BRAC Ivy Hill Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BRAC Ivy Hill Site clear sky chart from 2005-11-06 to 2024-11-10.

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% 5% 2 6% 4 4 4 4 6% 8% 42%	100810
	Night	15% 6% 2 5% 4 4 4 3 5% 7% 44%	98252
Transparency	Day	38% 7% 13% 22% 16% 4	104459
	Night	38% 6% 11% 22% 18% 5%	102742
Seeing	Day	18% 11% 24% 25% 16% 6%	15125
	Night	17% 8% 25% 32% 16% 2	25208

By Month

Percentage Cloud Cover SE (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% 4 1 4 4 3 4 3 5% 6% 48%	2836
	11th to 20th	Day	15% 6% 1 6% 4 5% 4 3 5% 6% 44%	2840
	21st to end	Day	21% 7% 2 6% 4 4 4 3 4 5% 40%	3124
February	1st to 10th	Day	13% 7% 2 5% 5% 5% 5% 4 7% 7% 41%	1710
	11th to 20th	Day	19% 5% 1 4 4 4 4 3 5% 7% 42%	1663
	21st to end	Day	20% 7% 3 6% 4 4 4 4 5% 6% 39%	1669
March	1st to 10th	Day	22% 8% 2 6% 3 3 3 3 5% 7% 37%	3435
	11th to 20th	Day	10% 5% 2 4 4 3 4 4 5% 7% 52%	3480
	21st to end	Day	19% 6% 3 7% 5% 4 4 3 6% 6% 38%	3815
April	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 5% 7% 50%	2197
	11th to 20th	Day	13% 6% 2 4 4 3 4 4 5% 7% 47%	2254
	21st to end	Day	16% 6% 1 6% 5% 3 4 3 5% 7% 43%	2351
May	1st to 10th	Day	16% 6% 2 6% 5% 6% 7% 5% 7% 8% 33%	4082
	11th to 20th	Day	21% 6% 2 6% 5% 4 4 4 5% 8% 35%	4081
	21st to end	Day	9% 4 2 5% 4 4 5% 5% 7% 10% 46%	4476
June	1st to 10th	Day	10% 4 1 5% 5% 5% 5% 5% 6% 10% 45%	2551
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 10% 41%	2650
	21st to end	Day	8% 4 2 5% 5% 5% 5% 5% 8% 11% 43%	2470
July	1st to 10th	Day	7% 5% 2 5% 5% 5% 6% 5% 9% 13% 39%	4120
	11th to 20th	Day	9% 5% 2 6% 5% 6% 5% 5% 8% 11% 37%	4120
	21st to end	Day	13% 7% 3 8% 6% 6% 6% 6% 8% 9% 29%	4466
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 5% 7% 11% 32%	4104
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 5% 7% 10% 40%	4017
	21st to end	Day	8% 4 2 5% 3 3 4 4 6% 10% 49%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 5% 7% 49%	2280
	11th to 20th	Day	15% 6% 2 5% 4 3 5% 4 5% 7% 45%	2123
	21st to end	Day	24% 6% 3 5% 4 4 3 3 5% 5% 38%	2066
October	1st to 10th	Day	16% 5% 2 5% 3 2 3 3 4 5% 53%	1900
	11th to 20th	Day	16% 4 1 4 3 2 3 2 4 6% 54%	1900
	21st to end	Day	14% 5% 2 5% 3 3 4 3 4 6% 53%	2090
November	1st to 10th	Day	13% 4 1 4 3 4 3 3 4 5% 55%	1947
	11th to 20th	Day	14% 5% 2 4 3 3 4 3 5% 6% 51%	1794
