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 2025-09-04.

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	18% 13% 4 9% 5% 4 4 3 5% 6% 27%	127207
	Night	18% 13% 5% 11% 6% 4 4 3 4 5% 27%	121614
Transparency	Day	57% 5% 11% 18% 8% 1	130786
	Night	58% 5% 11% 18% 8% 1	126122
Seeing	Day	30% 19% 30% 15% 5% 1	18541
	Night	27% 17% 31% 20% 5%	25799

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% 24% 8% 17% 9% 3 3 2 3 3 15%	3616
	11th to 20th	Day	12% 20% 9% 21% 7% 5% 4 3 4 4 12%	3723
	21st to end	Day	14% 13% 6% 12% 8% 7% 4 3 4 5% 23%	4466
February	1st to 10th	Day	17% 16% 5% 11% 9% 6% 5% 4 4 4 18%	1800
	11th to 20th	Day	17% 16% 6% 13% 6% 5% 4 3 5% 5% 19%	1753
	21st to end	Day	11% 17% 5% 9% 8% 5% 4 2 5% 6% 28%	1508
March	1st to 10th	Day	23% 17% 5% 12% 6% 5% 3 2 3 4 21%	4923
	11th to 20th	Day	14% 18% 5% 10% 5% 4 4 3 5% 5% 27%	4980
	21st to end	Day	22% 20% 5% 11% 5% 4 4 3 3 3 20%	5466
April	1st to 10th	Day	23% 17% 5% 8% 6% 3 3 2 4 5% 24%	2390
	11th to 20th	Day	19% 13% 3 7% 4 4 4 4 5% 7% 30%	2451
	21st to end	Day	22% 14% 3 7% 4 4 4 3 5% 6% 28%	2600
May	1st to 10th	Day	27% 16% 3 8% 4 4 4 3 5% 6% 19%	6024
	11th to 20th	Day	22% 12% 4 9% 5% 4 4 3 4 5% 28%	6318
	21st to end	Day	17% 13% 3 7% 5% 4 4 3 6% 8% 29%	6948
June	1st to 10th	Day	25% 10% 3 7% 6% 4 4 4 6% 6% 25%	2800
	11th to 20th	Day	20% 9% 3 7% 4 5% 3 3 5% 7% 33%	2929
	21st to end	Day	25% 10% 4 7% 6% 4 4 3 5% 6% 27%	3000
July	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 9% 27%	6692
	11th to 20th	Day	14% 9% 3 8% 5% 4 5% 4 5% 8% 34%	6360
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 7% 9% 35%	6713
August	1st to 10th	Day	21% 9% 3 8% 6% 4 5% 4 5% 7% 28%	4235
	11th to 20th	Day	14% 8% 3 8% 6% 5% 5% 4 7% 7% 34%	4175
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 43%	4492
September	1st to 10th	Day	17% 7% 2 5% 4 4 5% 3 5% 8% 41%	2316
	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	20% 15% 3 8% 6% 5% 5% 3 4 5% 27%	1617
	11th to 20th	Day	20% 21% 5% 8% 4 4 4 3 4 5% 23%	1600
	21st to end	Day	18% 18% 5% 10% 7% 4 4 5% 5% 5% 19%	1600
December	1st to 10th	Day	18% 16% 11% 16% 5% 3 4 3 4 5% 15%	2610
	11th to 20th	Day	23% 15% 6% 13% 7% 5% 4 2 4 3 18%	2996
	21st to end	Day	20% 12% 9% 16% 6% 4 4 3 3 4 18%	3982
January	1st to 10th	Night	14% 18% 7% 20% 7% 4 3 2 4 5% 16%	6349
	11th to 20th	Night	14% 18% 8% 22% 7% 5% 4 3 3 3 14%	6162
	21st to end	Night	13% 11% 5% 16% 11% 6% 5% 4 5% 5% 19%	6490
February	1st to 10th	Night	18% 15% 5% 13% 9% 6% 5% 4 4 4 17%	2600
