North Perth
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the North Perth clear sky chart from 2005-11-06 to 2026-07-01.

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	15% 12% 5% 10% 6% 5% 4 3 5% 6% 29%	138184
	Night	15% 13% 5% 11% 6% 5% 4 3 4 5% 28%	134204
Transparency	Day	54% 5% 11% 19% 10% 1	141761
	Night	57% 5% 10% 17% 10% 1	138716
Seeing	Day	21% 13% 29% 22% 11% 4	17169
	Night	20% 12% 30% 27% 10% 1	26321

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% 10% 20% 8% 4 3 3 3 5% 12%	4252
	11th to 20th	Day	9% 22% 12% 24% 12% 10% 2 1 2 2 4	5220
	21st to end	Day	13% 17% 7% 20% 11% 10% 6% 2 3 3 8%	5780
February	1st to 10th	Day	14% 19% 9% 15% 10% 6% 4 3 3 4 14%	1890
	11th to 20th	Day	13% 14% 9% 18% 10% 5% 4 4 4 5% 13%	1843
	21st to end	Day	15% 19% 7% 11% 8% 5% 4 3 5% 5% 17%	1692
March	1st to 10th	Day	17% 12% 6% 15% 8% 4 3 2 3 5% 25%	4941
	11th to 20th	Day	11% 15% 7% 11% 5% 4 4 4 5% 7% 27%	5090
	21st to end	Day	19% 15% 7% 11% 5% 4 4 3 4 5% 25%	5586
April	1st to 10th	Day	17% 16% 5% 9% 5% 4 4 3 5% 6% 27%	2410
	11th to 20th	Day	20% 12% 3 7% 3 4 4 3 4 6% 34%	2679
	21st to end	Day	19% 12% 4 7% 4 4 4 3 5% 7% 33%	2730
May	1st to 10th	Day	17% 9% 3 7% 5% 4 4 4 6% 7% 34%	7642
	11th to 20th	Day	16% 11% 3 7% 5% 4 5% 4 6% 7% 34%	7859
	21st to end	Day	18% 8% 2 6% 3 3 4 3 4 6% 43%	9060
June	1st to 10th	Day	15% 7% 2 6% 5% 4 5% 5% 6% 7% 38%	2971
	11th to 20th	Day	12% 6% 2 7% 4 5% 5% 5% 6% 7% 40%	3150
	21st to end	Day	16% 6% 2 6% 4 4 4 5% 6% 8% 39%	3150
July	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 5% 7% 8% 40%	6820
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 45%	6329
	21st to end	Day	9% 5% 2 6% 4 4 4 4 7% 8% 47%	6424
August	1st to 10th	Day	14% 6% 2 6% 6% 5% 6% 5% 6% 8% 35%	4234
	11th to 20th	Day	14% 7% 3 6% 5% 5% 6% 5% 8% 10% 32%	4174
	21st to end	Day	10% 5% 2 6% 5% 6% 5% 5% 7% 9% 40%	4611
September	1st to 10th	Day	12% 7% 2 8% 5% 6% 4 3 7% 8% 38%	2376
	11th to 20th	Day	11% 7% 2 5% 4 4 4 4 6% 8% 45%	2200
	21st to end	Day	17% 8% 3 7% 6% 4 5% 4 6% 7% 34%	2200
October	1st to 10th	Day	19% 8% 3 7% 4 4 4 3 5% 6% 35%	2024
	11th to 20th	Day	21% 13% 3 8% 5% 4 5% 4 6% 6% 25%	2000
	21st to end	Day	25% 16% 5% 7% 5% 5% 5% 4 4 6% 18%	2160
November	1st to 10th	Day	16% 18% 4 8% 5% 4 4 3 5% 7% 27%	1831
	11th to 20th	Day	15% 19% 6% 8% 6% 5% 5% 4 6% 6% 21%	1680
	21st to end	Day	17% 25% 8% 11% 5% 4 4 4 5% 5% 12%	1680
December	1st to 10th	Day	18% 28% 11% 14% 5% 3 4 3 3 3 7%	2680
	11th to 20th	Day	21% 22% 7% 15% 6% 4 3 2 4 3 11%	3066
	21st to end	Day	25% 21% 7% 12% 5% 4 4 3 4 4 11%	3750
January	1st to 10th	Night	13% 17% 9% 24% 8% 5% 4 3 4 5% 9%	8669
	11th to 20th	Night	10% 20% 10% 22% 13% 7% 5% 3 3 2 5%	7659
	21st to end	Night	11% 15% 8% 19% 12% 10% 6% 3 4 4 9%	8401
February	1st to 10th	Night	14% 18% 6% 15% 10% 5% 5% 4 4 4 15%	2730
