What are thermals? Thermals are columns of rising warm air

Transcription

1 THERMALS

2 What are thermals? Thermals are columns of rising warm air

3 What is required for a thermal? Solar radiation to warm the ground and surface features.

4 What is required for a thermal? Solar radiation to warm the ground and surface features. Surface differences to provide differential warming of the air.

5 What is required for a thermal? Solar radiation to warm the ground and surface features. Surface differences to provide differential warming of the air. Sufficient warming of the air to start it rising Trigger temperature.

6 What is required for a thermal? Solar radiation to warm the ground and surface features. Surface differences to provide differential warming of the air. Sufficient warming of the air to start it rising Trigger temperature. Remain warmer than its surroundings Instability.

7 What is required for a thermal? Solar radiation to warm the ground and surface features. Surface differences to provide differential warming of the air. Sufficient warming of the air to start it rising Trigger temperature. Remain warmer than its surroundings Instability. Dry air to give a reasonable cloudbase Dew point.

8 What is required for a thermal? Solar radiation to warm the ground and surface features. Surface differences to provide differential warming of the air. Sufficient warming of the air to start it rising Trigger temperature. Remain warmer than its surroundings Instability. Dry air to give a reasonable cloudbase Dew point. Limit to growth to prevent overdevelopment Stability.

9 Solar Radiation Solar radiation does not warm the air it passes through. Solar radiation warms any surface it hits. That surface warms anything in contact with it - conduction. The air above the surface warms by mixing - convection.

10 Surface differences Angle - south facing slopes receive more sunlight. Wind - surfaces in a wind shadow will warm better. Hard surfaces buildings, car parks, dry fields warm better. Foliage stores warmth for release later in the day. Wet surfaces do not warm.

11 Will the warm air rise? Is the warmed air warmer than its surroundings? Will it stay warmer than its surroundings? And for how long? All this depends on various Lapse Rates. A Lapse Rate is the rate at which temperature changes with altitude. Some are physical rules of the atmosphere, and one (the important one) is what is actually happening out there.

12 Lapse Rates There are four lapse rates to consider: Dry Adiabatic Lapse Rate (DALR). Saturated Adiabatic Lapse Rate (SALR). Environmental Lapse Rate (ELR) of a standard atmosphere. Environmental Lapse Rate (ELR) of the actual atmosphere.

13 DALR definition The dry adiabatic lapse rate (DALR) is the rate of temperature decrease with height for a parcel of dry or unsaturated air, rising under adiabatic conditions. Unsaturated air has less than 100% relative humidity; i.e. its actual temperature is higher than its dew point. The term adiabatic means that no heat transfer occurs into or out of the parcel. Air has low thermal conductivity, and the parcels of air involved are very large, so transfer of heat by conduction is negligibly small. Under these conditions when the air rises (for instance, by convection) it expands, because the pressure is lower at higher altitudes. As the air parcel expands, it pushes on the air around it, doing work. Since the parcel does work but gains no heat, it loses internal energy so that its temperature decreases. The rate of temperature decrease is 3.0 C per 1,000 ft. The reverse occurs for a sinking parcel of air.

14 DALR demonstration The Dry Adiabatic Lapse Rate (DALR) is the rate of temperature decrease of dry air with height. It is 3 o C per 1000 feet

15 SALR definition When the air is saturated with water vapour, at its dew point, the saturated adiabatic lapse rate (SALR) applies. This lapse rate varies strongly with temperature. A typical value is around 1.5 C per 1,000 ft. The reason for the difference between the dry and saturated adiabatic lapse rates is that latent heat is released when water condenses, thus decreasing the rate of temperature drop as altitude increases.

16 SALR demonstration The Saturated Adiabatic Lapse Rate (SALR) is the rate of temperature decrease of saturated air with height. It is 1.5 o C per 1000 feet

17 ELR definition The environmental lapse rate (ELR), is the rate of decrease of temperature with altitude in the stationary atmosphere at a given time and location. As an average, the International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 1.98 C per 1,000 ft from sea level to 36,090 ft. From 36,090 ft up to 65,620 ft the temperature is constant at 56.5 C, which is the lowest assumed temperature in the ISA. The standard atmosphere contains no moisture. Unlike the idealized ISA, the temperature of the actual atmosphere does not always fall at a uniform rate with height. For example, there can be an inversion layer in which the temperature increases with height.

18 ELR demonstration ISA Example of an actual atmosphere The Environmental Lapse Rate (ELR) for the International Standard Atmosphere (ISA) is 1.98 o C per 1000 feet The ELR of the actual atmosphere varies dramatically with meteorological conditions.

19 ELR an example Actual ELR is measured by taking dry and wet bulb thermometers (and other measuring equipment) to altitude on a balloon ascent. Dry bulb thermometer measures temperature. Wet bulb thermometer measures dew point.

20 Stability DALR SALR ELR 3 ELR 2 ELR ELR1 < SALR = absolutely stable SALR < ELR2 < DALR = conditionally unstable ELR3 > DALR = super-adiabatic lapse rate = absolutely unstable

21 What does all this give us? Air at 18 O will rise to about 500 feet and stop low level inversion Air at 20 O will rise above the inversion trigger temperature Air at 21 O will rise to about 4500 feet and stop top of thermal, and...cloudbase. Here things get a bit more complicated see next slide Air at 24 O will rise to about 5500 feet and stop high level inversion

22 What happens at cloudbase? The parcel of air that rose from the surface at 21 o has now cooled to about 7 o at 4500, where it meets the ELR and stops rising. However it also meets the dew point line, moisture condenses to form cloud. The parcel of air is now saturated and must follows the SALR, to the inversion at This is a day of short lived shallow clouds, before going blue.

23 Meteorologists make it difficult! That fairly simple graph of temperature against altitude is made difficult by the professionals, and is called a Tephigram or a Skew-T plot. The temperature gridlines are rotated through about 45 O clockwise. Altitude is plotted as pressure, at lower levels 1mb = 30ft, so 850mb is about 5000ft. Whole families of DALR and SALR are plotted. Moisture content lines are added at lower levels. A wind speed and direction indication is added.

24 Meteorologists make it difficult! A Tephigram or Skew-T plot

25 Rules of thumb General rules to spot a good thermal soaring day (but we all know that rules don t apply in Devon!): The passage of a cold front hours earlier - Instability A ridge of high pressure, anticyclonic curvature - Stability above North westerly wind < 10knots - cold, clear airmass Clear night, wet grass - low dew point, high cloudbase Cloudbase = (Tmax Tmin) x 400ft

26 Further study For a much deeper, but fairly simply, explanation of the analysis of Tephigrams visit Weatherjack s tutorial page at: