Lord Fairfax Community College
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lord Fairfax Community College clear sky chart from 2005-11-06 to 2024-01-13.

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 5% 5% 3 5% 5% 6% 7% 41%	74638
	Night	13% 6% 3 5% 5% 3 5% 5% 6% 7% 43%	75091
Transparency	Day	41% 4 12% 23% 16% 3	74354
	Night	40% 4 12% 24% 17% 3	74895
Seeing	Day	19% 12% 24% 24% 15% 6%	13788
	Night	17% 10% 25% 33% 14% 1	23518

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% 7% 3 7% 5% 4 5% 5% 5% 6% 42%	1646
	11th to 20th	Day	13% 7% 3 6% 5% 4 5% 5% 5% 6% 40%	1638
	21st to end	Day	13% 9% 3 8% 6% 3 4 4 5% 5% 40%	1782
February	1st to 10th	Day	12% 10% 3 6% 7% 4 5% 5% 6% 6% 37%	1620
	11th to 20th	Day	16% 7% 4 6% 4 3 5% 5% 5% 6% 39%	1573
	21st to end	Day	15% 8% 3 5% 6% 3 4 5% 5% 6% 40%	1507
March	1st to 10th	Day	12% 6% 3 4 5% 3 4 4 4 6% 49%	1980
	11th to 20th	Day	14% 7% 3 5% 5% 3 4 5% 5% 7% 41%	1980
	21st to end	Day	16% 7% 3 7% 5% 3 4 5% 6% 7% 37%	2178
April	1st to 10th	Day	10% 6% 3 6% 6% 3 6% 5% 6% 8% 41%	2144
	11th to 20th	Day	12% 6% 2 6% 5% 3 4 4 5% 6% 47%	2134
	21st to end	Day	18% 5% 2 6% 6% 3 5% 5% 5% 6% 39%	2200
May	1st to 10th	Day	17% 6% 2 5% 6% 4 5% 5% 6% 8% 37%	2340
	11th to 20th	Day	19% 5% 3 6% 6% 3 5% 4 6% 8% 35%	2322
	21st to end	Day	14% 6% 2 6% 6% 3 5% 5% 5% 8% 40%	2725
June	1st to 10th	Day	12% 4 2 5% 5% 4 5% 5% 7% 8% 43%	2520
	11th to 20th	Day	10% 5% 2 5% 5% 4 7% 8% 6% 10% 38%	2520
	21st to end	Day	10% 4 2 4 5% 4 5% 7% 7% 10% 44%	2520
July	1st to 10th	Day	8% 6% 2 6% 6% 4 6% 5% 7% 8% 41%	2520
	11th to 20th	Day	6% 3 3 5% 6% 4 7% 6% 9% 10% 42%	2340
	21st to end	Day	10% 5% 2 5% 6% 4 6% 7% 8% 10% 36%	2547
August	1st to 10th	Day	9% 5% 3 6% 6% 5% 7% 8% 7% 9% 35%	2327
	11th to 20th	Day	11% 4 2 6% 6% 4 6% 7% 6% 9% 39%	2267
	21st to end	Day	10% 5% 2 5% 6% 3 5% 6% 7% 9% 42%	2376
September	1st to 10th	Day	15% 5% 2 5% 6% 3 4 3 5% 7% 45%	2160
	11th to 20th	Day	12% 5% 2 5% 5% 2 5% 4 6% 8% 47%	2111
	21st to end	Day	19% 6% 2 5% 5% 3 5% 4 5% 6% 40%	1904
October	1st to 10th	Day	16% 6% 2 4 5% 3 3 4 5% 7% 46%	1800
	11th to 20th	Day	15% 4 2 5% 4 2 4 4 5% 6% 47%	1800
	21st to end	Day	14% 5% 2 4 4 2 4 4 4 6% 49%	1980
November	1st to 10th	Day	10% 5% 2 4 5% 2 4 4 4 7% 52%	1840
	11th to 20th	Day	15% 5% 3 4 5% 3 4 5% 5% 6% 47%	1615
	21st to end	Day	14% 5% 2 4 5% 3 5% 5% 4 6% 48%	1520
December	1st to 10th	Day	15% 7% 3 6% 4 3 6% 5% 6% 7% 38%	1520
	11th to 20th	Day	15% 6% 2 5% 6% 3 5% 5% 5% 7% 42%	1910
	21st to end	Day	17% 6% 2 5% 6% 4 6% 6% 6% 8% 34%	2772
January	1st to 10th	Night	12% 7% 3 6% 5% 3 5% 5% 6% 7% 44%	2516
	11th to 20th	Night	16% 7% 3 6% 5% 4 5% 5% 5% 6% 39%	2367
	21st to end	Night	13% 8% 3 5% 5% 2 4 4 5% 7% 43%	2574
February	1st to 10th	Night	14% 7% 4 5% 6% 3 4 5% 5% 7% 40%	2340
	11th to 20th	Night	15% 8% 3 5% 5% 3 5% 6% 5% 6% 40%	2273
