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 2025-02-21.

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% 29%	108497
	Night	14% 11% 4 11% 7% 5% 5% 4 5% 6% 27%	110075
Transparency	Day	53% 5% 11% 19% 10% 1	112026
	Night	56% 5% 11% 18% 9% 1	114710
Seeing	Day	23% 17% 28% 20% 9% 3	15019
	Night	21% 13% 26% 29% 10% 1	24106

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% 13% 9% 23% 10% 4 4 3 4 4 16%	3597
	11th to 20th	Day	8% 13% 6% 18% 9% 6% 5% 4 5% 5% 21%	3570
	21st to end	Day	9% 14% 6% 13% 7% 7% 5% 4 5% 5% 25%	3960
February	1st to 10th	Day	11% 12% 6% 12% 8% 8% 5% 5% 6% 6% 22%	1735
	11th to 20th	Day	9% 12% 5% 16% 10% 7% 4 4 5% 5% 22%	1948
	21st to end	Day	12% 14% 4 12% 8% 6% 5% 4 5% 6% 25%	1570
March	1st to 10th	Day	20% 15% 5% 12% 6% 5% 4 3 4 4 21%	3116
	11th to 20th	Day	11% 16% 7% 13% 7% 5% 5% 3 4 5% 24%	3243
	21st to end	Day	16% 12% 6% 12% 5% 4 4 3 4 4 30%	4166
April	1st to 10th	Day	18% 15% 3 9% 5% 4 3 3 4 6% 29%	2280
	11th to 20th	Day	16% 11% 4 8% 5% 5% 4 4 5% 6% 32%	2254
	21st to end	Day	21% 12% 3 7% 5% 4 5% 4 5% 6% 28%	2460
May	1st to 10th	Day	21% 12% 3 8% 5% 5% 5% 3 5% 6% 26%	4160
	11th to 20th	Day	15% 9% 3 8% 7% 6% 5% 4 6% 8% 28%	4398
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	4837
June	1st to 10th	Day	22% 7% 2 8% 5% 5% 5% 4 6% 7% 28%	2660
	11th to 20th	Day	17% 6% 2 7% 6% 5% 5% 4 6% 9% 34%	2660
	21st to end	Day	18% 9% 3 8% 6% 5% 5% 5% 6% 8% 28%	2660
July	1st to 10th	Day	11% 6% 2 7% 4 5% 6% 5% 8% 10% 35%	4440
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 5% 9% 11% 34%	4440
	21st to end	Day	13% 6% 3 8% 5% 5% 6% 5% 7% 8% 35%	4814
August	1st to 10th	Day	14% 7% 3 8% 6% 6% 6% 6% 7% 9% 30%	4375
	11th to 20th	Day	13% 7% 3 8% 5% 6% 6% 5% 8% 10% 29%	3949
	21st to end	Day	12% 6% 2 7% 4 4 5% 5% 7% 10% 38%	4180
September	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 9% 44%	2280
	11th to 20th	Day	14% 7% 2 5% 4 3 4 3 5% 7% 46%	2148
	21st to end	Day	14% 8% 3 7% 4 4 6% 4 6% 8% 38%	2090
October	1st to 10th	Day	20% 10% 3 6% 5% 5% 4 3 5% 7% 31%	2045
	11th to 20th	Day	13% 12% 3 7% 5% 5% 5% 4 7% 7% 32%	1826
	21st to end	Day	19% 16% 4 8% 5% 5% 5% 3 4 5% 26%	1881
November	1st to 10th	Day	11% 11% 4 8% 6% 5% 4 4 6% 8% 33%	1761
	11th to 20th	Day	14% 14% 4 11% 6% 5% 5% 4 5% 6% 27%	1800
	21st to end	Day	16% 13% 5% 11% 6% 4 5% 4 5% 6% 24%	1648
December	1st to 10th	Day	11% 16% 7% 22% 8% 6% 5% 3 5% 5% 12%	2600
	11th to 20th	Day	15% 13% 6% 11% 7% 6% 4 4 6% 6% 24%	2986
	21st to end	Day	16% 14% 6% 12% 5% 5% 6% 4 5% 5% 21%	3960
January	1st to 10th	Night	10% 12% 7% 21% 12% 6% 5% 3 5% 5% 14%	6349
	11th to 20th	Night	7% 14% 6% 19% 11% 8% 6% 5% 6% 6% 13%	6315
	21st to end	Night	10% 13% 5% 13% 8% 7% 6% 4 5% 6% 22%	6996
February	1st to 10th	Night	10% 12% 5% 14% 9% 7% 6% 4 6% 6% 21%	2660
	11th to 20th	Night	9% 10% 5% 13% 10% 8% 6% 5% 6% 7% 22%	2338