	11th to 20th	Night	13% 15% 7% 14% 9% 5% 5% 4 5% 5% 18%	2663
	21st to end	Night	14% 15% 6% 14% 7% 5% 4 4 5% 5% 22%	2112
March	1st to 10th	Night	13% 12% 5% 11% 7% 5% 4 4 5% 5% 28%	5119
	11th to 20th	Night	9% 12% 5% 11% 5% 4 5% 4 6% 8% 31%	5089
	21st to end	Night	19% 11% 5% 10% 5% 4 3 3 3 4 32%	5588
April	1st to 10th	Night	19% 14% 3 9% 5% 3 4 4 4 6% 28%	2210
	11th to 20th	Night	18% 11% 3 7% 4 4 4 3 4 7% 35%	1909
	21st to end	Night	16% 11% 3 8% 6% 4 5% 3 6% 6% 33%	1890
May	1st to 10th	Night	17% 8% 2 7% 4 4 4 3 5% 6% 39%	5257
	11th to 20th	Night	17% 10% 3 6% 4 5% 3 3 4 6% 39%	5000
	21st to end	Night	18% 7% 1 4 3 3 3 3 5% 6% 46%	5114
June	1st to 10th	Night	13% 6% 2 6% 5% 5% 4 3 5% 7% 43%	1643
	11th to 20th	Night	11% 7% 2 7% 4 4 6% 3 7% 11% 39%	1470
	21st to end	Night	12% 7% 2 6% 4 4 5% 3 5% 7% 45%	1470
July	1st to 10th	Night	8% 6% 2 6% 5% 4 5% 4 5% 8% 47%	3160
	11th to 20th	Night	14% 6% 2 4 3 3 4 3 5% 7% 50%	3620
	21st to end	Night	9% 6% 2 5% 4 4 4 4 5% 7% 50%	4428
August	1st to 10th	Night	12% 6% 2 6% 5% 4 4 4 5% 6% 45%	2915
	11th to 20th	Night	15% 8% 2 6% 4 4 4 3 5% 9% 41%	2875
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 7% 48%	3252
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 46%	2020
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 6% 54%	2200
	21st to end	Night	17% 8% 3 6% 4 4 4 3 5% 8% 39%	2200
October	1st to 10th	Night	16% 8% 2 6% 5% 5% 5% 3 5% 7% 38%	2375
	11th to 20th	Night	22% 11% 3 7% 5% 5% 4 4 5% 7% 29%	2400
	21st to end	Night	23% 13% 4 8% 6% 4 4 3 5% 6% 24%	2678
November	1st to 10th	Night	16% 13% 3 8% 5% 5% 4 4 5% 6% 31%	2684
	11th to 20th	Night	15% 18% 5% 9% 5% 5% 6% 4 5% 6% 21%	2940
	21st to end	Night	19% 20% 7% 10% 6% 4 4 3 4 6% 16%	2940
December	1st to 10th	Night	14% 24% 9% 13% 6% 6% 5% 4 5% 5% 9%	4720
	11th to 20th	Night	13% 26% 9% 15% 6% 4 4 3 4 4 14%	5348
	21st to end	Night	22% 21% 9% 14% 5% 4 4 3 4 4 12%	7456

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	76% 4 7% 8% 5%	4253
	11th to 20th	Day	91% 2 2 3 2	5254
	21st to end	Day	83% 3 7% 4 3	5780
February	1st to 10th	Day	75% 5% 6% 11% 3	1890
	11th to 20th	Day	71% 6% 7% 11% 4 1	1852
	21st to end	Day	69% 5% 10% 10% 5% 1	1692
March	1st to 10th	Day	63% 3 14% 12% 7% 1	4944
	11th to 20th	Day	56% 5% 14% 17% 6% 2	5090
	21st to end	Day	62% 5% 10% 15% 8% 1	5586
April	1st to 10th	Day	59% 5% 9% 16% 10%	2410
	11th to 20th	Day	51% 6% 10% 20% 12% 1	2679
	21st to end	Day	52% 5% 11% 19% 11% 2	2730
May	1st to 10th	Day	47% 6% 12% 20% 12% 4	7651
	11th to 20th	Day	48% 6% 10% 20% 15% 1	7863
	21st to end	Day	43% 5% 8% 25% 16% 3	9061
June	1st to 10th	Day	42% 7% 12% 22% 15% 1	2971
	11th to 20th	Day	41% 7% 12% 24% 15% 1	3150
	21st to end	Day	42% 7% 13% 25% 13% 1	3150
July	1st to 10th	Day	40% 5% 13% 23% 18% 1	6747
	11th to 20th	Day	36% 5% 11% 31% 16% 1	6249
	21st to end	Day	32% 6% 14% 28% 19% 1	6240
