CAAL Farnham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CAAL Farnham clear sky chart from 2005-11-06 to 2026-06-01.

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	18% 11% 4 9% 5% 5% 5% 4 5% 7% 28%	132100
	Night	18% 14% 4 10% 5% 4 4 3 5% 5% 27%	131059
Transparency	Day	56% 5% 12% 19% 8% 1	135556
	Night	59% 5% 10% 17% 8% 1	135677
Seeing	Day	25% 14% 28% 21% 9% 2	16560
	Night	26% 11% 28% 26% 9% 1	25556

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	17% 21% 8% 15% 5% 5% 4 3 3 4 15%	4661
	11th to 20th	Day	11% 18% 8% 20% 8% 6% 5% 3 3 4 13%	4650
	21st to end	Day	11% 15% 5% 13% 10% 8% 5% 4 5% 6% 18%	5124
February	1st to 10th	Day	17% 16% 5% 12% 7% 6% 5% 3 4 5% 21%	1806
	11th to 20th	Day	14% 18% 6% 13% 5% 5% 5% 3 4 5% 22%	1999
	21st to end	Day	16% 17% 4 8% 8% 6% 4 3 5% 6% 22%	1730
March	1st to 10th	Day	19% 15% 6% 11% 5% 5% 6% 3 4 5% 21%	4576
	11th to 20th	Day	14% 15% 4 9% 5% 4 4 4 5% 6% 29%	4767
	21st to end	Day	20% 14% 5% 9% 4 4 4 3 4 4 29%	6102
April	1st to 10th	Day	23% 14% 3 8% 5% 4 5% 3 5% 5% 27%	2520
	11th to 20th	Day	22% 10% 3 7% 4 4 4 3 4 5% 34%	2494
	21st to end	Day	24% 9% 3 7% 5% 4 4 3 4 6% 31%	2725
May	1st to 10th	Day	29% 11% 3 8% 5% 4 5% 3 4 6% 22%	6249
	11th to 20th	Day	20% 7% 3 8% 5% 5% 5% 4 6% 7% 29%	6458
	21st to end	Day	22% 8% 3 7% 5% 4 5% 4 6% 7% 30%	7103
June	1st to 10th	Day	24% 8% 2 7% 4 4 4 4 6% 7% 30%	2800
	11th to 20th	Day	16% 5% 3 6% 4 5% 6% 4 7% 8% 36%	2800
	21st to end	Day	23% 7% 3 7% 4 4 4 4 5% 6% 31%	2800
July	1st to 10th	Day	14% 6% 3 8% 5% 5% 6% 5% 7% 9% 30%	6360
	11th to 20th	Day	13% 5% 2 6% 4 5% 5% 5% 8% 9% 38%	6360
	21st to end	Day	13% 6% 2 5% 4 4 4 4 6% 10% 43%	6926
August	1st to 10th	Day	16% 7% 3 7% 5% 5% 6% 4 7% 9% 30%	4563
	11th to 20th	Day	16% 6% 3 7% 5% 4 5% 3 6% 9% 36%	4122
	21st to end	Day	16% 7% 3 6% 3 4 4 4 6% 8% 39%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 4 5% 4 5% 7% 43%	2400
	11th to 20th	Day	16% 5% 2 6% 3 4 4 3 4 7% 47%	2281
	21st to end	Day	17% 7% 2 6% 4 4 4 3 7% 8% 39%	2200
October	1st to 10th	Day	22% 7% 3 8% 5% 4 4 3 6% 7% 32%	2117
	11th to 20th	Day	21% 8% 3 7% 6% 4 5% 3 6% 6% 30%	1923
	21st to end	Day	26% 13% 3 7% 5% 4 4 3 4 5% 26%	1980
November	1st to 10th	Day	21% 14% 3 7% 5% 4 5% 3 4 5% 29%	1851
	11th to 20th	Day	18% 16% 5% 9% 5% 4 5% 4 5% 6% 25%	1867
	21st to end	Day	23% 17% 5% 9% 6% 4 4 3 6% 5% 19%	1680
December	1st to 10th	Day	19% 23% 6% 15% 6% 5% 4 2 4 3 14%	2680
	11th to 20th	Day	21% 15% 6% 10% 5% 4 4 3 5% 5% 21%	3066
	21st to end	Day	22% 17% 5% 11% 6% 5% 5% 4 6% 5% 16%	4048
January	1st to 10th	Night	16% 24% 8% 13% 5% 4 3 2 3 4 18%	8234
	11th to 20th	Night	12% 21% 9% 18% 7% 6% 5% 3 4 4 12%	8235
	21st to end	Night	12% 15% 5% 14% 10% 6% 4 3 4 5% 21%	9057
February	1st to 10th	Night	16% 17% 6% 14% 7% 5% 4 3 5% 5% 19%	2809
	11th to 20th	Night	15% 16% 5% 13% 7% 5% 5% 3 5% 4 22%	2505
