Irma
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Irma clear sky chart from 2005-11-06 to 2024-01-12.

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	10% 9% 4 7% 7% 4 6% 6% 6% 8% 34%	74654
	Night	8% 9% 4 8% 7% 4 6% 6% 6% 8% 34%	75071
Transparency	Day	47% 6% 17% 22% 7% 1	74335
	Night	47% 6% 16% 21% 9% 1	74908
Seeing	Day	13% 8% 27% 31% 17% 5%	14391
	Night	12% 6% 29% 36% 16% 2	23641

By Month

Percentage Cloud Cover NW (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	7% 11% 6% 10% 10% 6% 6% 6% 6% 7% 25%	1329
	11th to 20th	Day	9% 10% 5% 10% 10% 4 6% 6% 7% 7% 27%	1267
	21st to end	Day	6% 15% 7% 12% 10% 4 6% 5% 5% 7% 23%	1426
February	1st to 10th	Day	8% 11% 5% 13% 10% 5% 6% 7% 6% 7% 22%	1486
	11th to 20th	Day	7% 13% 6% 10% 8% 4 6% 5% 8% 9% 24%	1575
	21st to end	Day	6% 12% 5% 10% 10% 6% 7% 6% 8% 10% 21%	1332
March	1st to 10th	Day	8% 13% 6% 10% 10% 5% 7% 6% 7% 8% 20%	1823
	11th to 20th	Day	7% 13% 5% 9% 8% 3 5% 5% 6% 7% 33%	1979
	21st to end	Day	10% 12% 6% 12% 8% 5% 5% 6% 5% 8% 25%	2178
April	1st to 10th	Day	8% 11% 5% 9% 7% 4 6% 6% 6% 8% 30%	2244
	11th to 20th	Day	12% 11% 5% 9% 6% 3 5% 6% 6% 7% 30%	2310
	21st to end	Day	14% 11% 4 6% 5% 4 6% 6% 5% 8% 32%	2366
May	1st to 10th	Day	10% 10% 3 8% 8% 5% 6% 6% 6% 8% 31%	2554
	11th to 20th	Day	9% 7% 2 7% 6% 4 6% 5% 7% 9% 39%	2654
	21st to end	Day	15% 7% 3 6% 6% 3 6% 6% 6% 7% 33%	2917
June	1st to 10th	Day	14% 8% 3 6% 6% 4 6% 6% 7% 8% 31%	2700
	11th to 20th	Day	20% 9% 3 7% 7% 4 7% 7% 6% 7% 24%	2700
	21st to end	Day	11% 6% 2 5% 6% 4 6% 6% 8% 10% 35%	2700
July	1st to 10th	Day	8% 4 2 5% 5% 4 6% 6% 7% 10% 42%	2700
	11th to 20th	Day	11% 5% 2 5% 6% 4 6% 6% 7% 11% 37%	2700
	21st to end	Day	8% 5% 2 5% 5% 4 4 5% 8% 9% 46%	2938
August	1st to 10th	Day	9% 5% 3 5% 6% 4 6% 5% 7% 9% 40%	2631
	11th to 20th	Day	9% 5% 3 5% 5% 3 5% 5% 6% 8% 44%	2299
	21st to end	Day	10% 6% 2 5% 5% 3 5% 5% 6% 8% 45%	2574
September	1st to 10th	Day	11% 6% 2 6% 6% 4 5% 6% 5% 8% 42%	2257
	11th to 20th	Day	15% 8% 3 5% 5% 3 3 4 5% 7% 42%	2029
	21st to end	Day	10% 6% 2 5% 5% 3 4 5% 5% 8% 48%	1980
October	1st to 10th	Day	11% 10% 4 7% 5% 4 5% 5% 5% 6% 39%	1809
	11th to 20th	Day	10% 9% 4 6% 6% 3 4 5% 5% 8% 40%	1746
	21st to end	Day	8% 13% 5% 8% 7% 4 5% 5% 6% 8% 32%	1692
November	1st to 10th	Day	12% 13% 5% 8% 9% 4 7% 6% 5% 7% 26%	1488
