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 2026-06-16.

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	14% 10% 4 10% 6% 5% 5% 4 6% 7% 28%	137546
	Night	13% 11% 4 11% 7% 6% 5% 4 5% 6% 27%	134484
Transparency	Day	54% 5% 11% 20% 10% 1	141029
	Night	57% 5% 11% 18% 9% 1	139090
Seeing	Day	24% 17% 28% 20% 9% 3	16749
	Night	21% 13% 26% 29% 10% 1	25721

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	9% 12% 8% 22% 12% 6% 5% 3 4 4 16%	4661
	11th to 20th	Day	7% 13% 6% 19% 11% 7% 5% 4 5% 5% 18%	4650
	21st to end	Day	8% 13% 5% 13% 10% 9% 6% 4 6% 5% 22%	5124
February	1st to 10th	Day	10% 11% 5% 12% 8% 9% 5% 5% 6% 6% 23%	1822
	11th to 20th	Day	8% 12% 5% 16% 10% 7% 4 4 5% 5% 23%	2048
	21st to end	Day	11% 13% 5% 13% 8% 6% 5% 4 5% 6% 24%	1730
March	1st to 10th	Day	17% 15% 5% 15% 6% 6% 4 3 4 4 23%	4576
	11th to 20th	Day	11% 13% 6% 11% 6% 5% 5% 4 6% 6% 30%	4747
	21st to end	Day	18% 12% 6% 11% 5% 4 4 4 4 5% 27%	6102
April	1st to 10th	Day	18% 15% 3 9% 5% 4 3 3 4 6% 29%	2520
	11th to 20th	Day	16% 12% 4 8% 5% 5% 5% 4 5% 5% 31%	2494
	21st to end	Day	20% 12% 3 7% 5% 4 5% 4 5% 6% 30%	2719
May	1st to 10th	Day	25% 13% 3 8% 5% 5% 5% 3 5% 6% 21%	7734
	11th to 20th	Day	15% 9% 3 8% 6% 6% 5% 4 6% 8% 29%	8221
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 4 7% 7% 30%	9060
June	1st to 10th	Day	21% 7% 2 8% 5% 5% 5% 4 6% 7% 28%	2940
	11th to 20th	Day	16% 6% 2 7% 6% 5% 5% 4 6% 9% 34%	2870
	21st to end	Day	18% 8% 3 8% 6% 5% 5% 5% 6% 8% 29%	2800
July	1st to 10th	Day	10% 7% 2 7% 5% 6% 6% 5% 8% 10% 33%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 5% 8% 10% 35%	6360
	21st to end	Day	12% 5% 2 7% 4 5% 5% 5% 7% 9% 38%	6926
August	1st to 10th	Day	13% 7% 3 8% 6% 6% 6% 6% 7% 9% 31%	4514
	11th to 20th	Day	13% 7% 3 8% 5% 6% 5% 5% 8% 10% 30%	4076
	21st to end	Day	11% 6% 2 7% 4 4 5% 5% 7% 10% 38%	4312
September	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 45%	2400
	11th to 20th	Day	13% 7% 2 5% 4 4 4 3 5% 7% 46%	2261
	21st to end	Day	14% 8% 3 6% 4 4 6% 4 6% 8% 38%	2200
October	1st to 10th	Day	19% 10% 3 6% 5% 5% 4 4 5% 7% 33%	2152
	11th to 20th	Day	13% 12% 3 7% 5% 5% 5% 4 7% 8% 32%	1922
	21st to end	Day	19% 16% 4 8% 5% 5% 5% 3 4 5% 25%	1980
November	1st to 10th	Day	12% 12% 4 8% 6% 5% 4 4 6% 7% 32%	1851
	11th to 20th	Day	13% 15% 4 11% 6% 5% 5% 3 5% 6% 27%	1890
	21st to end	Day	16% 13% 5% 12% 6% 5% 5% 4 5% 6% 23%	1730
December	1st to 10th	Day	11% 16% 7% 21% 8% 6% 5% 3 5% 5% 12%	2680
	11th to 20th	Day	15% 13% 6% 11% 7% 6% 4 4 6% 6% 24%	3066
	21st to end	Day	15% 14% 6% 12% 6% 5% 6% 4 5% 5% 21%	4048
January	1st to 10th	Night	9% 11% 7% 21% 12% 6% 5% 3 5% 5% 14%	8234
	11th to 20th	Night	8% 14% 6% 19% 11% 8% 6% 5% 6% 5% 13%	8235
	21st to end	Night	9% 11% 5% 13% 9% 11% 7% 5% 5% 6% 19%	9057
February	1st to 10th	Night	9% 12% 5% 15% 9% 8% 6% 4 6% 6% 21%	2793
	11th to 20th	Night	8% 10% 5% 13% 10% 8% 6% 5% 6% 7% 22%	2458
