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 2025-03-27.

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 4 4 5% 7% 41%	108839
	Night	15% 8% 3 6% 4 4 4 3 5% 6% 43%	110639
Transparency	Day	41% 5% 11% 24% 17% 2	112484
	Night	41% 4 10% 23% 18% 3	115135
Seeing	Day	16% 8% 24% 25% 19% 8%	15965
	Night	17% 7% 26% 31% 16% 2	25939

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	15% 11% 6% 10% 5% 5% 4 4 4 5% 30%	3617
	11th to 20th	Day	13% 8% 4 11% 7% 5% 5% 3 5% 6% 33%	3620
	21st to end	Day	20% 13% 4 8% 4 3 3 3 4 5% 33%	4414
February	1st to 10th	Day	17% 10% 4 7% 5% 4 4 3 4 6% 36%	1985
	11th to 20th	Day	17% 10% 4 9% 4 4 3 3 5% 5% 36%	1948
	21st to end	Day	14% 10% 4 8% 5% 4 4 3 5% 7% 36%	1650
March	1st to 10th	Day	23% 13% 3 6% 4 3 3 2 3 5% 36%	3220
	11th to 20th	Day	14% 7% 3 5% 3 3 3 3 6% 6% 46%	3372
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 6% 7% 37%	4237
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%	1910
	21st to end	Day	18% 7% 2 6% 4 3 3 3 4 5% 45%	1929
November	1st to 10th	Day	8% 6% 2 5% 4 4 5% 2 5% 6% 54%	1762
	11th to 20th	Day	15% 7% 2 6% 4 4 4 2 5% 6% 45%	1800
	21st to end	Day	17% 10% 3 8% 3 3 3 3 4 5% 40%	1800
December	1st to 10th	Day	12% 10% 3 6% 4 3 3 3 3 5% 49%	2851
	11th to 20th	Day	14% 10% 5% 10% 6% 4 4 3 5% 6% 33%	3008
	21st to end	Day	28% 14% 5% 10% 5% 3 4 2 3 4 22%	3982
January	1st to 10th	Night	17% 12% 6% 8% 4 4 4 4 5% 5% 31%	6348
	11th to 20th	Night	15% 12% 4 8% 4 5% 5% 3 4 4 36%	6360
	21st to end	Night	19% 10% 4 6% 4 4 3 3 4 6% 36%	6559
February	1st to 10th	Night	20% 11% 4 7% 5% 4 3 4 5% 5% 31%	2412
	11th to 20th	Night	17% 9% 3 8% 4 4 4 3 4 5% 39%	2338
	21st to end	Night	18% 10% 3 7% 5% 3 3 3 5% 4 39%	1980
March	1st to 10th	Night	23% 10% 3 6% 4 4 3 3 4 5% 35%	3853
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 6% 50%	3836
	21st to end	Night	18% 7% 2 7% 5% 4 4 3 5% 6% 38%	3532
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% 54%	2267
	21st to end	Night	17% 7% 3 6% 3 3 3 2 4 6% 45%	2669
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 53%	2536
	11th to 20th	Night	14% 6% 2 5% 3 3 4 2 4 7% 49%	2600
	21st to end	Night	19% 7% 3 5% 4 4 3 2 5% 6% 44%	2600
December	1st to 10th	Night	18% 9% 3 6% 3 3 3 3 4 5% 44%	4316
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 6% 41%	5199
	21st to end	Night	29% 13% 3 8% 5% 4 4 3 3 4 24%	6996

