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 2024-07-03.

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	16% 11% 4 9% 5% 5% 5% 4 5% 7% 30%	90007
	Night	15% 11% 4 9% 5% 4 4 4 5% 6% 31%	89178
Transparency	Day	54% 4 12% 20% 10% 1	89734
	Night	53% 4 11% 20% 11% 1	89005
Seeing	Day	21% 13% 30% 21% 11% 4	14331
	Night	20% 14% 30% 25% 10% 1	23453

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	16% 20% 8% 11% 6% 4 3 3 2 6% 22%	2530
	11th to 20th	Day	11% 21% 9% 17% 8% 4 4 3 5% 6% 12%	2840
	21st to end	Day	20% 21% 8% 14% 6% 4 5% 3 3 4 13%	3124
February	1st to 10th	Day	15% 18% 5% 13% 9% 6% 5% 3 4 5% 17%	1710
	11th to 20th	Day	14% 14% 6% 14% 7% 7% 4 3 5% 6% 20%	1663
	21st to end	Day	18% 15% 5% 11% 6% 4 5% 3 5% 5% 22%	1498
March	1st to 10th	Day	20% 14% 5% 9% 5% 4 4 3 4 5% 28%	3418
	11th to 20th	Day	11% 15% 6% 10% 6% 5% 5% 4 4 6% 28%	3480
	21st to end	Day	17% 11% 3 8% 6% 4 5% 4 6% 5% 31%	3815
April	1st to 10th	Day	18% 13% 5% 9% 5% 4 4 3 5% 7% 28%	2209
	11th to 20th	Day	19% 12% 3 7% 4 4 4 3 4 6% 35%	2369
	21st to end	Day	20% 11% 3 7% 5% 5% 4 4 5% 6% 32%	2470
May	1st to 10th	Day	18% 8% 3 8% 4 4 4 3 6% 7% 33%	4108
	11th to 20th	Day	16% 9% 3 9% 5% 5% 5% 4 6% 8% 30%	4266
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 7% 46%	4837
June	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 9% 36%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 5% 7% 9% 39%	2732
	21st to end	Day	14% 5% 2 6% 5% 5% 5% 4 6% 8% 38%	2850
July	1st to 10th	Day	8% 5% 3 5% 5% 4 5% 4 7% 8% 45%	2687
	11th to 20th	Day	11% 6% 2 6% 6% 5% 5% 5% 8% 9% 37%	2476
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 8% 9% 38%	2547
August	1st to 10th	Day	10% 6% 2 6% 5% 5% 7% 5% 6% 10% 38%	2327
	11th to 20th	Day	10% 6% 2 8% 6% 5% 6% 5% 6% 9% 36%	2303
	21st to end	Day	12% 6% 2 8% 5% 5% 6% 4 6% 8% 38%	2507
September	1st to 10th	Day	13% 6% 3 6% 6% 4 5% 4 7% 7% 39%	2160
	11th to 20th	Day	13% 6% 3 6% 4 4 4 3 6% 8% 43%	2004
	21st to end	Day	15% 7% 3 7% 4 6% 5% 4 6% 8% 35%	1974
October	1st to 10th	Day	19% 10% 3 8% 4 5% 5% 3 5% 5% 33%	1800
	11th to 20th	Day	23% 10% 4 10% 5% 4 4 3 5% 6% 28%	1800
	21st to end	Day	25% 14% 3 8% 5% 3 5% 4 5% 7% 21%	1980
November	1st to 10th	Day	16% 12% 4 8% 5% 5% 4 4 6% 7% 30%	1625
	11th to 20th	Day	16% 15% 4 9% 6% 4 5% 4 6% 8% 24%	1520
	21st to end	Day	19% 17% 7% 10% 5% 4 4 3 6% 6% 19%	1520
December	1st to 10th	Day	15% 20% 7% 12% 7% 5% 5% 4 4 5% 15%	1520
	11th to 20th	Day	18% 19% 5% 11% 7% 5% 5% 5% 4 5% 18%	1906
	21st to end	Day	28% 20% 6% 13% 6% 5% 4 2 4 3 8%	2772
January	1st to 10th	Night	17% 18% 8% 13% 6% 4 4 4 4 5% 17%	4401
	11th to 20th	Night	14% 15% 8% 16% 10% 8% 6% 5% 5% 4 11%	4120
	21st to end	Night	21% 16% 9% 12% 5% 4 4 3 4 6% 18%	4532
February	1st to 10th	Night	14% 15% 5% 11% 8% 5% 6% 4 5% 6% 20%	2470
	11th to 20th	Night	14% 14% 5% 11% 7% 4 5% 4 5% 6% 24%	2403
