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 2024-01-12.

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	18% 12% 4 7% 6% 3 4 4 5% 6% 30%	74785
	Night	17% 13% 5% 8% 6% 3 4 4 4 6% 29%	74916
Transparency	Day	55% 6% 10% 18% 10% 1	74485
	Night	57% 6% 10% 17% 9% 1	74752
Seeing	Day	25% 16% 30% 19% 8% 2	13887
	Night	25% 13% 30% 24% 8% 1	23056

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	14% 23% 10% 13% 7% 3 3 3 4 4 15%	1328
	11th to 20th	Day	16% 18% 9% 12% 6% 3 5% 3 4 4 21%	1427
	21st to end	Day	15% 17% 7% 12% 7% 5% 4 5% 4 5% 20%	1782
February	1st to 10th	Day	13% 19% 7% 13% 9% 3 5% 4 4 5% 17%	1620
	11th to 20th	Day	12% 19% 8% 10% 7% 3 4 4 4 5% 24%	1573
	21st to end	Day	16% 20% 7% 10% 9% 3 5% 3 3 5% 20%	1344
March	1st to 10th	Day	15% 14% 5% 8% 7% 4 6% 4 4 5% 27%	1965
	11th to 20th	Day	14% 13% 6% 9% 9% 4 4 4 5% 4 29%	1980
	21st to end	Day	18% 11% 4 7% 5% 3 4 4 4 5% 35%	2178
April	1st to 10th	Day	21% 15% 5% 8% 5% 2 4 4 5% 6% 27%	2134
	11th to 20th	Day	21% 11% 4 5% 5% 2 3 3 5% 6% 35%	2253
	21st to end	Day	22% 12% 4 6% 5% 2 4 3 3 6% 33%	2340
May	1st to 10th	Day	20% 9% 3 6% 6% 3 4 4 6% 7% 33%	2394
	11th to 20th	Day	19% 9% 3 6% 5% 3 4 5% 5% 7% 35%	2478
	21st to end	Day	16% 7% 2 5% 4 3 4 5% 5% 8% 40%	2725
June	1st to 10th	Day	21% 7% 3 6% 6% 3 4 5% 5% 7% 34%	2605
	11th to 20th	Day	16% 6% 3 5% 5% 3 5% 5% 6% 8% 38%	2700
	21st to end	Day	20% 7% 3 6% 5% 3 5% 5% 6% 7% 32%	2700
July	1st to 10th	Day	14% 5% 2 5% 5% 4 6% 4 6% 7% 43%	2700
	11th to 20th	Day	15% 6% 3 6% 6% 4 5% 5% 6% 8% 37%	2645
	21st to end	Day	15% 6% 3 6% 5% 2 5% 6% 6% 8% 37%	2672
August	1st to 10th	Day	15% 7% 3 6% 6% 3 5% 5% 6% 9% 36%	2327
	11th to 20th	Day	16% 6% 2 6% 7% 3 5% 6% 6% 7% 36%	2303
	21st to end	Day	16% 6% 3 6% 6% 3 5% 5% 6% 8% 36%	2514
September	1st to 10th	Day	16% 8% 3 6% 6% 3 4 4 4 8% 38%	2160
	11th to 20th	Day	18% 7% 2 5% 4 3 5% 5% 5% 8% 40%	2061
	21st to end	Day	25% 7% 3 7% 5% 4 4 4 4 6% 32%	1942
October	1st to 10th	Day	21% 10% 4 6% 5% 3 4 5% 4 6% 33%	1800
	11th to 20th	Day	26% 15% 3 7% 7% 3 4 4 4 5% 23%	1800
	21st to end	Day	31% 15% 5% 6% 6% 3 4 3 4 4 19%	1961
November	1st to 10th	Day	20% 16% 5% 7% 6% 3 4 4 5% 6% 24%	1508
	11th to 20th	Day	21% 17% 6% 9% 6% 3 5% 4 4 6% 20%	1520
	21st to end	Day	23% 21% 8% 10% 6% 4 3 3 3 4 14%	1520
December	1st to 10th	Day	18% 24% 8% 11% 6% 3 6% 5% 4 4 12%	1520
	11th to 20th	Day	20% 18% 7% 11% 7% 3 4 4 4 4 19%	1875
	21st to end	Day	22% 23% 9% 13% 7% 3 3 2 3 3 13%	2431
January	1st to 10th	Night	17% 21% 9% 11% 7% 3 4 4 5% 4 16%	2838
	11th to 20th	Night	17% 21% 6% 10% 8% 3 5% 4 4 4 18%	2566
	21st to end	Night	13% 15% 7% 11% 8% 4 5% 4 4 5% 25%	2574
February	1st to 10th	Night	13% 16% 9% 11% 9% 4 5% 5% 4 4 19%	2340
	11th to 20th	Night	12% 14% 8% 12% 7% 4 5% 4 4 5% 25%	2273
	21st to end	Night	14% 17% 7% 11% 8% 5% 5% 4 4 4 22%	1911
