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 2024-11-08.

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%	101761
	Night	16% 14% 5% 9% 5% 4 4 3 4 6% 30%	97320
Transparency	Day	54% 5% 11% 20% 9% 1	105326
	Night	55% 5% 11% 18% 9% 1	101833
Seeing	Day	25% 16% 31% 19% 8% 2	15059
	Night	24% 13% 30% 24% 8% 1	24043

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% 25% 13% 14% 4 3 3 2 3 3 16%	2208
	11th to 20th	Day	11% 16% 8% 20% 7% 6% 5% 3 5% 4 15%	2471
	21st to end	Day	16% 24% 10% 12% 4 5% 4 2 3 4 16%	3124
February	1st to 10th	Day	13% 21% 6% 14% 8% 5% 4 3 4 5% 18%	1710
	11th to 20th	Day	11% 20% 8% 12% 6% 4 4 3 5% 4 24%	1663
	21st to end	Day	15% 21% 6% 12% 7% 5% 4 2 4 4 21%	1426
March	1st to 10th	Day	19% 17% 4 8% 5% 5% 5% 3 4 5% 25%	3420
	11th to 20th	Day	11% 17% 8% 12% 6% 5% 4 3 5% 5% 25%	3480
	21st to end	Day	13% 14% 6% 9% 4 5% 4 3 3 5% 33%	3815
April	1st to 10th	Day	20% 16% 4 8% 4 3 4 3 5% 6% 27%	2253
	11th to 20th	Day	22% 12% 3 6% 4 3 3 3 5% 5% 34%	2380
	21st to end	Day	22% 12% 4 7% 4 3 3 2 3 5% 33%	2470
May	1st to 10th	Day	20% 10% 3 7% 5% 5% 4 3 5% 7% 31%	4204
	11th to 20th	Day	15% 8% 3 7% 4 4 4 4 6% 8% 38%	4398
	21st to end	Day	20% 6% 2 5% 4 4 3 3 5% 7% 41%	4837
June	1st to 10th	Day	20% 8% 3 7% 6% 4 4 4 5% 7% 33%	2750
	11th to 20th	Day	15% 6% 3 6% 5% 5% 4 4 6% 8% 38%	2850
	21st to end	Day	21% 8% 3 7% 4 5% 4 4 6% 6% 31%	2850
July	1st to 10th	Day	17% 6% 2 7% 5% 5% 5% 4 6% 8% 34%	4760
	11th to 20th	Day	15% 6% 3 7% 5% 4 5% 4 6% 8% 38%	4649
	21st to end	Day	13% 6% 2 6% 4 3 4 3 5% 7% 47%	4675
August	1st to 10th	Day	13% 7% 3 7% 5% 6% 6% 4 7% 9% 34%	4104
	11th to 20th	Day	19% 7% 2 6% 5% 4 4 4 6% 8% 35%	4044
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 44%	4416
September	1st to 10th	Day	16% 8% 3 7% 5% 3 4 3 4 7% 39%	2280
	11th to 20th	Day	17% 7% 2 5% 3 3 5% 3 5% 7% 42%	2175
	21st to end	Day	24% 8% 2 8% 4 5% 4 2 4 5% 33%	2049
October	1st to 10th	Day	20% 10% 3 8% 4 4 4 3 4 6% 33%	1900
	11th to 20th	Day	25% 16% 3 8% 5% 4 4 3 4 5% 24%	1900
	21st to end	Day	31% 16% 5% 7% 5% 4 3 2 4 4 20%	2069
November	1st to 10th	Day	20% 17% 4 8% 5% 4 4 3 5% 6% 23%	1565
	11th to 20th	Day	19% 19% 5% 10% 5% 5% 4 3 4 6% 20%	1520
	21st to end	Day	23% 22% 7% 10% 5% 6% 3 3 3 3 14%	1520
December	1st to 10th	Day	17% 26% 7% 12% 5% 4 5% 3 4 4 12%	1520
	11th to 20th	Day	20% 19% 6% 13% 5% 4 4 2 4 4 18%	1875
	21st to end	Day	22% 24% 8% 15% 5% 3 2 2 3 3 13%	2431
January	1st to 10th	Night	15% 21% 9% 16% 6% 4 4 3 4 4 13%	4748
	11th to 20th	Night	12% 17% 6% 17% 8% 6% 5% 4 5% 5% 16%	4486
	21st to end	Night	14% 24% 10% 13% 5% 5% 3 2 3 3 17%	4532