	11th to 20th	Day	7% 12% 7% 13% 8% 3 4 5% 6% 8% 28%	1487
	21st to end	Day	7% 10% 6% 11% 9% 4 6% 5% 6% 9% 28%	1330
December	1st to 10th	Day	7% 12% 6% 9% 9% 6% 8% 7% 7% 6% 23%	1330
	11th to 20th	Day	7% 9% 4 11% 9% 5% 8% 6% 8% 8% 25%	1682
	21st to end	Day	5% 8% 4 7% 7% 4 5% 4 6% 8% 43%	2442
January	1st to 10th	Night	7% 9% 6% 10% 9% 6% 7% 8% 7% 7% 24%	2836
	11th to 20th	Night	5% 11% 6% 10% 9% 5% 6% 5% 6% 8% 30%	2722
	21st to end	Night	6% 13% 6% 12% 8% 4 8% 5% 6% 7% 24%	2948
February	1st to 10th	Night	6% 8% 4 12% 10% 5% 7% 7% 7% 7% 26%	2473
	11th to 20th	Night	6% 10% 5% 9% 9% 6% 6% 8% 8% 7% 27%	2272
	21st to end	Night	4 8% 5% 9% 10% 4 7% 7% 7% 9% 31%	1924
March	1st to 10th	Night	7% 10% 5% 11% 9% 5% 6% 7% 6% 8% 27%	2137
	11th to 20th	Night	7% 10% 4 9% 8% 4 5% 6% 6% 9% 31%	1980
	21st to end	Night	9% 11% 5% 9% 7% 5% 5% 5% 6% 9% 29%	2178
April	1st to 10th	Night	7% 8% 4 6% 7% 4 5% 6% 6% 9% 38%	1714
	11th to 20th	Night	9% 9% 4 9% 8% 4 6% 7% 7% 7% 28%	1601
	21st to end	Night	13% 11% 4 7% 6% 3 4 5% 6% 7% 36%	1594
May	1st to 10th	Night	11% 8% 3 8% 7% 4 7% 5% 6% 8% 33%	1404
	11th to 20th	Night	9% 8% 3 7% 6% 4 6% 6% 7% 7% 40%	1239
	21st to end	Night	17% 7% 3 5% 6% 4 6% 4 6% 9% 33%	1358
June	1st to 10th	Night	12% 9% 4 7% 7% 5% 5% 7% 6% 7% 31%	1260
	11th to 20th	Night	17% 8% 3 6% 9% 4 5% 7% 7% 7% 27%	1260
	21st to end	Night	8% 3 2 5% 6% 5% 7% 8% 7% 9% 40%	1260
July	1st to 10th	Night	8% 7% 3 5% 5% 2 5% 7% 6% 10% 44%	1260
	11th to 20th	Night	9% 4 3 4 5% 3 6% 6% 8% 11% 41%	1260
	21st to end	Night	8% 3 2 4 4 2 5% 5% 7% 9% 51%	1379
August	1st to 10th	Night	7% 5% 2 5% 6% 4 6% 7% 9% 10% 39%	1304
	11th to 20th	Night	11% 5% 3 5% 5% 4 5% 5% 6% 9% 43%	1602
	21st to end	Night	8% 5% 2 6% 5% 3 4 5% 6% 8% 48%	1782
September	1st to 10th	Night	10% 5% 2 5% 5% 3 5% 4 6% 8% 46%	1702
	11th to 20th	Night	11% 6% 2 4 5% 3 4 5% 5% 7% 48%	1930
	21st to end	Night	7% 6% 2 5% 5% 3 5% 4 6% 8% 49%	1980
October	1st to 10th	Night	10% 7% 4 7% 6% 4 5% 5% 6% 8% 39%	2150
	11th to 20th	Night	9% 8% 3 7% 6% 3 4 6% 6% 8% 41%	2211
	21st to end	Night	8% 10% 4 9% 7% 4 6% 5% 6% 7% 35%	2645
November	1st to 10th	Night	12% 13% 6% 8% 6% 3 6% 5% 6% 6% 29%	2590
	11th to 20th	Night	8% 10% 5% 11% 10% 5% 5% 6% 6% 8% 26%	2692
	21st to end	Night	5% 10% 6% 11% 9% 4 6% 5% 6% 8% 31%	2850
