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 2024-02-05.

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	16% 12% 5% 8% 6% 3 5% 5% 5% 7% 28%	78349
	Night	16% 13% 6% 9% 7% 3 5% 4 4 6% 27%	80501
Transparency	Day	55% 7% 11% 17% 9% 1	78057
	Night	58% 6% 10% 16% 9% 1	80328
Seeing	Day	20% 12% 28% 23% 12% 5%	13961
	Night	20% 11% 29% 27% 11% 1	23306

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% 20% 12% 15% 7% 2 3 2 3 5% 15%	2282
	11th to 20th	Day	12% 20% 11% 19% 13% 7% 5% 2 2 3 6%	2814
	21st to end	Day	19% 20% 10% 15% 8% 3 4 3 3 3 10%	3124
February	1st to 10th	Day	15% 19% 10% 13% 11% 5% 4 3 4 4 13%	1656
	11th to 20th	Day	14% 13% 9% 16% 11% 4 4 6% 4 5% 14%	1573
	21st to end	Day	16% 19% 8% 10% 8% 4 5% 4 4 5% 18%	1447
March	1st to 10th	Day	14% 13% 6% 11% 8% 5% 5% 4 4 5% 26%	1931
	11th to 20th	Day	12% 14% 7% 10% 7% 4 4 4 4 5% 28%	1980
	21st to end	Day	18% 13% 6% 7% 6% 3 5% 4 4 6% 28%	2178
April	1st to 10th	Day	18% 15% 6% 7% 6% 3 4 5% 5% 6% 26%	2065
	11th to 20th	Day	19% 12% 3 6% 4 3 4 4 4 7% 35%	2292
	21st to end	Day	21% 11% 4 7% 4 3 4 4 4 6% 32%	2340
May	1st to 10th	Day	18% 9% 4 6% 6% 3 4 4 5% 7% 33%	2340
	11th to 20th	Day	18% 8% 3 5% 5% 3 5% 3 5% 6% 38%	2366
	21st to end	Day	14% 4 2 6% 5% 3 5% 5% 5% 9% 41%	2725
June	1st to 10th	Day	15% 6% 3 5% 6% 3 6% 6% 5% 8% 38%	2546
	11th to 20th	Day	13% 6% 3 6% 4 3 6% 5% 6% 7% 42%	2700
	21st to end	Day	16% 5% 2 5% 4 3 5% 6% 6% 8% 40%	2700
July	1st to 10th	Day	10% 5% 2 4 4 3 5% 6% 6% 9% 46%	2700
	11th to 20th	Day	12% 6% 3 6% 6% 3 6% 5% 6% 9% 38%	2517
	21st to end	Day	11% 6% 2 6% 5% 3 5% 5% 7% 10% 39%	2547
August	1st to 10th	Day	11% 6% 2 5% 7% 4 6% 7% 7% 10% 36%	2327
	11th to 20th	Day	13% 5% 3 5% 7% 4 6% 6% 7% 10% 34%	2303
	21st to end	Day	14% 6% 2 5% 6% 4 5% 5% 6% 9% 39%	2540
September	1st to 10th	Day	13% 7% 3 7% 6% 4 5% 4 7% 9% 37%	2139
	11th to 20th	Day	12% 7% 3 4 5% 4 4 5% 6% 8% 41%	1980
	21st to end	Day	18% 7% 4 5% 7% 2 5% 5% 6% 7% 34%	1980
October	1st to 10th	Day	21% 8% 3 7% 5% 3 4 5% 5% 6% 32%	1822
	11th to 20th	Day	23% 13% 4 7% 6% 3 5% 5% 6% 6% 22%	1800
	21st to end	Day	27% 16% 6% 6% 6% 4 5% 5% 4 5% 17%	1944
November	1st to 10th	Day	16% 17% 4 7% 6% 3 4 4 5% 7% 27%	1653
	11th to 20th	Day	14% 18% 7% 7% 7% 4 4 5% 6% 7% 20%	1520
	21st to end	Day	19% 22% 9% 9% 6% 4 4 5% 5% 6% 13%	1520
December	1st to 10th	Day	15% 24% 12% 15% 6% 3 4 4 4 4 9%	1520
	11th to 20th	Day	18% 19% 10% 14% 7% 4 4 4 4 4 13%	1906
	21st to end	Day	27% 21% 9% 13% 6% 3 3 4 4 3 8%	2572
January	1st to 10th	Night	15% 20% 10% 19% 8% 4 4 3 3 4 10%	4670
	11th to 20th	Night	13% 16% 9% 18% 14% 6% 7% 4 3 2 7%	4140
	21st to end	Night	20% 17% 10% 14% 11% 4 4 4 3 3 10%	4532
February	1st to 10th	Night	15% 18% 7% 14% 11% 4 6% 4 4 4 14%	2399
	11th to 20th	Night	15% 15% 9% 12% 9% 3 5% 5% 5% 4 17%	2273
