Lapse rates and microclimate

Lapse rates and microclimate

Describe the changes in temperature with height through the lower layers of the atmosphere. They are often shown in diagrammatic form to show atmospheric stability or instability.

Environmental lapse rate:

This is the expected decrease in temperature with height through the lower atmosphere, approximately 6.5 degrees per 1000m. It varies according to height, time of year, and over different surfaces.

Adiabatic lapse rate:

Used to explain what occurs as a parcel of air rises, decreases in pressure and temperature, but increases in volume. The conditions are reversed if a parcel of air moves towards Earth.

Adiabatic lapse rates can be either dry or saturated.

Dry adiabatic lapse rate (DALR):

This is the rate a parcel of air cools at as it rises (or warms if falling) if condensation does not occur. The rate is approximately 1 degree per 100m and is shown on the diagram beneath:

Lapse rates and microclimate

Saturated Adiabatic Lapse Rate (SALR):

The rate at which air cools if it has risen sufficiently to reach dew point temperature, and condensation occurs. The rate of cooling is slower than the DALR because of the release of latent heat. The SALR varies from 4 degrees per 1000m to 9 degrees per 1000m but the average is 5.5 degrees. The reason for the variation is because of the ability of warmer air to hold more moisture, and thus release more latent heat after condensation.

This is where a parcel of air rises and cools at a quicker rate than the air surrounding it. The parcel of air is colder and denser than its surroundings so cannot rise further and sinks. Condensation and then subsequent rainfall does not occur. Stability is most commonly associated with high pressure and anticyclones.

The diagram below shows a graph of stable air conditions: (The DALR lies to the right of the ELR).

Lapse rates and microclimate

On warm summer day's high levels of insolation can create high surface temperatures. The air above such localised surfaces is then heated by conduction, leading to a high lapse rate. The air rises and cools less quickly than its surroundings. If it remains warmer than its surroundings, the air parcel will continue to rise. If dew point is reached, clouds and thunderstorms may result.

Instability is shown in the diagram below:

Lapse rates and microclimate

Unstable air:

When lifting stimulus (ground heating) is relaxed, air continues to rise.

Vertical currents dominate.

Cumulonimbus clouds and thunderstorms.

SALR:

If rising air is saturated, its temperature falls at about 6 degrees per 1000m. Latent heat is released due to condensation, so decrease is slower.

ELR:

Temperature of air decreases with height at an average rate of 6.5 degrees per 1000m.

DALR:

When a mass of air rises, its temperature decreases adiabatically. The air expands and loses heat energy as it rises and if it is unsaturated, it loses 10 degrees for every 1000m of ascent.

Conditional instability

This occurs where the continuation of air rising to form condensation, clouds and precipitation, is conditional on something (for example, mountains and fronts). The state of instability is based on the air being forced to rise initially.

Temperatures usually decrease with height through the lower atmosphere (troposphere). A temperature inversion is when the normal conditions are reversed and warm air lies above cold air. It acts to trap air in the lower layers, and can cause the build up of pollutants and smog as in Los Angeles.

Lapse rates and microclimate

These are climates that exist over small areas, where the conditions of shelter, temperature, precipitation, humidity, winds, pressure and clouds are different to the general surroundings.

The most common from of microclimate studied in geography is that of urban microclimate.

Urban microclimates

Many urban areas demonstrate distinctive climatic characteristics that differ from the norm. The reasons for this are due to the man made structures that reflect and absorb temperatures differently to natural surroundings. Urban areas also generate more dust, and condensation nuclei. The main changes are outlined in the chart below:

Climate feature: Distinctive urban features:
Temperature Buildings retain and release heat more strongly than in rural areas, temperatures are higher than in surrounding areas. Tarmac surfaces are heated intensely and then strongly heat the air above it.
Winds Two main processes occur as a result of high-rise buildings. Wind can be 'channelled' between high-rise buildings increasing velocities. Buildings also act as windbreaks, reducing velocities.
Sunlight Temperatures may be higher than surrounding areas, but actual sunlight amounts are lower. High-rise buildings block out light and higher amounts of dust reflect and absorb sunlight.
Precipitation Greater amounts than other locations. Concrete surfaces lead to convection currents, and cloud formation.
Humidity Lower in urban areas as higher temperatures can hold greater amounts of water vapour.
Clouds Thicker and more frequent than in rural areas.

The urban heat island

This is the term used to describe the higher temperatures found in urban locations than in the surrounding countryside. It is mainly a result of high levels of pollution and heat being released into the atmosphere.

Key points

  1. Less energy needed for evapotranspiration, due to concrete surfaces, artificial drainage, and reduced wind speeds.
  2. Pollution and smog retain energy.
  3. Excessive amounts of fossil fuels burnt for industry and business, can be greater than inputs from the Sun.
  4. Buildings retain and conduct heat easily.
  5. Wind speeds are lower as a result of building height and roughness of urban surfaces.

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