Grayville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grayville clear sky chart from 2005-11-06 to 2026-07-31.

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	13% 7% 3 7% 5% 4 5% 4 6% 8% 40%	137894
	Night	14% 8% 3 6% 4 4 4 3 5% 7% 42%	135330
Transparency	Day	42% 4 11% 24% 16% 2	141494
	Night	42% 4 11% 23% 18% 3	139807
Seeing	Day	16% 8% 24% 25% 19% 8%	17905
	Night	17% 7% 26% 31% 16% 2	27651

By Month

Percentage Cloud Cover SE (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% 10% 5% 10% 5% 5% 5% 4 4 6% 33%	4691
	11th to 20th	Day	10% 8% 3 10% 6% 4 5% 3 5% 7% 38%	4710
	21st to end	Day	18% 14% 4 9% 5% 4 4 3 4 5% 31%	5724
February	1st to 10th	Day	16% 10% 4 8% 5% 4 4 3 4 6% 36%	2085
	11th to 20th	Day	17% 11% 4 8% 4 4 3 3 5% 5% 36%	2048
	21st to end	Day	14% 10% 4 8% 5% 4 4 3 5% 7% 36%	1730
March	1st to 10th	Day	19% 11% 3 6% 4 3 2 2 3 4 43%	4580
	11th to 20th	Day	12% 8% 3 6% 3 3 3 3 6% 7% 46%	4787
	21st to end	Day	14% 8% 3 7% 4 4 5% 4 6% 7% 37%	6099
April	1st to 10th	Day	13% 7% 3 6% 6% 4 4 4 4 10% 41%	2520
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 3 5% 7% 41%	2494
	21st to end	Day	17% 6% 2 5% 5% 4 5% 3 5% 9% 39%	2538
May	1st to 10th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 32%	7616
	11th to 20th	Day	16% 7% 2 7% 5% 4 4 4 5% 7% 39%	7626
	21st to end	Day	17% 8% 3 8% 5% 5% 5% 4 7% 8% 30%	8806
June	1st to 10th	Day	10% 6% 2 5% 5% 4 5% 4 5% 8% 46%	2940
	11th to 20th	Day	6% 4 2 5% 5% 5% 6% 5% 6% 9% 46%	2940
	21st to end	Day	7% 5% 2 6% 5% 5% 4 4 7% 10% 46%	2940
July	1st to 10th	Day	10% 4 2 6% 4 4 5% 4 6% 11% 44%	6500
	11th to 20th	Day	7% 5% 2 7% 5% 6% 6% 5% 8% 11% 39%	6500
	21st to end	Day	8% 6% 3 7% 6% 6% 7% 6% 8% 10% 34%	7033
August	1st to 10th	Day	6% 4 1 4 3 4 4 4 6% 9% 54%	4007
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 9% 43%	3849
	21st to end	Day	5% 4 2 5% 4 4 5% 4 6% 9% 55%	4312
September	1st to 10th	Day	10% 5% 2 5% 4 3 5% 4 6% 7% 48%	2400
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 3 5% 7% 51%	2250
	21st to end	Day	11% 5% 2 5% 4 4 4 2 5% 7% 52%	2200
October	1st to 10th	Day	10% 4 1 5% 3 3 2 3 5% 6% 57%	2200
	11th to 20th	Day	13% 5% 2 4 4 3 3 4 4 6% 51%	2010
	21st to end	Day	18% 7% 2 5% 3 3 3 3 4 5% 45%	2030
November	1st to 10th	Day	8% 6% 2 5% 4 4 5% 2 4 6% 53%	1852
	11th to 20th	Day	15% 7% 2 7% 4 4 4 2 5% 6% 45%	1890
	21st to end	Day	18% 10% 3 7% 3 3 3 3 4 5% 40%	1890
