Saint John's University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Saint John's University Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 38%	101863
	Night	12% 9% 3 8% 5% 4 4 4 5% 7% 39%	97232
Transparency	Day	44% 4 13% 26% 11% 1	105475
	Night	44% 4 13% 26% 13% 2	101750
Seeing	Day	16% 10% 31% 27% 13% 3	16058
	Night	15% 9% 32% 32% 11% 1	25924

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	11% 15% 4 14% 8% 6% 5% 3 5% 6% 23%	2218
	11th to 20th	Day	5% 9% 5% 15% 10% 8% 8% 6% 7% 6% 22%	2584
	21st to end	Day	6% 17% 9% 13% 6% 5% 6% 4 5% 7% 23%	3135
February	1st to 10th	Day	8% 12% 6% 9% 7% 6% 4 4 4 6% 35%	1710
	11th to 20th	Day	7% 10% 4 11% 8% 5% 5% 4 6% 7% 34%	1664
	21st to end	Day	8% 12% 4 12% 7% 5% 4 4 6% 6% 32%	1418
March	1st to 10th	Day	11% 12% 4 8% 6% 5% 5% 4 5% 6% 35%	3436
	11th to 20th	Day	9% 9% 3 8% 4 5% 5% 4 5% 8% 40%	3480
	21st to end	Day	21% 13% 4 10% 7% 5% 5% 3 5% 5% 23%	3814
April	1st to 10th	Day	17% 10% 3 8% 4 4 3 3 5% 7% 35%	2261
	11th to 20th	Day	18% 13% 4 8% 4 4 4 3 4 6% 31%	2345
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 5% 5% 32%	2470
May	1st to 10th	Day	17% 7% 2 7% 4 4 4 4 5% 7% 39%	4196
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 44%	4399
	21st to end	Day	17% 7% 3 7% 4 5% 6% 4 6% 9% 34%	4824
June	1st to 10th	Day	12% 6% 2 6% 3 4 4 4 6% 8% 45%	2703
	11th to 20th	Day	14% 6% 3 7% 5% 5% 4 4 6% 8% 38%	2850
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 4 8% 11% 40%	2850
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 10% 38%	4750
	11th to 20th	Day	5% 3 1 4 3 3 4 4 6% 9% 58%	4576
	21st to end	Day	6% 4 2 5% 3 5% 5% 5% 7% 9% 48%	4674
August	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 7% 8% 44%	4104
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 5% 7% 10% 40%	4041
	21st to end	Day	10% 7% 2 6% 4 4 4 4 5% 8% 46%	4422
September	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 5% 9% 51%	2280
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 6% 8% 46%	2185
	21st to end	Day	14% 6% 2 6% 3 3 4 3 5% 7% 46%	2043
October	1st to 10th	Day	17% 8% 2 6% 3 3 3 4 5% 6% 42%	1900
	11th to 20th	Day	13% 11% 3 7% 4 3 4 3 5% 7% 39%	1900
	21st to end	Day	17% 10% 3 7% 4 3 4 3 4 7% 37%	2085
November	1st to 10th	Day	14% 12% 2 7% 6% 4 4 3 6% 6% 37%	1537
	11th to 20th	Day	9% 13% 7% 12% 5% 5% 4 3 5% 6% 29%	1520
	21st to end	Day	13% 10% 4 8% 5% 3 5% 4 6% 7% 35%	1520
December	1st to 10th	Day	9% 13% 5% 11% 7% 7% 5% 4 5% 6% 27%	1520
	11th to 20th	Day	10% 15% 6% 12% 6% 5% 4 3 5% 7% 28%	1918
	21st to end	Day	22% 15% 5% 16% 5% 3 4 2 4 5% 20%	2531
January	1st to 10th	Night	10% 12% 6% 16% 7% 4 4 3 5% 6% 28%	4737
	11th to 20th	Night	6% 12% 5% 12% 8% 8% 9% 7% 7% 6% 21%	4365
	21st to end	Night	7% 20% 8% 11% 5% 5% 4 4 5% 6% 25%	4532
February	1st to 10th	Night	6% 11% 5% 9% 6% 5% 4 4 6% 8% 35%	2470
	11th to 20th	Night	5% 9% 4 9% 5% 4 4 4 6% 8% 40%	2403
	21st to end	Night	11% 11% 3 8% 5% 5% 5% 4 5% 6% 36%	2040
March	1st to 10th	Night	8% 8% 3 7% 4 4 4 3 5% 7% 47%	3429
	11th to 20th	Night	8% 8% 3 8% 5% 4 5% 4 5% 7% 43%	3480
	21st to end	Night	23% 13% 3 11% 5% 3 3 2 4 4 27%	3817
April	1st to 10th	Night	14% 10% 4 7% 5% 4 4 3 5% 6% 39%	1919
	11th to 20th	Night	18% 11% 4 7% 4 3 3 3 5% 6% 37%	1783
	21st to end	Night	18% 10% 3 7% 4 3 3 4 5% 6% 36%	1710
May	1st to 10th	Night	14% 9% 3 7% 4 4 4 3 5% 6% 43%	2709
	11th to 20th	Night	15% 6% 1 4 4 5% 4 3 5% 7% 46%	2506
	21st to end	Night	17% 7% 2 6% 4 3 4 3 5% 7% 41%	2743
June	1st to 10th	Night	10% 5% 1 5% 3 4 5% 4 6% 8% 48%	1472
	11th to 20th	Night	8% 5% 2 5% 5% 4 4 5% 6% 8% 48%	1330
	21st to end	Night	9% 3 3 5% 4 3 5% 3 5% 7% 53%	1330
July	1st to 10th	Night	10% 4 1 4 4 4 5% 5% 8% 11% 44%	2220
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 9% 62%	2392
	21st to end	Night	5% 4 2 5% 4 3 5% 4 6% 8% 56%	2872
August	1st to 10th	Night	11% 6% 2 5% 4 2 4 3 5% 7% 51%	2836
	11th to 20th	Night	13% 5% 2 4 3 4 5% 4 5% 7% 48%	2798
	21st to end	Night	9% 6% 2 6% 3 3 4 3 6% 9% 48%	3228
September	1st to 10th	Night	8% 3 1 5% 3 3 3 4 5% 9% 55%	1900
	11th to 20th	Night	9% 6% 1 5% 3 3 4 3 4 6% 55%	1993
	21st to end	Night	13% 5% 2 5% 3 3 3 3 5% 6% 52%	2118
October	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 5% 6% 49%	2280
	11th to 20th	Night	14% 9% 2 6% 4 3 4 3 5% 6% 44%	2280
	21st to end	Night	16% 10% 3 8% 4 4 3 3 5% 7% 37%	2508
November	1st to 10th	Night	12% 9% 3 7% 4 4 4 4 6% 7% 40%	2641
	11th to 20th	Night	12% 10% 5% 11% 5% 4 4 4 4 6% 34%	2660
	21st to end	Night	12% 9% 3 8% 5% 4 4 3 5% 7% 40%	2660
December	1st to 10th	Night	9% 11% 5% 11% 7% 5% 5% 3 6% 6% 31%	2660
	11th to 20th	Night	10% 13% 5% 11% 7% 5% 5% 5% 5% 6% 27%	3279
	21st to end	Night	21% 15% 5% 12% 6% 4 4 3 4 5% 21%	5132

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	62% 5% 11% 17% 5%	2219
	11th to 20th	Day	60% 3 13% 20% 5%	2584
	21st to end	Day	61% 4 12% 17% 6%	3135
February	1st to 10th	Day	51% 3 12% 23% 9% 1	1710
	11th to 20th	Day	50% 5% 12% 25% 7% 1	1673
	21st to end	Day	53% 4 11% 21% 11% 1	1418
March	1st to 10th	Day	50% 4 13% 25% 8% 1	3436
	11th to 20th	Day	42% 4 12% 32% 10% 1	3480
	21st to end	Day	65% 3 9% 17% 5%	3814
April	1st to 10th	Day	49% 3 12% 21% 14% 1	2261
	11th to 20th	Day	55% 4 8% 20% 12% 1	2345
	21st to end	Day	55% 3 11% 21% 10% 1	2470
May	1st to 10th	Day	44% 5% 11% 25% 15% 1	4234
	11th to 20th	Day	39% 3 10% 31% 13% 2	4403
	21st to end	Day	46% 4 12% 29% 8% 1	4837
June	1st to 10th	Day	37% 4 15% 31% 12% 1	2703
	11th to 20th	Day	43% 6% 14% 21% 14% 1	2850
	21st to end	Day	36% 5% 18% 28% 11% 2	2850
July	1st to 10th	Day	38% 7% 19% 23% 13%	4676
	11th to 20th	Day	23% 5% 17% 39% 15% 2	4496
	21st to end	Day	30% 6% 16% 33% 14% 1	4527
August	1st to 10th	Day	38% 5% 12% 32% 12% 2	4090
