Perth
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perth clear sky chart from 2005-11-06 to 2024-10-20.

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% 10% 3 8% 5% 5% 4 4 5% 7% 33%	101449
	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 32%	97244
Transparency	Day	50% 7% 12% 21% 10% 1	101207
	Night	52% 7% 11% 19% 10% 1	97106
Seeing	Day	23% 17% 30% 20% 8% 2	14444
	Night	23% 14% 31% 23% 8% 1	23542

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	15% 17% 5% 11% 6% 5% 4 3 4 4 24%	2517
	11th to 20th	Day	13% 17% 6% 13% 7% 6% 5% 3 4 4 22%	2520
	21st to end	Day	17% 18% 7% 13% 7% 6% 5% 3 3 4 17%	2964
February	1st to 10th	Day	16% 15% 4 12% 7% 5% 5% 3 5% 5% 23%	1710
	11th to 20th	Day	12% 15% 5% 11% 6% 5% 4 3 4 5% 28%	1663
	21st to end	Day	20% 14% 4 10% 6% 6% 4 3 5% 5% 23%	1562
March	1st to 10th	Day	20% 14% 4 9% 5% 5% 4 3 4 4 28%	3169
	11th to 20th	Day	12% 16% 5% 8% 6% 6% 5% 3 5% 5% 30%	3480
	21st to end	Day	13% 10% 3 7% 5% 3 4 3 5% 6% 42%	3996
April	1st to 10th	Day	20% 13% 3 7% 4 4 4 3 5% 6% 32%	2280
	11th to 20th	Day	21% 9% 3 7% 4 4 3 2 4 5% 36%	2254
	21st to end	Day	24% 9% 3 7% 4 4 4 3 4 5% 32%	2390
May	1st to 10th	Day	20% 8% 3 8% 5% 5% 4 4 6% 6% 32%	4426
	11th to 20th	Day	16% 9% 3 8% 5% 4 4 3 5% 8% 33%	4398
	21st to end	Day	14% 6% 2 6% 4 5% 5% 4 6% 8% 40%	4837
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	2660
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 6% 9% 37%	2660
	21st to end	Day	18% 7% 3 7% 5% 5% 5% 5% 5% 8% 32%	2660
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	13% 5% 2 6% 4 5% 5% 4 7% 10% 40%	4440
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 8% 41%	4814
August	1st to 10th	Day	17% 8% 3 7% 5% 4 4 4 6% 7% 35%	4273
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 6% 9% 37%	4014
	21st to end	Day	15% 5% 2 6% 4 4 4 4 6% 9% 39%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 6% 4 4 5% 8% 42%	2280
	11th to 20th	Day	12% 4 3 6% 4 4 4 4 5% 7% 47%	2280
	21st to end	Day	19% 5% 2 6% 6% 4 3 4 5% 6% 40%	2019
October	1st to 10th	Day	20% 9% 3 7% 5% 3 3 3 5% 6% 36%	1900
	11th to 20th	Day	20% 10% 3 8% 6% 4 4 4 5% 7% 30%	1890
	21st to end	Day	27% 10% 3 8% 5% 3 4 3 5% 5% 26%	1864
November	1st to 10th	Day	15% 9% 2 7% 6% 6% 5% 4 6% 6% 34%	1671
	11th to 20th	Day	17% 14% 3 9% 6% 4 4 3 5% 5% 29%	1520
	21st to end	Day	20% 16% 4 9% 6% 5% 5% 3 5% 5% 21%	1520
December	1st to 10th	Day	22% 17% 5% 10% 7% 5% 4 4 5% 5% 17%	1520
	11th to 20th	Day	21% 15% 6% 8% 5% 4 4 3 4 5% 25%	1906
	21st to end	Day	27% 17% 5% 11% 5% 3 3 2 3 3 20%	2772
January	1st to 10th	Night	17% 15% 8% 11% 6% 5% 4 3 5% 5% 21%	4429
	11th to 20th	Night	13% 16% 5% 14% 8% 6% 5% 3 4 5% 21%	4440
	21st to end	Night	18% 18% 6% 12% 7% 6% 4 2 4 4 20%	4690
February	1st to 10th	Night	15% 17% 5% 11% 6% 5% 5% 3 6% 5% 22%	2470