	21st to end	Night	9% 12% 5% 13% 7% 6% 5% 4 5% 6% 28%	2076
March	1st to 10th	Night	16% 14% 5% 11% 8% 7% 5% 4 5% 5% 21%	5484
	11th to 20th	Night	12% 12% 5% 10% 6% 6% 5% 3 5% 6% 29%	5414
	21st to end	Night	16% 15% 5% 12% 5% 4 4 3 4 4 27%	5072
April	1st to 10th	Night	18% 12% 5% 9% 4 4 5% 3 5% 6% 29%	2100
	11th to 20th	Night	17% 12% 3 7% 5% 4 4 4 5% 6% 32%	2075
	21st to end	Night	18% 10% 3 7% 5% 4 4 4 5% 6% 34%	1901
May	1st to 10th	Night	24% 12% 3 9% 5% 4 4 3 5% 6% 25%	5208
	11th to 20th	Night	16% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	4639
	21st to end	Night	17% 8% 3 7% 5% 5% 4 4 6% 8% 33%	5114
June	1st to 10th	Night	17% 7% 3 7% 6% 4 5% 4 6% 7% 34%	1680
	11th to 20th	Night	17% 6% 3 6% 5% 5% 4 4 6% 7% 38%	1640
	21st to end	Night	17% 6% 3 7% 5% 4 4 5% 6% 7% 35%	1600
July	1st to 10th	Night	11% 6% 2 8% 5% 5% 6% 5% 7% 10% 34%	3600
	11th to 20th	Night	13% 5% 3 9% 5% 5% 6% 5% 6% 8% 35%	3600
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 5% 6% 9% 43%	3926
August	1st to 10th	Night	15% 8% 3 8% 5% 5% 5% 4 6% 6% 34%	2603
	11th to 20th	Night	11% 8% 3 8% 5% 5% 4 3 6% 8% 40%	2967
	21st to end	Night	11% 5% 2 6% 5% 4 5% 6% 7% 8% 41%	3586
September	1st to 10th	Night	11% 7% 2 6% 4 4 4 4 5% 7% 47%	2000
	11th to 20th	Night	12% 7% 3 4 3 4 4 4 5% 6% 48%	2138
	21st to end	Night	13% 7% 2 6% 4 4 5% 5% 5% 7% 41%	2200
October	1st to 10th	Night	18% 9% 3 6% 4 5% 4 4 5% 8% 35%	2245
	11th to 20th	Night	14% 11% 3 8% 5% 5% 4 3 6% 7% 33%	2476
	21st to end	Night	19% 13% 4 8% 5% 5% 5% 4 5% 6% 26%	2860
November	1st to 10th	Night	14% 13% 3 9% 5% 4 5% 3 6% 5% 31%	2667
	11th to 20th	Night	14% 12% 5% 9% 6% 5% 4 4 5% 6% 30%	2730
	21st to end	Night	14% 11% 6% 13% 6% 6% 5% 4 5% 6% 24%	2888
December	1st to 10th	Night	13% 12% 8% 21% 8% 6% 5% 4 5% 5% 13%	4720
	11th to 20th	Night	17% 14% 6% 12% 7% 6% 5% 4 5% 7% 18%	5348
	21st to end	Night	15% 14% 6% 15% 7% 6% 4 4 4 7% 16%	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	73% 4 7% 13% 3	4662
	11th to 20th	Day	67% 4 7% 13% 7% 2	4680
	21st to end	Day	63% 4 7% 15% 11% 1	5124
February	1st to 10th	Day	60% 6% 8% 16% 7% 2	1822
	11th to 20th	Day	63% 5% 9% 16% 7% 1	2057
	21st to end	Day	61% 5% 10% 14% 10% 2	1730
March	1st to 10th	Day	67% 3 6% 10% 9% 4	4579
	11th to 20th	Day	53% 4 14% 17% 10% 2	4747
	21st to end	Day	60% 3 5% 15% 16% 2	6102
April	1st to 10th	Day	58% 3 9% 18% 10% 2	2520
	11th to 20th	Day	54% 5% 9% 17% 14% 1	2494
	21st to end	Day	56% 4 8% 17% 11% 2	2719
May	1st to 10th	Day	65% 4 7% 15% 7% 1	7746
	11th to 20th	Day	52% 5% 14% 19% 8% 1	8225
	21st to end	Day	51% 5% 10% 25% 8%	9061
June	1st to 10th	Day	53% 7% 12% 18% 10%	2940
	11th to 20th	Day	47% 6% 11% 22% 13% 1	2870
	21st to end	Day	53% 8% 11% 19% 9% 1	2800
July	1st to 10th	Day	44% 4 17% 24% 10%	6290
	11th to 20th	Day	42% 6% 17% 26% 10% 1	6285
	21st to end	Day	41% 5% 12% 32% 9%	6728