	21st to end	Day	15% 5% 1 4 3 3 4 3 5% 8% 50%	1520
December	1st to 10th	Day	18% 6% 2 6% 4 4 5% 5% 6% 7% 38%	1520
	11th to 20th	Day	17% 5% 2 4 3 2 3 2 4 6% 53%	1910
	21st to end	Day	21% 5% 2 6% 3 4 4 3 6% 8% 38%	3069
January	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 4 6% 50%	4109
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 4 5% 49%	4120
	21st to end	Night	24% 6% 2 5% 4 3 3 3 4 5% 42%	4532
February	1st to 10th	Night	14% 6% 2 5% 4 3 4 4 6% 7% 45%	2470
	11th to 20th	Night	20% 7% 2 5% 3 3 4 3 4 5% 43%	2403
	21st to end	Night	16% 7% 2 5% 5% 3 3 4 5% 6% 43%	1779
March	1st to 10th	Night	22% 8% 3 7% 4 4 4 3 5% 6% 35%	3418
	11th to 20th	Night	11% 4 1 4 3 3 4 4 6% 8% 53%	3480
	21st to end	Night	17% 6% 2 5% 4 3 3 3 4 7% 48%	3817
April	1st to 10th	Night	14% 7% 1 5% 4 4 4 3 5% 7% 46%	1981
	11th to 20th	Night	12% 5% 1 5% 4 3 4 3 5% 7% 50%	1875
	21st to end	Night	15% 5% 2 5% 3 4 5% 3 6% 7% 44%	1844
May	1st to 10th	Night	17% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	2813
	11th to 20th	Night	24% 8% 2 6% 4 3 4 3 4 6% 36%	2812
	21st to end	Night	11% 4 2 5% 4 4 5% 4 7% 9% 46%	3079
June	1st to 10th	Night	10% 4 1 6% 4 4 4 5% 7% 11% 43%	1615
	11th to 20th	Night	10% 5% 2 6% 6% 4 4 3 7% 9% 43%	1520
	21st to end	Night	8% 4 2 5% 5% 5% 5% 4 7% 11% 45%	1705
July	1st to 10th	Night	6% 5% 2 7% 5% 4 5% 5% 8% 10% 42%	2840
	11th to 20th	Night	9% 7% 2 6% 4 5% 6% 5% 8% 10% 39%	2840
	21st to end	Night	12% 8% 3 9% 6% 6% 6% 5% 7% 9% 30%	3087
August	1st to 10th	Night	16% 7% 2 8% 4 5% 6% 4 6% 10% 31%	2825
	11th to 20th	Night	9% 6% 2 7% 5% 6% 6% 5% 8% 8% 39%	2785
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 45%	3469
September	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 6% 7% 46%	1900
	11th to 20th	Night	16% 6% 2 5% 5% 4 4 4 5% 6% 43%	2052
	21st to end	Night	25% 7% 3 6% 4 3 4 2 4 5% 38%	2114
October	1st to 10th	Night	17% 6% 1 5% 3 3 3 3 3 6% 50%	2280
	11th to 20th	Night	15% 5% 1 4 3 4 3 2 4 5% 52%	2280
	21st to end	Night	14% 4 1 3 3 3 3 3 4 6% 56%	2508
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 4 53%	2336
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 4 6% 51%	2379
	21st to end	Night	13% 3 2 5% 4 4 3 3 5% 7% 50%	2658
December	1st to 10th	Night	19% 7% 3 5% 3 3 4 3 4 6% 43%	2660
	11th to 20th	Night	16% 6% 2 5% 4 4 3 3 4 6% 47%	3285
	21st to end	Night	18% 6% 2 5% 4 4 4 4 6% 8% 39%	4582

