Trussville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Trussville clear sky chart from 2005-11-06 to 2024-11-28.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	101050
	Night	12% 7% 2 6% 4 4 4 4 5% 7% 44%	98450
Transparency	Day	38% 7% 11% 19% 18% 6%	104856
	Night	36% 7% 10% 20% 20% 7%	102770
Seeing	Day	16% 10% 24% 24% 18% 8%	14627
	Night	15% 8% 23% 29% 20% 4	24619

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	16% 13% 4 8% 4 4 4 3 5% 4 36%	2836
	11th to 20th	Day	14% 8% 2 6% 4 4 3 3 4 4 49%	2940
	21st to end	Day	25% 8% 2 4 3 3 2 2 3 6% 41%	3476
February	1st to 10th	Day	19% 10% 3 6% 4 3 4 3 5% 5% 38%	1900
	11th to 20th	Day	23% 9% 2 4 3 3 4 3 4 4 40%	1848
	21st to end	Day	20% 8% 2 8% 5% 4 4 3 4 5% 37%	1570
March	1st to 10th	Day	23% 9% 3 6% 5% 4 4 3 5% 6% 34%	3120
	11th to 20th	Day	15% 8% 3 6% 3 4 4 3 5% 6% 41%	3350
	21st to end	Day	16% 6% 2 7% 4 5% 4 4 5% 7% 38%	3973
April	1st to 10th	Day	12% 6% 3 6% 5% 4 4 3 5% 6% 46%	2280
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 3 5% 6% 45%	2254
	21st to end	Day	8% 4 3 5% 4 5% 4 5% 5% 8% 49%	2280
May	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 7% 8% 42%	4045
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 37%	4081
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 9% 43%	4476
June	1st to 10th	Day	10% 6% 3 8% 6% 5% 7% 5% 7% 10% 32%	2470
	11th to 20th	Day	6% 3 1 6% 5% 5% 7% 5% 8% 12% 42%	2470
	21st to end	Day	7% 5% 2 6% 6% 6% 6% 5% 8% 10% 39%	2470
July	1st to 10th	Day	7% 6% 3 8% 7% 8% 8% 6% 9% 11% 28%	4120
	11th to 20th	Day	8% 5% 2 8% 6% 7% 7% 7% 9% 11% 29%	4120
	21st to end	Day	11% 9% 4 10% 7% 8% 8% 6% 9% 8% 21%	4406
August	1st to 10th	Day	4 2 1 4 4 5% 5% 5% 8% 11% 49%	3785
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 4 7% 10% 46%	3729
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 6% 8% 50%	4180
September	1st to 10th	Day	10% 5% 2 7% 5% 6% 6% 4 6% 8% 40%	2223
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 4 6% 6% 42%	2090
	21st to end	Day	11% 6% 2 6% 4 4 4 4 5% 6% 50%	2090
October	1st to 10th	Day	10% 4 2 5% 4 5% 3 4 5% 5% 54%	2090
	11th to 20th	Day	9% 5% 2 4 4 2 3 3 5% 7% 56%	2090
	21st to end	Day	12% 5% 1 6% 3 3 3 3 3 6% 55%	1966
November	1st to 10th	Day	12% 7% 2 5% 3 3 3 3 4 5% 52%	1762
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 4 6% 52%	1800
	21st to end	Day	14% 7% 2 6% 4 3 3 4 5% 6% 46%	1773
December	1st to 10th	Day	20% 9% 2 6% 4 3 4 3 4 5% 39%	1710
	11th to 20th	Day	18% 8% 3 6% 5% 4 3 3 4 4 43%	2153
	21st to end	Day	19% 10% 4 7% 4 4 4 3 5% 6% 34%	3124
January	1st to 10th	Night	17% 13% 3 5% 3 4 4 3 5% 5% 39%	4109
	11th to 20th	Night	16% 9% 4 5% 3 3 3 3 4 5% 44%	4016
	21st to end	Night	21% 13% 3 6% 3 3 4 2 3 3 39%	4180
February	1st to 10th	Night	16% 8% 3 7% 4 4 4 3 4 5% 42%	2280
	11th to 20th	Night	20% 8% 3 6% 4 3 3 2 3 4 43%	2218
