St. Croix Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Croix Observatory clear sky chart from 2005-11-06 to 2024-03-23.

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	19% 12% 5% 8% 6% 3 4 4 5% 6% 28%	79085
	Night	19% 11% 5% 9% 7% 3 4 4 4 6% 27%	80820
Transparency	Day	57% 6% 11% 18% 8% 1	78790
	Night	58% 6% 11% 17% 7% 1	80650
Seeing	Day	30% 19% 30% 15% 5% 1	16416
	Night	27% 17% 31% 20% 5%	23977

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	14% 18% 7% 13% 10% 3 3 4 3 4 20%	2526
	11th to 20th	Day	14% 15% 6% 18% 10% 4 6% 4 4 5% 14%	2623
	21st to end	Day	17% 13% 7% 12% 10% 5% 5% 4 4 5% 20%	3124
February	1st to 10th	Day	18% 15% 6% 11% 10% 5% 5% 5% 4 4 17%	1710
	11th to 20th	Day	17% 15% 7% 11% 8% 4 4 4 5% 5% 19%	1663
	21st to end	Day	10% 14% 7% 8% 9% 4 4 3 4 7% 29%	1435
March	1st to 10th	Day	16% 20% 7% 9% 8% 3 3 3 3 4 24%	2079
	11th to 20th	Day	13% 13% 7% 12% 8% 3 4 4 4 4 28%	2090
	21st to end	Day	17% 19% 6% 8% 5% 2 3 3 3 4 30%	2200
April	1st to 10th	Day	22% 16% 5% 8% 7% 2 4 3 4 5% 24%	2150
	11th to 20th	Day	20% 12% 3 6% 4 3 4 4 5% 7% 32%	2203
	21st to end	Day	23% 13% 3 6% 6% 3 5% 4 5% 6% 26%	2340
May	1st to 10th	Day	23% 14% 3 7% 5% 3 4 4 4 6% 25%	2394
	11th to 20th	Day	22% 11% 3 6% 4 2 4 4 5% 7% 33%	2478
	21st to end	Day	20% 11% 3 6% 5% 3 5% 4 5% 7% 30%	2724
June	1st to 10th	Day	26% 9% 3 6% 7% 3 4 5% 5% 6% 25%	2520
	11th to 20th	Day	22% 9% 3 6% 5% 4 4 4 4 7% 32%	2635
	21st to end	Day	24% 9% 4 7% 6% 3 4 4 5% 6% 27%	2700
July	1st to 10th	Day	18% 6% 3 5% 5% 3 5% 4 6% 10% 35%	2656
	11th to 20th	Day	14% 8% 4 6% 5% 4 5% 5% 7% 9% 33%	2520
	21st to end	Day	19% 9% 4 6% 6% 4 5% 5% 7% 8% 27%	2668
August	1st to 10th	Day	18% 6% 3 5% 6% 3 5% 6% 6% 8% 34%	2327
	11th to 20th	Day	17% 8% 3 6% 7% 3 5% 5% 6% 7% 32%	2304
	21st to end	Day	15% 6% 3 6% 4 3 3 5% 5% 8% 42%	2480
September	1st to 10th	Day	18% 6% 2 4 5% 3 4 4 4 8% 41%	2160
	11th to 20th	Day	19% 6% 3 5% 5% 3 3 3 5% 6% 41%	2088
	21st to end	Day	19% 7% 3 4 4 3 3 4 4 6% 42%	1924
October	1st to 10th	Day	18% 7% 3 5% 6% 3 5% 4 4 7% 38%	1800
	11th to 20th	Day	19% 10% 3 5% 5% 3 5% 5% 5% 8% 32%	1800
	21st to end	Day	21% 13% 4 7% 6% 3 4 5% 4 4 29%	1980
November	1st to 10th	Day	21% 13% 3 7% 7% 4 4 4 4 5% 28%	1535
	11th to 20th	Day	19% 18% 5% 8% 4 3 5% 4 3 5% 25%	1520
	21st to end	Day	18% 15% 6% 10% 7% 4 4 5% 5% 5% 21%	1520
December	1st to 10th	Day	24% 17% 9% 10% 5% 2 4 3 4 5% 16%	1520
	11th to 20th	Day	27% 15% 7% 10% 9% 3 5% 4 4 3 13%	1906
	21st to end	Day	23% 13% 10% 13% 7% 3 4 3 3 4 16%	2783
January	1st to 10th	Night	15% 12% 7% 16% 10% 3 4 4 5% 6% 19%	4429
	11th to 20th	Night	16% 15% 6% 18% 9% 4 5% 4 4 4 15%	4343
	21st to end	Night	17% 12% 6% 13% 11% 4 5% 4 4 6% 17%	4532
February	1st to 10th	Night	19% 14% 6% 11% 10% 4 6% 5% 4 5% 18%	2470
	11th to 20th	Night	17% 14% 5% 10% 10% 5% 4 5% 5% 6% 18%	2403
