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 2024-10-20.

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% 4 4 4 5% 8% 41%	100913
	Night	14% 8% 3 6% 4 4 4 3 5% 6% 43%	97812
Transparency	Day	41% 5% 11% 25% 15% 2	100703
	Night	42% 5% 11% 24% 16% 3	97694
Seeing	Day	15% 8% 23% 25% 20% 9%	15483
	Night	17% 7% 26% 31% 16% 2	25261

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% 12% 6% 8% 5% 4 4 3 4 6% 34%	2527
	11th to 20th	Day	17% 8% 3 10% 8% 5% 6% 4 5% 5% 29%	2530
	21st to end	Day	25% 18% 4 8% 4 3 3 2 3 5% 25%	3075
February	1st to 10th	Day	17% 10% 4 8% 5% 4 4 3 4 6% 35%	1885
	11th to 20th	Day	16% 11% 4 8% 4 4 3 3 5% 5% 36%	1848
	21st to end	Day	15% 10% 4 8% 5% 4 4 3 6% 7% 35%	1570
March	1st to 10th	Day	23% 13% 3 6% 4 3 3 2 3 5% 36%	3120
	11th to 20th	Day	14% 7% 3 5% 3 3 3 3 6% 7% 46%	3268
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 6% 7% 36%	4165
April	1st to 10th	Day	12% 7% 3 6% 6% 4 4 4 4 10% 42%	2280
	11th to 20th	Day	13% 8% 3 7% 5% 4 4 3 5% 7% 42%	2254
	21st to end	Day	18% 6% 2 5% 5% 4 5% 3 5% 8% 39%	2296
May	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 5% 7% 10% 31%	4082
	11th to 20th	Day	18% 7% 2 7% 5% 4 4 4 5% 7% 38%	4081
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 5% 6% 9% 41%	4695
June	1st to 10th	Day	10% 6% 2 5% 5% 4 5% 4 5% 9% 46%	2660
	11th to 20th	Day	6% 4 2 5% 5% 5% 6% 5% 7% 9% 48%	2660
	21st to end	Day	6% 5% 2 6% 5% 6% 5% 4 7% 10% 46%	2660
July	1st to 10th	Day	12% 4 2 5% 4 4 5% 4 6% 10% 42%	4440
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 4 7% 11% 48%	4440
	21st to end	Day	9% 7% 3 7% 7% 7% 7% 5% 8% 9% 31%	4795
August	1st to 10th	Day	6% 4 1 4 3 4 4 4 6% 9% 54%	3884
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 9% 43%	3729
	21st to end	Day	5% 4 2 5% 4 4 5% 4 6% 8% 55%	4180
September	1st to 10th	Day	10% 5% 2 6% 4 3 5% 4 5% 7% 48%	2280
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 3 5% 8% 50%	2138
	21st to end	Day	11% 5% 1 5% 4 4 4 2 5% 7% 52%	2090
October	1st to 10th	Day	10% 4 1 5% 3 3 2 3 5% 6% 57%	2090
	11th to 20th	Day	13% 5% 2 5% 4 3 3 4 4 6% 50%	1900
	21st to end	Day	18% 7% 2 6% 4 3 3 3 4 5% 44%	1828
November	1st to 10th	Day	8% 6% 2 4 4 4 5% 2 4 6% 55%	1672
	11th to 20th	Day	15% 7% 2 6% 4 4 4 2 5% 6% 45%	1710
	21st to end	Day	17% 9% 3 7% 3 3 3 3 4 5% 41%	1710
December	1st to 10th	Day	17% 12% 3 7% 5% 4 4 3 3 5% 38%	1670
	11th to 20th	Day	15% 12% 5% 8% 5% 4 5% 3 5% 6% 32%	1918
	21st to end	Day	23% 14% 5% 10% 5% 3 4 3 3 5% 24%	2783
January	1st to 10th	Night	17% 12% 4 8% 4 4 4 4 5% 5% 33%	4428
	11th to 20th	Night	15% 12% 5% 9% 5% 5% 6% 3 4 4 31%	4440
	21st to end	Night	24% 14% 6% 7% 4 3 3 3 4 6% 26%	4587
February	1st to 10th	Night	20% 11% 4 7% 5% 4 3 4 5% 5% 32%	2292
	11th to 20th	Night	15% 9% 3 8% 5% 4 4 3 4 5% 40%	2218
	21st to end	Night	19% 11% 3 7% 5% 3 3 3 5% 5% 37%	1884
March	1st to 10th	Night	24% 10% 3 6% 4 4 3 3 4 5% 34%	3733
	11th to 20th	Night	12% 6% 2 6% 4 3 3 3 4 6% 50%	3719
	21st to end	Night	18% 7% 2 7% 5% 4 4 3 5% 6% 38%	3466
April	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 5% 6% 46%	1900
	11th to 20th	Night	17% 7% 2 5% 3 3 4 4 4 6% 44%	1875
