Brundish Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brundish Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 30%	131692
	Night	14% 11% 4 10% 5% 5% 4 4 5% 6% 31%	129119
Transparency	Day	52% 4 11% 20% 11% 1	135286
	Night	52% 4 11% 20% 11% 1	133613
Seeing	Day	22% 14% 30% 21% 10% 4	16375
	Night	20% 14% 30% 25% 10% 1	25554

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	15% 17% 8% 17% 6% 4 4 3 3 5% 19%	4702
	11th to 20th	Day	9% 23% 11% 21% 9% 7% 3 2 3 4 7%	5246
	21st to end	Day	13% 16% 6% 19% 9% 6% 5% 3 4 4 14%	5780
February	1st to 10th	Day	14% 18% 6% 13% 8% 6% 5% 3 3 5% 18%	1890
	11th to 20th	Day	14% 13% 7% 14% 8% 7% 4 3 5% 6% 19%	1798
	21st to end	Day	17% 16% 5% 11% 6% 4 5% 3 5% 5% 21%	1575
March	1st to 10th	Day	18% 12% 6% 10% 7% 4 3 3 4 5% 28%	4918
	11th to 20th	Day	12% 13% 5% 9% 5% 5% 5% 4 5% 8% 29%	4980
	21st to end	Day	19% 13% 4 9% 5% 4 4 3 5% 5% 30%	5465
April	1st to 10th	Day	18% 13% 5% 9% 5% 4 4 3 5% 7% 28%	2326
	11th to 20th	Day	18% 12% 2 7% 4 5% 4 3 4 6% 34%	2495
	21st to end	Day	20% 11% 3 7% 5% 5% 4 4 5% 6% 32%	2600
May	1st to 10th	Day	18% 10% 3 8% 4 4 4 3 6% 6% 33%	5888
	11th to 20th	Day	17% 10% 3 8% 4 5% 4 3 5% 7% 32%	6130
	21st to end	Day	18% 7% 2 6% 4 5% 5% 3 5% 6% 38%	6949
June	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 9% 37%	2800
	11th to 20th	Day	12% 6% 2 7% 6% 4 5% 5% 7% 9% 38%	2876
	21st to end	Day	14% 5% 2 6% 5% 5% 5% 4 6% 8% 38%	3000
July	1st to 10th	Day	10% 5% 3 6% 5% 5% 5% 4 7% 10% 40%	6691
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	6232
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 47%	6424
August	1st to 10th	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 37%	4234
	11th to 20th	Day	12% 8% 2 8% 5% 5% 5% 5% 7% 9% 34%	4174
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 5% 6% 9% 40%	4548
September	1st to 10th	Day	12% 6% 3 6% 6% 4 5% 4 7% 8% 40%	2400
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 6% 8% 45%	2226
	21st to end	Day	15% 7% 3 8% 4 6% 5% 4 6% 8% 34%	2192
October	1st to 10th	Day	18% 9% 3 7% 4 4 4 3 5% 5% 36%	2000
	11th to 20th	Day	21% 9% 4 9% 4 4 4 4 5% 6% 29%	2000
	21st to end	Day	24% 13% 3 8% 5% 3 5% 5% 5% 7% 21%	2200
November	1st to 10th	Day	16% 11% 3 8% 5% 5% 5% 4 6% 7% 30%	1799
	11th to 20th	Day	15% 15% 4 10% 6% 4 5% 4 6% 7% 25%	1680
	21st to end	Day	18% 19% 7% 10% 6% 4 4 3 5% 6% 18%	1680
December	1st to 10th	Day	17% 23% 8% 12% 6% 5% 5% 4 4 5% 12%	2680
	11th to 20th	Day	21% 21% 5% 10% 7% 4 4 3 4 4 17%	3066
	21st to end	Day	26% 19% 6% 12% 6% 5% 4 3 4 4 11%	4048
January	1st to 10th	Night	14% 15% 7% 17% 7% 5% 4 4 5% 4 18%	8178
	11th to 20th	Night	10% 18% 8% 18% 11% 7% 5% 4 4 4 10%	7639
	21st to end	Night	13% 13% 7% 15% 8% 6% 5% 3 5% 7% 17%	8401
February	1st to 10th	Night	14% 15% 5% 11% 8% 5% 6% 4 5% 6% 21%	2730
	11th to 20th	Night	14% 14% 5% 11% 7% 5% 5% 4 5% 6% 23%	2599