	11th to 20th	Night	13% 15% 5% 10% 6% 5% 5% 4 4 6% 29%	2403
	21st to end	Night	15% 15% 4 9% 6% 5% 4 3 5% 5% 28%	1891
March	1st to 10th	Night	20% 12% 4 9% 5% 4 4 3 4 4 32%	3670
	11th to 20th	Night	17% 17% 4 8% 5% 4 4 3 5% 6% 26%	3479
	21st to end	Night	14% 11% 3 7% 5% 3 3 3 5% 6% 40%	3636
April	1st to 10th	Night	21% 9% 3 8% 5% 4 3 4 4 5% 34%	1900
	11th to 20th	Night	20% 9% 3 7% 5% 4 3 3 4 5% 37%	1875
	21st to end	Night	20% 11% 3 7% 5% 4 4 3 5% 5% 33%	1805
May	1st to 10th	Night	22% 10% 3 8% 4 4 5% 4 5% 7% 29%	2514
	11th to 20th	Night	19% 9% 3 7% 4 3 4 4 5% 6% 37%	2495
	21st to end	Night	14% 7% 2 6% 4 4 5% 3 6% 8% 42%	2738
June	1st to 10th	Night	16% 7% 3 7% 5% 6% 5% 4 6% 7% 34%	1520
	11th to 20th	Night	13% 7% 2 5% 5% 4 5% 4 6% 8% 41%	1520
	21st to end	Night	18% 6% 3 7% 5% 5% 5% 4 6% 8% 35%	1520
July	1st to 10th	Night	10% 7% 3 7% 7% 5% 6% 5% 6% 9% 36%	2520
	11th to 20th	Night	13% 6% 2 6% 3 5% 5% 4 5% 7% 44%	2520
	21st to end	Night	9% 6% 2 6% 4 5% 5% 4 6% 7% 44%	2739
August	1st to 10th	Night	11% 7% 2 7% 4 4 4 4 6% 8% 42%	2653
	11th to 20th	Night	11% 8% 3 6% 4 4 4 3 6% 7% 42%	2785
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	3473
September	1st to 10th	Night	11% 6% 2 6% 5% 4 4 3 4 7% 46%	1900
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 5% 7% 51%	1900
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 6% 43%	2156
October	1st to 10th	Night	17% 8% 3 7% 4 4 4 4 4 8% 37%	2280
	11th to 20th	Night	21% 9% 3 7% 5% 4 4 3 5% 7% 33%	2275
	21st to end	Night	24% 10% 4 8% 5% 4 4 4 4 6% 28%	2491
November	1st to 10th	Night	19% 10% 3 7% 4 3 5% 4 3 6% 36%	2407
	11th to 20th	Night	17% 12% 4 8% 5% 4 5% 3 5% 6% 32%	2658
	21st to end	Night	21% 14% 4 8% 6% 4 4 3 5% 7% 24%	2660
December	1st to 10th	Night	20% 19% 5% 11% 6% 4 3 4 4 5% 18%	2660
	11th to 20th	Night	21% 15% 4 9% 5% 5% 3 3 4 4 27%	3288
	21st to end	Night	30% 17% 4 12% 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	60% 8% 14% 11% 6% 2	2518
	11th to 20th	Day	65% 6% 10% 11% 8% 1	2520
	21st to end	Day	71% 6% 9% 9% 5% 1	2964
February	1st to 10th	Day	61% 6% 9% 18% 5% 1	1710
	11th to 20th	Day	58% 5% 11% 16% 10% 1	1672
	21st to end	Day	62% 6% 10% 11% 10% 1	1562
March	1st to 10th	Day	60% 4 10% 17% 9% 1	3172
	11th to 20th	Day	54% 7% 13% 15% 10% 2	3480
	21st to end	Day	43% 5% 8% 24% 16% 4	3996
April	1st to 10th	Day	54% 5% 7% 18% 13% 3	2280
	11th to 20th	Day	51% 3 8% 20% 16% 2	2254
	21st to end	Day	54% 6% 8% 15% 14% 4	2390
May	1st to 10th	Day	50% 7% 10% 21% 11% 2	4440
	11th to 20th	Day	48% 8% 12% 21% 9% 1	4402
	21st to end	Day	40% 7% 12% 24% 15% 2	4837
June	1st to 10th	Day	52% 7% 9% 19% 12% 1	2660
	11th to 20th	Day	41% 8% 11% 24% 13% 2	2660
	21st to end	Day	48% 8% 12% 19% 11% 1	2660
July	1st to 10th	Day	40% 8% 14% 26% 12% 1	4370
	11th to 20th	Day	37% 8% 15% 31% 9%	4365
	21st to end	Day	37% 9% 12% 30% 11%	4661
