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Wind chill factor takes into account wind speeds and humidity to assess how the human body actually feels temperature.

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Cloud is an aggregate of very small water droplets, ice particles or a mixture of both. In most cases cloud water droplets have a diameter less than 200 microns as particles larger than this generally fall out of the atmosphere as rain or drizzle.

The large scale distribution of cloud is best measured from space in either the visible or infra red part of the spectrum. This satellite imagery is revealing in many ways, but it has its limitations for the measurement of cloud, in particular:

Extensive high cloud can mask important features in the distribution of low cloud

It is often not possible to distinguish between shallow low cloud and fog

There is no way of measuring cloud base from space, an element of critical importance for aviation and other purposes

To meet these shortcomings cloud base measurements are made from all surface synoptic stations.

How we measure cloud type

The standard method for classifying clouds is based on the World Meteorological Organisation criteria. The classification is made in terms of the level of the cloud ( high, medium or low) and various cloud genera specified by the cloud form and its features.

Common types of cloud include cumulus, cumulonimbus, stratocumulus and stratus at low levels; altocumulus, nimbostratus and altostratus at medium levels and cirrus, cirrocumulus and cirrostratus at high levels. As there are no automated systems for the detection of cloud type, its reporting is only possible where the human observer is present.

How we measure cloud amount

At any time different types of cloud at different heights above the ground may be visible from an observing station. To cater for such variability both the total cloud amount and partial cloud amounts are reported. Total cloud amount is the fraction of the sky covered by cloud of any type or height above the ground. Partial cloud amount is the fraction of the sky covered by each type or layer of cloud as if it was the only type or layer of cloud in the sky. If a number of different layers of cloud are present the partial amount of each one will be reported. The sum of partial cloud amounts may exceed the total cloud amount or indeed exceed 100% of the sky.

Cloud amount is reported in oktas or eighths with the additional convention that:

0 oktas represents the complete absence of cloud

1 okta represents a cloud amount of 1 eighth or less, but not zero

7 oktas represents a cloud amount of 7 eighths or more, but not full cloud cover

8 oktas represents full cloud cover with no breaks

9 oktas represents sky obscured by fog or other meteorological phenomena

A cloud base recorder ( se e below) is used to estimate partial cloud amount. For each layer of cloud identified by the instrument a time weighted average is used to derive the average amount.

Such measurements are by their nature only representative of the cloud that has passed directly above the instrument, not of the total sky that is on view from the station. Total cloud amount is only reported from stations where the human observer is present.

How we measure cloud base height

Cloud base is readily measured by instrument to a reasonable level of accuracy. The type of cloud base recorder used widely at synoptic observing stations is shown below.

It employs a pulsed diode laser LIDAR ( ligh t detection and ranging) technology whereby short laser pulses ( e y e safe) are sent out in a vertical or near vertical direction. The backscatter caused by reflection from the surface of cloud, precipitation or other particles is analysed to determine the height of the cloud base.

Many modern cloud base recorders are capable of detecting up to three cloud layers simultaneously.

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Cloud is an aggregate of very small water droplets, ice particles or a mixture of both. In most cases cloud water droplets have a diameter less than 200 microns as particles larger than this generally fall out of the atmosphere as rain or drizzle.

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Humidity is a measurement of the amount of water vapour in the air. Find out more about the different types of humidity.

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Aspect affect on climate

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Climate describes the temperature, precipitation, and other weather conditions of a certain area. The climate of an area describes how these variables may be over a long period, for instance the South of France in the summer is hot and dry.

However this is not always the case as there are some days when it is raining and fairly cool. These daily changes do not affect the overall climate of the area, and are described as the weather of the area for that day.
The map below shows the main climatic regions of the world:
Many factors affect the climate of different areas around the world. Themain ones are described below:

Altitude: Temperature falls the higher youare. The loss in temperature is approximately 10°C for every kilometre yourise. This fall in temperature with height is due to the fact that the airat higher altitudes is thinner. There are fewer molecules available to receive and retain heat. Wind chill may also mean that the temperatures inupland areas are lower than expected.

Aspect: This relates to the direction in which a place is facing. Aspect only really affects local climate, not global ones. In the Northern Hemisphere, south facing slopes receive far more sunlight than north facing ones. These are therefore much better for agriculture and often settlement will locate there due to the better aspect.

Distance from the Sea: The sea can have agreat influence over climates in maritime regions. The sea takes much longer to heat up but retains that heat far better than the land. Consequently the land is often warmer than the sea during the day, however the situation is reversed at night.

This means that places like Britain, which are near the sea, have relatively cool days and warm nights, as well as warm summers and mild winters. The influence ofthe sea diminishes as you move further inland, for instance to central Europe, where warmer winters and cooler summers will be experienced.

Latitude: on global climates. Lines of Latitude are those that run horizontally around the world. The most famous line of latitude is the Equator. Using the lines of latitude we can easily divide the world in to specific climatic sections.

Between 23.5 South and 23.5 North is the region called thetropics. These receive the most heat from the sun because itis almost directly overhead and so the solar radiation does not have to travel through very much of the atmosphere. It heats a very concentrated area of the surface of the ground.

Between 23.5 and 66.5 in both the Northern and Southern Hemispheres are the temperate regions. These have reasonably hot summers and mild winters.

Above 66.5 in both hemispheres are the Polar Regions, which are very cold. They receive sunlight for only half the year due to the angle of the Earth on its rotation. During the months when they do have sunlight, it travels to them at a very shallow angle, meaning that it has to travel through a lot of the atmosphere and has to heat a wide area of the surface.

Ocean Currents: Many areas beside the sea are affected by ocean currents. These can be either warm or cold currents. The United Kingdom is directly in the path of the Gulf Stream and the Northern Atlantic Stream, which bring warm water from the Caribbean across the Atlantic to our shores. This current alone is the reason why our climate is mild compared to places of similar latitude like Newfoundland.

Prevailing Winds: The prevailing (most common) wind in the United Kingdom comes form the South West. If a prevailing wind travels over a warm surface it will bring warm weather, whilst if it travels over a cold surface it will bring cold weather.

The south-westerly winds in Britain travel across the Atlantic. In the summer this means that the air is cooled, whilst in the winter the water warms the air, bringing warmer winters than we might expect.

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The global distribution of climate Climate describes the temperature, precipitation, and other weather conditions of a certain area. The climate of an area describes how these variables may be over a long period, for instance the South of France in the summer is hot and dry.

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There are 3 main types of rainfall: relief, frontal and convectional. Describe each and give conditions when the occur. (10 marks)

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There are 3 main types of rainfall: relief, frontal and convectional. Describe each and give conditions when the occur. (10 marks)

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Adiabatic Lapse Rates - Altitude

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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.

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

Vertical currents dominate.

Cumulonimbus clouds and thunderstorms.

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.

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

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.

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.

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:

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.

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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IntroductionDescribe 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.