Nelsons Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nelsons Bay Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	127380
	Night	16% 14% 5% 10% 5% 4 4 3 4 5% 29%	123000
Transparency	Day	54% 5% 10% 20% 10% 1	130928
	Night	57% 5% 10% 18% 9% 1	127529
Seeing	Day	25% 16% 31% 19% 7% 2	16383
	Night	24% 13% 30% 24% 8% 1	25349

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	11% 19% 10% 15% 7% 7% 3 3 4 4 17%	3158
	11th to 20th	Day	10% 19% 7% 18% 6% 6% 5% 4 5% 4 14%	3524
	21st to end	Day	13% 21% 9% 16% 5% 5% 4 3 3 4 17%	4466
February	1st to 10th	Day	13% 21% 6% 14% 8% 5% 4 3 4 5% 18%	1800
	11th to 20th	Day	11% 19% 8% 13% 7% 4 4 3 5% 4 23%	1753
	21st to end	Day	15% 21% 6% 13% 7% 5% 4 2 4 4 20%	1499
March	1st to 10th	Day	19% 16% 4 10% 4 5% 4 3 4 5% 25%	4920
	11th to 20th	Day	13% 14% 6% 11% 6% 4 4 3 6% 7% 25%	4980
	21st to end	Day	17% 17% 6% 9% 5% 4 3 2 3 4 29%	5465
April	1st to 10th	Day	20% 16% 4 8% 4 3 4 3 5% 6% 28%	2372
	11th to 20th	Day	22% 13% 3 6% 4 3 3 3 5% 5% 34%	2507
	21st to end	Day	22% 12% 4 7% 4 3 3 2 3 5% 33%	2600
May	1st to 10th	Day	18% 11% 3 7% 5% 5% 4 3 5% 6% 32%	6026
	11th to 20th	Day	14% 9% 3 7% 4 4 3 4 5% 7% 39%	6318
	21st to end	Day	23% 10% 3 6% 4 4 3 3 4 6% 35%	6949
June	1st to 10th	Day	20% 8% 3 7% 6% 4 4 4 5% 7% 33%	2895
	11th to 20th	Day	16% 6% 3 7% 5% 5% 4 4 6% 8% 38%	3000
	21st to end	Day	21% 8% 3 7% 4 5% 4 4 6% 7% 31%	3000
July	1st to 10th	Day	17% 6% 2 7% 5% 5% 5% 4 6% 8% 35%	6819
	11th to 20th	Day	15% 5% 2 6% 4 4 4 3 5% 7% 44%	6667
	21st to end	Day	11% 5% 2 5% 3 4 4 3 5% 8% 48%	6731
August	1st to 10th	Day	12% 7% 3 7% 5% 6% 6% 4 7% 9% 34%	4234
	11th to 20th	Day	19% 7% 2 6% 5% 4 4 4 6% 8% 35%	4174
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 44%	4555
September	1st to 10th	Day	16% 8% 3 7% 5% 3 4 3 4 7% 39%	2400
	11th to 20th	Day	16% 7% 2 5% 3 3 5% 3 5% 8% 43%	2289
	21st to end	Day	23% 9% 3 8% 4 5% 4 2 4 6% 33%	2157
October	1st to 10th	Day	19% 10% 3 7% 4 4 4 4 4 6% 35%	2000
	11th to 20th	Day	24% 16% 3 8% 5% 4 4 3 4 5% 25%	2000
	21st to end	Day	30% 16% 5% 8% 5% 4 3 2 4 3 20%	2178
November	1st to 10th	Day	20% 17% 4 9% 4 4 4 3 5% 6% 23%	1670
	11th to 20th	Day	19% 20% 6% 10% 5% 4 4 3 4 6% 20%	1656
	21st to end	Day	23% 23% 7% 11% 5% 6% 3 3 3 3 13%	1600
December	1st to 10th	Day	16% 24% 8% 17% 6% 5% 5% 3 3 3 10%	2600
	11th to 20th	Day	19% 19% 6% 13% 6% 5% 3 2 3 3 20%	2942
	21st to end	Day	21% 22% 8% 15% 5% 3 3 2 4 4 13%	3476
January	1st to 10th	Night	12% 19% 8% 15% 8% 6% 4 3 4 4 16%	6808
	11th to 20th	Night	11% 19% 6% 18% 8% 6% 4 3 4 4 15%	6361
	21st to end	Night	11% 22% 10% 16% 5% 6% 4 3 4 4 16%	6490
February	1st to 10th	Night	12% 18% 7% 12% 8% 5% 5% 3 5% 4 21%	2600
	11th to 20th	Night	12% 15% 8% 14% 6% 5% 4 3 5% 4 24%	2533
	21st to end	Night	13% 18% 6% 12% 7% 6% 4 3 4 5% 21%	2130