March	1st to 10th	Night	13% 13% 6% 8% 7% 3 4 4 4 6% 32%	1981
	11th to 20th	Night	14% 14% 5% 9% 5% 3 4 4 4 6% 31%	1979
	21st to end	Night	15% 11% 4 6% 6% 3 4 4 4 5% 38%	2178
April	1st to 10th	Night	22% 14% 5% 6% 5% 3 4 4 4 6% 29%	1825
	11th to 20th	Night	19% 10% 4 7% 5% 3 4 3 3 4 38%	1670
	21st to end	Night	21% 12% 3 6% 6% 3 4 4 4 5% 31%	1620
May	1st to 10th	Night	19% 7% 3 4 5% 3 4 5% 5% 7% 39%	1565
	11th to 20th	Night	15% 7% 2 5% 5% 4 5% 5% 6% 8% 39%	1415
	21st to end	Night	13% 6% 3 6% 6% 2 4 4 5% 8% 43%	1550
June	1st to 10th	Night	13% 9% 3 6% 7% 3 5% 5% 5% 6% 37%	1350
	11th to 20th	Night	16% 5% 3 5% 3 3 5% 4 6% 8% 44%	1260
	21st to end	Night	16% 7% 3 6% 4 3 3 5% 7% 8% 38%	1260
July	1st to 10th	Night	12% 5% 2 4 3 2 4 4 6% 9% 50%	1260
	11th to 20th	Night	13% 5% 3 5% 5% 3 4 6% 5% 8% 44%	1314
	21st to end	Night	11% 5% 2 4 6% 3 5% 5% 6% 8% 43%	1643
August	1st to 10th	Night	14% 5% 2 5% 4 3 5% 5% 6% 7% 44%	1611
	11th to 20th	Night	13% 4 2 5% 6% 3 5% 5% 5% 8% 44%	1598
	21st to end	Night	16% 6% 3 5% 6% 4 4 4 5% 6% 42%	1823
September	1st to 10th	Night	14% 5% 2 4 5% 3 4 4 5% 7% 47%	1800
	11th to 20th	Night	15% 7% 3 4 4 3 4 3 5% 7% 45%	1897
	21st to end	Night	18% 7% 2 6% 4 3 4 4 4 7% 40%	2017
October	1st to 10th	Night	19% 9% 4 5% 6% 3 4 4 4 6% 36%	2160
	11th to 20th	Night	26% 12% 3 7% 5% 4 5% 4 4 5% 24%	2160
	21st to end	Night	27% 15% 4 7% 6% 4 4 3 4 5% 22%	2394
November	1st to 10th	Night	21% 14% 4 7% 5% 3 4 4 5% 5% 29%	2569
	11th to 20th	Night	19% 14% 6% 7% 7% 3 4 5% 5% 5% 25%	2660
	21st to end	Night	23% 19% 6% 8% 7% 3 4 4 4 5% 16%	2660
December	1st to 10th	Night	18% 23% 10% 11% 8% 3 4 3 3 3 13%	2660
	11th to 20th	Night	18% 22% 8% 10% 7% 3 4 3 3 4 16%	3299
	21st to end	Night	21% 21% 8% 13% 8% 3 3 3 3 4 13%	5236

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	74% 4 8% 11% 4	1329
	11th to 20th	Day	68% 5% 6% 10% 9% 2	1427
	21st to end	Day	68% 4 6% 11% 10% 1	1782
February	1st to 10th	Day	71% 4 7% 12% 6% 1	1620
	11th to 20th	Day	64% 3 9% 13% 9% 1	1582
	21st to end	Day	69% 5% 6% 13% 7%	1344
March	1st to 10th	Day	60% 5% 11% 12% 11% 1	1968
	11th to 20th	Day	58% 4 9% 17% 10% 2	1980
	21st to end	Day	52% 4 8% 17% 14% 4	2178
April	1st to 10th	Day	59% 5% 11% 14% 10% 2	2134
	11th to 20th	Day	52% 4 9% 22% 13% 1	2253
	21st to end	Day	55% 4 7% 16% 14% 3	2340
May	1st to 10th	Day	50% 6% 9% 19% 14% 3	2394
	11th to 20th	Day	49% 6% 9% 21% 13% 2	2482
	21st to end	Day	42% 7% 12% 22% 13% 3	2725
June	1st to 10th	Day	49% 6% 10% 22% 13%	2605
	11th to 20th	Day	44% 7% 9% 24% 14% 1	2700
	21st to end	Day	50% 7% 10% 20% 11% 1	2700
July	1st to 10th	Day	38% 8% 12% 27% 14% 1	2626
	11th to 20th	Day	43% 6% 12% 27% 11% 1	2564
	21st to end	Day	43% 8% 11% 27% 11%	2523
August	1st to 10th	Day	45% 8% 12% 24% 11% 1	2314
	11th to 20th	Day	45% 9% 13% 24% 9% 1	2303
