Sydney
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sydney clear sky chart from 2005-11-06 to 2025-10-19.

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 10% 6% 5% 5% 4 5% 6% 26%	127040
	Night	18% 12% 4 10% 6% 5% 5% 4 5% 6% 25%	122919
Transparency	Day	55% 8% 13% 16% 6%	130491
	Night	59% 8% 12% 15% 6%	127567
Seeing	Day	23% 9% 24% 28% 13% 2	19054
	Night	26% 10% 26% 27% 10% 1	26283

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	12% 18% 7% 19% 7% 6% 6% 3 4 5% 13%	3616
	11th to 20th	Day	10% 15% 8% 16% 8% 7% 6% 4 5% 6% 16%	3591
	21st to end	Day	10% 10% 6% 15% 7% 7% 5% 4 6% 7% 22%	4004
February	1st to 10th	Day	17% 11% 5% 14% 9% 6% 5% 5% 6% 7% 15%	1800
	11th to 20th	Day	14% 13% 5% 11% 7% 6% 6% 5% 6% 7% 20%	1773
	21st to end	Day	10% 15% 4 9% 7% 5% 4 4 6% 6% 29%	1650
March	1st to 10th	Day	18% 12% 4 12% 8% 7% 5% 3 4 5% 23%	4474
	11th to 20th	Day	11% 16% 6% 13% 8% 6% 6% 4 5% 6% 18%	4882
	21st to end	Day	21% 17% 7% 13% 6% 5% 4 3 4 4 15%	5831
April	1st to 10th	Day	22% 13% 5% 8% 6% 5% 5% 3 5% 5% 24%	2400
	11th to 20th	Day	21% 12% 3 7% 4 3 3 4 4 6% 32%	2374
	21st to end	Day	24% 13% 3 7% 5% 4 4 3 5% 5% 27%	2576
May	1st to 10th	Day	25% 16% 3 7% 4 4 4 3 4 5% 25%	6345
	11th to 20th	Day	21% 12% 3 6% 5% 4 4 3 5% 6% 31%	6318
	21st to end	Day	18% 11% 3 7% 4 4 4 3 4 7% 34%	6948
June	1st to 10th	Day	29% 12% 3 7% 4 4 4 3 5% 6% 23%	2800
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 4 5% 7% 32%	2800
	21st to end	Day	25% 11% 4 7% 5% 4 4 4 6% 6% 24%	2800
July	1st to 10th	Day	18% 9% 3 10% 6% 6% 5% 4 6% 8% 26%	6360
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 4 6% 8% 37%	6360
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 7% 9% 37%	6927
August	1st to 10th	Day	16% 8% 3 9% 5% 6% 6% 5% 7% 8% 27%	4564
	11th to 20th	Day	19% 8% 3 8% 5% 5% 4 4 7% 8% 29%	4152
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	4312
September	1st to 10th	Day	18% 7% 3 6% 4 5% 5% 4 6% 7% 37%	2400
	11th to 20th	Day	20% 8% 3 6% 5% 4 4 3 5% 6% 35%	2376
	21st to end	Day	18% 8% 3 6% 4 4 5% 4 5% 6% 38%	2170
October	1st to 10th	Day	19% 9% 4 6% 4 4 4 4 6% 8% 31%	2000
	11th to 20th	Day	20% 11% 4 8% 6% 4 5% 4 6% 6% 26%	1980
	21st to end	Day	23% 14% 4 10% 6% 5% 5% 3 5% 6% 20%	1894
November	1st to 10th	Day	21% 15% 4 10% 6% 5% 5% 3 6% 6% 20%	1760
	11th to 20th	Day	22% 17% 5% 11% 6% 5% 5% 4 5% 6% 14%	1615
	21st to end	Day	16% 18% 4 10% 7% 6% 7% 4 6% 6% 15%	1600
December	1st to 10th	Day	14% 14% 6% 11% 8% 7% 6% 4 6% 6% 19%	2610
	11th to 20th	Day	22% 16% 5% 13% 5% 5% 5% 3 4 5% 15%	2996
	21st to end	Day	19% 14% 8% 17% 7% 6% 6% 4 4 4 11%	3982
January	1st to 10th	Night	17% 16% 7% 16% 7% 6% 5% 4 4 5% 15%	6349
	11th to 20th	Night	14% 16% 9% 19% 7% 5% 5% 4 5% 5% 11%	6314
	21st to end	Night	8% 11% 6% 18% 11% 7% 6% 4 6% 6% 17%	6968
February	1st to 10th	Night	17% 13% 4 13% 7% 6% 6% 4 6% 6% 18%	2600