	11th to 20th	Night	16% 15% 5% 12% 10% 5% 4 4 5% 5% 19%	2533
	21st to end	Night	11% 16% 4 12% 6% 5% 4 3 5% 4 28%	2120
March	1st to 10th	Night	22% 16% 5% 12% 8% 5% 3 3 3 4 19%	4915
	11th to 20th	Night	17% 17% 7% 8% 4 4 4 3 4 5% 27%	4979
	21st to end	Night	24% 18% 6% 11% 4 3 3 2 3 3 23%	5466
April	1st to 10th	Night	22% 15% 4 7% 4 4 4 3 5% 5% 26%	2010
	11th to 20th	Night	21% 14% 3 6% 5% 4 3 3 4 7% 31%	1896
	21st to end	Night	21% 15% 3 6% 5% 3 4 4 4 5% 30%	1800
May	1st to 10th	Night	24% 17% 4 9% 5% 5% 4 3 4 4 22%	3909
	11th to 20th	Night	23% 11% 3 7% 4 4 4 3 5% 6% 30%	3596
	21st to end	Night	16% 14% 3 7% 4 4 4 4 5% 7% 31%	3948
June	1st to 10th	Night	24% 9% 3 7% 4 3 5% 4 5% 8% 29%	1600
	11th to 20th	Night	19% 9% 3 7% 4 5% 4 3 5% 6% 36%	1484
	21st to end	Night	22% 9% 3 8% 6% 5% 4 5% 5% 7% 26%	1400
July	1st to 10th	Night	18% 9% 4 9% 6% 5% 6% 5% 6% 8% 25%	3224
	11th to 20th	Night	15% 8% 2 6% 4 4 5% 4 6% 9% 37%	3620
	21st to end	Night	13% 7% 3 7% 4 4 4 3 7% 8% 40%	4136
August	1st to 10th	Night	18% 8% 3 7% 4 4 5% 4 6% 7% 34%	2915
	11th to 20th	Night	13% 9% 3 8% 5% 6% 4 4 5% 7% 36%	2872
	21st to end	Night	13% 5% 2 5% 3 3 5% 3 5% 7% 49%	3406
September	1st to 10th	Night	13% 7% 2 5% 5% 4 4 4 5% 8% 44%	1931
	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% 5% 31%	2676
	11th to 20th	Night	19% 19% 4 10% 6% 5% 4 4 4 5% 22%	2800
	21st to end	Night	20% 16% 5% 9% 6% 5% 4 3 4 5% 23%	2800
December	1st to 10th	Night	15% 14% 7% 18% 6% 5% 4 3 4 4 19%	4580
	11th to 20th	Night	20% 15% 5% 13% 7% 4 4 3 4 3 22%	5213
	21st to end	Night	21% 15% 7% 12% 7% 3 3 2 3 3 24%	6996

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	73% 5% 11% 7% 3	3617
	11th to 20th	Day	74% 7% 8% 10% 2	3752
	21st to end	Day	60% 6% 9% 12% 11% 2	4466
February	1st to 10th	Day	69% 4 8% 12% 6% 1	1800
	11th to 20th	Day	67% 5% 9% 12% 6% 1	1761
	21st to end	Day	56% 5% 11% 17% 9% 2	1508
March	1st to 10th	Day	69% 3 7% 12% 8% 2	4927
	11th to 20th	Day	57% 7% 12% 18% 6%	4980
	21st to end	Day	70% 3 5% 14% 8% 1	5466
April	1st to 10th	Day	64% 3 6% 13% 11% 2	2390
	11th to 20th	Day	54% 5% 8% 20% 11% 1	2451
	21st to end	Day	56% 5% 9% 17% 11% 2	2600
May	1st to 10th	Day	66% 5% 9% 14% 6%	6035
	11th to 20th	Day	59% 5% 10% 20% 5%	6322
	21st to end	Day	52% 7% 13% 18% 10%	6948
June	1st to 10th	Day	58% 6% 11% 18% 6% 1	2800
	11th to 20th	Day	51% 5% 12% 20% 10% 1	2929
	21st to end	Day	58% 6% 11% 15% 9% 1	3000
July	1st to 10th	Day	54% 5% 15% 16% 9% 1	6619