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	35% 5% 9% 28% 18% 6%	2837
	11th to 20th	Day	40% 5% 7% 20% 23% 5%	2840
	21st to end	Day	46% 6% 7% 21% 17% 3	3124
February	1st to 10th	Day	39% 7% 11% 23% 16% 3	1710
	11th to 20th	Day	41% 5% 9% 22% 18% 4	1672
	21st to end	Day	46% 6% 12% 15% 17% 5%	1669
March	1st to 10th	Day	46% 5% 9% 20% 13% 6%	3435
	11th to 20th	Day	30% 7% 9% 25% 20% 10%	3480
	21st to end	Day	45% 7% 10% 19% 16% 3	3815
April	1st to 10th	Day	33% 5% 12% 23% 19% 8%	2197
	11th to 20th	Day	35% 6% 12% 22% 20% 6%	2254
	21st to end	Day	41% 7% 11% 23% 14% 4	2351
May	1st to 10th	Day	45% 9% 13% 19% 12% 2	4120
	11th to 20th	Day	47% 9% 11% 23% 10% 1	4085
	21st to end	Day	30% 11% 17% 21% 19% 3	4489
June	1st to 10th	Day	33% 10% 17% 26% 14% 1	2551
	11th to 20th	Day	35% 11% 16% 22% 16% 1	2650
	21st to end	Day	34% 11% 16% 23% 14% 2	2470
July	1st to 10th	Day	34% 10% 22% 18% 10% 5%	4055
	11th to 20th	Day	38% 8% 20% 23% 10% 1	4051
	21st to end	Day	49% 9% 13% 19% 10%	4324
August	1st to 10th	Day	45% 7% 18% 23% 6% 1	4091
	11th to 20th	Day	37% 8% 16% 28% 10% 1	4056
	21st to end	Day	31% 6% 20% 27% 15% 1	4180
September	1st to 10th	Day	34% 9% 13% 19% 21% 4	2280
	11th to 20th	Day	39% 7% 11% 24% 18% 3	2123
	21st to end	Day	48% 7% 9% 16% 18% 2	2066
October	1st to 10th	Day	34% 3 8% 19% 29% 6%	3160
	11th to 20th	Day	25% 3 7% 23% 28% 14%	3140
	21st to end	Day	23% 3 12% 27% 24% 12%	3428
November	1st to 10th	Day	32% 4 10% 23% 27% 5%	1947
	11th to 20th	Day	34% 5% 9% 23% 21% 8%	1794
	21st to end	Day	33% 5% 10% 23% 24% 4	1520
December	1st to 10th	Day	43% 8% 11% 19% 16% 4	1520
	11th to 20th	Day	34% 4 9% 25% 21% 8%	1906
	21st to end	Day	42% 6% 12% 25% 13% 2	3069
January	1st to 10th	Night	36% 3 9% 24% 23% 4	4109
	11th to 20th	Night	38% 5% 9% 19% 24% 5%	4120
	21st to end	Night	45% 5% 6% 24% 15% 5%	4532
February	1st to 10th	Night	37% 6% 12% 23% 19% 2	2470
	11th to 20th	Night	43% 4 10% 19% 18% 6%	2405
	21st to end	Night	41% 6% 10% 23% 15% 6%	1779
March	1st to 10th	Night	48% 7% 8% 18% 15% 4	3418
	11th to 20th	Night	29% 6% 9% 20% 27% 9%	3480
	21st to end	Night	38% 5% 10% 23% 19% 5%	3817
April	1st to 10th	Night	38% 5% 11% 18% 22% 5%	1981
	11th to 20th	Night	33% 7% 11% 22% 22% 6%	1875
	21st to end	Night	38% 7% 12% 23% 17% 3	1844
May	1st to 10th	Night	46% 8% 13% 19% 12% 2	2840
	11th to 20th	Night	48% 8% 10% 21% 12% 1	2813
	21st to end	Night	31% 9% 18% 25% 13% 4	3087
June	1st to 10th	Night	33% 9% 16% 27% 15% 1	1615
	11th to 20th	Night	37% 7% 17% 21% 16% 2	1520
	21st to end	Night	31% 10% 17% 21% 17% 4	1705
July	1st to 10th	Night	37% 9% 21% 17% 13% 3	2795
	11th to 20th	Night	39% 7% 22% 23% 8% 1	2786
	21st to end	Night	50% 7% 14% 20% 9%	2988
August	1st to 10th	Night	49% 7% 17% 20% 7%	2814
	11th to 20th	Night	40% 7% 16% 26% 11% 1	2812
	21st to end	Night	35% 7% 14% 33% 9% 2	3469
September	1st to 10th	Night	37% 8% 14% 18% 18% 6%	1900
	11th to 20th	Night	40% 8% 10% 21% 19% 2	2052
	21st to end	Night	50% 5% 8% 18% 15% 3	2114
October	1st to 10th	Night	39% 3 9% 19% 23% 6%	3800
	11th to 20th	Night	27% 3 7% 21% 30% 12%	3779
	21st to end	Night	20% 3 8% 29% 28% 12%	4120
November	1st to 10th	Night	35% 4 9% 21% 25% 7%	2336
	11th to 20th	Night	34% 5% 8% 21% 24% 8%	2379
	21st to end	Night	34% 5% 9% 25% 21% 6%	2658
December	1st to 10th	Night	43% 5% 9% 24% 16% 4	2660
	11th to 20th	Night	38% 4 9% 21% 21% 7%	3288
	21st to end	Night	41% 5% 12% 22% 18% 2	4582

