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 2024-09-29.

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% 4 9% 5% 5% 4 4 5% 7% 30%	101331
	Night	15% 13% 5% 10% 6% 4 4 3 5% 6% 29%	96914
Transparency	Day	53% 6% 11% 18% 10% 1	101090
	Night	55% 6% 11% 17% 10% 1	96775
Seeing	Day	20% 12% 29% 23% 12% 5%	14919
	Night	20% 11% 30% 27% 11% 1	24055

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% 16% 6% 3 3 2 3 5% 15%	2282
	11th to 20th	Day	11% 21% 11% 21% 11% 9% 3 1 2 3 6%	2814
	21st to end	Day	19% 22% 9% 17% 6% 4 4 3 3 3 10%	3124
February	1st to 10th	Day	15% 20% 9% 14% 9% 6% 4 3 3 4 14%	1710
	11th to 20th	Day	13% 14% 9% 17% 10% 5% 4 4 4 5% 14%	1663
	21st to end	Day	15% 20% 6% 11% 7% 5% 4 3 4 5% 18%	1535
March	1st to 10th	Day	18% 15% 5% 12% 6% 5% 3 2 3 5% 27%	3362
	11th to 20th	Day	10% 17% 9% 12% 5% 5% 4 3 4 6% 26%	3480
	21st to end	Day	16% 14% 5% 11% 5% 4 4 3 5% 5% 26%	3815
April	1st to 10th	Day	17% 16% 5% 9% 5% 4 4 4 5% 6% 26%	2180
	11th to 20th	Day	19% 12% 2 7% 3 4 4 3 4 6% 34%	2421
	21st to end	Day	20% 12% 4 7% 4 4 3 3 4 6% 32%	2470
May	1st to 10th	Day	19% 8% 3 6% 5% 5% 4 3 6% 7% 33%	4108
	11th to 20th	Day	18% 9% 2 7% 5% 4 5% 3 6% 7% 33%	4202
	21st to end	Day	15% 4 2 6% 4 4 4 4 5% 7% 46%	4837
June	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 5% 6% 7% 37%	2688
	11th to 20th	Day	12% 6% 2 6% 3 5% 5% 5% 6% 7% 42%	2850
	21st to end	Day	17% 6% 2 6% 4 4 4 5% 6% 8% 38%	2850
July	1st to 10th	Day	12% 6% 2 6% 5% 5% 6% 5% 8% 8% 36%	4760
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 4 7% 8% 42%	4423
	21st to end	Day	10% 6% 2 6% 5% 4 5% 5% 7% 9% 42%	4466
August	1st to 10th	Day	14% 7% 2 6% 6% 5% 6% 5% 6% 8% 35%	4104
	11th to 20th	Day	14% 7% 3 6% 5% 5% 6% 5% 8% 10% 32%	4044
	21st to end	Day	10% 5% 2 6% 5% 6% 5% 5% 7% 9% 40%	4470
September	1st to 10th	Day	12% 7% 2 8% 5% 6% 4 3 7% 8% 38%	2257
	11th to 20th	Day	12% 7% 2 5% 4 5% 4 4 6% 8% 43%	2090
	21st to end	Day	17% 8% 3 7% 6% 4 5% 4 6% 7% 33%	2069
October	1st to 10th	Day	20% 9% 3 8% 4 5% 4 3 5% 6% 32%	1822
	11th to 20th	Day	23% 14% 3 8% 5% 4 5% 4 6% 6% 22%	1800
	21st to end	Day	27% 17% 5% 7% 5% 5% 5% 4 4 5% 17%	1944
November	1st to 10th	Day	16% 18% 4 8% 6% 4 4 3 5% 7% 27%	1653
	11th to 20th	Day	14% 19% 6% 8% 6% 5% 5% 4 6% 7% 20%	1520
	21st to end	Day	18% 23% 8% 10% 5% 4 3 5% 5% 5% 13%	1520
December	1st to 10th	Day	15% 26% 10% 15% 6% 4 4 2 4 4 9%	1520
	11th to 20th	Day	18% 21% 8% 16% 5% 5% 4 3 4 4 13%	1906
	21st to end	Day	27% 22% 8% 14% 5% 4 3 3 4 3 8%	2572
January	1st to 10th	Night	15% 21% 10% 20% 6% 5% 3 3 3 4 10%	4670
	11th to 20th	Night	13% 17% 9% 20% 12% 8% 6% 3 3 2 6%	4140
	21st to end	Night	19% 18% 9% 17% 8% 5% 4 3 3 3 10%	4532
February	1st to 10th	Night	15% 19% 6% 14% 10% 5% 5% 4 4 4 15%	2470
	11th to 20th	Night	13% 16% 8% 14% 8% 5% 5% 4 5% 5% 17%	2403
	21st to end	Night	15% 14% 6% 14% 7% 5% 4 4 5% 5% 22%	1918
March	1st to 10th	Night	14% 12% 5% 10% 5% 4 4 3 4 6% 32%	3478
	11th to 20th	Night	11% 13% 5% 11% 5% 4 4 4 5% 6% 32%	3479
	21st to end	Night	15% 10% 4 10% 5% 5% 3 3 4 5% 36%	3817
April	1st to 10th	Night	20% 14% 3 8% 5% 3 4 4 4 6% 29%	2000
