Melville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Melville clear sky chart from 2005-11-06 to 2025-10-19.

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	10% 8% 3 9% 5% 5% 5% 4 6% 8% 37%	131517
	Night	8% 9% 4 9% 6% 5% 5% 4 6% 7% 36%	126984
Transparency	Day	44% 6% 15% 24% 9% 1	131229
	Night	46% 6% 14% 23% 10% 1	126852
Seeing	Day	14% 8% 29% 29% 16% 4	17632
	Night	13% 7% 31% 34% 13% 2	26987

By Month

Percentage Cloud Cover NW (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	5% 14% 7% 17% 9% 7% 5% 4 7% 8% 17%	3271
	11th to 20th	Day	4 8% 4 12% 10% 9% 8% 7% 10% 8% 21%	3616
	21st to end	Day	6% 12% 6% 13% 7% 5% 6% 3 5% 7% 29%	4004
February	1st to 10th	Day	8% 14% 6% 12% 7% 7% 7% 6% 5% 6% 23%	1600
	11th to 20th	Day	6% 10% 3 11% 6% 8% 9% 4 6% 7% 29%	1735
	21st to end	Day	5% 11% 4 12% 8% 7% 6% 6% 7% 8% 27%	1650
March	1st to 10th	Day	7% 13% 5% 15% 7% 6% 4 3 6% 8% 27%	4500
	11th to 20th	Day	8% 14% 6% 15% 7% 5% 4 4 5% 6% 28%	4622
	21st to end	Day	11% 11% 4 10% 6% 6% 5% 4 5% 6% 32%	5969
April	1st to 10th	Day	8% 10% 4 8% 5% 5% 5% 4 6% 7% 38%	2400
	11th to 20th	Day	15% 15% 5% 8% 6% 4 4 4 5% 7% 28%	2482
	21st to end	Day	13% 9% 3 8% 5% 5% 5% 4 6% 8% 36%	2680
May	1st to 10th	Day	15% 8% 2 5% 3 4 4 4 5% 7% 43%	6340
	11th to 20th	Day	18% 8% 2 6% 5% 5% 5% 4 6% 8% 32%	6361
	21st to end	Day	11% 7% 2 6% 4 5% 4 4 5% 7% 46%	7428
June	1st to 10th	Day	15% 7% 2 6% 5% 4 4 5% 8% 8% 37%	3000
	11th to 20th	Day	20% 8% 3 8% 5% 5% 5% 4 7% 7% 29%	3169
	21st to end	Day	14% 6% 2 5% 5% 5% 5% 5% 6% 9% 38%	3200
July	1st to 10th	Day	6% 4 2 7% 5% 6% 6% 5% 8% 9% 41%	7164
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 44%	6573
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 9% 52%	6898
August	1st to 10th	Day	8% 5% 3 6% 5% 5% 5% 5% 7% 9% 44%	4552
	11th to 20th	Day	7% 3 2 5% 4 4 6% 5% 8% 11% 45%	4456
	21st to end	Day	12% 5% 2 6% 3 4 4 3 5% 8% 48%	4492
September	1st to 10th	Day	11% 4 2 6% 3 5% 5% 4 5% 8% 48%	2400
	11th to 20th	Day	13% 8% 3 7% 4 4 4 3 5% 7% 41%	2320
	21st to end	Day	12% 7% 2 5% 4 3 3 3 4 6% 51%	2200
October	1st to 10th	Day	11% 9% 2 7% 4 4 4 3 4 7% 45%	3371
	11th to 20th	Day	8% 7% 2 6% 3 4 4 4 6% 9% 47%	2954
	21st to end	Day	8% 11% 5% 10% 5% 5% 5% 4 6% 6% 37%	3091
November	1st to 10th	Day	17% 14% 5% 10% 5% 5% 5% 3 5% 5% 27%	1714
	11th to 20th	Day	9% 14% 6% 12% 8% 6% 4 4 5% 5% 25%	1478
	21st to end	Day	8% 18% 5% 11% 7% 5% 4 4 4 5% 29%	1400
December	1st to 10th	Day	6% 11% 6% 15% 9% 7% 6% 4 6% 6% 22%	2290
	11th to 20th	Day	7% 14% 6% 16% 7% 7% 5% 3 5% 8% 22%	2639
	21st to end	Day	4 9% 4 13% 8% 5% 4 3 5% 6% 40%	3498
January	1st to 10th	Night	4 12% 5% 17% 10% 8% 6% 4 7% 7% 21%	6678
	11th to 20th	Night	4 9% 3 14% 10% 9% 10% 6% 9% 8% 18%	6302
	21st to end	Night	5% 11% 5% 14% 8% 6% 5% 4 6% 7% 27%	6974
February	1st to 10th	Night	9% 12% 4 11% 8% 6% 5% 5% 6% 7% 28%	2800
