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 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	17% 7% 2 6% 4 4 4 4 5% 7% 40%	108737
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 40%	110605
Transparency	Day	42% 7% 12% 22% 15% 2	112305
	Night	43% 6% 12% 21% 15% 2	115188
Seeing	Day	19% 13% 31% 26% 9% 2	15423
	Night	21% 12% 32% 26% 9% 1	24422

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	14% 6% 2 6% 5% 5% 5% 5% 6% 7% 39%	3597
	11th to 20th	Day	15% 7% 2 6% 3 3 3 3 5% 6% 47%	3570
	21st to end	Day	16% 7% 2 5% 4 4 4 3 4 5% 47%	3960
February	1st to 10th	Day	17% 8% 3 6% 4 4 4 4 6% 7% 38%	1823
	11th to 20th	Day	14% 7% 2 7% 5% 4 4 4 6% 7% 41%	1948
	21st to end	Day	15% 11% 3 7% 4 4 4 3 5% 7% 38%	1650
March	1st to 10th	Day	26% 12% 2 5% 4 4 4 4 4 5% 30%	3216
	11th to 20th	Day	16% 8% 2 6% 4 3 4 4 5% 7% 41%	3476
	21st to end	Day	24% 8% 3 6% 3 4 4 3 6% 6% 34%	4058
April	1st to 10th	Day	20% 9% 2 6% 6% 4 4 2 4 6% 37%	2280
	11th to 20th	Day	17% 7% 2 6% 4 5% 4 4 6% 7% 38%	2300
	21st to end	Day	21% 7% 2 7% 4 4 4 3 4 7% 37%	2470
May	1st to 10th	Day	23% 11% 3 7% 5% 4 4 4 5% 6% 30%	4106
	11th to 20th	Day	23% 7% 2 7% 5% 4 3 3 4 6% 35%	4161
	21st to end	Day	16% 5% 2 5% 4 4 4 4 5% 8% 42%	4837
June	1st to 10th	Day	20% 8% 3 6% 5% 3 4 4 6% 7% 34%	2660
	11th to 20th	Day	16% 5% 2 5% 4 5% 4 5% 6% 8% 41%	2660
	21st to end	Day	19% 7% 2 6% 5% 4 6% 4 6% 7% 34%	2660
July	1st to 10th	Day	13% 6% 2 7% 5% 6% 5% 4 7% 9% 36%	4440
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 5% 7% 10% 41%	4440
	21st to end	Day	17% 6% 3 7% 5% 4 5% 4 6% 8% 37%	4814
August	1st to 10th	Day	14% 6% 2 7% 4 5% 5% 5% 6% 8% 37%	4147
	11th to 20th	Day	16% 8% 3 6% 5% 4 4 4 6% 7% 36%	4045
	21st to end	Day	8% 3 1 5% 5% 5% 5% 4 7% 9% 48%	4214
September	1st to 10th	Day	12% 5% 2 5% 4 3 5% 4 6% 7% 47%	2261
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 4 5% 53%	2090
	21st to end	Day	16% 6% 2 6% 3 3 4 3 4 7% 46%	2090
October	1st to 10th	Day	21% 6% 3 5% 4 4 3 3 5% 5% 41%	2090
	11th to 20th	Day	15% 5% 2 6% 5% 3 3 3 5% 7% 46%	1844
	21st to end	Day	21% 7% 2 5% 4 3 3 4 5% 5% 41%	1881
November	1st to 10th	Day	16% 6% 2 5% 3 3 4 3 4 6% 49%	1761
	11th to 20th	Day	18% 6% 2 4 3 2 3 3 5% 5% 49%	1800
	21st to end	Day	21% 7% 2 4 3 3 3 4 4 6% 44%	1800
December	1st to 10th	Day	15% 9% 2 5% 3 3 4 3 5% 6% 46%	2642
	11th to 20th	Day	20% 5% 2 4 4 3 3 3 4 6% 47%	2986
	21st to end	Day	22% 9% 3 5% 4 3 3 3 4 5% 37%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 6% 7% 45%	6349
	11th to 20th	Night	13% 9% 4 9% 5% 5% 4 4 5% 5% 38%	6315
	21st to end	Night	16% 9% 2 6% 5% 4 4 3 5% 6% 40%	6996
February	1st to 10th	Night	15% 9% 3 7% 5% 3 5% 4 5% 5% 38%	2570
	11th to 20th	Night	14% 9% 3 6% 5% 4 4 3 6% 6% 41%	2338
	21st to end	Night	16% 10% 3 4 4 4 4 3 5% 6% 41%	1980
March	1st to 10th	Night	29% 10% 2 5% 3 3 4 3 4 4 34%	3844