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	57% 3 11% 20% 7% 1	3618
	11th to 20th	Day	53% 4 9% 22% 11% 1	3620
	21st to end	Day	55% 3 9% 17% 11% 5%	4414
February	1st to 10th	Day	50% 3 11% 22% 11% 2	1985
	11th to 20th	Day	51% 4 9% 20% 14% 2	1958
	21st to end	Day	50% 3 9% 21% 13% 4	1650
March	1st to 10th	Day	54% 3 5% 19% 15% 4	3220
	11th to 20th	Day	38% 4 11% 28% 16% 3	3372
	21st to end	Day	44% 6% 13% 23% 14% 1	4237
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	18% 3 5% 26% 43% 4	3480
	11th to 20th	Day	25% 3 8% 25% 35% 5%	3150
	21st to end	Day	33% 3 6% 25% 29% 4	3154
November	1st to 10th	Day	33% 4 9% 30% 17% 6%	1762
	11th to 20th	Day	41% 4 7% 25% 19% 5%	1800
	21st to end	Day	48% 3 8% 24% 16% 2	1800
December	1st to 10th	Day	41% 2 13% 27% 16%	2851
	11th to 20th	Day	53% 3 11% 21% 11% 1	3005
	21st to end	Day	68% 3 11% 12% 6%	3982
January	1st to 10th	Night	54% 3 11% 22% 8% 1	6348
	11th to 20th	Night	53% 3 7% 19% 15% 3	6360
	21st to end	Night	51% 3 8% 19% 18% 1	6559
February	1st to 10th	Night	55% 4 9% 21% 10% 1	2412
	11th to 20th	Night	49% 4 9% 22% 15% 2	2339
	21st to end	Night	48% 4 9% 21% 15% 3	1980
March	1st to 10th	Night	52% 4 8% 18% 16% 2	3853
	11th to 20th	Night	35% 5% 13% 23% 18% 6%	3836
	21st to end	Night	47% 5% 11% 25% 11% 1	3532
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	18% 3 5% 16% 50% 8%	3480
	11th to 20th	Night	24% 2 5% 26% 37% 6%	3766
	21st to end	Night	32% 2 8% 25% 25% 8%	4394
November	1st to 10th	Night	34% 3 8% 24% 24% 6%	2536
	11th to 20th	Night	36% 4 8% 23% 24% 5%	2600
	21st to end	Night	43% 3 9% 24% 18% 2	2600
December	1st to 10th	Night	44% 3 10% 24% 18% 1	4316
	11th to 20th	Night	46% 3 12% 22% 15% 1	5201
	21st to end	Night	64% 3 10% 16% 6% 1	6996

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% 16% 32% 18% 8% 1	300
	11th to 20th	Day	24% 16% 35% 20% 4	300
	21st to end	Day	21% 17% 41% 14% 7%	330
February	1st to 10th	Day	24% 12% 44% 15% 4 1	300
	11th to 20th	Day	26% 18% 40% 13% 3 1	300
	21st to end	Day	27% 14% 35% 20% 4	249
March	1st to 10th	Day	22% 17% 34% 20% 6%	300
	11th to 20th	Day	27% 18% 33% 17% 5% 1	331
	21st to end	Day	27% 13% 35% 17% 8% 1	530
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% 33% 25% 12% 7%	310
	21st to end	Day	24% 9% 32% 26% 9% 1	330
November	1st to 10th	Day	12% 8% 33% 26% 16% 5%	300
	11th to 20th	Day	15% 18% 29% 24% 12% 1	300
	21st to end	Day	26% 12% 27% 24% 11% 1	300
December	1st to 10th	Day	19% 18% 32% 22% 7% 3	300
	11th to 20th	Day	23% 13% 32% 24% 8% 1	300
	21st to end	Day	32% 15% 31% 17% 5%	330
January	1st to 10th	Night	23% 13% 34% 26% 4	1000
	11th to 20th	Night	26% 13% 35% 22% 4	1000
	21st to end	Night	20% 11% 38% 28% 3	1100
February	1st to 10th	Night	26% 11% 36% 25% 3	812
	11th to 20th	Night	22% 14% 35% 26% 2	783
	21st to end	Night	25% 11% 38% 24% 2	660
March	1st to 10th	Night	23% 13% 32% 28% 4	800
	11th to 20th	Night	20% 12% 33% 29% 7% 1	788
	21st to end	Night	23% 9% 29% 30% 7% 1	647
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	760
	21st to end	Night	21% 7% 31% 29% 11%	836
November	1st to 10th	Night	16% 7% 28% 34% 14% 1	780
	11th to 20th	Night	17% 9% 31% 32% 11%	800
	21st to end	Night	22% 11% 28% 30% 10%	800
December	1st to 10th	Night	24% 8% 32% 31% 5%	840
	11th to 20th	Night	24% 8% 31% 30% 7% 1	1000
	21st to end	Night	29% 12% 33% 23% 3	1100

Page generated at 2025-03-24 08:04:31. Version ac07

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