Monitor Pass
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Monitor Pass clear sky chart from 2005-11-06 to 2025-12-03.

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	2 3 1 4 3 3 3 3 5% 6% 66%	135560
	Night	2 3 2 4 3 3 3 3 5% 7% 66%	132527
Transparency	Day	13% 37% 4 12% 19% 15%	130544
	Night	14% 37% 5% 14% 19% 12%	128522
Seeing	Day	10% 18% 29% 25% 13% 4	16477
	Night	5% 9% 26% 35% 20% 5%	25866

By Month

Percentage Cloud Cover SW (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 3 2 4 3 4 5% 5% 7% 9% 59%	3639
	11th to 20th	Day	1 3 2 5% 4 3 4 4 6% 6% 63%	3750
	21st to end	Day	3 7% 3 7% 4 4 5% 4 7% 9% 47%	4510
February	1st to 10th	Day	2 4 1 5% 3 4 3 4 8% 7% 58%	1834
	11th to 20th	Day	1 5% 3 5% 3 4 5% 5% 7% 11% 51%	1950
	21st to end	Day	1 4 2 6% 4 3 4 4 6% 8% 59%	1650
March	1st to 10th	Day	1 8% 3 7% 5% 5% 5% 5% 8% 10% 42%	4518
	11th to 20th	Day	4 2 4 3 3 4 3 6% 9% 60%	4885
	21st to end	Day	1 8% 3 6% 3 4 4 3 6% 7% 55%	5870
April	1st to 10th	Day	2 4 2 6% 5% 4 5% 4 6% 8% 55%	2400
	11th to 20th	Day	2 5% 1 4 4 3 4 3 6% 7% 61%	2369
	21st to end	Day	3 6% 2 4 2 3 3 3 5% 7% 61%	2400
May	1st to 10th	Day	4 5% 1 3 3 2 3 2 3 5% 68%	5704
	11th to 20th	Day	2 2 1 3 2 2 3 3 4 5% 72%	6088
	21st to end	Day	3 3 1 4 3 3 3 3 4 6% 68%	6902
June	1st to 10th	Day	3 3 2 4 3 3 3 4 5% 7% 64%	2800
	11th to 20th	Day	2 2 1 3 2 2 2 2 4 5% 77%	2800
	21st to end	Day	2 2 1 3 2 2 3 3 4 4 74%	2800
July	1st to 10th	Day	1 1 2 2 2 2 2 2 2 84%	6319
	11th to 20th	Day	3 2 1 4 3 3 3 3 4 5% 71%	6320
	21st to end	Day	1 2 1 3 3 3 4 4 5% 6% 68%	6719
August	1st to 10th	Day	1 2 1 2 2 2 2 2 3 4 80%	5649
	11th to 20th	Day	1 1 2 2 2 2 2 2 3 84%	5364
	21st to end	Day	2 2 1 4 3 4 3 2 3 3 71%	5984
September	1st to 10th	Day	2 1 1 2 1 2 3 2 4 5% 77%	2400
	11th to 20th	Day	3 2 1 3 2 3 3 2 4 5% 73%	2354
	21st to end	Day	2 2 1 3 2 2 3 1 3 4 78%	2200
October	1st to 10th	Day	3 2 1 3 2 3 3 3 4 5% 70%	3376
	11th to 20th	Day	3 3 1 2 2 2 2 1 2 4 79%	3261
	21st to end	Day	1 3 1 4 2 2 3 3 5% 7% 69%	3464
November	1st to 10th	Day	2 4 1 3 2 3 3 3 6% 7% 64%	1854
	11th to 20th	Day	2 4 1 3 3 3 4 3 6% 8% 63%	1890