	21st to end	Night	9% 12% 5% 13% 7% 7% 5% 4 5% 6% 29%	1885
March	1st to 10th	Night	19% 15% 4 8% 8% 5% 5% 3 5% 5% 24%	3724
	11th to 20th	Night	12% 14% 6% 11% 7% 6% 4 3 4 5% 28%	3698
	21st to end	Night	14% 16% 5% 11% 5% 4 4 3 3 4 31%	3466
April	1st to 10th	Night	18% 12% 5% 8% 4 4 5% 3 5% 6% 29%	1900
	11th to 20th	Night	17% 11% 3 7% 5% 4 4 4 5% 6% 33%	1875
	21st to end	Night	19% 11% 3 8% 5% 4 4 4 5% 6% 33%	1720
May	1st to 10th	Night	23% 12% 3 7% 4 5% 4 3 6% 7% 27%	2810
	11th to 20th	Night	18% 9% 3 8% 5% 5% 4 4 6% 6% 33%	2495
	21st to end	Night	14% 6% 2 5% 5% 5% 5% 4 6% 9% 39%	2738
June	1st to 10th	Night	18% 7% 3 7% 6% 4 6% 4 6% 7% 32%	1520
	11th to 20th	Night	18% 6% 3 6% 5% 5% 4 4 6% 7% 38%	1520
	21st to end	Night	17% 7% 2 7% 5% 4 4 5% 6% 7% 34%	1520
July	1st to 10th	Night	12% 6% 2 8% 5% 6% 6% 5% 7% 9% 33%	2520
	11th to 20th	Night	14% 5% 2 7% 4 4 5% 5% 7% 8% 38%	2520
	21st to end	Night	10% 7% 3 8% 5% 6% 5% 5% 6% 8% 38%	2739
August	1st to 10th	Night	15% 8% 3 8% 5% 5% 5% 4 7% 6% 33%	2523
	11th to 20th	Night	11% 8% 3 8% 5% 5% 4 4 6% 8% 39%	2874
	21st to end	Night	11% 5% 2 6% 5% 4 5% 6% 7% 8% 41%	3476
September	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 5% 7% 46%	1900
	11th to 20th	Night	12% 7% 3 4 3 4 4 4 5% 6% 48%	2031
	21st to end	Night	13% 7% 2 6% 4 4 5% 5% 5% 7% 41%	2090
October	1st to 10th	Night	19% 10% 3 6% 5% 5% 5% 4 5% 8% 32%	2133
	11th to 20th	Night	15% 12% 3 8% 5% 5% 4 3 6% 7% 33%	2352
	21st to end	Night	19% 13% 4 8% 5% 5% 4 4 5% 6% 26%	2717
November	1st to 10th	Night	13% 13% 3 10% 5% 4 6% 4 6% 6% 32%	2537
	11th to 20th	Night	14% 11% 5% 9% 6% 5% 4 4 5% 6% 30%	2600
	21st to end	Night	14% 11% 6% 13% 6% 6% 5% 4 5% 6% 24%	2750
December	1st to 10th	Night	13% 12% 8% 21% 8% 6% 5% 4 5% 5% 13%	4580
	11th to 20th	Night	18% 15% 6% 12% 6% 6% 5% 4 5% 7% 18%	5208
	21st to end	Night	15% 14% 6% 15% 7% 6% 4 4 4 7% 16%	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	71% 5% 7% 14% 3	3598
	11th to 20th	Day	64% 5% 7% 14% 7% 2	3600
	21st to end	Day	61% 4 8% 15% 12% 1	3960
February	1st to 10th	Day	61% 7% 8% 16% 7% 2	1735
	11th to 20th	Day	63% 5% 9% 15% 7% 1	1957
	21st to end	Day	59% 5% 10% 14% 10% 2	1570
March	1st to 10th	Day	68% 4 7% 10% 9% 1	3119
	11th to 20th	Day	61% 6% 9% 12% 10% 2	3243
	21st to end	Day	58% 3 5% 15% 16% 2	4166
April	1st to 10th	Day	58% 3 9% 18% 10% 2	2280
	11th to 20th	Day	52% 6% 9% 18% 14% 1	2254
	21st to end	Day	58% 5% 9% 16% 11% 2	2460
May	1st to 10th	Day	59% 6% 9% 18% 7% 1	4174
	11th to 20th	Day	51% 9% 14% 16% 8% 1	4402
	21st to end	Day	41% 9% 13% 27% 11% 1	4837
June	1st to 10th	Day	53% 7% 13% 17% 9%	2660
	11th to 20th	Day	47% 6% 10% 21% 13% 1	2660
	21st to end	Day	54% 8% 11% 19% 8% 1	2660
July	1st to 10th	Day	42% 5% 16% 26% 11%	4370
	11th to 20th	Day	41% 6% 19% 28% 6%	4365
	21st to end	Day	45% 7% 12% 26% 10%	4661