March	1st to 10th	Night	15% 15% 5% 10% 5% 4 4 3 5% 5% 29%	4919
	11th to 20th	Night	11% 16% 7% 10% 4 4 4 4 5% 7% 26%	4979
	21st to end	Night	19% 13% 5% 8% 6% 4 4 3 3 4 30%	5467
April	1st to 10th	Night	21% 15% 4 7% 4 4 3 4 4 5% 29%	2027
	11th to 20th	Night	19% 10% 3 7% 4 4 3 3 4 5% 37%	1858
	21st to end	Night	21% 12% 3 8% 5% 4 3 4 5% 5% 30%	1800
May	1st to 10th	Night	17% 9% 3 6% 4 4 4 4 6% 7% 36%	3900
	11th to 20th	Night	11% 7% 2 6% 4 4 4 4 6% 9% 42%	3575
	21st to end	Night	17% 10% 3 7% 5% 4 4 3 5% 7% 36%	3926
June	1st to 10th	Night	13% 10% 2 7% 6% 5% 5% 4 6% 6% 36%	1500
	11th to 20th	Night	16% 5% 2 5% 3 5% 4 4 5% 8% 43%	1400
	21st to end	Night	17% 8% 2 7% 4 4 3 4 6% 7% 37%	1400
July	1st to 10th	Night	16% 6% 2 7% 4 4 4 3 5% 7% 41%	3160
	11th to 20th	Night	16% 6% 2 5% 3 3 4 4 4 6% 47%	3292
	21st to end	Night	12% 6% 2 5% 4 3 3 3 4 8% 50%	4104
August	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 6% 8% 44%	2915
	11th to 20th	Night	16% 6% 2 6% 4 3 4 3 5% 7% 43%	2875
	21st to end	Night	15% 5% 2 5% 4 3 3 3 4 7% 49%	3309
September	1st to 10th	Night	15% 5% 2 5% 4 4 4 3 5% 7% 47%	2000
	11th to 20th	Night	13% 7% 2 5% 4 4 3 2 5% 7% 48%	2109
	21st to end	Night	18% 8% 2 7% 4 4 3 3 4 6% 41%	2242
October	1st to 10th	Night	18% 8% 3 7% 4 4 4 4 4 6% 38%	2400
	11th to 20th	Night	25% 13% 3 7% 4 6% 4 3 4 5% 25%	2400
	21st to end	Night	25% 15% 4 8% 5% 5% 3 3 4 5% 23%	2660
November	1st to 10th	Night	20% 16% 4 7% 4 4 4 3 4 5% 28%	2847
	11th to 20th	Night	18% 16% 6% 9% 5% 4 4 3 5% 5% 24%	2899
	21st to end	Night	23% 21% 5% 10% 6% 4 4 3 4 5% 15%	2800
December	1st to 10th	Night	16% 25% 7% 14% 7% 5% 5% 3 5% 4 10%	4580
	11th to 20th	Night	16% 23% 7% 14% 6% 4 3 3 3 4 16%	5233
	21st to end	Night	20% 21% 7% 16% 5% 4 3 3 3 4 15%	7502

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	72% 3 8% 12% 5%	3159
	11th to 20th	Day	73% 5% 6% 9% 7% 1	3554
	21st to end	Day	72% 4 5% 10% 8% 1	4466
February	1st to 10th	Day	69% 4 7% 13% 6% 1	1800
	11th to 20th	Day	65% 3 9% 13% 9% 1	1762
	21st to end	Day	69% 5% 6% 13% 7%	1499
March	1st to 10th	Day	63% 4 9% 8% 14% 2	4923
	11th to 20th	Day	58% 4 13% 15% 8% 2	4980
	21st to end	Day	60% 3 9% 18% 10% 2	5465
April	1st to 10th	Day	58% 5% 10% 16% 10% 2	2372
	11th to 20th	Day	53% 4 8% 21% 13% 1	2507
	21st to end	Day	55% 4 8% 16% 14% 3	2600
May	1st to 10th	Day	52% 5% 9% 19% 10% 5%	6035
	11th to 20th	Day	44% 6% 10% 23% 15% 1	6322
	21st to end	Day	52% 5% 9% 18% 14% 3	6949
June	1st to 10th	Day	50% 6% 9% 21% 13%	2895
	11th to 20th	Day	44% 6% 10% 25% 14% 1	3000
	21st to end	Day	51% 7% 11% 19% 11% 1	3000
July	1st to 10th	Day	46% 5% 14% 25% 9% 1	6746
	11th to 20th	Day	39% 4 12% 36% 9%	6586
	21st to end	Day	34% 4 14% 34% 14%	6537
August	1st to 10th	Day	45% 6% 12% 28% 9%	4221
	11th to 20th	Day	46% 6% 12% 26% 9%	4213