	21st to end	Day	1 3 1 4 3 3 4 4 6% 9% 62%	1869
December	1st to 10th	Day	1 3 1 3 2 2 4 3 5% 9% 67%	2636
	11th to 20th	Day	2 5% 2 6% 5% 5% 5% 5% 8% 9% 46%	3028
	21st to end	Day	2 4 3 7% 4 5% 5% 5% 8% 14% 42%	4004
January	1st to 10th	Night	1 5% 2 5% 3 3 4 4 7% 10% 57%	6306
	11th to 20th	Night	1 4 2 5% 3 3 4 4 5% 6% 61%	6210
	21st to end	Night	2 7% 3 7% 4 4 5% 3 6% 7% 52%	6468
February	1st to 10th	Night	1 5% 3 6% 4 4 4 4 6% 7% 57%	2565
	11th to 20th	Night	1 5% 2 5% 3 4 4 4 7% 10% 54%	2338
	21st to end	Night	1 4 2 4 3 3 4 4 6% 9% 60%	1980
March	1st to 10th	Night	2 7% 3 6% 4 4 4 4 6% 9% 50%	5376
	11th to 20th	Night	1 4 2 5% 3 5% 5% 4 6% 8% 58%	5098
	21st to end	Night	1 6% 2 6% 4 4 5% 4 6% 10% 51%	5104
April	1st to 10th	Night	2 4 1 4 3 3 4 4 7% 9% 58%	2000
	11th to 20th	Night	1 4 2 3 3 4 3 4 5% 7% 64%	1979
	21st to end	Night	4 4 1 3 3 2 3 3 4 7% 66%	2000
May	1st to 10th	Night	3 3 1 3 2 2 2 2 3 5% 74%	4270
	11th to 20th	Night	1 3 1 3 2 2 3 2 3 5% 74%	3810
	21st to end	Night	2 1 1 2 2 2 3 2 3 5% 77%	3972
June	1st to 10th	Night	5% 3 1 3 2 3 2 3 4 6% 69%	1600
	11th to 20th	Night	1 1 1 1 2 2 2 2 3 4 82%	1600
	21st to end	Night	2 2 1 2 1 2 2 2 2 3 81%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 91%	3640
	11th to 20th	Night	2 2 1 3 3 2 2 2 3 4 77%	3640
	21st to end	Night	1 1 1 2 2 2 2 3 4 6% 75%	4122
August	1st to 10th	Night	1 1 1 2 2 1 2 2 3 4 82%	4168
	11th to 20th	Night	1 1 1 1 2 2 1 3 4 84%	4506
	21st to end	Night	2 2 1 3 3 2 2 2 3 3 77%	5015
September	1st to 10th	Night	1 1 1 1 1 2 2 2 4 85%	2000
	11th to 20th	Night	2 2 1 2 2 2 3 2 4 4 77%	2045
	21st to end	Night	2 2 1 3 2 2 2 3 4 4 78%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 77%	3820
	11th to 20th	Night	1 2 1 2 1 2 2 1 3 3 81%	3897
	21st to end	Night	2 4 1 3 2 3 3 2 4 6% 70%	4341
November	1st to 10th	Night	2 3 1 3 2 3 2 3 6% 8% 67%	2665
	11th to 20th	Night	1 4 1 3 3 3 3 3 6% 9% 65%	2730
	21st to end	Night	2 3 1 4 3 3 3 3 6% 7% 65%	2746
December	1st to 10th	Night	1 3 1 3 2 3 3 4 5% 7% 67%	4588
	11th to 20th	Night	2 4 2 4 4 4 5% 5% 9% 13% 47%	5176
	21st to end	Night	2 6% 4 8% 5% 5% 6% 5% 8% 10% 41%	6952