	11th to 20th	Night	19% 11% 3 6% 4 4 3 3 4 7% 36%	1725
	21st to end	Night	16% 11% 3 8% 6% 4 4 3 5% 6% 33%	1710
May	1st to 10th	Night	15% 8% 2 7% 4 3 5% 3 5% 7% 41%	2833
	11th to 20th	Night	16% 9% 3 6% 5% 5% 4 4 5% 6% 37%	2690
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 7% 50%	2738
June	1st to 10th	Night	14% 7% 2 6% 5% 5% 4 3 6% 7% 41%	1487
	11th to 20th	Night	11% 7% 2 6% 5% 4 5% 4 7% 10% 40%	1330
	21st to end	Night	13% 7% 2 6% 4 4 5% 3 5% 7% 45%	1330
July	1st to 10th	Night	6% 5% 2 5% 5% 4 5% 4 5% 8% 50%	2210
	11th to 20th	Night	13% 5% 1 4 3 3 4 2 4 7% 53%	2535
	21st to end	Night	7% 6% 2 6% 4 5% 5% 4 6% 7% 48%	3087
August	1st to 10th	Night	13% 6% 2 6% 5% 4 4 4 5% 6% 45%	2825
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 5% 9% 41%	2785
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 7% 47%	3152
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 6% 7% 47%	1919
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 6% 52%	2090
	21st to end	Night	17% 8% 2 6% 4 4 4 3 6% 8% 39%	2079
October	1st to 10th	Night	17% 9% 3 7% 5% 5% 5% 3 5% 7% 35%	2137
	11th to 20th	Night	23% 11% 3 8% 5% 5% 4 4 5% 6% 27%	2160
	21st to end	Night	25% 14% 4 8% 6% 4 4 3 5% 5% 22%	2410
November	1st to 10th	Night	16% 13% 3 8% 5% 5% 4 4 5% 6% 32%	2422
	11th to 20th	Night	16% 17% 5% 9% 5% 5% 6% 4 5% 6% 22%	2660
	21st to end	Night	18% 20% 6% 10% 6% 4 4 3 4 6% 17%	2660
December	1st to 10th	Night	19% 24% 8% 11% 6% 5% 4 3 4 5% 11%	2660
	11th to 20th	Night	13% 23% 9% 13% 6% 4 4 4 5% 5% 15%	3288
	21st to end	Night	25% 22% 9% 14% 5% 4 3 3 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	75% 6% 6% 10% 3	1710
	11th to 20th	Day	70% 6% 7% 11% 4 1	1672
	21st to end	Day	68% 5% 10% 11% 5% 1	1535
March	1st to 10th	Day	62% 4 11% 15% 7% 1	3365
	11th to 20th	Day	59% 6% 12% 17% 6% 1	3480
	21st to end	Day	57% 6% 11% 14% 9% 2	3815
April	1st to 10th	Day	58% 5% 10% 16% 10%	2180
	11th to 20th	Day	50% 6% 10% 19% 13% 1	2421
	21st to end	Day	53% 6% 11% 18% 11% 2	2470
May	1st to 10th	Day	46% 8% 13% 19% 13% 2	4120
	11th to 20th	Day	48% 8% 14% 22% 7% 1	4206
	21st to end	Day	37% 7% 12% 20% 19% 5%	4837
June	1st to 10th	Day	43% 8% 13% 22% 14% 1	2688
	11th to 20th	Day	39% 7% 12% 24% 16% 1	2850
	21st to end	Day	43% 7% 13% 24% 13%	2850
July	1st to 10th	Day	42% 6% 14% 20% 16% 2	4686
	11th to 20th	Day	37% 7% 12% 32% 12% 1	4343
	21st to end	Day	36% 7% 14% 23% 18% 2	4324
August	1st to 10th	Day	45% 8% 12% 23% 12% 1	4091
	11th to 20th	Day	44% 7% 16% 23% 10%	4083
	21st to end	Day	38% 6% 14% 30% 11% 1	4470
September	1st to 10th	Day	43% 7% 14% 24% 11% 2	2257
	11th to 20th	Day	37% 7% 14% 28% 14%	2090
	21st to end	Day	48% 7% 13% 23% 8% 1	2069
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	72% 6% 8% 12% 2	2470
	11th to 20th	Night	67% 7% 9% 13% 5%	2405
	21st to end	Night	63% 6% 10% 13% 8%	1918
March	1st to 10th	Night	52% 6% 15% 19% 7% 1	3488
	11th to 20th	Night	50% 7% 13% 19% 9% 1	3480
	21st to end	Night	49% 6% 10% 21% 12% 1	3817
April	1st to 10th	Night	56% 6% 8% 17% 13% 1	2000
	11th to 20th	Night	49% 6% 8% 19% 16% 2	1725
	21st to end	Night	51% 6% 9% 17% 15% 2	1710
