862

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	63% 7% 10% 12% 7% 1	2539
	11th to 20th	Day	56% 9% 17% 15% 2 2	2540
	21st to end	Day	73% 6% 9% 9% 2	2794
February	1st to 10th	Day	52% 9% 19% 14% 5% 1	1621
	11th to 20th	Day	46% 8% 19% 18% 9% 1	1674
	21st to end	Day	47% 10% 17% 17% 8% 1	1570
March	1st to 10th	Day	39% 9% 14% 24% 14% 1	3138
	11th to 20th	Day	26% 6% 17% 41% 9%	3354
	21st to end	Day	42% 11% 18% 17% 10% 1	4103
April	1st to 10th	Day	42% 9% 16% 22% 9% 2	2280
	11th to 20th	Day	37% 8% 17% 21% 13% 4	2252
	21st to end	Day	38% 9% 13% 22% 15% 4	2538
May	1st to 10th	Day	27% 7% 11% 23% 28% 4	4420
	11th to 20th	Day	23% 9% 14% 32% 19% 4	4380
	21st to end	Day	42% 12% 14% 22% 10% 1	4814
June	1st to 10th	Day	38% 8% 11% 21% 16% 6%	2660
	11th to 20th	Day	33% 8% 12% 26% 17% 5%	2660
	21st to end	Day	24% 8% 11% 27% 23% 7%	2660
July	1st to 10th	Day	12% 3 5% 16% 28% 36%	4351
	11th to 20th	Day	12% 2 4 12% 25% 44%	4345
	21st to end	Day	12% 2 4 21% 33% 27%	4639
August	1st to 10th	Day	13% 3 5% 23% 36% 19%	4384
	11th to 20th	Day	16% 3 13% 35% 27% 6%	4251
	21st to end	Day	23% 5% 12% 28% 26% 7%	4184
September	1st to 10th	Day	15% 5% 12% 29% 31% 9%	2280
	11th to 20th	Day	27% 6% 12% 25% 24% 6%	2266
	21st to end	Day	26% 7% 17% 28% 17% 5%	2068
October	1st to 10th	Day	23% 5% 16% 35% 20% 1	3180
	11th to 20th	Day	33% 8% 18% 25% 15% 1	3116
	21st to end	Day	37% 7% 15% 25% 14% 2	3101
November	1st to 10th	Day	47% 9% 15% 18% 9% 1	1719
	11th to 20th	Day	49% 10% 16% 15% 8% 2	1600
	21st to end	Day	56% 10% 15% 12% 6% 2	1576
December	1st to 10th	Day	55% 7% 16% 16% 6%	1520
	11th to 20th	Day	66% 9% 13% 8% 5%	1928
	21st to end	Day	68% 7% 10% 10% 5%	2794
January	1st to 10th	Night	65% 7% 12% 12% 4 1	4406
	11th to 20th	Night	53% 8% 17% 17% 5%	4420
	21st to end	Night	76% 6% 8% 7% 2	4862
February	1st to 10th	Night	51% 10% 18% 15% 5% 2	2556
	11th to 20th	Night	44% 8% 19% 19% 8% 1	2403
	21st to end	Night	44% 11% 15% 19% 9% 2	1884
March	1st to 10th	Night	38% 9% 16% 23% 13% 2	3736
	11th to 20th	Night	27% 6% 17% 37% 12% 1	3624
	21st to end	Night	46% 10% 18% 21% 4 1	3550
April	1st to 10th	Night	39% 8% 16% 26% 10% 2	1900
	11th to 20th	Night	33% 8% 15% 24% 17% 3	1879
	21st to end	Night	32% 6% 14% 29% 17% 3	1639
May	1st to 10th	Night	26% 6% 11% 24% 30% 3	2540
	11th to 20th	Night	21% 9% 11% 33% 24% 2	2516
	21st to end	Night	27% 9% 17% 33% 13% 1	2762
June	1st to 10th	Night	32% 7% 13% 25% 19% 5%	1520
	11th to 20th	Night	28% 7% 15% 28% 18% 4	1520
	21st to end	Night	22% 5% 13% 29% 26% 6%	1520
July	1st to 10th	Night	10% 3 5% 16% 31% 35%	2500
	11th to 20th	Night	8% 3 5% 15% 26% 44%	2492
	21st to end	Night	9% 2 4 23% 32% 30%	2674
August	1st to 10th	Night	17% 4 6% 23% 30% 20%	2521
	11th to 20th	Night	15% 3 17% 34% 25% 6%	2643
	21st to end	Night	21% 4 11% 29% 27% 8%	3493
September	1st to 10th	Night	17% 5% 10% 28% 31% 9%	1900
	11th to 20th	Night	23% 6% 11% 27% 26% 8%	1914
	21st to end	Night	23% 5% 16% 34% 16% 6%	2109
October	1st to 10th	Night	23% 4 17% 35% 19% 1	3800
	11th to 20th	Night	31% 6% 18% 27% 16% 1	3807
	21st to end	Night	39% 6% 15% 27% 12% 2	4457
November	1st to 10th	Night	45% 8% 19% 16% 9% 3	2578
	11th to 20th	Night	48% 10% 16% 16% 7% 3	2800
	21st to end	Night	56% 8% 15% 13% 6% 1	2758
December	1st to 10th	Night	55% 7% 16% 19% 3	2660
	11th to 20th	Night	67% 10% 11% 8% 3	3276
	21st to end	Night	67% 6% 9% 11% 6% 1	4862