	11th to 20th	Day	48% 7% 14% 23% 8%	6284
	21st to end	Day	43% 8% 17% 24% 8%	6521
August	1st to 10th	Day	57% 5% 12% 18% 8%	4222
	11th to 20th	Day	47% 7% 15% 23% 8%	4214
	21st to end	Day	36% 7% 13% 27% 16% 1	4492
September	1st to 10th	Day	43% 5% 13% 25% 13% 2	2316
	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% 7% 1	1617
	11th to 20th	Day	62% 7% 11% 14% 5% 1	1600
	21st to end	Day	65% 7% 12% 12% 4	1600
December	1st to 10th	Day	71% 5% 7% 15% 2	2610
	11th to 20th	Day	70% 5% 7% 13% 5%	2995
	21st to end	Day	70% 3 11% 12% 5%	3982
January	1st to 10th	Night	69% 6% 9% 13% 2	6349
	11th to 20th	Night	74% 6% 7% 10% 3	6207
	21st to end	Night	63% 6% 10% 13% 7% 1	6490
February	1st to 10th	Night	69% 5% 8% 11% 7% 1	2600
	11th to 20th	Night	66% 5% 11% 13% 5% 1	2536
	21st to end	Night	58% 5% 10% 19% 8% 1	2120
March	1st to 10th	Night	70% 3 5% 12% 9% 1	4924
	11th to 20th	Night	58% 5% 11% 18% 7% 1	4980
	21st to end	Night	67% 3 6% 16% 7% 1	5466
April	1st to 10th	Night	60% 5% 8% 14% 10% 3	2010
	11th to 20th	Night	55% 4 8% 20% 12% 1	1896
	21st to end	Night	56% 4 10% 16% 11% 2	1800
May	1st to 10th	Night	66% 4 9% 13% 7% 1	3915
	11th to 20th	Night	55% 5% 10% 22% 7% 2	3597
	21st to end	Night	53% 6% 15% 20% 6% 1	3949
June	1st to 10th	Night	54% 6% 13% 20% 7% 1	1600
	11th to 20th	Night	49% 4 13% 21% 12% 1	1484
	21st to end	Night	57% 8% 12% 16% 7% 1	1400
July	1st to 10th	Night	56% 7% 13% 16% 7% 1	3188
	11th to 20th	Night	44% 7% 16% 21% 11% 1	3572
	21st to end	Night	41% 7% 17% 23% 12% 1	4015
August	1st to 10th	Night	49% 7% 15% 23% 6%	2903
	11th to 20th	Night	47% 6% 17% 22% 8%	2899
	21st to end	Night	33% 5% 18% 27% 16% 1	3405
September	1st to 10th	Night	39% 5% 13% 27% 14% 2	1931
	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	54% 5% 13% 18% 9% 1	2676
	11th to 20th	Night	62% 6% 10% 15% 6%	2800
	21st to end	Night	63% 6% 14% 14% 3	2800
December	1st to 10th	Night	67% 5% 9% 17% 2	4580
	11th to 20th	Night	66% 4 9% 16% 5% 1	5215
	21st to end	Night	65% 4 10% 16% 5%	6996

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	35% 23% 30% 12% 1	400
	11th to 20th	Day	36% 21% 28% 13% 4	400
	21st to end	Day	27% 26% 33% 12% 3	440
February	1st to 10th	Day	34% 23% 29% 12% 2	400
	11th to 20th	Day	39% 24% 25% 9% 2	396
	21st to end	Day	28% 25% 31% 12% 2 1	331
March	1st to 10th	Day	35% 24% 30% 9% 2	498
	11th to 20th	Day	30% 28% 28% 9% 4	500
	21st to end	Day	37% 29% 23% 9% 3	550
April	1st to 10th	Day	43% 23% 26% 6% 1	500