	11th to 20th	Night	15% 13% 4 11% 7% 7% 6% 4 6% 5% 21%	2512
	21st to end	Night	10% 14% 5% 13% 7% 5% 4 4 5% 6% 25%	1980
March	1st to 10th	Night	20% 13% 4 9% 6% 5% 5% 3 4 5% 24%	5366
	11th to 20th	Night	16% 16% 6% 10% 5% 5% 5% 3 4 5% 24%	5075
	21st to end	Night	25% 15% 6% 10% 6% 5% 4 3 3 3 19%	5100
April	1st to 10th	Night	22% 12% 4 8% 5% 4 4 3 6% 6% 26%	2000
	11th to 20th	Night	20% 12% 3 8% 4 3 4 3 6% 5% 33%	1976
	21st to end	Night	24% 11% 3 7% 5% 4 3 3 4 5% 32%	1819
May	1st to 10th	Night	27% 13% 3 6% 4 4 4 3 5% 6% 24%	3615
	11th to 20th	Night	19% 13% 3 7% 4 4 4 3 5% 7% 32%	3596
	21st to end	Night	16% 10% 3 7% 4 4 4 4 6% 8% 34%	3948
June	1st to 10th	Night	31% 10% 3 6% 4 5% 4 3 4 6% 25%	1600
	11th to 20th	Night	22% 7% 2 5% 4 4 4 4 5% 6% 37%	1600
	21st to end	Night	25% 9% 3 7% 6% 4 3 4 6% 7% 27%	1600
July	1st to 10th	Night	17% 9% 4 10% 6% 6% 6% 5% 7% 8% 23%	3619
	11th to 20th	Night	13% 8% 3 8% 4 4 5% 4 6% 8% 37%	3620
	21st to end	Night	16% 7% 3 7% 5% 4 4 4 6% 8% 34%	3948
August	1st to 10th	Night	13% 8% 3 8% 6% 7% 5% 4 7% 8% 30%	2596
	11th to 20th	Night	15% 8% 3 8% 6% 5% 5% 4 6% 8% 31%	2893
	21st to end	Night	15% 6% 2 6% 4 4 4 3 6% 8% 43%	3586
September	1st to 10th	Night	15% 7% 3 6% 5% 5% 4 3 5% 7% 40%	2000
	11th to 20th	Night	19% 9% 2 5% 4 3 4 4 5% 5% 39%	2023
	21st to end	Night	18% 8% 2 8% 5% 3 4 3 5% 7% 37%	2225
October	1st to 10th	Night	18% 6% 3 8% 4 4 5% 3 7% 8% 34%	2400
	11th to 20th	Night	21% 9% 3 7% 4 4 5% 3 6% 7% 31%	2377
	21st to end	Night	24% 13% 3 9% 5% 5% 5% 3 4 5% 24%	2703
November	1st to 10th	Night	18% 12% 4 9% 6% 5% 5% 4 5% 6% 25%	2538
	11th to 20th	Night	24% 14% 5% 9% 6% 5% 5% 4 5% 6% 17%	2784
	21st to end	Night	21% 16% 4 8% 6% 5% 5% 5% 7% 6% 17%	2800
December	1st to 10th	Night	16% 12% 5% 9% 5% 6% 5% 4 6% 7% 25%	4580
	11th to 20th	Night	19% 15% 6% 12% 6% 5% 5% 4 4 4 20%	5213
	21st to end	Night	21% 14% 6% 15% 6% 4 4 3 4 5% 17%	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	74% 8% 10% 6% 3	3617
	11th to 20th	Day	63% 10% 14% 10% 3	3620
	21st to end	Day	60% 10% 10% 14% 5% 1	4004
February	1st to 10th	Day	70% 9% 9% 7% 5%	1800
	11th to 20th	Day	60% 11% 12% 11% 5% 1	1781
	21st to end	Day	54% 9% 12% 14% 9% 2	1650
March	1st to 10th	Day	59% 7% 9% 16% 9% 1	4478
	11th to 20th	Day	58% 10% 16% 12% 4	4882
	21st to end	Day	66% 7% 11% 11% 5% 1	5831
April	1st to 10th	Day	57% 8% 11% 14% 8% 2	2400
	11th to 20th	Day	52% 5% 11% 19% 12% 1	2374
	21st to end	Day	55% 6% 12% 15% 10% 1	2576
May	1st to 10th	Day	59% 7% 13% 16% 6% 1	6357
	11th to 20th	Day	50% 7% 15% 20% 8%	6322
	21st to end	Day	48% 8% 14% 22% 7%	6948
June	1st to 10th	Day	60% 9% 10% 15% 5% 1	2800
	11th to 20th	Day	51% 8% 14% 19% 8% 1	2800
	21st to end	Day	59% 10% 11% 13% 7%	2800
July	1st to 10th	Day	55% 10% 15% 13% 7%	6290
	11th to 20th	Day	41% 10% 19% 24% 6%	6284