February	1st to 10th	Night	13% 18% 7% 12% 8% 5% 5% 3 5% 4 20%	2470
	11th to 20th	Night	12% 15% 8% 13% 6% 5% 4 3 5% 4 24%	2403
	21st to end	Night	13% 19% 6% 12% 7% 6% 4 3 4 5% 22%	2027
March	1st to 10th	Night	14% 15% 4 10% 5% 4 4 3 5% 6% 30%	3419
	11th to 20th	Night	12% 18% 8% 9% 4 4 4 3 4 5% 28%	3479
	21st to end	Night	13% 12% 5% 8% 7% 5% 4 3 4 5% 34%	3817
April	1st to 10th	Night	21% 15% 4 7% 4 3 3 4 4 5% 29%	1926
	11th to 20th	Night	20% 10% 3 7% 4 4 3 3 3 5% 37%	1764
	21st to end	Night	22% 12% 3 7% 5% 4 3 3 5% 5% 31%	1710
May	1st to 10th	Night	17% 8% 3 6% 5% 4 4 4 5% 7% 38%	2722
	11th to 20th	Night	13% 7% 2 6% 5% 5% 5% 3 6% 9% 39%	2495
	21st to end	Night	13% 7% 3 7% 5% 4 4 4 5% 8% 41%	2738
June	1st to 10th	Night	14% 10% 2 7% 6% 5% 5% 4 6% 6% 36%	1425
	11th to 20th	Night	15% 5% 2 5% 3 4 4 3 5% 8% 44%	1330
	21st to end	Night	17% 8% 2 6% 4 4 3 4 6% 7% 38%	1330
July	1st to 10th	Night	14% 7% 2 7% 4 3 3 3 5% 7% 45%	2210
	11th to 20th	Night	17% 7% 2 5% 3 3 4 4 5% 6% 45%	2303
	21st to end	Night	11% 6% 2 5% 4 4 4 4 5% 8% 48%	2861
August	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 7% 44%	2825
	11th to 20th	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	2785
	21st to end	Night	15% 5% 2 5% 4 4 3 3 4 7% 49%	3207
September	1st to 10th	Night	14% 5% 2 5% 4 4 4 3 5% 7% 48%	1900
	11th to 20th	Night	14% 7% 2 5% 4 4 3 2 5% 7% 47%	2003
	21st to end	Night	18% 8% 2 7% 4 4 4 3 4 6% 41%	2130
October	1st to 10th	Night	19% 9% 3 7% 4 4 4 4 4 6% 36%	2280
	11th to 20th	Night	24% 13% 3 8% 4 6% 5% 3 4 5% 25%	2280
	21st to end	Night	26% 16% 4 8% 5% 5% 3 3 4 5% 22%	2527
November	1st to 10th	Night	21% 16% 4 7% 4 4 4 3 4 5% 29%	2673
	11th to 20th	Night	18% 16% 5% 9% 6% 4 4 3 5% 5% 24%	2660
	21st to end	Night	23% 20% 5% 10% 6% 4 4 3 4 5% 16%	2660
December	1st to 10th	Night	18% 26% 7% 12% 7% 4 4 2 4 3 13%	2660
	11th to 20th	Night	18% 24% 7% 12% 6% 4 4 3 4 4 16%	3299
	21st to end	Night	21% 22% 8% 16% 5% 4 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% 5% 8% 9% 4	2209
	11th to 20th	Day	72% 6% 8% 8% 5% 1	2471
	21st to end	Day	74% 4 6% 9% 6% 1	3124
February	1st to 10th	Day	69% 4 7% 13% 5% 1	1710
	11th to 20th	Day	64% 3 9% 13% 9% 1	1672
	21st to end	Day	68% 5% 6% 13% 7%	1426
March	1st to 10th	Day	61% 5% 12% 10% 11% 1	3423
	11th to 20th	Day	61% 5% 11% 15% 8% 1	3480
	21st to end	Day	54% 3 10% 19% 11% 2	3815
April	1st to 10th	Day	58% 5% 11% 15% 9% 2	2253
	11th to 20th	Day	53% 4 8% 21% 13% 1	2380
	21st to end	Day	55% 4 8% 16% 14% 3	2470
May	1st to 10th	Day	51% 5% 9% 21% 12% 2	4216
	11th to 20th	Day	43% 7% 11% 29% 9% 1	4402
