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 2024-02-15.

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 6% 5% 3 5% 4 5% 7% 41%	78366
	Night	15% 7% 3 5% 5% 3 4 4 5% 7% 42%	80758
Transparency	Day	43% 5% 11% 23% 15% 3	78069
	Night	43% 5% 10% 23% 16% 3	80576
Seeing	Day	16% 8% 23% 25% 20% 9%	14466
	Night	17% 7% 26% 31% 16% 2	24447

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% 11% 6% 7% 5% 3 4 5% 4 6% 34%	2527
	11th to 20th	Day	17% 8% 4 9% 9% 4 6% 5% 5% 6% 29%	2530
	21st to end	Day	25% 17% 5% 7% 4 2 3 3 3 5% 25%	3075
February	1st to 10th	Day	17% 9% 5% 6% 5% 3 4 4 4 6% 36%	1885
	11th to 20th	Day	17% 10% 4 7% 5% 3 3 4 5% 6% 35%	1788
	21st to end	Day	15% 10% 4 7% 6% 2 5% 4 5% 7% 33%	1480
March	1st to 10th	Day	17% 8% 3 6% 5% 3 4 4 3 6% 42%	1800
	11th to 20th	Day	20% 9% 4 6% 5% 3 4 4 5% 6% 35%	1852
	21st to end	Day	21% 9% 4 6% 6% 3 5% 4 6% 6% 30%	2376
April	1st to 10th	Day	11% 5% 3 5% 6% 3 4 5% 4 10% 43%	2160
	11th to 20th	Day	14% 8% 3 6% 6% 3 4 4 5% 6% 41%	2134
	21st to end	Day	18% 6% 3 4 6% 3 5% 4 5% 8% 39%	2175
May	1st to 10th	Day	17% 6% 3 7% 6% 3 4 5% 5% 7% 37%	2340
	11th to 20th	Day	17% 7% 3 5% 6% 3 4 5% 5% 7% 39%	2301
	21st to end	Day	9% 5% 3 6% 6% 4 6% 7% 7% 10% 36%	2652
June	1st to 10th	Day	9% 5% 2 4 5% 3 6% 5% 6% 9% 47%	2520
	11th to 20th	Day	6% 3 3 5% 5% 4 6% 6% 7% 9% 48%	2520
	21st to end	Day	6% 4 3 5% 5% 4 6% 5% 6% 10% 46%	2520
July	1st to 10th	Day	11% 4 2 5% 5% 3 6% 6% 7% 12% 38%	2520
	11th to 20th	Day	6% 4 2 5% 4 3 6% 5% 7% 11% 47%	2520
	21st to end	Day	8% 4 2 4 6% 4 6% 6% 8% 10% 43%	2738
August	1st to 10th	Day	7% 4 2 4 5% 3 7% 6% 6% 9% 46%	2205
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 5% 7% 10% 43%	2125
	21st to end	Day	6% 5% 2 6% 5% 3 5% 5% 6% 8% 50%	2376
September	1st to 10th	Day	11% 5% 3 5% 5% 2 4 5% 6% 7% 47%	2160
	11th to 20th	Day	8% 5% 2 5% 6% 3 5% 4 5% 8% 50%	2026
	21st to end	Day	10% 4 2 4 4 3 4 3 5% 7% 54%	1980
October	1st to 10th	Day	11% 4 2 4 4 3 3 4 5% 6% 56%	1980
	11th to 20th	Day	14% 5% 2 4 5% 2 4 4 4 7% 49%	1810
	21st to end	Day	18% 7% 2 5% 5% 2 3 4 4 6% 44%	1828
November	1st to 10th	Day	8% 5% 3 4 4 3 5% 3 4 6% 55%	1672
	11th to 20th	Day	16% 6% 3 6% 4 3 4 3 5% 5% 45%	1710
	21st to end	Day	18% 9% 3 7% 4 2 4 4 4 5% 41%	1710
December	1st to 10th	Day	17% 10% 4 6% 5% 3 4 4 3 5% 38%	1670
	11th to 20th	Day	15% 11% 5% 7% 6% 3 5% 4 5% 6% 32%	1918
	21st to end	Day	23% 14% 6% 9% 6% 3 4 4 3 5% 25%	2783
January	1st to 10th	Night	18% 11% 4 7% 5% 3 4 5% 5% 6% 33%	4428
	11th to 20th	Night	15% 12% 5% 8% 6% 4 7% 4 4 5% 31%	4440
	21st to end	Night	24% 13% 6% 6% 5% 2 3 3 4 6% 26%	4587
February	1st to 10th	Night	20% 10% 5% 7% 5% 3 4 5% 4 5% 32%	2292
	11th to 20th	Night	16% 8% 4 7% 6% 3 4 4 5% 5% 39%	2153
	21st to end	Night	20% 10% 3 6% 5% 2 4 3 5% 5% 36%	1776
