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 2024-11-05.

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	19% 12% 4 9% 6% 5% 5% 4 5% 7% 26%	101610
	Night	19% 12% 4 10% 6% 5% 4 4 5% 6% 25%	97572
Transparency	Day	55% 9% 14% 15% 6%	105079
	Night	59% 9% 12% 14% 5%	102203
Seeing	Day	24% 9% 24% 28% 12% 2	17712
	Night	26% 10% 26% 27% 10% 1	24997

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% 12% 7% 21% 7% 6% 6% 3 4 6% 16%	2526
	11th to 20th	Day	11% 11% 5% 17% 8% 7% 6% 4 6% 7% 18%	2530
	21st to end	Day	14% 11% 7% 13% 7% 7% 5% 4 5% 7% 20%	2792
February	1st to 10th	Day	18% 11% 5% 14% 8% 6% 5% 5% 6% 7% 15%	1710
	11th to 20th	Day	15% 13% 6% 11% 7% 5% 6% 5% 5% 7% 20%	1682
	21st to end	Day	9% 15% 5% 9% 7% 5% 4 4 6% 6% 29%	1570
March	1st to 10th	Day	22% 12% 4 8% 6% 6% 4 3 3 5% 28%	3114
	11th to 20th	Day	13% 18% 7% 13% 8% 6% 5% 4 5% 6% 16%	3410
	21st to end	Day	24% 19% 7% 10% 5% 5% 4 3 3 4 16%	4069
April	1st to 10th	Day	21% 13% 4 8% 6% 5% 5% 3 5% 5% 24%	2280
	11th to 20th	Day	21% 12% 3 7% 4 3 3 4 4 6% 33%	2254
	21st to end	Day	25% 12% 3 7% 5% 4 4 3 5% 5% 28%	2447
May	1st to 10th	Day	27% 17% 3 7% 4 3 3 2 4 5% 24%	4425
	11th to 20th	Day	21% 13% 3 6% 5% 4 4 3 5% 6% 30%	4398
	21st to end	Day	19% 10% 3 7% 4 5% 5% 4 5% 9% 29%	4836
June	1st to 10th	Day	30% 12% 3 7% 4 4 4 3 5% 6% 21%	2660
	11th to 20th	Day	20% 9% 3 7% 5% 4 4 4 5% 7% 32%	2660
	21st to end	Day	26% 11% 4 7% 5% 4 4 4 6% 6% 24%	2660
July	1st to 10th	Day	20% 8% 3 9% 6% 5% 5% 3 6% 7% 28%	4440
	11th to 20th	Day	14% 6% 3 8% 6% 5% 6% 5% 7% 9% 31%	4440
	21st to end	Day	14% 7% 3 8% 5% 5% 5% 4 6% 8% 36%	4815
August	1st to 10th	Day	16% 8% 3 9% 6% 6% 6% 5% 8% 9% 25%	4424
	11th to 20th	Day	19% 8% 3 8% 5% 5% 4 4 7% 8% 28%	4023
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	4180
September	1st to 10th	Day	19% 7% 3 6% 4 5% 5% 4 6% 7% 36%	2280
	11th to 20th	Day	20% 9% 3 7% 5% 4 4 3 5% 6% 35%	2257
	21st to end	Day	19% 8% 3 6% 5% 4 5% 4 5% 6% 37%	2062
October	1st to 10th	Day	20% 10% 4 7% 4 4 4 5% 6% 8% 30%	1900
	11th to 20th	Day	20% 11% 4 8% 6% 4 5% 4 6% 6% 25%	1900
	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% 4 4 6% 6% 20%	1708
	11th to 20th	Day	20% 16% 5% 11% 7% 6% 5% 4 5% 6% 15%	1535
	21st to end	Day	15% 18% 4 10% 7% 6% 7% 4 7% 6% 16%	1520
December	1st to 10th	Day	21% 17% 6% 10% 7% 7% 6% 5% 5% 5% 11%	1520
	11th to 20th	Day	26% 19% 5% 13% 6% 6% 5% 3 4 5% 9%	1906
	21st to end	Day	21% 16% 7% 12% 6% 5% 5% 4 4 5% 14%	2783
January	1st to 10th	Night	11% 13% 7% 17% 7% 6% 5% 4 5% 6% 20%	4429
	11th to 20th	Night	17% 12% 5% 19% 7% 5% 6% 4 5% 6% 13%	4440
	21st to end	Night	11% 13% 7% 17% 8% 5% 4 3 6% 7% 19%	4870
February	1st to 10th	Night	18% 13% 4 12% 7% 6% 6% 4 6% 6% 18%	2470
	11th to 20th	Night	15% 13% 4 11% 7% 7% 6% 4 6% 5% 21%	2384
