Past IEM Features tagged: sounding

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Cold Air Above Us

24 May 2017 05:33 AM
Tuesday was a return to chilly weather that has plagued our recent stretch of weather. Even with these chilly temperatures, convective clouds were able to form and bring brief shots of rain to the state and even a few reports of funnel clouds. The reason is that the air above us was relatively cool as well, so any bit of heating that the sunshine was able to produce resulted in rising parcels quickly reaching their lifted condensation levels and forming clouds. This rapid vertical motion with low cloud bases also promotes funnel clouds. Anyway, the featured plot shows mandatory sounding level temperature percentiles for the sounding Tuesday evening from Omaha. The lower atmosphere profile is dominated by cold percentiles around 10% and less. Interestingly, this reverses at levels above the tropopause.

Voting:
Good: 6
Bad: 1
Abstain: 1

Tags:   sounding  

High Heights

20 Jun 2016 05:35 AM
The big story in weather over much of the country are the extreme high temperatures being reported with 120 degrees being reported in some isolated locations over the desert southwestern US. This heat extends vertically in the atmosphere as well with a very strong ridge in place supporting this heat wave. Twice a day, vertical soundings of the atmosphere are sampled by weather balloons launched from a number of locations around the world. The featured chart looks at the reported height above sea level that a given mandatory pressure level was reported to be at on Sunday morning and the percentile rank of that value against the period of record for the Omaha area. Many of these levels are at record high heights for June as indicated by the percentiles near 100.

Voting:
Good: 9
Bad: 1
Abstain: 1

Tags:   sounding  


Cold above us as well

19 Feb 2015 05:41 AM
Even with plenty of sunshine on Wednesday, temperatures struggled well below freezing. There is currently plenty of cold air at the surface and above our heads as shown by the featured chart of 850 hPa temperatures for the Omaha sounding site. We've had two periods of near record warmth at this level followed by our current period at near record cold. This level is just a few thousand feet above our heads and represents air that is potentially mixed down during the say as the sun mixes the lower atmosphere.

Voting:
Good: 18
Bad: 7
Abstain: 4

Tags:   sounding  


500 hPa Temperatures

10 Jun 2014 05:38 AM
The featured chart presents a climatology of 500 hPa (millibar) temperatures from the morning sounding for Omaha along with this year's observations. This pressure level in the atmosphere is some over 3 miles above the ground surface and is one of the important levels meteorologists look at to forecast the weather. The chart clearly shows the significant dip around mid May that accompanied the very cold temperatures. Recently temperatures have been on the decrease, which helps promote thunderstorm activity as colder temperatures aloft create unstable situations.

Voting:
Good: 14
Bad: 2
Abstain: 4

Tags:   sounding  


Cold above us too

25 Mar 2014 05:37 AM
Near surface air temperatures have been a struggle recently and the same goes for the air just a few thousand feet above our head. The featured chart displays a weekly climatology of 850 hPa morning sounding temperatures from the Omaha site. The black line is a time series for this year and the dip below the 5th percentile this past weekend coincides with the cold temperatures we have experienced. With such cold air above us, any heating the sun is able to do of the surface will likely drive clouds and as we experienced yesterday, convective snow showers. Having warmer air above our heads allows near surface air to warm to higher temperatures.

Voting:
Good: 16
Bad: 3
Abstain: 2

Tags:   sounding  


Freezing Rain Profiles

20 Dec 2013 05:39 AM
Much of Iowa experienced freezing rain on Thursday evening and into Friday morning. Freezing rain happens when liquid water, in the form of rain, is able to reach the surface and then freeze as the physical surface and air temperature are both below freezing. In this situation, the layer of below freezing air temperature needs to be shallow otherwise the rain drops will freeze and hit the ground as sleet. The featured chart looks lower atmosphere soundings from the Omaha site during events of freezing rain reported at the Omaha Airport. The cases shown depict the classic freezing rain signature of having a shallow layer of freezing temperature air near the surface with another layer of above freezing temperature air above it. The left hand plot presents the profile of temperature for the 45 cases used in this analysis, including the event from Thursday evening. The right hand plot depicts the level of the warmest temperature for each of those cases. The warm layer of air is centered just a few thousand feet above the ground at temperatures typically above freezing. What would be snow falls into this warm layer, melts and reaches the ground as liquid water. Note: since the case selection was based off of automated data, some of the profiles shown may not have been producing freezing rain.