December	1st to 10th	Night	8% 11% 6% 11% 9% 5% 6% 6% 6% 8% 25%	2850
	11th to 20th	Night	6% 11% 6% 12% 10% 5% 6% 6% 6% 8% 23%	3510
	21st to end	Night	3 7% 4 7% 7% 4 6% 5% 6% 7% 45%	5214

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	57% 6% 14% 16% 7%	1329
	11th to 20th	Day	54% 7% 14% 15% 8% 1	1267
	21st to end	Day	61% 5% 10% 16% 7% 1	1426
February	1st to 10th	Day	60% 5% 12% 17% 6%	1486
	11th to 20th	Day	56% 5% 15% 17% 7%	1584
	21st to end	Day	56% 5% 13% 18% 7%	1332
March	1st to 10th	Day	60% 6% 16% 14% 4	1823
	11th to 20th	Day	49% 5% 16% 20% 10% 1	1980
	21st to end	Day	58% 6% 14% 18% 4	2178
April	1st to 10th	Day	51% 6% 14% 20% 9% 1	2244
	11th to 20th	Day	51% 5% 14% 21% 8% 1	2310
	21st to end	Day	48% 6% 15% 21% 9% 1	2366
May	1st to 10th	Day	50% 6% 13% 21% 10%	2554
	11th to 20th	Day	41% 5% 14% 28% 11% 1	2658
	21st to end	Day	48% 6% 14% 20% 11%	2918
June	1st to 10th	Day	49% 7% 14% 22% 8%	2700
	11th to 20th	Day	56% 8% 16% 16% 4 1	2700
	21st to end	Day	41% 8% 24% 20% 6% 1	2700
July	1st to 10th	Day	36% 8% 24% 26% 6% 1	2626
	11th to 20th	Day	40% 8% 25% 24% 3	2619
	21st to end	Day	33% 7% 25% 30% 5%	2775
August	1st to 10th	Day	38% 9% 23% 25% 4	2613
	11th to 20th	Day	35% 7% 24% 25% 8%	2299
	21st to end	Day	35% 6% 21% 30% 7% 1	2574
September	1st to 10th	Day	39% 6% 20% 24% 9% 3	2257
	11th to 20th	Day	43% 4 17% 28% 8%	2029
	21st to end	Day	34% 7% 17% 30% 10% 2	1980
October	1st to 10th	Day	46% 4 17% 24% 8%	1809
	11th to 20th	Day	43% 5% 17% 25% 10%	1746
	21st to end	Day	49% 6% 14% 21% 8%	1692
November	1st to 10th	Day	57% 5% 13% 18% 5% 1	1488
	11th to 20th	Day	56% 6% 17% 19% 3	1487
	21st to end	Day	52% 6% 18% 18% 5% 1	1330
December	1st to 10th	Day	58% 7% 13% 15% 6% 1	1330
	11th to 20th	Day	54% 7% 17% 17% 5%	1684
	21st to end	Day	41% 4 12% 28% 15% 1	2442
January	1st to 10th	Night	56% 7% 14% 16% 6% 1	2836
	11th to 20th	Night	52% 5% 17% 18% 6% 1	2722
	21st to end	Night	60% 6% 13% 15% 7%	2948
February	1st to 10th	Night	54% 6% 14% 18% 8% 1	2473
	11th to 20th	Night	52% 6% 14% 18% 9%	2273
	21st to end	Night	48% 6% 14% 22% 9% 1	1924
March	1st to 10th	Night	53% 6% 16% 18% 6%	2137
	11th to 20th	Night	48% 6% 12% 24% 9% 1	1980
	21st to end	Night	51% 7% 15% 19% 8%	2178
April	1st to 10th	Night	41% 6% 15% 23% 14% 1	1714
	11th to 20th	Night	52% 6% 11% 18% 11% 2	1601
