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 2026-07-16.

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	16% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	138565
	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 31%	134103
Transparency	Day	50% 6% 11% 21% 10% 1	142058
	Night	52% 6% 11% 19% 10% 1	138699
Seeing	Day	23% 17% 31% 19% 8% 2	16590
	Night	23% 14% 31% 23% 8% 1	25704

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	13% 17% 6% 13% 7% 5% 5% 3 4 5% 21%	4661
	11th to 20th	Day	11% 19% 8% 15% 6% 6% 5% 3 4 4 19%	4650
	21st to end	Day	11% 15% 7% 14% 8% 8% 6% 3 5% 5% 19%	5482
February	1st to 10th	Day	16% 14% 4 12% 7% 5% 5% 3 5% 5% 23%	1890
	11th to 20th	Day	12% 15% 5% 11% 7% 5% 5% 3 5% 5% 27%	1843
	21st to end	Day	19% 14% 4 11% 6% 6% 4 3 5% 5% 22%	1722
March	1st to 10th	Day	19% 14% 4 11% 5% 4 4 3 4 5% 27%	4658
	11th to 20th	Day	11% 13% 4 8% 5% 6% 6% 4 6% 7% 30%	5090
	21st to end	Day	18% 11% 4 8% 5% 3 3 3 4 4 37%	5856
April	1st to 10th	Day	21% 13% 3 7% 5% 4 4 3 4 6% 31%	2520
	11th to 20th	Day	22% 10% 3 7% 4 4 3 2 4 5% 36%	2494
	21st to end	Day	23% 10% 3 7% 4 4 4 3 4 5% 32%	2642
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 3 6% 6% 28%	8237
	11th to 20th	Day	16% 8% 3 8% 5% 4 5% 4 5% 8% 34%	8221
	21st to end	Day	20% 9% 3 6% 4 4 4 3 5% 7% 36%	9060
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 4 4 6% 7% 31%	2940
	11th to 20th	Day	14% 6% 2 6% 5% 5% 6% 5% 6% 9% 37%	2940
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 5% 5% 8% 33%	2940
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 5% 7% 10% 35%	6500
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 7% 9% 43%	6430
	21st to end	Day	10% 5% 2 6% 4 4 4 5% 7% 10% 44%	6926
August	1st to 10th	Day	16% 7% 3 7% 5% 4 4 4 6% 7% 37%	4408
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 6% 9% 37%	4143
	21st to end	Day	14% 5% 2 6% 4 4 4 4 6% 9% 39%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 6% 4 4 5% 8% 42%	2400
	11th to 20th	Day	12% 4 2 6% 4 4 4 4 6% 7% 48%	2400
	21st to end	Day	19% 5% 2 6% 6% 4 4 4 5% 6% 40%	2125
October	1st to 10th	Day	19% 8% 3 7% 5% 3 3 3 5% 6% 38%	2000
	11th to 20th	Day	20% 10% 3 8% 5% 4 5% 4 5% 7% 30%	2000
	21st to end	Day	26% 10% 3 8% 5% 4 4 3 5% 5% 27%	2070
November	1st to 10th	Day	15% 10% 3 7% 6% 5% 5% 4 6% 6% 33%	1851
	11th to 20th	Day	16% 14% 3 10% 6% 4 4 3 5% 5% 29%	1680
	21st to end	Day	20% 16% 4 9% 6% 5% 5% 3 5% 5% 21%	1680
December	1st to 10th	Day	21% 18% 6% 11% 6% 5% 4 4 5% 5% 16%	2680
	11th to 20th	Day	18% 18% 5% 11% 6% 4 4 3 4 4 24%	3066
	21st to end	Day	22% 17% 5% 12% 5% 4 4 2 3 4 22%	4048
January	1st to 10th	Night	13% 17% 8% 14% 7% 5% 4 3 4 5% 21%	8234
	11th to 20th	Night	12% 18% 7% 16% 7% 6% 5% 3 4 5% 18%	8235
	21st to end	Night	12% 15% 6% 12% 7% 6% 5% 3 5% 5% 22%	8683
February	1st to 10th	Night	14% 16% 5% 11% 6% 5% 5% 4 6% 5% 23%	2730
