The tropics do not cover the entire world!
When flying at night in the central United States, particularily during the summer months, it is not at all uncommon to have dissipating lines of thunderstorms which contain sections that top out well below normal jet cruising levels. The capability to judge the height of these storms greatly enhances situational awareness and the ability to choose the most sensible route across the line. Since some of these lines may literally stretch from Texas to Iowa, (several hundred miles) determining a low spot will often be one of the strategies employed to decide the best location to cross. It is sometimes not possible to cross in a location where NO precipitation returns are present on the wx RADAR.
Using the tilt to determine the areas of highest and lowest height of returns is one of the actions which may be performed in determining the most and least favorable places to cross the line. This can be a good confirmation of what is indicated by other information sources.
The specific techniques used will vary according to the particular characteristics and specifications of the installed system but the following
general technique works well to further verify suspected "low' or "soft" spots in the storm line in the aircraft I fly. Remember, this information is best used along with all other available information, not by itself.
With the range set to 100 miles, note the location of precipitation returns of interest, then tilt the antenna down until ground returns are noted at the 60 mile range. Note the selected angle. Now tilt the antenna up until the precipitation returns in your selected "soft spot" disappear from the screen. Note the tilt angle selected and find the difference between the two angles. Each degree of tilt equals aproximately 1 mile of height above the ground at 60 miles, so if it took 5 degrees of tilt difference to make the returns disappear, the precipitation tops are at aproximately 30,000' above the ground if they are at 60 miles. Adjust the tops estimate by 1,000' for each 10 miles of range difference from 60 miles. Higher if closer, and lower if further.
Look, this isn't anything more than a second, third or fourth way to verify what you think is happening ahead. I would never rely on this method as a sole source of information to make a decision as to where a storm line will be crossed. Precipitation returns by azimuth and intensity, lightning detection, reports from preceeding traffic, ATC RADAR reports and route suggestions, downlinked doppler RADAR feeds, and yes, good old visual observation and assessment are all arrows in the quiver. The more of these tools you have, the better. Each can be used in conjunction with the others as an aid in deciding the most appropriate course of action to take in crossing a line of storms which cannot be entirely circumnavigated. It's done all summer here, many thousands of times. I find the antenna tilt function to be one of several tools which can be used as an aid in enhancing situational awareness. Please remember that cumiluform cloud tops may extend well above airborn wx RADAR precipitation returns. Never rely soley on these precipitation top estimates to determine a course of action.
Best regards,
Westhawk