	11th to 20th	Night	13% 9% 2 8% 5% 5% 5% 4 5% 6% 37%	3703
	21st to end	Night	25% 8% 3 5% 4 3 4 4 6% 6% 33%	3711
April	1st to 10th	Night	20% 9% 2 6% 5% 4 4 3 5% 6% 37%	1900
	11th to 20th	Night	20% 8% 2 6% 3 3 3 4 4 6% 40%	1829
	21st to end	Night	22% 7% 2 6% 4 4 4 4 5% 5% 38%	1710
May	1st to 10th	Night	24% 12% 3 7% 4 4 4 3 5% 7% 26%	2834
	11th to 20th	Night	19% 9% 3 8% 5% 6% 5% 4 6% 8% 27%	2762
	21st to end	Night	18% 7% 2 6% 4 3 4 3 5% 7% 40%	2738
June	1st to 10th	Night	20% 9% 3 6% 5% 5% 4 3 5% 7% 35%	1520
	11th to 20th	Night	20% 5% 2 5% 4 4 4 4 7% 8% 36%	1520
	21st to end	Night	20% 6% 3 6% 5% 4 5% 4 5% 7% 35%	1520
July	1st to 10th	Night	12% 7% 2 7% 5% 6% 6% 4 8% 10% 34%	2520
	11th to 20th	Night	11% 6% 2 6% 5% 5% 5% 5% 8% 9% 39%	2520
	21st to end	Night	16% 6% 3 7% 4 5% 5% 4 6% 7% 39%	2739
August	1st to 10th	Night	20% 6% 2 6% 5% 6% 5% 4 5% 6% 34%	2752
	11th to 20th	Night	15% 8% 2 7% 5% 5% 6% 4 7% 8% 33%	2782
	21st to end	Night	11% 4 2 4 4 4 4 3 6% 8% 50%	3438
September	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 5% 7% 49%	1919
	11th to 20th	Night	14% 6% 1 4 3 3 3 3 4 7% 52%	2090
	21st to end	Night	16% 5% 2 5% 4 3 4 3 5% 7% 46%	2090
October	1st to 10th	Night	21% 6% 2 6% 4 3 4 3 5% 5% 40%	2090
	11th to 20th	Night	16% 5% 2 6% 4 5% 4 4 5% 6% 44%	2332
	21st to end	Night	23% 7% 2 6% 4 3 3 3 5% 5% 40%	2717
November	1st to 10th	Night	14% 6% 2 6% 4 3 4 3 4 5% 48%	2537
	11th to 20th	Night	18% 6% 2 5% 4 3 4 2 4 6% 47%	2600
	21st to end	Night	17% 7% 2 4 4 4 4 3 4 6% 44%	2600
December	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 4 6% 41%	4536
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 4 4 42%	5208
	21st to end	Night	20% 9% 3 6% 4 4 3 3 4 4 39%	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	41% 6% 10% 26% 16% 1	3598
	11th to 20th	Day	38% 5% 12% 24% 18% 3	3600
	21st to end	Day	41% 5% 8% 22% 20% 4	3960
February	1st to 10th	Day	45% 6% 13% 20% 12% 3	1823
	11th to 20th	Day	42% 6% 13% 20% 15% 4	1957
	21st to end	Day	46% 6% 11% 20% 14% 3	1650
March	1st to 10th	Day	55% 6% 10% 14% 12% 3	3219
	11th to 20th	Day	39% 8% 15% 21% 13% 4	3476
	21st to end	Day	50% 5% 6% 15% 17% 7%	4058
April	1st to 10th	Day	48% 6% 8% 17% 17% 4	2280
	11th to 20th	Day	42% 8% 12% 20% 15% 3	2300
	21st to end	Day	46% 8% 8% 17% 18% 3	2470
May	1st to 10th	Day	52% 11% 12% 15% 10% 1	4120
	11th to 20th	Day	49% 9% 15% 15% 10% 2	4165
	21st to end	Day	35% 9% 15% 22% 17% 1	4837
June	1st to 10th	Day	47% 8% 13% 20% 11% 1	2660
	11th to 20th	Day	39% 9% 14% 20% 15% 3	2660
	21st to end	Day	46% 10% 13% 19% 12% 1	2660
July	1st to 10th	Day	43% 8% 13% 21% 14% 1	4370
	11th to 20th	Day	36% 8% 16% 26% 13% 1	4365
	21st to end	Day	43% 8% 13% 22% 12% 2	4661
August	1st to 10th	Day	44% 9% 16% 22% 9%	4133
	11th to 20th	Day	45% 7% 14% 24% 9% 1	4084
	21st to end	Day	28% 9% 17% 27% 17% 2	4214