	21st to end	Night	16% 9% 3 6% 6% 3 5% 4 4 6% 39%	1747
March	1st to 10th	Night	12% 5% 2 4 4 3 5% 5% 5% 5% 49%	1980
	11th to 20th	Night	17% 6% 2 5% 4 3 4 4 5% 7% 43%	1980
	21st to end	Night	15% 7% 3 5% 5% 3 4 4 5% 6% 42%	2178
April	1st to 10th	Night	13% 7% 3 6% 6% 3 4 5% 6% 7% 40%	1814
	11th to 20th	Night	11% 5% 2 5% 6% 3 4 4 6% 8% 45%	1775
	21st to end	Night	16% 4 2 4 5% 2 5% 5% 5% 7% 44%	1775
May	1st to 10th	Night	16% 6% 2 4 5% 3 5% 5% 5% 7% 41%	1620
	11th to 20th	Night	15% 7% 3 6% 5% 3 5% 5% 6% 7% 39%	1568
	21st to end	Night	13% 5% 2 4 4 4 5% 5% 6% 10% 42%	1550
June	1st to 10th	Night	10% 3 2 4 5% 3 5% 7% 6% 10% 45%	1440
	11th to 20th	Night	10% 5% 3 4 6% 3 5% 5% 6% 9% 43%	1440
	21st to end	Night	8% 3 2 5% 5% 3 6% 5% 9% 11% 43%	1440
July	1st to 10th	Night	9% 5% 2 5% 6% 4 7% 5% 6% 10% 41%	1440
	11th to 20th	Night	6% 3 2 4 6% 4 6% 5% 7% 12% 46%	1615
	21st to end	Night	7% 4 2 5% 5% 3 6% 7% 8% 11% 42%	1771
August	1st to 10th	Night	7% 5% 2 6% 6% 5% 5% 6% 8% 9% 41%	1611
	11th to 20th	Night	10% 6% 3 5% 6% 4 5% 5% 6% 9% 42%	1616
	21st to end	Night	11% 3 3 4 7% 3 6% 5% 6% 9% 43%	1980
September	1st to 10th	Night	13% 6% 2 5% 5% 3 5% 5% 6% 7% 45%	1800
	11th to 20th	Night	11% 5% 2 5% 5% 3 4 3 4 6% 52%	1844
	21st to end	Night	18% 6% 2 4 4 3 4 4 4 6% 46%	2055
October	1st to 10th	Night	19% 5% 2 3 4 2 4 4 4 6% 47%	2160
	11th to 20th	Night	14% 4 2 4 4 2 4 4 5% 5% 51%	2160
	21st to end	Night	14% 5% 2 4 4 3 3 5% 5% 6% 50%	2376
November	1st to 10th	Night	10% 6% 2 4 5% 2 4 4 5% 6% 52%	2235
	11th to 20th	Night	13% 6% 3 5% 4 2 4 4 5% 7% 47%	2562
	21st to end	Night	13% 5% 2 5% 5% 3 4 4 5% 7% 46%	2660
December	1st to 10th	Night	15% 7% 3 6% 5% 3 4 4 5% 6% 41%	2660
	11th to 20th	Night	15% 7% 3 5% 5% 3 4 5% 5% 7% 42%	3285
	21st to end	Night	15% 6% 3 6% 6% 4 5% 6% 7% 8% 34%	4884

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	44% 4 8% 24% 17% 3	1647
	11th to 20th	Day	45% 3 13% 23% 15% 1	1638
	21st to end	Day	47% 3 10% 23% 15% 3	1782
February	1st to 10th	Day	47% 4 13% 21% 12% 3	1620
	11th to 20th	Day	46% 4 9% 23% 16% 2	1582
	21st to end	Day	45% 3 11% 20% 17% 4	1507
March	1st to 10th	Day	38% 3 10% 26% 20% 2	1980
	11th to 20th	Day	44% 3 10% 20% 18% 5%	1980
	21st to end	Day	47% 4 11% 19% 15% 4	2178
April	1st to 10th	Day	40% 5% 12% 19% 19% 5%	2144
	11th to 20th	Day	38% 4 10% 22% 20% 6%	2134
	21st to end	Day	45% 5% 10% 22% 14% 4	2200
May	1st to 10th	Day	45% 5% 13% 21% 14% 2	2340
	11th to 20th	Day	47% 5% 13% 19% 12% 3	2326
	21st to end	Day	42% 5% 15% 26% 11% 2	2725
June	1st to 10th	Day	38% 5% 14% 25% 16% 1	2520
	11th to 20th	Day	39% 8% 16% 22% 14% 2	2520
	21st to end	Day	35% 5% 16% 27% 15% 2	2520
July	1st to 10th	Day	40% 6% 13% 23% 16% 3	2450
	11th to 20th	Day	35% 7% 14% 27% 15% 2	2271
	21st to end	Day	40% 7% 13% 22% 17% 2	2405
August	1st to 10th	Day	43% 6% 15% 24% 11% 1	2314
	11th to 20th	Day	41% 7% 14% 24% 14% 1	2267
