Tidewater Area
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tidewater Area clear sky chart from 2005-11-06 to 2026-08-30.

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% 6% 2 6% 4 4 5% 4 6% 8% 41%	143411
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 43%	138831
Transparency	Day	39% 6% 13% 22% 16% 3	147004
	Night	39% 6% 11% 22% 18% 4	143310
Seeing	Day	16% 11% 27% 26% 15% 5%	17689
	Night	16% 9% 28% 31% 14% 2	27227

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	12% 5% 2 5% 3 3 3 3 4 5% 55%	4943
	11th to 20th	Day	13% 6% 2 5% 3 3 3 2 4 4 56%	5280
	21st to end	Day	12% 9% 3 7% 4 4 4 3 4 5% 44%	5780
February	1st to 10th	Day	13% 8% 2 6% 3 5% 5% 4 6% 6% 42%	1890
	11th to 20th	Day	20% 7% 2 6% 4 3 4 3 4 5% 43%	2026
	21st to end	Day	16% 8% 2 6% 4 4 3 4 5% 8% 39%	1730
March	1st to 10th	Day	20% 8% 2 6% 4 3 4 3 4 6% 40%	4850
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 4 6% 52%	5090
	21st to end	Day	13% 6% 2 5% 3 4 4 3 5% 7% 47%	5772
April	1st to 10th	Day	13% 5% 1 5% 4 3 3 3 5% 8% 50%	2520
	11th to 20th	Day	10% 4 1 6% 5% 4 4 3 5% 7% 51%	2494
	21st to end	Day	12% 6% 2 6% 5% 4 4 4 6% 7% 45%	2520
May	1st to 10th	Day	15% 6% 2 6% 4 5% 5% 4 6% 9% 38%	7021
	11th to 20th	Day	15% 6% 2 7% 4 4 5% 3 5% 8% 41%	7817
	21st to end	Day	16% 7% 3 7% 4 4 5% 4 5% 7% 38%	9045
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 10% 40%	2940
	11th to 20th	Day	11% 5% 2 5% 5% 4 5% 5% 7% 10% 39%	2940
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 5% 7% 12% 39%	2940
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 9% 12% 37%	8280
	11th to 20th	Day	11% 7% 3 9% 6% 6% 7% 5% 8% 10% 28%	7860
	21st to end	Day	11% 6% 3 8% 6% 6% 6% 5% 8% 10% 30%	8340
August	1st to 10th	Day	16% 9% 4 9% 6% 6% 6% 5% 7% 10% 22%	4303
	11th to 20th	Day	9% 6% 3 8% 5% 7% 6% 5% 8% 9% 34%	3970
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	4420
September	1st to 10th	Day	14% 5% 2 6% 5% 5% 5% 4 7% 9% 37%	2400
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 4 5% 7% 38%	2400
	21st to end	Day	22% 7% 3 6% 4 4 5% 4 5% 7% 34%	2147
October	1st to 10th	Day	17% 6% 2 5% 4 4 5% 3 4 5% 45%	2000
	11th to 20th	Day	15% 5% 3 5% 3 4 5% 3 4 5% 49%	2000
	21st to end	Day	13% 5% 2 6% 3 3 4 3 4 6% 51%	2200
November	1st to 10th	Day	15% 6% 2 6% 3 3 4 3 4 6% 49%	1952
	11th to 20th	Day	15% 7% 2 4 4 3 4 2 4 6% 49%	1890
	21st to end	Day	13% 5% 1 6% 4 4 4 3 5% 6% 50%	1853
December	1st to 10th	Day	11% 5% 3 5% 4 4 3 2 5% 5% 52%	2680
	11th to 20th	Day	14% 8% 3 7% 4 4 4 3 4 5% 44%	3070
	21st to end	Day	13% 5% 2 5% 4 3 3 4 5% 7% 49%	4048
January	1st to 10th	Night	11% 6% 2 6% 3 3 4 3 5% 6% 50%	7934
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 5% 53%	7680
	21st to end	Night	16% 7% 2 7% 4 4 3 3 4 6% 44%	8401
February	1st to 10th	Night	17% 7% 2 6% 4 3 3 4 6% 5% 43%	2730
	11th to 20th	Night	21% 7% 3 5% 3 3 4 3 4 5% 40%	2479
