Tidewater Farm Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tidewater Farm Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	18% 7% 2 6% 4 4 4 4 5% 7% 39%	137772
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 39%	135251
Transparency	Day	43% 6% 12% 22% 14% 2	141294
	Night	44% 6% 12% 21% 15% 2	139805
Seeing	Day	20% 13% 31% 25% 9% 2	17208
	Night	21% 12% 32% 26% 9% 1	26006

By Month

Percentage Cloud Cover NE (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% 7% 3 8% 5% 5% 5% 5% 6% 7% 38%	4661
	11th to 20th	Day	16% 9% 2 6% 3 3 4 3 4 5% 45%	4650
	21st to end	Day	13% 8% 2 5% 4 3 4 3 4 5% 49%	5124
February	1st to 10th	Day	16% 8% 3 6% 4 4 4 4 6% 7% 38%	1915
	11th to 20th	Day	13% 7% 2 7% 5% 4 4 4 6% 7% 42%	2048
	21st to end	Day	15% 11% 3 7% 4 4 4 3 5% 6% 38%	1730
March	1st to 10th	Day	23% 10% 2 5% 4 4 4 4 5% 5% 34%	4576
	11th to 20th	Day	17% 11% 3 6% 4 3 4 3 5% 6% 39%	4920
	21st to end	Day	29% 7% 2 5% 3 3 4 4 5% 6% 31%	5838
April	1st to 10th	Day	20% 9% 2 6% 5% 4 4 2 4 6% 37%	2520
	11th to 20th	Day	17% 7% 2 6% 4 5% 5% 4 6% 6% 38%	2545
	21st to end	Day	20% 7% 3 7% 4 4 4 3 4 7% 38%	2730
May	1st to 10th	Day	27% 9% 3 6% 4 4 4 3 5% 6% 29%	7639
	11th to 20th	Day	20% 7% 2 7% 5% 4 4 3 5% 7% 35%	7761
	21st to end	Day	21% 7% 2 7% 4 5% 5% 4 6% 7% 33%	9060
June	1st to 10th	Day	19% 7% 3 6% 5% 4 4 4 6% 8% 34%	2940
	11th to 20th	Day	15% 5% 2 5% 5% 4 4 4 6% 8% 41%	2940
	21st to end	Day	18% 7% 2 6% 5% 5% 6% 4 6% 7% 35%	2940
July	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 9% 33%	6500
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 10% 41%	6500
	21st to end	Day	14% 5% 2 6% 4 3 4 4 5% 7% 45%	7066
August	1st to 10th	Day	14% 6% 2 6% 4 5% 5% 5% 6% 7% 38%	4278
	11th to 20th	Day	16% 8% 3 6% 4 4 4 4 6% 7% 37%	4175
	21st to end	Day	8% 3 1 5% 5% 5% 5% 4 7% 9% 48%	4347
September	1st to 10th	Day	12% 5% 2 4 4 3 5% 4 6% 7% 48%	2380
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 4 6% 53%	2200
	21st to end	Day	16% 6% 2 5% 3 3 4 3 5% 7% 46%	2200
October	1st to 10th	Day	20% 6% 3 5% 3 4 3 3 4 5% 43%	2200
	11th to 20th	Day	16% 5% 2 6% 4 3 3 3 5% 7% 46%	1941
	21st to end	Day	21% 7% 2 5% 4 3 3 4 5% 5% 41%	1980
November	1st to 10th	Day	16% 6% 2 6% 3 3 4 3 4 6% 49%	1851
	11th to 20th	Day	18% 6% 2 4 3 2 3 3 5% 5% 49%	1890
	21st to end	Day	21% 7% 2 4 3 3 3 3 4 6% 43%	1890
December	1st to 10th	Day	15% 9% 2 5% 3 3 4 3 5% 6% 46%	2723
	11th to 20th	Day	20% 5% 2 4 4 3 3 3 4 6% 47%	3066
	21st to end	Day	22% 9% 2 5% 4 4 3 3 4 5% 37%	4048
January	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 45%	8234
	11th to 20th	Night	15% 10% 3 9% 5% 5% 4 4 5% 6% 35%	8235
	21st to end	Night	14% 8% 2 6% 4 4 5% 3 5% 6% 43%	9057
February	1st to 10th	Night	15% 9% 3 7% 5% 4 5% 4 5% 6% 39%	2698
	11th to 20th	Night	14% 8% 3 6% 5% 4 4 3 5% 6% 41%	2458
	21st to end	Night	16% 10% 2 4 4 4 4 3 5% 6% 41%	2076