	21st to end	Day	43% 6% 12% 18% 17% 3	4837
June	1st to 10th	Day	50% 6% 10% 21% 13%	2750
	11th to 20th	Day	44% 6% 10% 24% 14% 1	2850
	21st to end	Day	51% 7% 11% 19% 11% 1	2850
July	1st to 10th	Day	47% 6% 14% 23% 10% 1	4686
	11th to 20th	Day	42% 5% 14% 30% 8%	4568
	21st to end	Day	36% 5% 11% 30% 17%	4526
August	1st to 10th	Day	45% 6% 12% 28% 8%	4091
	11th to 20th	Day	46% 6% 12% 26% 9%	4083
	21st to end	Day	37% 5% 12% 34% 11%	4416
September	1st to 10th	Day	45% 6% 12% 24% 12%	2280
	11th to 20th	Day	42% 6% 12% 29% 11%	2175
	21st to end	Day	54% 5% 10% 23% 8%	2049
October	1st to 10th	Day	50% 5% 11% 23% 9% 1	3160
	11th to 20th	Day	55% 6% 12% 22% 6%	3140
	21st to end	Day	67% 4 11% 13% 4	3383
November	1st to 10th	Day	61% 6% 12% 16% 5%	1565
	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	74% 6% 8% 7% 4	4748
	11th to 20th	Night	68% 8% 10% 9% 4 1	4486
	21st to end	Night	74% 3 6% 10% 6% 1	4532
February	1st to 10th	Night	66% 5% 9% 12% 6% 1	2470
	11th to 20th	Night	62% 4 11% 16% 6% 1	2405
	21st to end	Night	65% 6% 8% 14% 7% 1	2027
March	1st to 10th	Night	54% 6% 12% 15% 12% 1	3429
	11th to 20th	Night	57% 6% 13% 15% 8% 2	3480
	21st to end	Night	52% 4 9% 21% 11% 2	3817
April	1st to 10th	Night	57% 5% 8% 17% 11% 2	1926
	11th to 20th	Night	50% 5% 6% 21% 15% 2	1764
	21st to end	Night	55% 5% 7% 14% 16% 3	1710
May	1st to 10th	Night	44% 7% 11% 25% 12% 1	2729
	11th to 20th	Night	40% 7% 12% 24% 15% 2	2496
	21st to end	Night	41% 6% 11% 24% 17% 2	2739
June	1st to 10th	Night	47% 8% 10% 22% 11% 3	1425
	11th to 20th	Night	38% 7% 10% 27% 18% 1	1330
	21st to end	Night	44% 7% 12% 23% 12% 2	1330
July	1st to 10th	Night	39% 4 14% 26% 14% 2	2175
	11th to 20th	Night	40% 5% 14% 32% 9% 1	2261
	21st to end	Night	35% 6% 10% 30% 18% 1	2770
August	1st to 10th	Night	38% 5% 16% 25% 14% 2	2814
	11th to 20th	Night	41% 5% 18% 26% 9% 1	2812
	21st to end	Night	37% 4 12% 33% 12% 1	3207
September	1st to 10th	Night	37% 7% 13% 29% 13% 1	1900
	11th to 20th	Night	38% 6% 15% 29% 13%	2003
	21st to end	Night	44% 5% 15% 26% 11%	2130
October	1st to 10th	Night	43% 5% 9% 29% 14%	3800
	11th to 20th	Night	54% 4 13% 22% 7%	3779
	21st to end	Night	60% 5% 17% 13% 5%	4151
November	1st to 10th	Night	57% 5% 12% 19% 6%	2673
	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	22% 23% 31% 19% 5%	204
	21st to end	Day	27% 17% 34% 17% 4 1	297
February	1st to 10th	Day	33% 25% 32% 8% 1 1	270
	11th to 20th	Day	25% 26% 38% 10% 1	270
	21st to end	Day	39% 25% 28% 8%	225
March	1st to 10th	Day	27% 22% 34% 14% 3	448
	11th to 20th	Day	22% 21% 38% 13% 5% 1	460