	21st to end	Night	16% 13% 8% 12% 8% 4 4 4 4 5% 22%	1808
March	1st to 10th	Night	13% 10% 5% 11% 8% 4 5% 4 4 5% 30%	2016
	11th to 20th	Night	14% 13% 4 9% 7% 3 5% 4 5% 6% 30%	1979
	21st to end	Night	17% 11% 5% 7% 6% 4 4 3 4 4 34%	2178
April	1st to 10th	Night	20% 13% 4 7% 5% 2 5% 5% 4 6% 29%	1895
	11th to 20th	Night	18% 10% 3 6% 4 3 4 4 4 7% 36%	1633
	21st to end	Night	17% 11% 4 7% 7% 2 5% 4 5% 7% 33%	1620
May	1st to 10th	Night	16% 9% 3 6% 5% 2 5% 5% 5% 8% 37%	1620
	11th to 20th	Night	16% 8% 2 4 5% 3 4 5% 4 7% 41%	1526
	21st to end	Night	9% 4 2 4 5% 3 5% 5% 6% 8% 49%	1550
June	1st to 10th	Night	13% 5% 3 5% 5% 3 5% 4 6% 7% 43%	1409
	11th to 20th	Night	11% 7% 2 5% 5% 3 5% 5% 6% 11% 41%	1260
	21st to end	Night	13% 6% 2 5% 5% 2 5% 4 5% 7% 46%	1260
July	1st to 10th	Night	7% 4 2 4 4 3 4 4 5% 9% 56%	1260
	11th to 20th	Night	11% 4 2 4 4 2 5% 5% 5% 8% 50%	1437
	21st to end	Night	9% 4 2 5% 5% 3 5% 5% 6% 7% 48%	1771
August	1st to 10th	Night	9% 6% 2 4 6% 3 5% 6% 6% 8% 46%	1611
	11th to 20th	Night	8% 5% 2 5% 5% 3 4 5% 6% 9% 48%	1598
	21st to end	Night	12% 5% 2 5% 6% 3 4 4 5% 8% 45%	1796
September	1st to 10th	Night	12% 6% 2 5% 5% 3 5% 4 6% 7% 46%	1818
	11th to 20th	Night	12% 4 2 4 5% 2 4 4 5% 6% 51%	1980
	21st to end	Night	16% 8% 3 5% 5% 3 4 4 5% 8% 40%	1980
October	1st to 10th	Night	17% 8% 3 5% 6% 4 5% 4 5% 7% 35%	2137
	11th to 20th	Night	23% 10% 3 7% 6% 4 4 5% 5% 6% 27%	2160
	21st to end	Night	25% 13% 4 7% 7% 3 4 4 5% 6% 22%	2410
November	1st to 10th	Night	16% 12% 3 7% 5% 3 5% 4 5% 6% 32%	2422
	11th to 20th	Night	16% 16% 6% 8% 6% 4 6% 5% 5% 6% 22%	2660
	21st to end	Night	19% 18% 7% 9% 7% 3 5% 4 4 6% 18%	2660
December	1st to 10th	Night	19% 22% 10% 10% 6% 3 5% 4 4 5% 12%	2660
	11th to 20th	Night	13% 21% 10% 12% 7% 3 5% 5% 4 5% 15%	3288
	21st to end	Night	26% 21% 9% 13% 6% 3 4 4 3 3 8%	5085

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	73% 5% 8% 9% 5%	2283
	11th to 20th	Day	86% 4 3 4 2	2814
	21st to end	Day	81% 5% 7% 5% 3	3124
February	1st to 10th	Day	76% 5% 6% 10% 3	1656
	11th to 20th	Day	70% 6% 8% 11% 4 1	1582
	21st to end	Day	69% 5% 10% 11% 5% 1	1447
March	1st to 10th	Day	61% 5% 12% 14% 7% 1	1934
	11th to 20th	Day	58% 6% 9% 16% 9% 2	1980
	21st to end	Day	57% 4 9% 16% 11% 2	2178
April	1st to 10th	Day	59% 5% 10% 16% 10%	2065
	11th to 20th	Day	50% 6% 11% 19% 13% 1	2292
	21st to end	Day	53% 5% 11% 18% 11% 2	2340
May	1st to 10th	Day	49% 6% 10% 22% 13% 1	2340
	11th to 20th	Day	47% 5% 11% 23% 12% 1	2370
	21st to end	Day	40% 8% 15% 22% 14% 2	2725
June	1st to 10th	Day	42% 8% 13% 22% 14% 1	2546
	11th to 20th	Day	40% 7% 12% 25% 15% 1	2700
	21st to end	Day	42% 7% 13% 24% 13% 1	2700
July	1st to 10th	Day	34% 8% 13% 24% 19% 2	2626
	11th to 20th	Day	39% 9% 14% 26% 12% 1	2437
	21st to end	Day	37% 9% 13% 25% 14% 2	2405
August	1st to 10th	Day	39% 10% 16% 25% 9% 2	2314