	21st to end	Day	40% 8% 21% 25% 6%	6730
August	1st to 10th	Day	49% 11% 17% 17% 5%	4550
	11th to 20th	Day	47% 9% 18% 22% 4	4192
	21st to end	Day	38% 9% 16% 28% 10%	4312
September	1st to 10th	Day	45% 9% 13% 23% 9% 1	2400
	11th to 20th	Day	48% 7% 14% 21% 8% 1	2376
	21st to end	Day	44% 8% 14% 24% 9%	2170
October	1st to 10th	Day	56% 6% 14% 16% 7%	3260
	11th to 20th	Day	47% 7% 13% 23% 9%	3220
	21st to end	Day	58% 9% 11% 16% 6%	3105
November	1st to 10th	Day	63% 9% 10% 14% 3	1760
	11th to 20th	Day	70% 9% 10% 8% 3	1615
	21st to end	Day	68% 9% 13% 7% 3	1600
December	1st to 10th	Day	63% 8% 7% 15% 6%	2610
	11th to 20th	Day	69% 8% 11% 10% 2	2995
	21st to end	Day	74% 9% 9% 5% 2	3982
January	1st to 10th	Night	71% 13% 9% 6% 1	6349
	11th to 20th	Night	74% 10% 9% 5% 2	6360
	21st to end	Night	64% 11% 10% 10% 5%	6968
February	1st to 10th	Night	70% 8% 10% 8% 4	2600
	11th to 20th	Night	65% 9% 11% 11% 4 1	2515
	21st to end	Night	60% 9% 10% 12% 8% 1	1980
March	1st to 10th	Night	61% 7% 7% 16% 8% 2	5375
	11th to 20th	Night	61% 7% 13% 13% 5%	5076
	21st to end	Night	69% 6% 7% 13% 4 1	5100
April	1st to 10th	Night	58% 6% 9% 16% 11% 1	2000
	11th to 20th	Night	53% 8% 9% 18% 10% 1	1976
	21st to end	Night	56% 5% 11% 16% 11% 1	1819
May	1st to 10th	Night	58% 7% 12% 14% 8%	3620
	11th to 20th	Night	50% 6% 14% 22% 8% 1	3597
	21st to end	Night	47% 9% 14% 23% 7%	3949
June	1st to 10th	Night	64% 5% 9% 15% 7% 1	1600
	11th to 20th	Night	49% 7% 11% 21% 12%	1600
	21st to end	Night	58% 10% 12% 13% 8%	1600
July	1st to 10th	Night	57% 8% 14% 15% 6%	3581
	11th to 20th	Night	44% 8% 16% 21% 10% 1	3572
	21st to end	Night	47% 7% 17% 23% 6%	3831
August	1st to 10th	Night	50% 9% 18% 18% 6%	2585
	11th to 20th	Night	48% 10% 19% 19% 4	2919
	21st to end	Night	41% 7% 16% 29% 7%	3585
September	1st to 10th	Night	45% 9% 14% 22% 10%	2000
	11th to 20th	Night	46% 7% 14% 22% 9% 1	2023
	21st to end	Night	50% 8% 11% 23% 8%	2225
October	1st to 10th	Night	51% 7% 12% 18% 11%	3920
	11th to 20th	Night	48% 9% 13% 24% 6%	3876
	21st to end	Night	57% 6% 16% 15% 6%	4453
November	1st to 10th	Night	63% 9% 13% 12% 4	2538
	11th to 20th	Night	68% 10% 11% 8% 3	2784
	21st to end	Night	68% 11% 12% 7% 2	2800
December	1st to 10th	Night	59% 9% 9% 16% 7%	4580
	11th to 20th	Night	69% 8% 9% 9% 4 1	5215
	21st to end	Night	73% 7% 11% 9% 1	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	25% 18% 34% 18% 4 1	400
	11th to 20th	Day	24% 16% 31% 19% 9% 2	400
	21st to end	Day	20% 14% 40% 23% 4	440
February	1st to 10th	Day	25% 18% 34% 17% 6%	400
	11th to 20th	Day	25% 20% 32% 19% 4	396
	21st to end	Day	15% 11% 38% 24% 9% 1	331
March	1st to 10th	Day	18% 15% 36% 25% 6%	400
	11th to 20th	Day	14% 13% 33% 28% 11% 1	478
	21st to end	Day	25% 13% 28% 22% 9% 2	550
April	1st to 10th	Day	27% 12% 35% 19% 7% 1	500
	11th to 20th	Day	25% 11% 26% 28% 10%	492