	21st to end	Night	19% 7% 2 6% 5% 4 4 4 6% 6% 38%	1884
March	1st to 10th	Night	24% 11% 2 6% 4 4 3 2 4 6% 35%	3733
	11th to 20th	Night	12% 6% 2 5% 4 3 4 4 6% 7% 48%	3608
	21st to end	Night	15% 7% 3 8% 4 4 4 3 4 5% 40%	3672
April	1st to 10th	Night	12% 5% 1 6% 4 4 5% 4 6% 8% 45%	1900
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 7% 49%	1875
	21st to end	Night	10% 5% 2 6% 4 3 4 4 4 6% 52%	1900
May	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 6% 8% 43%	2847
	11th to 20th	Night	16% 7% 2 6% 5% 4 4 4 6% 7% 38%	2812
	21st to end	Night	5% 3 1 5% 5% 5% 5% 5% 8% 11% 46%	3079
June	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 6% 9% 10% 38%	1710
	11th to 20th	Night	4 3 1 3 5% 3 5% 5% 7% 12% 51%	1710
	21st to end	Night	5% 4 2 6% 5% 5% 5% 4 7% 12% 45%	1710
July	1st to 10th	Night	6% 5% 2 8% 7% 6% 7% 5% 8% 11% 33%	2840
	11th to 20th	Night	8% 5% 2 7% 5% 5% 6% 5% 9% 14% 36%	2840
	21st to end	Night	9% 8% 4 10% 8% 6% 6% 6% 7% 10% 25%	3142
August	1st to 10th	Night	2 2 1 4 4 4 5% 3 7% 11% 57%	3144
	11th to 20th	Night	6% 4 2 4 4 4 4 4 6% 9% 53%	3100
	21st to end	Night	6% 4 1 4 3 4 5% 4 6% 8% 55%	3476
September	1st to 10th	Night	8% 6% 3 7% 5% 6% 5% 3 6% 8% 44%	1936
	11th to 20th	Night	12% 6% 2 6% 4 5% 4 4 6% 7% 45%	2090
	21st to end	Night	7% 4 2 6% 4 4 4 4 5% 7% 53%	2090
October	1st to 10th	Night	8% 4 1 4 3 3 2 3 5% 5% 61%	2090
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 4 6% 56%	2090
	21st to end	Night	13% 6% 2 6% 3 2 3 2 3 4 56%	2627
November	1st to 10th	Night	12% 5% 2 4 3 3 3 3 5% 5% 56%	2536
	11th to 20th	Night	12% 6% 2 4 3 3 2 3 4 6% 54%	2600
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 6% 49%	2562
December	1st to 10th	Night	18% 8% 2 6% 4 4 4 3 5% 6% 39%	2470
	11th to 20th	Night	17% 8% 3 5% 4 3 3 2 4 5% 47%	3042
	21st to end	Night	18% 8% 3 7% 4 4 4 4 5% 7% 33%	4532

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	49% 5% 5% 14% 22% 5%	2837
	11th to 20th	Day	38% 5% 7% 25% 21% 3	2940
	21st to end	Day	46% 3 7% 18% 20% 6%	3476
February	1st to 10th	Day	48% 4 8% 18% 18% 4	1900
	11th to 20th	Day	47% 5% 5% 17% 20% 7%	1858
	21st to end	Day	50% 5% 6% 12% 21% 7%	1570
March	1st to 10th	Day	50% 6% 10% 11% 17% 7%	3120
	11th to 20th	Day	41% 6% 8% 17% 21% 8%	3350
	21st to end	Day	43% 7% 10% 16% 19% 5%	3973
April	1st to 10th	Day	38% 5% 11% 19% 19% 8%	2280
	11th to 20th	Day	39% 5% 7% 19% 22% 7%	2254
	21st to end	Day	32% 6% 10% 25% 23% 3	2280
May	1st to 10th	Day	34% 11% 13% 25% 16% 2	4083
	11th to 20th	Day	40% 8% 14% 26% 11% 1	4085
	21st to end	Day	34% 9% 13% 27% 16% 1	4489
June	1st to 10th	Day	42% 11% 15% 17% 14% 1	2470
	11th to 20th	Day	31% 10% 17% 25% 16% 2	2470
	21st to end	Day	35% 11% 18% 21% 11% 4	2470
July	1st to 10th	Day	42% 14% 22% 17% 5% 1	4055
	11th to 20th	Day	42% 13% 20% 20% 5% 1	4051
	21st to end	Day	55% 13% 14% 14% 4	4265
August	1st to 10th	Day	23% 10% 14% 35% 18% 1	3773