	21st to end	Day	46% 7% 13% 23% 10%	2514
September	1st to 10th	Day	45% 6% 12% 24% 12%	2160
	11th to 20th	Day	43% 7% 12% 28% 9%	2061
	21st to end	Day	55% 5% 10% 24% 6%	1942
October	1st to 10th	Day	52% 7% 11% 18% 11% 1	1800
	11th to 20th	Day	65% 6% 11% 12% 5% 1	1800
	21st to end	Day	70% 5% 8% 10% 6%	1961
November	1st to 10th	Day	61% 6% 12% 16% 5%	1508
	11th to 20th	Day	65% 7% 11% 10% 6% 1	1520
	21st to end	Day	76% 5% 9% 8% 3	1520
December	1st to 10th	Day	74% 6% 9% 8% 2 1	1520
	11th to 20th	Day	71% 5% 7% 11% 5%	1875
	21st to end	Day	79% 3 6% 10% 2	2431
January	1st to 10th	Night	72% 5% 9% 9% 5%	2838
	11th to 20th	Night	70% 6% 7% 10% 7% 1	2566
	21st to end	Night	63% 3 7% 15% 10% 1	2574
February	1st to 10th	Night	68% 5% 9% 11% 6% 1	2340
	11th to 20th	Night	62% 4 11% 16% 6% 1	2275
	21st to end	Night	65% 5% 8% 14% 6% 1	1911
March	1st to 10th	Night	54% 5% 11% 15% 13% 2	1991
	11th to 20th	Night	54% 6% 11% 16% 11% 2	1980
	21st to end	Night	49% 5% 9% 19% 14% 3	2178
April	1st to 10th	Night	57% 5% 8% 17% 11% 2	1825
	11th to 20th	Night	50% 5% 6% 21% 16% 2	1670
	21st to end	Night	55% 5% 7% 14% 15% 3	1620
May	1st to 10th	Night	44% 6% 10% 22% 15% 2	1565
	11th to 20th	Night	42% 6% 10% 21% 18% 3	1416
	21st to end	Night	39% 7% 13% 22% 16% 3	1551
June	1st to 10th	Night	46% 8% 9% 22% 11% 3	1350
	11th to 20th	Night	38% 6% 10% 27% 18% 1	1260
	21st to end	Night	44% 7% 13% 23% 12% 2	1260
July	1st to 10th	Night	31% 6% 13% 29% 18% 3	1225
	11th to 20th	Night	37% 8% 13% 28% 13% 1	1272
	21st to end	Night	38% 9% 14% 26% 12% 1	1552
August	1st to 10th	Night	37% 8% 16% 25% 13% 1	1600
	11th to 20th	Night	37% 8% 17% 26% 11% 1	1598
	21st to end	Night	43% 7% 14% 24% 11% 1	1823
September	1st to 10th	Night	37% 7% 13% 28% 13% 1	1800
	11th to 20th	Night	40% 6% 15% 28% 11%	1897
	21st to end	Night	44% 6% 15% 27% 9%	2017
October	1st to 10th	Night	49% 7% 10% 21% 12%	2160
	11th to 20th	Night	63% 6% 10% 16% 5%	2160
	21st to end	Night	67% 6% 8% 12% 6%	2394
November	1st to 10th	Night	56% 6% 13% 20% 5%	2569
	11th to 20th	Night	60% 6% 13% 14% 6% 1	2660
	21st to end	Night	71% 7% 11% 8% 3	2660
December	1st to 10th	Night	76% 5% 8% 8% 3	2660
	11th to 20th	Night	72% 5% 7% 11% 5%	3299
	21st to end	Night	78% 4 7% 9% 3	5236

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	27% 28% 23% 15% 6% 1	180
	11th to 20th	Day	23% 21% 33% 18% 5%	186
	21st to end	Day	28% 16% 35% 16% 4 1	264
February	1st to 10th	Day	35% 26% 28% 8% 1 2	240
	11th to 20th	Day	27% 27% 36% 9% 1	240
	21st to end	Day	38% 26% 27% 8% 1	198
March	1st to 10th	Day	27% 22% 34% 14% 3	408
	11th to 20th	Day	23% 20% 37% 13% 5% 1	420
	21st to end	Day	31% 19% 30% 16% 3 1	462
April	1st to 10th	Day	27% 20% 32% 16% 5% 1	420
	11th to 20th	Day	34% 18% 21% 16% 8% 2	412
	21st to end	Day	29% 19% 32% 13% 6% 1	420
May	1st to 10th	Day	28% 15% 28% 20% 7% 3	423
