Fort Falstaff Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fort Falstaff Observatory clear sky chart from 2005-11-06 to 2024-01-31.

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	15% 10% 4 8% 7% 4 5% 5% 5% 7% 30%	74923
	Night	15% 10% 4 8% 7% 4 5% 5% 6% 7% 30%	75203
Transparency	Day	53% 6% 11% 19% 10% 1	74628
	Night	54% 6% 11% 19% 9% 1	75032
Seeing	Day	23% 17% 28% 20% 9% 3	13677
	Night	21% 13% 26% 29% 10% 1	22771

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	10% 11% 7% 15% 9% 4 6% 6% 5% 7% 20%	1517
	11th to 20th	Day	13% 13% 5% 12% 10% 4 5% 5% 5% 4 23%	1520
	21st to end	Day	9% 10% 6% 11% 9% 4 6% 6% 6% 6% 24%	1664
February	1st to 10th	Day	12% 11% 7% 10% 8% 6% 7% 6% 6% 7% 21%	1562
	11th to 20th	Day	9% 12% 6% 13% 10% 5% 6% 5% 5% 5% 23%	1748
	21st to end	Day	12% 13% 5% 11% 8% 5% 6% 5% 5% 5% 24%	1480
March	1st to 10th	Day	12% 12% 5% 11% 9% 4 7% 4 5% 5% 26%	1797
	11th to 20th	Day	14% 14% 6% 9% 8% 3 4 4 4 6% 27%	1841
	21st to end	Day	16% 11% 6% 9% 7% 4 4 4 4 5% 30%	2376
April	1st to 10th	Day	18% 13% 5% 8% 6% 3 4 3 4 6% 30%	2160
	11th to 20th	Day	16% 9% 4 7% 6% 3 5% 5% 5% 6% 33%	2134
	21st to end	Day	22% 12% 4 6% 6% 3 5% 5% 4 6% 28%	2330
May	1st to 10th	Day	19% 11% 4 7% 6% 3 5% 5% 4 5% 31%	2366
	11th to 20th	Day	17% 8% 3 6% 6% 4 4 5% 5% 7% 36%	2478
	21st to end	Day	15% 7% 3 6% 6% 4 5% 5% 6% 8% 34%	2725
June	1st to 10th	Day	23% 7% 2 7% 6% 4 6% 5% 5% 7% 28%	2520
	11th to 20th	Day	17% 6% 3 6% 6% 3 5% 6% 5% 9% 34%	2520
	21st to end	Day	18% 8% 4 7% 7% 3 6% 5% 6% 8% 28%	2520
July	1st to 10th	Day	14% 7% 2 5% 5% 3 6% 6% 6% 8% 38%	2520
	11th to 20th	Day	11% 5% 3 7% 8% 4 7% 6% 7% 10% 33%	2520
	21st to end	Day	15% 5% 3 6% 6% 4 6% 7% 6% 9% 33%	2744
August	1st to 10th	Day	12% 6% 3 7% 7% 4 6% 6% 6% 9% 35%	2485
	11th to 20th	Day	12% 6% 3 6% 6% 4 6% 5% 7% 10% 33%	2250
	21st to end	Day	13% 5% 3 6% 5% 4 5% 6% 7% 10% 37%	2376
September	1st to 10th	Day	13% 6% 2 4 5% 3 4 5% 6% 9% 42%	2160
	11th to 20th	Day	15% 7% 2 5% 4 2 3 4 4 7% 46%	2035
	21st to end	Day	13% 8% 3 5% 5% 3 5% 6% 5% 8% 39%	1980
October	1st to 10th	Day	21% 10% 3 6% 6% 4 4 5% 5% 6% 30%	1938
	11th to 20th	Day	13% 12% 3 7% 6% 4 5% 5% 7% 7% 31%	1730
	21st to end	Day	21% 16% 4 7% 6% 4 6% 4 4 5% 24%	1782
November	1st to 10th	Day	12% 10% 4 7% 7% 4 5% 5% 6% 7% 33%	1671
	11th to 20th	Day	15% 13% 6% 9% 8% 3 5% 5% 5% 6% 26%	1710
	21st to end	Day	16% 11% 6% 11% 7% 3 6% 5% 5% 6% 24%	1566
December	1st to 10th	Day	16% 15% 6% 12% 8% 4 6% 4 5% 7% 16%	1520
	11th to 20th	Day	18% 14% 6% 9% 6% 4 3 4 5% 6% 24%	1906
	21st to end	Day	20% 13% 6% 12% 8% 4 6% 5% 5% 4 18%	2772
January	1st to 10th	Night	12% 10% 5% 12% 10% 5% 8% 7% 7% 7% 16%	2649
	11th to 20th	Night	12% 13% 5% 11% 10% 5% 7% 6% 6% 7% 18%	2660
	21st to end	Night	9% 9% 6% 12% 10% 4 7% 6% 6% 7% 23%	2919
February	1st to 10th	Night	10% 11% 6% 13% 10% 6% 7% 5% 6% 6% 20%	2395