	11th to 20th	Day	31% 9% 17% 25% 16% 2	3765
	21st to end	Day	30% 8% 16% 20% 23% 4	4179
September	1st to 10th	Day	40% 9% 14% 17% 15% 6%	2223
	11th to 20th	Day	40% 9% 11% 19% 18% 4	2090
	21st to end	Day	35% 6% 11% 20% 22% 7%	2090
October	1st to 10th	Day	23% 4 7% 12% 34% 21%	3480
	11th to 20th	Day	19% 2 5% 15% 31% 28%	3458
	21st to end	Day	24% 4 6% 19% 35% 12%	3203
November	1st to 10th	Day	35% 4 7% 17% 27% 10%	1762
	11th to 20th	Day	33% 5% 7% 18% 27% 11%	1800
	21st to end	Day	38% 5% 7% 16% 25% 8%	1773
December	1st to 10th	Day	48% 5% 8% 15% 19% 6%	1710
	11th to 20th	Day	46% 3 6% 17% 21% 6%	2150
	21st to end	Day	48% 6% 8% 20% 17% 2	3124
January	1st to 10th	Night	47% 4 6% 16% 25% 3	4109
	11th to 20th	Night	42% 4 5% 22% 18% 7%	4016
	21st to end	Night	50% 3 4 15% 22% 5%	4180
February	1st to 10th	Night	45% 4 6% 18% 20% 7%	2280
	11th to 20th	Night	46% 4 5% 17% 18% 10%	2219
	21st to end	Night	45% 6% 7% 13% 20% 9%	1884
March	1st to 10th	Night	50% 5% 9% 14% 15% 8%	3733
	11th to 20th	Night	34% 6% 9% 19% 22% 11%	3608
	21st to end	Night	45% 5% 7% 20% 19% 4	3672
April	1st to 10th	Night	35% 6% 12% 21% 19% 7%	1900
	11th to 20th	Night	35% 5% 8% 19% 22% 12%	1875
	21st to end	Night	31% 6% 11% 24% 22% 6%	1900
May	1st to 10th	Night	34% 12% 12% 19% 20% 3	2874
	11th to 20th	Night	41% 8% 15% 23% 11% 1	2813
	21st to end	Night	25% 12% 15% 30% 16% 3	3087
June	1st to 10th	Night	36% 11% 14% 20% 17% 2	1710
	11th to 20th	Night	22% 10% 22% 27% 16% 3	1710
	21st to end	Night	31% 11% 16% 22% 17% 4	1710
July	1st to 10th	Night	41% 12% 22% 17% 6% 2	2795
	11th to 20th	Night	33% 13% 26% 21% 6% 1	2786
	21st to end	Night	51% 13% 16% 14% 6% 1	3042
August	1st to 10th	Night	19% 8% 14% 41% 16% 2	3132
	11th to 20th	Night	26% 9% 17% 30% 15% 3	3130
	21st to end	Night	25% 8% 14% 22% 26% 4	3476
September	1st to 10th	Night	37% 9% 16% 16% 16% 6%	1936
	11th to 20th	Night	37% 9% 10% 19% 20% 5%	2090
	21st to end	Night	29% 6% 10% 23% 23% 8%	2090
October	1st to 10th	Night	21% 4 7% 16% 30% 22%	3480
	11th to 20th	Night	18% 3 5% 16% 29% 28%	3461
	21st to end	Night	23% 3 4 22% 32% 16%	4328
November	1st to 10th	Night	31% 5% 8% 19% 25% 13%	2536
	11th to 20th	Night	32% 4 6% 17% 28% 13%	2600
	21st to end	Night	37% 5% 7% 19% 24% 9%	2562
December	1st to 10th	Night	45% 6% 8% 17% 20% 5%	2470
	11th to 20th	Night	41% 4 8% 15% 28% 5%	3044
	21st to end	Night	46% 5% 9% 19% 18% 3	4532

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	26% 27% 30% 14% 3	270
	11th to 20th	Day	30% 22% 27% 16% 4 1	270
	21st to end	Day	24% 27% 35% 10% 3	297
February	1st to 10th	Day	28% 22% 35% 12% 2	270
	11th to 20th	Day	39% 20% 28% 11% 2	270
	21st to end	Day	32% 27% 35% 5% 1	225
March	1st to 10th	Day	28% 21% 32% 14% 4	270
	11th to 20th	Day	22% 21% 34% 16% 7%	382
	21st to end	Day	22% 19% 33% 20% 6%	506
April	1st to 10th	Day	23% 13% 31% 22% 10% 1	460