	11th to 20th	Day	38% 5% 15% 32% 9%	4080
	21st to end	Day	36% 5% 16% 30% 13% 1	4421
September	1st to 10th	Day	31% 4 14% 37% 13%	2280
	11th to 20th	Day	36% 5% 18% 32% 9% 1	2185
	21st to end	Day	38% 4 11% 34% 11% 1	2043
October	1st to 10th	Day	32% 3 10% 33% 21% 1	3160
	11th to 20th	Day	38% 4 12% 23% 21% 2	3140
	21st to end	Day	47% 2 14% 28% 9%	3412
November	1st to 10th	Day	48% 4 12% 21% 13% 3	1537
	11th to 20th	Day	55% 3 12% 19% 10% 1	1520
	21st to end	Day	49% 4 16% 23% 7% 2	1520
December	1st to 10th	Day	57% 4 15% 18% 6%	1520
	11th to 20th	Day	57% 3 17% 19% 4	1915
	21st to end	Day	68% 2 9% 14% 6%	2531
January	1st to 10th	Night	58% 3 12% 20% 6% 1	4737
	11th to 20th	Night	62% 3 13% 16% 6%	4365
	21st to end	Night	60% 3 10% 21% 5% 1	4532
February	1st to 10th	Night	47% 3 13% 24% 11% 2	2470
	11th to 20th	Night	42% 5% 15% 25% 11% 2	2404
	21st to end	Night	47% 4 13% 22% 13% 2	2040
March	1st to 10th	Night	37% 3 14% 27% 17% 2	3429
	11th to 20th	Night	40% 3 10% 30% 16% 1	3480
	21st to end	Night	61% 3 7% 21% 6% 1	3817
April	1st to 10th	Night	47% 3 10% 25% 13% 1	1919
	11th to 20th	Night	50% 3 7% 23% 16% 2	1783
	21st to end	Night	49% 4 9% 23% 13% 2	1710
May	1st to 10th	Night	42% 3 11% 26% 15% 3	2736
	11th to 20th	Night	38% 4 10% 25% 20% 3	2507
	21st to end	Night	42% 4 14% 25% 14% 1	2751
June	1st to 10th	Night	32% 5% 16% 30% 14% 2	1472
	11th to 20th	Night	33% 5% 17% 27% 16% 3	1330
	21st to end	Night	30% 5% 16% 29% 17% 3	1330
July	1st to 10th	Night	32% 7% 22% 22% 16% 1	2185
	11th to 20th	Night	19% 4 14% 41% 19% 3	2349
	21st to end	Night	24% 7% 17% 35% 15% 3	2781
August	1st to 10th	Night	34% 4 13% 30% 14% 5%	2826
	11th to 20th	Night	35% 5% 15% 32% 12% 1	2825
	21st to end	Night	33% 6% 18% 30% 12% 2	3228
September	1st to 10th	Night	26% 5% 17% 34% 17% 1	1900
	11th to 20th	Night	30% 4 17% 33% 16%	1993
	21st to end	Night	32% 3 12% 35% 16% 1	2118
October	1st to 10th	Night	24% 2 7% 37% 26% 4	3800
	11th to 20th	Night	34% 3 11% 28% 21% 2	3779
	21st to end	Night	42% 3 15% 27% 12%	4120
November	1st to 10th	Night	42% 3 14% 25% 13% 2	2641
	11th to 20th	Night	52% 4 13% 20% 11% 1	2660
	21st to end	Night	44% 4 18% 25% 7% 2	2660
December	1st to 10th	Night	54% 3 16% 21% 7%	2660
	11th to 20th	Night	56% 4 14% 18% 6% 2	3281
	21st to end	Night	67% 3 10% 16% 5%	5132

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	15% 11% 29% 30% 13% 1	270
	11th to 20th	Day	14% 11% 39% 31% 6%	270
	21st to end	Day	13% 7% 40% 24% 13% 2	297
February	1st to 10th	Day	14% 17% 41% 25% 3	270
	11th to 20th	Day	14% 19% 39% 21% 7%	270
	21st to end	Day	18% 11% 42% 27% 2	225
March	1st to 10th	Day	18% 15% 39% 22% 5% 1	270
	11th to 20th	Day	20% 10% 36% 23% 9% 2	270
	21st to end	Day	26% 13% 26% 23% 10% 2	297
April	1st to 10th	Day	27% 15% 29% 22% 7%	441
	11th to 20th	Day	27% 15% 26% 21% 10% 1	543
	21st to end	Day	25% 12% 32% 20% 9% 2	650