	11th to 20th	Day	37% 27% 25% 9% 3	492
	21st to end	Day	34% 23% 26% 13% 3	500
May	1st to 10th	Day	39% 21% 27% 9% 3 1	563
	11th to 20th	Day	28% 20% 29% 17% 5% 1	724
	21st to end	Day	31% 17% 33% 15% 3 1	790
June	1st to 10th	Day	34% 16% 30% 14% 5%	730
	11th to 20th	Day	28% 14% 33% 19% 5% 1	730
	21st to end	Day	33% 13% 30% 17% 5% 1	730
July	1st to 10th	Day	24% 10% 37% 22% 7% 1	720
	11th to 20th	Day	19% 10% 37% 24% 8% 1	718
	21st to end	Day	21% 12% 35% 25% 6% 1	708
August	1st to 10th	Day	15% 9% 40% 27% 7% 2	526
	11th to 20th	Day	22% 10% 35% 19% 11% 3	519
	21st to end	Day	17% 16% 34% 21% 9% 4	583
September	1st to 10th	Day	18% 15% 35% 23% 7% 1	502
	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	31% 20% 28% 15% 5% 1	395
	11th to 20th	Day	42% 18% 26% 10% 4 1	400
	21st to end	Day	36% 24% 26% 11% 2 1	400
December	1st to 10th	Day	39% 21% 27% 11% 2 1	400
	11th to 20th	Day	41% 24% 25% 9% 2	400
	21st to end	Day	34% 24% 29% 9% 3 1	440
January	1st to 10th	Night	31% 21% 31% 15% 1	900
	11th to 20th	Night	30% 22% 31% 15% 3	868
	21st to end	Night	26% 24% 32% 16% 2	880
February	1st to 10th	Night	32% 21% 31% 15% 2	800
	11th to 20th	Night	35% 26% 26% 12% 2	783
	21st to end	Night	23% 24% 31% 18% 4	660
March	1st to 10th	Night	30% 25% 29% 14% 2	800
	11th to 20th	Night	27% 20% 31% 18% 4	800
	21st to end	Night	30% 18% 33% 15% 4	880
April	1st to 10th	Night	35% 19% 28% 16% 3	610
	11th to 20th	Night	31% 20% 30% 15% 3 1	594
	21st to end	Night	30% 18% 30% 17% 4	600
May	1st to 10th	Night	31% 16% 32% 17% 4 1	600
	11th to 20th	Night	29% 13% 30% 23% 5% 1	592
	21st to end	Night	22% 15% 40% 21% 2	649
June	1st to 10th	Night	30% 12% 33% 22% 3	600
	11th to 20th	Night	24% 12% 34% 26% 4	484
	21st to end	Night	26% 7% 31% 29% 7%	400
July	1st to 10th	Night	18% 6% 35% 32% 8%	414
	11th to 20th	Night	18% 7% 35% 32% 8%	582
	21st to end	Night	17% 7% 34% 33% 8%	656
August	1st to 10th	Night	17% 6% 31% 35% 10% 1	598
	11th to 20th	Night	17% 8% 27% 34% 12% 1	592
	21st to end	Night	16% 8% 30% 34% 12%	660
September	1st to 10th	Night	17% 9% 35% 29% 10% 1	580
	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	30% 16% 29% 19% 5% 1	870
	11th to 20th	Night	32% 20% 26% 17% 4	900
	21st to end	Night	34% 20% 26% 16% 3	900
December	1st to 10th	Night	34% 19% 30% 14% 3	900
	11th to 20th	Night	34% 23% 29% 12% 2	900
	21st to end	Night	33% 21% 27% 16% 2 1	990

Page generated at 2025-09-01 02:51:03. Version ac07

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