	11th to 20th	Night	9% 9% 5% 11% 11% 6% 8% 6% 6% 7% 22%	2098
	21st to end	Night	9% 11% 5% 12% 8% 6% 6% 5% 5% 6% 28%	1776
March	1st to 10th	Night	10% 12% 6% 9% 10% 5% 7% 5% 5% 5% 26%	2150
	11th to 20th	Night	13% 11% 5% 10% 8% 5% 6% 4 5% 6% 29%	2114
	21st to end	Night	13% 12% 4 7% 7% 4 5% 5% 4 5% 34%	1980
April	1st to 10th	Night	18% 11% 6% 8% 5% 3 4 4 5% 6% 30%	1800
	11th to 20th	Night	17% 9% 4 7% 6% 3 4 5% 5% 7% 34%	1775
	21st to end	Night	19% 10% 3 7% 5% 3 4 5% 5% 6% 33%	1630
May	1st to 10th	Night	19% 10% 4 7% 5% 3 4 4 6% 7% 31%	1610
	11th to 20th	Night	15% 7% 4 6% 4 3 4 5% 6% 8% 38%	1415
	21st to end	Night	15% 6% 3 5% 6% 3 5% 5% 7% 8% 38%	1550
June	1st to 10th	Night	18% 7% 3 6% 6% 3 6% 5% 6% 7% 33%	1440
	11th to 20th	Night	19% 5% 3 5% 5% 4 5% 5% 5% 7% 38%	1440
	21st to end	Night	17% 5% 3 6% 6% 3 5% 6% 6% 9% 35%	1440
July	1st to 10th	Night	15% 5% 2 5% 5% 3 4 5% 6% 9% 41%	1440
	11th to 20th	Night	12% 5% 2 5% 6% 3 5% 5% 7% 9% 41%	1440
	21st to end	Night	10% 5% 3 5% 4 3 5% 6% 7% 9% 41%	1574
August	1st to 10th	Night	10% 5% 2 4 5% 4 5% 6% 7% 8% 43%	1435
	11th to 20th	Night	12% 6% 3 5% 5% 3 4 4 7% 9% 43%	1646
	21st to end	Night	14% 6% 3 4 5% 2 4 5% 6% 7% 44%	1980
September	1st to 10th	Night	12% 6% 2 5% 5% 3 4 5% 5% 7% 46%	1800
	11th to 20th	Night	13% 7% 3 3 4 3 4 5% 5% 6% 47%	1924
	21st to end	Night	14% 7% 2 5% 5% 2 5% 6% 5% 7% 42%	1980
October	1st to 10th	Night	19% 9% 3 5% 5% 3 5% 5% 5% 7% 33%	2020
	11th to 20th	Night	15% 11% 3 7% 6% 3 5% 5% 6% 7% 32%	2228
	21st to end	Night	20% 13% 4 7% 6% 4 4 6% 5% 6% 25%	2574
November	1st to 10th	Night	13% 12% 4 8% 7% 3 5% 5% 6% 6% 32%	2407
	11th to 20th	Night	14% 9% 6% 9% 7% 4 4 5% 5% 6% 30%	2470
	21st to end	Night	14% 10% 7% 11% 7% 4 5% 5% 5% 6% 25%	2612
December	1st to 10th	Night	16% 13% 7% 13% 10% 4 6% 6% 5% 6% 15%	2660
	11th to 20th	Night	18% 13% 7% 11% 7% 4 5% 5% 5% 6% 19%	3288
	21st to end	Night	20% 12% 6% 15% 8% 4 5% 4 4 5% 17%	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	62% 7% 8% 17% 6%	1518
	11th to 20th	Day	63% 7% 9% 12% 8% 1	1520
	21st to end	Day	59% 5% 11% 15% 9% 1	1664
February	1st to 10th	Day	62% 7% 8% 15% 6% 2	1562
	11th to 20th	Day	62% 6% 9% 15% 7% 1	1757
	21st to end	Day	61% 5% 10% 12% 11% 2	1480
March	1st to 10th	Day	61% 5% 9% 14% 9% 2	1800
	11th to 20th	Day	58% 5% 10% 13% 12% 3	1841
	21st to end	Day	57% 4 7% 17% 12% 3	2376
April	1st to 10th	Day	57% 3 9% 18% 10% 2	2160
	11th to 20th	Day	51% 6% 10% 18% 14% 1	2134
	21st to end	Day	58% 5% 9% 16% 10% 2	2330
May	1st to 10th	Day	55% 5% 7% 19% 11% 2	2366
	11th to 20th	Day	49% 5% 9% 21% 15% 2	2482
	21st to end	Day	46% 8% 14% 20% 10% 1	2725
June	1st to 10th	Day	54% 7% 13% 16% 9%	2520
	11th to 20th	Day	47% 7% 10% 21% 14% 1	2520
	21st to end	Day	53% 8% 11% 18% 8% 1	2520
July	1st to 10th	Day	42% 7% 14% 24% 13%	2450
	11th to 20th	Day	44% 9% 16% 23% 9%	2445