May	1st to 10th	Day	21% 13% 34% 23% 8% 2	653
	11th to 20th	Day	19% 10% 31% 26% 12% 2	675
	21st to end	Day	19% 7% 30% 29% 12% 3	734
June	1st to 10th	Day	11% 9% 27% 31% 20% 3	680
	11th to 20th	Day	15% 9% 34% 29% 12% 1	680
	21st to end	Day	13% 7% 33% 28% 15% 4	680
July	1st to 10th	Day	8% 4 25% 34% 20% 10%	671
	11th to 20th	Day	9% 6% 28% 30% 20% 7%	668
	21st to end	Day	7% 5% 24% 35% 23% 6%	739
August	1st to 10th	Day	9% 5% 24% 33% 24% 5%	676
	11th to 20th	Day	13% 6% 22% 30% 24% 5%	665
	21st to end	Day	12% 6% 25% 33% 20% 4	690
September	1st to 10th	Day	12% 5% 30% 34% 16% 3	490
	11th to 20th	Day	10% 4 28% 35% 18% 4	395
	21st to end	Day	19% 11% 31% 23% 12% 4	300
October	1st to 10th	Day	19% 10% 33% 22% 12% 4	300
	11th to 20th	Day	15% 18% 36% 23% 7% 1	300
	21st to end	Day	22% 18% 37% 17% 6% 1	330
November	1st to 10th	Day	23% 14% 28% 26% 6% 2	282
	11th to 20th	Day	14% 19% 43% 21% 2	270
	21st to end	Day	21% 14% 39% 21% 6%	270
December	1st to 10th	Day	15% 11% 43% 22% 8% 1	270
	11th to 20th	Day	19% 16% 38% 23% 4	270
	21st to end	Day	30% 12% 34% 21% 3 1	297
January	1st to 10th	Night	10% 12% 37% 30% 9% 2	950
	11th to 20th	Night	17% 10% 37% 28% 8% 1	950
	21st to end	Night	16% 12% 35% 29% 8%	1045
February	1st to 10th	Night	11% 12% 39% 29% 8% 1	950
	11th to 20th	Night	11% 13% 40% 28% 7% 1	929
	21st to end	Night	16% 11% 33% 31% 8% 1	784
March	1st to 10th	Night	15% 10% 38% 27% 9% 1	580
	11th to 20th	Night	16% 10% 40% 26% 6% 1	570
	21st to end	Night	22% 9% 31% 26% 10% 2	627
April	1st to 10th	Night	19% 11% 37% 26% 6%	570
	11th to 20th	Night	22% 12% 29% 28% 8%	563
	21st to end	Night	22% 8% 32% 29% 8% 2	570
May	1st to 10th	Night	17% 10% 35% 29% 8% 1	570
	11th to 20th	Night	16% 9% 30% 34% 11% 1	561
	21st to end	Night	15% 7% 26% 38% 13%	617
June	1st to 10th	Night	13% 5% 27% 39% 16% 1	570
	11th to 20th	Night	11% 4 34% 41% 9%	570
	21st to end	Night	9% 6% 27% 43% 15% 1	570
July	1st to 10th	Night	4 3 23% 44% 26% 1	555
	11th to 20th	Night	6% 4 25% 45% 18% 2	552
	21st to end	Night	4 3 24% 46% 21% 2	624
August	1st to 10th	Night	10% 3 18% 47% 21% 1	567
	11th to 20th	Night	12% 3 17% 41% 24% 2	564
	21st to end	Night	9% 4 24% 37% 24% 1	627
September	1st to 10th	Night	10% 5% 26% 39% 18% 2	570
	11th to 20th	Night	11% 6% 30% 33% 19% 1	570
	21st to end	Night	14% 10% 32% 29% 13% 2	598
October	1st to 10th	Night	16% 9% 34% 29% 11% 1	760
	11th to 20th	Night	19% 10% 34% 30% 7% 1	760
	21st to end	Night	22% 13% 32% 27% 7%	836
November	1st to 10th	Night	17% 11% 32% 31% 8% 1	950
	11th to 20th	Night	18% 13% 35% 26% 8% 1	950
	21st to end	Night	17% 14% 38% 25% 7%	950
December	1st to 10th	Night	14% 12% 35% 30% 8% 1	950
	11th to 20th	Night	19% 9% 38% 28% 6% 1	950
	21st to end	Night	24% 11% 33% 26% 5% 1	1045

Page generated at 2024-11-03 01:11:28. Version ac07

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