	21st to end	Night	11% 14% 5% 12% 7% 5% 4 4 5% 6% 26%	1884
March	1st to 10th	Night	21% 12% 4 8% 6% 4 4 3 5% 5% 27%	3726
	11th to 20th	Night	19% 19% 5% 10% 5% 5% 4 3 4 5% 21%	3547
	21st to end	Night	25% 15% 6% 11% 6% 5% 4 3 3 3 20%	3562
April	1st to 10th	Night	22% 12% 4 8% 5% 4 4 3 6% 6% 26%	1900
	11th to 20th	Night	19% 12% 4 8% 4 3 4 3 6% 5% 33%	1876
	21st to end	Night	24% 10% 3 7% 5% 4 3 3 4 5% 32%	1729
May	1st to 10th	Night	28% 12% 3 6% 4 4 4 3 5% 6% 25%	2525
	11th to 20th	Night	19% 11% 3 8% 4 4 3 3 5% 7% 33%	2506
	21st to end	Night	17% 11% 3 7% 4 4 4 4 6% 8% 30%	2749
June	1st to 10th	Night	32% 11% 3 6% 4 5% 4 3 4 6% 23%	1520
	11th to 20th	Night	22% 8% 2 6% 4 3 4 3 5% 6% 36%	1520
	21st to end	Night	26% 9% 3 7% 6% 4 3 4 6% 7% 26%	1520
July	1st to 10th	Night	20% 8% 4 9% 5% 5% 5% 4 7% 8% 26%	2530
	11th to 20th	Night	14% 8% 4 9% 5% 4 5% 4 7% 8% 31%	2530
	21st to end	Night	15% 8% 3 7% 5% 4 4 4 6% 7% 36%	2750
August	1st to 10th	Night	13% 8% 3 8% 7% 7% 5% 5% 7% 8% 29%	2516
	11th to 20th	Night	15% 8% 3 8% 6% 5% 5% 4 6% 8% 31%	2803
	21st to end	Night	15% 6% 2 6% 4 4 4 3 6% 8% 43%	3476
September	1st to 10th	Night	16% 7% 3 6% 5% 5% 4 3 5% 7% 39%	1900
	11th to 20th	Night	20% 9% 2 5% 4 3 4 3 5% 5% 39%	1922
	21st to end	Night	19% 9% 2 8% 5% 4 4 3 5% 7% 36%	2114
October	1st to 10th	Night	18% 6% 3 8% 4 4 5% 3 7% 8% 33%	2280
	11th to 20th	Night	22% 9% 3 7% 4 4 5% 3 6% 7% 31%	2280
	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% 6% 5% 4 5% 6% 25%	2470
	11th to 20th	Night	22% 14% 5% 10% 6% 5% 5% 5% 5% 6% 18%	2644
	21st to end	Night	20% 15% 4 8% 6% 5% 5% 5% 7% 6% 18%	2660
December	1st to 10th	Night	21% 17% 5% 10% 6% 6% 5% 5% 6% 6% 14%	2660
	11th to 20th	Night	24% 19% 6% 12% 6% 4 4 4 4 4 13%	3293
	21st to end	Night	26% 16% 6% 13% 5% 4 4 3 4 4 16%	4884

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	71% 10% 11% 6% 2	2527
	11th to 20th	Day	60% 12% 13% 12% 3	2530
	21st to end	Day	63% 11% 12% 7% 6% 1	2792
February	1st to 10th	Day	70% 9% 9% 7% 5%	1710
	11th to 20th	Day	61% 11% 12% 11% 4	1690
	21st to end	Day	54% 9% 12% 14% 9% 2	1570
March	1st to 10th	Day	57% 6% 9% 19% 9%	3118
	11th to 20th	Day	58% 12% 12% 13% 4	3410
	21st to end	Day	66% 8% 11% 9% 5% 1	4069
April	1st to 10th	Day	57% 8% 11% 14% 8% 2	2280
	11th to 20th	Day	52% 6% 11% 19% 12% 1	2254
	21st to end	Day	55% 7% 12% 14% 11% 1	2447
May	1st to 10th	Day	60% 7% 13% 15% 5% 1	4440
	11th to 20th	Day	52% 9% 15% 18% 6%	4402
	21st to end	Day	50% 10% 17% 18% 5% 1	4836
June	1st to 10th	Day	61% 9% 10% 15% 5%	2660
	11th to 20th	Day	52% 8% 14% 18% 8% 1	2660
	21st to end	Day	60% 10% 11% 13% 6%	2660
July	1st to 10th	Day	55% 10% 14% 12% 9%	4370
	11th to 20th	Day	43% 13% 21% 18% 5%	4364
	21st to end	Day	41% 9% 21% 23% 6%	4662
August	1st to 10th	Day	51% 11% 18% 16% 4	4410