August	1st to 10th	Day	49% 6% 15% 21% 8%	4361
	11th to 20th	Day	48% 4 18% 24% 5%	3988
	21st to end	Day	40% 5% 18% 23% 13% 1	4180
September	1st to 10th	Day	37% 6% 12% 31% 13% 1	2280
	11th to 20th	Day	39% 4 10% 31% 16% 1	2148
	21st to end	Day	43% 7% 15% 22% 11% 2	2090
October	1st to 10th	Day	48% 4 11% 25% 12%	3396
	11th to 20th	Day	47% 5% 13% 23% 11%	3016
	21st to end	Day	48% 4 11% 27% 9% 1	3081
November	1st to 10th	Day	49% 7% 13% 20% 11%	1761
	11th to 20th	Day	58% 5% 11% 13% 11% 2	1800
	21st to end	Day	60% 6% 13% 15% 4	1648
December	1st to 10th	Day	76% 4 9% 8% 2	2600
	11th to 20th	Day	61% 4 8% 18% 9%	2986
	21st to end	Day	65% 4 7% 11% 11% 1	3960
January	1st to 10th	Night	71% 6% 11% 9% 3	6349
	11th to 20th	Night	70% 6% 9% 9% 4 1	6360
	21st to end	Night	63% 4 8% 13% 12% 1	6996
February	1st to 10th	Night	62% 6% 10% 14% 8% 1	2660
	11th to 20th	Night	59% 7% 9% 16% 9% 1	2340
	21st to end	Night	56% 5% 11% 17% 9% 2	1885
March	1st to 10th	Night	62% 5% 9% 14% 9% 2	3734
	11th to 20th	Night	59% 5% 8% 16% 11% 1	3699
	21st to end	Night	58% 4 5% 14% 16% 2	3466
April	1st to 10th	Night	56% 4 8% 19% 12% 2	1900
	11th to 20th	Night	52% 6% 9% 21% 12%	1875
	21st to end	Night	51% 6% 9% 18% 13% 3	1720
May	1st to 10th	Night	56% 6% 9% 18% 9% 1	2816
	11th to 20th	Night	51% 9% 12% 18% 9% 1	2496
	21st to end	Night	39% 9% 16% 20% 14% 1	2739
June	1st to 10th	Night	49% 9% 11% 21% 10%	1520
	11th to 20th	Night	45% 7% 14% 20% 11% 2	1520
	21st to end	Night	47% 7% 13% 23% 9% 1	1520
July	1st to 10th	Night	46% 6% 15% 20% 12% 1	2480
	11th to 20th	Night	41% 6% 16% 28% 9%	2472
	21st to end	Night	44% 6% 12% 28% 10%	2651
August	1st to 10th	Night	50% 6% 13% 23% 7%	2513
	11th to 20th	Night	44% 5% 19% 22% 9%	2901
	21st to end	Night	38% 5% 19% 25% 12%	3476
September	1st to 10th	Night	37% 7% 15% 30% 11%	1900
	11th to 20th	Night	37% 5% 14% 30% 13% 1	2031
	21st to end	Night	40% 6% 14% 24% 13% 2	2090
October	1st to 10th	Night	52% 5% 11% 24% 8% 1	3562
	11th to 20th	Night	49% 4 10% 23% 13%	3901
	21st to end	Night	50% 3 13% 25% 8% 1	4467
November	1st to 10th	Night	53% 6% 11% 19% 9% 2	2537
	11th to 20th	Night	54% 5% 10% 19% 11% 2	2600
	21st to end	Night	60% 6% 12% 17% 5%	2750
December	1st to 10th	Night	75% 4 9% 10% 2	4580
	11th to 20th	Night	67% 4 7% 12% 9%	5208
	21st to end	Night	69% 3 7% 13% 7% 1	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	21% 26% 30% 16% 7% 1	200
	11th to 20th	Day	27% 29% 25% 14% 6% 1	200
	21st to end	Day	23% 22% 30% 18% 8%	220
February	1st to 10th	Day	34% 27% 28% 8% 3	267
	11th to 20th	Day	29% 21% 33% 15% 3	496
	21st to end	Day	29% 22% 29% 16% 4	382
March	1st to 10th	Day	32% 16% 30% 18% 3	460
	11th to 20th	Day	28% 23% 29% 14% 5% 1	460
	21st to end	Day	31% 21% 25% 14% 7% 2	506
April	1st to 10th	Day	33% 18% 26% 14% 7% 1	460
	11th to 20th	Day	32% 22% 23% 15% 5% 2	452
	21st to end	Day	33% 21% 25% 12% 8% 2	460