	21st to end	Night	15% 8% 2 5% 4 4 3 3 4 5% 46%	2076
March	1st to 10th	Night	20% 6% 2 6% 4 4 4 3 4 5% 42%	5217
	11th to 20th	Night	13% 6% 3 6% 4 4 4 3 4 6% 48%	5090
	21st to end	Night	15% 8% 2 5% 3 4 4 3 4 5% 47%	5401
April	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 7% 45%	2100
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 7% 50%	2075
	21st to end	Night	14% 5% 2 5% 5% 4 3 4 5% 6% 47%	2100
May	1st to 10th	Night	12% 7% 3 8% 6% 5% 5% 4 6% 9% 36%	5833
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 5% 7% 42%	5097
	21st to end	Night	18% 8% 3 7% 5% 4 4 4 5% 7% 35%	5108
June	1st to 10th	Night	10% 6% 3 4 5% 5% 5% 4 8% 10% 42%	1680
	11th to 20th	Night	8% 6% 3 7% 5% 5% 5% 5% 6% 11% 40%	1680
	21st to end	Night	7% 4 3 6% 5% 5% 5% 4 6% 9% 45%	1680
July	1st to 10th	Night	7% 5% 2 6% 4 5% 6% 5% 9% 14% 38%	4678
	11th to 20th	Night	9% 7% 4 8% 5% 5% 6% 5% 8% 12% 30%	5091
	21st to end	Night	14% 8% 3 8% 5% 6% 5% 4 7% 9% 31%	5736
August	1st to 10th	Night	17% 9% 3 8% 6% 5% 5% 4 7% 8% 29%	3034
	11th to 20th	Night	9% 7% 3 8% 6% 5% 5% 4 6% 9% 38%	3297
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 7% 8% 44%	3677
September	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 6% 8% 47%	2000
	11th to 20th	Night	14% 6% 2 8% 4 4 4 4 4 7% 44%	2000
	21st to end	Night	20% 7% 3 8% 5% 4 4 3 4 8% 35%	2247
October	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 4 6% 49%	2400
	11th to 20th	Night	16% 5% 2 6% 4 3 3 2 4 6% 50%	2400
	21st to end	Night	14% 4 2 5% 3 3 4 3 4 5% 52%	2640
November	1st to 10th	Night	15% 6% 2 5% 4 3 3 3 5% 6% 48%	2563
	11th to 20th	Night	14% 7% 3 5% 4 3 3 3 5% 6% 47%	2730
	21st to end	Night	12% 5% 2 5% 4 3 4 3 5% 6% 51%	2762
December	1st to 10th	Night	10% 5% 3 6% 4 4 4 3 4 5% 53%	4720
	11th to 20th	Night	16% 8% 3 7% 5% 6% 5% 3 5% 6% 38%	5345
	21st to end	Night	12% 5% 2 6% 4 4 4 3 5% 7% 49%	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	32% 3 12% 26% 18% 9%	4944
	11th to 20th	Day	34% 3 5% 16% 35% 8%	5280
	21st to end	Day	43% 4 8% 25% 17% 4	5780
February	1st to 10th	Day	42% 4 13% 24% 14% 3	1890
	11th to 20th	Day	44% 5% 9% 18% 19% 5%	2035
	21st to end	Day	42% 7% 11% 19% 16% 5%	1730
March	1st to 10th	Day	44% 5% 8% 20% 16% 7%	4850
	11th to 20th	Day	34% 4 9% 19% 25% 10%	5090
	21st to end	Day	37% 5% 14% 19% 20% 5%	5772
April	1st to 10th	Day	33% 5% 15% 21% 21% 5%	2520
	11th to 20th	Day	33% 5% 12% 25% 19% 5%	2494
	21st to end	Day	37% 7% 13% 23% 16% 4	2520
May	1st to 10th	Day	43% 7% 16% 19% 13% 2	7053
	11th to 20th	Day	40% 6% 14% 24% 14% 1	7821
	21st to end	Day	44% 7% 12% 21% 14% 2	9061
June	1st to 10th	Day	35% 10% 16% 23% 15% 2	2940
	11th to 20th	Day	36% 9% 18% 23% 13% 1	2940
	21st to end	Day	34% 10% 17% 24% 13% 1	2940
July	1st to 10th	Day	36% 9% 18% 27% 8% 1	8210
	11th to 20th	Day	47% 9% 20% 17% 6%	7788
	21st to end	Day	44% 8% 13% 22% 12%	8156
August	1st to 10th	Day	55% 8% 17% 16% 4	4290
