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 2024-11-04.

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% 13% 4 9% 5% 4 4 3 5% 6% 27%	102098
	Night	19% 13% 4 10% 6% 4 4 3 5% 6% 27%	96982
Transparency	Day	56% 6% 11% 18% 9% 1	105694
	Night	57% 5% 11% 18% 8% 1	101475
Seeing	Day	30% 19% 30% 15% 5% 1	17362
	Night	27% 17% 31% 20% 5%	24692

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% 19% 6% 14% 9% 4 3 3 3 4 20%	2526
	11th to 20th	Day	14% 16% 6% 21% 7% 6% 5% 3 4 5% 14%	2623
	21st to end	Day	17% 13% 7% 13% 8% 5% 4 3 4 5% 20%	3124
February	1st to 10th	Day	17% 16% 6% 11% 9% 6% 5% 4 4 4 17%	1710
	11th to 20th	Day	17% 17% 6% 12% 7% 5% 4 3 5% 5% 19%	1663
	21st to end	Day	10% 17% 5% 9% 8% 5% 4 2 5% 6% 28%	1435
March	1st to 10th	Day	24% 18% 4 7% 5% 5% 3 3 3 4 23%	3423
	11th to 20th	Day	12% 22% 6% 12% 6% 4 4 3 4 4 22%	3480
	21st to end	Day	26% 20% 5% 10% 4 4 3 2 3 3 20%	3816
April	1st to 10th	Day	22% 18% 5% 8% 6% 4 3 2 4 5% 24%	2270
	11th to 20th	Day	19% 13% 3 7% 4 4 4 3 5% 7% 31%	2327
	21st to end	Day	22% 14% 3 6% 4 4 4 3 5% 6% 28%	2470
May	1st to 10th	Day	24% 18% 3 8% 4 3 4 3 5% 7% 21%	4202
	11th to 20th	Day	21% 12% 3 8% 4 4 4 4 5% 6% 30%	4398
	21st to end	Day	16% 10% 3 7% 5% 5% 5% 4 7% 9% 30%	4836
June	1st to 10th	Day	26% 10% 3 7% 6% 4 4 4 5% 6% 24%	2660
	11th to 20th	Day	21% 10% 3 7% 4 5% 3 3 5% 7% 33%	2782
	21st to end	Day	25% 10% 4 7% 6% 4 4 3 5% 6% 26%	2850
July	1st to 10th	Day	22% 8% 3 6% 5% 5% 5% 4 6% 8% 30%	4674
	11th to 20th	Day	15% 9% 3 8% 4 5% 5% 4 6% 8% 31%	4440
	21st to end	Day	14% 9% 3 7% 5% 5% 5% 4 7% 9% 30%	4671
August	1st to 10th	Day	21% 9% 4 8% 6% 4 5% 4 5% 7% 27%	4105
	11th to 20th	Day	14% 8% 3 8% 6% 5% 5% 4 7% 7% 33%	4045
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 43%	4354
September	1st to 10th	Day	17% 7% 2 5% 4 4 4 3 4 8% 41%	2280
	11th to 20th	Day	19% 6% 2 6% 4 4 4 3 5% 6% 42%	2204
	21st to end	Day	19% 8% 3 5% 3 4 3 3 5% 6% 41%	2030
October	1st to 10th	Day	18% 7% 3 7% 4 4 6% 3 4 7% 36%	1900
	11th to 20th	Day	19% 10% 3 6% 4 4 5% 4 5% 7% 33%	1900
	21st to end	Day	20% 14% 4 8% 5% 4 4 4 3 4 30%	2090
November	1st to 10th	Day	21% 14% 3 8% 6% 5% 5% 3 4 5% 27%	1561
	11th to 20th	Day	19% 19% 5% 8% 4 4 4 3 4 5% 25%	1520
	21st to end	Day	18% 17% 5% 11% 7% 4 4 5% 5% 5% 20%	1520
December	1st to 10th	Day	23% 19% 8% 10% 5% 3 4 3 4 5% 16%	1520
	11th to 20th	Day	27% 17% 6% 11% 8% 5% 4 3 4 3 12%	1906
	21st to end	Day	23% 13% 10% 14% 5% 4 3 3 3 4 16%	2783
January	1st to 10th	Night	14% 13% 6% 19% 8% 4 4 3 5% 5% 19%	4429
	11th to 20th	Night	16% 16% 5% 20% 7% 5% 4 3 4 4 15%	4343
	21st to end	Night	17% 13% 5% 15% 9% 5% 5% 3 5% 5% 17%	4532
February	1st to 10th	Night	18% 16% 4 13% 9% 6% 5% 4 4 4 17%	2470
	11th to 20th	Night	17% 15% 5% 12% 10% 5% 4 4 5% 5% 18%	2403
	21st to end	Night	11% 16% 5% 12% 7% 5% 4 3 5% 4 28%	2017
March	1st to 10th	Night	22% 15% 4 11% 7% 5% 3 3 3 4 22%	3415
	11th to 20th	Night	16% 21% 8% 9% 4 4 4 3 4 5% 20%	3479
	21st to end	Night	24% 17% 6% 12% 5% 3 2 2 3 3 23%	3816
April	1st to 10th	Night	22% 14% 4 7% 5% 4 4 3 5% 5% 27%	1910