	21st to end	Night	46% 5% 13% 23% 11% 3	1594
May	1st to 10th	Night	48% 5% 10% 25% 11% 1	1404
	11th to 20th	Night	40% 5% 12% 27% 14% 1	1239
	21st to end	Night	49% 4 11% 25% 9% 2	1358
June	1st to 10th	Night	48% 5% 14% 24% 8% 1	1260
	11th to 20th	Night	52% 10% 14% 18% 7%	1260
	21st to end	Night	37% 9% 24% 23% 6% 1	1260
July	1st to 10th	Night	33% 8% 21% 28% 8% 1	1225
	11th to 20th	Night	34% 10% 25% 22% 8% 1	1218
	21st to end	Night	26% 8% 26% 31% 9%	1302
August	1st to 10th	Night	35% 11% 23% 26% 6%	1297
	11th to 20th	Night	37% 6% 22% 27% 7% 1	1602
	21st to end	Night	33% 5% 15% 33% 12% 1	1782
September	1st to 10th	Night	35% 6% 20% 24% 10% 5%	1702
	11th to 20th	Night	35% 5% 16% 31% 10% 2	1930
	21st to end	Night	32% 7% 14% 31% 13% 2	1980
October	1st to 10th	Night	42% 6% 15% 26% 9% 1	2150
	11th to 20th	Night	41% 5% 16% 28% 10%	2211
	21st to end	Night	48% 6% 14% 25% 7% 1	2645
November	1st to 10th	Night	54% 6% 14% 18% 8% 1	2590
	11th to 20th	Night	56% 6% 16% 16% 6% 1	2692
	21st to end	Night	51% 6% 20% 18% 5%	2850
December	1st to 10th	Night	57% 5% 12% 17% 8% 2	2850
	11th to 20th	Night	59% 7% 16% 11% 6%	3507
	21st to end	Night	39% 4 15% 22% 18% 2	5214

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	9% 4 32% 33% 18% 4	180
	11th to 20th	Day	8% 9% 29% 32% 20% 2	162
	21st to end	Day	13% 6% 36% 28% 14% 3	176
February	1st to 10th	Day	11% 8% 38% 31% 9% 3	181
	11th to 20th	Day	13% 6% 31% 33% 14% 2	240
	21st to end	Day	10% 5% 28% 33% 17% 8%	198
March	1st to 10th	Day	13% 9% 31% 30% 16% 2	392
	11th to 20th	Day	11% 6% 35% 31% 13% 5%	420
	21st to end	Day	13% 8% 28% 31% 17% 3	462
April	1st to 10th	Day	10% 11% 36% 30% 9% 3	420
	11th to 20th	Day	16% 8% 29% 31% 13% 3	412
	21st to end	Day	17% 11% 28% 30% 11% 2	446
May	1st to 10th	Day	14% 9% 24% 31% 17% 5%	603
	11th to 20th	Day	9% 8% 24% 33% 19% 7%	625
	21st to end	Day	16% 7% 22% 28% 18% 8%	679
June	1st to 10th	Day	14% 8% 26% 36% 14% 3	630
	11th to 20th	Day	20% 8% 25% 29% 15% 4	630
	21st to end	Day	11% 7% 22% 30% 21% 9%	630
July	1st to 10th	Day	10% 5% 17% 33% 26% 8%	621
	11th to 20th	Day	12% 6% 24% 33% 20% 6%	618
	21st to end	Day	8% 4 22% 29% 28% 8%	684
August	1st to 10th	Day	10% 5% 19% 31% 24% 9%	591
	11th to 20th	Day	9% 4 25% 33% 22% 6%	437
	21st to end	Day	12% 5% 22% 30% 21% 10%	495