	21st to end	Day	37% 5% 12% 34% 10%	4555
September	1st to 10th	Day	45% 5% 13% 24% 12%	2400
	11th to 20th	Day	40% 6% 12% 30% 11%	2289
	21st to end	Day	54% 4 10% 23% 8%	2157
October	1st to 10th	Day	49% 5% 12% 24% 9% 1	3260
	11th to 20th	Day	55% 6% 11% 22% 6%	3240
	21st to end	Day	67% 4 11% 14% 5%	3492
November	1st to 10th	Day	62% 6% 11% 17% 5%	1670
	11th to 20th	Day	66% 6% 11% 11% 6% 1	1656
	21st to end	Day	76% 5% 9% 8% 3	1600
December	1st to 10th	Day	79% 4 9% 6% 1	2600
	11th to 20th	Day	71% 3 6% 13% 7%	2942
	21st to end	Day	77% 3 4 12% 4	3476
January	1st to 10th	Night	71% 4 7% 12% 5%	6808
	11th to 20th	Night	71% 6% 8% 7% 7%	6406
	21st to end	Night	73% 3 5% 9% 9% 1	6490
February	1st to 10th	Night	66% 5% 9% 13% 7% 1	2600
	11th to 20th	Night	63% 4 10% 15% 6% 1	2535
	21st to end	Night	65% 6% 8% 14% 6% 1	2130
March	1st to 10th	Night	56% 5% 9% 13% 13% 4	4929
	11th to 20th	Night	57% 5% 14% 15% 8% 1	4980
	21st to end	Night	58% 3 7% 18% 12% 2	5467
April	1st to 10th	Night	57% 5% 8% 18% 11% 2	2027
	11th to 20th	Night	51% 5% 6% 21% 15% 2	1858
	21st to end	Night	56% 5% 7% 14% 15% 2	1800
May	1st to 10th	Night	46% 6% 8% 23% 13% 4	3907
	11th to 20th	Night	38% 7% 12% 22% 19% 2	3576
	21st to end	Night	47% 5% 10% 23% 14% 2	3927
June	1st to 10th	Night	46% 8% 9% 22% 12% 3	1500
	11th to 20th	Night	39% 7% 10% 27% 18% 1	1400
	21st to end	Night	45% 7% 12% 23% 12% 1	1400
July	1st to 10th	Night	42% 4 14% 28% 10% 1	3125
	11th to 20th	Night	39% 4 12% 33% 12% 1	3250
	21st to end	Night	34% 5% 10% 37% 14%	3984
August	1st to 10th	Night	38% 5% 16% 25% 14% 2	2904
	11th to 20th	Night	41% 5% 18% 25% 10% 1	2902
	21st to end	Night	37% 4 12% 34% 12% 1	3309
September	1st to 10th	Night	37% 6% 14% 29% 12% 1	2000
	11th to 20th	Night	37% 6% 15% 29% 13%	2109
	21st to end	Night	44% 5% 14% 25% 12%	2242
October	1st to 10th	Night	42% 5% 9% 30% 14%	3920
	11th to 20th	Night	54% 4 13% 22% 7%	3899
	21st to end	Night	59% 5% 17% 13% 6%	4284
November	1st to 10th	Night	57% 5% 12% 19% 5%	2847
	11th to 20th	Night	61% 6% 12% 14% 6% 1	2899
	21st to end	Night	71% 7% 11% 8% 3	2800
December	1st to 10th	Night	78% 4 11% 6% 2	4580
	11th to 20th	Night	73% 4 6% 10% 8%	5233
	21st to end	Night	75% 3 6% 12% 4	7502

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	25% 27% 27% 16% 5% 1	200
	11th to 20th	Day	23% 22% 32% 19% 5%	227
	21st to end	Day	27% 17% 34% 17% 4 1	330
February	1st to 10th	Day	34% 26% 31% 7% 1 1	300
	11th to 20th	Day	26% 27% 36% 9% 1	300
	21st to end	Day	37% 24% 29% 9%	249
March	1st to 10th	Day	28% 22% 34% 13% 3	488
	11th to 20th	Day	23% 21% 38% 13% 4 1	500
	21st to end	Day	31% 21% 29% 15% 3 1	550
April	1st to 10th	Day	29% 20% 31% 14% 4 1	500
	11th to 20th	Day	37% 17% 22% 15% 8% 2	492
	21st to end	Day	30% 19% 31% 12% 6% 1	500
May	1st to 10th	Day	28% 15% 30% 18% 6% 2	503