March	1st to 10th	Night	17% 7% 3 6% 5% 3 4 4 5% 5% 43%	2160
	11th to 20th	Night	16% 8% 3 6% 6% 3 4 4 4 6% 38%	2121
	21st to end	Night	21% 7% 3 6% 5% 3 4 4 5% 6% 34%	1980
April	1st to 10th	Night	12% 5% 2 5% 5% 3 5% 4 5% 7% 47%	1800
	11th to 20th	Night	17% 6% 2 4 4 2 5% 5% 4 7% 44%	1775
	21st to end	Night	18% 6% 4 6% 6% 2 4 4 5% 7% 39%	1782
May	1st to 10th	Night	17% 6% 3 5% 5% 3 4 4 5% 7% 42%	1620
	11th to 20th	Night	12% 6% 3 6% 5% 3 5% 5% 5% 6% 42%	1592
	21st to end	Night	6% 4 3 6% 7% 4 7% 6% 6% 10% 41%	1620
June	1st to 10th	Night	9% 6% 2 5% 4 3 4 5% 6% 8% 49%	1440
	11th to 20th	Night	6% 3 1 4 5% 3 5% 5% 6% 8% 54%	1440
	21st to end	Night	5% 3 2 4 4 3 5% 5% 7% 10% 53%	1440
July	1st to 10th	Night	7% 4 1 4 4 3 5% 5% 6% 11% 49%	1440
	11th to 20th	Night	6% 3 1 4 3 3 5% 4 5% 9% 56%	1440
	21st to end	Night	8% 4 2 4 5% 3 4 5% 6% 9% 50%	1579
August	1st to 10th	Night	7% 3 2 4 4 2 4 4 7% 10% 52%	1729
	11th to 20th	Night	4 4 2 5% 4 4 6% 6% 6% 9% 50%	1776
	21st to end	Night	6% 4 2 3 5% 3 4 3 5% 8% 55%	1980
September	1st to 10th	Night	10% 5% 2 4 4 3 3 4 4 6% 54%	1800
	11th to 20th	Night	8% 4 2 5% 4 2 5% 5% 5% 9% 52%	1916
	21st to end	Night	11% 4 2 3 4 2 4 5% 6% 7% 52%	1980
October	1st to 10th	Night	9% 3 2 4 4 2 3 4 5% 5% 59%	1980
	11th to 20th	Night	15% 5% 2 4 4 2 3 3 3 7% 52%	2147
	21st to end	Night	18% 8% 3 5% 4 2 3 3 4 6% 45%	2528
November	1st to 10th	Night	12% 5% 2 4 4 2 3 4 4 5% 54%	2406
	11th to 20th	Night	14% 5% 3 5% 4 2 3 3 4 6% 50%	2470
	21st to end	Night	19% 6% 2 4 4 3 3 3 5% 6% 45%	2470
December	1st to 10th	Night	19% 10% 3 5% 4 3 4 4 5% 7% 36%	2508
	11th to 20th	Night	18% 8% 4 6% 5% 3 4 4 5% 6% 38%	3279
	21st to end	Night	24% 13% 4 8% 7% 4 4 4 4 5% 24%	4884

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	53% 4 12% 22% 8% 1	2528
	11th to 20th	Day	57% 4 11% 21% 8%	2530
	21st to end	Day	64% 3 8% 18% 6% 1	3075
February	1st to 10th	Day	51% 4 10% 22% 11% 2	1885
	11th to 20th	Day	51% 4 10% 22% 12% 2	1798
	21st to end	Day	52% 4 9% 21% 11% 3	1480
March	1st to 10th	Day	46% 4 8% 24% 17% 2	1800
	11th to 20th	Day	52% 4 12% 20% 9% 2	1852
	21st to end	Day	54% 5% 10% 21% 9% 1	2376
April	1st to 10th	Day	39% 6% 13% 26% 16% 1	2160
	11th to 20th	Day	44% 4 11% 22% 16% 3	2134
	21st to end	Day	44% 5% 11% 25% 12% 2	2175
May	1st to 10th	Day	48% 6% 12% 24% 10% 1	2340
	11th to 20th	Day	45% 7% 12% 19% 15% 1	2305
	21st to end	Day	41% 9% 15% 21% 13% 3	2652
June	1st to 10th	Day	34% 7% 11% 25% 20% 3	2520
	11th to 20th	Day	31% 9% 11% 26% 20% 3	2520
	21st to end	Day	32% 7% 15% 23% 16% 6%	2520
July	1st to 10th	Day	37% 9% 13% 23% 14% 4	2451
	11th to 20th	Day	30% 8% 15% 26% 18% 3	2445
	21st to end	Day	33% 9% 14% 27% 15% 2	2586
August	1st to 10th	Day	33% 7% 15% 29% 15% 1	2192
	11th to 20th	Day	35% 7% 14% 26% 16% 2	2125
	21st to end	Day	32% 5% 14% 25% 18% 7%	2376