	21st to end	Night	18% 7% 3 7% 5% 4 4 3 5% 7% 39%	1881
May	1st to 10th	Night	17% 9% 2 6% 4 5% 4 3 5% 7% 37%	2813
	11th to 20th	Night	15% 7% 2 6% 4 3 4 4 4 6% 44%	2812
	21st to end	Night	9% 5% 3 7% 5% 5% 5% 4 7% 9% 41%	2851
June	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 6% 8% 48%	1520
	11th to 20th	Night	6% 4 1 5% 4 4 4 4 6% 8% 54%	1520
	21st to end	Night	5% 4 1 4 4 4 5% 4 8% 9% 53%	1520
July	1st to 10th	Night	11% 5% 2 5% 3 4 4 4 5% 8% 50%	2530
	11th to 20th	Night	5% 3 1 4 3 3 4 3 5% 8% 59%	2530
	21st to end	Night	10% 7% 3 8% 6% 5% 6% 5% 7% 9% 35%	2766
August	1st to 10th	Night	5% 3 2 4 3 3 4 3 6% 9% 59%	3041
	11th to 20th	Night	6% 4 2 5% 3 4 5% 4 6% 8% 52%	3100
	21st to end	Night	4 3 1 4 3 4 4 3 6% 9% 58%	3476
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 4 6% 55%	1900
	11th to 20th	Night	8% 4 2 6% 3 4 4 4 5% 9% 52%	2023
	21st to end	Night	12% 4 1 4 4 3 4 4 6% 7% 50%	2090
October	1st to 10th	Night	9% 3 2 5% 3 3 3 3 5% 5% 60%	2090
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 6% 53%	2262
	21st to end	Night	18% 8% 3 6% 3 3 3 2 4 6% 45%	2528
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 54%	2406
	11th to 20th	Night	14% 6% 3 5% 3 3 4 2 4 6% 50%	2470
	21st to end	Night	19% 7% 2 5% 4 4 3 2 5% 5% 44%	2470
December	1st to 10th	Night	19% 10% 3 6% 4 4 4 4 5% 7% 36%	2508
	11th to 20th	Night	18% 9% 4 6% 4 4 4 3 5% 5% 38%	3279
	21st to end	Night	23% 14% 3 9% 6% 5% 4 3 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	50% 4 10% 21% 14% 2	1858
	21st to end	Day	51% 4 9% 21% 12% 4	1570
March	1st to 10th	Day	55% 3 5% 19% 14% 4	3120
	11th to 20th	Day	38% 4 12% 28% 16% 3	3268
	21st to end	Day	44% 6% 13% 22% 13% 1	4165
April	1st to 10th	Day	40% 6% 12% 25% 16% 1	2280
	11th to 20th	Day	43% 4 11% 23% 16% 3	2254
	21st to end	Day	44% 5% 11% 26% 12% 2	2296
May	1st to 10th	Day	45% 9% 14% 22% 9% 1	4120
	11th to 20th	Day	45% 8% 10% 24% 13% 1	4085
	21st to end	Day	38% 7% 12% 19% 22% 2	4708
June	1st to 10th	Day	35% 7% 11% 25% 19% 3	2660
	11th to 20th	Day	31% 8% 11% 25% 21% 3	2660
	21st to end	Day	32% 7% 15% 23% 16% 6%	2660
July	1st to 10th	Day	38% 5% 11% 27% 16% 3	4371
	11th to 20th	Day	30% 6% 13% 33% 17% 2	4365
	21st to end	Day	47% 6% 16% 21% 9% 1	4644
August	1st to 10th	Day	27% 5% 15% 36% 17% 1	3871
	11th to 20th	Day	36% 5% 14% 25% 19% 1	3765
	21st to end	Day	28% 3 13% 35% 17% 4	4179
September	1st to 10th	Day	35% 5% 9% 25% 20% 6%	2280
	11th to 20th	Day	33% 5% 9% 27% 23% 3	2138
	21st to end	Day	33% 4 7% 25% 27% 4	2090
October	1st to 10th	Day	28% 4 9% 27% 28% 5%	2090
	11th to 20th	Day	35% 4 8% 25% 23% 5%	1900
	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	47% 4 9% 23% 15% 2	2219
	21st to end	Night	50% 4 10% 19% 14% 3	1884
March	1st to 10th	Night	52% 4 8% 18% 16% 2	3733
	11th to 20th	Night	35% 5% 13% 23% 18% 6%	3719
	21st to end	Night	47% 5% 11% 25% 11% 1	3466
April	1st to 10th	Night	37% 6% 10% 27% 16% 3	1900
	11th to 20th	Night	40% 4 9% 22% 19% 5%	1875
	21st to end	Night	45% 5% 10% 22% 15% 2	1881
May	1st to 10th	Night	44% 8% 16% 23% 8% 1	2840
	11th to 20th	Night	41% 7% 11% 25% 14% 1	2813
	21st to end	Night	36% 8% 14% 22% 17% 3	2859