August	1st to 10th	Day	47% 6% 14% 23% 10% 1	4259
	11th to 20th	Day	41% 7% 15% 26% 9% 1	4053
	21st to end	Day	39% 6% 15% 28% 11%	4180
September	1st to 10th	Day	40% 7% 13% 27% 12% 1	2280
	11th to 20th	Day	35% 7% 12% 30% 15%	2280
	21st to end	Day	44% 6% 14% 25% 11%	2019
October	1st to 10th	Day	49% 6% 12% 23% 11% 1	1900
	11th to 20th	Day	54% 8% 12% 18% 7% 1	1890
	21st to end	Day	59% 6% 11% 16% 7% 1	1864
November	1st to 10th	Day	49% 6% 16% 20% 8% 1	1671
	11th to 20th	Day	56% 6% 12% 15% 10% 1	1520
	21st to end	Day	64% 8% 11% 10% 7%	1520
December	1st to 10th	Day	68% 7% 12% 10% 3	1520
	11th to 20th	Day	62% 7% 14% 12% 4	1906
	21st to end	Day	71% 5% 7% 14% 3	2772
January	1st to 10th	Night	64% 8% 13% 12% 3	4429
	11th to 20th	Night	65% 8% 12% 11% 4 1	4440
	21st to end	Night	69% 6% 9% 10% 6% 1	4690
February	1st to 10th	Night	61% 6% 11% 16% 6% 1	2470
	11th to 20th	Night	57% 5% 10% 19% 8%	2405
	21st to end	Night	56% 7% 9% 17% 10% 1	1891
March	1st to 10th	Night	56% 4 9% 21% 9% 1	3680
	11th to 20th	Night	57% 6% 13% 15% 8% 1	3480
	21st to end	Night	44% 6% 7% 24% 17% 3	3636
April	1st to 10th	Night	52% 5% 7% 20% 13% 2	1900
	11th to 20th	Night	50% 5% 7% 19% 17% 1	1875
	21st to end	Night	52% 6% 8% 15% 16% 3	1805
May	1st to 10th	Night	52% 8% 9% 19% 10% 3	2520
	11th to 20th	Night	46% 9% 10% 23% 10% 1	2496
	21st to end	Night	38% 7% 14% 21% 18% 2	2739
June	1st to 10th	Night	46% 8% 13% 20% 12% 1	1520
	11th to 20th	Night	39% 7% 12% 22% 17% 2	1520
	21st to end	Night	46% 9% 13% 19% 13% 1	1520
July	1st to 10th	Night	40% 9% 14% 21% 14% 2	2480
	11th to 20th	Night	37% 8% 13% 33% 10%	2472
	21st to end	Night	36% 9% 14% 24% 15% 1	2651
August	1st to 10th	Night	38% 7% 14% 26% 13% 1	2643
	11th to 20th	Night	39% 6% 15% 30% 9%	2812
	21st to end	Night	35% 9% 17% 27% 11% 1	3473
September	1st to 10th	Night	36% 8% 12% 29% 14% 1	1900
	11th to 20th	Night	32% 7% 13% 34% 12% 1	1900
	21st to end	Night	41% 6% 11% 30% 11% 1	2156
October	1st to 10th	Night	47% 6% 10% 22% 14% 1	2280
	11th to 20th	Night	51% 6% 14% 20% 8% 1	2275
	21st to end	Night	56% 7% 12% 15% 9%	2491
November	1st to 10th	Night	48% 6% 14% 21% 10% 1	2407
	11th to 20th	Night	52% 6% 12% 18% 10% 2	2658
	21st to end	Night	60% 7% 12% 16% 5%	2660
December	1st to 10th	Night	68% 6% 10% 12% 4	2660
	11th to 20th	Night	61% 6% 10% 15% 8% 1	3288
	21st to end	Night	72% 6% 8% 12% 2	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	26% 22% 36% 12% 4	180
	11th to 20th	Day	32% 22% 28% 16% 2	180
	21st to end	Day	27% 18% 34% 18% 3	252
February	1st to 10th	Day	31% 27% 28% 12% 1	270
	11th to 20th	Day	26% 32% 35% 7% 1	270
	21st to end	Day	33% 23% 32% 11% 2	374
March	1st to 10th	Day	27% 21% 32% 17% 3	460
	11th to 20th	Day	28% 20% 32% 15% 4	460
	21st to end	Day	31% 23% 27% 13% 5% 1	506
April	1st to 10th	Day	31% 22% 27% 13% 6%	460
	11th to 20th	Day	32% 21% 21% 15% 7% 2	452