	11th to 20th	Night	13% 16% 5% 11% 6% 5% 5% 4 4 6% 28%	2663
	21st to end	Night	14% 16% 4 9% 5% 5% 4 3 6% 6% 27%	2083
March	1st to 10th	Night	21% 14% 3 9% 5% 4 4 3 4 3 30%	5401
	11th to 20th	Night	13% 13% 3 7% 5% 5% 5% 4 7% 8% 30%	5089
	21st to end	Night	18% 11% 3 9% 5% 4 4 3 5% 6% 34%	5318
April	1st to 10th	Night	21% 10% 3 8% 5% 4 3 4 4 6% 33%	2100
	11th to 20th	Night	20% 10% 3 7% 5% 4 3 3 4 5% 36%	2075
	21st to end	Night	20% 10% 3 7% 5% 4 4 3 5% 6% 34%	1995
May	1st to 10th	Night	22% 10% 3 8% 5% 5% 5% 4 6% 7% 24%	4660
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 5% 7% 39%	4639
	21st to end	Night	18% 8% 2 6% 4 4 4 4 6% 8% 36%	5114
June	1st to 10th	Night	16% 7% 2 6% 5% 5% 5% 4 6% 7% 35%	1680
	11th to 20th	Night	14% 7% 2 5% 5% 4 5% 4 5% 8% 42%	1680
	21st to end	Night	17% 6% 2 7% 5% 5% 5% 4 6% 8% 35%	1680
July	1st to 10th	Night	12% 6% 3 7% 6% 5% 5% 5% 7% 9% 36%	3680
	11th to 20th	Night	11% 6% 2 6% 3 5% 5% 4 5% 7% 45%	3640
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 7% 48%	3926
August	1st to 10th	Night	11% 7% 2 7% 4 4 4 4 6% 8% 43%	2738
	11th to 20th	Night	11% 7% 3 6% 4 4 4 3 6% 7% 43%	2875
	21st to end	Night	10% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	3583
September	1st to 10th	Night	11% 6% 2 6% 5% 4 4 3 5% 7% 47%	2000
	11th to 20th	Night	13% 5% 1 4 3 3 3 3 5% 7% 51%	2000
	21st to end	Night	17% 6% 3 6% 4 4 3 3 5% 6% 43%	2270
October	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 4 8% 39%	2400
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 3 6% 7% 33%	2400
	21st to end	Night	23% 10% 4 7% 4 4 4 4 5% 6% 30%	2769
November	1st to 10th	Night	18% 11% 3 7% 4 4 5% 4 4 6% 35%	2667
	11th to 20th	Night	16% 12% 5% 8% 5% 4 5% 3 5% 5% 32%	2938
	21st to end	Night	20% 15% 4 9% 6% 4 4 3 5% 7% 24%	2940
December	1st to 10th	Night	19% 18% 5% 12% 7% 5% 5% 4 4 5% 17%	4720
	11th to 20th	Night	22% 14% 5% 9% 5% 5% 4 3 4 4 26%	5348
	21st to end	Night	26% 17% 4 12% 5% 4 3 3 4 4 19%	7150

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	63% 7% 12% 13% 4 1	4662
	11th to 20th	Day	68% 5% 9% 9% 8% 1	4680
	21st to end	Day	67% 6% 8% 10% 9% 1	5482
February	1st to 10th	Day	61% 6% 9% 18% 6% 1	1890
	11th to 20th	Day	58% 5% 11% 17% 9% 1	1852
	21st to end	Day	64% 5% 10% 11% 9% 1	1722
March	1st to 10th	Day	59% 4 10% 13% 12% 2	4661
	11th to 20th	Day	51% 6% 17% 14% 11% 1	5090
	21st to end	Day	50% 4 6% 19% 18% 3	5856
April	1st to 10th	Day	55% 5% 7% 17% 13% 3	2520
	11th to 20th	Day	52% 3 7% 20% 16% 2	2494
	21st to end	Day	54% 5% 8% 16% 14% 3	2642
May	1st to 10th	Day	54% 7% 9% 19% 10% 2	8249
	11th to 20th	Day	47% 8% 12% 22% 11% 1	8225
	21st to end	Day	47% 6% 10% 24% 11% 1	9061
June	1st to 10th	Day	51% 6% 9% 19% 13% 1	2940
	11th to 20th	Day	42% 8% 11% 25% 12% 2	2940
	21st to end	Day	47% 8% 13% 20% 11% 1	2940
July	1st to 10th	Day	41% 8% 14% 26% 10%	6430
	11th to 20th	Day	35% 8% 14% 28% 15%	6355