	21st to end	Night	11% 14% 6% 11% 7% 4 5% 3 5% 5% 28%	2017
March	1st to 10th	Night	17% 14% 6% 9% 8% 4 5% 4 4 5% 25%	2086
	11th to 20th	Night	17% 11% 7% 8% 6% 4 5% 5% 4 7% 26%	2089
	21st to end	Night	16% 15% 5% 9% 6% 3 3 4 4 5% 31%	2201
April	1st to 10th	Night	23% 13% 4 7% 5% 3 5% 4 5% 5% 26%	1810
	11th to 20th	Night	22% 13% 3 6% 5% 3 4 4 4 7% 31%	1705
	21st to end	Night	22% 13% 4 5% 5% 3 3 5% 5% 5% 29%	1620
May	1st to 10th	Night	22% 11% 3 7% 5% 4 5% 5% 4 5% 29%	1566
	11th to 20th	Night	23% 9% 3 5% 5% 3 4 3 5% 6% 33%	1416
	21st to end	Night	15% 9% 3 6% 6% 4 5% 5% 5% 8% 32%	1550
June	1st to 10th	Night	25% 8% 3 5% 5% 3 5% 5% 5% 8% 29%	1440
	11th to 20th	Night	20% 8% 4 6% 5% 3 4 4 5% 6% 34%	1336
	21st to end	Night	22% 8% 4 7% 7% 3 5% 5% 5% 7% 27%	1260
July	1st to 10th	Night	16% 5% 4 5% 5% 3 4 6% 7% 9% 35%	1282
	11th to 20th	Night	15% 7% 3 7% 5% 3 6% 6% 7% 10% 33%	1440
	21st to end	Night	15% 7% 3 7% 7% 3 5% 5% 7% 8% 33%	1647
August	1st to 10th	Night	15% 6% 3 5% 4 4 5% 6% 5% 8% 40%	1612
	11th to 20th	Night	15% 9% 3 7% 6% 4 5% 5% 6% 7% 34%	1596
	21st to end	Night	15% 6% 2 5% 5% 3 5% 4 4 8% 43%	1876
September	1st to 10th	Night	13% 7% 3 5% 5% 3 4 5% 5% 8% 43%	1800
	11th to 20th	Night	17% 7% 2 5% 4 2 4 4 5% 6% 43%	1867
	21st to end	Night	18% 7% 3 5% 5% 2 4 4 5% 7% 40%	2035
October	1st to 10th	Night	18% 7% 2 5% 5% 3 5% 4 5% 6% 40%	2160
	11th to 20th	Night	20% 6% 2 6% 5% 4 4 5% 5% 7% 36%	2160
	21st to end	Night	24% 11% 4 6% 5% 3 5% 4 4 6% 30%	2376
November	1st to 10th	Night	21% 11% 4 7% 6% 2 4 4 5% 6% 31%	2539
	11th to 20th	Night	18% 16% 5% 9% 6% 4 5% 4 4 6% 23%	2660
	21st to end	Night	19% 15% 5% 9% 7% 4 5% 4 4 5% 24%	2660
December	1st to 10th	Night	23% 14% 6% 12% 8% 4 5% 4 4 4 17%	2660
	11th to 20th	Night	27% 15% 6% 11% 8% 3 4 3 3 3 16%	3293
	21st to end	Night	26% 14% 8% 11% 6% 3 3 3 3 4 20%	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	65% 6% 16% 10% 2	2527
	11th to 20th	Day	70% 7% 8% 13% 2	2623
	21st to end	Day	63% 7% 9% 13% 8%	3124
February	1st to 10th	Day	69% 4 8% 12% 6% 1	1710
	11th to 20th	Day	68% 5% 9% 12% 6% 1	1671
	21st to end	Day	56% 5% 11% 17% 9% 2	1435
March	1st to 10th	Day	66% 4 6% 15% 8% 1	2083
	11th to 20th	Day	60% 4 10% 16% 9% 1	2090
	21st to end	Day	60% 4 7% 18% 10% 2	2200
April	1st to 10th	Day	65% 3 7% 13% 10% 2	2150
	11th to 20th	Day	52% 5% 9% 20% 12% 1	2203
	21st to end	Day	58% 5% 9% 17% 9% 2	2340
May	1st to 10th	Day	60% 4 9% 17% 9% 1	2394
	11th to 20th	Day	52% 5% 10% 21% 12%	2482
	21st to end	Day	54% 6% 11% 20% 8% 1	2724
June	1st to 10th	Day	59% 5% 10% 18% 6%	2520
	11th to 20th	Day	53% 5% 11% 20% 11% 1	2635
	21st to end	Day	57% 7% 11% 15% 9%	2700
July	1st to 10th	Day	46% 7% 15% 21% 11% 1	2583
	11th to 20th	Day	47% 7% 15% 23% 7% 1	2444
	21st to end	Day	52% 9% 15% 20% 5% 1	2519