September	1st to 10th	Day	36% 6% 9% 25% 19% 5%	2160
	11th to 20th	Day	33% 5% 9% 27% 23% 3	2026
	21st to end	Day	31% 4 7% 26% 28% 4	1980
October	1st to 10th	Day	29% 4 9% 27% 26% 5%	1980
	11th to 20th	Day	37% 4 8% 25% 21% 5%	1810
	21st to end	Day	42% 4 8% 25% 18% 3	1828
November	1st to 10th	Day	32% 4 9% 31% 17% 6%	1672
	11th to 20th	Day	41% 4 7% 25% 19% 5%	1710
	21st to end	Day	47% 3 8% 24% 16% 2	1710
December	1st to 10th	Day	51% 3 11% 22% 12% 1	1670
	11th to 20th	Day	53% 4 10% 23% 10% 1	1915
	21st to end	Day	64% 4 11% 13% 8%	2783
January	1st to 10th	Night	51% 4 12% 22% 10% 1	4428
	11th to 20th	Night	56% 4 9% 20% 10% 1	4440
	21st to end	Night	61% 3 8% 18% 9% 1	4587
February	1st to 10th	Night	54% 4 9% 21% 10% 1	2292
	11th to 20th	Night	48% 4 9% 23% 14% 2	2154
	21st to end	Night	51% 4 10% 19% 13% 3	1776
March	1st to 10th	Night	44% 4 8% 21% 20% 4	2160
	11th to 20th	Night	48% 5% 12% 19% 14% 3	2121
	21st to end	Night	52% 4 13% 20% 10% 2	1980
April	1st to 10th	Night	36% 6% 10% 27% 17% 3	1800
	11th to 20th	Night	41% 4 9% 22% 19% 6%	1775
	21st to end	Night	45% 5% 10% 22% 15% 2	1782
May	1st to 10th	Night	43% 5% 11% 26% 13% 1	1620
	11th to 20th	Night	43% 7% 11% 21% 17% 2	1593
	21st to end	Night	37% 8% 14% 21% 16% 5%	1621
June	1st to 10th	Night	32% 5% 12% 25% 23% 3	1440
	11th to 20th	Night	26% 8% 12% 23% 26% 5%	1440
	21st to end	Night	25% 7% 16% 25% 20% 7%	1440
July	1st to 10th	Night	29% 9% 16% 25% 17% 5%	1400
	11th to 20th	Night	25% 8% 13% 29% 19% 5%	1392
	21st to end	Night	30% 7% 11% 30% 20% 2	1491
August	1st to 10th	Night	26% 7% 18% 27% 19% 3	1718
	11th to 20th	Night	29% 7% 12% 28% 20% 3	1776
	21st to end	Night	27% 6% 14% 26% 21% 6%	1980
September	1st to 10th	Night	32% 4 7% 29% 22% 6%	1800
	11th to 20th	Night	30% 5% 7% 27% 26% 5%	1916
	21st to end	Night	30% 5% 9% 24% 24% 7%	1980
October	1st to 10th	Night	28% 5% 9% 23% 29% 6%	1980
	11th to 20th	Night	35% 3 6% 26% 24% 7%	2147
	21st to end	Night	43% 3 7% 25% 17% 5%	2528
November	1st to 10th	Night	33% 3 8% 25% 24% 6%	2406
	11th to 20th	Night	36% 3 8% 23% 24% 5%	2470
	21st to end	Night	42% 3 9% 25% 18% 2	2470
December	1st to 10th	Night	48% 4 12% 22% 13% 1	2508
	11th to 20th	Night	49% 4 9% 22% 15% 1	3281
	21st to end	Night	63% 4 9% 14% 8% 2	4884

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	24% 15% 31% 19% 9% 1	270
	11th to 20th	Day	26% 14% 34% 20% 5%	270
	21st to end	Day	23% 18% 38% 14% 7%	297
February	1st to 10th	Day	25% 12% 41% 17% 4 1	270
	11th to 20th	Day	26% 18% 38% 15% 3	252
	21st to end	Day	29% 11% 34% 20% 6%	198
March	1st to 10th	Day	22% 15% 38% 19% 6%	240
	11th to 20th	Day	28% 17% 30% 18% 6% 1	267
	21st to end	Day	28% 12% 34% 17% 9% 1	462
April	1st to 10th	Day	15% 14% 34% 26% 9% 2	420
	11th to 20th	Day	20% 13% 27% 23% 15% 2	412
	21st to end	Day	21% 10% 33% 23% 9% 3	420