March	1st to 10th	Night	25% 9% 2 5% 3 4 4 3 4 5% 36%	5484
	11th to 20th	Night	15% 10% 3 7% 5% 4 5% 3 5% 6% 37%	5259
	21st to end	Night	26% 8% 2 5% 4 3 4 4 5% 6% 32%	5321
April	1st to 10th	Night	21% 9% 2 6% 4 4 4 3 5% 6% 37%	2100
	11th to 20th	Night	20% 8% 2 6% 4 3 3 4 4 6% 39%	2024
	21st to end	Night	21% 7% 3 6% 4 4 4 3 5% 6% 38%	1890
May	1st to 10th	Night	29% 10% 3 6% 3 4 3 3 5% 6% 28%	5259
	11th to 20th	Night	18% 8% 3 7% 5% 5% 5% 4 5% 8% 32%	5142
	21st to end	Night	21% 8% 3 7% 5% 4 4 4 6% 7% 31%	5114
June	1st to 10th	Night	20% 8% 2 6% 5% 5% 3 3 4 7% 36%	1680
	11th to 20th	Night	19% 5% 1 5% 4 4 5% 4 7% 9% 37%	1680
	21st to end	Night	20% 6% 3 6% 5% 4 5% 4 5% 7% 36%	1680
July	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 4 7% 10% 36%	3680
	11th to 20th	Night	11% 8% 2 5% 4 5% 5% 5% 7% 9% 38%	3680
	21st to end	Night	12% 5% 2 6% 3 5% 4 4 5% 6% 48%	4006
August	1st to 10th	Night	20% 6% 2 6% 5% 6% 5% 4 5% 6% 35%	2840
	11th to 20th	Night	14% 8% 3 8% 5% 5% 6% 4 7% 8% 34%	2872
	21st to end	Night	11% 4 2 4 4 4 4 3 6% 8% 50%	3547
September	1st to 10th	Night	12% 6% 2 5% 4 4 3 3 5% 7% 49%	2020
	11th to 20th	Night	14% 6% 1 4 3 3 4 3 4 7% 52%	2200
	21st to end	Night	16% 6% 2 5% 4 4 4 3 4 7% 46%	2200
October	1st to 10th	Night	20% 6% 2 6% 4 3 4 3 5% 5% 42%	2200
	11th to 20th	Night	16% 5% 2 6% 4 4 4 3 5% 7% 44%	2455
	21st to end	Night	22% 7% 2 6% 4 4 3 3 5% 5% 40%	2860
November	1st to 10th	Night	14% 6% 2 6% 4 3 4 3 4 5% 48%	2667
	11th to 20th	Night	17% 6% 2 5% 4 3 4 2 4 6% 47%	2730
	21st to end	Night	17% 7% 2 5% 4 4 4 3 4 7% 44%	2730
December	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 4 6% 41%	4675
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 4 4 42%	5348
	21st to end	Night	20% 9% 3 6% 4 4 3 3 4 4 39%	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	42% 5% 12% 25% 15% 1	4662
	11th to 20th	Day	41% 5% 11% 22% 18% 3	4680
	21st to end	Day	38% 5% 9% 24% 22% 3	5124
February	1st to 10th	Day	45% 6% 13% 21% 13% 3	1915
	11th to 20th	Day	41% 6% 13% 21% 15% 3	2057
	21st to end	Day	46% 6% 11% 20% 14% 3	1730
March	1st to 10th	Day	51% 5% 9% 13% 12% 9%	4579
	11th to 20th	Day	44% 6% 15% 22% 10% 3	4920
	21st to end	Day	52% 4 6% 15% 16% 6%	5838
April	1st to 10th	Day	48% 5% 8% 17% 17% 4	2520
	11th to 20th	Day	43% 8% 12% 19% 15% 3	2545
	21st to end	Day	46% 7% 8% 18% 18% 3	2730
May	1st to 10th	Day	54% 8% 11% 15% 10% 1	7651
	11th to 20th	Day	48% 7% 15% 18% 10% 2	7765
	21st to end	Day	48% 7% 12% 21% 11% 1	9061
June	1st to 10th	Day	47% 8% 12% 21% 11% 1	2940
	11th to 20th	Day	39% 8% 14% 20% 15% 3	2940
	21st to end	Day	45% 9% 13% 20% 12% 1	2940
July	1st to 10th	Day	46% 8% 13% 20% 13% 1	6430
	11th to 20th	Day	36% 8% 16% 25% 14% 1	6425
	21st to end	Day	37% 7% 13% 28% 14% 1	6868
August	1st to 10th	Day	43% 9% 16% 22% 9% 1	4264
	11th to 20th	Day	44% 7% 14% 25% 9% 1	4214