August	1st to 10th	Day	44% 8% 13% 23% 12% 1	4221
	11th to 20th	Day	44% 7% 16% 23% 10%	4213
	21st to end	Day	38% 6% 14% 30% 11% 1	4611
September	1st to 10th	Day	42% 7% 14% 25% 11% 2	2376
	11th to 20th	Day	36% 7% 14% 29% 14%	2200
	21st to end	Day	47% 7% 13% 23% 9% 1	2200
October	1st to 10th	Day	45% 6% 11% 22% 15% 1	3297
	11th to 20th	Day	44% 5% 13% 22% 14%	3240
	21st to end	Day	57% 6% 14% 17% 6%	3474
November	1st to 10th	Day	57% 7% 10% 18% 7% 1	1831
	11th to 20th	Day	61% 8% 10% 13% 7% 1	1680
	21st to end	Day	73% 6% 9% 9% 2	1680
December	1st to 10th	Day	82% 5% 6% 6% 1	2680
	11th to 20th	Day	78% 4 7% 8% 2	3066
	21st to end	Day	78% 4 6% 7% 5%	3750
January	1st to 10th	Night	77% 5% 7% 8% 3	8669
	11th to 20th	Night	84% 5% 4 4 2	7700
	21st to end	Night	79% 4 6% 6% 4	8401
February	1st to 10th	Night	71% 6% 7% 12% 3	2730
	11th to 20th	Night	67% 6% 8% 13% 5%	2665
	21st to end	Night	63% 6% 10% 13% 7%	2112
March	1st to 10th	Night	57% 5% 13% 16% 9% 1	5129
	11th to 20th	Night	49% 6% 16% 19% 9% 1	5090
	21st to end	Night	55% 5% 9% 17% 12% 2	5588
April	1st to 10th	Night	57% 5% 8% 16% 12% 1	2210
	11th to 20th	Night	50% 5% 8% 19% 16% 1	1909
	21st to end	Night	50% 6% 10% 17% 15% 2	1890
May	1st to 10th	Night	45% 5% 10% 18% 15% 6%	5264
	11th to 20th	Night	46% 6% 10% 20% 15% 2	5001
	21st to end	Night	39% 5% 9% 26% 19% 2	5115
June	1st to 10th	Night	39% 6% 9% 26% 18% 2	1643
	11th to 20th	Night	39% 6% 13% 25% 15% 2	1470
	21st to end	Night	39% 5% 13% 26% 17% 1	1470
July	1st to 10th	Night	35% 5% 13% 23% 19% 4	3125
	11th to 20th	Night	35% 5% 10% 28% 20% 2	3577
	21st to end	Night	34% 6% 11% 28% 20% 2	4297
August	1st to 10th	Night	39% 5% 12% 27% 16% 1	2904
	11th to 20th	Night	40% 6% 15% 26% 12% 1	2902
	21st to end	Night	36% 6% 10% 32% 14% 1	3252
September	1st to 10th	Night	37% 7% 12% 29% 14% 2	2020
	11th to 20th	Night	30% 7% 12% 31% 19% 1	2200
	21st to end	Night	44% 7% 13% 25% 11% 1	2200
October	1st to 10th	Night	35% 6% 8% 27% 22% 1	3882
	11th to 20th	Night	47% 5% 13% 20% 14%	3899
	21st to end	Night	49% 6% 15% 21% 8%	4314
November	1st to 10th	Night	53% 7% 11% 20% 9%	2684
	11th to 20th	Night	62% 7% 12% 13% 5% 1	2940
	21st to end	Night	68% 6% 12% 10% 3	2940
December	1st to 10th	Night	77% 6% 10% 6% 1	4720
	11th to 20th	Night	75% 4 8% 9% 4	5348
	21st to end	Night	77% 5% 7% 7% 4	7456

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% 20% 33% 17% 4 1	249
	11th to 20th	Day	25% 21% 38% 13% 2 1	330
	21st to end	Day	29% 18% 31% 18% 3 1	363
February	1st to 10th	Day	27% 19% 38% 12% 4	330
	11th to 20th	Day	23% 22% 41% 14% 1	330
	21st to end	Day	30% 26% 29% 13% 1	273
March	1st to 10th	Day	27% 16% 37% 13% 7%	487
	11th to 20th	Day	24% 17% 33% 19% 5% 1	540
	21st to end	Day	29% 13% 34% 18% 5% 1	594
April	1st to 10th	Day	27% 17% 32% 17% 6% 1	540
	11th to 20th	Day	29% 17% 26% 15% 10% 4	532
	21st to end	Day	25% 13% 33% 21% 6% 2	540