	11th to 20th	Day	48% 9% 18% 21% 4	4063
	21st to end	Day	38% 9% 15% 28% 10%	4180
September	1st to 10th	Day	46% 9% 14% 22% 9% 1	2280
	11th to 20th	Day	48% 7% 14% 21% 8% 1	2257
	21st to end	Day	45% 8% 15% 23% 8%	2062
October	1st to 10th	Day	57% 6% 14% 16% 7%	3160
	11th to 20th	Day	47% 8% 13% 23% 9%	3140
	21st to end	Day	58% 9% 11% 16% 6%	3105
November	1st to 10th	Day	64% 9% 9% 14% 3	1708
	11th to 20th	Day	69% 10% 10% 8% 3	1535
	21st to end	Day	67% 10% 13% 7% 3	1520
December	1st to 10th	Day	76% 10% 8% 4 2	1520
	11th to 20th	Day	78% 8% 7% 6% 1	1905
	21st to end	Day	72% 10% 12% 4 2	2783
January	1st to 10th	Night	66% 17% 11% 4 2	4429
	11th to 20th	Night	72% 13% 9% 4 2	4440
	21st to end	Night	64% 11% 12% 8% 5%	4870
February	1st to 10th	Night	70% 8% 10% 8% 3	2470
	11th to 20th	Night	65% 9% 11% 12% 4 1	2387
	21st to end	Night	59% 9% 11% 12% 8% 1	1884
March	1st to 10th	Night	59% 6% 9% 19% 7%	3735
	11th to 20th	Night	64% 7% 10% 13% 4 1	3548
	21st to end	Night	68% 6% 8% 12% 5% 2	3562
April	1st to 10th	Night	58% 6% 9% 16% 10% 1	1900
	11th to 20th	Night	53% 8% 10% 18% 10% 1	1876
	21st to end	Night	56% 5% 10% 16% 12% 1	1729
May	1st to 10th	Night	57% 9% 15% 13% 6%	2530
	11th to 20th	Night	51% 7% 14% 20% 7% 1	2507
	21st to end	Night	49% 11% 17% 18% 5%	2750
June	1st to 10th	Night	66% 5% 9% 14% 5% 1	1520
	11th to 20th	Night	50% 8% 11% 20% 11%	1520
	21st to end	Night	59% 10% 11% 13% 7%	1520
July	1st to 10th	Night	56% 9% 13% 14% 8%	2491
	11th to 20th	Night	50% 10% 16% 14% 9% 1	2482
	21st to end	Night	45% 8% 19% 21% 8%	2663
August	1st to 10th	Night	51% 9% 18% 17% 4	2505
	11th to 20th	Night	48% 10% 20% 19% 3	2829
	21st to end	Night	41% 8% 15% 29% 7%	3475
September	1st to 10th	Night	45% 9% 14% 22% 10%	1900
	11th to 20th	Night	47% 7% 14% 22% 9% 1	1922
	21st to end	Night	51% 8% 11% 21% 8% 1	2114
October	1st to 10th	Night	52% 7% 13% 18% 10%	3800
	11th to 20th	Night	48% 9% 13% 24% 6%	3779
	21st to end	Night	57% 6% 16% 15% 6%	4453
November	1st to 10th	Night	63% 9% 13% 12% 4	2470
	11th to 20th	Night	67% 10% 11% 8% 3	2644
	21st to end	Night	68% 11% 12% 7% 2	2660
December	1st to 10th	Night	73% 9% 9% 6% 2	2660
	11th to 20th	Night	78% 7% 8% 4 2	3295
	21st to end	Night	75% 6% 11% 6% 1	4884

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% 19% 35% 18% 4	370
	11th to 20th	Day	25% 16% 32% 19% 6% 1	370
	21st to end	Day	21% 13% 38% 24% 4	407
February	1st to 10th	Day	26% 17% 34% 16% 6%	370
	11th to 20th	Day	25% 20% 32% 19% 4	366
	21st to end	Day	14% 12% 40% 24% 9% 1	307
March	1st to 10th	Day	19% 14% 37% 24% 6%	370
	11th to 20th	Day	15% 14% 32% 26% 12% 1	440
	21st to end	Day	26% 14% 28% 21% 9% 2	506
April	1st to 10th	Day	27% 12% 35% 17% 8% 1	460
	11th to 20th	Day	25% 11% 27% 27% 10%	452
	21st to end	Day	27% 8% 24% 27% 11% 2	560