	21st to end	Night	15% 13% 5% 11% 6% 5% 4 4 5% 6% 25%	1956
March	1st to 10th	Night	16% 10% 4 8% 5% 5% 4 4 5% 7% 31%	3421
	11th to 20th	Night	11% 12% 3 9% 5% 4 4 4 5% 7% 37%	3479
	21st to end	Night	16% 9% 3 7% 4 5% 4 3 5% 5% 40%	3817
April	1st to 10th	Night	19% 11% 3 6% 5% 5% 4 3 5% 5% 32%	1970
	11th to 20th	Night	17% 10% 3 7% 5% 4 4 3 5% 6% 37%	1777
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 5% 7% 36%	1710
May	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 6% 7% 40%	2833
	11th to 20th	Night	13% 9% 3 8% 6% 5% 5% 4 6% 7% 35%	2623
	21st to end	Night	10% 5% 2 5% 3 4 4 4 5% 9% 50%	2738
June	1st to 10th	Night	12% 6% 2 6% 4 5% 5% 5% 6% 8% 43%	1520
	11th to 20th	Night	11% 4 2 5% 4 6% 5% 4 7% 8% 43%	1440
	21st to end	Night	12% 6% 2 6% 5% 3 4 4 5% 8% 46%	1330
July	1st to 10th	Night	5% 4 1 4 4 4 3 3 4 8% 58%	1314
	11th to 20th	Night	9% 5% 1 4 4 4 4 4 6% 9% 49%	1481
	21st to end	Night	8% 5% 2 4 4 4 5% 4 7% 8% 49%	1771
August	1st to 10th	Night	7% 5% 2 6% 5% 4 4 4 6% 9% 48%	1611
	11th to 20th	Night	7% 4 2 5% 4 4 5% 3 7% 10% 50%	1598
	21st to end	Night	11% 5% 2 6% 5% 4 4 3 6% 7% 48%	1831
September	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 7% 7% 46%	1800
	11th to 20th	Night	11% 5% 2 5% 3 2 3 4 6% 7% 52%	1952
	21st to end	Night	15% 6% 3 6% 4 4 3 4 5% 8% 41%	1985
October	1st to 10th	Night	14% 9% 3 7% 5% 4 4 3 6% 7% 38%	2160
	11th to 20th	Night	18% 10% 3 7% 5% 4 4 3 5% 7% 34%	2160
	21st to end	Night	22% 10% 3 8% 5% 5% 5% 3 5% 6% 27%	2376
November	1st to 10th	Night	15% 9% 3 8% 5% 4 4 3 5% 7% 37%	2447
	11th to 20th	Night	16% 12% 4 9% 5% 5% 5% 4 6% 7% 27%	2660
	21st to end	Night	19% 14% 4 8% 5% 5% 4 4 5% 6% 24%	2660
December	1st to 10th	Night	19% 17% 6% 9% 6% 5% 5% 4 5% 6% 18%	2660
	11th to 20th	Night	17% 18% 5% 12% 6% 4 4 3 5% 5% 20%	3288
	21st to end	Night	27% 21% 7% 13% 5% 4 4 2 4 4 9%	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	67% 2 11% 11% 8%	2531
	11th to 20th	Day	75% 2 8% 10% 4	2840
	21st to end	Day	77% 2 8% 8% 4 1	3124
February	1st to 10th	Day	71% 4 7% 14% 4 1	1710
	11th to 20th	Day	65% 3 7% 16% 8% 2	1672
	21st to end	Day	65% 2 10% 15% 8% 1	1498
March	1st to 10th	Day	61% 3 10% 19% 7% 1	3421
	11th to 20th	Day	57% 4 12% 19% 7% 2	3480
	21st to end	Day	53% 3 9% 21% 11% 2	3815
April	1st to 10th	Day	57% 4 10% 18% 11% 1	2209
	11th to 20th	Day	51% 3 11% 21% 13% 1	2369
	21st to end	Day	54% 4 9% 20% 11% 2	2470
May	1st to 10th	Day	50% 3 12% 21% 12% 2	4120
	11th to 20th	Day	52% 4 14% 22% 7% 1	4270
	21st to end	Day	39% 3 13% 23% 18% 4	4837
June	1st to 10th	Day	44% 5% 12% 23% 15% 1	2660
	11th to 20th	Day	40% 5% 14% 25% 15% 1	2732
	21st to end	Day	43% 4 13% 25% 15%	2850
July	1st to 10th	Day	34% 5% 12% 28% 18% 3	2615
	11th to 20th	Day	40% 5% 15% 28% 11% 1	2401
	21st to end	Day	41% 7% 12% 24% 14% 2	2405