Voting:
Good: 83
Bad: 16
Abstain: 12

Tags:   freezingrain   sounding  


540 Thickness

22 Oct 2013 12:01 AM
A forecasting rule of thumb is to look at something called the "540 thickness" to delineate snow versus rain events. The technique involves subtracting the height of the 1000 mb surface from the 500 mb surface. When this depth of air, or thickness, falls below 540 decameters (5400 meters), the air is typically cold enough to support snow. The featured chart presents the combination of this thickness value vs near surface air temperature for events where rain or snow were reported by the Omaha airport weather station. There are many caveats to this chart including exact timing issues and changes in automated reporting techniques for rain + snow. The application of this chart to today's weather is that thicknesses are well below 540 over eastern Iowa and closer to 540 over western Iowa. Snow is more likely the further east into the falling precipitation you go!

Voting:
Good: 50
Bad: 5

Tags:   forecasting   sounding   snow  


Severe 500mb Flow

19 Sep 2013 05:43 AM
Severe weather looks to be a possibility today with the passage of a cold front. The featured chart presents the combination of 500 hPa flow as observed by the Omaha sounding on September days with severe thunderstorm and/or tornado warnings issued for Iowa. The polar chart shows the wind direction and speed with the size of the dots indicating the number of warnings issued that day. 500 hPa (same unit as millibar) flow is important to thunderstorms as changing wind speed and/or direction is necessary with increasing height to separate updrafts and downdrafts in long lived thunderstorms. During the summertime, wind shear is often weak and so a downdraft will often choke out the updraft of a storm and limit its lifetime. The chart shows that at least 10-20 knots of wind speed are typically needed for severe events. The forecasted wind at 500 hPa this evening for Omaha appears to be around 30-40 knots from the WSW.

Voting:
Good: 74
Bad: 11

Tags:   sounding  


Lapse Rates

14 Aug 2013 05:26 AM
Atmospheric soundings provide twice daily vertical profiles of the troposphere and lower stratosphere. The lapse rate is calculated from this profile as the rate of temperature change with height. The featured chart presents the monthly distribution of the lapse rate between 850 and 500 hPa (millibar) pressure levels for the Omaha sounding site. This is roughly from a typical cloud base to midway vertically through the cloud. When this lapse rate is below 6 °C per kilometer, the atmosphere is in a stable condition meaning vertical temperature gradients are not large enough to drive robust motions due to buoyancy. When values are between about 6 and 9.5, the atmosphere is conditionally unstable and vertical motions are likely to occur. Values above about 9.5 are said to be absolutely unstable and vertical mixing will happen quickly. The chart shows the strongest instabilities during the summer season, which is also the time of year for thunderstorms. Of course, there is a direct relationship between thunderstorms and lapse rates. Stability increases (lower lapse rate) in the winter time and so we see fewer thunderstorms. For comparison, recent lapse rates have been in the mid 5s, which is not high enough for robust storms.