	21st to end	Day	27% 9% 23% 29% 11% 2	605
May	1st to 10th	Day	30% 8% 25% 25% 8% 3	703
	11th to 20th	Day	21% 6% 22% 31% 15% 5%	724
	21st to end	Day	23% 6% 21% 37% 12% 1	790
June	1st to 10th	Day	34% 6% 18% 28% 12% 2	730
	11th to 20th	Day	19% 6% 20% 36% 18% 2	730
	21st to end	Day	26% 4 18% 34% 16% 3	730
July	1st to 10th	Day	20% 3 13% 40% 22% 2	720
	11th to 20th	Day	18% 3 13% 41% 22% 3	718
	21st to end	Day	21% 2 15% 37% 21% 3	795
August	1st to 10th	Day	16% 3 14% 40% 24% 3	725
	11th to 20th	Day	20% 3 16% 33% 21% 7%	550
	21st to end	Day	16% 6% 17% 36% 21% 3	583
September	1st to 10th	Day	16% 6% 22% 36% 17% 3	530
	11th to 20th	Day	23% 8% 22% 28% 16% 3	516
	21st to end	Day	24% 6% 27% 26% 14% 4	420
October	1st to 10th	Day	23% 6% 20% 34% 15% 2	420
	11th to 20th	Day	24% 10% 28% 18% 17% 3	414
	21st to end	Day	31% 11% 26% 23% 8% 1	429
November	1st to 10th	Day	26% 17% 25% 23% 8% 2	395
	11th to 20th	Day	36% 15% 27% 18% 4 1	400
	21st to end	Day	30% 18% 28% 19% 4 1	400
December	1st to 10th	Day	31% 13% 34% 18% 4 1	400
	11th to 20th	Day	29% 22% 30% 16% 3 1	400
	21st to end	Day	22% 17% 35% 17% 7% 1	440
January	1st to 10th	Night	26% 18% 32% 19% 5%	900
	11th to 20th	Night	25% 15% 35% 19% 5% 1	900
	21st to end	Night	19% 17% 37% 21% 6%	983
February	1st to 10th	Night	26% 15% 33% 21% 5% 1	800
	11th to 20th	Night	22% 20% 35% 20% 3	783
	21st to end	Night	17% 13% 36% 25% 9% 1	660
March	1st to 10th	Night	24% 11% 35% 23% 7%	800
	11th to 20th	Night	21% 11% 29% 29% 10% 1	800
	21st to end	Night	27% 10% 30% 25% 6% 2	721
April	1st to 10th	Night	27% 11% 30% 26% 5%	600
	11th to 20th	Night	26% 6% 27% 31% 9% 1	594
	21st to end	Night	31% 5% 27% 25% 11% 1	600
May	1st to 10th	Night	33% 5% 23% 25% 11% 4	600
	11th to 20th	Night	26% 5% 20% 31% 15% 3	592
	21st to end	Night	22% 5% 23% 36% 12% 1	649
June	1st to 10th	Night	32% 4 22% 30% 11% 1	600
	11th to 20th	Night	25% 5% 16% 37% 16% 1	600
	21st to end	Night	27% 3 17% 40% 13% 1	600
July	1st to 10th	Night	22% 4 15% 39% 19% 2	586
	11th to 20th	Night	17% 4 16% 45% 18% 1	582
	21st to end	Night	23% 2 14% 39% 21% 1	656
August	1st to 10th	Night	19% 3 14% 41% 21% 2	598
	11th to 20th	Night	20% 2 14% 39% 22% 3	592
	21st to end	Night	18% 5% 17% 39% 21%	660
September	1st to 10th	Night	20% 6% 22% 34% 17% 3	600
	11th to 20th	Night	25% 5% 25% 30% 15% 1	600
	21st to end	Night	24% 6% 25% 33% 10% 2	625
October	1st to 10th	Night	23% 7% 28% 29% 13% 1	800
	11th to 20th	Night	29% 8% 24% 25% 12% 2	793
	21st to end	Night	33% 11% 28% 20% 7% 1	939
November	1st to 10th	Night	31% 15% 26% 22% 5% 1	880
	11th to 20th	Night	35% 15% 26% 17% 6% 1	900
	21st to end	Night	34% 14% 30% 19% 3	900
December	1st to 10th	Night	31% 16% 31% 16% 6%	900
	11th to 20th	Night	32% 21% 28% 16% 3 1	900
	21st to end	Night	28% 16% 32% 18% 5% 1	990

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