	21st to end	Day	38% 6% 16% 25% 13% 3	2376
September	1st to 10th	Day	41% 4 13% 20% 19% 4	2160
	11th to 20th	Day	37% 4 9% 28% 20% 3	2111
	21st to end	Day	45% 4 7% 23% 18% 2	1904
October	1st to 10th	Day	38% 3 12% 26% 17% 4	1800
	11th to 20th	Day	39% 3 9% 26% 21% 3	1800
	21st to end	Day	37% 3 8% 26% 22% 4	1980
November	1st to 10th	Day	34% 3 11% 25% 23% 4	1840
	11th to 20th	Day	40% 2 10% 21% 22% 5%	1615
	21st to end	Day	39% 3 11% 28% 18% 2	1520
December	1st to 10th	Day	46% 4 13% 24% 11% 2	1520
	11th to 20th	Day	42% 3 10% 29% 12% 4	1906
	21st to end	Day	47% 4 14% 24% 11% 1	2772
January	1st to 10th	Night	40% 4 10% 23% 18% 4	2516
	11th to 20th	Night	45% 4 12% 23% 14% 2	2367
	21st to end	Night	42% 3 10% 26% 18% 2	2574
February	1st to 10th	Night	44% 4 12% 24% 14% 1	2340
	11th to 20th	Night	45% 4 9% 22% 16% 4	2275
	21st to end	Night	48% 3 10% 21% 14% 4	1747
March	1st to 10th	Night	36% 4 11% 29% 17% 3	1980
	11th to 20th	Night	42% 3 11% 22% 17% 4	1980
	21st to end	Night	43% 4 11% 21% 16% 5%	2178
April	1st to 10th	Night	42% 5% 10% 21% 18% 4	1814
	11th to 20th	Night	38% 4 12% 22% 19% 6%	1775
	21st to end	Night	40% 5% 11% 26% 13% 4	1775
May	1st to 10th	Night	43% 5% 13% 22% 14% 4	1620
	11th to 20th	Night	45% 6% 13% 20% 15% 1	1569
	21st to end	Night	38% 5% 19% 25% 11% 1	1551
June	1st to 10th	Night	34% 7% 12% 29% 15% 2	1440
	11th to 20th	Night	37% 6% 16% 24% 14% 1	1440
	21st to end	Night	33% 6% 18% 25% 17% 2	1440
July	1st to 10th	Night	39% 6% 17% 21% 13% 4	1400
	11th to 20th	Night	31% 8% 18% 25% 16% 3	1562
	21st to end	Night	31% 7% 19% 25% 16% 1	1672
August	1st to 10th	Night	37% 7% 17% 25% 13% 1	1600
	11th to 20th	Night	39% 6% 13% 26% 15% 1	1616
	21st to end	Night	37% 5% 14% 26% 14% 3	1980
September	1st to 10th	Night	38% 5% 12% 22% 18% 4	1800
	11th to 20th	Night	34% 4 8% 30% 22% 2	1844
	21st to end	Night	41% 4 10% 24% 19% 2	2055
October	1st to 10th	Night	40% 3 11% 26% 18% 2	2160
	11th to 20th	Night	35% 4 9% 27% 23% 3	2160
	21st to end	Night	36% 3 9% 26% 24% 3	2376
November	1st to 10th	Night	33% 3 11% 26% 22% 5%	2235
	11th to 20th	Night	38% 3 7% 25% 22% 6%	2562
	21st to end	Night	38% 3 11% 25% 19% 3	2660
December	1st to 10th	Night	45% 4 10% 26% 12% 2	2660
	11th to 20th	Night	43% 3 10% 24% 17% 3	3288
	21st to end	Night	46% 4 16% 21% 13% 1	4884

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	28% 25% 26% 17% 4	241
	11th to 20th	Day	24% 25% 29% 17% 5%	246
	21st to end	Day	24% 28% 30% 13% 4 2	264
February	1st to 10th	Day	28% 23% 33% 10% 5%	240
	11th to 20th	Day	35% 27% 31% 7%	240
	21st to end	Day	29% 20% 29% 20% 2	337
March	1st to 10th	Day	23% 26% 25% 23% 3	420
	11th to 20th	Day	24% 22% 29% 16% 6% 2	420
	21st to end	Day	22% 18% 32% 21% 5% 2	462
April	1st to 10th	Day	20% 20% 29% 20% 9% 1	420
	11th to 20th	Day	17% 17% 26% 25% 10% 5%	412