	11th to 20th	Day	42% 9% 16% 22% 10%	2303
	21st to end	Day	41% 7% 14% 25% 12% 1	2540
September	1st to 10th	Day	44% 7% 14% 23% 11% 2	2139
	11th to 20th	Day	39% 7% 14% 28% 12% 1	1980
	21st to end	Day	47% 7% 14% 24% 8% 1	1980
October	1st to 10th	Day	52% 6% 12% 17% 12% 2	1822
	11th to 20th	Day	60% 7% 12% 15% 6% 1	1800
	21st to end	Day	67% 8% 9% 10% 5%	1944
November	1st to 10th	Day	56% 8% 11% 18% 7% 1	1653
	11th to 20th	Day	61% 9% 11% 12% 6% 1	1520
	21st to end	Day	71% 7% 10% 9% 3	1520
December	1st to 10th	Day	79% 6% 7% 7% 2	1520
	11th to 20th	Day	75% 6% 9% 8% 2	1906
	21st to end	Day	82% 5% 8% 5% 1	2572
January	1st to 10th	Night	79% 5% 7% 6% 2	4670
	11th to 20th	Night	81% 8% 5% 4 2	4140
	21st to end	Night	78% 6% 7% 6% 3	4532
February	1st to 10th	Night	73% 6% 7% 11% 2	2399
	11th to 20th	Night	67% 7% 8% 13% 5%	2275
	21st to end	Night	65% 6% 9% 12% 8%	1808
March	1st to 10th	Night	55% 6% 11% 17% 9% 1	2026
	11th to 20th	Night	54% 6% 9% 19% 10% 2	1980
	21st to end	Night	54% 4 8% 19% 13% 2	2178
April	1st to 10th	Night	56% 5% 8% 17% 13% 1	1895
	11th to 20th	Night	48% 6% 8% 19% 17% 2	1633
	21st to end	Night	51% 5% 9% 17% 15% 3	1620
May	1st to 10th	Night	46% 6% 9% 21% 16% 2	1620
	11th to 20th	Night	43% 5% 10% 22% 17% 3	1527
	21st to end	Night	31% 8% 15% 26% 16% 4	1551
June	1st to 10th	Night	39% 6% 10% 26% 17% 2	1409
	11th to 20th	Night	38% 6% 13% 24% 17% 3	1260
	21st to end	Night	38% 5% 13% 26% 17% 1	1260
July	1st to 10th	Night	26% 6% 13% 28% 24% 4	1225
	11th to 20th	Night	33% 6% 13% 29% 18% 1	1394
	21st to end	Night	33% 9% 15% 25% 16% 2	1672
August	1st to 10th	Night	35% 7% 14% 30% 13% 1	1600
	11th to 20th	Night	31% 8% 15% 29% 15% 1	1598
	21st to end	Night	38% 7% 12% 28% 14% 1	1796
September	1st to 10th	Night	38% 7% 13% 27% 14% 2	1818
	11th to 20th	Night	33% 7% 12% 31% 17% 1	1980
	21st to end	Night	42% 7% 14% 26% 11%	1980
October	1st to 10th	Night	48% 7% 10% 19% 15% 1	2137
	11th to 20th	Night	56% 7% 11% 19% 7%	2160
	21st to end	Night	62% 6% 10% 13% 8%	2410
November	1st to 10th	Night	52% 7% 11% 20% 9%	2422
	11th to 20th	Night	61% 7% 12% 13% 5% 1	2660
	21st to end	Night	67% 7% 13% 10% 3	2660
December	1st to 10th	Night	76% 7% 9% 6% 2	2660
	11th to 20th	Night	72% 5% 12% 9% 2	3288
	21st to end	Night	82% 5% 8% 5% 1	5085

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	23% 19% 33% 20% 4 1	203
	11th to 20th	Day	23% 17% 41% 16% 2 1	270
	21st to end	Day	28% 16% 31% 21% 3 1	297
February	1st to 10th	Day	27% 17% 40% 10% 4	252
	11th to 20th	Day	22% 21% 45% 11% 1	240
	21st to end	Day	31% 27% 29% 12% 1	198
March	1st to 10th	Day	26% 15% 38% 13% 8%	374
	11th to 20th	Day	20% 15% 36% 20% 6% 2	420
	21st to end	Day	28% 11% 34% 20% 6% 1	462
April	1st to 10th	Day	23% 16% 34% 19% 7% 1	420
	11th to 20th	Day	26% 14% 28% 16% 11% 5%	412