	21st to end	Night	15% 12% 5% 12% 6% 5% 5% 4 5% 6% 25%	2055
March	1st to 10th	Night	15% 11% 5% 9% 5% 5% 5% 4 5% 7% 30%	4921
	11th to 20th	Night	9% 9% 3 9% 5% 4 4 5% 7% 8% 36%	4979
	21st to end	Night	19% 10% 3 7% 4 4 3 3 5% 5% 36%	5467
April	1st to 10th	Night	19% 11% 4 7% 5% 5% 4 3 5% 5% 32%	2073
	11th to 20th	Night	17% 10% 3 7% 5% 4 4 3 5% 6% 36%	1871
	21st to end	Night	15% 10% 3 7% 5% 4 4 4 5% 7% 36%	1800
May	1st to 10th	Night	18% 8% 3 7% 4 4 5% 3 6% 7% 36%	4053
	11th to 20th	Night	13% 9% 3 8% 5% 5% 5% 4 5% 7% 37%	3759
	21st to end	Night	17% 7% 2 5% 3 3 3 3 5% 8% 44%	3926
June	1st to 10th	Night	12% 6% 2 6% 4 5% 5% 4 6% 7% 44%	1600
	11th to 20th	Night	11% 5% 2 5% 4 5% 5% 4 7% 8% 42%	1516
	21st to end	Night	11% 6% 2 6% 5% 3 4 4 5% 8% 46%	1400
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 5% 9% 49%	3265
	11th to 20th	Night	10% 6% 2 5% 4 4 3 3 5% 8% 50%	3725
	21st to end	Night	8% 6% 2 5% 3 4 4 3 6% 8% 52%	4428
August	1st to 10th	Night	10% 5% 2 7% 4 4 4 3 5% 8% 46%	2915
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 6% 9% 45%	2875
	21st to end	Night	11% 4 2 5% 4 4 4 3 6% 6% 49%	3317
September	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 7% 7% 48%	2000
	11th to 20th	Night	10% 5% 1 5% 3 3 3 3 6% 7% 54%	2170
	21st to end	Night	16% 6% 3 6% 4 4 4 4 5% 7% 40%	2207
October	1st to 10th	Night	13% 9% 2 7% 4 4 4 3 6% 7% 42%	2400
	11th to 20th	Night	18% 10% 3 7% 5% 4 4 3 5% 6% 35%	2400
	21st to end	Night	21% 10% 3 8% 5% 5% 5% 3 5% 6% 30%	2640
November	1st to 10th	Night	15% 10% 3 8% 4 4 4 4 5% 7% 36%	2712
	11th to 20th	Night	15% 13% 4 9% 5% 5% 5% 4 6% 7% 26%	2940
	21st to end	Night	20% 14% 4 9% 5% 5% 4 4 5% 6% 23%	2940
December	1st to 10th	Night	14% 19% 8% 12% 6% 5% 5% 4 6% 6% 16%	4720
	11th to 20th	Night	16% 21% 6% 12% 7% 5% 4 3 4 5% 18%	5348
	21st to end	Night	23% 20% 7% 13% 5% 4 4 3 4 4 13%	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	69% 3 9% 9% 9% 1	4703
	11th to 20th	Day	83% 2 6% 6% 3	5280
	21st to end	Day	73% 3 10% 8% 5%	5780
February	1st to 10th	Day	70% 4 7% 14% 4 1	1890
	11th to 20th	Day	66% 3 7% 15% 7% 2	1807
	21st to end	Day	66% 2 10% 14% 7% 1	1575
March	1st to 10th	Day	59% 3 13% 15% 10% 1	4921
	11th to 20th	Day	53% 4 15% 19% 7% 3	4980
	21st to end	Day	57% 3 8% 20% 11% 1	5465
April	1st to 10th	Day	57% 3 10% 18% 11% 1	2326
	11th to 20th	Day	52% 3 10% 21% 13% 1	2495
	21st to end	Day	53% 4 9% 20% 11% 3	2600
May	1st to 10th	Day	52% 3 11% 18% 10% 6%	5897
	11th to 20th	Day	52% 4 11% 18% 13% 1	6134
	21st to end	Day	47% 4 11% 20% 14% 3	6949
June	1st to 10th	Day	44% 5% 11% 23% 16% 1	2800
	11th to 20th	Day	41% 5% 14% 25% 14% 1	2876
	21st to end	Day	42% 5% 13% 25% 15%	3000
July	1st to 10th	Day	37% 6% 14% 26% 16% 2	6620
	11th to 20th	Day	34% 5% 13% 32% 16% 1	6157
	21st to end	Day	33% 7% 13% 29% 18% 1	6240