	21st to end	Day	29% 23% 28% 13% 6% 2	460
May	1st to 10th	Day	24% 17% 32% 19% 6% 2	463
	11th to 20th	Day	20% 16% 35% 20% 8% 2	487
	21st to end	Day	19% 16% 33% 21% 10% 2	529
June	1st to 10th	Day	23% 13% 34% 21% 8% 1	490
	11th to 20th	Day	15% 18% 33% 22% 10% 2	490
	21st to end	Day	21% 16% 31% 19% 11% 2	490
July	1st to 10th	Day	13% 9% 31% 31% 14% 3	485
	11th to 20th	Day	12% 13% 34% 24% 14% 4	484
	21st to end	Day	14% 8% 33% 30% 11% 4	532
August	1st to 10th	Day	16% 10% 29% 30% 11% 3	486
	11th to 20th	Day	14% 8% 28% 30% 14% 6%	479
	21st to end	Day	14% 12% 33% 24% 11% 7%	539
September	1st to 10th	Day	16% 13% 29% 24% 12% 6%	490
	11th to 20th	Day	12% 9% 24% 24% 19% 11%	490
	21st to end	Day	20% 12% 28% 24% 11% 4	490
October	1st to 10th	Day	25% 16% 26% 19% 10% 4	490
	11th to 20th	Day	27% 21% 28% 17% 6% 2	486
	21st to end	Day	31% 19% 27% 20% 3	437
November	1st to 10th	Day	27% 17% 30% 19% 6%	365
	11th to 20th	Day	29% 24% 29% 12% 6% 1	180
	21st to end	Day	29% 23% 33% 11% 3	180
December	1st to 10th	Day	30% 21% 35% 12% 2	180
	11th to 20th	Day	34% 26% 28% 9% 2 1	180
	21st to end	Day	33% 23% 28% 12% 3 1	198
January	1st to 10th	Night	28% 20% 31% 17% 4	760
	11th to 20th	Night	31% 17% 29% 19% 4 1	760
	21st to end	Night	21% 15% 37% 23% 5%	836
February	1st to 10th	Night	25% 22% 33% 18% 3	760
	11th to 20th	Night	22% 20% 39% 16% 3	743
	21st to end	Night	24% 22% 32% 18% 4	628
March	1st to 10th	Night	25% 17% 33% 21% 4	760
	11th to 20th	Night	26% 21% 30% 19% 4 1	760
	21st to end	Night	23% 15% 32% 21% 7% 1	836
April	1st to 10th	Night	27% 15% 33% 22% 3	760
	11th to 20th	Night	26% 19% 29% 19% 6% 1	751
	21st to end	Night	26% 15% 29% 22% 8% 1	665
May	1st to 10th	Night	25% 11% 27% 26% 10% 1	380
	11th to 20th	Night	18% 12% 34% 29% 6% 1	374
	21st to end	Night	14% 9% 39% 26% 12%	411
June	1st to 10th	Night	17% 9% 30% 35% 8%	380
	11th to 20th	Night	16% 9% 32% 32% 9% 1	380
	21st to end	Night	20% 6% 33% 28% 13%	380
July	1st to 10th	Night	8% 6% 34% 38% 13% 1	370
	11th to 20th	Night	11% 7% 28% 39% 14% 1	368
	21st to end	Night	10% 5% 30% 43% 12% 1	416
August	1st to 10th	Night	12% 5% 30% 34% 19%	460
	11th to 20th	Night	11% 4 24% 34% 24% 2	564
	21st to end	Night	12% 8% 31% 31% 16% 2	833
September	1st to 10th	Night	14% 7% 33% 30% 16% 1	760
	11th to 20th	Night	17% 7% 21% 29% 22% 4	760
	21st to end	Night	19% 10% 26% 25% 19% 2	760
October	1st to 10th	Night	22% 13% 27% 23% 14% 2	760
	11th to 20th	Night	26% 13% 30% 23% 8% 1	759
	21st to end	Night	29% 16% 27% 23% 5%	792
November	1st to 10th	Night	27% 15% 30% 20% 8%	740
	11th to 20th	Night	25% 19% 31% 16% 9% 1	760
	21st to end	Night	29% 18% 29% 18% 6%	760
December	1st to 10th	Night	31% 18% 32% 17% 3	760
	11th to 20th	Night	28% 18% 31% 18% 3 1	760
	21st to end	Night	35% 17% 29% 18% 2	836

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