	21st to end	Day	28% 8% 17% 27% 18% 2	4347
September	1st to 10th	Day	34% 7% 13% 24% 20% 1	2380
	11th to 20th	Day	33% 6% 9% 27% 22% 2	2200
	21st to end	Day	39% 6% 10% 23% 19% 2	2200
October	1st to 10th	Day	42% 6% 9% 23% 17% 2	3590
	11th to 20th	Day	31% 4 8% 31% 22% 3	3131
	21st to end	Day	33% 5% 9% 30% 20% 3	3180
November	1st to 10th	Day	38% 5% 14% 25% 17% 2	1851
	11th to 20th	Day	38% 4 10% 22% 22% 3	1890
	21st to end	Day	43% 5% 15% 22% 14% 1	1890
December	1st to 10th	Day	40% 4 14% 29% 12% 1	2723
	11th to 20th	Day	39% 4 15% 18% 18% 6%	3066
	21st to end	Day	48% 5% 8% 20% 14% 5%	4048
January	1st to 10th	Night	39% 4 12% 29% 15% 2	8234
	11th to 20th	Night	49% 5% 10% 19% 13% 4	8280
	21st to end	Night	43% 5% 8% 20% 19% 5%	9057
February	1st to 10th	Night	45% 7% 12% 19% 14% 3	2698
	11th to 20th	Night	43% 5% 12% 21% 16% 3	2460
	21st to end	Night	43% 6% 10% 25% 13% 4	2076
March	1st to 10th	Night	50% 5% 8% 15% 14% 8%	5494
	11th to 20th	Night	46% 7% 14% 17% 13% 2	5260
	21st to end	Night	52% 4 7% 12% 20% 6%	5321
April	1st to 10th	Night	47% 5% 11% 17% 16% 4	2100
	11th to 20th	Night	46% 4 9% 20% 18% 2	2024
	21st to end	Night	46% 6% 10% 17% 18% 3	1890
May	1st to 10th	Night	56% 7% 11% 14% 11% 1	5265
	11th to 20th	Night	49% 7% 14% 20% 9% 1	5143
	21st to end	Night	51% 6% 12% 21% 8% 1	5115
June	1st to 10th	Night	48% 6% 11% 23% 12% 1	1680
	11th to 20th	Night	41% 10% 13% 19% 14% 3	1680
	21st to end	Night	45% 8% 13% 21% 11% 1	1680
July	1st to 10th	Night	43% 7% 13% 22% 15%	3640
	11th to 20th	Night	40% 7% 17% 20% 16%	3632
	21st to end	Night	36% 6% 12% 31% 15% 1	3889
August	1st to 10th	Night	49% 8% 12% 19% 11% 1	2830
	11th to 20th	Night	45% 9% 15% 19% 11% 1	2899
	21st to end	Night	30% 7% 15% 26% 21% 1	3547
September	1st to 10th	Night	34% 5% 14% 26% 20% 1	2020
	11th to 20th	Night	33% 5% 10% 26% 23% 2	2200
	21st to end	Night	39% 6% 10% 25% 18% 2	2200
October	1st to 10th	Night	41% 6% 15% 21% 14% 2	3590
	11th to 20th	Night	28% 5% 11% 28% 25% 3	3991
	21st to end	Night	35% 4 11% 30% 17% 2	4610
November	1st to 10th	Night	37% 6% 11% 25% 18% 3	2667
	11th to 20th	Night	40% 4 11% 22% 20% 3	2730
	21st to end	Night	41% 5% 15% 26% 12% 1	2730
December	1st to 10th	Night	45% 4 11% 28% 11% 1	4675
	11th to 20th	Night	45% 4 14% 15% 20% 2	5348
	21st to end	Night	47% 5% 9% 22% 16% 2	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	24% 22% 31% 20% 3	220
	11th to 20th	Day	25% 25% 31% 16% 2 1	220
	21st to end	Day	20% 25% 35% 16% 4	242
February	1st to 10th	Day	25% 20% 37% 16% 1	417
	11th to 20th	Day	23% 24% 36% 14% 2	536
	21st to end	Day	22% 21% 36% 18% 3	446
March	1st to 10th	Day	26% 18% 35% 17% 3	540
	11th to 20th	Day	22% 24% 36% 16% 3 1	540
	21st to end	Day	24% 21% 36% 13% 4 2	594
April	1st to 10th	Day	26% 21% 31% 16% 5% 1	540
	11th to 20th	Day	22% 19% 35% 17% 6%	532