	21st to end	Night	17% 16% 5% 10% 6% 5% 4 2 3 4 28%	2076
March	1st to 10th	Night	21% 14% 4 9% 6% 4 3 3 4 4 28%	5484
	11th to 20th	Night	16% 16% 3 8% 6% 4 4 3 5% 6% 27%	5409
	21st to end	Night	17% 17% 5% 11% 5% 4 4 3 3 4 28%	5072
April	1st to 10th	Night	23% 12% 4 7% 5% 5% 4 3 4 5% 27%	2100
	11th to 20th	Night	25% 11% 3 7% 4 4 3 3 4 5% 32%	2075
	21st to end	Night	22% 10% 3 7% 4 3 4 3 4 5% 35%	1895
May	1st to 10th	Night	27% 11% 4 8% 5% 4 4 4 5% 6% 23%	3942
	11th to 20th	Night	24% 8% 3 8% 5% 4 4 3 5% 6% 29%	3655
	21st to end	Night	20% 10% 3 8% 5% 4 4 4 4 6% 31%	4014
June	1st to 10th	Night	25% 7% 2 5% 5% 4 5% 4 5% 7% 31%	1600
	11th to 20th	Night	17% 7% 2 7% 5% 4 4 4 5% 8% 38%	1600
	21st to end	Night	19% 9% 2 7% 6% 4 4 4 6% 7% 33%	1600
July	1st to 10th	Night	14% 8% 2 8% 5% 6% 5% 5% 7% 8% 31%	3600
	11th to 20th	Night	16% 7% 2 7% 4 4 4 4 6% 8% 38%	3600
	21st to end	Night	11% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	3926
August	1st to 10th	Night	20% 8% 2 7% 4 4 4 3 5% 7% 34%	2586
	11th to 20th	Night	16% 8% 3 8% 5% 5% 5% 4 6% 6% 36%	2890
	21st to end	Night	17% 6% 2 6% 4 4 5% 4 5% 6% 40%	3586
September	1st to 10th	Night	15% 6% 3 7% 4 3 4 4 6% 6% 42%	2000
	11th to 20th	Night	14% 5% 2 6% 4 3 4 3 5% 7% 47%	2118
	21st to end	Night	19% 7% 2 5% 5% 4 4 3 5% 5% 42%	2200
October	1st to 10th	Night	20% 7% 3 7% 4 4 4 3 5% 7% 36%	2280
	11th to 20th	Night	22% 9% 3 8% 5% 4 3 3 5% 6% 31%	2475
	21st to end	Night	26% 11% 3 8% 4 4 4 3 4 5% 27%	2860
November	1st to 10th	Night	22% 13% 3 6% 4 5% 4 4 5% 6% 29%	2667
	11th to 20th	Night	22% 13% 4 8% 5% 3 4 3 4 4 28%	2751
	21st to end	Night	22% 17% 4 10% 4 4 4 3 4 4 24%	2940
December	1st to 10th	Night	19% 24% 8% 13% 6% 4 3 2 3 4 12%	4720
	11th to 20th	Night	23% 17% 6% 13% 5% 4 3 2 3 3 20%	5348
	21st to end	Night	23% 18% 5% 10% 5% 4 3 3 6% 6% 18%	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	74% 5% 10% 10% 1	4662
	11th to 20th	Day	77% 5% 7% 7% 4	4680
	21st to end	Day	67% 6% 8% 10% 8%	5124
February	1st to 10th	Day	67% 6% 9% 13% 5% 1	1806
	11th to 20th	Day	65% 5% 13% 13% 4 1	2008
	21st to end	Day	64% 5% 10% 13% 6% 1	1730
March	1st to 10th	Day	67% 5% 7% 12% 7% 2	4579
	11th to 20th	Day	55% 5% 15% 15% 9% 1	4767
	21st to end	Day	59% 4 8% 16% 13% 1	6102
April	1st to 10th	Day	60% 5% 8% 17% 8% 1	2520
	11th to 20th	Day	53% 5% 9% 19% 12% 1	2494
	21st to end	Day	56% 5% 10% 17% 10% 2	2725
May	1st to 10th	Day	63% 5% 8% 16% 6% 1	6260
	11th to 20th	Day	53% 8% 15% 16% 8% 1	6462
	21st to end	Day	52% 7% 11% 20% 10% 1	7103
June	1st to 10th	Day	52% 7% 12% 18% 9% 1	2800
	11th to 20th	Day	44% 7% 13% 23% 11% 2	2800
	21st to end	Day	53% 7% 10% 20% 8% 1	2800
July	1st to 10th	Day	49% 5% 15% 24% 7%	6290
	11th to 20th	Day	42% 5% 14% 29% 9%	6285
	21st to end	Day	39% 5% 13% 35% 7%	6728