August	1st to 10th	Day	47% 6% 16% 21% 10%	2314
	11th to 20th	Day	49% 7% 15% 20% 8%	2304
	21st to end	Day	41% 6% 13% 27% 12% 1	2480
September	1st to 10th	Day	42% 5% 14% 25% 13% 2	2160
	11th to 20th	Day	44% 4 11% 23% 15% 1	2088
	21st to end	Day	43% 5% 10% 25% 17%	1924
October	1st to 10th	Day	46% 6% 10% 27% 12%	1800
	11th to 20th	Day	50% 7% 13% 23% 7%	1800
	21st to end	Day	58% 5% 9% 20% 7% 1	1980
November	1st to 10th	Day	59% 6% 8% 19% 8% 1	1535
	11th to 20th	Day	60% 7% 12% 15% 5% 1	1520
	21st to end	Day	63% 8% 12% 13% 4	1520
December	1st to 10th	Day	71% 4 9% 13% 4	1520
	11th to 20th	Day	75% 5% 6% 10% 3 1	1905
	21st to end	Day	71% 4 12% 10% 3	2783
January	1st to 10th	Night	63% 8% 11% 16% 2	4429
	11th to 20th	Night	71% 7% 8% 10% 4	4343
	21st to end	Night	66% 7% 9% 13% 5%	4532
February	1st to 10th	Night	69% 5% 8% 11% 6% 1	2470
	11th to 20th	Night	66% 5% 11% 12% 5% 1	2406
	21st to end	Night	58% 5% 10% 18% 8% 1	2017
March	1st to 10th	Night	62% 5% 9% 16% 8% 1	2095
	11th to 20th	Night	58% 5% 11% 15% 10% 1	2090
	21st to end	Night	57% 5% 9% 19% 9% 2	2201
April	1st to 10th	Night	60% 5% 8% 14% 10% 3	1810
	11th to 20th	Night	55% 4 8% 18% 13% 1	1705
	21st to end	Night	57% 4 10% 16% 11% 2	1620
May	1st to 10th	Night	58% 5% 10% 19% 8% 1	1566
	11th to 20th	Night	52% 4 10% 22% 10% 1	1417
	21st to end	Night	50% 8% 15% 20% 6% 1	1551
June	1st to 10th	Night	55% 6% 14% 20% 6%	1440
	11th to 20th	Night	51% 5% 12% 19% 12% 1	1336
	21st to end	Night	56% 8% 12% 17% 7% 1	1260
July	1st to 10th	Night	44% 9% 19% 20% 9%	1246
	11th to 20th	Night	46% 8% 17% 19% 9% 1	1392
	21st to end	Night	47% 7% 19% 18% 7% 1	1556
August	1st to 10th	Night	43% 6% 17% 27% 8%	1600
	11th to 20th	Night	48% 7% 17% 19% 8%	1596
	21st to end	Night	40% 6% 17% 27% 11%	1876
September	1st to 10th	Night	39% 6% 14% 27% 12% 2	1800
	11th to 20th	Night	41% 4 12% 28% 14% 1	1867
	21st to end	Night	44% 5% 11% 25% 13% 1	2035
October	1st to 10th	Night	46% 4 11% 25% 13% 1	2160
	11th to 20th	Night	48% 5% 12% 28% 7%	2160
	21st to end	Night	55% 5% 11% 17% 11%	2376
November	1st to 10th	Night	54% 5% 13% 18% 10% 1	2539
	11th to 20th	Night	61% 6% 11% 16% 6%	2660
	21st to end	Night	62% 6% 15% 14% 4	2660
December	1st to 10th	Night	70% 5% 10% 12% 2	2660
	11th to 20th	Night	73% 5% 7% 11% 4	3295
	21st to end	Night	68% 4 10% 15% 3	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	34% 22% 31% 12% 1	370
	11th to 20th	Day	36% 21% 28% 11% 3	370
	21st to end	Day	27% 25% 33% 13% 3	407
February	1st to 10th	Day	33% 24% 29% 12% 2	370
	11th to 20th	Day	39% 25% 24% 10% 2	366
	21st to end	Day	28% 26% 31% 12% 3 1	307
March	1st to 10th	Day	34% 24% 30% 9% 2	458
	11th to 20th	Day	31% 28% 27% 10% 4	460
	21st to end	Day	35% 27% 24% 10% 3	470
April	1st to 10th	Day	43% 22% 27% 6% 1	420
	11th to 20th	Day	33% 28% 26% 9% 3	412
