Urban Areas and Precipitation

Precipitation Remind yourself how the principle of convection rainfall works… Remind yourself how the principle of convection rainfall works…

As seen in the previous lesson, tarmac and concrete absorbs more of the suns energy than grass and soil. This heat is released and increases the principles of convection rainfall. As seen in the previous lesson, tarmac and concrete absorbs more of the suns energy than grass and soil. This heat is released and increases the principles of convection rainfall. As cities experience growth in their footprint (urban sprawl) the surface area increases, absorbing more heat and heating the air over a greater area. As cities experience growth in their footprint (urban sprawl) the surface area increases, absorbing more heat and heating the air over a greater area. Baltimore USA >>>>>> (Yellow area is urban) Baltimore USA >>>>>> (Yellow area is urban)

The result is more frequent rainfall and due to the increased heat = increased updraft = increased chance of cumulonimbus clouds = more intense rain and thunderstorms. The result is more frequent rainfall and due to the increased heat = increased updraft = increased chance of cumulonimbus clouds = more intense rain and thunderstorms. Tall buildings also fuel thunderstorm formation as wind is forced to rise. Tall buildings also fuel thunderstorm formation as wind is forced to rise. A third element is the increase in particles from urban activities. This adds potential microscopic nuclei to the atmosphere, an essential part of the precipitation ladder. A third element is the increase in particles from urban activities. This adds potential microscopic nuclei to the atmosphere, an essential part of the precipitation ladder.

Research has shown that Baltimore received 30% more rainfall than would be expected if there was no urban area there. Research has shown that Baltimore received 30% more rainfall than would be expected if there was no urban area there. This makes flooding more likely (more rain, more intense rain, combined with the impermeable urban surfaces stopping infiltration) This makes flooding more likely (more rain, more intense rain, combined with the impermeable urban surfaces stopping infiltration) In U.S. cities, summer rainfall is 9–27% greater than in rural areas, the incidence of thunderstorms is 10–42% greater, and hailstorms are more frequent by over 67% In U.S. cities, summer rainfall is 9–27% greater than in rural areas, the incidence of thunderstorms is 10–42% greater, and hailstorms are more frequent by over 67%

Read the case study on Baltimore Read the case study on Baltimore 08/pues-cit php 08/pues-cit php 08/pues-cit php 08/pues-cit php And the more complicated article on Atlanta And the more complicated article on Atlanta atlanta/atlanta-s-urban-heat-island-enhances- thunderstorms atlanta/atlanta-s-urban-heat-island-enhances- thunderstorms atlanta/atlanta-s-urban-heat-island-enhances- thunderstorms atlanta/atlanta-s-urban-heat-island-enhances- thunderstorms

So, urban areas cause more rain, intense rain and intense thunderstorms. So, urban areas cause more rain, intense rain and intense thunderstorms. They can also cause fog. They can also cause fog.

FOG Comes in many forms…..

WHAT YOU NEED TO KNOW Definition of fog Definition of fog Under what conditions does fog form? Under what conditions does fog form? The different types of fog The different types of fog

Definition Natural phenomenon that can be regarded as a hazard. It is an obscurity at the surface where visibility is below 100m. It is water suspended in the surface layer of the atmosphere. Natural phenomenon that can be regarded as a hazard. It is an obscurity at the surface where visibility is below 100m. It is water suspended in the surface layer of the atmosphere. MIST = Less dense fog MIST = Less dense fog

Airlines <1000m visibility = fog <1000m visibility = fog 1000m-2000m = mist 1000m-2000m = mist HAZE = Obscurity caused by dust or smoke. Cooling of mist leads to dense fog, cooling of haze does not HAZE = Obscurity caused by dust or smoke. Cooling of mist leads to dense fog, cooling of haze does not SMOG = Fog mixed with smoke or pollutants such as sulphur dioxide. Reaction between sunlight and oxides. SMOG = Fog mixed with smoke or pollutants such as sulphur dioxide. Reaction between sunlight and oxides.