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	27% 26% 25% 16% 5% 1	270
	11th to 20th	Day	31% 20% 34% 13% 1	270
	21st to end	Day	30% 22% 34% 11% 4	297
February	1st to 10th	Day	21% 23% 36% 14% 5%	270
	11th to 20th	Day	40% 20% 31% 6% 3	270
	21st to end	Day	30% 20% 31% 16% 4	339
March	1st to 10th	Day	26% 16% 33% 20% 5%	460
	11th to 20th	Day	21% 22% 34% 18% 5%	460
	21st to end	Day	23% 16% 34% 21% 5% 1	506
April	1st to 10th	Day	20% 23% 27% 22% 7% 1	460
	11th to 20th	Day	20% 16% 26% 24% 11% 4	452
	21st to end	Day	22% 12% 32% 21% 11% 2	460
May	1st to 10th	Day	21% 13% 31% 22% 11% 3	463
	11th to 20th	Day	19% 11% 24% 26% 16% 4	487
	21st to end	Day	17% 4 21% 29% 20% 8%	529
June	1st to 10th	Day	11% 4 29% 31% 18% 6%	490
	11th to 20th	Day	11% 6% 19% 34% 20% 11%	490
	21st to end	Day	10% 3 25% 32% 23% 7%	490
July	1st to 10th	Day	9% 4 15% 28% 30% 13%	485
	11th to 20th	Day	6% 2 12% 37% 31% 13%	484
	21st to end	Day	11% 2 17% 32% 27% 11%	532
August	1st to 10th	Day	14% 3 19% 26% 25% 13%	486
	11th to 20th	Day	11% 3 16% 30% 26% 14%	479
	21st to end	Day	11% 3 10% 28% 27% 21%	539
September	1st to 10th	Day	13% 5% 13% 24% 29% 17%	490
	11th to 20th	Day	17% 5% 13% 28% 27% 11%	490
	21st to end	Day	29% 8% 17% 25% 17% 4	490
October	1st to 10th	Day	21% 6% 17% 31% 19% 6%	490
	11th to 20th	Day	14% 13% 23% 26% 20% 4	490
	21st to end	Day	15% 11% 27% 31% 12% 4	539
November	1st to 10th	Day	19% 8% 27% 30% 13% 3	491
	11th to 20th	Day	20% 16% 26% 24% 11% 2	354
	21st to end	Day	20% 19% 27% 22% 9% 2	180
December	1st to 10th	Day	24% 17% 35% 17% 8%	180
	11th to 20th	Day	24% 13% 28% 25% 7% 2	180
	21st to end	Day	28% 14% 32% 18% 6% 1	283
January	1st to 10th	Night	21% 16% 34% 26% 4	760
	11th to 20th	Night	18% 17% 32% 31% 2	760
	21st to end	Night	19% 13% 40% 25% 3	836
February	1st to 10th	Night	17% 14% 39% 28% 3	760
	11th to 20th	Night	26% 16% 36% 20% 1	743
	21st to end	Night	19% 16% 37% 26% 2	628
March	1st to 10th	Night	17% 14% 34% 30% 5%	760
	11th to 20th	Night	22% 14% 32% 25% 7% 1	760
	21st to end	Night	18% 12% 34% 30% 6%	836
April	1st to 10th	Night	18% 16% 34% 27% 6%	760
	11th to 20th	Night	15% 12% 30% 31% 11% 1	751
	21st to end	Night	17% 6% 31% 37% 9%	704
May	1st to 10th	Night	15% 9% 29% 32% 14% 1	570
	11th to 20th	Night	20% 6% 21% 35% 16% 2	561
	21st to end	Night	17% 2 12% 34% 32% 4	617
June	1st to 10th	Night	9% 2 15% 44% 27% 3	475
	11th to 20th	Night	12% 2 14% 39% 28% 4	380
	21st to end	Night	10% 2 11% 45% 30% 4	565
July	1st to 10th	Night	12% 1 10% 35% 37% 5%	555
	11th to 20th	Night	6% 1 7% 33% 48% 5%	552
	21st to end	Night	10% 1 8% 44% 34% 4	624
August	1st to 10th	Night	14% 3 8% 38% 33% 5%	567
	11th to 20th	Night	14% 2 9% 36% 35% 4	564
	21st to end	Night	12% 2 10% 34% 37% 5%	829
September	1st to 10th	Night	14% 2 10% 33% 35% 7%	760
	11th to 20th	Night	17% 2 12% 32% 32% 5%	760
	21st to end	Night	27% 4 15% 32% 21% 1	760
October	1st to 10th	Night	19% 5% 17% 30% 25% 4	760
	11th to 20th	Night	16% 8% 24% 34% 17% 2	760
	21st to end	Night	16% 7% 24% 38% 14% 2	836
November	1st to 10th	Night	16% 7% 26% 36% 13% 2	779
	11th to 20th	Night	17% 10% 29% 32% 12% 1	760
	21st to end	Night	15% 11% 29% 34% 10% 1	760
December	1st to 10th	Night	22% 11% 32% 28% 6%	760
	11th to 20th	Night	17% 9% 36% 32% 5%	760
	21st to end	Night	22% 14% 30% 31% 4	836

Page generated at 2024-11-07 06:55:44. Version ac07

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