	21st to end	Day	26% 14% 28% 19% 11% 2	420
May	1st to 10th	Day	23% 13% 29% 18% 11% 4	423
	11th to 20th	Day	22% 11% 27% 23% 12% 4	447
	21st to end	Day	19% 6% 23% 28% 19% 7%	485
June	1st to 10th	Day	16% 8% 22% 28% 20% 6%	450
	11th to 20th	Day	15% 7% 26% 26% 19% 7%	450
	21st to end	Day	13% 4 26% 30% 22% 6%	450
July	1st to 10th	Day	11% 2 18% 30% 26% 13%	445
	11th to 20th	Day	6% 2 18% 33% 30% 11%	444
	21st to end	Day	14% 3 20% 31% 25% 8%	488
August	1st to 10th	Day	10% 4 19% 25% 30% 12%	446
	11th to 20th	Day	13% 3 14% 32% 25% 14%	439
	21st to end	Day	9% 1 19% 30% 24% 16%	495
September	1st to 10th	Day	18% 6% 15% 23% 24% 14%	450
	11th to 20th	Day	14% 3 16% 31% 23% 13%	450
	21st to end	Day	24% 9% 18% 26% 18% 5%	450
October	1st to 10th	Day	19% 6% 18% 27% 20% 9%	450
	11th to 20th	Day	14% 14% 26% 26% 16% 4	450
	21st to end	Day	18% 12% 33% 23% 11% 3	495
November	1st to 10th	Day	17% 11% 27% 27% 15% 3	446
	11th to 20th	Day	19% 17% 26% 25% 11% 1	275
	21st to end	Day	18% 19% 34% 22% 6% 1	180
December	1st to 10th	Day	21% 19% 32% 18% 9% 1	180
	11th to 20th	Day	26% 13% 30% 23% 8%	180
	21st to end	Day	21% 14% 30% 25% 10% 1	198
January	1st to 10th	Night	17% 18% 34% 27% 4	760
	11th to 20th	Night	21% 14% 30% 30% 5%	729
	21st to end	Night	18% 17% 37% 27% 2	792
February	1st to 10th	Night	17% 17% 33% 31% 2	720
	11th to 20th	Night	22% 21% 32% 23% 1	703
	21st to end	Night	24% 16% 34% 23% 3	592
March	1st to 10th	Night	17% 19% 36% 25% 3	720
	11th to 20th	Night	23% 18% 28% 24% 5% 1	720
	21st to end	Night	18% 16% 35% 24% 6% 1	792
April	1st to 10th	Night	18% 20% 32% 25% 5%	720
	11th to 20th	Night	16% 12% 33% 27% 11% 1	711
	21st to end	Night	18% 12% 28% 33% 8%	695
May	1st to 10th	Night	21% 5% 25% 32% 15% 1	540
	11th to 20th	Night	19% 5% 23% 37% 16% 1	507
	21st to end	Night	15% 3 17% 42% 21% 2	389
June	1st to 10th	Night	11% 3 23% 39% 24% 1	360
	11th to 20th	Night	10% 5% 16% 43% 23% 3	360
	21st to end	Night	9% 4 16% 49% 21% 1	360
July	1st to 10th	Night	11% 2 12% 39% 31% 5%	350
	11th to 20th	Night	7% 1 8% 38% 41% 4	518
	21st to end	Night	8% 1 9% 45% 32% 4	591
August	1st to 10th	Night	9% 2 7% 45% 34% 4	537
	11th to 20th	Night	11% 2 9% 42% 33% 4	552
	21st to end	Night	12% 1 12% 40% 30% 4	792
September	1st to 10th	Night	17% 3 10% 35% 31% 5%	720
	11th to 20th	Night	13% 2 15% 36% 30% 5%	720
	21st to end	Night	21% 4 17% 33% 23% 2	720
October	1st to 10th	Night	22% 5% 14% 33% 23% 3	720
	11th to 20th	Night	16% 9% 23% 35% 15% 1	720
	21st to end	Night	18% 11% 25% 35% 10% 1	792
November	1st to 10th	Night	14% 9% 26% 37% 12% 1	740
	11th to 20th	Night	17% 14% 28% 30% 9% 1	760
	21st to end	Night	17% 14% 29% 32% 8% 1	760
December	1st to 10th	Night	20% 14% 30% 30% 7%	760
	11th to 20th	Night	18% 9% 34% 35% 4	760
	21st to end	Night	16% 13% 28% 39% 4	836

Page generated at 2024-01-10 01:25:02. Version ac07

Copyright 2024 Attilla Danko