	11th to 20th	Day	42% 8% 16% 22% 12%	4006
	21st to end	Day	36% 8% 14% 25% 16% 2	4420
September	1st to 10th	Day	40% 9% 15% 18% 16% 3	2400
	11th to 20th	Day	45% 6% 10% 21% 16% 2	2400
	21st to end	Day	48% 7% 11% 19% 13% 2	2147
October	1st to 10th	Day	33% 4 9% 25% 26% 3	3260
	11th to 20th	Day	28% 4 6% 21% 33% 9%	3240
	21st to end	Day	23% 4 9% 28% 28% 9%	3538
November	1st to 10th	Day	36% 5% 10% 26% 19% 3	1952
	11th to 20th	Day	37% 4 13% 21% 20% 5%	1890
	21st to end	Day	36% 4 10% 25% 21% 4	1853
December	1st to 10th	Day	35% 4 9% 28% 21% 3	2680
	11th to 20th	Day	43% 4 13% 20% 15% 5%	3066
	21st to end	Day	35% 4 13% 30% 17% 1	4048
January	1st to 10th	Night	34% 4 11% 23% 21% 7%	7935
	11th to 20th	Night	35% 4 5% 19% 30% 8%	7680
	21st to end	Night	42% 4 7% 24% 19% 5%	8401
February	1st to 10th	Night	42% 5% 10% 22% 17% 3	2730
	11th to 20th	Night	46% 4 9% 20% 16% 5%	2481
	21st to end	Night	41% 5% 9% 21% 18% 7%	2076
March	1st to 10th	Night	44% 5% 9% 18% 17% 8%	5217
	11th to 20th	Night	38% 5% 9% 16% 22% 10%	5090
	21st to end	Night	40% 5% 9% 24% 16% 6%	5401
April	1st to 10th	Night	38% 6% 12% 19% 22% 4	2100
	11th to 20th	Night	33% 6% 12% 25% 19% 5%	2075
	21st to end	Night	37% 6% 12% 24% 18% 3	2100
May	1st to 10th	Night	43% 8% 15% 22% 10% 1	5863
	11th to 20th	Night	42% 6% 13% 26% 11% 1	5098
	21st to end	Night	47% 8% 12% 18% 12% 2	5115
June	1st to 10th	Night	34% 9% 15% 24% 16% 1	1680
	11th to 20th	Night	37% 11% 17% 21% 13% 1	1680
	21st to end	Night	33% 10% 16% 25% 14% 2	1680
July	1st to 10th	Night	35% 10% 19% 25% 10% 1	4640
	11th to 20th	Night	44% 10% 21% 16% 8%	5044
	21st to end	Night	48% 8% 13% 18% 12%	5606
August	1st to 10th	Night	51% 7% 19% 19% 4	3023
	11th to 20th	Night	41% 8% 15% 24% 12%	3327
	21st to end	Night	35% 7% 12% 27% 18% 1	3677
September	1st to 10th	Night	35% 9% 17% 21% 15% 3	2000
	11th to 20th	Night	42% 6% 11% 21% 18% 2	2000
	21st to end	Night	48% 7% 12% 18% 14% 2	2247
October	1st to 10th	Night	37% 5% 8% 24% 24% 2	3920
	11th to 20th	Night	29% 4 6% 19% 32% 10%	3899
	21st to end	Night	22% 3 7% 30% 27% 12%	4252
November	1st to 10th	Night	38% 5% 9% 22% 23% 3	2563
	11th to 20th	Night	39% 5% 10% 20% 18% 7%	2730
	21st to end	Night	34% 6% 10% 25% 22% 4	2762
December	1st to 10th	Night	34% 4 8% 24% 25% 4	4720
	11th to 20th	Night	48% 4 11% 16% 15% 6%	5348
	21st to end	Night	34% 6% 15% 27% 16% 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	18% 29% 35% 14% 4	272
	11th to 20th	Day	20% 26% 37% 15% 2	330
	21st to end	Day	20% 20% 39% 17% 3 1	363
February	1st to 10th	Day	20% 21% 38% 15% 5%	330
	11th to 20th	Day	29% 21% 35% 14% 2	514
	21st to end	Day	20% 21% 36% 19% 2 1	446
March	1st to 10th	Day	18% 20% 38% 19% 5%	540
	11th to 20th	Day	20% 21% 36% 18% 4 1	540
	21st to end	Day	16% 18% 41% 20% 5% 1	594
April	1st to 10th	Day	15% 19% 40% 18% 6% 1	540
	11th to 20th	Day	12% 16% 36% 24% 9% 2	532