September	1st to 10th	Day	34% 8% 13% 24% 20% 1	2261
	11th to 20th	Day	34% 6% 9% 27% 22% 2	2090
	21st to end	Day	39% 6% 11% 23% 20% 2	2090
October	1st to 10th	Day	42% 6% 10% 23% 17% 2	3480
	11th to 20th	Day	31% 4 8% 31% 23% 3	3034
	21st to end	Day	33% 5% 9% 30% 21% 3	3081
November	1st to 10th	Day	37% 5% 14% 25% 17% 2	1761
	11th to 20th	Day	38% 5% 10% 21% 23% 3	1800
	21st to end	Day	43% 5% 15% 22% 14% 1	1800
December	1st to 10th	Day	40% 5% 14% 30% 11% 1	2642
	11th to 20th	Day	39% 4 15% 18% 18% 6%	2986
	21st to end	Day	49% 5% 8% 20% 14% 5%	3960
January	1st to 10th	Night	37% 5% 11% 32% 14% 2	6349
	11th to 20th	Night	46% 6% 12% 21% 12% 3	6360
	21st to end	Night	45% 5% 9% 18% 17% 5%	6996
February	1st to 10th	Night	46% 7% 13% 18% 13% 3	2570
	11th to 20th	Night	44% 5% 12% 20% 16% 3	2340
	21st to end	Night	43% 6% 10% 24% 13% 4	1980
March	1st to 10th	Night	53% 5% 10% 17% 11% 4	3854
	11th to 20th	Night	43% 9% 12% 17% 16% 2	3704
	21st to end	Night	50% 4 9% 13% 18% 7%	3711
April	1st to 10th	Night	47% 6% 11% 17% 16% 4	1900
	11th to 20th	Night	45% 4 10% 20% 19% 2	1829
	21st to end	Night	47% 6% 10% 17% 16% 3	1710
May	1st to 10th	Night	55% 10% 11% 15% 8% 2	2840
	11th to 20th	Night	52% 10% 16% 13% 8% 2	2763
	21st to end	Night	41% 8% 16% 19% 14% 2	2739
June	1st to 10th	Night	49% 6% 11% 21% 12% 1	1520
	11th to 20th	Night	42% 9% 13% 19% 14% 3	1520
	21st to end	Night	45% 9% 13% 21% 11% 1	1520
July	1st to 10th	Night	44% 8% 13% 21% 13% 1	2480
	11th to 20th	Night	38% 8% 17% 23% 14%	2472
	21st to end	Night	43% 7% 12% 23% 14% 1	2651
August	1st to 10th	Night	50% 8% 12% 19% 10% 1	2742
	11th to 20th	Night	45% 10% 16% 19% 11%	2809
	21st to end	Night	30% 7% 16% 26% 21% 1	3438
September	1st to 10th	Night	35% 6% 14% 25% 19% 1	1919
	11th to 20th	Night	34% 5% 11% 26% 23% 2	2090
	21st to end	Night	38% 6% 10% 25% 19% 2	2090
October	1st to 10th	Night	42% 7% 15% 21% 13% 2	3480
	11th to 20th	Night	27% 5% 11% 28% 26% 3	3868
	21st to end	Night	35% 4 12% 30% 18% 2	4467
November	1st to 10th	Night	37% 7% 11% 24% 18% 3	2537
	11th to 20th	Night	41% 4 11% 22% 20% 3	2600
	21st to end	Night	41% 5% 15% 26% 12% 2	2600
December	1st to 10th	Night	45% 4 11% 28% 11% 1	4536
	11th to 20th	Night	45% 4 14% 15% 20% 1	5208
	21st to end	Night	47% 5% 9% 22% 16% 2	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	26% 23% 29% 20% 4	200
	11th to 20th	Day	23% 25% 32% 18% 3 1	200
	21st to end	Day	18% 23% 37% 17% 4	220
February	1st to 10th	Day	25% 20% 37% 16% 1	385
	11th to 20th	Day	24% 25% 35% 14% 2	496
	21st to end	Day	21% 21% 36% 19% 4	414
March	1st to 10th	Day	26% 18% 35% 18% 3	500
	11th to 20th	Day	20% 23% 37% 16% 3 1	500
	21st to end	Day	23% 19% 37% 15% 5% 2	530
April	1st to 10th	Day	23% 20% 31% 18% 6% 1	460
	11th to 20th	Day	20% 18% 36% 18% 7%	452
	21st to end	Day	24% 10% 37% 20% 8% 1	460