August	1st to 10th	Day	40% 6% 16% 26% 11% 2	2314
	11th to 20th	Day	44% 5% 16% 25% 11%	2303
	21st to end	Day	42% 5% 13% 27% 12% 1	2507
September	1st to 10th	Day	42% 5% 14% 26% 11% 1	2160
	11th to 20th	Day	38% 5% 13% 30% 13% 1	2004
	21st to end	Day	45% 5% 14% 26% 9% 1	1974
October	1st to 10th	Day	52% 5% 12% 18% 12% 2	1800
	11th to 20th	Day	57% 4 11% 19% 8% 1	1800
	21st to end	Day	63% 4 11% 15% 6%	1980
November	1st to 10th	Day	53% 4 12% 21% 8% 1	1625
	11th to 20th	Day	58% 5% 13% 15% 8% 1	1520
	21st to end	Day	66% 3 12% 13% 5%	1520
December	1st to 10th	Day	71% 4 10% 11% 4	1520
	11th to 20th	Day	69% 4 11% 10% 6%	1906
	21st to end	Day	83% 2 6% 8% 2	2772
January	1st to 10th	Night	69% 3 10% 12% 5% 1	4401
	11th to 20th	Night	76% 3 9% 9% 3	4120
	21st to end	Night	70% 2 11% 13% 4 1	4532
February	1st to 10th	Night	64% 4 9% 18% 4 1	2470
	11th to 20th	Night	60% 3 10% 17% 9% 1	2405
	21st to end	Night	59% 3 11% 18% 8%	1956
March	1st to 10th	Night	53% 4 13% 23% 7% 1	3431
	11th to 20th	Night	47% 3 14% 22% 12% 2	3480
	21st to end	Night	47% 2 8% 24% 17% 2	3817
April	1st to 10th	Night	54% 3 8% 20% 14% 1	1970
	11th to 20th	Night	48% 5% 8% 22% 16% 2	1777
	21st to end	Night	49% 3 9% 21% 15% 2	1710
May	1st to 10th	Night	45% 3 11% 21% 15% 4	2840
	11th to 20th	Night	48% 4 14% 23% 9% 2	2624
	21st to end	Night	33% 4 14% 24% 23% 3	2739
June	1st to 10th	Night	38% 5% 11% 27% 17% 1	1520
	11th to 20th	Night	37% 4 12% 26% 18% 2	1440
	21st to end	Night	37% 4 14% 28% 17% 1	1330
July	1st to 10th	Night	25% 5% 12% 28% 26% 4	1277
	11th to 20th	Night	32% 4 13% 32% 17% 2	1433
	21st to end	Night	32% 5% 16% 28% 16% 3	1672
August	1st to 10th	Night	33% 5% 17% 32% 12% 2	1600
	11th to 20th	Night	29% 5% 15% 34% 15% 1	1598
	21st to end	Night	36% 5% 12% 31% 15% 2	1831
September	1st to 10th	Night	36% 5% 14% 28% 15% 1	1800
	11th to 20th	Night	31% 4 11% 34% 19% 1	1952
	21st to end	Night	42% 4 14% 27% 13% 1	1985
October	1st to 10th	Night	46% 4 10% 24% 15% 1	2160
	11th to 20th	Night	51% 3 12% 24% 9% 1	2160
	21st to end	Night	57% 4 9% 19% 10%	2376
November	1st to 10th	Night	47% 4 12% 23% 13% 1	2447
	11th to 20th	Night	56% 4 13% 18% 8% 1	2660
	21st to end	Night	59% 5% 16% 14% 5% 1	2660
December	1st to 10th	Night	66% 4 12% 14% 4	2660
	11th to 20th	Night	66% 5% 11% 14% 4	3288
	21st to end	Night	81% 2 8% 7% 2	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	26% 18% 34% 16% 5% 1	186
	11th to 20th	Day	26% 27% 32% 11% 4	270
	21st to end	Day	28% 22% 31% 16% 3	297
February	1st to 10th	Day	31% 22% 34% 10% 3	270
	11th to 20th	Day	24% 27% 41% 8%	270
	21st to end	Day	37% 24% 30% 8% 1	225
March	1st to 10th	Day	29% 16% 31% 19% 5%	446
	11th to 20th	Day	26% 17% 35% 16% 4 1	460
	21st to end	Day	31% 14% 33% 17% 5% 1	506
April	1st to 10th	Day	25% 19% 33% 17% 6%	460
	11th to 20th	Day	33% 16% 25% 17% 8% 2	452
