The Atmosphere from Space: Weather

Introduction to Earth Observation

What can we measure from space?

• The weather is the atmospheric state at a specific space or time
• This is related to physical properties such as temperature, wind and humidity
• These quantities vary from day-to-day and across the world leading to all different types of weather
• Our weather happens in the troposphere, the densest and lowermost portion of the atmosphere

What is the weather?

• Until the 1960s, meteorologists relied on in situ ('in place') observations
• First satellites that took images of Earth were around 1960
• We now have wider coverage due to a series of geostationary satellites
• More sophisticated instruments increase the quality and reliability of the data
• We have more than 100k measurements for different observations every day which improves modelling and forecasting

A brief history...

• Altitude of ~800km
• Higher resolution
• Passes near to or directly over the pole on each orbit
• Global coverage due to Earth rotating underneath
• Can take anywhere from a day to several weeks to get imagery from the same location

Polar-Orbiting Satellites

• Altitude of ~36000km above the Equator
• Give frequent imagery (~every 30 mins) over a fixed view
• Orbit in sync with the Earth’s rotation
• Surface receivers only have to point in one direction

Geostationary Satellites

FUN FACT

At the surface we usually experience a pressure of around 1000 hPa or 100,000 Pa - that’s equivalent to the weight of around 1kg. If you went 10m underwater you would feel an additional pressure of 1000 hPa

• Air pressure is a result of the weight of all the air above it
• Normally we think of air moving from high pressure to low pressure down the pressure gradient
• However, the Earth's rotation and friction causes wind to deviate from the pressure gradient!
• In the Northern Hemisphere and at mid-latitudes (for example in Europe) this causes winds to go clockwise around areas of high pressure and anticlockwise around low pressure (opposite in Southern Hemisphere)
• High pressure tends to be associated with fair weather, and low pressure is associated with unsettled and often wetter weather

Pressure and the Wind

• Isobars are a useful way of showing how air pressure varies across space
• Each isobar (line) connects points with the same pressure (this idea is very similar to contour lines on a map which join up points of equal height)
• As you move from one isobar to the next one the pressure goes up or down by a constant amount
• If you keep moving down the isobars (smaller numbers) you will find an isobar that doesn’t contain any others, this is a local area of lowest pressure
• If you keep going up the isobars you will find a local area of highest pressure
• In the real world the isobars rarely form nice neat circles

Isobars

• Fronts are boundaries of air masses. The Coriolis force, resulting from Earth’s rotation coils up the air boundaries, forming the characteristic spiral arms of fronts which we see around an area of low pressure
• Areas of high pressure have few fronts near them

Fronts

• Warm front: A warm air mass is pushed up and over the cold air in front of it
• Clouds and light rain often form ahead of the front - there tend to be clear conditions behind it

Warm Fronts

• Cold front: A cold air mass catches up to warm air forcing it to rise over the colder air
• This often leads to storm (cumulonimbus) clouds and lots of rainfall, with cold weather coming after the cold front passes by

Cold Fronts

Stationary front:

• Air masses run alongside one another - the boundary doesn’t move
• The differences in temperature and wind on either side of the front produce cloudy and often rainy/snowy weather, especially if near a region of low pressure

Stationary and Occluded Fronts

• Temperature can be calculated from the amount (and frequency) of light energy that the atmosphere emits back into space
• The way in which this light bends (refracts) can be used to determine the temperature and amount of water vapour in the air

Temperature & Humidity

• Clouds are important as they carry large amounts of water and heat around the Earth
• They also affect how much of the Sun’s energy reaches the ground and how much escapes
• Infrared measurements can determine the height of a cloud and visible imagery gives an idea of their thickness
• The amount of water and ice in a cloud can be found using radar
• Combining all of these together can tell us what type of cloud is present and what kind of weather it could bring

Clouds and Rain

• Wind moves air around and so can bring warm/cold air, clouds and rain from place to place
• Wind can be inferred from the movement of clouds and other atmospheric properties
• Wind can also be calculated using pressure measurements. Physics equations explain how air moves between regions of high and low pressure on our rotating planet
• Frequent imaging is critical as wind direction and speed can change very rapidly
• Radar backscatter from the sea surface can give us information about the wind speed over the ocean

Wind

You have all had a go at using satellite data to interpret the weather - it’s hard!

How do forecasters make accurate weather predictions?:

• Satellite data, in situ observations and models allow gaps in data to be filled t
• Looking at past events allows us to learn how the atmosphere behaves. What we learn can be used in simulations to predict the weather based on current conditions
• With more instruments and satellites in use, we have more frequent and better quality data about current atmospheric conditions. This improves the quality of forecasts
• Scientists analyse data, techniques and models. They assess how good our forecasts are and try to understand their limitations and uncertainties

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We discovered what a satellite is and how it is used to measure properties of the atmosphere

Lesson Recap 1

Features of UK weather

Lesson Recap 2

How satellite data can be used to make a forecast

Lesson Recap 3