December	1st to 10th	Day	12% 10% 3 7% 4 3 3 3 3 5% 48%	2939
	11th to 20th	Day	14% 10% 5% 10% 5% 4 4 3 5% 6% 33%	3088
	21st to end	Day	27% 14% 5% 10% 5% 3 4 3 3 4 22%	4070
January	1st to 10th	Night	16% 11% 5% 8% 3 4 4 4 5% 6% 35%	8232
	11th to 20th	Night	12% 11% 4 9% 4 5% 5% 3 4 5% 39%	8280
	21st to end	Night	18% 10% 4 7% 4 4 3 3 4 6% 37%	8485
February	1st to 10th	Night	19% 11% 4 8% 5% 4 3 4 5% 5% 32%	2532
	11th to 20th	Night	17% 9% 3 8% 4 4 4 3 4 5% 39%	2458
	21st to end	Night	18% 10% 2 7% 5% 3 3 3 5% 4 39%	2076
March	1st to 10th	Night	21% 10% 3 6% 4 4 4 3 4 5% 35%	5493
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 5% 6% 50%	5434
	21st to end	Night	17% 7% 3 8% 6% 5% 4 4 5% 6% 36%	5072
April	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 46%	2100
	11th to 20th	Night	16% 7% 2 5% 3 3 4 4 5% 6% 44%	2075
	21st to end	Night	17% 7% 3 7% 4 4 4 3 5% 7% 39%	2079
May	1st to 10th	Night	15% 8% 3 8% 5% 5% 4 3 6% 8% 34%	5237
	11th to 20th	Night	13% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	5234
	21st to end	Night	16% 7% 3 8% 5% 5% 5% 4 6% 8% 32%	5351
June	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 6% 8% 47%	1680
	11th to 20th	Night	6% 4 1 6% 4 4 5% 4 5% 8% 52%	1680
	21st to end	Night	6% 4 1 4 4 4 5% 4 7% 9% 52%	1680
July	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 49%	3698
	11th to 20th	Night	7% 4 2 6% 4 4 5% 4 6% 9% 50%	3697
	21st to end	Night	7% 5% 2 7% 5% 4 6% 5% 8% 12% 39%	4043
August	1st to 10th	Night	6% 3 2 5% 3 3 4 3 6% 9% 58%	3138
	11th to 20th	Night	6% 4 2 5% 3 4 5% 4 6% 8% 53%	3200
	21st to end	Night	4 3 1 4 3 4 4 3 6% 9% 59%	3586
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 6% 54%	2000
	11th to 20th	Night	8% 4 2 6% 3 4 4 4 5% 9% 52%	2130
	21st to end	Night	12% 5% 1 5% 4 3 4 4 6% 7% 50%	2200
October	1st to 10th	Night	9% 3 2 5% 2 3 3 3 5% 5% 60%	2200
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 6% 54%	2387
	21st to end	Night	18% 8% 3 5% 3 3 3 2 4 6% 45%	2810
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 5% 53%	2666
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 7% 50%	2730
	21st to end	Night	19% 7% 3 5% 4 4 3 2 5% 5% 43%	2730
December	1st to 10th	Night	18% 9% 3 6% 3 3 3 3 4 5% 43%	4448
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 6% 41%	5339
	21st to end	Night	29% 13% 3 8% 5% 4 4 3 3 4 25%	7150