	11th to 20th	Night	5% 9% 3 10% 7% 7% 8% 5% 7% 8% 32%	2571
	21st to end	Night	3 10% 4 11% 8% 6% 6% 4 5% 8% 35%	1980
March	1st to 10th	Night	6% 12% 5% 13% 6% 6% 5% 4 7% 9% 28%	5374
	11th to 20th	Night	7% 12% 4 9% 6% 5% 4 4 6% 7% 35%	5340
	21st to end	Night	8% 12% 4 10% 7% 6% 6% 4 6% 6% 33%	4984
April	1st to 10th	Night	5% 8% 2 7% 5% 5% 5% 4 7% 8% 43%	2000
	11th to 20th	Night	14% 11% 4 8% 4 5% 5% 3 5% 5% 35%	1869
	21st to end	Night	15% 9% 3 6% 5% 4 4 4 6% 7% 37%	1716
May	1st to 10th	Night	12% 7% 2 5% 4 4 4 4 5% 7% 45%	3640
	11th to 20th	Night	15% 7% 3 7% 5% 5% 6% 5% 7% 8% 32%	3555
	21st to end	Night	14% 6% 2 5% 4 3 3 3 5% 7% 48%	3470
June	1st to 10th	Night	13% 6% 2 6% 5% 4 4 3 5% 9% 41%	1400
	11th to 20th	Night	16% 6% 3 6% 4 3 5% 3 6% 8% 40%	1226
	21st to end	Night	12% 5% 2 5% 5% 4 4 5% 6% 8% 44%	1200
July	1st to 10th	Night	6% 3 2 6% 4 4 5% 4 8% 9% 48%	2784
	11th to 20th	Night	10% 3 2 5% 3 3 4 3 6% 9% 52%	3403
	21st to end	Night	6% 3 1 4 2 3 3 3 5% 8% 62%	3968
August	1st to 10th	Night	6% 5% 2 5% 4 4 6% 4 6% 9% 50%	2608
	11th to 20th	Night	6% 3 2 5% 4 6% 4 4 7% 8% 51%	2602
	21st to end	Night	8% 5% 2 5% 4 3 4 4 5% 8% 52%	3414
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 6% 7% 52%	2000
	11th to 20th	Night	12% 7% 3 7% 4 3 4 4 6% 7% 44%	2080
	21st to end	Night	10% 6% 2 5% 3 4 4 3 6% 7% 50%	2200
October	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 5% 8% 48%	3813
	11th to 20th	Night	6% 7% 3 6% 4 5% 5% 5% 7% 10% 42%	4118
	21st to end	Night	9% 9% 4 7% 4 4 4 4 5% 8% 43%	4467
November	1st to 10th	Night	15% 12% 4 9% 8% 5% 4 4 5% 6% 30%	2580
	11th to 20th	Night	9% 14% 6% 12% 7% 5% 5% 4 5% 5% 28%	2920
	21st to end	Night	9% 12% 5% 12% 7% 5% 6% 4 5% 7% 28%	3000
December	1st to 10th	Night	7% 13% 5% 15% 10% 9% 7% 5% 5% 6% 18%	4900
	11th to 20th	Night	7% 14% 5% 11% 6% 6% 7% 5% 6% 7% 25%	5568
	21st to end	Night	5% 7% 3 13% 9% 6% 5% 4 5% 5% 38%	7480

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	64% 6% 12% 12% 6%	3271
	11th to 20th	Day	55% 6% 13% 18% 8%	3640
	21st to end	Day	54% 6% 15% 20% 5%	4004
February	1st to 10th	Day	60% 6% 13% 13% 7% 1	1600
	11th to 20th	Day	51% 7% 9% 21% 12%	1744
	21st to end	Day	52% 8% 12% 18% 10%	1650
March	1st to 10th	Day	57% 6% 15% 16% 7%	4500
	11th to 20th	Day	57% 7% 13% 16% 7%	4623
	21st to end	Day	51% 6% 11% 21% 12% 1	5969
April	1st to 10th	Day	45% 7% 15% 21% 11% 1	2400
	11th to 20th	Day	56% 5% 13% 16% 8% 1	2482
	21st to end	Day	45% 7% 15% 24% 9% 1	2680
May	1st to 10th	Day	40% 4 12% 25% 18%	6340
	11th to 20th	Day	48% 6% 13% 22% 10% 1	6365
	21st to end	Day	36% 6% 10% 25% 22% 1	7428
June	1st to 10th	Day	41% 8% 15% 26% 9% 1	3000
	11th to 20th	Day	51% 8% 14% 18% 8% 1	3169
	21st to end	Day	41% 9% 17% 25% 8%	3200
July	1st to 10th	Day	34% 9% 23% 27% 6%	7086
	11th to 20th	Day	34% 7% 17% 36% 5%	6492
	21st to end	Day	28% 6% 23% 33% 10% 1	6705