	21st to end	Day	33% 8% 13% 35% 10%	6728
August	1st to 10th	Day	45% 6% 14% 24% 10% 1	4394
	11th to 20th	Day	41% 7% 15% 27% 9% 1	4182
	21st to end	Day	39% 6% 15% 28% 11%	4312
September	1st to 10th	Day	39% 7% 13% 28% 12% 1	2400
	11th to 20th	Day	34% 7% 12% 31% 15%	2400
	21st to end	Day	45% 6% 13% 24% 11%	2125
October	1st to 10th	Day	42% 6% 12% 31% 8%	3260
	11th to 20th	Day	44% 6% 14% 25% 10% 1	3240
	21st to end	Day	48% 7% 16% 22% 6%	3321
November	1st to 10th	Day	50% 6% 15% 21% 8%	1851
	11th to 20th	Day	56% 6% 12% 16% 10% 1	1680
	21st to end	Day	64% 8% 11% 10% 7%	1680
December	1st to 10th	Day	69% 6% 13% 9% 2	2680
	11th to 20th	Day	64% 6% 10% 13% 6%	3066
	21st to end	Day	67% 5% 8% 15% 6%	4048
January	1st to 10th	Night	65% 6% 11% 13% 5%	8234
	11th to 20th	Night	68% 6% 9% 10% 7%	8280
	21st to end	Night	62% 6% 8% 13% 11% 1	8683
February	1st to 10th	Night	60% 6% 10% 16% 6% 1	2730
	11th to 20th	Night	58% 5% 10% 19% 8% 1	2665
	21st to end	Night	57% 6% 9% 17% 10% 1	2083
March	1st to 10th	Night	59% 4 7% 16% 10% 4	5411
	11th to 20th	Night	49% 7% 18% 16% 8% 2	5090
	21st to end	Night	50% 5% 7% 22% 13% 2	5318
April	1st to 10th	Night	53% 5% 8% 20% 13% 2	2100
	11th to 20th	Night	51% 5% 7% 19% 17% 1	2075
	21st to end	Night	51% 6% 7% 16% 16% 3	1995
May	1st to 10th	Night	55% 8% 9% 18% 8% 2	4666
	11th to 20th	Night	43% 7% 12% 22% 14% 1	4640
	21st to end	Night	45% 6% 13% 20% 16% 1	5115
June	1st to 10th	Night	45% 8% 12% 20% 14% 1	1680
	11th to 20th	Night	39% 7% 12% 23% 17% 2	1680
	21st to end	Night	46% 9% 13% 20% 12% 1	1680
July	1st to 10th	Night	40% 8% 16% 19% 15% 1	3640
	11th to 20th	Night	35% 8% 14% 31% 13%	3592
	21st to end	Night	33% 8% 12% 31% 15% 1	3809
August	1st to 10th	Night	38% 7% 14% 27% 13% 1	2728
	11th to 20th	Night	39% 6% 15% 30% 9% 1	2902
	21st to end	Night	35% 9% 16% 28% 11% 1	3583
September	1st to 10th	Night	36% 8% 12% 30% 14% 1	2000
	11th to 20th	Night	32% 7% 13% 35% 12% 1	2000
	21st to end	Night	41% 6% 11% 29% 12% 1	2270
October	1st to 10th	Night	39% 6% 12% 29% 14%	3920
	11th to 20th	Night	43% 6% 16% 26% 9%	3899
	21st to end	Night	47% 6% 15% 24% 8%	4468
November	1st to 10th	Night	49% 6% 14% 21% 9% 1	2667
	11th to 20th	Night	52% 6% 11% 19% 10% 1	2938
	21st to end	Night	61% 6% 11% 16% 5%	2940
December	1st to 10th	Night	68% 6% 12% 11% 3	4720
	11th to 20th	Night	61% 6% 9% 14% 9% 1	5348
	21st to end	Night	68% 5% 7% 11% 8%	7150

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	24% 21% 40% 11% 4	220
	11th to 20th	Day	31% 21% 30% 14% 3	220
	21st to end	Day	25% 22% 33% 17% 3	308
February	1st to 10th	Day	30% 24% 30% 14% 2	330
	11th to 20th	Day	26% 31% 33% 9% 1	330
	21st to end	Day	31% 24% 34% 10% 1	438
March	1st to 10th	Day	27% 23% 31% 15% 3	540
	11th to 20th	Day	32% 19% 31% 14% 3	540
	21st to end	Day	32% 23% 27% 12% 5% 1	594
April	1st to 10th	Day	33% 24% 26% 11% 5%	540
	11th to 20th	Day	34% 21% 21% 15% 7% 2	532