	11th to 20th	Day	17% 17% 30% 23% 12% 1	452
	21st to end	Day	13% 15% 30% 28% 12% 1	460
May	1st to 10th	Day	14% 9% 29% 28% 15% 5%	463
	11th to 20th	Day	9% 7% 30% 28% 18% 8%	487
	21st to end	Day	10% 6% 23% 28% 20% 12%	529
June	1st to 10th	Day	12% 5% 20% 30% 22% 11%	490
	11th to 20th	Day	7% 4 17% 27% 28% 19%	490
	21st to end	Day	10% 3 14% 31% 27% 15%	490
July	1st to 10th	Day	6% 2 11% 27% 32% 22%	485
	11th to 20th	Day	7% 3 8% 23% 32% 27%	484
	21st to end	Day	8% 2 12% 24% 32% 22%	532
August	1st to 10th	Day	9% 2 9% 26% 33% 20%	486
	11th to 20th	Day	8% 3 11% 29% 35% 14%	479
	21st to end	Day	13% 3 11% 27% 31% 16%	539
September	1st to 10th	Day	9% 3 13% 33% 29% 12%	490
	11th to 20th	Day	13% 5% 15% 28% 31% 9%	490
	21st to end	Day	12% 4 20% 29% 28% 7%	490
October	1st to 10th	Day	10% 6% 27% 32% 19% 6%	490
	11th to 20th	Day	11% 9% 28% 33% 18% 1	490
	21st to end	Day	18% 10% 26% 31% 14% 1	363
November	1st to 10th	Day	19% 11% 33% 27% 7% 3	300
	11th to 20th	Day	17% 17% 40% 20% 6%	300
	21st to end	Day	15% 20% 35% 21% 8% 1	291
December	1st to 10th	Day	29% 19% 31% 18% 3	270
	11th to 20th	Day	24% 21% 34% 14% 5% 1	270
	21st to end	Day	31% 16% 33% 16% 4	297
January	1st to 10th	Night	22% 16% 36% 22% 4	760
	11th to 20th	Night	23% 17% 30% 24% 7%	760
	21st to end	Night	20% 17% 37% 24% 3	836
February	1st to 10th	Night	21% 13% 34% 29% 4	760
	11th to 20th	Night	26% 14% 34% 23% 3	743
	21st to end	Night	22% 18% 36% 23% 2	628
March	1st to 10th	Night	21% 14% 30% 29% 5%	760
	11th to 20th	Night	17% 13% 37% 24% 8% 1	760
	21st to end	Night	17% 14% 33% 27% 9%	755
April	1st to 10th	Night	14% 13% 32% 28% 12%	570
	11th to 20th	Night	15% 11% 32% 28% 13% 1	563
	21st to end	Night	12% 10% 28% 31% 18% 1	570
May	1st to 10th	Night	11% 8% 26% 32% 21% 3	570
	11th to 20th	Night	10% 3 24% 34% 24% 5%	561
	21st to end	Night	7% 2 13% 36% 36% 6%	617
June	1st to 10th	Night	8% 2 13% 38% 33% 6%	570
	11th to 20th	Night	4 1 10% 29% 41% 14%	570
	21st to end	Night	5% 2 10% 30% 42% 10%	570
July	1st to 10th	Night	5% 3 6% 28% 42% 16%	555
	11th to 20th	Night	6% 1 5% 23% 46% 19%	552
	21st to end	Night	7% 1 8% 27% 42% 14%	624
August	1st to 10th	Night	5% 1 4 28% 48% 14%	567
	11th to 20th	Night	10% 1 8% 29% 42% 10%	564
	21st to end	Night	11% 2 8% 29% 39% 11%	627
September	1st to 10th	Night	10% 2 12% 31% 38% 8%	606
	11th to 20th	Night	14% 3 12% 30% 37% 5%	760
	21st to end	Night	9% 2 15% 35% 37% 3	760
October	1st to 10th	Night	10% 4 21% 35% 24% 6%	760
	11th to 20th	Night	12% 6% 24% 36% 20% 2	760
	21st to end	Night	15% 8% 21% 34% 19% 2	836
November	1st to 10th	Night	15% 7% 21% 37% 18% 3	780
	11th to 20th	Night	15% 9% 30% 32% 12% 1	800
	21st to end	Night	16% 10% 30% 32% 11% 1	789
December	1st to 10th	Night	22% 13% 29% 27% 8% 1	760
	11th to 20th	Night	24% 11% 30% 26% 9%	760
	21st to end	Night	24% 13% 30% 26% 6%	836

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