Voting:
Good: 57
Bad: 4

Tags:   sounding   lapserate  


13C Forecasting Rule

06 Aug 2013 05:35 AM
When the sun heats the ground, the ground heats the lower atmosphere which drives a mixing process with warm air rising and cold air sinking. This mixing process grows vertically creating a well mixed layer. If this mixing encounters colder air aloft, relatively more heating needs to occur to continue raising the temperature of the mixed layer. If this mixing encounters warmer air aloft, the heating process can accelerate as temperatures are already warm. Atmospheric soundings sample the atmosphere twice per day. The morning sounding provides a sampling of the air above our heads that may be mixed down during the day. A informal rule of forecasting in the plains is to add 13 Celsius to the 850 hPa (millibar) level temperature to provide an afternoon high temperature. For example, the morning sounding on Monday for Omaha had 16.2°C at 850 hPa and when you add 13°C to that reading it yields a high temperature of 85°F. The actual high was 87°F! The featured chart looks at the monthly distribution of change in afternoon high temperatures from the 850 hPa morning temperature. The green bar represents 13°C. The box plots provide detail on the temperature distributions and for the warm six months of the year, this relationship closely approximates the monthly mean. The cold months are dominated by horizontal advection processes and also have decreased solar heating, so this rule does not apply well then.

Voting:
Good: 99
Bad: 7

Tags:   sounding   forecasting  


Cold above our heads

29 Jul 2013 05:43 AM
The past few days have been rather remarkable for July with record lows being set over parts of the midwest. Each day the sun heats the ground, which warms the air near to the ground. This warmed air vertically mixes growing what is known as the planetary boundary layer. The air temperature we experience near the ground is a result of the strength of the surface heating and the mixed air down from above. The featured chart shows a climatology of air temperatures at the 850 hPa pressure level for the sounding site near Minneapolis for its 12 UTC (6/7 AM CT). This level is typically included in the planetary boundary layer. For this recent event, the temperature at this level (2.2 centigrade) was the coldest on record for the site in July.

Voting:
Good: 108
Bad: 16

Tags:   sounding  


click the image for any hope of being able to read it

Easterly winds aloft

17 Jul 2013 05:17 AM
The weather pattern earlier this week was a bit strange with the southern plains experiencing a storm system that traveled west! The winds well above the ground surface, that help guide storm systems, were actually from the east. The featured chart looks at the frequency of easterly winds at a height of 200 hPa (millibars), which is about 7- 8 miles above the earth surface. The chart presents 12 wind roses, one for each month for the sounding site near Omaha. The site launches a weather balloon twice per day, providing a vertical profile of temperature, humidity, pressure, and winds. The wind roses provide frequencies of wind direction and speed. Westerlies clearly dominate for all months of the year. Each month is labelled with the percentage of observations that are primarily easterly. The highest percentages (although meager) are in the summer time, which is when the jet stream (very strong westerlies) is furthest away permitting such flow to occur. So while such events are very rare, this is the time of year when they most frequently happen.

Voting:
Good: 84
Bad: 16

Tags:   sounding  


Sounding from twisterdata.com, webcam image from Nevada, IA

Thin Clouds

10 May 2012 05:56 AM
If you live in Iowa, you may have noticed that the clouds yesterday afternoon were very thin (having short vertical extent). The featured chart presents a vertical sounding of the atmosphere over Ames based on model data from the RAP model. The red line is temperature and green line is dew point. As the sun heats the ground, the lower part of the atmosphere warms and mixes vertically until it reaches air that is relatively much warmer and unwilling to mix down. The sounding shows that this level was very well defined with a sharp increase in temperature and rapid decrease in moisture. This caused the vertical extent of the clouds that developed at the top of the mixed lower atmosphere to be very flat.

Voting:
Good: 30
Bad: 7

Tags:   sounding  


click image for better view

Lots of water

07 Jul 2010 06:07 AM
The featured chart is from the excellent precipitable water climatology page on the Rapid City, SD NWS website. The chart displays the observed monthly ranges of the amount of water in the atmosphere. On Monday evening, the Davenport sounding had well over 2 inches of water observed, which is near the maximum end of the range. All of this water in the atmosphere makes for efficient rainers as slow moving storms are able to quickly process all of that water in the column.

Voting:
Good: 20
Bad: 8

Tags:   sounding