	21st to end	Day	16% 12% 35% 26% 9% 1	540
May	1st to 10th	Day	18% 14% 34% 23% 9% 2	543
	11th to 20th	Day	18% 7% 30% 30% 11% 3	567
	21st to end	Day	20% 7% 23% 27% 18% 5%	617
June	1st to 10th	Day	11% 5% 25% 32% 23% 5%	570
	11th to 20th	Day	12% 8% 20% 32% 22% 6%	570
	21st to end	Day	9% 3 21% 39% 23% 6%	570
July	1st to 10th	Day	10% 1 11% 37% 31% 8%	565
	11th to 20th	Day	10% 3 10% 32% 34% 11%	564
	21st to end	Day	9% 3 17% 35% 28% 7%	620
August	1st to 10th	Day	15% 3 15% 36% 23% 7%	566
	11th to 20th	Day	10% 4 14% 32% 27% 14%	559
	21st to end	Day	12% 4 19% 28% 26% 11%	619
September	1st to 10th	Day	15% 3 22% 27% 19% 14%	530
	11th to 20th	Day	18% 7% 14% 31% 21% 8%	530
	21st to end	Day	24% 7% 22% 23% 18% 5%	530
October	1st to 10th	Day	19% 5% 21% 26% 20% 8%	530
	11th to 20th	Day	19% 12% 25% 26% 17% 3	530
	21st to end	Day	16% 11% 31% 25% 13% 5%	583
November	1st to 10th	Day	17% 11% 31% 24% 13% 4	480
	11th to 20th	Day	19% 15% 31% 24% 8% 2	430
	21st to end	Day	15% 19% 29% 24% 12% 2	393
December	1st to 10th	Day	23% 15% 42% 14% 5% 1	220
	11th to 20th	Day	15% 19% 38% 21% 6% 1	220
	21st to end	Day	21% 17% 33% 19% 8% 1	242
January	1st to 10th	Night	18% 19% 35% 24% 5%	840
	11th to 20th	Night	21% 16% 34% 24% 5%	840
	21st to end	Night	18% 15% 40% 24% 2	924
February	1st to 10th	Night	21% 11% 40% 24% 3	840
	11th to 20th	Night	25% 16% 39% 17% 2	823
	21st to end	Night	19% 16% 38% 23% 4	692
March	1st to 10th	Night	18% 12% 37% 27% 5%	840
	11th to 20th	Night	19% 12% 41% 22% 6%	840
	21st to end	Night	19% 13% 38% 26% 4	924
April	1st to 10th	Night	17% 13% 41% 23% 6%	840
	11th to 20th	Night	13% 11% 37% 29% 10%	831
	21st to end	Night	17% 6% 34% 35% 7% 1	840
May	1st to 10th	Night	16% 10% 33% 33% 7% 1	840
	11th to 20th	Night	16% 4 29% 39% 11% 1	577
	21st to end	Night	17% 3 16% 41% 21% 2	455
June	1st to 10th	Night	11% 3 19% 45% 19% 2	420
	11th to 20th	Night	7% 4 19% 45% 23% 2	420
	21st to end	Night	8% 2 16% 49% 24% 1	420
July	1st to 10th	Night	8% 2 13% 47% 27% 2	410
	11th to 20th	Night	8% 2 9% 41% 34% 6%	547
	21st to end	Night	10% 2 14% 42% 29% 3	690
August	1st to 10th	Night	11% 1 13% 41% 30% 3	643
	11th to 20th	Night	10% 3 12% 40% 31% 4	830
	21st to end	Night	12% 2 12% 35% 34% 5%	917
September	1st to 10th	Night	13% 2 15% 37% 27% 7%	800
	11th to 20th	Night	16% 4 15% 33% 28% 5%	800
	21st to end	Night	22% 4 19% 31% 23% 1	800
October	1st to 10th	Night	15% 5% 18% 32% 25% 5%	800
	11th to 20th	Night	19% 8% 24% 31% 17% 2	800
	21st to end	Night	16% 8% 25% 31% 16% 3	880
November	1st to 10th	Night	16% 8% 27% 30% 15% 3	820
	11th to 20th	Night	17% 11% 33% 27% 11% 1	840
	21st to end	Night	14% 11% 31% 31% 13% 1	840
December	1st to 10th	Night	19% 13% 33% 26% 8%	840
	11th to 20th	Night	17% 11% 37% 29% 7%	840
	21st to end	Night	18% 11% 33% 32% 7%	924

Page generated at 2026-08-27 04:07:12. Version ac07

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