	21st to end	Day	30% 23% 28% 12% 5% 2	540
May	1st to 10th	Day	25% 18% 32% 17% 6% 2	543
	11th to 20th	Day	19% 16% 37% 18% 8% 2	567
	21st to end	Day	22% 16% 32% 20% 9% 1	617
June	1st to 10th	Day	22% 13% 36% 20% 8% 1	570
	11th to 20th	Day	16% 19% 35% 20% 9% 2	570
	21st to end	Day	20% 16% 32% 19% 11% 2	570
July	1st to 10th	Day	12% 10% 32% 30% 13% 3	565
	11th to 20th	Day	12% 13% 34% 25% 13% 3	544
	21st to end	Day	14% 10% 34% 29% 10% 4	576
August	1st to 10th	Day	15% 9% 29% 32% 12% 3	526
	11th to 20th	Day	14% 8% 28% 30% 14% 6%	519
	21st to end	Day	15% 11% 32% 24% 11% 6%	583
September	1st to 10th	Day	15% 14% 28% 24% 12% 6%	530
	11th to 20th	Day	11% 9% 24% 26% 19% 10%	530
	21st to end	Day	19% 13% 29% 23% 12% 4	530
October	1st to 10th	Day	23% 16% 26% 19% 11% 5%	530
	11th to 20th	Day	26% 21% 30% 15% 6% 1	530
	21st to end	Day	29% 20% 28% 20% 4	511
November	1st to 10th	Day	29% 19% 29% 18% 5%	425
	11th to 20th	Day	26% 23% 32% 12% 6% 1	220
	21st to end	Day	31% 24% 32% 10% 3	220
December	1st to 10th	Day	32% 19% 35% 12% 3	220
	11th to 20th	Day	35% 26% 29% 8% 2	220
	21st to end	Day	33% 21% 29% 13% 4	242
January	1st to 10th	Night	27% 20% 33% 17% 3	840
	11th to 20th	Night	31% 17% 29% 19% 4	840
	21st to end	Night	21% 16% 37% 22% 5%	924
February	1st to 10th	Night	24% 21% 33% 19% 3	840
	11th to 20th	Night	22% 20% 38% 17% 3	823
	21st to end	Night	24% 22% 32% 18% 3	692
March	1st to 10th	Night	26% 18% 33% 20% 4	840
	11th to 20th	Night	26% 20% 30% 19% 4	840
	21st to end	Night	25% 16% 32% 20% 7% 1	924
April	1st to 10th	Night	28% 16% 33% 21% 2	840
	11th to 20th	Night	26% 19% 29% 19% 6% 1	831
	21st to end	Night	26% 15% 30% 22% 7% 1	735
May	1st to 10th	Night	25% 11% 28% 25% 9% 1	420
	11th to 20th	Night	18% 14% 34% 28% 6%	414
	21st to end	Night	15% 9% 39% 25% 11%	455
June	1st to 10th	Night	17% 9% 31% 35% 8%	420
	11th to 20th	Night	16% 11% 34% 30% 9% 1	420
	21st to end	Night	19% 7% 33% 29% 12%	420
July	1st to 10th	Night	8% 7% 35% 38% 12%	410
	11th to 20th	Night	11% 7% 28% 40% 13% 1	398
	21st to end	Night	9% 5% 30% 43% 12% 1	438
August	1st to 10th	Night	12% 5% 29% 34% 19%	485
	11th to 20th	Night	11% 4 24% 34% 24% 2	594
	21st to end	Night	12% 8% 31% 31% 16% 2	877
September	1st to 10th	Night	13% 7% 33% 30% 16% 1	800
	11th to 20th	Night	16% 7% 21% 30% 22% 4	800
	21st to end	Night	19% 10% 26% 25% 18% 2	800
October	1st to 10th	Night	21% 12% 27% 23% 15% 2	800
	11th to 20th	Night	25% 13% 31% 22% 8% 1	800
	21st to end	Night	28% 16% 27% 23% 5%	880
November	1st to 10th	Night	27% 16% 29% 20% 7%	820
	11th to 20th	Night	24% 19% 32% 16% 9%	840
	21st to end	Night	29% 19% 29% 18% 6%	840
December	1st to 10th	Night	29% 19% 32% 17% 3	840
	11th to 20th	Night	28% 19% 31% 18% 3	840
	21st to end	Night	34% 17% 28% 18% 2	924

Page generated at 2026-07-13 03:14:53. Version ac07

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