	21st to end	Day	44% 9% 14% 22% 10%	2591
August	1st to 10th	Day	44% 8% 16% 22% 9%	2471
	11th to 20th	Day	45% 7% 18% 22% 9%	2250
	21st to end	Day	41% 7% 14% 24% 13% 1	2376
September	1st to 10th	Day	38% 7% 13% 29% 13% 1	2160
	11th to 20th	Day	40% 4 9% 30% 16% 1	2035
	21st to end	Day	42% 7% 15% 22% 11% 2	1980
October	1st to 10th	Day	54% 5% 10% 19% 11% 1	1938
	11th to 20th	Day	48% 8% 14% 22% 6%	1730
	21st to end	Day	64% 4 8% 15% 7% 1	1782
November	1st to 10th	Day	49% 7% 13% 19% 11%	1671
	11th to 20th	Day	59% 5% 11% 12% 10% 2	1710
	21st to end	Day	60% 7% 14% 15% 4 1	1566
December	1st to 10th	Day	68% 6% 10% 12% 4	1520
	11th to 20th	Day	61% 5% 10% 17% 8%	1906
	21st to end	Day	70% 5% 8% 10% 7%	2772
January	1st to 10th	Night	63% 8% 10% 12% 6%	2649
	11th to 20th	Night	63% 8% 11% 12% 6%	2660
	21st to end	Night	59% 6% 10% 15% 9% 1	2919
February	1st to 10th	Night	63% 6% 10% 12% 8% 1	2395
	11th to 20th	Night	59% 7% 9% 16% 9% 1	2100
	21st to end	Night	57% 5% 11% 17% 9% 2	1776
March	1st to 10th	Night	60% 5% 8% 16% 9% 2	2160
	11th to 20th	Night	57% 5% 10% 17% 10% 2	2115
	21st to end	Night	52% 5% 8% 18% 14% 3	1980
April	1st to 10th	Night	56% 4 8% 19% 12% 2	1800
	11th to 20th	Night	50% 6% 9% 21% 13%	1775
	21st to end	Night	52% 6% 9% 18% 12% 3	1630
May	1st to 10th	Night	52% 5% 7% 18% 15% 2	1610
	11th to 20th	Night	45% 6% 10% 20% 16% 2	1416
	21st to end	Night	43% 8% 15% 21% 12% 1	1551
June	1st to 10th	Night	50% 8% 11% 20% 10%	1440
	11th to 20th	Night	46% 7% 14% 20% 12% 2	1440
	21st to end	Night	45% 8% 14% 24% 9%	1440
July	1st to 10th	Night	38% 10% 15% 22% 14% 1	1400
	11th to 20th	Night	36% 9% 16% 28% 10%	1392
	21st to end	Night	37% 8% 16% 27% 11% 1	1486
August	1st to 10th	Night	36% 9% 17% 28% 10%	1425
	11th to 20th	Night	39% 6% 18% 26% 11%	1646
	21st to end	Night	38% 8% 16% 27% 12%	1980
September	1st to 10th	Night	37% 7% 16% 29% 10%	1800
	11th to 20th	Night	38% 6% 13% 29% 13% 1	1924
	21st to end	Night	40% 6% 14% 24% 13% 2	1980
October	1st to 10th	Night	51% 6% 11% 20% 11% 1	2020
	11th to 20th	Night	50% 7% 13% 22% 7%	2228
	21st to end	Night	60% 6% 10% 15% 8% 1	2574
November	1st to 10th	Night	53% 7% 11% 18% 9% 2	2407
	11th to 20th	Night	54% 5% 10% 19% 10% 2	2470
	21st to end	Night	60% 6% 12% 16% 5%	2612
December	1st to 10th	Night	70% 7% 9% 11% 3	2660
	11th to 20th	Night	65% 7% 9% 12% 7%	3288
	21st to end	Night	72% 4 7% 14% 3	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	20% 25% 29% 18% 7% 1	180
	11th to 20th	Day	28% 29% 23% 13% 5% 1	180
	21st to end	Day	21% 20% 32% 17% 10%	196
February	1st to 10th	Day	36% 29% 26% 8% 2	214
	11th to 20th	Day	27% 21% 33% 15% 3	416
	21st to end	Day	29% 23% 27% 16% 4	346
March	1st to 10th	Day	30% 17% 32% 18% 4	420
	11th to 20th	Day	28% 22% 29% 15% 5% 1	420
	21st to end	Day	30% 19% 26% 15% 8% 2	462
April	1st to 10th	Day	32% 18% 27% 15% 7% 1	420
