Mount Nemo Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Nemo Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	14% 10% 3 9% 5% 5% 5% 4 6% 7% 33%	114347
	Night	14% 9% 4 8% 5% 5% 5% 4 6% 7% 34%	114864
Transparency	Day	47% 7% 14% 20% 11% 1	117987
	Night	47% 7% 13% 20% 12% 1	119387
Seeing	Day	18% 12% 26% 26% 14% 5%	15225
	Night	18% 11% 28% 29% 13% 2	24643

By Month

Percentage Cloud Cover NE (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	10% 14% 6% 14% 6% 5% 4 3 5% 5% 27%	3597
	11th to 20th	Day	9% 18% 8% 18% 7% 6% 5% 4 4 6% 14%	4008
	21st to end	Day	17% 15% 6% 15% 6% 5% 5% 3 4 5% 18%	4466
February	1st to 10th	Day	14% 15% 5% 12% 8% 6% 5% 4 6% 5% 21%	1800
	11th to 20th	Day	13% 14% 5% 11% 7% 5% 5% 4 5% 7% 23%	1753
	21st to end	Day	18% 12% 4 11% 5% 4 5% 4 5% 6% 25%	1559
March	1st to 10th	Day	17% 9% 4 11% 6% 4 4 3 5% 5% 31%	4932
	11th to 20th	Day	10% 11% 4 9% 6% 4 5% 4 6% 8% 31%	4980
	21st to end	Day	16% 10% 4 8% 5% 5% 5% 4 6% 5% 32%	5465
April	1st to 10th	Day	16% 12% 4 8% 5% 5% 5% 4 5% 5% 30%	2340
	11th to 20th	Day	17% 10% 2 8% 5% 4 5% 4 4 7% 36%	2469
	21st to end	Day	18% 9% 3 8% 5% 4 5% 4 5% 6% 34%	2600
May	1st to 10th	Day	17% 8% 3 8% 4 4 5% 4 5% 7% 35%	4238
	11th to 20th	Day	15% 10% 3 8% 6% 5% 4 4 6% 7% 31%	4434
	21st to end	Day	11% 5% 2 6% 4 4 4 3 6% 8% 46%	4907
June	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 7% 40%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 40%	2660
	21st to end	Day	12% 5% 2 6% 5% 5% 6% 4 6% 8% 41%	2792
July	1st to 10th	Day	9% 4 2 5% 4 5% 6% 5% 8% 10% 41%	4468
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 4 8% 10% 44%	4386
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	4466
August	1st to 10th	Day	16% 6% 2 6% 4 4 4 4 6% 8% 38%	4104
	11th to 20th	Day	9% 6% 3 7% 6% 5% 6% 5% 7% 10% 37%	4044
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	4379
September	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 3 6% 8% 42%	2280
	11th to 20th	Day	10% 6% 2 5% 5% 4 4 3 6% 8% 47%	2138
	21st to end	Day	14% 6% 2 7% 6% 6% 6% 5% 6% 8% 34%	2069
October	1st to 10th	Day	18% 9% 2 6% 5% 4 5% 3 6% 7% 35%	1900
	11th to 20th	Day	18% 9% 3 8% 6% 5% 4 4 5% 7% 31%	1900
	21st to end	Day	22% 12% 2 8% 6% 6% 5% 4 5% 6% 25%	2090
November	1st to 10th	Day	12% 9% 3 9% 5% 6% 6% 5% 6% 7% 32%	1717
	11th to 20th	Day	14% 11% 4 10% 6% 5% 6% 4 6% 8% 26%	1600
	21st to end	Day	16% 15% 5% 7% 6% 5% 5% 5% 7% 6% 23%	1600
December	1st to 10th	Day	16% 17% 5% 11% 7% 5% 6% 4 7% 7% 15%	2600
	11th to 20th	Day	22% 15% 4 9% 5% 4 4 4 5% 6% 22%	2986
	21st to end	Day	25% 17% 6% 12% 6% 5% 4 3 4 4 13%	3960
January	1st to 10th	Night	13% 11% 4 12% 6% 5% 5% 4 5% 7% 28%	6349
	11th to 20th	Night	11% 12% 5% 15% 9% 6% 6% 4 6% 7% 18%	5859
	21st to end	Night	16% 12% 6% 11% 6% 5% 4 4 5% 7% 25%	6490
February	1st to 10th	Night	14% 12% 5% 10% 7% 5% 5% 4 6% 7% 25%	2600