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	16% 28% 15% 23% 17% 1	3639
	11th to 20th	Day	17% 26% 8% 15% 25% 9%	3750
	21st to end	Day	25% 29% 7% 12% 17% 10%	4510
February	1st to 10th	Day	10% 63% 6% 11% 7% 3	1834
	11th to 20th	Day	11% 60% 4 11% 11% 3	1960
	21st to end	Day	11% 61% 4 12% 8% 4	1650
March	1st to 10th	Day	29% 29% 4 16% 17% 5%	4518
	11th to 20th	Day	15% 29% 5% 26% 23% 2	4885
	21st to end	Day	23% 29% 6% 19% 20% 3	5870
April	1st to 10th	Day	13% 62% 4 10% 10% 2	2400
	11th to 20th	Day	9% 62% 3 11% 12% 4	2372
	21st to end	Day	10% 62% 5% 11% 10% 2	2400
May	1st to 10th	Day	15% 28% 3 15% 33% 5%	5704
	11th to 20th	Day	11% 27% 3 16% 33% 9%	6091
	21st to end	Day	11% 29% 2 10% 36% 11%	6904
June	1st to 10th	Day	7% 61% 3 9% 14% 6%	2800
	11th to 20th	Day	4 61% 2 9% 13% 12%	2800
	21st to end	Day	5% 59% 1 3 11% 21%	2800
July	1st to 10th	Day	7% 25% 1 2 12% 54%	6251
	11th to 20th	Day	15% 25% 1 4 7% 49%	6245
	21st to end	Day	14% 24% 1 11% 22% 28%	6523
August	1st to 10th	Day	11% 35% 1 5% 18% 30%	4111
	11th to 20th	Day	5% 35% 1 2 10% 47%	3880
	21st to end	Day	8% 35% 1 6% 24% 27%	4311
September	1st to 10th	Day	6% 56% 2 3 17% 17%	2400
	11th to 20th	Day	8% 56% 2 9% 16% 9%	2354
	21st to end	Day	8% 56% 1 7% 17% 11%	2200
October	1st to 10th	Day	10% 35% 1 5% 25% 23%	3376
	11th to 20th	Day	8% 34% 1 8% 38% 10%	3261
	21st to end	Day	13% 35% 3 17% 23% 9%	3464
November	1st to 10th	Day	7% 52% 4 13% 18% 5%	1854
	11th to 20th	Day	11% 54% 4 11% 15% 4	1890
	21st to end	Day	11% 55% 7% 12% 10% 5%	1869
December	1st to 10th	Day	10% 35% 7% 17% 24% 6%	2636
	11th to 20th	Day	25% 32% 11% 13% 13% 6%	3028
	21st to end	Day	22% 28% 16% 21% 11% 2	4004
January	1st to 10th	Night	19% 27% 15% 21% 16% 2	6306
	11th to 20th	Night	19% 26% 7% 17% 26% 6%	6210
	21st to end	Night	24% 28% 8% 14% 19% 8%	6468
February	1st to 10th	Night	12% 63% 7% 9% 6% 4	2565
	11th to 20th	Night	10% 60% 5% 11% 11% 2	2339
	21st to end	Night	8% 61% 4 14% 9% 4	1980
March	1st to 10th	Night	23% 29% 5% 17% 21% 5%	5376
	11th to 20th	Night	21% 29% 5% 35% 10% 1	5098
	21st to end	Night	24% 29% 12% 24% 9% 2	5104
April	1st to 10th	Night	10% 62% 6% 13% 9% 1	2000
	11th to 20th	Night	8% 62% 3 13% 10% 3	1979
	21st to end	Night	8% 64% 5% 14% 8% 1	2000
May	1st to 10th	Night	12% 28% 3 25% 29% 3	4267
	11th to 20th	Night	12% 28% 3 19% 30% 7%	3810
	21st to end	Night	7% 27% 2 14% 44% 5%	3972
June	1st to 10th	Night	6% 61% 3 10% 15% 5%	1600
	11th to 20th	Night	2 61% 3 9% 14% 11%	1600
	21st to end	Night	5% 59% 1 3 13% 19%	1600
July	1st to 10th	Night	4 24% 1 2 13% 57%	3600
	11th to 20th	Night	12% 24% 1 3 8% 51%	3592
	21st to end	Night	10% 24% 1 16% 18% 31%	4006
August	1st to 10th	Night	9% 34% 1 5% 23% 27%	3026
	11th to 20th	Night	3 35% 4 11% 47%	3256
	21st to end	Night	6% 34% 1 8% 27% 24%	3608
September	1st to 10th	Night	4 55% 1 4 19% 16%	2000
	11th to 20th	Night	7% 56% 2 9% 18% 7%	2045
	21st to end	Night	7% 56% 1 9% 16% 12%	2200
October	1st to 10th	Night	8% 34% 2 6% 27% 24%	3820
	11th to 20th	Night	6% 35% 2 9% 37% 11%	3897
	21st to end	Night	13% 35% 2 14% 27% 9%	4341
November	1st to 10th	Night	6% 52% 5% 15% 16% 6%	2665
	11th to 20th	Night	10% 53% 3 13% 16% 3	2730
	21st to end	Night	12% 54% 6% 11% 13% 4	2746
December	1st to 10th	Night	12% 35% 7% 18% 19% 9%	4588
	11th to 20th	Night	21% 33% 10% 16% 17% 3	5176
	21st to end	Night	29% 28% 11% 17% 11% 3	6952

