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 2025-03-12.

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% 4 4 5% 7% 32%	108935
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	110051
Transparency	Day	50% 7% 12% 21% 10% 1	112474
	Night	52% 6% 11% 19% 10% 1	114676
Seeing	Day	23% 17% 30% 19% 8% 2	14821
	Night	23% 14% 31% 23% 8% 1	24114

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% 17% 6% 13% 7% 5% 5% 3 4 5% 22%	3597
	11th to 20th	Day	11% 17% 7% 14% 7% 5% 5% 3 5% 5% 22%	3570
	21st to end	Day	14% 17% 7% 14% 7% 6% 5% 3 4 5% 20%	4236
February	1st to 10th	Day	16% 15% 5% 12% 7% 5% 5% 4 5% 5% 23%	1800
	11th to 20th	Day	12% 15% 5% 11% 7% 5% 4 3 5% 5% 28%	1753
	21st to end	Day	19% 14% 4 10% 6% 6% 4 3 5% 5% 23%	1642
March	1st to 10th	Day	20% 14% 4 9% 5% 5% 4 3 4 4 28%	3271
	11th to 20th	Day	12% 16% 5% 8% 6% 6% 5% 3 5% 5% 30%	3491
	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% 5% 4 4 4 5% 7% 30%	1900
	21st to end	Day	26% 10% 3 8% 5% 4 4 3 5% 5% 27%	1967
November	1st to 10th	Day	15% 9% 2 7% 6% 5% 6% 4 6% 6% 34%	1761
	11th to 20th	Day	17% 14% 3 9% 6% 4 4 3 5% 6% 30%	1600
	21st to end	Day	21% 15% 4 9% 6% 5% 5% 3 5% 5% 20%	1600
December	1st to 10th	Day	21% 17% 6% 11% 6% 5% 4 4 5% 5% 16%	2600
	11th to 20th	Day	18% 18% 5% 11% 6% 4 4 2 4 4 24%	2986
	21st to end	Day	22% 17% 5% 12% 5% 4 4 2 3 4 22%	3960
January	1st to 10th	Night	14% 17% 8% 13% 7% 5% 4 3 4 5% 21%	6349
	11th to 20th	Night	11% 16% 7% 15% 7% 6% 5% 3 4 5% 20%	6315
	21st to end	Night	15% 17% 6% 12% 7% 6% 4 3 5% 4 20%	6704
February	1st to 10th	Night	14% 17% 5% 11% 6% 5% 5% 4 6% 5% 23%	2600
	11th to 20th	Night	13% 15% 5% 10% 6% 5% 5% 4 4 6% 28%	2533
	21st to end	Night	14% 16% 4 9% 5% 5% 4 3 6% 6% 28%	1987
March	1st to 10th	Night	20% 13% 4 9% 5% 4 4 3 4 4 32%	3788
	11th to 20th	Night	17% 17% 4 8% 5% 4 4 3 5% 6% 27%	3497
	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%	2280
	21st to end	Night	23% 10% 4 8% 4 4 4 4 5% 6% 28%	2630
November	1st to 10th	Night	18% 10% 3 7% 4 4 5% 4 3 6% 36%	2537
	11th to 20th	Night	16% 12% 4 8% 5% 4 5% 3 5% 6% 32%	2798
	21st to end	Night	20% 14% 4 8% 6% 4 4 3 5% 7% 24%	2800
December	1st to 10th	Night	19% 18% 5% 12% 7% 5% 5% 4 4 5% 17%	4580
	11th to 20th	Night	22% 14% 5% 9% 5% 5% 4 3 4 4 26%	5208
	21st to end	Night	26% 17% 4 12% 5% 4 3 3 4 4 19%	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	62% 7% 11% 14% 5% 1	3598
	11th to 20th	Day	65% 6% 10% 11% 8% 1	3600
	21st to end	Day	66% 6% 8% 10% 8% 1	4236
February	1st to 10th	Day	61% 6% 9% 18% 6% 1	1800
	11th to 20th	Day	58% 5% 11% 16% 9% 1	1762
	21st to end	Day	63% 5% 10% 11% 10% 1	1642
March	1st to 10th	Day	59% 4 10% 16% 9% 1	3274
	11th to 20th	Day	54% 7% 13% 15% 10% 2	3491
	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	43% 6% 12% 31% 7%	3160