For Fog to Form….. Air MUST be moist (close to saturation) Air MUST be moist (close to saturation) The air must be forced to cool The air must be forced to cool How it cools makes different types of fog How it cools makes different types of fog Fog is essentially a cloud Fog is essentially a cloud

Radiation Fog Radiation fog is formed by the cooling of land due to long wave radiation along with conditions of clear sky. The cool ground then produces condensation in the nearby air. In perfect calm the fog layer can be less than a metre deep but turbulence can promote a thicker layer. Radiation fog is common in autumn and usually does not last long past sunrise. Radiation fog is formed by the cooling of land due to long wave radiation along with conditions of clear sky. The cool ground then produces condensation in the nearby air. In perfect calm the fog layer can be less than a metre deep but turbulence can promote a thicker layer. Radiation fog is common in autumn and usually does not last long past sunrise. Explain how this could occur in a city?

Advection Fog Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passes over an area with significant snow pack. It's most common at sea when tropical air encounters cooler higher-latitude waters, or in areas of upswelling, such as along the California coast Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passes over an area with significant snow pack. It's most common at sea when tropical air encounters cooler higher-latitude waters, or in areas of upswelling, such as along the California coast Explain how this could occur in a city?

Advection Fog

Precipitation Fog This fog type generally occurs when rain falling from warm air aloft evaporates at or near the surface under light wind conditions. The evaporating precipitation as it falls through colder air thus increasing the surface air's moisture burden until condensation is achieved. This fog type generally occurs when rain falling from warm air aloft evaporates at or near the surface under light wind conditions. The evaporating precipitation as it falls through colder air thus increasing the surface air's moisture burden until condensation is achieved.

Other Fog The next few slides are not necessarily associated with Urban Areas. The next few slides are not necessarily associated with Urban Areas. However, you might want to read them as you might be interested in other types of fog, and it does explain why certain conditions are found at a local scale (some of which affect Stroud) However, you might want to read them as you might be interested in other types of fog, and it does explain why certain conditions are found at a local scale (some of which affect Stroud) You should also read the ‘Formation of Fog’ document on the VLE. It has good detail on fog and some great info on the next lesson (Photochemical Smog) You should also read the ‘Formation of Fog’ document on the VLE. It has good detail on fog and some great info on the next lesson (Photochemical Smog)

Up Slope Fog When air tries to move over a terrain obstacle, it cools to some degree as it rises, the degree of cooling depending on the amount of rise. During that cooling, if the air temperature falls below the dew point, the resulting condensation will form a cloud. If that cloud hugs the ground, it becomes fog at that surface. For example, an air mass moving over a water body may gain moisture until it is very near its saturation level. While still over the water, it may not cool enough to reach condensation, but when forced over the shore, the rise from water level may cause enough cooling to form a cloud/fog. When air tries to move over a terrain obstacle, it cools to some degree as it rises, the degree of cooling depending on the amount of rise. During that cooling, if the air temperature falls below the dew point, the resulting condensation will form a cloud. If that cloud hugs the ground, it becomes fog at that surface. For example, an air mass moving over a water body may gain moisture until it is very near its saturation level. While still over the water, it may not cool enough to reach condensation, but when forced over the shore, the rise from water level may cause enough cooling to form a cloud/fog.

Valley Fog Valley fogs form when the air near the terrain heights cools — usually by radiation at night — and descends through its greater density into the surrounding valleys, flowing like water. Pooled in the valley, the cold air may condense the water vapour present into a fog which fills the valleys to a depth. Valley fogs form when the air near the terrain heights cools — usually by radiation at night — and descends through its greater density into the surrounding valleys, flowing like water. Pooled in the valley, the cold air may condense the water vapour present into a fog which fills the valleys to a depth. E.g. Temperature Inversion E.g. Temperature Inversion

DEW As air cools, it has the ability to hold less moisture As air cools, it has the ability to hold less moisture Leaves may be colder than the surrounding air, and so any ‘vapour’ touching it may condense and form dew. Leaves may be colder than the surrounding air, and so any ‘vapour’ touching it may condense and form dew. The lowest few inches of the atmosphere are likely to have more moisture than that above it. Where does this come from? The lowest few inches of the atmosphere are likely to have more moisture than that above it. Where does this come from? Explain why wind makes dew unlikely to form? Explain why wind makes dew unlikely to form?

DEW

FROST NOT simply Dew that gets frozen NOT simply Dew that gets frozen SUBLIMATION (Vapour – Ice) Gas – Solid missing out the liquid stage SUBLIMATION (Vapour – Ice) Gas – Solid missing out the liquid stage

FROST Calm winds Calm winds Temperature below freezing at grass level Temperature below freezing at grass level Radiation cooling (clear skies) Radiation cooling (clear skies) 100% relative humidity 100% relative humidity