Isobaric Surfaces METR DEC2009 Radiosondes are the main instrument for measuring the state of the atmosphere aloft. Isobaric maps (upper air maps) show observations on a constant-pressure “surface”. A common isobaric map is the 500 mb map (about half way up in the troposphere). The height of isobaric surfaces aloft vary from place to place. – Contours of constant height above sea level help us visualize the height pattern. Contours of constant height above sea level We can interpret height contours as if they were isobars. – Winds are faster where height contours are closer together. Winds are faster where height contours are closer together

Winds and Pressure Patterns Aloft and Lower Tropospheric Temperature Patterns The areas aloft with the fastest winds (say, > 60 knots; can be > 200 knots) tend to occur in the midlatitudes (30°-60° latitude).midlatitudes (30°-60° latitude) Areas with fastest winds aloft form a narrow belt around the midlatitudes. This is the jet stream.jet stream The jet stream has wobbles in it. These wobbles tend to migrate eastward (typically mph).jet stream has wobbles in it wobbles tend to migrate eastward

The fastest winds tend to occur where the PG is largest, which is also where the isobaric surfaces slope the most steeply (which is where height contours on an isobaric map are closest together).fastest winds tend to occur where the PG is largest The height of an isobaric surface aloft depends on the (average) temperature below that isobaric surface.height of an isobaric surface aloft temperature below that isobaric surface – Colder air in the lower troposphere creates lower heights (lower pressure) aloft. – Warmer air in the lower troposphere creates higher heights (and higher pressure) aloft.

Globally, it’s colder at higher latitudes (closer to the poles) and warmer at lower latitudes (closer to the equator). Globally, it’s colder at higher latitudes (closer to the poles) and warmer at lower latitudes (closer to the equator) – So heights (pressures) aloft are lower at high latitudes and higher at lower latitudes. – So contours of height (pressure) aloft tend to oriented east/west (though with wobbles in them). – So geostrophic winds, which “blow” parallel to height/pressure contours, generally blow eastward, northeastward, or southeastward.

In the transition area between low and high latitudes--at midlatitudes--the temperature gradient is largest.at midlatitudes--the temperature gradient is largest – So the height gradient (pressure gradient) aloft is largest at midlatitudes. – So winds aloft are fastest at midlatitudes (and thus the jet streams).

At midlatitudes, there are also east-west variations in temperature in the lower troposphere: “tongues” of colder air from higher latitudes “protruding” equatorward alternate with tongues of warmer air protruding poleward.“tongues” of colder air from higher latitudes “protruding” equatorward alternate with tongues of warmer air protruding poleward – So there are east-west variations in heights (pressure) aloft. – These features in the lower troposphere temperature pattern make height contours aloft wavy. The wavy features we call troughs (lower heights/pressure) and ridges (higher heights/pressure).height contours aloft wavy – The (geostrophic) winds follow these contours, so the jet stream has waves (troughs, ridges) in it, too.jet stream has waves (troughs, ridges)

Wind and Pressure Patterns On horizontal surfaces (such as at sea level), pressure varies from place to place.pressure varies from place to place – Maps with isobars drawn on them help us visualize the spatial pressure pattern. Maps with isobars drawn on them Pressure differences between places create a net force—the pressure-gradient (PG) force--on air, pushing toward lower pressure. The PG force pushes air into motion. – The strength of the PG force is greater where the pressure gradient (PG) is larger. – On a weather map, the spacing of isobars allows us to tell about the relative size of the PG and hence the PG force.

Once air is moving, the rotation of the earth affects the motion by apparently trying to deflect it. We account for this effect by inventing a Coriolis force.rotation of the earth affects the motion – The Coriolis force is stronger when the wind is faster. – The Coriolis force pushes on moving objects (including air) to their right in the N. Hem. and to their left in the S. Hem., but not at all at the equator.

Together, the PG force and Coriolis force tend to drive the wind close to geostrophic balance.PG force and Coriolis force tend to drive the wind close to geostrophic balance – A wind where the balance is exactly achieved is the geostrophic wind. – The observed winds aloft are usually close to the geostrophic wind. Winds aloft tend to blow toward the east, northeast, or southeast. Winds aloft tend to blow toward the east, northeast, or southeast

Friction with the Earth ’ s surface Near the earth’s surface, friction is a third important force (within the “friction layer”). – Friction opposes the wind, trying to slow it down. – Friction is larger over land than over water. (Land is “rougher” than water.) The 3-way combination of PG force, Coriolis force, and friction drives winds across isobars at an angle. The 3-way combination of PG force, Coriolis force, and friction drives winds across isobars at an angle

As a result, surface winds tend to converge into low-pressure areas and diverge out of high-pressure areas. (We don’t see this aloft, though!) As a result, air tends to move upward out of surface low-pressure areas and sink (subside) into surface high-pressure areas. Regardless of the combination of forces acting on air, winds tend to be faster where the PG (and hence PG force) is stronger.