June	1st to 10th	Night	32% 5% 12% 25% 23% 3	1520
	11th to 20th	Night	26% 7% 11% 23% 27% 6%	1520
	21st to end	Night	25% 6% 16% 25% 20% 7%	1520
July	1st to 10th	Night	33% 6% 10% 27% 19% 5%	2490
	11th to 20th	Night	24% 6% 12% 36% 20% 3	2482
	21st to end	Night	45% 4 15% 22% 12% 1	2678
August	1st to 10th	Night	23% 5% 15% 40% 15% 2	3030
	11th to 20th	Night	29% 5% 13% 30% 22% 1	3130
	21st to end	Night	23% 4 13% 33% 23% 3	3476
September	1st to 10th	Night	32% 4 7% 28% 24% 6%	1900
	11th to 20th	Night	30% 5% 7% 27% 26% 5%	2023
	21st to end	Night	32% 5% 9% 23% 23% 7%	2090
October	1st to 10th	Night	27% 5% 8% 22% 32% 6%	2090
	11th to 20th	Night	34% 3 6% 26% 25% 7%	2262
	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	25% 17% 39% 14% 4 1	270
	21st to end	Day	29% 13% 34% 19% 5%	225
March	1st to 10th	Day	23% 17% 36% 19% 6%	270
	11th to 20th	Day	26% 18% 33% 17% 6% 1	299
	21st to end	Day	27% 12% 34% 18% 8% 1	506
April	1st to 10th	Day	17% 14% 33% 26% 8% 2	460
	11th to 20th	Day	20% 13% 28% 23% 14% 2	452
	21st to end	Day	20% 12% 33% 23% 9% 3	460
May	1st to 10th	Day	19% 11% 28% 29% 9% 4	463
	11th to 20th	Day	20% 4 25% 28% 15% 8%	487
	21st to end	Day	10% 5% 26% 27% 20% 12%	657
June	1st to 10th	Day	13% 6% 17% 25% 28% 11%	680
	11th to 20th	Day	6% 3 20% 33% 27% 11%	680
	21st to end	Day	9% 3 22% 33% 22% 11%	680
July	1st to 10th	Day	12% 3 12% 24% 27% 23%	671
	11th to 20th	Day	7% 3 13% 31% 31% 15%	668
	21st to end	Day	8% 2 11% 30% 34% 14%	733
August	1st to 10th	Day	8% 3 10% 24% 35% 19%	486
	11th to 20th	Day	13% 3 11% 28% 33% 12%	479
	21st to end	Day	7% 2 10% 23% 36% 22%	539
September	1st to 10th	Day	9% 3 11% 27% 36% 14%	490
	11th to 20th	Day	9% 3 8% 27% 38% 16%	490
	21st to end	Day	15% 4 18% 26% 24% 13%	490
October	1st to 10th	Day	10% 5% 19% 34% 22% 9%	490
	11th to 20th	Day	12% 11% 34% 24% 12% 6%	307
	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	743
	21st to end	Night	26% 11% 37% 23% 2	628
March	1st to 10th	Night	23% 12% 33% 27% 4	760
	11th to 20th	Night	20% 11% 33% 28% 7% 1	749
	21st to end	Night	24% 9% 29% 30% 7% 1	627
April	1st to 10th	Night	16% 9% 33% 34% 8%	570
	11th to 20th	Night	20% 10% 28% 28% 13% 1	563
	21st to end	Night	19% 5% 35% 30% 10% 1	570
May	1st to 10th	Night	18% 6% 29% 35% 12% 1	570
	11th to 20th	Night	15% 5% 24% 35% 19% 2	561
	21st to end	Night	8% 2 20% 38% 27% 5%	617
June	1st to 10th	Night	12% 2 16% 42% 25% 4	570
	11th to 20th	Night	8% 2 17% 44% 27% 3	570
	21st to end	Night	5% 1 21% 40% 29% 3	570
July	1st to 10th	Night	10% 1 8% 32% 39% 10%	555
	11th to 20th	Night	7% 1 9% 36% 43% 5%	552
	21st to end	Night	10% 2 8% 37% 37% 7%	624
August	1st to 10th	Night	7% 2 7% 37% 40% 8%	567
	11th to 20th	Night	6% 1 9% 38% 41% 4	564
	21st to end	Night	6% 1 8% 35% 43% 7%	627
September	1st to 10th	Night	11% 2 11% 34% 36% 7%	570
	11th to 20th	Night	11% 2 10% 35% 35% 6%	693
	21st to end	Night	13% 4 18% 31% 28% 5%	760
October	1st to 10th	Night	10% 4 19% 36% 26% 5%	760
	11th to 20th	Night	17% 6% 29% 30% 16% 2	759
	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

Page generated at 2024-10-17 12:09:40. Version ac07

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