August	1st to 10th	Day	43% 6% 13% 24% 13% 1	4221
	11th to 20th	Day	45% 5% 15% 26% 9%	4213
	21st to end	Day	37% 6% 13% 33% 10% 1	4548
September	1st to 10th	Day	40% 5% 14% 27% 12% 1	2400
	11th to 20th	Day	36% 5% 13% 32% 14% 1	2226
	21st to end	Day	46% 5% 13% 25% 9% 1	2192
October	1st to 10th	Day	42% 6% 13% 25% 13% 1	3260
	11th to 20th	Day	43% 5% 14% 21% 17%	3240
	21st to end	Day	53% 5% 14% 20% 7%	3538
November	1st to 10th	Day	53% 5% 12% 21% 8% 1	1799
	11th to 20th	Day	58% 5% 13% 16% 8% 1	1680
	21st to end	Day	67% 3 12% 13% 5%	1680
December	1st to 10th	Day	74% 5% 10% 9% 3	2680
	11th to 20th	Day	71% 3 9% 10% 6%	3066
	21st to end	Day	79% 3 6% 7% 5%	4048
January	1st to 10th	Night	66% 5% 10% 13% 6%	8178
	11th to 20th	Night	77% 3 7% 8% 4	7680
	21st to end	Night	67% 4 11% 11% 7%	8401
February	1st to 10th	Night	62% 4 9% 18% 5% 1	2730
	11th to 20th	Night	60% 4 10% 17% 9% 1	2601
	21st to end	Night	59% 3 11% 18% 8%	2055
March	1st to 10th	Night	53% 4 14% 17% 11% 2	4931
	11th to 20th	Night	45% 4 17% 22% 11% 2	4980
	21st to end	Night	51% 3 8% 20% 17% 2	5467
April	1st to 10th	Night	55% 3 8% 19% 14% 1	2073
	11th to 20th	Night	49% 4 7% 22% 16% 2	1871
	21st to end	Night	48% 3 9% 21% 16% 2	1800
May	1st to 10th	Night	48% 3 11% 16% 13% 8%	4060
	11th to 20th	Night	47% 5% 12% 21% 12% 2	3760
	21st to end	Night	41% 3 13% 24% 16% 2	3927
June	1st to 10th	Night	38% 5% 11% 28% 17% 2	1600
	11th to 20th	Night	37% 4 12% 26% 18% 2	1516
	21st to end	Night	37% 4 15% 27% 17% 1	1400
July	1st to 10th	Night	31% 6% 14% 24% 21% 3	3228
	11th to 20th	Night	32% 5% 11% 29% 20% 2	3677
	21st to end	Night	31% 6% 11% 29% 20% 2	4297
August	1st to 10th	Night	36% 5% 14% 26% 16% 2	2904
	11th to 20th	Night	36% 5% 16% 31% 12% 1	2902
	21st to end	Night	33% 5% 12% 33% 15% 1	3317
September	1st to 10th	Night	35% 5% 13% 31% 15% 1	2000
	11th to 20th	Night	29% 4 12% 34% 20% 1	2170
	21st to end	Night	44% 4 13% 26% 13% 1	2207
October	1st to 10th	Night	33% 4 10% 29% 23% 1	3920
	11th to 20th	Night	45% 4 11% 25% 15%	3899
	21st to end	Night	43% 5% 15% 27% 10%	4252
November	1st to 10th	Night	47% 4 12% 23% 13% 1	2712
	11th to 20th	Night	56% 4 13% 18% 8% 1	2940
	21st to end	Night	60% 5% 15% 14% 5%	2940
December	1st to 10th	Night	69% 5% 11% 13% 3	4720
	11th to 20th	Night	69% 4 9% 12% 6%	5348
	21st to end	Night	76% 3 9% 8% 5%	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	28% 20% 34% 14% 4 1	228
	11th to 20th	Day	29% 27% 30% 11% 3	330
	21st to end	Day	29% 23% 31% 15% 3	363
February	1st to 10th	Day	33% 20% 32% 11% 4	330
	11th to 20th	Day	27% 26% 39% 8%	315
	21st to end	Day	36% 24% 31% 8% 1	249
March	1st to 10th	Day	30% 16% 31% 18% 5%	486
	11th to 20th	Day	27% 18% 34% 16% 4 1	500
	21st to end	Day	32% 14% 32% 16% 4 1	550
April	1st to 10th	Day	26% 19% 32% 16% 6%	500
	11th to 20th	Day	32% 16% 24% 17% 8% 2	492
	21st to end	Day	25% 14% 35% 17% 7% 1	500