May	1st to 10th	Night	42% 7% 11% 20% 15% 5%	2840
	11th to 20th	Night	45% 7% 13% 20% 12% 2	2691
	21st to end	Night	31% 9% 12% 23% 22% 3	2739
June	1st to 10th	Night	40% 6% 9% 26% 16% 2	1487
	11th to 20th	Night	38% 6% 13% 24% 16% 2	1330
	21st to end	Night	39% 5% 13% 26% 17% 1	1330
July	1st to 10th	Night	32% 5% 14% 25% 19% 6%	2175
	11th to 20th	Night	32% 5% 12% 33% 18% 1	2492
	21st to end	Night	34% 7% 14% 26% 17% 1	2988
August	1st to 10th	Night	39% 5% 11% 27% 16% 1	2814
	11th to 20th	Night	40% 6% 15% 26% 11% 1	2812
	21st to end	Night	36% 6% 10% 32% 15% 1	3152
September	1st to 10th	Night	37% 7% 12% 27% 15% 2	1919
	11th to 20th	Night	32% 7% 11% 30% 20% 1	2090
	21st to end	Night	44% 7% 13% 26% 10%	2079
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	26% 19% 40% 11% 4	270
	11th to 20th	Day	21% 21% 43% 14% 1	270
	21st to end	Day	30% 26% 29% 13% 1	225
March	1st to 10th	Day	26% 15% 38% 14% 8%	412
	11th to 20th	Day	20% 16% 36% 20% 6% 2	460
	21st to end	Day	27% 11% 34% 21% 5% 1	506
April	1st to 10th	Day	24% 16% 35% 18% 7% 1	460
	11th to 20th	Day	27% 16% 27% 15% 10% 5%	452
	21st to end	Day	25% 13% 32% 21% 7% 2	460
May	1st to 10th	Day	23% 12% 27% 24% 11% 3	463
	11th to 20th	Day	21% 11% 24% 24% 15% 5%	487
	21st to end	Day	16% 8% 25% 28% 16% 7%	529
June	1st to 10th	Day	16% 10% 28% 30% 13% 3	518
	11th to 20th	Day	14% 10% 31% 27% 16% 3	680
	21st to end	Day	18% 7% 29% 31% 12% 3	680
July	1st to 10th	Day	10% 8% 21% 30% 22% 8%	671
	11th to 20th	Day	15% 7% 24% 28% 18% 8%	569
	21st to end	Day	12% 2 28% 32% 19% 7%	532
August	1st to 10th	Day	12% 5% 23% 27% 23% 10%	486
	11th to 20th	Day	14% 4 19% 27% 24% 12%	479
	21st to end	Day	16% 6% 21% 28% 19% 10%	539
September	1st to 10th	Day	12% 9% 22% 26% 19% 12%	490
	11th to 20th	Day	9% 6% 21% 25% 26% 14%	490
	21st to end	Day	16% 12% 29% 21% 14% 7%	483
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	23% 17% 36% 21% 3	760
	11th to 20th	Night	21% 15% 36% 23% 5%	743
	21st to end	Night	22% 17% 35% 22% 3	628
March	1st to 10th	Night	19% 14% 38% 22% 7%	760
	11th to 20th	Night	19% 16% 38% 21% 6% 1	760
	21st to end	Night	22% 12% 32% 25% 9%	836
April	1st to 10th	Night	26% 15% 30% 24% 4	760
	11th to 20th	Night	24% 11% 32% 22% 10% 1	601
	21st to end	Night	21% 10% 30% 30% 9%	570
May	1st to 10th	Night	19% 5% 30% 32% 13% 1	570
	11th to 20th	Night	19% 6% 27% 36% 10% 2	491
	21st to end	Night	12% 4 28% 39% 17%	411
June	1st to 10th	Night	14% 6% 28% 38% 14% 1	380
	11th to 20th	Night	12% 6% 29% 39% 13%	380
	21st to end	Night	14% 6% 24% 39% 15% 1	380
July	1st to 10th	Night	7% 3 17% 50% 23% 1	370
	11th to 20th	Night	11% 3 23% 37% 24% 2	458
	21st to end	Night	11% 2 17% 46% 23% 1	624
August	1st to 10th	Night	12% 3 14% 43% 26% 2	567
	11th to 20th	Night	11% 3 18% 35% 29% 4	564
	21st to end	Night	13% 3 22% 33% 25% 3	642
September	1st to 10th	Night	13% 6% 21% 34% 21% 4	760
	11th to 20th	Night	12% 4 20% 34% 25% 5%	760
	21st to end	Night	19% 8% 22% 31% 16% 4	756
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

Page generated at 2024-09-26 09:52:23. Version ac07

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