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	55% 3 12% 23% 6% 1	4692
	11th to 20th	Day	47% 3 9% 26% 13% 2	4710
	21st to end	Day	58% 2 10% 16% 9% 5%	5724
February	1st to 10th	Day	50% 3 12% 23% 11% 2	2085
	11th to 20th	Day	51% 4 9% 21% 14% 2	2058
	21st to end	Day	50% 3 9% 22% 13% 3	1730
March	1st to 10th	Day	48% 3 4 20% 22% 4	4580
	11th to 20th	Day	38% 3 10% 28% 18% 3	4787
	21st to end	Day	45% 5% 12% 25% 13% 1	6101
April	1st to 10th	Day	41% 5% 13% 24% 16% 1	2520
	11th to 20th	Day	43% 4 11% 23% 15% 3	2494
	21st to end	Day	43% 5% 11% 27% 13% 2	2538
May	1st to 10th	Day	48% 6% 12% 21% 11%	7651
	11th to 20th	Day	44% 5% 11% 22% 17% 1	7630
	21st to end	Day	50% 5% 11% 18% 14% 1	8820
June	1st to 10th	Day	35% 6% 11% 24% 19% 3	2940
	11th to 20th	Day	33% 8% 11% 25% 20% 3	2940
	21st to end	Day	33% 7% 15% 24% 16% 5%	2940
July	1st to 10th	Day	35% 5% 11% 28% 18% 2	6431
	11th to 20th	Day	37% 6% 18% 26% 13% 1	6425
	21st to end	Day	43% 6% 19% 24% 8% 1	6837
August	1st to 10th	Day	27% 5% 15% 37% 16% 1	3994
	11th to 20th	Day	35% 5% 15% 25% 18% 1	3885
	21st to end	Day	27% 3 13% 35% 18% 4	4311
September	1st to 10th	Day	34% 5% 9% 26% 21% 6%	2400
	11th to 20th	Day	32% 5% 10% 27% 24% 3	2250
	21st to end	Day	33% 4 7% 24% 28% 4	2200
October	1st to 10th	Day	18% 3 5% 26% 43% 4	3590
	11th to 20th	Day	24% 3 8% 25% 35% 5%	3250
	21st to end	Day	33% 3 6% 25% 29% 4	3255
November	1st to 10th	Day	34% 4 9% 30% 17% 6%	1852
	11th to 20th	Day	41% 3 7% 25% 18% 5%	1890
	21st to end	Day	49% 3 8% 23% 15% 2	1890
December	1st to 10th	Day	42% 2 13% 27% 16%	2939
	11th to 20th	Day	53% 3 11% 22% 11% 1	3085
	21st to end	Day	68% 3 12% 12% 6%	4070
January	1st to 10th	Night	51% 3 14% 24% 8% 1	8233
	11th to 20th	Night	49% 3 6% 20% 19% 3	8280
	21st to end	Night	50% 2 10% 20% 16% 1	8485
February	1st to 10th	Night	54% 4 9% 21% 10% 1	2532
	11th to 20th	Night	48% 4 8% 24% 14% 2	2459
	21st to end	Night	48% 4 9% 22% 14% 3	2076
March	1st to 10th	Night	51% 3 6% 18% 18% 3	5493
	11th to 20th	Night	36% 4 10% 24% 21% 6%	5434
	21st to end	Night	50% 4 11% 24% 10% 1	5072
April	1st to 10th	Night	38% 6% 11% 27% 15% 3	2100
	11th to 20th	Night	41% 4 10% 21% 19% 5%	2075
	21st to end	Night	44% 5% 11% 23% 15% 2	2079
May	1st to 10th	Night	48% 5% 14% 23% 9% 1	5264
	11th to 20th	Night	44% 6% 11% 22% 15% 1	5235
	21st to end	Night	48% 5% 14% 20% 11% 2	5361
June	1st to 10th	Night	33% 5% 12% 25% 22% 3	1680
	11th to 20th	Night	28% 7% 11% 23% 26% 5%	1680
	21st to end	Night	26% 6% 16% 25% 20% 7%	1680
July	1st to 10th	Night	34% 6% 9% 25% 22% 3	3660
	11th to 20th	Night	31% 7% 17% 29% 14% 2	3652
	21st to end	Night	38% 4 21% 25% 11% 1	3928
August	1st to 10th	Night	23% 5% 15% 40% 15% 2	3127
	11th to 20th	Night	29% 5% 13% 30% 21% 1	3230
	21st to end	Night	22% 4 13% 33% 24% 3	3586
September	1st to 10th	Night	32% 4 7% 28% 24% 6%	2000
	11th to 20th	Night	29% 5% 8% 27% 26% 5%	2130
	21st to end	Night	33% 5% 9% 22% 24% 7%	2200
October	1st to 10th	Night	18% 3 5% 16% 50% 8%	3590
	11th to 20th	Night	24% 2 5% 26% 37% 6%	3886
	21st to end	Night	33% 2 7% 24% 26% 8%	4535
November	1st to 10th	Night	34% 3 8% 25% 24% 5%	2666
	11th to 20th	Night	36% 4 8% 24% 23% 5%	2730
	21st to end	Night	44% 3 9% 24% 17% 2	2730
December	1st to 10th	Night	45% 3 10% 24% 17% 1	4448
	11th to 20th	Night	46% 3 12% 22% 15% 1	5341
	21st to end	Night	64% 3 10% 16% 6% 1	7150