	11th to 20th	Day	45% 6% 14% 25% 10%	3140
	21st to end	Day	48% 7% 16% 22% 6%	3218
November	1st to 10th	Day	49% 6% 15% 21% 8% 1	1761
	11th to 20th	Day	56% 6% 12% 16% 10% 1	1600
	21st to end	Day	64% 8% 11% 10% 7%	1600
December	1st to 10th	Day	69% 7% 13% 9% 2	2600
	11th to 20th	Day	64% 6% 10% 13% 6%	2986
	21st to end	Day	67% 5% 8% 15% 6%	3960
January	1st to 10th	Night	65% 6% 11% 13% 5%	6349
	11th to 20th	Night	66% 7% 11% 11% 6%	6360
	21st to end	Night	66% 5% 8% 9% 10% 1	6704
February	1st to 10th	Night	61% 6% 11% 16% 6% 1	2600
	11th to 20th	Night	58% 5% 10% 19% 8% 1	2535
	21st to end	Night	56% 6% 10% 18% 10% 1	1987
March	1st to 10th	Night	56% 4 9% 20% 9% 1	3798
	11th to 20th	Night	57% 6% 14% 15% 8% 1	3498
	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	40% 6% 12% 29% 13%	3800
	11th to 20th	Night	43% 6% 16% 26% 9%	3779
	21st to end	Night	47% 7% 15% 24% 8%	4329
November	1st to 10th	Night	48% 6% 14% 21% 10% 1	2537
	11th to 20th	Night	52% 6% 11% 19% 10% 1	2798
	21st to end	Night	61% 7% 11% 16% 5%	2800
December	1st to 10th	Night	69% 6% 12% 11% 2	4580
	11th to 20th	Night	62% 6% 9% 14% 9% 1	5208
	21st to end	Night	68% 5% 7% 11% 9%	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% 22% 39% 11% 4	200
	11th to 20th	Day	30% 21% 31% 16% 3	200
	21st to end	Day	26% 21% 34% 16% 3	280
February	1st to 10th	Day	33% 26% 28% 12% 1	300
	11th to 20th	Day	27% 32% 32% 8% 1	300
	21st to end	Day	32% 23% 33% 10% 1	406
March	1st to 10th	Day	28% 21% 31% 16% 3	500
	11th to 20th	Day	28% 20% 32% 15% 4	464
	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	26% 21% 29% 16% 6% 1	490
	21st to end	Day	30% 20% 27% 19% 4	474
November	1st to 10th	Day	27% 18% 30% 19% 6%	395
	11th to 20th	Day	27% 24% 31% 12% 6% 1	200
	21st to end	Day	31% 24% 32% 12% 3	200
December	1st to 10th	Day	31% 20% 35% 13% 3	200
	11th to 20th	Day	35% 26% 28% 9% 3 1	200
	21st to end	Day	31% 21% 29% 14% 4	220
January	1st to 10th	Night	28% 20% 33% 17% 3	800
	11th to 20th	Night	30% 17% 29% 19% 4 1	800
	21st to end	Night	20% 16% 37% 22% 4	880
February	1st to 10th	Night	25% 22% 33% 18% 2	800
	11th to 20th	Night	23% 20% 38% 16% 3	783
	21st to end	Night	24% 22% 32% 19% 3	660
March	1st to 10th	Night	26% 17% 33% 20% 4	800
	11th to 20th	Night	26% 21% 30% 19% 4 1	767
	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	760
	21st to end	Night	29% 16% 27% 23% 5%	836
November	1st to 10th	Night	27% 16% 29% 20% 7%	780
	11th to 20th	Night	24% 19% 32% 16% 9% 1	800
	21st to end	Night	29% 19% 29% 18% 6%	800
December	1st to 10th	Night	30% 18% 32% 17% 3	800
	11th to 20th	Night	28% 18% 32% 18% 3 1	800
	21st to end	Night	34% 16% 29% 19% 2	880

Page generated at 2025-03-09 08:08:47. Version ac07

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