May	1st to 10th	Day	19% 11% 28% 29% 9% 4	423
	11th to 20th	Day	20% 4 26% 28% 15% 7%	447
	21st to end	Day	11% 4 24% 28% 21% 13%	606
June	1st to 10th	Day	12% 5% 16% 25% 30% 12%	630
	11th to 20th	Day	6% 3 20% 32% 27% 11%	630
	21st to end	Day	8% 3 22% 33% 22% 11%	630
July	1st to 10th	Day	12% 3 10% 23% 28% 24%	621
	11th to 20th	Day	8% 4 12% 29% 30% 17%	618
	21st to end	Day	8% 2 11% 29% 35% 15%	679
August	1st to 10th	Day	8% 3 9% 24% 36% 19%	446
	11th to 20th	Day	14% 3 9% 27% 34% 13%	439
	21st to end	Day	8% 2 11% 23% 35% 22%	495
September	1st to 10th	Day	10% 3 12% 25% 36% 14%	450
	11th to 20th	Day	9% 3 7% 28% 37% 16%	450
	21st to end	Day	13% 4 19% 25% 26% 14%	450
October	1st to 10th	Day	11% 5% 20% 36% 21% 8%	450
	11th to 20th	Day	13% 11% 35% 25% 11% 4	280
	21st to end	Day	26% 9% 31% 26% 8% 1	297
November	1st to 10th	Day	13% 7% 33% 26% 16% 5%	270
	11th to 20th	Day	16% 19% 27% 25% 13% 1	270
	21st to end	Day	24% 11% 28% 24% 11% 1	270
December	1st to 10th	Day	19% 18% 31% 22% 7% 3	270
	11th to 20th	Day	23% 13% 32% 24% 7% 1	270
	21st to end	Day	29% 16% 32% 17% 6%	297
January	1st to 10th	Night	22% 13% 34% 26% 4	950
	11th to 20th	Night	26% 14% 34% 22% 4	950
	21st to end	Night	20% 12% 38% 28% 3	1045
February	1st to 10th	Night	26% 10% 35% 26% 3	772
	11th to 20th	Night	21% 14% 36% 27% 2	722
	21st to end	Night	27% 11% 37% 23% 2	592
March	1st to 10th	Night	22% 12% 33% 28% 4	720
	11th to 20th	Night	20% 11% 33% 28% 7% 1	710
	21st to end	Night	25% 8% 27% 31% 8% 1	594
April	1st to 10th	Night	16% 8% 32% 36% 8%	540
	11th to 20th	Night	20% 9% 28% 27% 14% 2	533
	21st to end	Night	20% 5% 33% 31% 10% 1	540
May	1st to 10th	Night	18% 6% 29% 34% 12% 1	540
	11th to 20th	Night	15% 5% 24% 35% 18% 3	531
	21st to end	Night	8% 2 20% 38% 27% 5%	584
June	1st to 10th	Night	12% 2 15% 41% 26% 4	540
	11th to 20th	Night	8% 2 17% 44% 27% 3	540
	21st to end	Night	6% 1 20% 40% 30% 4	540
July	1st to 10th	Night	10% 1 7% 32% 40% 10%	525
	11th to 20th	Night	7% 1 8% 34% 44% 5%	522
	21st to end	Night	9% 2 8% 36% 38% 7%	591
August	1st to 10th	Night	7% 2 7% 37% 39% 8%	537
	11th to 20th	Night	6% 1 9% 37% 43% 5%	534
	21st to end	Night	6% 1 8% 34% 43% 8%	594
September	1st to 10th	Night	11% 1 11% 34% 35% 8%	540
	11th to 20th	Night	11% 2 11% 35% 36% 6%	656
	21st to end	Night	12% 4 18% 32% 29% 6%	720
October	1st to 10th	Night	11% 4 20% 36% 25% 5%	720
	11th to 20th	Night	18% 6% 29% 31% 14% 2	720
	21st to end	Night	22% 7% 31% 28% 11% 1	792
November	1st to 10th	Night	15% 6% 28% 35% 15% 1	740
	11th to 20th	Night	17% 9% 31% 31% 12%	760
	21st to end	Night	22% 11% 27% 30% 10%	760
December	1st to 10th	Night	24% 8% 31% 31% 5%	798
	11th to 20th	Night	24% 8% 31% 30% 7% 1	950
	21st to end	Night	28% 12% 33% 23% 4	1045

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