The radar beam is the focused pulse of microwave energy transmitted from the antenna and later received after scattering off hydrometeors and other targets. Its shape, width, and elevation angle set the volume of atmosphere the radar can actually see at any given moment.
What the beam is made of
A weather radar transmits a short pulse of electromagnetic energy through a parabolic dish. The dish focuses that energy into a main lobe, and it is the width of that lobe that meteorologists refer to as the beam. On the WSR-88D, the S-band pulse operates near 2850 megahertz with a beamwidth close to one degree.
Because the pulse travels at the speed of light, the range to any target is set by how long the echo takes to return. Range gates near 250 meters long are stitched together along each radial to build the reflectivity and velocity fields.
Where the beam actually goes
Every scan is defined by an azimuth measured clockwise from true north and an elevation angle above the horizon. At each tilt, the beam sweeps a full circle and samples a thin conical slice of atmosphere. A 0.5 degree tilt hugs the low levels near the radar and gradually climbs into the mid-troposphere at long range.
The height at which the beam is intersecting a storm is called the echo height. This height grows with distance because of both the elevation angle and Earth's curvature, so a low-level circulation 150 kilometers from the radar may be several thousand feet above the ground even on the lowest tilt.
How forecasters use beam geometry
Reading a radar image well requires holding the beam geometry in mind. A low tilt is best for sampling near-surface features close in, while higher tilts are used to look at storm tops or the mid-level rotation of a mature updraft. Multiple tilts stacked together form the volume scan that supports composite products and vertical cross sections.
When a storm sits far from the radar, forecasters lean on nearer sites, dual-pol variables that survive averaging, and surface observations to fill in what the beam cannot resolve. When it sits close, they can trust fine structure like tight hook echoes and low-level velocity couplets.