September	1st to 10th	Day	13% 7% 33% 29% 14% 3	450
	11th to 20th	Day	19% 10% 24% 29% 14% 4	450
	21st to end	Day	15% 12% 27% 30% 12% 3	450
October	1st to 10th	Day	15% 9% 24% 33% 15% 4	279
	11th to 20th	Day	17% 7% 33% 28% 14% 1	270
	21st to end	Day	12% 11% 36% 27% 11% 3	297
November	1st to 10th	Day	17% 6% 34% 30% 12% 2	270
	11th to 20th	Day	11% 12% 38% 29% 9%	255
	21st to end	Day	8% 8% 32% 37% 10% 5%	180
December	1st to 10th	Day	11% 8% 31% 29% 16% 6%	180
	11th to 20th	Day	8% 12% 29% 29% 21% 1	180
	21st to end	Day	9% 5% 26% 37% 18% 5%	198
January	1st to 10th	Night	10% 7% 32% 37% 12% 2	950
	11th to 20th	Night	10% 8% 33% 35% 13% 2	908
	21st to end	Night	13% 11% 34% 32% 9% 1	968
February	1st to 10th	Night	8% 8% 25% 39% 17% 3	720
	11th to 20th	Night	11% 6% 33% 35% 14% 2	704
	21st to end	Night	7% 3 24% 40% 23% 3	592
March	1st to 10th	Night	9% 4 27% 42% 16% 2	720
	11th to 20th	Night	11% 4 32% 35% 16% 2	720
	21st to end	Night	12% 7% 28% 35% 16% 3	792
April	1st to 10th	Night	9% 8% 29% 35% 17% 2	568
	11th to 20th	Night	13% 7% 30% 37% 11% 2	533
	21st to end	Night	17% 8% 24% 37% 14%	540
May	1st to 10th	Night	15% 5% 25% 36% 16% 2	504
	11th to 20th	Night	12% 5% 24% 40% 18% 1	354
	21st to end	Night	20% 5% 23% 29% 21% 2	389
June	1st to 10th	Night	17% 6% 25% 37% 14% 2	360
	11th to 20th	Night	22% 5% 19% 34% 18% 3	360
	21st to end	Night	9% 4 23% 37% 24% 3	360
July	1st to 10th	Night	9% 5% 19% 40% 26%	350
	11th to 20th	Night	9% 4 22% 44% 20% 2	348
	21st to end	Night	7% 5% 18% 40% 25% 4	394
August	1st to 10th	Night	8% 5% 18% 38% 25% 7%	376
	11th to 20th	Night	12% 5% 18% 41% 22% 2	534
	21st to end	Night	9% 3 24% 40% 23% 1	594
September	1st to 10th	Night	12% 4 26% 37% 20% 1	540
	11th to 20th	Night	15% 5% 19% 43% 17% 1	670
	21st to end	Night	10% 4 31% 39% 15% 1	720
October	1st to 10th	Night	13% 4 30% 36% 16% 2	720
	11th to 20th	Night	13% 4 32% 36% 13% 2	720
	21st to end	Night	14% 8% 33% 35% 10% 1	863
November	1st to 10th	Night	19% 6% 31% 32% 10% 2	925
	11th to 20th	Night	13% 7% 34% 32% 11% 2	950
	21st to end	Night	8% 7% 37% 37% 10% 1	950
December	1st to 10th	Night	14% 7% 30% 30% 16% 4	950
	11th to 20th	Night	10% 10% 34% 32% 12% 1	950
	21st to end	Night	9% 4 33% 36% 16% 3	1045

Page generated at 2024-01-09 05:14:40. Version ac07

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