	11th to 20th	Day	24% 17% 29% 17% 10% 3	447
	21st to end	Day	21% 17% 29% 21% 10% 2	485
June	1st to 10th	Day	22% 15% 34% 22% 6% 1	535
	11th to 20th	Day	19% 14% 33% 23% 9% 2	630
	21st to end	Day	23% 13% 34% 20% 9% 1	630
July	1st to 10th	Day	13% 12% 32% 31% 10% 2	621
	11th to 20th	Day	17% 13% 26% 29% 12% 3	563
	21st to end	Day	14% 13% 30% 29% 12% 3	488
August	1st to 10th	Day	20% 7% 27% 29% 11% 6%	446
	11th to 20th	Day	17% 5% 27% 26% 17% 8%	439
	21st to end	Day	19% 12% 30% 24% 12% 4	495
September	1st to 10th	Day	18% 13% 30% 24% 10% 5%	450
	11th to 20th	Day	18% 10% 32% 24% 12% 5%	450
	21st to end	Day	27% 13% 24% 24% 10% 2	450
October	1st to 10th	Day	27% 13% 25% 22% 10% 2	450
	11th to 20th	Day	31% 17% 34% 12% 4 2	450
	21st to end	Day	41% 15% 29% 13% 1	476
November	1st to 10th	Day	33% 19% 32% 12% 3 1	191
	11th to 20th	Day	31% 21% 36% 9% 3 1	180
	21st to end	Day	36% 21% 32% 9% 1 1	180
December	1st to 10th	Day	29% 21% 36% 13% 2 1	180
	11th to 20th	Day	32% 23% 32% 10% 3 1	180
	21st to end	Day	31% 23% 27% 15% 3 1	198
January	1st to 10th	Night	29% 16% 34% 17% 4 1	950
	11th to 20th	Night	32% 15% 29% 20% 4 1	798
	21st to end	Night	22% 11% 34% 22% 8% 1	792
February	1st to 10th	Night	23% 22% 33% 18% 3	720
	11th to 20th	Night	21% 19% 33% 21% 6% 1	703
	21st to end	Night	24% 20% 32% 22% 3	592
March	1st to 10th	Night	23% 19% 33% 20% 4	720
	11th to 20th	Night	24% 20% 32% 18% 5% 1	720
	21st to end	Night	21% 14% 33% 25% 7% 1	792
April	1st to 10th	Night	32% 11% 32% 20% 5%	720
	11th to 20th	Night	27% 17% 25% 22% 9% 1	606
	21st to end	Night	28% 11% 25% 26% 9%	540
May	1st to 10th	Night	23% 7% 27% 33% 10% 1	485
	11th to 20th	Night	19% 11% 31% 31% 8%	354
	21st to end	Night	15% 10% 35% 32% 7%	389
June	1st to 10th	Night	16% 8% 33% 33% 11%	360
	11th to 20th	Night	19% 9% 24% 34% 13%	360
	21st to end	Night	19% 7% 28% 32% 13% 1	360
July	1st to 10th	Night	14% 3 27% 40% 15%	350
	11th to 20th	Night	15% 5% 26% 34% 19% 1	348
	21st to end	Night	13% 6% 30% 35% 15% 1	463
August	1st to 10th	Night	17% 3 23% 37% 19% 1	537
	11th to 20th	Night	13% 4 21% 38% 20% 4	534
	21st to end	Night	19% 8% 24% 30% 17% 2	635
September	1st to 10th	Night	17% 6% 29% 29% 17% 2	720
	11th to 20th	Night	18% 8% 27% 27% 17% 3	720
	21st to end	Night	22% 8% 24% 28% 16% 2	720
October	1st to 10th	Night	22% 13% 28% 25% 10% 1	720
	11th to 20th	Night	32% 13% 28% 20% 7% 1	720
	21st to end	Night	34% 15% 30% 17% 4	792
November	1st to 10th	Night	31% 12% 31% 19% 6% 1	740
	11th to 20th	Night	27% 18% 31% 18% 6%	760
	21st to end	Night	35% 14% 28% 18% 3 1	760
December	1st to 10th	Night	32% 18% 31% 15% 4	760
	11th to 20th	Night	31% 16% 33% 16% 3 1	771
	21st to end	Night	33% 16% 28% 19% 3	1045

Page generated at 2024-01-09 16:24:25. Version ac07

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