	11th to 20th	Day	31% 21% 24% 16% 6% 3	412
	21st to end	Day	34% 20% 25% 12% 8% 2	420
May	1st to 10th	Day	33% 20% 24% 14% 6% 3	423
	11th to 20th	Day	21% 18% 34% 19% 6% 3	447
	21st to end	Day	18% 18% 33% 22% 8% 2	485
June	1st to 10th	Day	27% 15% 31% 18% 7% 2	450
	11th to 20th	Day	20% 16% 30% 21% 10% 3	450
	21st to end	Day	19% 14% 35% 20% 10% 2	450
July	1st to 10th	Day	17% 13% 28% 26% 13% 3	445
	11th to 20th	Day	16% 8% 33% 30% 10% 3	444
	21st to end	Day	15% 8% 38% 28% 10% 2	488
August	1st to 10th	Day	13% 11% 31% 27% 14% 4	446
	11th to 20th	Day	17% 8% 28% 24% 16% 6%	439
	21st to end	Day	14% 15% 27% 23% 14% 7%	495
September	1st to 10th	Day	15% 12% 25% 25% 15% 8%	450
	11th to 20th	Day	17% 13% 25% 18% 16% 11%	450
	21st to end	Day	18% 14% 28% 22% 13% 6%	450
October	1st to 10th	Day	27% 13% 23% 19% 12% 7%	450
	11th to 20th	Day	20% 20% 24% 24% 10% 2	414
	21st to end	Day	35% 15% 23% 22% 6%	396
November	1st to 10th	Day	22% 18% 22% 25% 12% 1	365
	11th to 20th	Day	25% 23% 24% 20% 6% 2	370
	21st to end	Day	31% 16% 26% 21% 6%	226
December	1st to 10th	Day	23% 23% 27% 18% 9% 1	180
	11th to 20th	Day	32% 19% 30% 16% 3	180
	21st to end	Day	29% 22% 26% 18% 6%	198
January	1st to 10th	Night	20% 18% 32% 24% 5%	760
	11th to 20th	Night	24% 18% 27% 27% 4	760
	21st to end	Night	16% 18% 32% 29% 5%	835
February	1st to 10th	Night	20% 16% 33% 27% 4	720
	11th to 20th	Night	19% 20% 34% 23% 4	703
	21st to end	Night	21% 19% 31% 25% 5%	592
March	1st to 10th	Night	21% 16% 33% 26% 4	720
	11th to 20th	Night	23% 18% 28% 24% 7%	720
	21st to end	Night	25% 14% 28% 25% 8% 1	792
April	1st to 10th	Night	28% 16% 26% 24% 7%	720
	11th to 20th	Night	24% 19% 24% 23% 9%	711
	21st to end	Night	28% 15% 28% 23% 6%	550
May	1st to 10th	Night	25% 15% 27% 26% 7% 1	530
	11th to 20th	Night	18% 9% 33% 31% 8% 1	354
	21st to end	Night	17% 11% 23% 34% 14%	389
June	1st to 10th	Night	21% 8% 27% 34% 11%	360
	11th to 20th	Night	21% 6% 24% 35% 13% 1	360
	21st to end	Night	17% 6% 23% 41% 13% 1	360
July	1st to 10th	Night	16% 4 19% 44% 16% 1	350
	11th to 20th	Night	14% 2 20% 43% 20% 1	348
	21st to end	Night	13% 3 22% 44% 18%	394
August	1st to 10th	Night	12% 4 14% 44% 26%	361
	11th to 20th	Night	14% 6% 21% 36% 20% 3	582
	21st to end	Night	16% 6% 18% 35% 23% 2	792
September	1st to 10th	Night	15% 6% 23% 36% 18% 2	720
	11th to 20th	Night	18% 8% 17% 31% 22% 4	720
	21st to end	Night	18% 8% 24% 28% 19% 3	720
October	1st to 10th	Night	24% 10% 19% 29% 15% 2	720
	11th to 20th	Night	21% 12% 22% 30% 14% 1	720
	21st to end	Night	27% 15% 27% 25% 6%	792
November	1st to 10th	Night	22% 15% 25% 28% 9% 1	740
	11th to 20th	Night	22% 16% 29% 24% 9% 1	760
	21st to end	Night	25% 16% 25% 27% 7%	760
December	1st to 10th	Night	26% 18% 29% 22% 4 1	760
	11th to 20th	Night	27% 17% 24% 27% 5%	760
	21st to end	Night	26% 18% 28% 23% 4	836

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