	21st to end	Day	30% 21% 30% 16% 3 1	506
April	1st to 10th	Day	28% 20% 32% 15% 4 1	460
	11th to 20th	Day	37% 18% 20% 15% 8% 2	452
	21st to end	Day	30% 20% 31% 12% 6% 1	460
May	1st to 10th	Day	28% 15% 29% 19% 7% 2	463
	11th to 20th	Day	22% 17% 30% 18% 10% 2	487
	21st to end	Day	21% 17% 29% 20% 10% 2	529
June	1st to 10th	Day	22% 15% 33% 23% 6% 1	580
	11th to 20th	Day	19% 14% 35% 22% 9% 2	680
	21st to end	Day	24% 13% 34% 19% 8% 1	680
July	1st to 10th	Day	14% 12% 32% 29% 10% 1	671
	11th to 20th	Day	17% 13% 28% 28% 11% 3	609
	21st to end	Day	14% 12% 29% 30% 13% 3	532
August	1st to 10th	Day	19% 8% 27% 29% 12% 5%	486
	11th to 20th	Day	18% 5% 28% 24% 17% 8%	479
	21st to end	Day	18% 12% 30% 24% 12% 4	539
September	1st to 10th	Day	19% 13% 30% 23% 10% 5%	490
	11th to 20th	Day	17% 9% 30% 24% 14% 7%	490
	21st to end	Day	26% 12% 24% 24% 11% 3	490
October	1st to 10th	Day	26% 14% 27% 21% 10% 2	490
	11th to 20th	Day	29% 17% 36% 12% 4 2	490
	21st to end	Day	41% 15% 29% 13% 1	518
November	1st to 10th	Day	34% 18% 33% 11% 3	206
	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	31% 15% 29% 20% 4 1	832
	21st to end	Night	22% 11% 35% 22% 8% 1	836
February	1st to 10th	Night	23% 22% 33% 18% 4	760
	11th to 20th	Night	21% 19% 34% 21% 5% 1	743
	21st to end	Night	23% 20% 33% 21% 2	628
March	1st to 10th	Night	22% 20% 33% 20% 4	760
	11th to 20th	Night	23% 21% 33% 18% 5% 1	760
	21st to end	Night	21% 14% 34% 24% 6% 1	836
April	1st to 10th	Night	32% 11% 32% 20% 5%	760
	11th to 20th	Night	27% 17% 25% 22% 8% 1	640
	21st to end	Night	28% 12% 25% 25% 10%	570
May	1st to 10th	Night	22% 8% 28% 32% 9% 1	511
	11th to 20th	Night	18% 11% 31% 32% 8%	374
	21st to end	Night	16% 10% 34% 33% 7%	411
June	1st to 10th	Night	16% 8% 33% 32% 11%	380
	11th to 20th	Night	18% 9% 25% 34% 13%	380
	21st to end	Night	20% 8% 28% 31% 13% 1	380
July	1st to 10th	Night	14% 4 27% 40% 14%	370
	11th to 20th	Night	14% 5% 27% 35% 19% 1	368
	21st to end	Night	12% 6% 29% 36% 17% 1	489
August	1st to 10th	Night	16% 4 23% 37% 19% 1	567
	11th to 20th	Night	14% 5% 21% 37% 20% 3	564
	21st to end	Night	18% 8% 24% 31% 16% 2	671
September	1st to 10th	Night	17% 6% 29% 29% 17% 2	760
	11th to 20th	Night	17% 8% 26% 26% 20% 3	760
	21st to end	Night	22% 8% 24% 29% 16% 2	760
October	1st to 10th	Night	22% 13% 29% 25% 10% 1	760
	11th to 20th	Night	32% 12% 28% 21% 7% 1	760
	21st to end	Night	33% 15% 30% 17% 4	836
November	1st to 10th	Night	32% 12% 30% 19% 6% 1	771
	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 2024-11-05 08:15:54. Version ac07

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