May	1st to 10th	Day	32% 20% 24% 15% 6% 3	463
	11th to 20th	Day	20% 17% 33% 20% 7% 2	487
	21st to end	Day	17% 19% 34% 22% 8% 2	529
June	1st to 10th	Day	26% 16% 31% 18% 8% 2	490
	11th to 20th	Day	20% 16% 30% 21% 10% 2	490
	21st to end	Day	20% 16% 34% 19% 9% 2	490
July	1st to 10th	Day	16% 13% 29% 26% 13% 3	485
	11th to 20th	Day	16% 9% 33% 30% 10% 3	484
	21st to end	Day	14% 8% 37% 29% 10% 3	532
August	1st to 10th	Day	14% 11% 30% 27% 14% 4	486
	11th to 20th	Day	17% 9% 28% 24% 16% 6%	479
	21st to end	Day	14% 14% 26% 24% 15% 7%	539
September	1st to 10th	Day	15% 12% 26% 24% 15% 8%	490
	11th to 20th	Day	16% 12% 23% 18% 18% 14%	490
	21st to end	Day	18% 13% 27% 22% 13% 7%	490
October	1st to 10th	Day	26% 13% 23% 20% 12% 7%	490
	11th to 20th	Day	21% 19% 23% 24% 11% 3	450
	21st to end	Day	34% 15% 23% 22% 6%	429
November	1st to 10th	Day	23% 18% 22% 23% 12% 1	395
	11th to 20th	Day	25% 23% 24% 21% 7% 2	400
	21st to end	Day	33% 16% 26% 19% 5%	248
December	1st to 10th	Day	24% 21% 26% 20% 9% 1	200
	11th to 20th	Day	32% 20% 30% 15% 4	200
	21st to end	Day	27% 20% 27% 20% 5%	220
January	1st to 10th	Night	20% 19% 33% 23% 5%	800
	11th to 20th	Night	24% 18% 27% 27% 4	800
	21st to end	Night	16% 18% 33% 28% 4	880
February	1st to 10th	Night	19% 16% 33% 27% 4	800
	11th to 20th	Night	20% 19% 35% 22% 3	783
	21st to end	Night	21% 18% 31% 24% 5%	629
March	1st to 10th	Night	22% 15% 32% 26% 4	760
	11th to 20th	Night	23% 19% 28% 24% 7%	760
	21st to end	Night	25% 15% 28% 25% 8%	836
April	1st to 10th	Night	27% 16% 27% 24% 6%	760
	11th to 20th	Night	25% 19% 24% 23% 9%	751
	21st to end	Night	28% 15% 27% 23% 7%	580
May	1st to 10th	Night	25% 14% 27% 26% 7% 1	559
	11th to 20th	Night	18% 9% 33% 31% 8% 1	374
	21st to end	Night	17% 11% 24% 34% 14%	411
June	1st to 10th	Night	20% 8% 26% 36% 10%	380
	11th to 20th	Night	21% 6% 23% 36% 14% 1	380
	21st to end	Night	18% 6% 24% 39% 13% 1	380
July	1st to 10th	Night	16% 4 19% 45% 16% 1	370
	11th to 20th	Night	15% 2 19% 42% 20% 1	368
	21st to end	Night	14% 3 21% 44% 17%	416
August	1st to 10th	Night	13% 4 14% 44% 25%	382
	11th to 20th	Night	14% 6% 22% 35% 20% 3	615
	21st to end	Night	16% 6% 18% 36% 23% 2	836
September	1st to 10th	Night	15% 6% 23% 36% 18% 2	760
	11th to 20th	Night	18% 8% 16% 31% 23% 4	760
	21st to end	Night	18% 8% 23% 29% 19% 3	760
October	1st to 10th	Night	24% 10% 19% 30% 15% 2	760
	11th to 20th	Night	22% 11% 23% 30% 14% 1	760
	21st to end	Night	27% 14% 27% 26% 6%	836
November	1st to 10th	Night	23% 14% 26% 28% 9% 1	780
	11th to 20th	Night	22% 15% 28% 25% 9% 2	800
	21st to end	Night	25% 15% 25% 27% 7%	800
December	1st to 10th	Night	25% 18% 28% 23% 5% 1	800
	11th to 20th	Night	27% 17% 24% 27% 5%	800
	21st to end	Night	25% 18% 28% 24% 5%	880

Page generated at 2025-02-18 03:26:30. Version ac07

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