	11th to 20th	Night	21% 14% 3 6% 5% 3 3 3 4 7% 31%	1800
	21st to end	Night	21% 15% 3 6% 5% 4 4 4 5% 5% 30%	1710
May	1st to 10th	Night	22% 19% 4 8% 4 4 4 3 5% 5% 23%	2728
	11th to 20th	Night	21% 10% 3 7% 4 4 4 4 5% 6% 31%	2506
	21st to end	Night	15% 10% 4 9% 5% 5% 5% 4 5% 8% 29%	2749
June	1st to 10th	Night	24% 9% 3 7% 4 3 5% 4 5% 7% 28%	1520
	11th to 20th	Night	19% 9% 3 7% 4 5% 4 3 5% 6% 35%	1410
	21st to end	Night	22% 9% 3 8% 6% 5% 4 5% 5% 7% 26%	1330
July	1st to 10th	Night	18% 9% 3 8% 6% 5% 5% 5% 6% 7% 27%	2260
	11th to 20th	Night	16% 9% 3 6% 4 4 5% 4 7% 9% 33%	2530
	21st to end	Night	13% 7% 3 7% 4 4 4 3 6% 8% 41%	2886
August	1st to 10th	Night	19% 8% 3 7% 4 4 5% 4 6% 7% 33%	2825
	11th to 20th	Night	13% 9% 3 8% 5% 6% 4 4 5% 7% 36%	2782
	21st to end	Night	13% 5% 2 5% 4 3 5% 3 5% 8% 49%	3302
September	1st to 10th	Night	13% 7% 2 5% 5% 3 4 4 5% 8% 44%	1900
	11th to 20th	Night	17% 7% 2 6% 3 3 3 3 5% 6% 44%	1971
	21st to end	Night	18% 8% 3 6% 4 4 4 3 5% 7% 39%	2149
October	1st to 10th	Night	18% 7% 2 6% 4 4 5% 3 5% 6% 39%	2280
	11th to 20th	Night	19% 7% 2 6% 5% 5% 5% 3 4 7% 36%	2280
	21st to end	Night	23% 11% 3 6% 4 4 4 3 4 6% 31%	2508
November	1st to 10th	Night	20% 12% 3 8% 5% 3 4 3 5% 6% 31%	2585
	11th to 20th	Night	18% 17% 4 10% 6% 5% 5% 4 4 5% 23%	2660
	21st to end	Night	19% 15% 5% 10% 6% 5% 4 3 4 5% 23%	2660
December	1st to 10th	Night	22% 16% 5% 13% 7% 5% 4 3 4 4 17%	2660
	11th to 20th	Night	27% 16% 5% 13% 7% 4 3 3 3 3 16%	3293
	21st to end	Night	25% 15% 7% 11% 5% 4 3 3 3 3 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	65% 4 8% 16% 7% 1	3427
	11th to 20th	Day	63% 6% 9% 14% 6% 1	3480
	21st to end	Day	72% 3 5% 12% 7% 1	3816
April	1st to 10th	Day	64% 3 7% 13% 10% 2	2270
	11th to 20th	Day	53% 5% 9% 20% 12% 1	2327
	21st to end	Day	56% 5% 9% 17% 11% 2	2470
May	1st to 10th	Day	64% 4 10% 16% 6% 1	4216
	11th to 20th	Day	55% 6% 13% 19% 7%	4402
	21st to end	Day	48% 8% 17% 18% 9% 1	4836
June	1st to 10th	Day	59% 6% 11% 18% 6%	2660
	11th to 20th	Day	52% 5% 12% 20% 10% 1	2782
	21st to end	Day	58% 6% 11% 15% 9% 1	2850
July	1st to 10th	Day	53% 5% 13% 17% 12% 1	4601
	11th to 20th	Day	49% 8% 14% 21% 7% 1	4364
	21st to end	Day	48% 8% 16% 20% 7%	4522
August	1st to 10th	Day	58% 6% 12% 17% 7%	4092
	11th to 20th	Day	48% 7% 14% 23% 8%	4084
	21st to end	Day	36% 7% 13% 26% 17% 1	4354
September	1st to 10th	Day	43% 5% 13% 25% 13% 2	2280
	11th to 20th	Day	44% 4 12% 24% 15% 1	2204
	21st to end	Day	44% 5% 10% 25% 16%	2030
October	1st to 10th	Day	47% 7% 12% 23% 11%	3160
	11th to 20th	Day	45% 5% 10% 26% 14%	3140
	21st to end	Day	48% 5% 14% 21% 12% 1	3428
November	1st to 10th	Day	59% 6% 8% 19% 8% 1	1561
	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	68% 4 6% 15% 7% 1	3424
	11th to 20th	Night	65% 5% 8% 15% 7% 1	3480
	21st to end	Night	68% 3 7% 14% 7% 1	3816
April	1st to 10th	Night	60% 5% 8% 15% 10% 3	1910
	11th to 20th	Night	54% 4 8% 20% 13% 1	1800
	21st to end	Night	56% 4 10% 16% 12% 2	1710
May	1st to 10th	Night	63% 5% 13% 13% 6% 1	2734
	11th to 20th	Night	52% 5% 13% 21% 7% 1	2507