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	17% 21% 31% 21% 9% 3	200
	11th to 20th	Day	13% 21% 27% 22% 13% 4	245
	21st to end	Day	8% 20% 30% 28% 11% 3	440
February	1st to 10th	Day	19% 18% 29% 21% 10% 3	417
	11th to 20th	Day	13% 23% 32% 23% 8% 1	495
	21st to end	Day	10% 22% 35% 25% 7% 1	414
March	1st to 10th	Day	16% 26% 31% 20% 6% 1	500
	11th to 20th	Day	16% 26% 28% 19% 9% 2	500
	21st to end	Day	16% 25% 31% 20% 7% 1	550
April	1st to 10th	Day	13% 25% 31% 19% 10% 2	500
	11th to 20th	Day	18% 24% 28% 19% 8% 3	492
	21st to end	Day	12% 25% 30% 20% 9% 3	500
May	1st to 10th	Day	14% 25% 30% 20% 7% 4	503
	11th to 20th	Day	13% 23% 27% 22% 11% 4	528
	21st to end	Day	13% 17% 26% 24% 14% 6%	573
June	1st to 10th	Day	9% 19% 27% 24% 14% 5%	530
	11th to 20th	Day	9% 22% 33% 21% 10% 5%	530
	21st to end	Day	4 14% 33% 30% 14% 5%	530
July	1st to 10th	Day	3 14% 32% 30% 17% 4	526
	11th to 20th	Day	4 11% 30% 32% 17% 6%	524
	21st to end	Day	5% 10% 31% 27% 20% 6%	575
August	1st to 10th	Day	3 15% 30% 27% 20% 6%	527
	11th to 20th	Day	3 15% 30% 26% 17% 8%	517
	21st to end	Day	4 13% 31% 30% 17% 5%	583
September	1st to 10th	Day	5% 13% 28% 31% 18% 5%	530
	11th to 20th	Day	8% 16% 28% 30% 14% 5%	530
	21st to end	Day	6% 13% 30% 37% 10% 4	530
October	1st to 10th	Day	7% 15% 29% 27% 18% 4	530
	11th to 20th	Day	8% 15% 25% 31% 16% 5%	530
	21st to end	Day	9% 15% 28% 25% 19% 4	525
November	1st to 10th	Day	10% 16% 23% 22% 21% 8%	330
	11th to 20th	Day	10% 15% 25% 30% 14% 6%	330
	21st to end	Day	7% 13% 27% 29% 17% 5%	319
December	1st to 10th	Day	10% 17% 23% 22% 17% 11%	204
	11th to 20th	Day	10% 22% 28% 21% 15% 6%	200
	21st to end	Day	20% 13% 28% 23% 11% 4	220
January	1st to 10th	Night	6% 13% 34% 23% 15% 9%	800
	11th to 20th	Night	4 13% 28% 34% 14% 6%	800
	21st to end	Night	5% 8% 34% 31% 16% 6%	880
February	1st to 10th	Night	8% 11% 29% 33% 16% 3	800
	11th to 20th	Night	5% 13% 33% 29% 15% 5%	784
	21st to end	Night	5% 17% 29% 34% 12% 3	660
March	1st to 10th	Night	7% 15% 38% 26% 12% 3	800
	11th to 20th	Night	6% 19% 29% 25% 18% 3	800
	21st to end	Night	6% 19% 31% 30% 12% 1	880
April	1st to 10th	Night	7% 12% 33% 32% 15% 1	800
	11th to 20th	Night	7% 16% 28% 30% 16% 4	791
	21st to end	Night	7% 13% 29% 32% 16% 4	800
May	1st to 10th	Night	6% 12% 29% 33% 17% 3	679
	11th to 20th	Night	6% 9% 24% 42% 18% 1	484
	21st to end	Night	8% 5% 23% 38% 24% 3	433
June	1st to 10th	Night	5% 6% 22% 43% 22% 2	400
	11th to 20th	Night	2 6% 27% 41% 24% 1	400
	21st to end	Night	3 4 21% 47% 24% 3	400
July	1st to 10th	Night	1 2 18% 49% 28% 1	390
	11th to 20th	Night	2 1 19% 48% 29% 1	388
	21st to end	Night	3 2 13% 41% 36% 5%	502
August	1st to 10th	Night	3 1 18% 42% 33% 3	655
	11th to 20th	Night	2 4 20% 43% 26% 5%	792
	21st to end	Night	3 3 20% 44% 27% 4	880
September	1st to 10th	Night	1 3 21% 41% 29% 5%	800
	11th to 20th	Night	3 5% 20% 37% 29% 6%	800
	21st to end	Night	2 6% 21% 37% 27% 8%	800
October	1st to 10th	Night	5% 5% 22% 39% 21% 8%	800
	11th to 20th	Night	5% 7% 18% 40% 24% 6%	800
	21st to end	Night	4 6% 24% 31% 24% 11%	880
November	1st to 10th	Night	4 10% 24% 30% 24% 8%	820
	11th to 20th	Night	6% 9% 31% 33% 15% 6%	840
	21st to end	Night	8% 10% 25% 32% 21% 5%	840
December	1st to 10th	Night	5% 10% 32% 31% 17% 6%	808
	11th to 20th	Night	6% 13% 30% 32% 14% 6%	800
	21st to end	Night	9% 13% 33% 31% 11% 3	880

Page generated at 2025-11-29 22:00:54. Version ac07

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