August	1st to 10th	Day	35% 8% 18% 28% 10% 1	4536
	11th to 20th	Day	29% 8% 21% 35% 6%	4498
	21st to end	Day	34% 6% 17% 36% 7%	4491
September	1st to 10th	Day	33% 7% 14% 33% 12% 1	2400
	11th to 20th	Day	43% 6% 16% 25% 10% 1	2320
	21st to end	Day	36% 5% 13% 33% 13% 1	2200
October	1st to 10th	Day	40% 4 17% 31% 8% 1	3371
	11th to 20th	Day	34% 5% 17% 35% 8% 1	2954
	21st to end	Day	48% 5% 15% 24% 7%	3091
November	1st to 10th	Day	60% 5% 11% 16% 7% 1	1714
	11th to 20th	Day	61% 6% 13% 13% 6% 1	1478
	21st to end	Day	58% 6% 13% 19% 5%	1400
December	1st to 10th	Day	61% 8% 14% 13% 5%	2290
	11th to 20th	Day	61% 5% 17% 14% 2	2640
	21st to end	Day	47% 5% 9% 23% 12% 4	3498
January	1st to 10th	Night	62% 5% 11% 17% 5%	6678
	11th to 20th	Night	58% 7% 14% 15% 6%	6340
	21st to end	Night	55% 7% 18% 16% 4	6974
February	1st to 10th	Night	55% 5% 12% 18% 10% 1	2800
	11th to 20th	Night	46% 8% 13% 21% 12% 1	2572
	21st to end	Night	47% 6% 12% 24% 11%	1980
March	1st to 10th	Night	52% 5% 20% 17% 6%	5374
	11th to 20th	Night	47% 6% 13% 24% 9%	5340
	21st to end	Night	50% 5% 9% 23% 12% 1	4984
April	1st to 10th	Night	37% 7% 13% 24% 18% 1	2000
	11th to 20th	Night	50% 6% 10% 21% 12% 1	1869
	21st to end	Night	45% 7% 12% 22% 14%	1716
May	1st to 10th	Night	36% 5% 14% 23% 20% 1	3640
	11th to 20th	Night	47% 8% 13% 19% 12% 1	3555
	21st to end	Night	35% 6% 10% 23% 24% 2	3470
June	1st to 10th	Night	40% 6% 15% 23% 13% 2	1400
	11th to 20th	Night	41% 8% 17% 19% 13% 1	1226
	21st to end	Night	37% 8% 19% 28% 8% 1	1200
July	1st to 10th	Night	30% 7% 25% 31% 6%	2753
	11th to 20th	Night	29% 5% 18% 41% 7%	3361
	21st to end	Night	21% 5% 25% 31% 15% 2	3856
August	1st to 10th	Night	29% 8% 21% 30% 12%	2600
	11th to 20th	Night	29% 7% 19% 36% 9% 1	2626
	21st to end	Night	30% 6% 17% 36% 10%	3414
September	1st to 10th	Night	31% 7% 13% 33% 14% 2	2000
	11th to 20th	Night	38% 7% 16% 26% 12% 1	2080
	21st to end	Night	33% 5% 13% 32% 14% 2	2200
October	1st to 10th	Night	36% 5% 15% 32% 10% 2	3813
	11th to 20th	Night	36% 5% 14% 33% 11% 1	4118
	21st to end	Night	41% 5% 14% 31% 10%	4467
November	1st to 10th	Night	56% 6% 13% 16% 8% 1	2580
	11th to 20th	Night	59% 7% 14% 15% 5%	2920
	21st to end	Night	56% 7% 15% 18% 5%	3000
December	1st to 10th	Night	67% 6% 11% 12% 4	4900
	11th to 20th	Night	57% 6% 14% 18% 5%	5566
	21st to end	Night	48% 5% 9% 20% 14% 4	7480

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	12% 8% 30% 31% 16% 3	300
	11th to 20th	Day	6% 8% 39% 36% 8% 2	300
	21st to end	Day	15% 10% 30% 30% 14% 2	330
February	1st to 10th	Day	15% 9% 38% 27% 11% 1	300
	11th to 20th	Day	8% 7% 31% 35% 15% 5%	300
	21st to end	Day	8% 7% 30% 37% 15% 4	249
March	1st to 10th	Day	12% 9% 32% 31% 12% 3	300
	11th to 20th	Day	13% 12% 33% 25% 15% 2	300
	21st to end	Day	15% 9% 31% 25% 17% 2	550
April	1st to 10th	Day	9% 14% 40% 26% 10% 1	500
	11th to 20th	Day	21% 12% 30% 25% 11% 2	600