	21st to end	Night	52% 7% 17% 18% 5% 1	2750
June	1st to 10th	Night	55% 6% 14% 19% 6%	1520
	11th to 20th	Night	50% 5% 13% 20% 11% 1	1410
	21st to end	Night	57% 8% 12% 16% 7% 1	1330
July	1st to 10th	Night	54% 8% 13% 15% 9% 1	2224
	11th to 20th	Night	46% 8% 19% 17% 10%	2482
	21st to end	Night	43% 7% 16% 22% 12% 1	2795
August	1st to 10th	Night	50% 7% 15% 23% 5%	2813
	11th to 20th	Night	47% 6% 16% 22% 8%	2809
	21st to end	Night	33% 5% 18% 27% 16% 1	3301
September	1st to 10th	Night	39% 5% 13% 27% 14% 2	1900
	11th to 20th	Night	40% 4 12% 29% 14% 1	1971
	21st to end	Night	45% 5% 11% 25% 13% 1	2149
October	1st to 10th	Night	50% 4 12% 24% 11%	3800
	11th to 20th	Night	43% 4 11% 26% 15%	3779
	21st to end	Night	45% 3 14% 23% 15%	4120
November	1st to 10th	Night	53% 5% 13% 19% 10% 1	2585
	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	37% 27% 23% 9% 3	506
April	1st to 10th	Day	43% 22% 27% 6% 2	460
	11th to 20th	Day	35% 28% 26% 9% 3	452
	21st to end	Day	34% 22% 27% 14% 4	460
May	1st to 10th	Day	39% 21% 26% 9% 3 1	520
	11th to 20th	Day	28% 20% 29% 18% 5% 1	674
	21st to end	Day	31% 16% 34% 15% 3 1	735
June	1st to 10th	Day	34% 17% 30% 14% 5%	680
	11th to 20th	Day	29% 15% 32% 19% 5% 1	680
	21st to end	Day	33% 11% 31% 17% 6% 1	680
July	1st to 10th	Day	24% 10% 37% 22% 8% 1	670
	11th to 20th	Day	20% 10% 38% 23% 7% 1	668
	21st to end	Day	21% 13% 34% 25% 7% 1	658
August	1st to 10th	Day	16% 10% 40% 26% 7% 1	486
	11th to 20th	Day	23% 10% 34% 18% 11% 3	479
	21st to end	Day	17% 15% 33% 22% 9% 4	539
September	1st to 10th	Day	19% 15% 36% 23% 6% 1	490
	11th to 20th	Day	26% 15% 29% 18% 8% 3	490
	21st to end	Day	24% 16% 30% 19% 9% 2	457
October	1st to 10th	Day	24% 16% 32% 17% 9% 3	390
	11th to 20th	Day	27% 18% 32% 13% 7% 2	390
	21st to end	Day	31% 24% 25% 14% 4 1	429
November	1st to 10th	Day	30% 19% 29% 16% 5% 1	374
	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	30% 18% 32% 16% 4	836
April	1st to 10th	Night	33% 19% 28% 16% 3	580
	11th to 20th	Night	31% 20% 31% 15% 4 1	564
	21st to end	Night	30% 19% 30% 16% 4	570
May	1st to 10th	Night	30% 16% 32% 18% 4 1	570
	11th to 20th	Night	28% 13% 30% 23% 5% 1	562
	21st to end	Night	21% 15% 41% 21% 2	616
June	1st to 10th	Night	31% 12% 33% 22% 3	570
	11th to 20th	Night	24% 11% 35% 25% 4	460
	21st to end	Night	26% 7% 32% 28% 7%	380
July	1st to 10th	Night	18% 6% 35% 32% 8%	393
	11th to 20th	Night	17% 7% 36% 32% 7%	552
	21st to end	Night	17% 7% 34% 34% 8%	623
August	1st to 10th	Night	17% 6% 32% 34% 10% 1	568
	11th to 20th	Night	18% 8% 27% 34% 12% 1	562
	21st to end	Night	16% 8% 29% 34% 12%	627
September	1st to 10th	Night	17% 9% 35% 29% 9% 1	570
	11th to 20th	Night	21% 10% 28% 29% 12% 1	641
	21st to end	Night	23% 12% 30% 25% 8% 1	760
October	1st to 10th	Night	22% 11% 31% 26% 9% 1	760
	11th to 20th	Night	24% 15% 34% 19% 8% 1	760
	21st to end	Night	33% 17% 28% 18% 4	836
November	1st to 10th	Night	31% 15% 28% 21% 5% 1	838
	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 2024-11-01 23:47:37. Version ac07

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