August	1st to 10th	Day	50% 6% 16% 21% 6%	4549
	11th to 20th	Day	47% 6% 19% 22% 6%	4162
	21st to end	Day	44% 5% 18% 20% 13% 1	4312
September	1st to 10th	Day	40% 7% 14% 29% 10%	2400
	11th to 20th	Day	38% 5% 13% 32% 12%	2281
	21st to end	Day	43% 7% 15% 25% 10%	2200
October	1st to 10th	Day	46% 5% 14% 28% 7%	3442
	11th to 20th	Day	50% 5% 15% 25% 5%	3113
	21st to end	Day	52% 4 17% 20% 6% 1	3180
November	1st to 10th	Day	58% 6% 11% 18% 6%	1851
	11th to 20th	Day	62% 6% 12% 13% 8%	1867
	21st to end	Day	67% 6% 14% 9% 3	1680
December	1st to 10th	Day	77% 4 9% 9% 1	2680
	11th to 20th	Day	66% 5% 8% 15% 5%	3066
	21st to end	Day	69% 5% 9% 10% 6%	4048
January	1st to 10th	Night	73% 4 10% 10% 2	8234
	11th to 20th	Night	77% 5% 8% 7% 4	8280
	21st to end	Night	68% 3 8% 11% 10%	9057
February	1st to 10th	Night	68% 5% 8% 13% 5%	2809
	11th to 20th	Night	65% 5% 9% 15% 6%	2507
	21st to end	Night	62% 4 9% 14% 10% 1	2076
March	1st to 10th	Night	60% 4 8% 14% 10% 3	5494
	11th to 20th	Night	58% 5% 13% 14% 9% 1	5410
	21st to end	Night	61% 4 7% 18% 9% 1	5072
April	1st to 10th	Night	59% 5% 9% 17% 9% 1	2100
	11th to 20th	Night	56% 5% 7% 18% 12% 1	2075
	21st to end	Night	52% 5% 10% 18% 13% 3	1895
May	1st to 10th	Night	62% 6% 9% 16% 6% 1	3948
	11th to 20th	Night	54% 6% 11% 20% 8% 1	3656
	21st to end	Night	53% 6% 8% 19% 13% 1	4015
June	1st to 10th	Night	52% 7% 10% 20% 10% 1	1600
	11th to 20th	Night	45% 7% 12% 20% 13% 3	1600
	21st to end	Night	49% 7% 13% 18% 12% 1	1600
July	1st to 10th	Night	50% 4 16% 21% 8%	3560
	11th to 20th	Night	44% 5% 13% 28% 9%	3552
	21st to end	Night	42% 6% 14% 26% 11%	3809
August	1st to 10th	Night	50% 6% 13% 23% 8%	2576
	11th to 20th	Night	50% 6% 16% 20% 8%	2916
	21st to end	Night	44% 5% 13% 23% 14% 1	3586
September	1st to 10th	Night	41% 7% 12% 27% 13% 1	2000
	11th to 20th	Night	36% 6% 15% 30% 13%	2118
	21st to end	Night	44% 6% 11% 23% 15% 1	2200
October	1st to 10th	Night	50% 5% 11% 25% 10%	3722
	11th to 20th	Night	50% 4 11% 27% 9%	4024
	21st to end	Night	54% 4 12% 18% 11% 1	4610
November	1st to 10th	Night	57% 5% 12% 18% 6% 1	2667
	11th to 20th	Night	59% 5% 11% 14% 8% 2	2751
	21st to end	Night	65% 5% 12% 13% 4	2940
December	1st to 10th	Night	79% 4 8% 7% 2	4720
	11th to 20th	Night	71% 4 7% 11% 7%	5348
	21st to end	Night	67% 4 8% 13% 8%	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	31% 15% 30% 16% 7%	220
	11th to 20th	Day	27% 24% 34% 11% 3	220
	21st to end	Day	25% 17% 35% 16% 6%	242
February	1st to 10th	Day	29% 25% 29% 14% 2 1	286
	11th to 20th	Day	28% 22% 32% 16% 3	487
	21st to end	Day	26% 21% 30% 19% 3	446
March	1st to 10th	Day	28% 21% 27% 20% 4	540
	11th to 20th	Day	29% 18% 33% 15% 5% 1	540
	21st to end	Day	31% 21% 22% 20% 6% 1	594
April	1st to 10th	Day	35% 15% 28% 16% 6% 1	540
	11th to 20th	Day	35% 16% 25% 14% 7% 3	532
	21st to end	Day	32% 14% 32% 13% 8% 1	540