May	1st to 10th	Day	24% 13% 37% 19% 6% 1	463
	11th to 20th	Day	17% 11% 35% 29% 7% 1	487
	21st to end	Day	19% 7% 31% 32% 10% 1	529
June	1st to 10th	Day	20% 6% 30% 31% 12% 1	490
	11th to 20th	Day	13% 8% 35% 33% 8% 2	490
	21st to end	Day	18% 6% 36% 28% 10% 2	490
July	1st to 10th	Day	15% 5% 23% 36% 17% 3	485
	11th to 20th	Day	12% 7% 23% 40% 16% 2	484
	21st to end	Day	11% 3 27% 41% 15% 2	532
August	1st to 10th	Day	12% 4 25% 38% 18% 3	486
	11th to 20th	Day	15% 5% 24% 34% 18% 5%	479
	21st to end	Day	14% 3 24% 39% 16% 5%	539
September	1st to 10th	Day	14% 6% 26% 31% 17% 7%	490
	11th to 20th	Day	17% 7% 27% 27% 14% 8%	490
	21st to end	Day	16% 9% 30% 29% 11% 4	490
October	1st to 10th	Day	24% 10% 26% 25% 12% 3	490
	11th to 20th	Day	16% 14% 28% 28% 12% 2	419
	21st to end	Day	26% 10% 32% 22% 8%	429
November	1st to 10th	Day	15% 17% 32% 25% 10% 2	395
	11th to 20th	Day	24% 22% 29% 22% 4 1	400
	21st to end	Day	26% 20% 27% 21% 6% 1	400
December	1st to 10th	Day	20% 21% 35% 19% 6%	229
	11th to 20th	Day	23% 21% 39% 16% 2	200
	21st to end	Day	22% 21% 31% 20% 5% 1	220
January	1st to 10th	Night	21% 20% 40% 17% 3	800
	11th to 20th	Night	25% 17% 38% 18% 3	800
	21st to end	Night	19% 22% 38% 18% 4	880
February	1st to 10th	Night	23% 17% 38% 21% 1	800
	11th to 20th	Night	23% 20% 40% 15% 3	783
	21st to end	Night	21% 16% 37% 23% 3	660
March	1st to 10th	Night	25% 17% 37% 18% 3	800
	11th to 20th	Night	21% 16% 37% 20% 6%	800
	21st to end	Night	24% 15% 35% 21% 5%	863
April	1st to 10th	Night	24% 14% 34% 24% 4	760
	11th to 20th	Night	22% 12% 34% 25% 7%	705
	21st to end	Night	23% 10% 33% 29% 4 1	570
May	1st to 10th	Night	25% 7% 33% 26% 8% 1	570
	11th to 20th	Night	21% 7% 33% 30% 8% 1	537
	21st to end	Night	21% 6% 26% 36% 11%	411
June	1st to 10th	Night	22% 5% 26% 34% 12% 2	380
	11th to 20th	Night	20% 6% 24% 38% 12% 1	380
	21st to end	Night	20% 3 24% 40% 13%	380
July	1st to 10th	Night	17% 2 20% 44% 16% 1	370
	11th to 20th	Night	12% 3 17% 47% 20% 1	368
	21st to end	Night	13% 3 16% 48% 17% 2	416
August	1st to 10th	Night	15% 3 17% 41% 23% 1	520
	11th to 20th	Night	14% 4 20% 41% 18% 2	562
	21st to end	Night	14% 4 22% 35% 22% 3	811
September	1st to 10th	Night	14% 4 27% 33% 21% 1	760
	11th to 20th	Night	16% 5% 26% 29% 20% 4	760
	21st to end	Night	18% 4 27% 31% 17% 3	760
October	1st to 10th	Night	22% 7% 26% 28% 16% 1	760
	11th to 20th	Night	18% 10% 31% 29% 11% 1	760
	21st to end	Night	25% 11% 31% 25% 7%	836
November	1st to 10th	Night	16% 11% 36% 29% 8% 1	780
	11th to 20th	Night	20% 17% 34% 21% 6% 1	800
	21st to end	Night	22% 18% 33% 22% 4 1	800
December	1st to 10th	Night	24% 14% 40% 18% 4	800
	11th to 20th	Night	22% 19% 34% 21% 3	800
	21st to end	Night	25% 16% 36% 21% 2	880

Page generated at 2025-03-24 09:04:26. Version ac07

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