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	26% 4 14% 38% 12% 6%	180
	11th to 20th	Day	26% 7% 15% 29% 18% 5%	180
	21st to end	Day	29% 1 15% 35% 16% 3	198
February	1st to 10th	Day	12% 5% 30% 39% 13% 2	281
	11th to 20th	Day	16% 5% 31% 31% 13% 4	365
	21st to end	Day	9% 6% 31% 36% 15% 2	382
March	1st to 10th	Day	13% 8% 31% 31% 13% 4	460
	11th to 20th	Day	13% 10% 25% 25% 20% 6%	460
	21st to end	Day	11% 13% 32% 28% 14% 2	506
April	1st to 10th	Day	15% 14% 29% 26% 13% 3	460
	11th to 20th	Day	9% 17% 31% 26% 13% 5%	452
	21st to end	Day	8% 15% 30% 28% 16% 3	460
May	1st to 10th	Day	8% 13% 30% 32% 16% 2	463
	11th to 20th	Day	14% 9% 23% 29% 19% 6%	488
	21st to end	Day	11% 7% 25% 29% 19% 8%	529
June	1st to 10th	Day	10% 11% 28% 32% 15% 3	490
	11th to 20th	Day	8% 13% 31% 29% 16% 4	490
	21st to end	Day	5% 7% 22% 36% 24% 6%	490
July	1st to 10th	Day	5% 6% 21% 43% 22% 4	486
	11th to 20th	Day	3 8% 23% 35% 24% 7%	484
	21st to end	Day	3 4 22% 36% 27% 8%	531
August	1st to 10th	Day	5% 3 19% 34% 30% 9%	487
	11th to 20th	Day	2 3 18% 34% 31% 12%	477
	21st to end	Day	3 8% 21% 36% 23% 8%	539
September	1st to 10th	Day	4 5% 22% 32% 24% 13%	490
	11th to 20th	Day	6% 7% 21% 33% 26% 8%	490
	21st to end	Day	9% 9% 23% 32% 22% 5%	490
October	1st to 10th	Day	9% 8% 23% 30% 23% 7%	490
	11th to 20th	Day	9% 7% 24% 29% 23% 8%	470
	21st to end	Day	16% 4 25% 37% 14% 4	330
November	1st to 10th	Day	18% 6% 23% 36% 16% 1	277
	11th to 20th	Day	21% 5% 28% 25% 17% 6%	200
	21st to end	Day	19% 3 21% 32% 19% 6%	194
December	1st to 10th	Day	24% 5% 12% 31% 17% 12%	180
	11th to 20th	Day	19% 3 20% 43% 13% 2	180
	21st to end	Day	18% 3 13% 42% 18% 6%	198
January	1st to 10th	Night	23% 5% 21% 34% 16% 1	760
	11th to 20th	Night	23% 3 17% 37% 18% 3	760
	21st to end	Night	20% 3 16% 39% 20% 3	836
February	1st to 10th	Night	17% 3 21% 42% 14% 2	760
	11th to 20th	Night	14% 2 23% 41% 17% 2	744
	21st to end	Night	11% 5% 23% 41% 18% 2	628
March	1st to 10th	Night	15% 4 20% 42% 18% 2	760
	11th to 20th	Night	10% 4 23% 43% 19% 1	760
	21st to end	Night	11% 6% 24% 41% 17% 1	836
April	1st to 10th	Night	10% 5% 21% 42% 21% 1	760
	11th to 20th	Night	8% 7% 25% 38% 19% 2	751
	21st to end	Night	7% 6% 27% 43% 15% 2	499
May	1st to 10th	Night	5% 4 30% 39% 22%	380
	11th to 20th	Night	11% 4 19% 39% 26% 2	374
	21st to end	Night	9% 4 16% 41% 26% 4	411
June	1st to 10th	Night	9% 3 26% 39% 21% 2	380
	11th to 20th	Night	6% 6% 24% 40% 22% 1	380
	21st to end	Night	3 4 20% 48% 23% 1	380
July	1st to 10th	Night	3 4 19% 44% 29% 2	370
	11th to 20th	Night	2 1 22% 47% 26% 1	368
	21st to end	Night	2 15% 48% 34% 1	416
August	1st to 10th	Night	2 2 14% 40% 38% 4	378
	11th to 20th	Night	2 2 11% 40% 41% 5%	446
	21st to end	Night	4 4 16% 42% 32% 3	831
September	1st to 10th	Night	3 3 15% 45% 29% 5%	760
	11th to 20th	Night	7% 3 15% 33% 37% 7%	760
	21st to end	Night	7% 3 16% 45% 26% 3	760
October	1st to 10th	Night	10% 4 15% 43% 26% 2	760
	11th to 20th	Night	9% 3 23% 41% 22% 3	760
	21st to end	Night	16% 4 20% 40% 17% 3	836
November	1st to 10th	Night	20% 4 19% 44% 14% 1	780
	11th to 20th	Night	18% 3 25% 38% 15% 3	800
	21st to end	Night	21% 3 19% 37% 15% 4	788
December	1st to 10th	Night	18% 2 18% 39% 20% 3	760
	11th to 20th	Night	22% 3 19% 40% 15% 1	760
	21st to end	Night	26% 2 19% 43% 9% 1	836

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