	11th to 20th	Night	13% 11% 5% 11% 7% 5% 5% 4 5% 7% 27%	2533
	21st to end	Night	13% 11% 5% 10% 6% 5% 6% 4 6% 7% 27%	2067
March	1st to 10th	Night	16% 7% 4 8% 5% 4 5% 4 6% 7% 34%	4908
	11th to 20th	Night	9% 8% 4 7% 4 4 5% 4 7% 9% 39%	4979
	21st to end	Night	18% 9% 3 7% 4 4 4 3 4 5% 39%	5467
April	1st to 10th	Night	18% 9% 4 7% 5% 5% 4 5% 5% 6% 33%	2059
	11th to 20th	Night	16% 7% 3 8% 6% 5% 4 3 5% 6% 37%	1897
	21st to end	Night	15% 8% 3 6% 5% 5% 4 3 6% 6% 39%	1800
May	1st to 10th	Night	18% 7% 3 7% 4 4 4 3 5% 6% 39%	2923
	11th to 20th	Night	15% 9% 3 8% 5% 4 4 4 7% 8% 34%	2676
	21st to end	Night	10% 4 2 5% 4 4 4 4 6% 8% 50%	2784
June	1st to 10th	Night	11% 5% 2 6% 5% 5% 5% 4 6% 8% 43%	1520
	11th to 20th	Night	10% 5% 2 5% 4 5% 4 4 8% 8% 43%	1520
	21st to end	Night	10% 5% 2 5% 4 5% 4 5% 5% 7% 47%	1370
July	1st to 10th	Night	6% 4 2 6% 4 4 4 4 6% 8% 53%	2475
	11th to 20th	Night	10% 3 2 5% 4 4 4 3 6% 9% 51%	2570
	21st to end	Night	6% 4 2 5% 4 4 4 4 6% 9% 51%	3087
August	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 46%	2825
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 6% 8% 49%	2785
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 6% 7% 49%	3244
September	1st to 10th	Night	11% 5% 2 6% 4 5% 4 4 6% 7% 47%	1900
	11th to 20th	Night	10% 4 1 4 4 3 3 3 5% 8% 55%	2039
	21st to end	Night	15% 6% 3 7% 4 5% 4 4 6% 6% 41%	2110
October	1st to 10th	Night	16% 7% 2 6% 4 4 4 4 6% 6% 42%	2280
	11th to 20th	Night	16% 8% 3 7% 5% 5% 5% 4 6% 6% 36%	2280
	21st to end	Night	19% 9% 3 8% 4 5% 4 4 5% 6% 32%	2508
November	1st to 10th	Night	14% 7% 3 6% 5% 4 4 4 6% 7% 40%	2576
	11th to 20th	Night	14% 11% 3 7% 5% 5% 5% 5% 6% 8% 29%	2800
	21st to end	Night	18% 9% 3 8% 5% 5% 5% 4 6% 8% 27%	2800
December	1st to 10th	Night	12% 14% 5% 10% 6% 6% 6% 5% 8% 8% 20%	4580
	11th to 20th	Night	17% 16% 4 10% 5% 5% 5% 4 5% 6% 24%	5208
	21st to end	Night	25% 16% 6% 11% 5% 4 4 3 5% 5% 15%	6996

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	55% 10% 12% 11% 11% 1	3598
	11th to 20th	Day	69% 7% 11% 9% 4	4042
	21st to end	Day	66% 7% 12% 9% 6%	4466
February	1st to 10th	Day	64% 6% 10% 15% 4 1	1800
	11th to 20th	Day	61% 5% 11% 15% 7% 1	1762
	21st to end	Day	60% 5% 12% 15% 7% 1	1559
March	1st to 10th	Day	54% 6% 14% 15% 10% 1	4935
	11th to 20th	Day	48% 6% 18% 18% 7% 2	4980
	21st to end	Day	49% 6% 12% 21% 11% 1	5465
April	1st to 10th	Day	54% 4 11% 18% 12% 1	2340
	11th to 20th	Day	48% 5% 12% 20% 13% 1	2469
	21st to end	Day	50% 5% 12% 18% 12% 3	2600
May	1st to 10th	Day	46% 6% 14% 19% 12% 3	4250
	11th to 20th	Day	50% 9% 12% 19% 8% 1	4438
	21st to end	Day	35% 7% 13% 23% 19% 4	4907
June	1st to 10th	Day	41% 7% 11% 23% 16% 2	2660
	11th to 20th	Day	39% 7% 14% 23% 16% 1	2660
	21st to end	Day	39% 6% 14% 24% 17% 1	2792
July	1st to 10th	Day	33% 7% 16% 27% 15% 2	4398
	11th to 20th	Day	31% 8% 16% 31% 13% 1	4311
	21st to end	Day	34% 8% 15% 22% 19% 1	4324
August	1st to 10th	Day	42% 8% 14% 22% 13% 1	4091