May	1st to 10th	Day	32% 14% 29% 18% 7% 1	543
	11th to 20th	Day	23% 13% 26% 26% 10% 2	567
	21st to end	Day	22% 13% 33% 23% 8% 1	617
June	1st to 10th	Day	26% 13% 29% 23% 7% 2	530
	11th to 20th	Day	19% 11% 31% 27% 10% 1	530
	21st to end	Day	26% 9% 34% 20% 9% 2	530
July	1st to 10th	Day	15% 8% 29% 32% 13% 2	525
	11th to 20th	Day	17% 7% 32% 30% 11% 3	524
	21st to end	Day	19% 7% 29% 31% 13% 2	576
August	1st to 10th	Day	16% 7% 25% 32% 17% 2	526
	11th to 20th	Day	17% 7% 24% 30% 18% 5%	519
	21st to end	Day	19% 10% 25% 25% 15% 7%	583
September	1st to 10th	Day	16% 9% 30% 24% 15% 6%	530
	11th to 20th	Day	15% 9% 21% 25% 20% 10%	530
	21st to end	Day	18% 12% 25% 24% 16% 5%	530
October	1st to 10th	Day	22% 12% 29% 19% 13% 5%	530
	11th to 20th	Day	28% 17% 23% 20% 11% 1	487
	21st to end	Day	36% 15% 24% 19% 6%	462
November	1st to 10th	Day	33% 18% 26% 16% 6%	425
	11th to 20th	Day	30% 19% 25% 15% 9% 1	407
	21st to end	Day	33% 22% 29% 14% 2	220
December	1st to 10th	Day	36% 15% 24% 20% 4 1	220
	11th to 20th	Day	29% 25% 30% 12% 3	220
	21st to end	Day	32% 22% 26% 15% 4 1	242
January	1st to 10th	Night	30% 15% 30% 21% 3	840
	11th to 20th	Night	28% 16% 31% 19% 6%	840
	21st to end	Night	23% 13% 31% 25% 7%	924
February	1st to 10th	Night	26% 12% 33% 23% 5%	840
	11th to 20th	Night	24% 16% 34% 22% 4	823
	21st to end	Night	27% 13% 30% 26% 3	692
March	1st to 10th	Night	26% 15% 31% 24% 4	840
	11th to 20th	Night	31% 15% 26% 22% 6%	840
	21st to end	Night	28% 14% 27% 23% 6% 1	924
April	1st to 10th	Night	30% 13% 29% 23% 5%	840
	11th to 20th	Night	30% 14% 27% 22% 6%	831
	21st to end	Night	27% 14% 29% 23% 7%	635
May	1st to 10th	Night	26% 10% 32% 23% 8% 1	545
	11th to 20th	Night	24% 8% 32% 26% 9% 1	414
	21st to end	Night	21% 7% 32% 32% 7% 1	455
June	1st to 10th	Night	30% 7% 27% 27% 9% 1	400
	11th to 20th	Night	20% 7% 28% 33% 13%	400
	21st to end	Night	19% 8% 26% 36% 11% 1	400
July	1st to 10th	Night	14% 5% 29% 40% 11% 1	390
	11th to 20th	Night	13% 3 24% 43% 17% 1	388
	21st to end	Night	14% 4 25% 42% 15%	438
August	1st to 10th	Night	18% 4 18% 40% 19% 1	398
	11th to 20th	Night	16% 5% 19% 36% 22% 1	595
	21st to end	Night	18% 6% 21% 35% 19% 2	880
September	1st to 10th	Night	17% 7% 24% 35% 16% 1	800
	11th to 20th	Night	16% 7% 18% 33% 24% 3	800
	21st to end	Night	23% 6% 27% 26% 16% 2	800
October	1st to 10th	Night	23% 9% 26% 28% 13% 1	800
	11th to 20th	Night	28% 10% 26% 26% 8% 1	800
	21st to end	Night	34% 10% 29% 22% 5%	880
November	1st to 10th	Night	31% 13% 29% 21% 5% 1	820
	11th to 20th	Night	28% 16% 26% 20% 8% 1	840
	21st to end	Night	33% 16% 28% 18% 5% 1	840
December	1st to 10th	Night	34% 15% 27% 20% 4	840
	11th to 20th	Night	33% 14% 30% 17% 5% 1	840
	21st to end	Night	34% 16% 26% 19% 4	924

Page generated at 2026-05-29 01:53:31. Version ac07

Copyright © 2026 Attilla Danko.