May	1st to 10th	Day	30% 8% 26% 25% 8% 3	653
	11th to 20th	Day	21% 6% 22% 32% 15% 5%	674
	21st to end	Day	24% 7% 22% 37% 10% 1	735
June	1st to 10th	Day	36% 6% 18% 27% 11% 2	680
	11th to 20th	Day	20% 6% 21% 34% 18% 1	680
	21st to end	Day	27% 4 17% 33% 16% 3	680
July	1st to 10th	Day	20% 3 13% 40% 21% 2	670
	11th to 20th	Day	19% 3 13% 42% 22% 2	668
	21st to end	Day	21% 2 15% 37% 21% 3	740
August	1st to 10th	Day	16% 4 15% 40% 24% 2	675
	11th to 20th	Day	20% 3 17% 33% 21% 6%	509
	21st to end	Day	17% 6% 18% 35% 20% 4	539
September	1st to 10th	Day	18% 7% 22% 36% 14% 3	490
	11th to 20th	Day	24% 8% 22% 28% 15% 3	477
	21st to end	Day	25% 6% 26% 26% 14% 3	390
October	1st to 10th	Day	23% 7% 20% 34% 13% 2	390
	11th to 20th	Day	24% 10% 29% 18% 16% 3	390
	21st to end	Day	31% 11% 26% 23% 8% 1	429
November	1st to 10th	Day	27% 16% 24% 23% 8% 2	378
	11th to 20th	Day	33% 16% 28% 18% 4 1	370
	21st to end	Day	29% 19% 29% 19% 4 1	370
December	1st to 10th	Day	33% 13% 33% 16% 4 1	370
	11th to 20th	Day	29% 23% 30% 15% 3 1	370
	21st to end	Day	22% 17% 36% 16% 6% 1	407
January	1st to 10th	Night	25% 19% 32% 19% 4	850
	11th to 20th	Night	26% 16% 36% 19% 4	850
	21st to end	Night	20% 16% 36% 21% 6%	929
February	1st to 10th	Night	26% 15% 32% 22% 5% 1	760
	11th to 20th	Night	22% 20% 35% 21% 3	743
	21st to end	Night	17% 13% 36% 24% 9% 1	628
March	1st to 10th	Night	23% 11% 36% 23% 7%	760
	11th to 20th	Night	21% 11% 29% 28% 10% 1	760
	21st to end	Night	27% 11% 30% 25% 6% 2	685
April	1st to 10th	Night	27% 11% 29% 26% 6%	570
	11th to 20th	Night	26% 6% 28% 30% 9% 1	564
	21st to end	Night	31% 5% 27% 25% 11% 1	570
May	1st to 10th	Night	32% 5% 23% 25% 11% 4	570
	11th to 20th	Night	27% 4 20% 31% 15% 3	562
	21st to end	Night	23% 5% 24% 37% 11% 1	616
June	1st to 10th	Night	34% 4 22% 29% 10% 1	570
	11th to 20th	Night	26% 5% 16% 36% 15% 1	570
	21st to end	Night	27% 3 16% 40% 13% 1	570
July	1st to 10th	Night	23% 4 15% 39% 18% 2	556
	11th to 20th	Night	17% 4 16% 45% 17% 1	552
	21st to end	Night	23% 2 13% 39% 21% 1	623
August	1st to 10th	Night	19% 3 15% 41% 21% 2	568
	11th to 20th	Night	21% 2 13% 39% 21% 3	562
	21st to end	Night	18% 5% 16% 39% 22%	627
September	1st to 10th	Night	20% 6% 21% 34% 17% 2	570
	11th to 20th	Night	26% 5% 25% 29% 14% 1	570
	21st to end	Night	25% 6% 24% 32% 10% 2	594
October	1st to 10th	Night	23% 7% 28% 29% 12% 1	760
	11th to 20th	Night	29% 8% 24% 25% 12% 2	760
	21st to end	Night	33% 11% 28% 20% 7% 1	939
November	1st to 10th	Night	31% 14% 26% 23% 6% 1	854
	11th to 20th	Night	33% 16% 27% 16% 6% 1	850
	21st to end	Night	32% 15% 31% 18% 3	850
December	1st to 10th	Night	32% 16% 31% 15% 5%	850
	11th to 20th	Night	33% 21% 27% 16% 3 1	850
	21st to end	Night	28% 16% 33% 17% 5% 1	935

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