	21st to end	Day	25% 13% 35% 18% 7% 2	460
May	1st to 10th	Day	24% 13% 30% 21% 9% 4	463
	11th to 20th	Day	19% 11% 29% 26% 11% 4	487
	21st to end	Day	15% 10% 30% 27% 12% 6%	529
June	1st to 10th	Day	17% 14% 31% 24% 11% 3	490
	11th to 20th	Day	14% 11% 30% 30% 13% 3	562
	21st to end	Day	18% 8% 33% 28% 11% 3	680
July	1st to 10th	Day	9% 5% 28% 32% 19% 7%	590
	11th to 20th	Day	15% 7% 27% 25% 18% 8%	444
	21st to end	Day	10% 5% 34% 30% 15% 6%	488
August	1st to 10th	Day	11% 7% 23% 30% 20% 9%	446
	11th to 20th	Day	13% 4 23% 29% 20% 11%	439
	21st to end	Day	16% 6% 24% 27% 20% 8%	495
September	1st to 10th	Day	14% 9% 25% 26% 17% 9%	450
	11th to 20th	Day	12% 6% 23% 27% 20% 12%	450
	21st to end	Day	17% 14% 25% 23% 14% 8%	450
October	1st to 10th	Day	24% 14% 21% 22% 14% 5%	450
	11th to 20th	Day	26% 15% 36% 15% 7% 1	450
	21st to end	Day	33% 15% 29% 17% 6%	495
November	1st to 10th	Day	28% 18% 31% 16% 6%	253
	11th to 20th	Day	27% 19% 32% 14% 3 3	180
	21st to end	Day	27% 22% 36% 12% 3 1	180
December	1st to 10th	Day	32% 22% 31% 12% 4	180
	11th to 20th	Day	34% 20% 31% 14% 2	180
	21st to end	Day	30% 26% 32% 11% 1	198
January	1st to 10th	Night	23% 24% 33% 15% 4 1	760
	11th to 20th	Night	29% 20% 29% 18% 3	760
	21st to end	Night	21% 17% 37% 19% 6%	836
February	1st to 10th	Night	23% 24% 31% 20% 2	760
	11th to 20th	Night	23% 20% 35% 18% 4	743
	21st to end	Night	23% 23% 34% 17% 3	628
March	1st to 10th	Night	20% 17% 36% 22% 5%	760
	11th to 20th	Night	24% 18% 35% 18% 6%	760
	21st to end	Night	22% 16% 32% 22% 8% 1	836
April	1st to 10th	Night	26% 15% 33% 23% 3	760
	11th to 20th	Night	23% 15% 32% 21% 8% 1	653
	21st to end	Night	18% 12% 35% 27% 8%	570
May	1st to 10th	Night	19% 11% 29% 27% 12% 2	570
	11th to 20th	Night	16% 7% 31% 33% 12% 1	450
	21st to end	Night	11% 8% 25% 36% 19%	411
June	1st to 10th	Night	14% 9% 28% 36% 12% 1	380
	11th to 20th	Night	11% 8% 34% 34% 13% 1	380
	21st to end	Night	14% 6% 29% 36% 15%	380
July	1st to 10th	Night	6% 3 26% 43% 21% 2	354
	11th to 20th	Night	9% 5% 30% 35% 20% 2	389
	21st to end	Night	11% 2 26% 41% 19% 2	591
August	1st to 10th	Night	8% 4 21% 41% 23% 3	537
	11th to 20th	Night	8% 4 20% 37% 28% 3	534
	21st to end	Night	13% 5% 24% 32% 23% 3	643
September	1st to 10th	Night	13% 6% 23% 34% 19% 5%	720
	11th to 20th	Night	13% 5% 23% 33% 22% 5%	720
	21st to end	Night	18% 9% 21% 27% 19% 5%	720
October	1st to 10th	Night	18% 10% 28% 26% 16% 3	720
	11th to 20th	Night	23% 13% 30% 25% 8% 1	720
	21st to end	Night	26% 16% 31% 23% 4	792
November	1st to 10th	Night	22% 13% 30% 25% 9% 1	740
	11th to 20th	Night	23% 21% 28% 20% 7% 1	760
	21st to end	Night	28% 19% 30% 18% 5% 1	760
December	1st to 10th	Night	29% 20% 29% 16% 4	760
	11th to 20th	Night	26% 23% 32% 15% 4	760
	21st to end	Night	32% 18% 33% 15% 2	836

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