	11th to 20th	Day	39% 9% 17% 24% 10% 1	4083
	21st to end	Day	34% 8% 13% 32% 13% 1	4379
September	1st to 10th	Day	40% 7% 14% 27% 12% 1	2280
	11th to 20th	Day	33% 6% 14% 30% 16%	2138
	21st to end	Day	44% 7% 14% 25% 9% 1	2069
October	1st to 10th	Day	37% 7% 17% 24% 14% 1	3160
	11th to 20th	Day	40% 6% 14% 24% 16%	3140
	21st to end	Day	49% 6% 15% 22% 7%	3428
November	1st to 10th	Day	46% 7% 14% 23% 9% 1	1717
	11th to 20th	Day	53% 7% 16% 14% 9% 1	1600
	21st to end	Day	58% 6% 15% 15% 4 1	1600
December	1st to 10th	Day	64% 9% 14% 9% 3	2600
	11th to 20th	Day	61% 6% 13% 10% 10%	2986
	21st to end	Day	71% 9% 8% 7% 5%	3960
January	1st to 10th	Night	52% 9% 12% 18% 8% 1	6349
	11th to 20th	Night	61% 8% 14% 13% 4	5900
	21st to end	Night	58% 7% 11% 14% 9% 1	6490
February	1st to 10th	Night	59% 6% 12% 17% 5% 1	2600
	11th to 20th	Night	55% 6% 12% 17% 9% 1	2535
	21st to end	Night	55% 5% 13% 18% 8% 1	2067
March	1st to 10th	Night	47% 8% 15% 18% 11% 2	4918
	11th to 20th	Night	40% 5% 21% 21% 12% 1	4980
	21st to end	Night	47% 5% 10% 19% 17% 2	5467
April	1st to 10th	Night	52% 5% 11% 18% 12% 2	2059
	11th to 20th	Night	47% 5% 9% 21% 16% 3	1897
	21st to end	Night	46% 5% 10% 20% 16% 3	1800
May	1st to 10th	Night	47% 6% 11% 17% 15% 4	2930
	11th to 20th	Night	47% 11% 14% 19% 8% 2	2677
	21st to end	Night	30% 7% 14% 23% 23% 3	2785
June	1st to 10th	Night	38% 7% 10% 24% 19% 1	1520
	11th to 20th	Night	36% 6% 12% 25% 19% 2	1520
	21st to end	Night	35% 7% 14% 25% 18% 1	1370
July	1st to 10th	Night	29% 6% 13% 25% 23% 4	2435
	11th to 20th	Night	29% 7% 14% 31% 17% 2	2522
	21st to end	Night	28% 8% 17% 25% 19% 2	2988
August	1st to 10th	Night	36% 7% 12% 27% 16% 2	2814
	11th to 20th	Night	34% 6% 18% 29% 12% 1	2812
	21st to end	Night	30% 7% 14% 31% 16% 1	3244
September	1st to 10th	Night	37% 6% 13% 27% 16% 2	1900
	11th to 20th	Night	29% 5% 12% 34% 19% 1	2039
	21st to end	Night	43% 6% 12% 26% 12% 1	2110
October	1st to 10th	Night	32% 5% 11% 28% 23% 1	3800
	11th to 20th	Night	43% 5% 12% 25% 15%	3779
	21st to end	Night	39% 6% 15% 26% 12% 1	4120
November	1st to 10th	Night	42% 6% 13% 25% 14% 1	2576
	11th to 20th	Night	49% 8% 14% 19% 10% 1	2800
	21st to end	Night	54% 7% 17% 17% 5%	2800
December	1st to 10th	Night	56% 8% 18% 14% 4	4580
	11th to 20th	Night	57% 7% 11% 14% 10% 1	5208
	21st to end	Night	67% 8% 11% 9% 4	6996

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	18% 19% 34% 21% 8% 2	200
	11th to 20th	Day	25% 18% 37% 15% 5% 1	297
	21st to end	Day	24% 19% 31% 21% 5%	330
February	1st to 10th	Day	25% 18% 37% 16% 4	300
	11th to 20th	Day	23% 22% 36% 17% 2	300
	21st to end	Day	26% 26% 30% 15% 1	253
March	1st to 10th	Day	20% 15% 34% 23% 7%	500
	11th to 20th	Day	21% 18% 33% 20% 7% 1	500
	21st to end	Day	26% 13% 29% 23% 8% 1	550
April	1st to 10th	Day	20% 16% 28% 27% 8% 1	500
	11th to 20th	Day	25% 16% 25% 22% 9% 3	492