August	1st to 10th	Day	48% 6% 16% 22% 8%	4500
	11th to 20th	Day	48% 4 18% 24% 5%	4115
	21st to end	Day	40% 5% 18% 23% 13% 1	4312
September	1st to 10th	Day	37% 6% 12% 31% 13% 1	2400
	11th to 20th	Day	38% 4 9% 31% 17% 1	2261
	21st to end	Day	43% 7% 14% 22% 12% 2	2200
October	1st to 10th	Day	47% 3 11% 25% 13% 1	3503
	11th to 20th	Day	47% 5% 13% 24% 11%	3112
	21st to end	Day	49% 4 10% 26% 9% 1	3180
November	1st to 10th	Day	51% 7% 12% 19% 11%	1851
	11th to 20th	Day	59% 5% 11% 13% 10% 2	1890
	21st to end	Day	61% 6% 13% 15% 4	1730
December	1st to 10th	Day	75% 4 9% 9% 3	2680
	11th to 20th	Day	61% 4 8% 18% 9%	3066
	21st to end	Day	65% 4 7% 12% 11% 1	4048
January	1st to 10th	Night	70% 5% 10% 11% 3	8234
	11th to 20th	Night	72% 5% 8% 9% 6% 1	8280
	21st to end	Night	65% 3 7% 13% 10% 1	9057
February	1st to 10th	Night	61% 6% 9% 15% 8% 1	2793
	11th to 20th	Night	59% 6% 9% 17% 8% 1	2460
	21st to end	Night	57% 5% 10% 18% 9% 2	2076
March	1st to 10th	Night	66% 3 7% 12% 9% 2	5494
	11th to 20th	Night	56% 4 15% 14% 9% 1	5415
	21st to end	Night	61% 3 4 15% 15% 2	5072
April	1st to 10th	Night	57% 4 8% 18% 12% 2	2100
	11th to 20th	Night	53% 5% 9% 20% 13%	2075
	21st to end	Night	51% 6% 8% 19% 14% 3	1901
May	1st to 10th	Night	61% 4 10% 16% 8% 1	5214
	11th to 20th	Night	50% 7% 12% 22% 8% 1	4640
	21st to end	Night	49% 6% 11% 24% 10% 1	5115
June	1st to 10th	Night	49% 8% 11% 22% 11%	1680
	11th to 20th	Night	45% 7% 14% 21% 11% 2	1640
	21st to end	Night	46% 7% 13% 24% 9% 1	1600
July	1st to 10th	Night	45% 5% 18% 22% 9%	3560
	11th to 20th	Night	44% 5% 14% 26% 10%	3552
	21st to end	Night	39% 5% 15% 33% 9%	3809
August	1st to 10th	Night	49% 6% 14% 23% 7%	2593
	11th to 20th	Night	44% 4 19% 23% 10% 1	2994
	21st to end	Night	38% 5% 19% 25% 12%	3586
September	1st to 10th	Night	37% 7% 15% 30% 11%	2000
	11th to 20th	Night	36% 5% 14% 30% 14% 1	2138
	21st to end	Night	41% 6% 14% 24% 14% 2	2200
October	1st to 10th	Night	51% 5% 11% 24% 9% 1	3674
	11th to 20th	Night	49% 4 11% 23% 13%	4025
	21st to end	Night	51% 3 13% 25% 8% 1	4610
November	1st to 10th	Night	54% 6% 11% 19% 9% 1	2667
	11th to 20th	Night	55% 5% 10% 18% 10% 2	2730
	21st to end	Night	60% 5% 12% 16% 5%	2888
December	1st to 10th	Night	74% 4 9% 11% 2	4720
	11th to 20th	Night	67% 4 7% 12% 9%	5348
	21st to end	Night	70% 3 6% 13% 7% 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	21% 25% 29% 17% 7% 1	220
	11th to 20th	Day	25% 30% 27% 12% 5% 1	220
	21st to end	Day	23% 22% 30% 17% 7%	242
February	1st to 10th	Day	31% 27% 29% 10% 3 1	294
	11th to 20th	Day	27% 20% 32% 17% 3	536
	21st to end	Day	28% 22% 30% 15% 4	446
March	1st to 10th	Day	32% 18% 31% 17% 3	540
	11th to 20th	Day	30% 23% 27% 14% 5% 1	540
	21st to end	Day	32% 21% 25% 13% 6% 2	594
April	1st to 10th	Day	36% 19% 25% 14% 6% 1	540
	11th to 20th	Day	34% 21% 23% 14% 5% 2	532