	11th to 20th	Day	22% 17% 30% 19% 10% 2	527
	21st to end	Day	24% 16% 28% 21% 9% 2	573
June	1st to 10th	Day	22% 15% 34% 23% 6% 1	625
	11th to 20th	Day	20% 14% 35% 22% 8% 2	730
	21st to end	Day	25% 13% 35% 19% 8% 1	730
July	1st to 10th	Day	14% 12% 33% 29% 10% 1	721
	11th to 20th	Day	16% 13% 30% 28% 11% 3	656
	21st to end	Day	14% 12% 31% 28% 12% 3	576
August	1st to 10th	Day	18% 7% 27% 31% 12% 5%	526
	11th to 20th	Day	18% 5% 29% 25% 16% 7%	519
	21st to end	Day	17% 13% 30% 24% 12% 4	583
September	1st to 10th	Day	19% 12% 30% 23% 10% 5%	530
	11th to 20th	Day	16% 8% 29% 25% 14% 8%	530
	21st to end	Day	25% 12% 25% 25% 10% 3	530
October	1st to 10th	Day	25% 14% 27% 23% 10% 2	530
	11th to 20th	Day	29% 17% 36% 13% 4 2	530
	21st to end	Day	40% 14% 29% 14% 1	561
November	1st to 10th	Day	36% 17% 31% 12% 3	233
	11th to 20th	Day	31% 21% 36% 8% 3	214
	21st to end	Day	36% 20% 32% 10% 2 1	200
December	1st to 10th	Day	30% 20% 35% 14% 2 1	200
	11th to 20th	Day	33% 22% 33% 10% 3 1	200
	21st to end	Day	32% 22% 26% 16% 3 1	220
January	1st to 10th	Night	28% 16% 35% 17% 4 1	1000
	11th to 20th	Night	31% 15% 30% 20% 4 1	875
	21st to end	Night	23% 11% 34% 23% 8% 1	880
February	1st to 10th	Night	23% 22% 33% 18% 4	800
	11th to 20th	Night	21% 19% 34% 20% 5% 1	783
	21st to end	Night	23% 20% 34% 21% 2	660
March	1st to 10th	Night	23% 20% 33% 20% 4	800
	11th to 20th	Night	23% 21% 34% 18% 4 1	800
	21st to end	Night	22% 14% 33% 24% 7% 1	880
April	1st to 10th	Night	32% 12% 32% 19% 5%	800
	11th to 20th	Night	27% 18% 25% 22% 9% 1	674
	21st to end	Night	28% 12% 26% 24% 9%	600
May	1st to 10th	Night	22% 8% 29% 32% 9% 1	538
	11th to 20th	Night	18% 11% 33% 30% 8%	394
	21st to end	Night	17% 10% 33% 33% 7%	433
June	1st to 10th	Night	17% 9% 32% 33% 10%	400
	11th to 20th	Night	19% 10% 25% 34% 12%	400
	21st to end	Night	20% 8% 29% 31% 12% 1	400
July	1st to 10th	Night	15% 4 27% 41% 14%	390
	11th to 20th	Night	15% 5% 26% 35% 18% 1	388
	21st to end	Night	12% 6% 29% 36% 16% 1	515
August	1st to 10th	Night	15% 4 22% 39% 18% 1	597
	11th to 20th	Night	14% 4 21% 37% 20% 3	594
	21st to end	Night	19% 8% 24% 30% 17% 2	707
September	1st to 10th	Night	17% 7% 28% 29% 17% 2	800
	11th to 20th	Night	16% 7% 25% 27% 21% 3	800
	21st to end	Night	22% 8% 23% 30% 16% 2	800
October	1st to 10th	Night	21% 12% 29% 26% 11% 1	800
	11th to 20th	Night	32% 12% 28% 21% 7% 1	800
	21st to end	Night	33% 14% 30% 19% 4	880
November	1st to 10th	Night	32% 12% 30% 19% 6% 1	820
	11th to 20th	Night	28% 17% 31% 18% 6%	829
	21st to end	Night	35% 16% 28% 18% 3 1	800
December	1st to 10th	Night	32% 17% 31% 16% 4	800
	11th to 20th	Night	30% 16% 33% 16% 3 1	812
	21st to end	Night	33% 16% 29% 19% 3	1100

Page generated at 2025-11-15 00:56:01. Version ac07

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