May	1st to 10th	Day	22% 14% 30% 22% 9% 3	543
	11th to 20th	Day	21% 12% 25% 23% 14% 5%	567
	21st to end	Day	18% 10% 26% 26% 14% 6%	617
June	1st to 10th	Day	16% 11% 30% 29% 12% 3	601
	11th to 20th	Day	14% 11% 33% 25% 14% 2	780
	21st to end	Day	17% 7% 31% 31% 11% 3	780
July	1st to 10th	Day	10% 8% 22% 30% 21% 8%	722
	11th to 20th	Day	15% 7% 25% 28% 17% 7%	613
	21st to end	Day	12% 2 30% 32% 19% 6%	576
August	1st to 10th	Day	12% 4 22% 29% 23% 10%	526
	11th to 20th	Day	13% 4 20% 28% 23% 12%	519
	21st to end	Day	16% 6% 22% 27% 18% 10%	583
September	1st to 10th	Day	12% 9% 22% 25% 19% 12%	530
	11th to 20th	Day	9% 5% 20% 25% 27% 14%	530
	21st to end	Day	16% 12% 27% 23% 15% 7%	530
October	1st to 10th	Day	21% 9% 27% 24% 15% 5%	530
	11th to 20th	Day	23% 16% 34% 20% 5% 1	530
	21st to end	Day	30% 16% 30% 18% 4 1	543
November	1st to 10th	Day	29% 21% 28% 14% 7%	319
	11th to 20th	Day	27% 18% 35% 15% 2 3	220
	21st to end	Day	28% 23% 32% 13% 4	220
December	1st to 10th	Day	30% 25% 26% 15% 4	220
	11th to 20th	Day	31% 23% 32% 10% 3	220
	21st to end	Day	34% 20% 34% 10% 2	242
January	1st to 10th	Night	22% 18% 36% 18% 4 1	1050
	11th to 20th	Night	26% 16% 34% 20% 3	847
	21st to end	Night	21% 12% 39% 23% 5%	924
February	1st to 10th	Night	23% 18% 36% 20% 3	840
	11th to 20th	Night	22% 16% 36% 22% 4	823
	21st to end	Night	22% 17% 36% 22% 3	692
March	1st to 10th	Night	19% 15% 37% 21% 7%	840
	11th to 20th	Night	21% 16% 37% 20% 5%	840
	21st to end	Night	23% 13% 31% 23% 9%	924
April	1st to 10th	Night	26% 16% 31% 23% 4	840
	11th to 20th	Night	23% 12% 32% 22% 9% 1	665
	21st to end	Night	21% 10% 30% 31% 8%	630
May	1st to 10th	Night	19% 5% 31% 32% 13% 1	630
	11th to 20th	Night	20% 6% 28% 34% 10% 1	543
	21st to end	Night	15% 5% 29% 36% 16%	455
June	1st to 10th	Night	14% 5% 30% 37% 14% 1	420
	11th to 20th	Night	13% 6% 31% 38% 12%	420
	21st to end	Night	14% 6% 25% 40% 14% 1	420
July	1st to 10th	Night	7% 3 17% 51% 22% 1	390
	11th to 20th	Night	12% 3 23% 37% 23% 2	483
	21st to end	Night	11% 2 18% 46% 22% 1	657
August	1st to 10th	Night	11% 3 14% 43% 26% 2	597
	11th to 20th	Night	11% 3 18% 36% 28% 4	594
	21st to end	Night	12% 4 22% 34% 25% 3	676
September	1st to 10th	Night	13% 6% 21% 34% 22% 4	800
	11th to 20th	Night	11% 4 19% 35% 26% 5%	800
	21st to end	Night	19% 8% 22% 32% 16% 4	800
October	1st to 10th	Night	19% 9% 27% 29% 16% 2	800
	11th to 20th	Night	25% 13% 26% 26% 9% 1	800
	21st to end	Night	29% 12% 31% 24% 5%	880
November	1st to 10th	Night	24% 13% 29% 26% 8% 1	820
	11th to 20th	Night	23% 17% 33% 20% 7% 1	840
	21st to end	Night	28% 15% 32% 21% 3	840
December	1st to 10th	Night	29% 17% 33% 17% 4	840
	11th to 20th	Night	23% 20% 37% 16% 3	840
	21st to end	Night	30% 15% 35% 17% 3	1061

Page generated at 2026-06-28 02:10:43. Version ac07

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