	21st to end	Day	32% 21% 25% 12% 8% 2	540
May	1st to 10th	Day	35% 19% 24% 14% 5% 2	543
	11th to 20th	Day	22% 18% 32% 19% 7% 2	567
	21st to end	Day	19% 19% 33% 20% 7% 1	617
June	1st to 10th	Day	26% 14% 30% 19% 9% 2	570
	11th to 20th	Day	19% 16% 32% 21% 9% 2	550
	21st to end	Day	20% 15% 35% 20% 9% 2	530
July	1st to 10th	Day	16% 12% 30% 26% 13% 3	525
	11th to 20th	Day	15% 8% 33% 29% 11% 3	524
	21st to end	Day	15% 8% 37% 28% 10% 3	576
August	1st to 10th	Day	13% 10% 29% 29% 14% 6%	526
	11th to 20th	Day	16% 9% 28% 24% 16% 8%	519
	21st to end	Day	14% 14% 27% 24% 15% 7%	583
September	1st to 10th	Day	15% 12% 26% 24% 15% 9%	530
	11th to 20th	Day	15% 11% 24% 18% 19% 14%	530
	21st to end	Day	19% 12% 27% 22% 13% 6%	530
October	1st to 10th	Day	24% 12% 22% 21% 12% 8%	530
	11th to 20th	Day	21% 19% 23% 24% 10% 3	486
	21st to end	Day	34% 14% 23% 22% 7%	462
November	1st to 10th	Day	25% 18% 22% 23% 11% 1	425
	11th to 20th	Day	24% 22% 25% 21% 7% 2	430
	21st to end	Day	33% 17% 26% 19% 5%	270
December	1st to 10th	Day	24% 21% 25% 20% 9%	220
	11th to 20th	Day	31% 21% 31% 14% 3	220
	21st to end	Day	27% 21% 26% 21% 5%	242
January	1st to 10th	Night	20% 19% 33% 24% 5%	840
	11th to 20th	Night	24% 18% 27% 27% 4	840
	21st to end	Night	16% 18% 33% 27% 5% 1	924
February	1st to 10th	Night	18% 16% 33% 28% 5%	840
	11th to 20th	Night	20% 18% 35% 23% 4	823
	21st to end	Night	21% 17% 32% 25% 5%	692
March	1st to 10th	Night	22% 15% 33% 25% 4	840
	11th to 20th	Night	24% 19% 27% 23% 6%	840
	21st to end	Night	26% 14% 29% 24% 7%	924
April	1st to 10th	Night	28% 16% 28% 23% 6%	840
	11th to 20th	Night	27% 17% 25% 23% 8%	831
	21st to end	Night	27% 15% 29% 23% 6%	641
May	1st to 10th	Night	26% 14% 28% 26% 6% 1	617
	11th to 20th	Night	18% 9% 33% 32% 8%	414
	21st to end	Night	19% 11% 24% 33% 13%	455
June	1st to 10th	Night	20% 7% 25% 37% 11%	420
	11th to 20th	Night	21% 5% 26% 34% 13% 1	410
	21st to end	Night	17% 6% 24% 39% 13% 1	400
July	1st to 10th	Night	15% 4 18% 45% 17% 1	390
	11th to 20th	Night	15% 2 19% 42% 20% 1	388
	21st to end	Night	13% 3 21% 45% 17%	438
August	1st to 10th	Night	13% 3 13% 44% 27%	402
	11th to 20th	Night	14% 5% 22% 35% 21% 3	648
	21st to end	Night	16% 5% 18% 36% 22% 2	880
September	1st to 10th	Night	15% 6% 23% 36% 18% 2	800
	11th to 20th	Night	17% 7% 16% 32% 24% 4	800
	21st to end	Night	18% 8% 23% 29% 20% 2	800
October	1st to 10th	Night	23% 9% 19% 31% 16% 2	800
	11th to 20th	Night	22% 11% 22% 31% 13% 1	800
	21st to end	Night	27% 14% 27% 26% 7%	880
November	1st to 10th	Night	24% 14% 25% 27% 8% 1	820
	11th to 20th	Night	22% 15% 28% 24% 9% 1	840
	21st to end	Night	25% 15% 25% 28% 7%	840
December	1st to 10th	Night	25% 18% 28% 24% 4	840
	11th to 20th	Night	26% 18% 25% 26% 5%	840
	21st to end	Night	26% 19% 27% 24% 5%	924

Page generated at 2026-06-13 02:31:44. Version ac07

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