May	1st to 10th	Day	24% 14% 31% 19% 8% 4	503
	11th to 20th	Day	19% 12% 30% 25% 11% 4	527
	21st to end	Day	17% 11% 29% 25% 12% 6%	573
June	1st to 10th	Day	17% 14% 33% 23% 10% 3	530
	11th to 20th	Day	14% 11% 31% 29% 12% 3	606
	21st to end	Day	17% 9% 33% 28% 11% 3	730
July	1st to 10th	Day	11% 6% 29% 31% 18% 6%	673
	11th to 20th	Day	14% 7% 29% 26% 17% 7%	524
	21st to end	Day	10% 5% 34% 28% 17% 6%	576
August	1st to 10th	Day	12% 7% 22% 31% 19% 8%	526
	11th to 20th	Day	13% 6% 23% 27% 19% 11%	519
	21st to end	Day	15% 6% 25% 28% 19% 7%	583
September	1st to 10th	Day	13% 11% 24% 23% 19% 10%	530
	11th to 20th	Day	11% 5% 20% 28% 22% 14%	530
	21st to end	Day	17% 14% 25% 25% 12% 7%	530
October	1st to 10th	Day	22% 13% 22% 23% 14% 5%	530
	11th to 20th	Day	25% 15% 36% 15% 7% 1	530
	21st to end	Day	31% 16% 30% 17% 6%	583
November	1st to 10th	Day	32% 18% 31% 14% 5%	307
	11th to 20th	Day	29% 21% 31% 14% 3 3	220
	21st to end	Day	29% 24% 35% 10% 3	220
December	1st to 10th	Day	32% 22% 30% 12% 4	220
	11th to 20th	Day	35% 22% 29% 13% 1	220
	21st to end	Day	32% 26% 30% 10% 2	242
January	1st to 10th	Night	23% 24% 34% 15% 4	840
	11th to 20th	Night	29% 21% 29% 17% 3	840
	21st to end	Night	21% 19% 37% 18% 5%	924
February	1st to 10th	Night	23% 24% 31% 20% 2	840
	11th to 20th	Night	24% 20% 34% 18% 4	804
	21st to end	Night	22% 23% 35% 17% 3	660
March	1st to 10th	Night	21% 18% 36% 21% 5%	800
	11th to 20th	Night	24% 18% 36% 17% 6%	800
	21st to end	Night	23% 16% 31% 22% 7% 1	880
April	1st to 10th	Night	27% 15% 33% 22% 3	800
	11th to 20th	Night	24% 15% 32% 20% 8% 1	687
	21st to end	Night	18% 13% 35% 27% 7%	600
May	1st to 10th	Night	19% 11% 30% 27% 12% 2	600
	11th to 20th	Night	16% 7% 32% 33% 12% 1	474
	21st to end	Night	13% 8% 25% 35% 18%	433
June	1st to 10th	Night	14% 9% 30% 35% 12% 1	400
	11th to 20th	Night	12% 8% 34% 34% 13% 1	400
	21st to end	Night	14% 5% 30% 37% 14%	400
July	1st to 10th	Night	5% 4 25% 45% 19% 2	390
	11th to 20th	Night	9% 5% 29% 35% 20% 1	435
	21st to end	Night	11% 2 25% 40% 19% 2	657
August	1st to 10th	Night	8% 4 21% 41% 23% 2	597
	11th to 20th	Night	9% 4 21% 36% 27% 3	594
	21st to end	Night	13% 5% 24% 33% 23% 3	715
September	1st to 10th	Night	12% 7% 23% 34% 20% 5%	800
	11th to 20th	Night	12% 5% 21% 33% 24% 5%	800
	21st to end	Night	19% 9% 21% 28% 19% 5%	800
October	1st to 10th	Night	17% 9% 27% 27% 17% 3	800
	11th to 20th	Night	23% 13% 29% 26% 9% 1	800
	21st to end	Night	25% 16% 32% 23% 4	880
November	1st to 10th	Night	23% 14% 30% 24% 8% 1	820
	11th to 20th	Night	23% 22% 28% 20% 6% 1	840
	21st to end	Night	29% 19% 30% 17% 4	840
December	1st to 10th	Night	29% 21% 29% 16% 5%	840
	11th to 20th	Night	26% 24% 32% 15% 3	840
	21st to end	Night	32% 17% 33% 15% 2	924

Page generated at 2026-02-13 01:42:12. Version ac07

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