	21st to end	Day	22% 13% 31% 22% 8% 3	500
May	1st to 10th	Day	20% 12% 26% 26% 12% 4	503
	11th to 20th	Day	17% 9% 24% 28% 17% 5%	527
	21st to end	Day	13% 6% 24% 31% 18% 8%	549
June	1st to 10th	Day	14% 8% 26% 33% 15% 3	490
	11th to 20th	Day	12% 8% 23% 31% 21% 5%	490
	21st to end	Day	14% 10% 24% 32% 15% 5%	622
July	1st to 10th	Day	8% 4 19% 35% 23% 11%	500
	11th to 20th	Day	14% 5% 18% 30% 23% 11%	484
	21st to end	Day	9% 5% 20% 38% 21% 8%	532
August	1st to 10th	Day	11% 5% 17% 31% 23% 13%	486
	11th to 20th	Day	9% 6% 16% 30% 25% 13%	479
	21st to end	Day	11% 5% 20% 32% 21% 11%	539
September	1st to 10th	Day	11% 9% 19% 28% 21% 12%	490
	11th to 20th	Day	10% 4 15% 28% 27% 16%	490
	21st to end	Day	15% 11% 24% 26% 16% 8%	490
October	1st to 10th	Day	22% 12% 20% 22% 16% 8%	490
	11th to 20th	Day	19% 15% 29% 25% 11% 2	490
	21st to end	Day	29% 12% 28% 21% 9% 1	539
November	1st to 10th	Day	19% 21% 28% 20% 10% 2	293
	11th to 20th	Day	22% 21% 30% 20% 7% 2	200
	21st to end	Day	24% 18% 35% 18% 5% 1	200
December	1st to 10th	Day	20% 23% 32% 20% 5% 1	200
	11th to 20th	Day	26% 27% 27% 18% 3	200
	21st to end	Day	27% 17% 38% 14% 3 1	220
January	1st to 10th	Night	19% 18% 36% 20% 6% 1	800
	11th to 20th	Night	24% 18% 33% 20% 4	800
	21st to end	Night	19% 14% 37% 24% 6%	880
February	1st to 10th	Night	21% 17% 38% 21% 3	800
	11th to 20th	Night	20% 16% 35% 25% 3	783
	21st to end	Night	19% 17% 36% 24% 4	660
March	1st to 10th	Night	18% 13% 34% 28% 7%	800
	11th to 20th	Night	19% 14% 36% 24% 7%	800
	21st to end	Night	21% 11% 30% 29% 9% 1	880
April	1st to 10th	Night	21% 11% 32% 31% 6%	800
	11th to 20th	Night	20% 11% 31% 27% 10% 1	713
	21st to end	Night	19% 7% 26% 37% 11%	600
May	1st to 10th	Night	18% 5% 25% 34% 16% 3	600
	11th to 20th	Night	16% 5% 24% 38% 15% 2	456
	21st to end	Night	13% 5% 22% 33% 25% 2	423
June	1st to 10th	Night	13% 4 27% 39% 16% 2	380
	11th to 20th	Night	12% 6% 24% 40% 17% 1	380
	21st to end	Night	11% 4 21% 42% 21%	380
July	1st to 10th	Night	6% 2 20% 46% 23% 2	370
	11th to 20th	Night	9% 3 20% 42% 23% 3	392
	21st to end	Night	8% 2 16% 46% 25% 3	624
August	1st to 10th	Night	9% 3 15% 42% 28% 4	567
	11th to 20th	Night	8% 2 14% 37% 35% 4	564
	21st to end	Night	12% 3 19% 36% 28% 4	695
September	1st to 10th	Night	12% 6% 18% 34% 25% 6%	760
	11th to 20th	Night	11% 4 17% 32% 29% 6%	760
	21st to end	Night	17% 6% 20% 29% 24% 5%	760
October	1st to 10th	Night	19% 8% 24% 26% 20% 3	760
	11th to 20th	Night	19% 12% 29% 29% 10% 2	760
	21st to end	Night	25% 13% 27% 29% 6%	836
November	1st to 10th	Night	19% 13% 28% 29% 10% 2	780
	11th to 20th	Night	20% 15% 33% 24% 8% 2	800
	21st to end	Night	23% 14% 34% 23% 6%	800
December	1st to 10th	Night	24% 17% 33% 20% 6% 1	800
	11th to 20th	Night	23% 19% 36% 18% 5%	800
	21st to end	Night	28% 16% 31% 22% 4	880

Page generated at 2025-05-23 09:20:58. Version ac07

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