	21st to end	Day	24% 12% 37% 18% 8% 1	540
May	1st to 10th	Day	25% 14% 37% 17% 6% 1	543
	11th to 20th	Day	17% 12% 36% 27% 6% 2	567
	21st to end	Day	21% 8% 32% 29% 9% 1	617
June	1st to 10th	Day	19% 6% 32% 29% 12% 1	570
	11th to 20th	Day	15% 10% 36% 30% 8% 2	570
	21st to end	Day	18% 6% 36% 28% 10% 2	570
July	1st to 10th	Day	16% 5% 24% 36% 16% 3	565
	11th to 20th	Day	11% 8% 25% 38% 15% 2	564
	21st to end	Day	12% 5% 27% 39% 14% 3	616
August	1st to 10th	Day	11% 5% 27% 36% 18% 3	526
	11th to 20th	Day	14% 5% 24% 34% 17% 6%	519
	21st to end	Day	13% 4 26% 38% 16% 4	583
September	1st to 10th	Day	14% 6% 27% 31% 15% 7%	530
	11th to 20th	Day	16% 7% 27% 28% 14% 8%	530
	21st to end	Day	16% 9% 31% 28% 11% 5%	530
October	1st to 10th	Day	23% 11% 25% 25% 13% 4	530
	11th to 20th	Day	18% 13% 27% 27% 12% 2	452
	21st to end	Day	26% 11% 31% 23% 8%	462
November	1st to 10th	Day	16% 18% 32% 23% 9% 1	425
	11th to 20th	Day	23% 22% 30% 21% 4	430
	21st to end	Day	26% 20% 28% 20% 5%	430
December	1st to 10th	Day	22% 21% 34% 18% 6%	250
	11th to 20th	Day	25% 21% 38% 15% 1	220
	21st to end	Day	26% 21% 30% 18% 5% 1	242
January	1st to 10th	Night	21% 20% 40% 16% 3	840
	11th to 20th	Night	26% 18% 37% 17% 3	840
	21st to end	Night	19% 22% 37% 18% 3	924
February	1st to 10th	Night	22% 17% 38% 21% 1	840
	11th to 20th	Night	22% 19% 41% 15% 3	823
	21st to end	Night	22% 17% 36% 23% 3	692
March	1st to 10th	Night	25% 18% 37% 18% 3	840
	11th to 20th	Night	22% 16% 37% 19% 5%	840
	21st to end	Night	24% 16% 35% 20% 5%	924
April	1st to 10th	Night	25% 16% 33% 22% 4	840
	11th to 20th	Night	23% 13% 34% 24% 6%	780
	21st to end	Night	23% 10% 34% 28% 4	630
May	1st to 10th	Night	27% 7% 33% 25% 8% 1	630
	11th to 20th	Night	21% 8% 34% 29% 8% 1	594
	21st to end	Night	22% 6% 27% 35% 10%	455
June	1st to 10th	Night	21% 5% 25% 35% 12% 2	420
	11th to 20th	Night	19% 7% 25% 37% 12%	420
	21st to end	Night	20% 3 25% 40% 13%	420
July	1st to 10th	Night	17% 3 21% 45% 14% 1	410
	11th to 20th	Night	12% 4 18% 47% 18% 1	408
	21st to end	Night	13% 3 19% 46% 17% 2	458
August	1st to 10th	Night	15% 3 18% 41% 23% 1	548
	11th to 20th	Night	14% 5% 20% 41% 18% 3	592
	21st to end	Night	14% 4 23% 35% 22% 3	854
September	1st to 10th	Night	13% 4 27% 34% 20% 1	800
	11th to 20th	Night	16% 5% 26% 30% 20% 4	800
	21st to end	Night	18% 4 27% 31% 17% 3	800
October	1st to 10th	Night	21% 8% 26% 29% 15% 1	800
	11th to 20th	Night	18% 11% 30% 30% 11% 1	800
	21st to end	Night	25% 11% 32% 25% 7%	880
November	1st to 10th	Night	16% 12% 35% 28% 7% 1	820
	11th to 20th	Night	20% 17% 35% 21% 6% 1	840
	21st to end	Night	21% 19% 33% 22% 4 1	840
December	1st to 10th	Night	24% 15% 39% 18% 4	840
	11th to 20th	Night	23% 20% 34% 20% 3	840
	21st to end	Night	26% 16% 35% 20% 2	924

Page generated at 2026-07-28 03:25:23. Version ac07

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