	21st to end	Day	15% 10% 29% 28% 15% 2	700
May	1st to 10th	Day	15% 8% 28% 32% 15% 3	703
	11th to 20th	Day	16% 8% 27% 28% 16% 5%	725
	21st to end	Day	17% 6% 24% 30% 18% 6%	789
June	1st to 10th	Day	17% 11% 28% 27% 12% 5%	730
	11th to 20th	Day	20% 9% 28% 28% 12% 3	730
	21st to end	Day	14% 9% 25% 30% 17% 5%	730
July	1st to 10th	Day	8% 4 23% 32% 25% 7%	721
	11th to 20th	Day	12% 6% 25% 28% 23% 7%	718
	21st to end	Day	6% 7% 26% 31% 22% 8%	794
August	1st to 10th	Day	10% 5% 23% 35% 20% 7%	726
	11th to 20th	Day	9% 6% 23% 32% 22% 8%	715
	21st to end	Day	13% 7% 22% 35% 19% 4	698
September	1st to 10th	Day	12% 10% 31% 28% 15% 4	530
	11th to 20th	Day	19% 10% 25% 29% 13% 4	450
	21st to end	Day	17% 11% 29% 28% 14% 2	330
October	1st to 10th	Day	20% 7% 31% 23% 16% 3	330
	11th to 20th	Day	14% 14% 34% 23% 10% 5%	324
	21st to end	Day	14% 11% 38% 26% 8% 4	330
November	1st to 10th	Day	25% 6% 30% 25% 10% 4	300
	11th to 20th	Day	20% 10% 34% 28% 8%	300
	21st to end	Day	14% 9% 38% 29% 8% 1	300
December	1st to 10th	Day	9% 6% 41% 28% 13% 3	300
	11th to 20th	Day	14% 7% 44% 27% 6% 1	300
	21st to end	Day	11% 4 33% 31% 17% 4	330
January	1st to 10th	Night	10% 9% 36% 32% 12% 1	1000
	11th to 20th	Night	15% 9% 34% 32% 8% 1	1000
	21st to end	Night	13% 10% 40% 28% 9% 1	1100
February	1st to 10th	Night	14% 9% 30% 33% 13% 1	1000
	11th to 20th	Night	10% 6% 34% 36% 12% 2	853
	21st to end	Night	8% 7% 30% 35% 17% 3	660
March	1st to 10th	Night	14% 7% 30% 34% 14% 2	800
	11th to 20th	Night	11% 7% 35% 32% 14% 1	765
	21st to end	Night	13% 4 30% 36% 15% 3	660
April	1st to 10th	Night	8% 7% 33% 38% 12% 1	600
	11th to 20th	Night	18% 8% 33% 32% 9%	593
	21st to end	Night	19% 7% 32% 33% 10% 1	600
May	1st to 10th	Night	15% 6% 27% 37% 14% 2	600
	11th to 20th	Night	15% 7% 30% 33% 15%	591
	21st to end	Night	18% 6% 26% 33% 15% 1	650
June	1st to 10th	Night	17% 7% 30% 34% 12% 1	600
	11th to 20th	Night	19% 5% 31% 32% 12%	426
	21st to end	Night	15% 4 24% 39% 17% 1	400
July	1st to 10th	Night	9% 4 21% 40% 24% 1	414
	11th to 20th	Night	10% 4 27% 37% 20% 2	582
	21st to end	Night	7% 3 20% 47% 19% 4	657
August	1st to 10th	Night	6% 4 22% 45% 21% 2	597
	11th to 20th	Night	9% 5% 19% 39% 24% 4	594
	21st to end	Night	10% 5% 27% 41% 17% 1	660
September	1st to 10th	Night	11% 4 26% 37% 20% 2	600
	11th to 20th	Night	16% 5% 27% 38% 13% 1	600
	21st to end	Night	12% 8% 29% 37% 13% 2	600
October	1st to 10th	Night	19% 5% 25% 37% 13% 1	732
	11th to 20th	Night	13% 8% 31% 35% 12% 1	933
	21st to end	Night	16% 10% 32% 32% 8% 1	1045
November	1st to 10th	Night	20% 9% 33% 27% 10% 1	975
	11th to 20th	Night	15% 9% 38% 27% 10% 2	1000
	21st to end	Night	15% 10% 37% 30% 7% 1	1000
December	1st to 10th	Night	11% 7% 37% 29% 12% 4	1000
	11th to 20th	Night	12% 11% 34% 31% 11% 1	1000
	21st to end	Night	13% 5% 31% 33% 15% 2	1100

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