L2 The Motion of the Atmosphere

Introduction

• This lesson explores atmospheric heating and horizontal air movement, resulting in wind.

• The sun's energy drives air movement, but not directly.

Heating of the Atmosphere

• The atmosphere is transparent to solar radiation and acquires virtually no heat energy directly from the sun.

• Solar radiation heats the earth's surface, and non-uniform heating has meteorological consequences.

• Factors causing uneven heating:

◦ Spherical shape of the earth.

◦ The oblique angle of the atmosphere at different latitudes.

• Tropical areas receive a higher concentration of insolation (solar radiation) than polar regions.

• The earth's surface absorbs energy, which is then transferred to the troposphere through:

◦ Conduction (near the surface).

◦ Terrestrial radiation (especially at lower levels).

◦ Convection (vertical movement of air currents).

• The atmosphere is heated from below.

• The Earth's tilt causes hemispheres to receive varying solar energy concentrations based on Earth's position in its orbit, creating seasons.

• Surface temperatures are warmer at the equator due to concentrated solar energy.

Factors Affecting Atmospheric Heating

• Uneven surface heating leads to a circulation of air.

◦ Air at the tropics is warmed, becomes less dense, rises, and spreads out.

◦ Cold ground at the poles cools the air, increasing its density.

• This results in:

◦ High pressure at the poles.

◦ Low pressure in the tropics.

◦ Air moving from high to low pressure (surface wind) from the poles to the tropics.

• Modifying factors:

◦ Different surfaces have different heating properties.

▪ Land heats and cools quickly (hourly changes).

▪ Water changes temperature slowly (monthly changes).

▪ Oceans retain heat after summer and cool slowly through winter.

• Uneven heating of the earth's surface creates areas of high and low pressure.

• Air flows from high to low pressure, creating wind.

Measurement of Pressure

• Knowledge of pressure distribution is important for weather assessment and forecasting.

• Meteorological stations measure surface atmospheric pressure.

• Complicating factor: observing points are at different elevations (altitudes above sea level).

◦ Example: Blackpool Airport is 37 ft AMSL, Biggin Hill is about 600 ft AMSL.

◦ Elevation differences cause pressure differentials (approximately 27 ft = 1 mb).

• Direct plotting of surface pressure measurements is pointless due to different elevations.

• To create a uniform datum, observed pressure is increased using a standard formula to calculate the pressure at sea level.

• A chart of observed pressures is drawn with all figures based on a common sea-level datum.

• This sea level pressure datum is called QFF.

Isobars

• Pressure measurements are plotted on a chart.

• Lines are drawn joining points of equal sea-level pressure; these lines are called isobars.

• Isobars are similar to contour lines on a map, indicating:

◦ High pressure areas.

◦ Low pressure areas.

◦ Pressure gradients.

• 'Isobar' is derived from Greek words: 'isos' (equal) and 'baros' (weight) - lines of equal weight of atmosphere.

Horizontal Pressure Gradient

• Air flows down a pressure gradient from high to low pressure, like water flowing downhill.

• Steeper ground slope corresponds to closer contour lines.

• Steeper pressure gradient (close isobars) means a large pressure change over a short distance, indicating stronger winds.

• Slack pressure gradient (widely spaced isobars) indicates light winds.

• Relationship between isobar spacing and wind strength allows estimation of wind strength from weather charts.

Convergence and Divergence

• Surface low-pressure systems cause air to rush in from all directions.

• The air collides and rises, leading to convection and cloud formation.

• Rising air diverges in a higher-pressure system near the troposphere (an 'upper high').

• Surface high-pressure systems lead to divergence.

• Divergence leads to subsidence and an upper low near the troposphere.

• In active pressure systems, pressure patterns at the surface and the tropopause are generally opposite.

Coriolis Force

• Air doesn't flow directly from high to low pressure due to the Coriolis force.

• Coriolis force is a consequence of the earth's rotation.

• Imagine drawing a straight line from the center of a rotating disc; the rotation curves the line.

• Coriolis force acts similarly on moving particles.

• In the northern hemisphere, the force acts at 90° to the right of the moving particle.

• In the southern hemisphere, the force acts at 90° to the left.

• Coriolis force is strongest at the poles and zero at the equator.

• The Coriolis 'force' is not a Newtonian force but the result of linear and rotational movement.

• Often referred to as the Coriolis 'effect', named after Gustave-Gaspard Coriolis.

Geostrophic and Gradient Winds

• Air movement due to a pressure gradient is deflected by the Coriolis 'force' (to the right in the northern hemisphere).

• Deflection 'curves' the path of air particles.

• If the deflection is sufficient, air flows parallel to straight isobars.

• This occurs when the pressure gradient force equals the Coriolis 'force', resulting in the geostrophic wind.

• Geostrophic wind applies to straight isobars; curved isobars (around high or low pressures) result in the gradient wind.

• A practical feature of gradient wind is that for equally spaced isobars, wind speed around a low-pressure system is often less than around a high-pressure system.

Measurement of Wind

• Wind is measured and forecast as a velocity (direction and speed).

• Wind direction is the direction from which the wind is blowing.

• Weather forecasts/reports use degrees 'true', while ATSUs use degrees 'magnetic'.

• Wind speed is measured by an anemometer and reported in knots (KT, nautical miles per hour), sometimes metres per second (MPS) or kilometres per hour (KMH).

• The knot is standard in aviation, but metres per second are used in eastern Europe.

• Met surface pressure charts use isobars to estimate wind direction and spacing to estimate wind strength.

• Surface friction changes both speed and direction.

• A rule-of-thumb conversion: double the metres per second figure to obtain the speed in knots (10 MPS ≈ 20 KT).

• Wind velocity is reported as direction and speed (e.g.,