	21st to end	Day	34% 20% 27% 14% 4	420
May	1st to 10th	Day	40% 21% 26% 9% 3 1	477
	11th to 20th	Day	29% 20% 28% 18% 4 1	624
	21st to end	Day	32% 16% 34% 14% 3 1	680
June	1st to 10th	Day	33% 17% 30% 14% 5%	630
	11th to 20th	Day	30% 15% 31% 19% 5% 1	630
	21st to end	Day	31% 11% 32% 18% 6% 1	630
July	1st to 10th	Day	23% 10% 37% 23% 8% 1	620
	11th to 20th	Day	20% 10% 38% 23% 7% 2	618
	21st to end	Day	22% 12% 33% 25% 7% 1	608
August	1st to 10th	Day	15% 9% 40% 26% 8% 2	446
	11th to 20th	Day	24% 11% 34% 18% 11% 2	439
	21st to end	Day	18% 15% 32% 23% 9% 3	495
September	1st to 10th	Day	19% 14% 36% 24% 6% 1	450
	11th to 20th	Day	28% 16% 29% 18% 7% 3	450
	21st to end	Day	25% 17% 31% 17% 9% 2	421
October	1st to 10th	Day	24% 16% 33% 16% 9% 3	360
	11th to 20th	Day	27% 18% 33% 13% 7% 2	360
	21st to end	Day	33% 24% 24% 13% 5% 1	396
November	1st to 10th	Day	31% 19% 28% 16% 5% 1	365
	11th to 20th	Day	39% 17% 28% 11% 5% 1	370
	21st to end	Day	35% 25% 26% 11% 2 1	370
December	1st to 10th	Day	39% 21% 26% 11% 2 1	370
	11th to 20th	Day	41% 25% 25% 9% 1	370
	21st to end	Day	36% 24% 29% 9% 3 1	407
January	1st to 10th	Night	31% 20% 32% 15% 1	850
	11th to 20th	Night	30% 23% 31% 14% 2	822
	21st to end	Night	26% 23% 32% 16% 2	836
February	1st to 10th	Night	32% 20% 31% 15% 2	760
	11th to 20th	Night	34% 26% 26% 12% 2	743
	21st to end	Night	22% 24% 31% 18% 4	628
March	1st to 10th	Night	29% 25% 30% 14% 2	760
	11th to 20th	Night	27% 20% 31% 18% 5%	760
	21st to end	Night	29% 17% 33% 16% 4	801
April	1st to 10th	Night	34% 18% 28% 16% 3	550
	11th to 20th	Night	31% 19% 31% 15% 4 1	534
	21st to end	Night	31% 19% 29% 17% 4	540
May	1st to 10th	Night	30% 16% 32% 18% 4 1	540
	11th to 20th	Night	29% 13% 31% 23% 4	532
	21st to end	Night	21% 15% 42% 20% 2	583
June	1st to 10th	Night	30% 12% 32% 22% 3	540
	11th to 20th	Night	25% 11% 35% 25% 4	436
	21st to end	Night	26% 7% 32% 28% 8%	360
July	1st to 10th	Night	18% 5% 36% 33% 8%	371
	11th to 20th	Night	18% 7% 36% 32% 7%	522
	21st to end	Night	18% 7% 34% 33% 9%	590
August	1st to 10th	Night	17% 5% 33% 34% 10% 1	538
	11th to 20th	Night	19% 8% 28% 33% 11% 1	532
	21st to end	Night	17% 8% 29% 34% 12%	594
September	1st to 10th	Night	17% 9% 34% 30% 10% 1	540
	11th to 20th	Night	22% 11% 29% 27% 11% 1	607
	21st to end	Night	23% 12% 31% 24% 8% 1	720
October	1st to 10th	Night	22% 11% 31% 26% 9% 1	720
	11th to 20th	Night	23% 14% 35% 19% 8% 1	720
	21st to end	Night	34% 17% 28% 17% 4	792
November	1st to 10th	Night	31% 15% 28% 20% 5% 1	821
	11th to 20th	Night	29% 21% 27% 18% 4	850
	21st to end	Night	32% 20% 27% 17% 3	850
December	1st to 10th	Night	35% 19% 30% 14% 2	850
	11th to 20th	Night	35% 23% 29% 11% 2	850
	21st to end	Night	33% 21% 27% 16% 3 1	935

Page generated at 2024-03-20 00:31:04. Version ac07

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