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	22% 16% 33% 19% 8% 1	330
	11th to 20th	Day	23% 17% 36% 19% 4	330
	21st to end	Day	22% 17% 41% 14% 6%	363
February	1st to 10th	Day	23% 12% 42% 18% 4 1	330
	11th to 20th	Day	25% 18% 40% 12% 3 1	330
	21st to end	Day	26% 14% 37% 19% 4	273
March	1st to 10th	Day	23% 17% 36% 18% 6%	330
	11th to 20th	Day	26% 19% 33% 17% 5% 1	363
	21st to end	Day	25% 14% 36% 18% 7% 1	594
April	1st to 10th	Day	19% 13% 35% 24% 8% 1	540
	11th to 20th	Day	20% 14% 29% 23% 12% 2	532
	21st to end	Day	19% 11% 33% 24% 10% 3	540
May	1st to 10th	Day	20% 11% 29% 29% 8% 3	543
	11th to 20th	Day	20% 6% 27% 27% 14% 7%	567
	21st to end	Day	13% 5% 26% 27% 18% 11%	759
June	1st to 10th	Day	13% 5% 18% 26% 28% 10%	780
	11th to 20th	Day	7% 4 20% 34% 25% 10%	780
	21st to end	Day	9% 3 21% 33% 22% 11%	780
July	1st to 10th	Day	12% 3 11% 23% 28% 23%	771
	11th to 20th	Day	8% 3 12% 31% 30% 15%	768
	21st to end	Day	8% 3 12% 29% 34% 14%	838
August	1st to 10th	Day	9% 3 11% 24% 35% 19%	526
	11th to 20th	Day	12% 3 10% 27% 33% 14%	519
	21st to end	Day	7% 2 11% 24% 36% 21%	583
September	1st to 10th	Day	9% 3 12% 28% 35% 14%	530
	11th to 20th	Day	8% 3 8% 26% 38% 17%	530
	21st to end	Day	15% 4 19% 25% 25% 13%	530
October	1st to 10th	Day	10% 5% 18% 34% 24% 9%	530
	11th to 20th	Day	11% 11% 32% 26% 14% 6%	340
	21st to end	Day	26% 9% 31% 24% 9% 1	363
November	1st to 10th	Day	13% 8% 34% 26% 15% 4	330
	11th to 20th	Day	16% 18% 29% 23% 12% 1	330
	21st to end	Day	27% 12% 26% 24% 10% 1	330
December	1st to 10th	Day	19% 18% 33% 21% 6% 2	330
	11th to 20th	Day	23% 14% 33% 22% 7% 1	330
	21st to end	Day	31% 15% 33% 17% 5%	363
January	1st to 10th	Night	22% 12% 35% 26% 4	1050
	11th to 20th	Night	26% 14% 35% 22% 4	1050
	21st to end	Night	20% 12% 39% 28% 2	1155
February	1st to 10th	Night	25% 10% 37% 25% 3	852
	11th to 20th	Night	22% 14% 35% 26% 2	823
	21st to end	Night	25% 11% 39% 24% 2	692
March	1st to 10th	Night	24% 13% 32% 27% 4	840
	11th to 20th	Night	20% 12% 33% 28% 6%	827
	21st to end	Night	23% 10% 30% 30% 7% 1	693
April	1st to 10th	Night	17% 9% 34% 33% 7%	630
	11th to 20th	Night	19% 11% 30% 27% 12% 1	623
	21st to end	Night	18% 5% 34% 31% 10% 1	630
May	1st to 10th	Night	18% 6% 29% 35% 11% 1	630
	11th to 20th	Night	15% 6% 25% 35% 17% 2	621
	21st to end	Night	10% 2 21% 39% 25% 4	683
June	1st to 10th	Night	13% 2 16% 42% 24% 3	630
	11th to 20th	Night	8% 3 17% 44% 26% 2	630
	21st to end	Night	6% 1 19% 39% 31% 3	630
July	1st to 10th	Night	10% 1 7% 32% 41% 10%	615
	11th to 20th	Night	7% 1 8% 35% 43% 5%	612
	21st to end	Night	9% 2 8% 36% 38% 6%	687
August	1st to 10th	Night	8% 2 8% 36% 39% 8%	597
	11th to 20th	Night	6% 1 9% 37% 43% 5%	594
	21st to end	Night	6% 1 8% 36% 42% 7%	660
September	1st to 10th	Night	11% 2 11% 34% 36% 7%	600
	11th to 20th	Night	10% 2 10% 36% 35% 7%	730
	21st to end	Night	14% 4 17% 31% 29% 5%	800
October	1st to 10th	Night	10% 4 18% 37% 26% 5%	800
	11th to 20th	Night	16% 6% 28% 31% 16% 2	800
	21st to end	Night	22% 7% 32% 29% 10%	880
November	1st to 10th	Night	15% 7% 29% 35% 13% 1	820
	11th to 20th	Night	17% 9% 31% 32% 11%	840
	21st to end	Night	22% 11% 28% 29% 10%	840
December	1st to 10th	Night	24% 8% 32% 31% 5%	882
	11th to 20th	Night	24% 8% 32% 29% 6%	1050
	21st to end	Night	29% 12% 33% 23% 3	1155

Page generated at 2026-07-28 02:25:25. Version ac07

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