	21st to end	Day	26% 12% 32% 21% 7% 2	420
May	1st to 10th	Day	23% 12% 28% 23% 11% 3	423
	11th to 20th	Day	21% 11% 25% 23% 15% 5%	447
	21st to end	Day	16% 8% 24% 28% 17% 7%	485
June	1st to 10th	Day	16% 10% 27% 31% 13% 3	476
	11th to 20th	Day	14% 10% 30% 27% 17% 3	630
	21st to end	Day	17% 6% 29% 32% 13% 3	630
July	1st to 10th	Day	9% 7% 20% 31% 24% 9%	621
	11th to 20th	Day	16% 6% 23% 29% 18% 8%	525
	21st to end	Day	13% 2 29% 33% 17% 6%	488
August	1st to 10th	Day	11% 4 22% 28% 24% 11%	446
	11th to 20th	Day	13% 3 18% 29% 24% 12%	439
	21st to end	Day	16% 7% 21% 27% 19% 10%	495
September	1st to 10th	Day	13% 8% 21% 27% 19% 12%	450
	11th to 20th	Day	10% 6% 22% 26% 24% 12%	450
	21st to end	Day	16% 12% 27% 22% 15% 8%	450
October	1st to 10th	Day	23% 9% 27% 23% 14% 5%	450
	11th to 20th	Day	25% 16% 32% 20% 5% 1	450
	21st to end	Day	31% 16% 29% 18% 4 1	459
November	1st to 10th	Day	25% 22% 28% 17% 8%	261
	11th to 20th	Day	24% 18% 36% 16% 3 3	180
	21st to end	Day	27% 21% 35% 14% 4	180
December	1st to 10th	Day	29% 25% 27% 16% 4	180
	11th to 20th	Day	28% 26% 31% 12% 3	180
	21st to end	Day	32% 21% 36% 11% 1	198
January	1st to 10th	Night	23% 18% 35% 18% 5% 1	950
	11th to 20th	Night	26% 15% 33% 22% 3	767
	21st to end	Night	20% 10% 39% 24% 6%	836
February	1st to 10th	Night	24% 17% 37% 20% 3	739
	11th to 20th	Night	21% 15% 36% 23% 5%	703
	21st to end	Night	23% 17% 34% 22% 3	592
March	1st to 10th	Night	18% 14% 37% 23% 8%	720
	11th to 20th	Night	20% 15% 37% 21% 6% 1	720
	21st to end	Night	22% 12% 31% 25% 10% 1	792
April	1st to 10th	Night	25% 15% 30% 25% 5%	720
	11th to 20th	Night	23% 11% 32% 23% 10% 1	569
	21st to end	Night	21% 9% 30% 30% 9%	540
May	1st to 10th	Night	19% 5% 30% 31% 14% 1	540
	11th to 20th	Night	20% 5% 28% 35% 10% 2	465
	21st to end	Night	12% 4 28% 39% 18%	389
June	1st to 10th	Night	13% 4 29% 39% 14% 1	360
	11th to 20th	Night	13% 6% 28% 39% 14%	360
	21st to end	Night	14% 6% 24% 40% 16% 1	360
July	1st to 10th	Night	7% 3 15% 50% 24% 1	350
	11th to 20th	Night	10% 3 23% 38% 24% 2	433
	21st to end	Night	11% 3 17% 47% 22% 1	591
August	1st to 10th	Night	11% 3 13% 43% 27% 3	537
	11th to 20th	Night	10% 3 17% 36% 29% 5%	534
	21st to end	Night	13% 3 22% 33% 25% 3	608
September	1st to 10th	Night	13% 5% 21% 35% 21% 5%	720
	11th to 20th	Night	12% 5% 21% 33% 24% 5%	720
	21st to end	Night	18% 8% 22% 31% 17% 4	720
October	1st to 10th	Night	20% 8% 26% 27% 16% 2	720
	11th to 20th	Night	26% 13% 26% 26% 8% 1	720
	21st to end	Night	30% 11% 31% 23% 5%	792
November	1st to 10th	Night	23% 12% 29% 27% 8% 1	740
	11th to 20th	Night	22% 17% 34% 20% 7% 1	760
	21st to end	Night	27% 14% 33% 23% 4	760
December	1st to 10th	Night	30% 17% 32% 17% 4	760
	11th to 20th	Night	23% 19% 37% 17% 3 1	760
	21st to end	Night	30% 15% 35% 18% 2	959

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